Invading pines get help

What makes for a successful invasion?  Is it better to invade with a small, fast moving force or a large, but less mobile force?  Should the invaders be capable of operating independently, or should they have partners (or make partnerships easily) with the existing population? Should resources be allocated to defending the individuals that make up the invasion force, or instead be allocated to recruiting large numbers of less-well-defended invaders?  While military strategists are confounded by these questions, pine trees have solved them. The solution is:

Z-score = 23.39 – 0.63(SM)1/2 -3.88(JP)1/2 -1.09(SC)

This equation was derived about 25 years ago by Marcel Rejmánek and David Richardson who wanted to know what plant attributes were associated with whether pine trees invaded new areas successfully.  They contrasted 12 species that had made successful invasions with 12 species who were primarily noninvasive, and derived the z-score as a quantitative measure of what attributes the invasive species shared.  A higher z-score was correlated with higher invasiveness.  Qualitatively, this equation tells us that invasiveness is correlated with small seed mass (SM), a short juvenile period (JP) and a short interval of time between large seed crops (SC). 

Pine seeds vary in size and number across species. Credit: Jaime Moyano.

Shift to the present time (or at least the recent past). Jaime Moyano and his colleagues were puzzling over whether it was better for these invaders to be capable of operating independently, or whether they should depend on partners.  Ecologists had assumed that independence was a good idea for invaders, and had framed an “ideal weed hypothesis” that plant species that depend on mutualisms are less prone to invade.  Common mutualisms for plants include association with pollinators, seed dispersers and fungi (mycorrhizae).

Pinus contorta (lodgepole pine) invades a forest near Christchurch, New Zealand. Credit: Martin Nuñez.

Moyano and his colleagues tested a prediction of the ideal weed hypothesis by going through the literature to see whether pine species seedlings with higher invasiveness are less dependent on mutualisms with ectomycorrhizal fungi (EMF).  EMF are an association between plant roots and fungi in which the fungal hyphae form a sheath around the root’s exterior and suck up nutrients which they may share with the plant. To test this prediction, the researchers compiled a database of 1206 data points in 34 species based on studies where researchers evaluated how pine seedlings grew with and without EMF inoculation. For each study, they calculated an effect size of EMF as equal to the ln(EMFP/EMFA), where EMFP is seedling biomass with EMF present , and EMFA is seedling biomass with EMF absent.  So a higher effect size indicates that EMF improves seedling growth.

All the pieces were together – all that was left was to do the analysis.  The prediction of the ideal weed hypothesis was that the most invasive species – the species with the highest Z-score – would be expected to have the lowest EMF effect size (be less dependent on mutualism).  The researchers discovered…exactly the opposite.  In general, invasive pines depended heavily on EMF mutualisms to aid seedling growth, while non-invasive pines were less likely to benefit from the services of EMF (top graph below).

EMF effect size in relation to invasiveness (Z score) (top graph). EMF effect size in relation to seed mass (bottom graph).

In addition, the researchers discovered that species with smaller seeds benefitted more from EMF (bottom graph above).  Initially, they were puzzled by these findings that conflicted with conventional expectations.  But then it started making sense…

Parental investment theory tells us that parents have a limited amount of resources that they can allocate to their offspring.  Given this limitation, some plant species make a small number of large seeds that are endowed with large stores of nutrients that the baby can use while germinating and a thick seed coat to protect it.  The downside of this approach is that the large seed might not disperse very far from its parent and may get shaded out by it.  Other plant species make large numbers of very small seeds that are very poorly supplied with nutrients.  The upside of this approach is that the seeds can be blown to new locations that might be ripe for germination (pine seeds are equipped with wings that facilitate traveling in the breeze when released).  The downside of this approach is that germinating seeds might run out of nutrients before they establish themselves.  This selects for a strong dependence on quickly establishing mutualisms to facilitate nutrient intake from the environment. All pines trees ultimately establish EMF, but the smaller-seeded most invasive plants benefit more from EMF early in development, and thus can travel long distances and still get enough nutrients to invade new habitats.

Lodgepole pines invade a forest in Patagonia. This species produces numerous tiny seeds and is highly invasive. Credit: Martin Nuñez.

The question then becomes, how generalizable are these results to other species and other types of mutualisms? The pattern of large seeds showing decreasing response to EMF has been found in some plant families but not others. There are not a lot of data on the relationship between plant invasiveness and their dependence on other types of mutualisms such as pollinators and animal seed dispersers. Moyano and his colleagues caution us that many factors are involved in biological invasions, which makes it very difficult to anticipate which species will be successful invaders.  

note: the paper that describes this research is from the journal Ecology. The reference is Moyano, J., M. A. Rodriguez-Cabal, and M. A. Nunez. 2020. Highly invasive tree species are more dependent on mutualisms. Ecology 101(5):e02997. 10.1002/ecy.2997. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

Urchins in hot water

The fabled Mediterranean Sea is under stress from overfishing, pollution, rapid warming, and the associated proliferation of invasive species that thrive in the warming waters.  Two species of rabbitfish (Siganus luridus and Siganus rivulatus) crossed the Suez Canal into the Mediterranean Sea in the 20th century, and now make up about 95% of the herbivorous fish in rocky habitats along the Levant Basin off the Israeli coast.  These fish are voracious feeders on macroalgae that live in the Levant, and they have become much more abundant during the past 30 years in association with increased water temperatures of 2-3 degrees C.

luridusRoberto Pillon

The rabbitfish Siganus luridus. Credit: Roberto Pillon at Wikipedia.

While the Levant has been warming and rabbitfish have been proliferating, things have not gone very well for the purple sea urchin Paracentrotus lividus.  Previously, it had been a very important consumer of macroalgae within the Levant, but its population has collapsed within the past decades.  For his Masters program, Erez Yeruham decided to investigate why the sea urchin population collapsed.  Initially, he and his colleagues thought it was likely that sea urchins were competitively excluded by the invasive rabbitfish. These fish overgrazed much of the algal meadows, forming barren grounds along much of the Israeli coastline. However, during the experiments they did to check that out, they noted that sea urchin mortality occurred in two consecutive summers, but not in other seasons. That led them to explore how sea urchin survival was affected by both the impact of warming water and by competition with rabbitfish.

study site

Researchers construct cages to investigate to investigate the causes of sea urchin population collapse.  See description below. Credit Erez Yeruham.

To investigate competitive exclusion of sea urchins by rabbitfish, the researchers bolted 25 metal cages (50 x 50 x 20 cm) to the rocks approximately 9 meters below the surface of the sea. They set up six different treatments: (1) fish only (F), (2) fish and sea urchins (FU), (3) sea urchins only (U), (4) no fish nor sea urchins (N), (5) cage control – a partial cage that allowed access to organisms (CC), and (6) no cage – an open control plot marked with bolts (NC).  For the treatments with sea urchins (FU and U), the researchers introduced five sea urchins into each cage. For the treatments with fish (F and FU), the researchers cut oblong holes in the mesh large enough for rabbitfish to get through. There were five replicates of each experiment in the fall of 2011 and again in the spring of 2012.

Urchins

Metal cage with five sea urchins (upper left corner of cage). Credit: Erez Yeruham.

Yeruham and his colleagues discovered that fish drastically reduced the abundance of soft algae, but that urchins had no discernable effect.  The researchers suggest that sea urchin density in the cages was low enough that even though sea urchins were eating some soft algae, the effects were too small to be detected. Both fish and sea urchins had very little effect on the abundance of calcareous algae (algae with hard crusty surfaces).

