Introduced quolls quell melomys

October 25, 2021 by fredsingerecology

The cane toad has the distinction of being the world’s largest toad. It was introduced to Australia in 1935 to control cane beetles that were eating sugarcane (see A toxic brew: toad vs. quoll). Since their introduction, cane toads have expanded their range by over 2000 km from their release sites, which would be fine if all they did was eat cane beetles. As it turns out, they are worthless at eating cane beetles, but are very good at being eaten by many predators, including the northern quoll (Dasyurus hallucatus). This turns out poorly for the quolls, as cane toads are loaded with toxins, which quickly convert a cane-toad-fed quoll into a quoll corpse. Consequently, quoll populations are collapsing across much of Australia.

A northern quoll sports a radio collar. Credit: Chris Jolly

For his PhD work, Chris Jolly was hoping to explore whether he could use behavioral conditioning techniques to train quolls to avoid cane toads before he released them back into the environment. Unfortunately, while in captivity, the quolls lost their fear of predators, and became easy prey for dingoes, which resulted in a failed reintroduction to Kakadu National Park. In an interesting twist, another graduate student was releasing quolls onto Indian Island for a different purpose, and Jolly decided to regroup and focus his attention on how the prey population responded to a novel predator. In 2017, 54 quolls were released on the northern part of the island, which set up a natural experiment in which quolls were present in the north, and absent in central and southern Indian Island

Ben Phillips and John Moreen release the first batch of northern quolls on Indian Island (Kabarl), Northern Territory, Australia. Credit: Chris Jolly

Working with several researchers, Jolly established four research sites in the north and three in the south. At each one-hectare site, the researchers set up a 10 X 10 grid of live-traps which they baited with balls of peanut butter, rolled oats and honey (100 traps at each site). The target species was the grain-eating rodent, Melomys burtonia. Over the course of the study, 439 individual melomys were captured, weighed, sexed and implanted with a microchip for identification purposes. The researchers wanted to know how the presence of predaceous quolls influenced melomys abundance, and whether melomys adjusted behaviorally to quoll presence.

Research sites on Indian Island. Quolls were introduced to the northern part of the island in 2017.

They discovered that the three southern sites (without quolls) maintained relatively steady numbers of melomys throughout the study. In contrast, the four northern sites (with quolls) showed a sharp decrease in melomys abundance. Complicating the issue, a wildfire broke out in August 2017, affecting only the northern part of the island. The researchers believe this fire did not affect the melomys in any significant way, as wildfires are common in the area, and several previous studies have shown no effect of wildfires on melomys abundance.

Melomys population estimates at three southern sites (left graph) and four northern sites (right graph). The dashed orange line denotes quoll introduction, while the dashed red line indicates the wildfire in 2017. Error bars are 95% confidence intervals.

Shyness can be an adaptive behavior if predators are in your environment. Jolly and his colleagues wanted to know if there was any difference in the shyness (or conversely – the boldness) of melomys from the north (with quolls) and south (without quolls). They set up arenas that were baited with the aforementioned peanut butter balls, and placed a live-trap with a melomys at the door to the arena. The researchers then opened up the trap door and recorded whether the melomys entered the arena within 10 minutes.

Experimental setup testing melomys responses to open-field tests. Credit: Chris Jolly.

After 10 minutes, each melomys was rounded up and placed back in its trap, and a red plastic bowl was put into the arena. The trap was then reopened and the researchers recorded whether the melomys interacted with the red bowl.

Looking at the left graph, you can see that in 2017, north island melomys were much shyer than melomys from the predator-free south island. But by 2019, this difference was mostly gone. But when it comes to exploring a novel object (right graph), the northern melomys still retained some of their fear in comparison to southern melomys.

Figure 4

Left graph.Proportion (+ 95% confidence intervals) of melomys that emerged from live traps within 10 minutes in the open-field test. Right graph. Proportion of melomys that interacted with the novel object in the experiment that tested for neophobia (fear of novel objects).

Lastly, Jolly and his colleagues tested the effect of living with quolls on melomys foraging behavior. At nightfall, the researchers placed one wheat seed at 81 locations in each site. Control (unmanipulated) seeds were set out at 40 locations while seeds that had been stored with quoll fur (and presumably smelled like quoll) were set out at 41 locations. At daybreak, the researchers counted the number of remaining seeds, so they could calculate seed removal. In the first session conducted shortly after quoll release, they found no evidence of discrimination based on predator scent in either melomys population. But over time, the northern melomys began to discriminate based on quoll scent, while southern quolls continued to forage at the same rate on control and quoll-scented seeds.

Figure 5B

Mean seed take bias (the number of scented seeds – the number of control seeds) taken by north and south island melomys. Error bars are 95% confidence intervals.