Yeruham2

Mean (+ standard error) dry weight (grams) of soft and calcareous algae for the six experimental treatments.

The researchers compared the amount of food in sea urchin guts when they were caged by themselves, or in cages with fish access.  Sea urchins had 40% more food in their guts when fish were excluded (left graph below).  In addition, they had a 30% greater gonado-somatic index (GSI) when fish were excluded (right graph below – the GSI measures the relative size of the gonads – a high GSI indicates good health and high reproductive potential). So when rabbitfish could visit the cages, sea urchins ate much less and suffered poorer health.

Yeruham3

Mean dry organic gut content (left graph) and GSI (right graph) of sea urchins with and without fish.

The results of this experiment show that rabbitfish have strong competitive effects on sea urchin food intake and overall health. But do warmer waters also help to explain the collapse of sea urchin populations in the Levant?  And might thermal stress interact with food limitation to influence sea urchin health?  To answer these questions the researchers used seawater pumped in directly from the sea into tanks that housed eight sea urchins.  Five tanks received ambient temperature seawater, while five other tanks received water that was chilled by 2 deg. C to mimic water temperatures before sea urchin populations collapsed.  Each tank was divided in half by a partition so that four urchins could be fed (algae) three times a week, while the other four urchins were starved.

One important finding is that during the winter, feeding rates were similar when comparing sea urchins in ambient vs. chilled sea water (two left bars below – those differences are not statistically significant).  However, feeding rates plummeted in the summer when water temperatures exceeded 29 deg. C in the ambient-temperature sea water.

Yeruham4a

Mean algal consumption by sea urchins in ambient vs. chilled water during the winter (two left bars) and summer (two right bars).

Respiration rates (measured as oxygen consumption) are a good measure of metabolic performance. Highest respiration rates were measured in the winter with fed sea urchins (ambient was slightly higher than cold) and in the summer with cold fed sea urchins.  Most notably, when sea water temperatures increased above 29 deg. C in the summer, the respiration rates were very low, even in sea urchins that were well-fed.

Yeruham4b

Mean (+ standard error) respiration rate (measured as oxygen uptake) of starved and fed sea urchins in ambient vs. chilled water during the winter and summer.

What emerges from this series of experiments is that sea urchins feed much more poorly and have lower respiration rates at high temperatures, independent of the effects of competition with rabbitfish.  The researchers also found that survival rates were lower at elevated temperatures.  Yeruham and his colleagues conclude that the direct effects of high temperature and the indirect effects of competition with rabbitfish are important factors that together conspired to lead to the collapse of sea urchin populations in the Levant.  They expect that as sea temperatures increase, rabbitfish will become more dominant in other regions that are now a bit cooler than the Levant. As warming continues and competition increases, Yeruham and his colleagues predict that sea urchin populations will collapse in those somewhat cooler ecosystems as well, changing the structure and functioning of coastal ecosystems across the Mediterranean.

note: the paper that describes this research is from the journal Ecology. The reference is Yeruham, E.,  Shpigel, M.,  Abelson, A., and  Rilov, G..  2020.  Ocean warming and tropical invaders erode the performance of a key herbivore. Ecology  101( 2):e02925. 10.1002/ecy.2925. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

 

Stress frequency structures communities

COVID-19 has amplified our experience of stress, but even in a COVID-free world, we share with most other organisms a continuously stressful existence, highlighted by situations affecting our survival (e.g. getting food and not becoming someone else’s food) and our reproductive success.  Today we will discuss organisms that live in a very stressful environment – the subtidal zone off of the Galapagos islands – located just below the line demarcating the furthest extent of low tide.  One serious stress for subtidal organisms is coping with dramatically fluctuating ocean currents.  The speedy surgeonfish uses its powerful pectoral fins and slender, disc-shaped body to minimize drag, permitting feeding in high flow conditions brought about by powerful ocean waves.  In contrast, the broad-bodied torpedo-shaped parrotfish is unable to do so; for it, fast water is too much of a drag.

ALE_3

Yellowtail surgeonfish (Prionurus laticlavius) stand out as voracious herbivores that can feed even in the most wave-swept coastlines of the Galapagos Islands. Credit: Dr. Alejandro Perez-Matus.

Waters near the Galapagos Islands are enriched by upwelling equatorial currents, which provide nutrients to a diverse community of plankton and benthic (attached to the ocean bottom) algae.  These in turn support a high diversity of macroinvertebrates and herbivorous fish that feed on them, including the pencil urchin, Eucidaris galapagensis, a voracious feeder on algae, barnacles and coral. This species wedges itself among rocks and crevices during the day, and emerges to feed at night.  It attaches itself (and moves very slowly) using its tube feet.  Robert Lamb, Franz Smith and Jon Witman hypothesized that given the weak attachment strength of the pencil urchin’s tube feet, it might only be an effective feeder in locations where wave action was minimal.

IMG_0465

Robert Lamb bolts experimental cages to the rock as Eucidaris urchins stand guard at the sheltered side of Caamaño. Credit: Salome Buglass.

To explore how wave action might affect the subtidal community, the researchers set up two research locations at Caamaño and Las Palmas – both off the Galapagos Island of Santa Cruz.

LambFig1

Effect of wave action (exposed – dark bar, sheltered – light bar) on abundance of some of the important members of the subtidal community off of the island of Santa Cruz.

 

At each location, they chose an exposed site with strong wave action and a sheltered site that had much reduced wave action.  Mean flow speed was more than twice as fast at exposed sites than in sheltered sites. As you can see in the figure to your left, site differences in mean flow speed corresponded to differences in the subtidal community. Crustose coralline algae (red algae firmly attached to corals) were more common in sheltered sites (Figure A), while a variety of red and green macroalgae were more common at exposed sites (Figure B).  Surgeonfish (Figure C) and parrotfish (Figure D) were much more abundant in exposed areas, while pencil urchins were much more abundant in sheltered sites (Figure E).

 

 

 

 

 

Lamb and his colleagues wanted to know why these differences exist. They set up a series of exclosures within each of these sites using wire mesh cages to either allow fish, but not urchins (+ fish treatment), allow urchins but not fish (+ urchins), or exclude both groups of herbivores (- all).  They also had a control treatment that allowed all herbivores (+ all).

LambTreatments

In one experiment the researchers created sandwiches made up of the delectable green algae Ulva.  For five days, they ran six replicates of each treatment at exposed and sheltered sites at Caamaño and Las Palmas. Lamb and his colleagues then harvested the sandwiches, weighed them, and calculated the percent remaining of each sandwich.

LambUlvaSandwich

An Ulva sandwich

At exposed locations, urchins (without fish) consumed very little Ulva, while fish (without urchins) consumed about 2/3 of the Ulva (when compared to the –all controls). In contrast, at sheltered locations, urchins took some mighty significant bites from the Ulva sandwiches, while fish also ate substantial Ulva at Caamaño, but not at Las Palmas.

LambFig3

Percent of Ulva biomass remaining after five days of the Ulva sandwich experiment. Error bars are 1 SE.

In a related experiment, the researchers used the same cages to explore how macroalgal communities assemble themselves in the presence or absence of urchin and fish herbiores under different flow rates.  If this was not enough to consider, they also ran these experiments both during the cool season, when nutrient-rich ocean currents lead to high production, and during the warm season when production is usually lower.  Lamb and his colleagues bolted two 13 X 13 cm polycarbonate plates to the bottom of each cage, and after two months measured the abundance and type of algae that colonized each plate.

Several trends emerge.  First, macroalgae colonized much more effectively during the cool season.  Second, urchins profoundly reduced macroalgal colonization at sheltered sites, but had little effect at exposed sites.  In contrast, fish herbivory reduced macroalgal colonization at exposed sites at Caamaño but not Las Palmas, during the warm and cool season.

LambFig4

Effect of herbivores on macroalgal community assembly, as measured by amount of algae colonizing the polycarbonate plates after two weeks.

In addition, the researchers set up video cameras and were able to document herbivory by 17 fish species, with drastically higher herbivory rates at exposed sites.

Lamb and his colleagues conclude that the dominant herbivores switched between urchins in low flow sites and fish in exposed sites. Fish can leave the resource patch when stress (flow rate) is unusually high, and return when flow rate drops, while the slow-moving pencil urchins do not have that option. The researchers argue that in many ecosystems, consumer mobility in relation to the frequency of environmental stress can predict how consumers influence community structure and assembly.  They point out that the coupling of mobility effects with environmental stress is common throughout the natural world.  As examples, many shorebirds feed on marine organisms that become available during low tides, or also between crashing waves.  Large mammals in Africa can migrate long distances to escape drought-stricken areas, while smaller animals cannot undertake such long journeys.  In locally acidic regions of the Mediterranean Sea, many fish species can enter, feed and leave before experiencing toxic effects from the acid water, while slow-moving urchins are excluded from feeding in those habitats. Thus, while extreme environmental stress often decreases consumer activity, there are also times when it doesn’t.  In these cases, we need to understand how particular species will behave and perform in the stressful environment to predict how stress influences community structure and functioning.

note: the paper that describes this research is from the journal Ecology. The reference is Lamb, R. W.,  Smith, F., and  Witman, J. D..  2020.  Consumer mobility predicts impacts of herbivory across an environmental stress gradient. Ecology  101( 1):e02910. 10.1002/ecy.2910. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

Turkey mullein trichomes gobble up protective pollen

I’m always amazed at how brilliant plants can be.  For example Billy Krimmel and Ian Pearse showed in 2013 that many plant species exude sticky substances that entrap small arthropods, thereby attracting predators, which then rid these plants of many herbivores that might otherwise consume their leaves or reproductive structures. Jennifer Van Wyk joined this research group (which included Laure Crova) in graduate school. They were hunting for predatory hemipterans (true bugs) for a different experiment, which involved looking for them on turkey mullein (Croton setiger). They found plenty of predators, but almost no prey.  This was puzzling; what were these predators eating?  Intrigued, the researchers swabbed the turkey mullein leaves for pollen and found relatively vast quantities of pollen trapped in the trichomes (hairlike protuberances) of the leaves. Much of the pollen was from other species, and the researchers suspected that the trichomes were removing pollen from pollinators (primarily bees) that came to visit. Presumably the predators, which included spiders, hemipterans and ants, were attracted to this highly nutritious pollen.

VanWykTurkeyMullein

Turkey mullein with predaceous hemipteran on lower right leaf. Credit: Billy Krimmel – http://www.miridae.com/our-team

Van Wyk and her colleagues wondered whether pollen capture benefitted turkey mullein. If turkey mullein used pollen to attract predators, and predators ate herbivores, pollen extraction by trichomes would be an adaptation that formed part of turkey mullein’s defense strategy.  If this is true, supplementing turkey mullein with additional pollen should increase visitation by predators, and decrease herbivore abundance.  With fewer herbivores, the researchers predicted less leaf damage.

Wykdamagedturkeymullein

Turkey mullein with herbivore-damaged leaves

Supplementing turkey mullein with additional pollen presents its own set of problems – most importantly coming up with enough pollen – in particular pollen that predators want to eat (Van Wyk and her colleagues collected a pound of oak pollen, only to find that predacious bugs were not interested in it).  The researchers grew sunflowers in greenhouses, secured squash pollen from friends’ gardens and used tuning forks to vibrate pollen from tarweed flowers.

The researchers then set up experiments using 60 turkey mullein plants from one population in 2013 and 80 plants from another population in 2014. Nearby plants were paired up, with one member of the pair receiving 150 mg of supplemental pollen each week from mid-August to mid-September.  They surveyed all arthropods visible to the naked eye, and categorized each species as predator or herbivore based on its primary diet (many of the arthropods were actually omnivorous).

In accordance with expectations, predator abundance was substantially greater in the supplemented populations in both years of the study. Spiders showed the most consistent increase, while Orius (the minute pirate bug) increased significantly in the 2014 population. The 2014 population had fewer arthropods of all species, possibly because it was immediately adjacent to an agricultural field.

WykFig1

Mean predator abundance per plant in 2013 (top) and 2014 (bottom). Geocoris is a Genus of big-eyed bugs, while Orius is the minute pirate bug. ** p < 0.01, † p < 0.1. Error bars are 1 standard error.

The results are less clear-cut with herbivore abundance.  Fleahoppers were 18% less abundant on supplemented plants in 2014, and slightly (not significantly) less abundant in supplemented plants in 2013.  Plants with a greater number of spiders had fewer fleahoppers, suggesting that spiders were eating them (or scaring them away). The researchers were unable to measure the abundance of an important herbivore, the grey hairstreak caterpillar, which forages primarily at night, and retreats into the soil during the heat of the day.

WykFig2b

Mean number of fleahoppers  per plant in 2013 (left graph) and 2014 (right graph).  Blue bars indicated plants with supplemented pollen. * p < 0.05.

Lastly, supplemented plants suffered much less leaf damage than did unsupplemented plants.

WykFig2A

Mean number of damaged leaves per plant in 2013 (left graph) and 2014 (right graph).  Blue bars indicated plants with supplemented pollen. ** p < 0.01.

Taken together, these experiments indicate that turkey mullein uses its trichomes to capture pollen and attract a diverse army of predators, which reduce herbivore abundance and reduce damage to the plant.  It is possible that pollen supplementation could be used on a larger scale to reduce herbivore loads on agricultural crops.  More generally, it will be interesting to see whether other plants with sticky trichomes, such as the marijuana plant Cannabis sativa, also use their trichomes to attract predators and reduce herbivore abundance.

note: the paper that describes this research is from the journal Ecology. The reference is Van Wyk, J. I.,  Krimmel, B. A.,  Crova, L., and  Pearse, I. S..  2019.  Plants trap pollen to feed predatory arthropods as an indirect resistance against herbivory. Ecology  100( 11):e02867. 10.1002/ecy.2867. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Birds and plants team up and trade off

For many years, ecologists have been puzzling over the question of why the world is so green.  Given the abundance of herbivores in the world, it seems, on the surface, that plants don’t stand a chance. The famous naturalist/ecologist Aldo Leopold was one of the first scientists to emphasize the role of predators, which provide service for plants by eating herbivores (his example was wolves eating deer, ultimately preserving the plant community growing on a hillside).  As it turns out there are many different predator species providing these services. Colleen Nell began her PhD program with Kailen Mooney with a keen interest on how insectivorous birds locate their prey, and how this could affect the plants that are being attacked by herbivorous insects.

COYE common yellowthroat simple

 A Common Yellowthroat perches on Encelia californica. Credit: Sandrine Biziaux.