The researchers conclude that introduction of quolls as a novel predator influenced melomys in two distinct ways. First, quolls preyed on them and reduced melomys abundance. But equally important, quolls changed melomys behavior. Soon after quoll introduction, invaded melomys populations were substantially shyer than the non-invaded populations. But this changed over the next two years, with a reduction in general shyness in the invaded populations, and an increase in predator-scent aversion. In effect, melomys were fine-tuning their behavioral response to quoll invasion.

Unfortunately, the researchers can’t evaluate whether these behavioral changes result from learning, or from natural selection. Melomys has a short generation time, so natural selection could be strong, even over a short timespan. Unfortunately, because of low survival from one year to the next, there were not enough melomys to test for whether individual behavior changed over time as a result of learning. It is certainly plausible that natural selection and learning operate together to change melomys behavior following quoll introduction.

note: the paper that describes this research is from the journal Ecology. The reference is Jolly, C. J., A. S. Smart, J. Moreen, J. K. Webb, G. R. Gillespie, and B. L. Phillips. 2021. Trophic cascade driven by behavioral fine-tuning as naıve prey rapidly adjust to a novel predator. Ecology 102(7): e03363. 10.1002/ecy.3363. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2021 by the Ecological Society of America. All rights reserved.

Turkey mullein trichomes gobble up protective pollen

February 21, 2020 by fredsingerecology

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.

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.

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.

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.

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.

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

February 7, 2020 by fredsingerecology

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.

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.

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, 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.

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.

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.

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.

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.

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

January 27, 2020 by fredsingerecology

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.

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.

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.

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.

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).

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.

A saltier Great Salt Lake supports a shifting ecosystem

July 23, 2019 by fredsingerecology

In science, like many other fields, “who you know” can be critical to success. Eric Boyd from Montana State University was introduced to the Great Salt Lake (GSL) ecosystem by his colleague Bonnie Baxter, a professor at Westminster College and the Great Salt Lake Institute in Salt Lake City, Utah. Baxter was fascinated by microbialites- deposits of carbonate mud of diverse shape and structure, that harbor an impressive diversity and abundance of microorganisms. Some of these microorganisms are photosynthetic, using dissolved organic carbon from the water to build carbohydrates; as such they are the primary producers which feed the rest of the ecosystem. Baxter impressed upon Boyd the need to understand the ecosystem, which feeds huge populations of two consumer species, the brine fly Ephydra gracilis and the brine shrimp Artemia franciscana. Up to 10 million birds, representing about 250 species, feed on these two species over the course of a year.

Eric Boyd collects samples from the north arm of GSL. Credit: Bonnie Baxter.

In 1959 a railroad causeway was built that divided GSL into a south and north arm, which differ from each other in one critical way. The south arm receives freshwater input from three rivers, while the north arm’s only freshwater input is rain and snowmelt. Both arms are hypersaline; the south arm is 4-5 times saltier than typical ocean water, while the north arm is about twice as salty as the south arm. Boyd and Baxter recognized that these salinity differences were probably impacting the microbial communities in the two arms; in fact preliminary observations indicated that microbialite communities were no longer forming in the north arm. So when Melody Lindsay began her doctoral research with Boyd, she elected to investigate how salinity was influencing the microbialite communities in the lake.

Melody Lindsay (right) and Bonnie Baxter (left) planning to sample in the south arm of GSL. Credit: Jaimi Butler.

Lindsay and her colleagues collected samples of microbialite mats from the south arm of the lake where the salinity of the water measured 15.6% (as a comparison, typical ocean water is about 3.5%). At each of six salinity levels (8, 10, 15, 20, 25 and 30%), the researchers set up three microcosms of 150 ml of lakewater, which they then inoculated with 10 grams of homogenized microbial mat. They then sampled microbial diversity and abundance four and seven weeks after beginning the experiment.

Exposed microbialites along the south arm’s shoreline. Credit: Eric Boyd.

This experiment was conceptually simple, but technically a bit of a challenge. Microorganisms are difficult to identify and count; and in fact it is likely that some of the species were new to science. Fortunately, researchers can use molecular approaches (quantitative PCR) to measure the quantity of each type of 16S rRNA gene in each microcosm. Each species of microorganism has distinct rRNA genes, so different base sequences indicate different microorganisms. This allows researchers to estimate how much of each species is present. One restriction is that closely related species will have almost identical rRNA genes, so they may be difficult to distinguish from each other.

Overall, microorganism abundance was 152% greater after four weeks and 128% greater after seven weeks at the 15% salinity. Recall that these samples came from microbialites associated with 15.6% salinity, so this finding indicates good growth at the salinity which the microorganisms have recently experienced. Interestingly, microorganisms thrived even better at 10% salinity. But higher salinity levels, particularly 25% and 30%, were very detrimental to microbial growth.