Plants are not as poorly defended as you might expect (having sat on a prickly pear cactus I can  painfully attest to that).  In addition to thorns and other discouraging structures, many plants are armed with a variety of toxins that protect them against herbivores.  Thorns and toxins are examples of direct defenses.  But many plants use indirect defenses that involve attracting a predator to the site of attack.  Some plants emit volatile compounds that predators are attuned to; these compounds tell the predator that there is a yummy herbivore nearby.  Nell and Mooney recognized that plant morphology (shape and form) could also act as an indirect defense, making herbivorous insects more accessible to bird predators. They also recognized that we might expect a tradeoff between how much a plant invests in different types of defense.  For example, a plant that produces nasty thorns might not invest so much in a morphology attractive to predaceous birds.

pricklypearcawr_2

California Coastal Cactus Wren eating an orthopteran insect on a prickly pear cactus. Credit: Sandrine Biziaux.

What is a plant morphology that attracts birds?  The researchers hypothesized that birds might be attracted to a plant with simple branching patterns, so they could easily land on any branch that might be hosting a herbivorous insect (Encelia californica (first photo) has a simple or open branching pattern).  In contrast, birds might have a more difficult time foraging on insects that feed on structurally complex plants that host herbivorous insects which might be difficult to reach.

isocoma menziesii complex

Isocoma menziesii, a structurally complex plant. Credit: Colleen Nell.

The researchers chose nine common plant species from the coastal sage scrub ecosystem – a shrub-dominated ecosystem along the southern California coast. For each plant species they measured both its direct resistance and indirect resistance to herbivores.  Plants of each species were raised until they were four years old.  Then, for three months during bird breeding season, bird-protective mesh was placed over eight plants of each species, leaving five or six plants as unprotected controls.

IMG_3938

Kailen Mooney and Daniel Sheng lower bird-protective mesh over a plant. Credit: Colleen Nell.

After three months, the researchers vacuumed all of the arthropods from the plants, measured each arthropod, and classified it to Order or Family to evaluate whether the arthropod was herbaceous.

IMG_vaccum

Colleen Nell vacuums the arthropods from Artemisia californica. Credit: Colleen Nell.

Nell and Mooney evaluated the herbivore resistance of each plant species by measuring herbivore density in the bird-exclusion plants.  Relatively few herbivorous arthropods in plants that were protected from birds would indicate that these plants had strong direct defenses against herbivores.  The researchers also evaluated indirect defenses as the ratio of herbivore density on bird exclusion plants in comparison to controls (technically the ln[exclusion density/control density]).  A density of herbivores on plants protected from birds that is much greater than the density of herbivores on plants that allowed birds would indicate that birds are eating many herbivores. Finally, Nell and Mooney estimated plant complexity by counting the number of times a branch intersected an axis placed through the center of the plant at three different angles.  More intersecting branches indicated a more complex plant.

The researchers expected a tradeoff between direct and indirect defenses.  As predicted, as herbivore resistance (direct defense) increased, indirect defenses from birds decreased among the nine plant species.

NellFiga

Tradeoff between direct herbivore resistance and indirect defense by predaceous birds, for nine common plant species in the coastal sage scrub ecosystem.

The researchers also expected that more structurally complex plants would be less accessible to birds because complex branching would interfere with bird perching and foraging.  Thus Nell and Mooney predicted that structurally more complex plants would have weaker indirect defenses from birds, which is precisely what they discovered.

NellFigc

Indirect defenses (from birds) in relation to plant structural complexity .

Given that structurally complex plants received little benefit from birds, you might expect that they had greater direct defenses in the form of herbivore resistance.  Once again the data support this prediction.

NellFigb

Direct defenses (herbivore resistance) in relation to plant  structural complexity.

Initially, Nell was uncertain about whether increased plant complexity would deter insectivorous birds.  She points out that the top predators in this ecosystem are birds of prey that circle overhead in search of vulnerable birds to eat.  Structurally complex plants might provide refuge for insectivorous birds, which could result in them spending more time foraging in complex plants.  But the research showed the opposite trend. Plant complexity reduced the foraging efficiency of these small insectivorous birds, who prefer foraging on plants with relatively simple structure, which are easier to access and tend to host more prey.

note: the paper that describes this research is from the journal Ecology. The reference is Nell, C. S., and  Mooney, K. A..  2019.  Plant structural complexity mediates trade‐off in direct and indirect plant defense by birds. Ecology  100( 10):e02853. 10.1002/ecy.2853.  Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Tadpoles shun trout across time

At a young school child (so long ago I can’t recall exactly when) I was exposed to Ernst Haeckel’s dictum that “ontogeny recapitulates phylogeny.”  More interested in language than biology at the time, I thought “cool – three words that I’m clueless about.” Though biological thinking about ontogeny – the processes of growth and development – has changed since Haeckel’s time, interest has, if anything, grown more intense across disciplines. Tiffany Garcia has explored her lifelong fascination with ontogeny by focusing her research on amphibians, which are famous for their distinct stages of development, each with unique habitats and ecological requirements. Working with eggs and tadpoles of the Pacific chorus frog (Pseudacris regilla), Garcia and her colleagues investigated whether stress associated with the presence of predators during one developmental stage (for example an egg) would carry over to influence behavior or development of subsequent stages.

chorus frog

The Pacific chorus frog (Pseudacris regilla). Credit Brett Hanshew.

A tadpole’s anti-predator strategy can be influenced by other factors besides carry-over from earlier developmental stages.  For example, we might expect that tadpoles whose ancestors lived in association with predators for many generations might have evolved a different anti-predator strategy than did tadpoles whose ancestors lived in a less threatening environment (this would be an adaptive effect). Tadpoles may also show very short-term changes in behavior or development (this is termed plasticity) if exposed to a cue that indicated a possible predation threat.

ThreeCreeksLindsey

Collecting newly laid eggs at Three Creeks Lake. Credit: Lindsey Thurman

These three processes operate over very different time scales (long term – adaptive; intermediate term – carry-over; short term – plastic).  Garcia and her colleagues designed an experiment to explore how these processes might interact to influence a tadpole’s anti-predator strategy.  To investigate long term adaptive effects, the researchers collected newly laid (fertilized) eggs from lakes with and without rainbow trout (Oncorhynchus mykiss). They investigated carry-over effects by conditioning these eggs with four different environments during development: (1) trout odor, (2) cues from injured tadpoles (alarm cues), (3) trout odor paired with alarm cues, and (4) a water control (no odors nor cues).  The researchers created alarm cues by grinding up four juvenile tadpoles in 150 ml of water, and trout odor by housing 30 juvenile rainbow trout in a 200 L tank filled with well water.  They then conducted behavioral and developmental assays on tadpoles to see how adaptive, carry-over and plastic effects influenced tadpole growth, development and behavior.

GarciaFig2

Overview of the experimental design.

Garcia and her colleagues discovered that early exposure to trout odor had very little effect on growth and development, with body size and stage of development equivalent to that of controls.  In contrast exposure of eggs to tadpole alarm cues or to alarm cues + trout odor resulted in smaller, less developed fish (see table below).  In addition there was no effect of evolutionary history – eggs from lakes with and without trout showed similar patterns of growth and development.

GarciaTable1

Tadpole size and development in response to the four conditioning  treatments.  Higher Gosner stage numbers indicated more developed tadpoles. A tadpole hatches at Gosner stage 21 and begins metamorphosis at Gosner stage 42.

The next question is how do tadpoles respond behaviorally from exposure to different environments over the long, intermediate and short time scale?  To test tadpole anti-predator behavior, the researchers placed an individual tadpole into a tub that had a 6 X 8 cm piece of corrugated black plastic, which the tadpole could use as a refuge.  The researchers added to each tub one of the following: water (as a control (C)), tadpole alarm cues (AC), trout odor (TO), or alarm cues + trout odor (AC+TO).  After an acclimation period, a researcher noted the position of the tadpole (under the refuge or out in the open) every 20 minutes over a 3-hour time period.

There were no effects of evolutionary history on refuge use.  Tadpoles from lakes with and without trout showed similar patterns of refuge use.  However, embryonic conditioning to alarm cues and trout odor had a large effect on refuge use.  The left graph below shows the response of tadpoles from all four conditioning groups (C, AC, TO and TO+AC) to the addition of water.  As you can see, tadpoles that hatched from eggs that were conditioned with AC+TO were most likely to use refuges, while tadpoles from AC only or TO only eggs were somewhat more likely to use refuges. The pattern repeats itself when tadpole alarm cues are added to the water (second graph from left).  However when trout odor is added to the water, the responses are much more extreme, but follow the same pattern (third graph).  Lastly, when confronted with alarm cues and trout odor, tadpoles increase refuge use dramatically, but again show the same pattern, with tadpoles from control eggs using refuges the least, and tadpoles from eggs conditioned with alarm cues and trout odor using refuges the most (right graph).

GarciaFig6

Refuge use by tadpoles in response to embryonic conditioning and experimental exposure. C = water control, AC = tadpole alarm cue, TO = trout odor, and AC+TO = tadpole alarm cue and trout odor. Blue bars are means and gray bars are 95% confidence intervals.

There are two processes going on here.  First, over the short term, tadpoles are more responsive to the strongest cues, increasing refuge use when exposed to both tadpole alarm cues and trout odor.  Second, over the intermediate term, there is solid evidence for carry over effects.  Tadpoles that hatched from eggs conditioned with alarm cues and/or trout odor showed markedly increased refuge use than did tadpoles that hatched from control eggs.

These predator-induced responses impose a cost to the tadpoles.  Tadpoles exposed to alarm cues and trout odor while still in the egg were smaller and less developed, and probably metamorphosed into smaller frogs.  Many studies have shown that smaller frogs have reduced reproductive success.  The researchers recommend further studies to explore these trade-offs between survivorship, growth rate, development rate and size at metamorphosis. These studies are particularly essential, because rainbow trout are a non-native predator to these lakes.  Studies such as these allow conservation ecologists to understand the evolution and development of predator-prey interactions when novel species are introduced into an ecosystem.

note: the paper that describes this research is from the journal Ecology. The reference is Garcia, T. S.,  Bredeweg, E. M.,  Urbina, J., and  Ferrari, M. C. O..  2019.  Evaluating adaptive, carry‐over, and plastic antipredator responses across a temporal gradient in Pacific chorus frogs. Ecology  100( 11):e02825. 10.1002/ecy.2825.  Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

 

 

 

 

 

 

 

 

 

 

 

Tropical trophic cascade slows decomposers

In the rough and tumble natural world, consumers such as lions, lady bugs, llamas and lizards get most of the press, while producers such as peas, pumpkins and phytoplankton come in a close second.  Consumers earn their name because they get their energy from consuming other organisms, while producers produce their own energy (using photosynthesis or chemosynthesis) from inorganic molecules.  Often ignored in this ecosystem structural scheme are decomposers, which get their energy from breaking down the tissue of dead organisms.  They should not be ignored.  Much of the energy transferred through ecosystems passes through decomposers.

One reason they are overlooked is that most decomposers are tiny. Some of the largest decomposers are detritivores, which actually eat the dead materials (detritus), in contrast to other microbial decomposers such as bacteria and fungi.  Shredders are detritivores commonly found in streams and rivers; these aquatic insects eat portions of dead leaves and, in the process, shred them into much smaller pieces that energize other decomposers. Many researchers had noted that shredders were relatively rare in tropical streams, in part because there are many other larger consumers in the ecosystem that are willing to eat dead leaves and any shredders associated with them. Thus Troy Simon and his colleagues expected that shredders, such as the caddisfly, Phylloicus hansoni, would play, at best, a minor role in the streams they studied in the Northern Range Mountains in Trinidad.

stream

A typical headwater stream located in the Northern Range mountains of Trinidad. Waterfalls in the uppermost reaches of these streams act as a barrier to the upstream movement of guppies, but not killifish and crabs, which can move over land during periods of heavy rain. Credit: Joshua Goldberg.

We will discuss interactions between several species in these aquatic systems.  Trees are important producers as they shed leaves into the streams; these leaves are broken down by shredders such as the aforementioned caddisflies and also microbial decomposers.   The major consumers are omnivorous crabs, Eudaniela garmani, which eat leaves and caddisflies (and many other items), and two fish species. Killifish, Anablepsoides hartii, eat caddisflies, other invertebrates and also the occasional small fish (including fish eggs).

killifish

Hart’s killifish (Anablepsoides hartii) are primarily insectivorous and major consumers of leaf‐shredding caddisflies. Credit: Pierson Hill.

Guppies, Poecilia reticulata, are much smaller than killifish, maxing out at 32 mm long in comparison to the killifish maximum length of 100 mm.  But guppies are much more omnivorous, feeding on leaves, leaf-shredding insects and even killifish eggs and larvae.

guppies

Male (left) and female (right) Trinidadian guppy (Poecilia reticulata). Guppies are omnivorous, feeding broadly on detritus as well as plant and animal prey, including young killifish. Credit: Pierson Hill.

Amazingly, killifish can disperse over land, as can crabs (less amazingly).  This allows them to bypass barrier waterfalls during wet periods, which results in them being the only large consumer species above waterfalls in many Trinidad streams.  Guppies lack killifish dispersal abilities, so they are often confined to stream reaches below significant waterfalls.  These species, and their consumption patterns are highlighted in the figure below.

SimonFig1

Diagram of the two detrital-based food webs.  Above the waterfall is the KC reach, named after its two important consumers, killifish and crabs.  Below the waterfalls is the KCG reach, named after its three important consumers, killifish, crabs and guppies. Arrows show direction of energy flow within the ecosystem.

Simon and his colleagues wanted to know how interactions among all of these species influenced the rate of leaf decomposition.  The researchers constructed identical-size leaf packs of recently fallen leaves of the Guarumo tree, Cercropia peltata, and attached them to copper wire frames within each reach of the stream.  They periodically harvested a subset of the packs and measured the amount of decomposition by drying and weighing the leaves, and comparing this weight to the starting weight of the leaf pack.  In addition, they collected all invertebrates > 1 mm long from each leaf pack and identified them to species or genus.

To control the consumers involved in each interaction, Simon and his colleagues constructed underwater electric exclosures which created an electric field that convinced all fish and crabs to exit (and stay out) within 30 seconds of being turned on, but did not influence invertebrates in any detectable way.  Killifish are active day and night, guppies only during the day, and the researchers believed that crabs were active primarily at night. The researchers set up four treatments: control (C) with 24 hour access to consumers, experimental (E) with 24 hour exclusion of consumers, day-only exclusion (D) and night-only exclusion (N).  The researchers expected that the day-only exclusion treatments would selectively exclude guppies, while night-only exclusion would selectively exclude crabs. They then placed the leaf packs into each exclosure, turned on the current, and ran the experiment for 29 days.  Five replicates of each treatment were done above and below the waterfalls.

simonexperiment

Electric exclosures established in the stream. Leaf packs were tied to the copper frame and periodically harvested over the 29 days of the experiment. Rectangular tiles shown in treatment frames were part of a separate study. Credit: Troy N. Simon.