Change in abundance of 16S rRNA gene from microcosms incubated for four and seven seeks in comparison to abundance at week 0 for each salinity. Significant differences are comparisons with abundance at week 0. NS = no significant difference, * P<0.1, ** P<0.01, *** P<0.001, **** P<0.0001. Error bars = 1SE.

The researchers broke down their results into taxonomic Orders, based on the 16S rRNA sequence of each gene. The two most common Orders were Sphingobacteriales and Spirochaetales, which both grew best at low salinity. The next most common Orders were a cyanobacterium from the Order Croococcales, and an alga from the Order Naviculales. Species from these two taxonomic Orders are foundational to the ecosystem, because they are photosynthetic and relatively large. These dominant producers either directly, or indirectly, feed the rest of the ecosystem. Croococcales grew best at intermediate salinities (10-20%), while Naviculales did best at 8-15%, but also reasonably well at 20% salinity (see the figure below for a summary of the most common Orders).

Abundance of taxonomic Orders of Microorganisms incubated at different salinities at 4 and 7 weeks, in comparison to initial abundance (week 0 = yellow square). Darker green squares indicate a greater increase, and darker brown squares indicate a greater decrease in abundance. The most common Orders are on top, least common are on the bottom. Het = heterotroph, PP = primary producers, PhH = photoheterotroph.

Overall, primary productivity, as measured by how much dissolved organic carbon was taken up by the photosynthesizers, was greatest at 10 and 15%, and declined sharply above 20% salinity. In addition, brine shrimp, one of the two important animal consumers of microorganisms, hatched and survived best at the lowest salinities.

Two mating brine shrimp under the watchful eyes of an observer. Credit: Hans Hillewaert.

Lindsay and her colleagues conclude that conditions in the south arm are conducive to microbialite communities and the consumers they support. However, the north arm has much lower productivity, with salinity levels so high that salt is spontaneously crystalizing out of solution in some areas. Given that climate change models predict increased drought severity over the next century in the GSL region, it is very likely that salinity levels will rise throughout the lake. Over the same time period, humans are expected to increase water usage from the rivers that flow into the lake, which will further drop water levels in the lake, increase salinity in GSL, and dry out many of the microbial mats. This loss of ecosystem production is expected to cascade up the ecosystem, reduce brine shrimp abundance and ultimately the abundance and diversity of migratory birds that feed on them.

note: the paper that describes this research is from the journal Ecology. The reference is Lindsay, M. R., Johnston, R. E., Baxter, B. K., and Boyd, E. S. 2019. Effects of salinity on microbialite‐associated production in Great Salt Lake, Utah. Ecology 100( 3):e02611. 10.1002/ecy.2611. 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.

Quoll vs. toad: a toxic brew

May 6, 2019 by fredsingerecology

A native of Central and South America, the cane toad, Rhinella marina, was introduced to Australia in 1935 with great fanfare. The plan was for the voracious cane toad to eat all of the grey-backed cane beetles that were plaguing sugar cane plantations in northern Australia (a similar introduction had been successful in Puerto Rico). But the plan failed, in part because there was no cover from predators, so the toads were not enthusiastic about hanging out in sugar cane plantations, and in part because adult beetles live primarily near the tops of sugar cane, and cane toads are poor climbers.

A cane toad. Credit: Ben Philips

So now, northern Australia has a cane toad plague, which is wreaking havoc on ecosystems, and threatening many native species, including the northern quoll, Dasyurus hallucatus. These omnivorous marsupials eat fruit, invertebrates and small vertebrates. Unfortunately, their long list of food items includes cane toads, which are highly toxic to most consumers, having poison glands that contain bufotoxin, a composite of several very nasty chemicals. If a northern quoll eats a cane toad, it’s bye bye quoll.

Male captive born northern quoll_EllaKelly

A northern quoll. Credit: Ella Kelly.

Unfortunately most quolls have not gotten the message; huge numbers are dying, and populations are going extinct. As toads continue their invasion from north to south, more quoll populations, particularly those in northwestern Australia, will be at risk.

Map of Australia showing past (light shading) and recent (dark shading) northern quoll distribution, and present (solid line) and future (dashed line) cane toad distribution.

Some quolls show “toad-smart” behavior and don’t eat toads. Ella Kelly and Ben Phillips are trying to understand how this happens. This is particularly important because a few quoll populations have managed to survive the cane toad plague by virtue of being toad-smart (though 95% of quoll populations have gone extinct in the wake of the cane toad wave). The researchers reason that if there is a genetic basis to toad-smart behavior, it might be possible to introduce toad-smart individuals into populations that have not yet been overrun by cane toads. These individuals with toad-smart genes would breed and spread their genes through their adopted population. This strategy of targeted gene flow would give the recipient population the genetic variation needed, so that some individuals (those with toad-smart genes) would be more likely to survive the cane toad invasion. Over time toad-smart behavior would spread throughout the population via natural selection.