We’re finally ready for some data.  The two graphs on the left represent the downstream reach below the waterfalls, where killifish, crabs and guppies are naturally present (KCG).  The two graphs on the right represent the upstream reach above the falls, where only killifish and crabs are naturally present.  There was no evidence in the downstream reach that excluding consumers influenced decomposition rates (top left graph).  However, when consumers were present (C treatment) in the upstream reach, decomposition rates were reduced by about 40% in comparison to treatments when consumers were partially (D and N) or completely (E) excluded (top right graph).

SimonFig3

Mean (+SE) for (a,b) decay rate of Cecropia peltata leaves (percentage of mass lost per day) and (c,d)  biomass of Phylloicus hansoni (milligrams of dry mass per gram of Cecropia). 24-hour treatments allow full macroconsumer access [control (C)] or completely exclude macroconsumers [electric (E)]. Twelve-hour treatments exclude access to either diurnally active [day (D)] or nocturnally active [night (N)] macroconsumers. Different letters above the bars indicate statistically significant differences between the treatments.

The two bottom graphs above look at the biomass of the caddisfly, Phylloicus hansoni, which was easily the most abundant macroinvertebrate within the leaf packs.  There was no significant difference in caddisfly abundance below the waterfall regardless of treatment (bottom left graph above).  Above the waterfalls, caddisfly abundance was severely depressed in the controls (C) where killifish were free to feed on them (bottom right graph).

One piece of evidence that killifish ate caddisflies and depressed their abundance was that surviving caddisflies were much smaller in the control treatment leaf packs than in any of the experimental treatment leaf packs.  This suggests that  killifish with unimpeded access to caddisflies were picking off the largest individuals.

SimonFig4

Mean (+SE) caddisfly length in mm (y-axis) for each treatment, 

These findings support the hypothesis that a trophic cascade prevails in the KC reach, in which killifish eat caddisflies, thereby slowing down decomposition. But in the KCG reach, guppies eat killifish eggs and larvae and compete with them for resources, thereby reducing killifish abundance, and interfering with the establishment of a trophic cascade.

Lastly, the researchers explored whether the same trophic cascade operated in upper reaches but not in lower reaches of other streams in the area. Surveys of six streams indicate a definite “yes” answer, with Cecropia decay rate and caddisfly biomass much lower in the upper reaches.

SimonFig6

(Top) Mean (+SE) decay rate for Cecropia peltata
leaves (percentage of mass lost per day) and (b) caddisfly biomass (milligrams of dry mass per gram of Cecropia) in the landscape study (n = 6 streams). Different letters above bars indicate statistically significant differences  between treatments.

Surveys of each stream indicated that killifish were much more abundant in the upper reaches where guppies were not found, but guppies were much more prevalent in the lower reaches than were killifish.  These findings indicate that this detrital-based trophic cascade, with killifish eating caddisflies and thereby slowing down decomposition, is a general pattern in the upper reaches of these tropical streams.  However, Simon and his colleagues caution us that different streams will have different groups of organisms playing different ecological roles.  Thus the presence of detrital-based trophic cascades will depend on the particulars of which species are present and how abundant they are in a particular stream.

note: the paper that describes this research is from the journal Ecology. The reference is Simon, T. N., A. J. Binderup, A. S. Flecker, J. F. Gilliam, M. C. Marshall, S. A. Thomas, J. Travis, D. N. Reznick, and C. M. Pringle. 2019. Landscape patterns in top-down control of decomposition: omnivory disrupts a tropical detrital-based trophic cascade. Ecology 100(7):e02723. 10.1002/ecy.2723. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

 

Hot ants defend plants from elephants

I’ve lost a lot of sleep over ants.  As a spider researcher, I often placed ants on spiderwebs to lure my spiders out of their underground retreats and onto their webs. The problem was that these harvester ants (Pogonmyrmex species) were fierce, so to minimize damage to myself, I was forced to capture them in the very early morning, when they and (alas) I were very sluggish.

acacias_thorn copy

Swollen thorn (domatia) that serves as living quarters for acacia ants. Credit: T. Palmer.

Todd Palmer has worked with ants for many years, including research on ant-plant mutualisms in which acacia trees provide domatia (swollen thorns) as ant living quarters and extrafloral nectaries as ant food, while ants provide protection from herbivores such as elephants, kudus and steenboks.

Similar to my efforts with ants and spiders, Palmer wanted to reduce ant-induced damage to himself and his colleagues, so he often took advantage of early morning ant sluggishness for purposes of manipulating acacia trees. On the other hand, if he wanted to study aggressive responses, he learned that mid-day was best. Recognizing the daily patterns of ant activity got Palmer, Ryan Tamashiro (Palmer’s undergraduate research student) and Patrick Milligan (Palmer’s graduate student) thinking about how these different levels of activity would influence herbivores, many of which tend to be most active during dawn and dusk when temperatures are low and ants are relatively sluggish.

Elephant side

Elephants are major herbivores that can cause enormous damage to acacia trees. Credit: T. Palmer.

Four species of ants live in domatia on branches of Acacia drepanolobium, the dominant tree species at Mpala Research Centre in Laikipia, Kenya.

Acdr habitat

A grove of Acacia drepanolobium. Credit: T. Palmer.

In order of relative abundance, the ant species are Crematogaster mimosae (52%), C. sjostedti (18%), Tetraponera penzigi (16%) and C. nigriceps (15%).  Previous research showed that C. mimosae and C. nigriceps are the two most effective acacia defenders.

Cnigriceps copy

Crematogaster nigriceps on an acacia tree. Credit: T. Palmer.

Ants are poikilotherms, whose body temperature, and presumably their activity levels, fluctuate with environmental temperature.  As these ants live in acacia branches, the first order of business became to determine how branch temperature fluctuated with time of day during the 21 days of data collection.  Not surprisingly, branch temperature peaked at mid-day, and was lowest at dawn and dusk (temperatures were not measured during the night).

TamashiroFig S!

Variation in branch surface temperature with time of day. Horizontal bars are median values; boxes are first and third quartiles.

Tamashiro, Milligan and Palmer next asked how ant activity level related to branch temperature.  Different ant species don’t get along so well, so each tree hosted only one ant species.  For each tree surveyed, the researchers counted the number of ants that passed over a 5 cm branch segment during a 30 second time period (they did this twice for each tree),  The researchers discovered a strong correlation between branch surface temperature and baseline ant activity, with C. mimosae and C. nigriceps showing greatest activity levels at all temperatures, which increased sharply at higher temperatures.

TamashiroFig 1a

Ant activity levels in relation to branch surface temperature. Shaded areas are 95% confidence intervals for each species.

Do higher temperatures cause a stronger aggressive response to predators or other disturbances? Tamashiro and his colleagues tested this by rapidly sliding a gloved hand over a 15 cm segment of a branch three times and then resting the gloved hand on the branch for 30 s.  They then removed the glove and counted the number of ants that had swarmed onto the glove.  Again, C. mimosae and C. nigriceps showed the strongest aggressive response, which increased sharply with temperature

TamashiroFig 1b

Aggressive swarming by ants in relation to branch surface temperature. Shaded areas are 95% confidence intervals for each species.

While a gloved hand is a nice surrogate for predators, the researchers wanted to know how the ants would respond to a real predator, and whether the response was temperature dependent.  At the same time, they wanted to determine whether the predator would change its behavior in response to changes in ant defensive behavior at different temperatures.  They used eight somali goats (Capra aegagrus hircus) as their predators, and C. mimosae as the focal ant species for these trials.

Cpl. Paula M. Fitzgerald, USMC - United States Department of Defense

Somali goats in Ali Sabieh, Djibouti. Credit: Cpl. Paula M. Fitzgerald, USMC – United States Department of Defense.

The researchers chose eight trees of similar size for their experiment, and removed ants from four of the trees by spraying them with a short-lived insecticide, and preventing ant recolonization by spreading a layer of ultra-sticky solution (Tanglefoot) around the based of each treated tree.  Goats were allowed to feed for five minutes.

TamashiroFig2

Number of bites (top graph) and time spent feeding (bottom graph) by goats in relation to branch surface temperature. Shaded area is 95% confidence interval.

Tamashiro and his colleagues measured the number of bites taken (top graph) and the amount of time spent feeding (bottom graph) at different branch temperatures.  Both measures of goat feeding were not influenced by branch temperature if there were no ants on the trees (blue lines and points).  But if ants were present (red lines and points), goat feeding decreased sharply with increasing branch temperature, presumably reflecting more aggressive ant defense of the plants.

These findings have important implications for acacia trees, which are a critical species in the sub-Saharan ecosystem.  Previous research has shown that elephant damage is strongly influenced by the number of swarming ants on a particular tree; a greater number of swarming ants are associated with less elephant damage. Many vertebrate browsers feed throughout the day, but may feed preferentially at dawn and dusk, when temperatures are cooler and ant-defense is weakest. Browsing is particularly problematic for acacia saplings, which are usually attacked by small-bodied vertebrates such as steenbok, which forage primarily at night when ants are least active.  Thus the effectiveness of ant defense may be compromised by mismatches between vertebrate activity periods and ant activity periods.

note: the paper that describes this research is from the journal Ecology. The reference is Tamashiro, R. A., P. D. Milligan, and T. M. Palmer. 2019. Left out in the cold: temperature-dependence of defense in an African ant–plant mutualism. Ecology 100(6): e02712. 10.1002/ecy.2712 . Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

 

 

Mystifying trophic cascades

Within ecosystems, trophic cascades may occur when one species, usually a predator, has a negative effect on a second species (its prey), thereby having a positive effect on its prey’s prey. Today’s example considers the interaction between a group of predators (including several fish species, a sea snail and a sea star) their prey (the sea urchin Paracentrotus lividus) and sea urchin prey, which comprise numerous species of macroalgae that attach to the shallow ocean floor. These predators can negatively affect sea urchin populations either by eating them (consumptive effects), or by scaring them so they forage less efficiently (nonconsumptive effects). If sea urchins are less abundant or less aggressive foragers, the net indirect effect of a large population of fish, sea snails and sea stars will be an increase in macroalgal abundance.

Maldonado Halo

A large sea urchin grazing in a macroalgal community.  Notice the white halo surrounding the urchin, indicating that it has grazed all of the algae within that region. Credit: Albert Pessarrodona.

Many humans enjoy eating predatory fish, and we have overfished much of the ocean’s best fisheries including the shallow temperate rocky reefs (4 – 12 m deep) in the northwest Mediterranean Sea. Removing these predators has caused sea urchin populations to explode, overgrazing their favorite macroalgal food source, and ultimately leading to the formation of urchin barrens – large areas with little algal growth, low productivity and a small nondiverse assemblage of invertebrates and vertebrates.

DCIM112GOPRO

A sea urchin barrens whose macroalgae have been overgrazed by sea urchins. Credit: Albert Pessarrodona

Albert Pessarrodona became interested in this trophic cascade after years of diving in the Mediterranean. He noticed that in Marine Protected Areas, predatory fish abound and there are few visible urchins and lots of macroalgae. In nearby unprotected areas where fishing is permitted, urchins graze out in the open brazenly, and urchin barrens are common. He also wondered whether a second variable – sea urchin size – might play a role in this dynamic. Were large sea urchins relatively immune from predation by virtue of their large size and long spines, allowing them to forage out in the open even if predators were relatively common?

Urchinfig1

Interactions investigated in this study.  (a) Predators consume either small (left) or large (right) sea urchins (consumptive effects). (b) Sea urchins eat macroalgae. (c) Predators scare small or large sea urchins, reducing their foraging efficiency (nonconsumptive effects). (d) Predatory fish indirectly increase macroalgal abundance.

Pessarrodona and his research team used field and laboratory experiments to explore the relationship between sea urchin size and their survival and behavior in high-predator-risk and low-predator-risk conditions. High-risk was the Medes Islands Marine Reserve, which has had no fishing since 1983 and boasts a large, diverse assemblage of predatory fish, while low-risk was the nearby Montgri coast, which has a similar habitat structure, but allows fishing. The researchers tethered 40 urchins of varying sizes to the sea bottom (about 5m deep) in each of these regions, left them for 24 hours, and then collected the survivors to compare survival in relation to body size in high and low-risk conditions. They discovered that large urchins were much less likely to get eaten than were small urchins, and that the probability of getting eaten was substantially greater in the high-risk site.

UrchinFig3a

Probability of being eaten in relation to sea urchin size (cm) in high-risk (blue line) and low-risk (green line) habitats.

Pessarrodona and his colleagues followed this up by investigating whether the relatively predation-resistant large urchins were less fearful, and thus more likely to forage effectively, even in high-risk sites. Previous studies showed that sea urchins can evaluate risk using chemical cues given off by other urchins injured in a predatory attack, or given off by the actual predators. To explore the relationship between these cues and sea urchin behavior, the researchers put either large or small sea urchins into partitioned tanks with an injured sea urchin. Water flowed from one partition to the other, so the experimental sea urchins received chemical cues from the injured urchins. They also had a group of sea urchins placed in similar tanks without any injured sea urchins as controls. The experimental sea urchins were given seagrass to feed on, and the researchers calculated feeding rates based on how much food remained after seven days.

Small sea urchins were not deterred by the presence of an injured urchin (left graph below), while large sea urchins drastically reduced their feeding rates in response to the presence of an injured urchin (middle graph). This was startling as it flew in the face of the commonsense expectation that small sea urchins (most susceptible to predation) should be most fearful of predator cues. The researchers repeated the experiment (under slightly different conditions) placing an actual predator (a fearsome sea snail) on the other side of the partition. Again, large urchins showed drastically reduced foraging rates (right graph below).

UrchinFig4

Sea urchin responses to predation risk cues in the laboratory. When exposed to injured urchins – symbolized as having a triangle cut out – (A) small urchins did not reduce their grazing rate, while (B) large urchins drastically curtailed grazing. (C) When exposed to a predatory snail on the other side of a partition, large urchins sharply curtailed grazing. n.s = no significant difference, **P<0.01.

It turns out that large sea urchins are the critical players in this trophic cascade because they do much more damage to algal biomass than do the smaller urchins (we won’t go through the details of that research). The question then becomes how this plays out in natural ecosystems. Do consumptive and non-consumptive effects of predators in high-risk sites reduce sea urchin abundance and reduce the foraging levels of large sea urchins so that macroalgal cover is greatly enhanced? Pessarrodona and his colleagues surveyed high-risk and low-risk sites for sea urchin density and algal abundance. They set up 45 quadrats (40 X 40 cm) at each site, measured each sea urchin’s diameter, and estimated the abundance of each type of algae by harvesting a 20 X 20 cm subsample from each quadrat and drying and weighing the sample.