Targeted gene flow requires the trait to be influenced by genes. To test for a genetic basis to the toad-smart trait, Kelly and Phillips designed a common-garden experiment, capturing some quolls that had survived the cane toad invasion (toad-exposed), and others from regions that had not yet been exposed (toad-naïve). At Territory Wildlife Park, Northern Territory, Australia, the researchers bred these quolls to create three lines of offspring: Toad-exposed x toad-exposed, toad-exposed x toad-naïve (hybrids), and toad-naïve x toad-naïve. They raised these three lines under identical conditions at the park. Kelly and Phillips then asked, are there behavioral differences in how these three lines respond to cane toads?

Northern quoll captured in Northern Territory, Australia. Credit: Ella Kelly.

The researchers set up two experiments. First they asked, which would a quoll (that had never before experienced a cane toad) prefer to investigate if given the choice: a dead cane toad or a dead mouse? It turned out that the quoll offspring with two toad-exposed parents were somewhat more interested in mice than in cane toads. The same was true for the hybrids. However, the toads with two toad-naïve parents showed little preference.

Second, and more important, the researchers gave quolls from the three lines the opportunity to eat a toad leg (which does not have enough poison to harm the quoll). The results of this experiment were striking; offspring of toad-naïve parents were twice as likely to eat the toad leg than were offspring of toad-exposed parents, or hybrids with one parent of each type.

Proportion of toad-naive (both parents toad-naive), hybrid and toad-exposed (both parents toad-exposed) quoll offspring that ate a cane toad leg. Error bar = +/- 1 SE.

Kelly and Phillips conclude that toad-smart behavior is a genetically-based trait that has been under strong natural selection in populations of quolls that survived the cane toad invasion. Hybrid offspring behave similarly to the offspring of two toad-exposed parents, suggesting that toad-smart behavior has a dominance inheritance pattern. The researchers propose using targeted gene flow, in this case introducing toad-adapted individuals into populations prior to the arrival of cane toads. Recently, Kelly and Phillips released 54 offspring with toad-smart genetic backgrounds onto Indian Island, which is about 40 km from Darwin. The island has a large cane toad population, so the researchers will follow the introduced quoll population to see whether it is genetically equipped to survive in the presence of the cane toad scourge.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Kelly, E. and Phillips, B. L. (2019), Targeted gene flow and rapid adaptation in an endangered marsupial. Conservation Biology, 33: 112-121. doi:10.1111/cobi.13149. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2019 by the Society for Conservation Biology. All rights reserved.

Females are better speakers and better listeners than males – at least in plants

April 3, 2019 by fredsingerecology

My age puts me smack dab in the middle of the woo-woo generation, when many people engaged in activities, or shared in belief systems, that were criticized as unscientific, spacey or just plain bizarre. For example, talking to your plants was purported to make them bigger, greener or more florid. This hypothesis generated a huge number of science fair projects, but no clear answers (so far as I know – but I admit that I have not done the appropriate research!). But, it turns out that plants do talk to each other and to some animals. When attacked by herbivores, many plant species will emit volatile organic compounds (VOCs) into the air that can have two effects. First, these VOCs can alert nearby plants that herbivores are in the area, and that they should start producing defense compounds in their tissues that will repel these herbivores. Second, these VOCs can alert predators that herbivores are present, and they should swing by and eat them.

Several studies have shown that female and male plants may differ in several ways that could affect communication. Females typically invest more in reproduction, grow more slowly and invest more in defense against herbivory. Xoaquin Moreira and his colleagues wondered if sexual dimorphism in defense investment would result in differences between males and female in how they talk to each other. They chose the woody shrub Baccharis salicifolia, in which females grow more slowly but invest more in chemical defense and thus are infested by fewer herbivores than are males. They focused their study on chemical responses of the plant to the highly-specialized aphid Uroleucon macolai, which only feeds on two Baccharis species.

Baccharis salicifolia hosting an army of herbivorous aphids. Credit: X. Moreira.

The researchers used greenhouse experiments to explore how Baccharis uses VOCs for communication. To control aphid movement, each treatment was done in a mesh cage, with one centrally located VOC emitter plant (of either sex), and one female and one male receiver plant equally distant from the central plant. Control emitter plants were untreated, while herbivore-induced emitter plants were given 15 mature aphids, which fed and reproduced on the plants for 15 days. After 15 days Moreira and his colleagues removed all of the emitter plants and all of the aphids, and then inoculated each receiver plant with two adult aphids. The researchers measured aphid reproductive rate on the fifth day as their measure of aphid performance, or of plant resistance to aphids.

Emitter Baccharis salicifolia plant flanked by one male and one female receiver plant. Credit X. Moreira.

Aphids did much more poorly on male and female receiver plants that were associated with male herbivore-induced emitter plants (top graph below). This implies that these receiver plants became resistant to aphids as a result of their exposure to an airborne substance released by the male emitter plant. When the researchers used female emitter plants they found something very different. There was no effect on male receivers, but still a very strong effect on female receivers, which had a much lower aphid reproductive rate than the female plants exposed to untreated female emitter plants (bottom graph below).

Reproductive performance of aphids raised on control receiver plants (emitter plant with no aphids – clear bars) and herbivore-induced emitter plants (gray bars). Two left bars show performance on male receiver plants, while two right bars show performance on female receiver plants. Top graph shows data for male emitters and bottom graph shows data for female emitters. Error bars = 1 SE. *** indicates P < 0.001.

Showing differences between sexes in communication is important, but the next step is to figure out how this happens. In previous research, Moreira and his colleagues identified seven different VOCs that Baccharis emitted after aphid herbivory. So they explored whether there were differences between males and females in how much of each VOC they emitted in response to aphids. As before, they subjected some plants (of each sex) to herbivory and others were untreated controls. They then bagged each plant, and passed the collected vapors over a charcoal filter trap at a constant rate for an equal period of time. After extracting the substances from the charcoal, the researchers used a gas chromatograph to identify and quantify the VOCs.

Setup for collecting VOCs from Baccharis salicifolia. Credit X. Moreira.

The most impressive finding was a fivefold increase in pinocarvone release by female herbivore-induced plants in comparison to controls. In contrast, in males there was only a minor pinocarvone effect.

Relative increase in VOC emission following aphid attack in female (clear triangle) vs. male (filled triangle) Baccharis salicifolia. The induction effect is the log response ration (LRR) which is the natural log of (emission by the herbivore induced plants divided by the emission by the control plants). Error bars are 95% confidence intervals.

Having discovered that females emit much more pinocarvone than males, the next question was whether females are more sensitive to pinocarvone, or in fact to any of the other VOCs. So Moreira and his colleagues exposed plants to one of three treatments: 100 ul of pure pinocarvone, 100 ul of six VOCs including pinocarvone, and a control (no VOCs). They discovered that all experimental treatments reduced herbivory in comparison to the controls, but that there was no difference between males and females in how they responded.

Reproductive performance of aphids raised on female plants (left graph) or male plants (right graph) subjected to pinocarvone or a blend of six VOCs (including pinocarvone) in comparison to reproductive performance on untreated control plants (dashed line on top of each graph). Shading surrounding dashed line indicates 1 SE. Error bars are 1 SE.

This lack of different response between male and female plants to pinocarvone was a bit surprising; the researchers speculate that both males and females have pinocarvone receptors, but that female receptors are more sensitive (or numerous). If true, natural emissions of pinocarvone may suffice to induce a response in female but not male plants. But the artificial emitters may have released enough pinocarvone to stimulate male plants to respond as well. Clearly there is much more work to do here.

The researchers also wanted to know whether plants were more sensitive to VOCs produced by genetically identical plants (clones) in comparison to genetically-distant plants. They discovered no influence of genetic relatedness on plant response to herbivory. This is important, because from an evolutionary standpoint, there is no obvious reason why a plant would want to warn an unrelated plant that it was about to get eaten. An adaptive explanation is that relatives may tend to live near each other, so an emitter plant still benefits indirectly by promoting the survival of relatives who carry a proportion of genes identical to its own genetic constitution. One possible non-adaptive explanation is that a plant may use VOCs as a way of quickly communicating with itself, informing distant tissues that they need to produce defense compounds. Nearby plants may simply be eavesdropping on this conversation, and using it to their advantage.

note: the paper that describes this research is from the journal Ecology. The reference is Moreira, X., Nell, C. S., Meza‐Lopez, M. M., Rasmann, S. and Mooney, K. A. (2018), Specificity of plant–plant communication for Baccharis salicifolia sexes but not genotypes. Ecology, 99: 2731-2739. doi:10.1002/ecy.2534. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Dinoflagellates deter copepod consumption

February 18, 2019 by fredsingerecology

Those of us who enjoy eating seafood are dismayed by the dreaded red tide, which renders some of our favorite prey toxic to us. A red tide occurs when dinoflagellates and other algae increase sharply in abundance, often in response to upwelling of nutrients from the ocean floor. Many of these dinoflagellates are red or brownish-red in color, so large numbers of them floating on or near the surface give the ocean its characteristic red color. These dinoflagellates produce toxic compounds (in particular neurotoxins) that pass through the food web, ultimately contaminating fish, molluscs and many other groups of species.

Red tide at Isahaya Bay, Japan. Credit: Marufish/Flickr.