The findings were striking. Small and large sea urchins were much less abundant at high-risk sites than at low-risk sites (left graph below). At the same time, macroalgae were much more abundant at high-risk sites than at low-risk sites (right graph below).

UrchinFig5bc

(Left graph) Density of small and large sea urchins in high-risk and low-risk habitats. (Right graph) Biomass of macroalgae of different growth structures in high-risk and low-risk habitats. Canopy algae are taller than 10 cm, while turf algae are lower stature. Codium algae are generally not grazed by sea urchins. **P<0.01, ***P<0.001.

UrchinFig6a

Summary of interactions.  Arrow width indicates relative importance.

To summarize this system, predators reduce small sea urchin abundance by eating them (consumptive effects), and reduce large sea urchin foraging by intimidating them (nonconsumptive effects). The net indirect effect of predators on macroalgae is a function of these two effects. Large sea urchins are the major macroalgae consumers, but, of course, large sea urchins develop from small sea urchins.

The $64 question is why large sea urchins fear predators so much, while small (more vulnerable) urchins do not. The quick answer is that we don’t know. One possibility is that small sea urchins may be bolder in risky environments since they are more vulnerable to starvation (have fewer reserves), and also have lower reproductive potential since they are likely to die before they get large enough to reproduce. In contrast, large sea urchins can survive many days without food because of their large reserves. In addition, large urchins are close to sexual maturity, and thus may be unwilling to accept even a small risk to their well-being, which could interfere with them achieving reproductive success.

note: the paper that describes this research is from the journal Ecology. The reference is Pessarrodona, A.,  Boada, J.,  Pagès, J. F.,  Arthur, R., and  Alcoverro, T. 2019.  Consumptive and non‐consumptive effects of predators vary with the ontogeny of their prey. Ecology  100( 5):e02649. 10.1002/ecy.2649. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Spiders eat spiders sometimes

In human society, a guild is an association of craftsmen or merchants that work together to achieve a common goal. For example, 14thcentury Paris boasted over 350 different guilds, including drapiers (cloth makers), knife-makers, locksmiths, helmet-makers and harness-polishers. Ecological guilds are similar to human guilds, in that members of the same guild depend on the same resources for survival. But members of the same ecological guild are  different species, each of which uses a similar resource, or group of resources.  As we shall now discover, as in human guilds, members of ecological guilds don’t always get along very well.

A guild is part of a food web, which is a summary of the feeding relationships within a community.  Israel Leinbach, Kevin McCluney and John Sabo were interested in one particular part of a food web – the relationship between a large wolf spider (Hogna antelucana), a small wolf spider (Pardosa species) and a cricket (Gryllus alogus).  Both spiders are in the same guild, because they obtain their energy from similar sources – insect prey.  This cricket specializes on willow and cottonwood leaves that fall to the ground in the semi-arid floodplain of the San Pedro River in southeast Arizona. Under natural conditions, the researchers observed the large spiders eating both the small spiders and crickets.  However, they never observed the small spider eating the relatively large cricket (which averages 20 times its mass), though small spiders are delighted to eat many other (smaller) insect species.

spideeatscricket.jpg

A large wolf spider subdues and begins to consume a cricket.  Credit: Kevin McCluney.

The researchers argue that even though guild members specialize on similar resources, it is important to consider how other resources might influence the relationships among the species.  During the dry season, water is a critical limiting resource.  As it turns out, large spiders, crickets and small spiders are very different in how much energy and water they contain. From the table below you can see that the small Pardosa spiders are very low in water content, but pack a huge amount of energy into their tiny bodies.  Crickets of both sexes have a high water content, but contain a relatively small amount of energy in their large bodies. Thus small spiders have a much higher energy/water ratio than crickets or large spiders.

SpiderTable2

Mean (+/- 1 standard error) dry mass, energy, water and energy/water ratio of the three species discussed in this report.

When water is limiting, the large spiders might devote themselves to eating crickets to take advantage of their very high water content. But when water is not limiting, the large spiders would be expected to turn their attention to eating small wolf spiders, which are much dryer, but much higher in energy per unit body mass. The researchers reasoned that providing water to large spiders should increase the rate of intraguild predation (in this case large spiders eating small spiders).

SpiderFig1

Interactions among the three species when water is limiting (Control – left) and abundant (Experimental – right).  Black arrows are direct effects, while gray arrows show the direction of energy flux).

Leinbach and his colleagues set up a mesocosm experiment using 2 X 2 X 2 meter cages in which they experimentally manipulated community composition and water availability.

spidercages.jpg

Network of cages set up in the San Pedro River floodplain. Credit: Kevin McCluney.

All cages, except controls, received either one large male or female spider, two small spiders (sex unknown) and two crickets (again either male or female). Controls received no large spiders, and were used to establish a baseline survival rate for the two potential prey items (small spiders and crickets). To test for the effects of water availability on predation by large spiders the researchers placed water pillows that held approximately 30 ml of water into half of the enclosures. They predicted that large spiders would primarily eat energy-rich small spiders in cages with water pillows, but prefer water-rich crickets in cages without water pillows. The water pillows had minimal impact on cricket water levels as they got plenty of water from their food (green water-rich leaves)

spiderwaterpillow.jpg

A large wolf spider sucks water from the water pillow.  Credit: Kevin McCluney.

Leinbach and his colleagues used per capita interaction strength as their quantitative measure of predation effects.  If prey survival was lower in the experimental cages than  in the control, there was a negative interaction strength – indicating that large spiders were eating a particular prey type.  When the researchers provided them with water, large spiders of both sexes ate significantly more small spiders than they did  without water supplements.

SpiderFig4

Interaction strength (effect of predation) of large spider (Hogna antelucana) on the small spider (Pardosa species).  Both male and female large spiders have significant negative effects on small spiders when water is supplemented (blue bars), but have minimal effects without water supplements (gray bars).

But the story was very different with crickets.  The researchers expected that when supplemented with water, large spiders would bypass the water-rich crickets in favor of the energy-rich small spiders. Surprisingly, instead of crickets in cages with pillows surviving as well as controls, they actually survived better – at least male crickets did. One possible explanation is that spiders may emit odor (or other types of) cues that affect cricket behavior in a negative way, for example by causing them to feed more cautiously and inefficiently. Once the large spiders have killed the small spiders, there may be fewer spiders around to smell up the place, and crickets may feed more efficiently, and thus survive better.

spidermccluney.jpg

Israel Leinbach searches for spiders and crickets within a cage. Credit Kevin McCluney.

I asked Kevin McCluney if there were any other surprising findings, and he pointed out that large male and female spiders showed very similar consumption patterns.  He expected that females would need more energy because egg production is very energy demanding.  One explanation for this lack of difference is that large male spiders may expend considerable energy wandering around in search of sexually receptive females, and their overall energy needs may be similar to those of females. Balancing the demands of energy, water and sex may be equally demanding for both sexes of large spiders, and may lead to adaptive feeding on different levels of the food chain as environmental conditions shift.

note: the paper that describes this research is from the journal Ecology. The reference is Leinbach, I.,  McCluney, K. E., and Sabo, J. L. 2019. Predator water balance alters intraguild predation in a streamside food web. Ecology 100(4):e02635. 10.1002/ecy.2635. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.