Did toxicity arise in dinoflagellates to protect them from being eaten by predators – in particular by voracious copepods? The problem with this hypothesis is that copepods eat an entire dinoflagellate. Let’s imagine a dinoflagellate with a mutation that produces a toxic substance. At some point the dinoflagellate gets eaten, and the poor copepod consumer is exposed to the toxin. Maybe it dies and maybe it lives, but the important result is that the dinoflagellate dies, and its mutant genes are gone forever, along with the toxic trait. The only way toxicity will benefit the dinoflagellate individual, and thus spread throughout the dinoflagellate population, is if it increases the survival/reproductive success of individuals with the toxic trait. This can occur if copepods have some mechanism for detecting toxic dinoflagellates, and are therefore less likely to eat them.

Jiayi Xu and Thomas Kiørboe went looking for such a mechanism using 13 different species or strains of dinoflagellates that were presented to the copepod Temora longicornis. This copepod beats its legs to create an ocean current that moves water, and presumably dinoflagellates, in its direction, which it then eats. For their experiment, the researchers glued a hair to the dorsal surface of an individual copepod (very carefully), and they then attached the other side of the hair to a capillary tube, which was controlled by a micromanipulator. They placed these copepods into small aquaria, where the copepods continued to beat their legs, eat and engage in other bodily functions.

Aquarium with tethered copepod and recording equipment: Credit: J. Xu.

The researchers then added a measured amount of one type of dinoflagellate into the aquarium, and using high resolution videography, watched the copepods feed over the next 24 hours.

Tethered copepod beats its legs to attract a dinoflagellate (round blue circular cell). Credit: J. Xu.

Twelve of the dinoflegellate strains were known to be toxic, though they had several different types of poison. Protoceratium reticulatum was a nontoxic control species of dinoflagellate. As you can see below, on average, copepods ate more of the nontoxic P. reticulatum than they did of any of the toxic species.

Average dinoflagellate biomass ingested by the tethered copepods. P. reticulatum is the nontoxic control. Error bars are 1 SE.

Xu and Kiørboe identified two major mechanisms that underlie selectivity by the copepod predator. In many cases, the copepod successfully captured the prey, but then rejected it (top graph below). For one strain of A. tamarense prey, and a lesser extent for K. brevis prey, the predator simply fed less as a consequence of reducing the proportion of time that it beat its feeding legs (bottom graph below).

Copepod feeding behavior on 13 dinoflagellate prey species. Top graph is fraction of dinoflagellates rejected, while bottom graph is the proportion of time the copepods beats its feeding legs in the presence of a particular species/strain of dinoflagellate.

If you look at the very first graph in this post, which shows the average dinoflagellate biomass consumed, you will note that both strains of K. brevis (K8 and K9) are eaten very sparingly. The graphs just above show that the copepod rejects some K. brevis that it captures, and beats its legs a bit less often when presented with K. brevis. However, the rejection increase and leg beating decreases are not sufficient to account for the tremendous reduction in consumption. So something else must be going on. The researchers suspect that the copepod can identify K. breviscells from a distance, presumably through olfaction, and decide not to capture them. This mechanism warrants further exploration.

One surprising finding of this study is that the copepod responds differently to one strain of the same species (A. tamarense) than it does to the other strains. Xu and Kiorbe point out that previous studies of copepod/dinoflagellate interactions have identified other surprises. For example, there are cases where a dinoflagellate strain is toxic to one strain of copepod, but harmless to another copepod strain of the same species. Also, within a dinoflagellate species, one strain may have a very different distribution of toxins than does a second strain. So why does this degree of variation exist in this system?

The researchers argue that there may be an evolutionary arms race between copepods and dinoflagellates. The copepod adapts to the toxin of co-occurring dinoflagellates, becoming resistant to the toxin. This selects for dinoflagellates that produce a novel toxin that the copepod is sensitive to. Over time, the copepod evolves resistance to the second toxin as well, and so on… Because masses of ocean water and populations of both groups are constantly mixing, different species and strains are exposed to novel environments with high frequency. Evolution happens.

note: the paper that describes this research is from the journal Ecology. The reference is Xu, J. and Kiørboe, T. (2018), Toxic dinoflagellates produce true grazer deterrents. Ecology, 99: 2240-2249. doi:10.1002/ecy.2479. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Seaweed defense – location, location, location.

June 13, 2018 by fredsingerecology

If you’re ever feeling sorry for yourself, you should know that things could have been much worse; you could have been the brown seaweed, Silvetia compressa. So many problems! Ocean waves come crashing over you, threatening to pull you off your life-sustaining substrate. Ocean tides recede, exposing you to harsh sun and dangerously dry conditions. Perhaps worst of all, the fearsome predator Tegula funebralis eats away at your body, and you are powerless to defend yourself from its savage ravages.

Tegula snails chomp away on Silvetia seaweed in northern California. Credit: Emily Jones.

As it turns out, Silvetia is not so powerless after all. After being partially grazed by Tegula, the seaweed can induce defenses that reduce its palatability. From prior work, Emily Jones noticed that seaweed from northern California shorelines was much more sensitive to grazing than was seaweed from southern California shorelines. It took fewer grazing snails to elicit palatability reduction in northern Silvetia than it did in southern Silvetia. She decided to focus her PhD work with Jeremy Long on documenting these geographic differences, and figuring out why they exist.

Emily Matthews (near) and Grace Ha (far) survey snails and seaweed in a northern California site. Credit: Emily Jones.

Environmental conditions vary along the California coast. Northern seaweed populations experience cooler temperatures (air ~5-20 °C; water ~10-12 °C) and more nutrients (nitrate levels up to 40 umol/L) than do southern populations (air 5-37 °C; water ~14-20 °C; nitrate levels < 2 umol/L). In addition, Jones and Long surveyed Tegula abundance at three northern California and three southern California sites, counting every snail in 20 quadrats placed in the low, mid and high intertidal zone at each of the six sites (360 0.25 X 0.25m quadrats in total) . They discovered that seaweed was much more likely to encounter Tegula along northern coastlines.

Percent of plots with Tegula snails in northern sites (Stornetta, Moat Creek and Sea Ranch – blue bars) and southern sites (Coast, Calumet and Cabrillo – orange bars). High, Mid and Low refer to location within the intertidal zone (high is closest to shore and regularly exposed at low tide).

Given these differences in snail abundance, we can now understand why Silvetia is more sensitive in its northern range to Tegula grazing. But how strong are these differences in sensitivity? Jones and Long developed a simple paired-choice feeding preference assay to test for differences in palatability. At each location (north and south), the researchers gave test snails a choice between feeding on seaweed that had been previously grazed by either 1, 4, 7, 10 or 13 Tegula snails, or to feed on seaweed with no grazing history. The test snails grazed for five days, and the researchers measured the amount of seaweed consumed for each group. They discovered that even a little bit of previous grazing (the 1-snail treatment) made northern test snails prefer non-grazed northern Silvetia, while only high levels of previous grazing (the 10 and 13-snail treatments) had similar effects on southern snails tested on southern Silvetia.

Amount of previously-grazed and non-grazed Silvetia eaten by Tegula in paired choice tests. (Top) Northern Selvetia, (Bottom) Southern Silvetia. Error bars are 1SE. * indicates significant differences in consumption rate.

These findings raised the question of whether the cooler and more nutrient-rich environmental conditions at the northern site were somehow causing this difference in consumption of previously-grazed seaweed. The researchers designed a series of common garden experiments at the Bodega Marine Laboratory, in which seaweed from both locations were tested in the same environment. Silvetia was exposed to grazing by two snails, or by no snails for 14 days. When test snails were given the choice of non-grazed or previously-grazed northern Silvetia, they much preferred eating non-grazed Silvetia. In contrast, they showed no preference when given a similar choice between non-grazed or previously-grazed southern Silvetia. This indicates that seaweed from the north are responding more to grazing by reducing palatability than are seaweed from the southern locations.

Amount of previously-grazed and non-grazed northern and southern Silvetia eaten by Tegula in paired choice tests.

In theory, there could be a tradeoff between induced defenses, such as reduction in palatability in response to grazing, and constitutive defenses, which an organism expresses all of the time. Examples of constitutive defenses are thorns or spines in plants, and cryptic coloration or body shape in many insects. Jones and Long found no evidence for such a tradeoff; in contrast southern Silvetia actually had lower levels of constitutive defenses, as both northern and southern Tegula strongly preferred eating southern Silvetia in paired choice tests.

Amount of northern and southern Silvetia eaten by northern and southern Tegula in paired choice tests.

These geographic differences in seaweed sensitivity to grazing are probably due to long-term differences in environmental history. Southern Silvetia seaweeds live in stressful conditions (high temperatures and low nutrients), and the physiological cost of mounting an induced defense against low and moderate levels of grazing may be too high to be worthwhile. We also don’t know what the overall grazing rates are in the north versus the south, and importantly, how variable the grazing rates are in each location. Highly variable grazing rates would select for a strong set of induced responses, which could be turned on and off as needed, allowing seaweed, or any plant, to defend itself against new or more hungry herbivores moving into their environment.

note: the paper that describes this research is from the journal Ecology. The reference is Jones, Emily and Long, Jeremy D. 2018. Geographic variation in the sensitivity of an herbivore-induced seaweed defense. Ecology. doi: 10.1002/ecy.2407. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Plant communities bank against drought

May 3, 2018 by fredsingerecology

Many plants shed their young embryos (seeds) into the soil where they may accumulate in a dormant (non-growth) state over years before germinating (resuming growth and development). Ecologists describe this collection of seeds as a seed bank. Marina LaForgia describes how scientists were able to germinate and grow to maturity some 32,000 year old Silene stenophylla seeds that was stashed, probably by an ancient squirrel, in the permafrost! With increased climatic variation predicted by most climate models, she wanted to know how environmental variability might affect germination of particular groups of species within a community. In addition, she and her colleagues recognized that most ecological studies investigate community responses to disturbances by looking at the aboveground species. It stands to reason that we should consider the below-surface seed bank as a window to how a community might respond in the future.

Some seedlings coming up from the seed bank. Credit:Marina LaForgia.

Seed banks can be viewed as a bet-hedging strategy that spreads out germination over several (or many) years to reduce the probability of catastrophic population decline in response to one severe disturbance, such as drought, flood or fire. In some California annual grassland communities, species diversity is dominated by annual forbs – nonwoody flowering plants that are not grasses. Many forbs produce seeds that can lie dormant in the seed banks for several years. Though these forbs are the most diverse group, there are also about 15 species of exotic annual grasses that dominate the landscape in abundance and cover. These grasses dominate because they produce up to 60,000 seeds per m², they grow very quickly, and they build up a layer of thatch that suppresses native forbs. However, seeds from these grasses cannot lie dormant in the seed bank for very long.

Area of field site dominated by Delphinium (purple flower) and Lasthenia (yellow flower). Looking closely you can also see some tall grasses rising. Credit Marina LaForgia.

How is drought affecting these two major components of the plant community? LaForgia and her colleagues answered this question by collecting seeds from a northern California grassland at the University of California McLaughlin Natural Reserve in fall 2012 (beginning of the drought) and fall 2014 (near the end of the drought). They used a 5-cm diameter 10-cm deep cylindrical sampler to collect soil and associated seeds from 80 different plots. The researchers also used these same plots to estimate aboveground-cover, and to identify the aboveground species that were present. The research team germinated and identified more than 11,000 seeds.

Plants germinating in the greenhouse. Credit Marina LaForgia.

The researchers knew from previous work on aboveground vegetation that exotic annual grasses declined very sharply in response to drought. In contrast, the native forbs did relatively well, in part depending on their specific leaf area (SLA) – a measure of relative leaf size, with low SLA plants conserving water more efficiently. It seemed reasonable that these same patterns would be reflected belowground. Recall that most grass seeds are incapable of extended dormancy, while many forbs can remain dormant for several years. Consequently, LaForgia and her colleagues expected that grass abundance in the seed bank would decline more sharply than would forb abundance. In addition, they expected that high SLA forbs would not do as well as low SLA forbs during drought.

The researchers discovered very sharp differences between the two groups over the course of the drought. Exotic annual grasses declined sharply in the seed bank, while native annual forb abundance tripled. Aboveground cover of grasses declined considerably, while aboveground cover of forbs increased modestly. Clearly the exotic grasses were suffering from the drought, while the forbs were doing quite well.

(a) Seed bank abundance of grasses (red circles) and forbs (blue triangles) at beginning of drought (2012) and near end of drought (2014). (b) Percent cover of grasses (red circles) and forbs (blue triangles) at beginning of drought (2012) and near end of drought (2014). Data are based on samples from 80 plots. Error bars indicate one standard error.

We can see these differences on an individual species basis, with most of the grasses declining modestly or sharply in abundance, while most of the forbs increased.

Mean change in seed bank abundance per species based on 15 exotic grass species and 81 native forb species.

It is not surprising that the grasses do so poorly during the drought. Presumably, less water causes poorer germination, growth, survival and seed production. In addition, because grass seeds have a low capacity for dormancy, grass abundance will tend to decrease in the seed bank very quickly with such a low infusion of new seeds.

But why are the forbs actually doing better with less water available to them? One explanation is that grass abundance and cover declined sharply, causing the forbs to experience reduced competition with grasses that might otherwise inhibit their growth, development and reproductive success. The tripling of native forbs in the seed bank was much greater than the 14% increase in aboveground forb cover. The researchers reason that the drought caused many of the forb seeds to remain dormant, leading to them building up in the seed bank. This was particularly the case for low SLA forbs, which increased much more than did high SLA forbs in the seed bank.

We can understand exotic grass behavior in the context of their place of origin – the Mediterranean basin, which tends to have wet winters. In that environment, natural selection favored individuals that germinated quickly, grew fast and made lots of babies. Since their introduction to California in the mid 1800s, 2014 was the driest year on record. It will be fascinating to see if these exotic grasses can recover when, and if, wetter conditions return. Can we bank on it?

note: the paper that describes this research is from the journal Ecology. The reference is LaForgia, M.L., Spasojevic, M.J., Case, E.J., Latimer, A.M. and Harrison, S.P., 2018. Seed banks of native forbs, but not exotic grasses, increase during extreme drought. Ecology99 (4): 896-903. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

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How new research expands our understanding of the natural world

Category Archives: Evolution