Vacation’s changing tides

Cindy and I and our dog (Cheyanne) recently returned from a two+ week vacation at North Carolina’s Outer Banks.  We stayed in Avon, which is about eight miles north of the iconic Cape Hatteras lighthouse in a large house with a great ocean view.  We got a large house, because we thought our kids might join us, but it turns out that one disadvantage of kids getting older is that their lives become more complex.  Anyhow, several friends stayed with us for a few days, and a grand time was had by all.

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Cheyanne and I ponder the ocean’s vastness. Credit: Cindy Miller

But the point of this post is the trip home.  On Friday, we packed everything into our car, including Cheyanne, and began the eight-hour drive back to our home in Radford, VA.  At Rodanthe (about 15 miles north), traffic just stopped.  We sat in our car for a few minutes, disembarked, and spoke with many people walking by, who told us that the road (NC12) was flooded and covered with sand.  We had heard rumors of flooding, but since the sun was out and the wind relatively calm, we assumed that was all in the past.  Apparently the flooding was so bad that a motor home and the boat it was towing got totally caught up in the sand and water, and was wedged so efficiently that they could not even be towed out until serious excavation happened. That was not going to happen until Saturday.

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Moving the dunes off of the road. Credit: Cindy Miller.

Saturday at 6 PM we got the call that the road was open and we could head home.  We repacked the car, re-experienced Cheyanne’s baleful look, and set out, with an ETA of 3 AM at the earliest.  Alas the high tide came in, water breached the dunes, and a very kind police officer knocked on our window, imploring us to return to Avon and wait for a better day.  Cheyanne gave him a baleful look, but we obeyed.

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Reconstituted dunes.  Notice the tire tracks left by earth-moving machines. Credit: Cindy Miller.

The next morning we set out again; by now we could pack a car in just a few minutes.  Our peanut butter on toast dinner of the previous night had left us a bit peckish, so we stopped off for some pastries and cappuccinos. We headed north once again and this time we were able to pass through the Rodanthe flood, and several others along the way.  The water level was high, but our car had good ground clearance and our escape was relatively uneventful, but done at sub-breakneck speed.

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Riding away through Rodanthe’s rising tides. Credit: Cindy Miller.

 

Why was this happening?  The weather was beautiful – no rain, no wind and sunny skies.  It just doesn’t get any nicer than this at the Outer Banks.  As it turns out, there were two provocateurs.  First, there was subtropical storm Melissa several hundred miles to our east, passing harmlessly out to sea, but increasing sea levels.  Second, there was almost a full moon, which also tends to increase sea levels.  But that’s it!

That shouldn’t be enough.  In past years those two events might cause waves to crash to the dunes with increased vigor, but would not cause them to breach the dunes and spill onto the roads.  But those were past years, and now is the present, and sea levels along the North Carolina coast have risen by about one foot in the past 50 years.  Here are some data from Wilmington, NC – about 150 miles south of Avon.

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Rising sea levels measured at Wilmington, North Carolina. Credit: National Oceanic and Atmospheric Administration and SeaLevelRise.org

You should note two things.  First, there is substantial year-to-year variation in sea levels. Second, rates of sea level rise are accelerating.  Scientists at the National Oceanic and Atmospheric Administration and the US Army Corps of Engineers expect this trend to continue.  Here is the prognosticated change in sea levels between now and 2050 at Oregon Inlet (just a few miles north of Rodanthe).

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Forecast sea level change between 2016 and 2050. Credit: National Oceanic and Atmospheric Administration, U. S. Army Corps of Engineers and SeaLevelRise.org

This is very bad.  I’ve been vacationing at the Outer Banks for about 25 years; it has become a part of who I am.  I don’t want to give up on this spectacular part of the world, but we must act.  We cannot continue sticking our heads in the sand (which we can now oftentimes find on NC12), pretending that climate change is a construct of the liberal press or elite intelligentsia.

The first step in dealing with a problem is acknowledging that it exists. Climate change is here, and its impact is increasing. An estimated 50 million climate change refugees around the globe are being forced to abandon their homes. More will follow, including our neighbors from North Carolina’s Outer Banks. For their sake, and ours, let’s acknowledge the problem, and focus our resources, energies and talents to reducing the damage in the short term, and dealing with the causes of climate change over the next decades and centuries.

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.

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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.

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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?

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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.

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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).

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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).

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(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.

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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.

Fast living vs. slow and steady

Fast living makes headlines, as evidenced by such notables as Freddie Mercury, Paul Walker and Lamar Odom.  Unfortunately the first two are dead while Odom was narrowly brought back from a near-death experience – all were victims of their fast life styles.  Like humans, some birds live fast and die young, while others live slow, but may survive to relatively ripe old ages.

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The tiny rifleman (Acanthisitta chloris), reintroduced in Tiritiri Matangi, New-Zealand.  This endangered bird feeds on insects that it gleans from tree trunks.  It often has two clutches of 2-5 young per year. Credit: Simon Ducatez.

Simon Ducatez studied invasive cane toads with Rick Shine in Australia, and became interested in why some species were more likely than others to successfully invade new habitat.  The problem for answering that question is that most invasions are not studied until after the invasive species becomes established; by that time it may be too late to identify exactly what factors were responsible for the successful invasion. On his first visit to New-Zealand in 2016, Ducatez discovered ecosanctuaries – enclosed wildlife reserves where invasive predators are eliminated, and native animals (mostly birds) are introduced. He realised that these introductions could provide invaluable information on why species thrive or fail to become established in a new environment. At about the same time, a colleague drew his attention to a database developed by the Lincoln Park Zoo (LPZ) in Chicago, Illinois, which contains data on hundreds of intentional release events (translocation attempts), including information on the survival and reproduction of the released individuals. Analyzing how a species life history could affect the survival and reproduction of these voluntarily introduced populations would provide answers useful for restoration biologists who wish to return native species to habits where they were now extinct, and to ecologists who want to identify the factors promoting biological invasions.

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The relatively massive and flightless south island takahe (Porphyrio hochstetteri), reintroduced in New-Zealand.  This bird was thought to be extinct but was rediscovered in 1948 and has benefited from active restoration programs. Credit: Simon Ducatez.

Life history traits are adaptations that influence growth, survivorship and reproduction of individuals of a particular species.  For each species in the LPZ dataset, Ducatez and Shine used the bird literature to gather data on body mass and four life history traits: maximum lifespan, clutch size, number of clutches per year, and age at first reproduction. They then used a statistical procedure – principle components analysis – which described each species based on their life history strategy.  Fast life styles were associated with small bodies, short lifespans, large clutch size and number, and early reproduction.  Slow life styles were associated with large bodies, long lifespans, small clutch size and number, and delayed reproduction. Ducatez and Shine then asked a simple question based on 1249 translocation events in the LPZ dataset – how do fast life style birds perform in comparison to slow life style birds following translocation?

It turns out that slow life style birds are much better at surviving translocation than are fast life style birds, at least when measured in the short term (one week) and the medium term (one month).

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Association between life style as measured by principle component analysis (PC1 on X-axis) and survival (proportion of translocated individuals still alive on Y-axis). The left graph is survival to one week, while the right graph is survival to one month. 

In contrast, following translocation fast life style birds are more likely to attempt breeding and successfully breed than are slow life style birds.

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Association between life style and probability of attempting breeding (left graph) and successfully breeding (right graph).

Ducatez and Shine suggest that both restoration biologists and invasion ecologists could use these findings to address major questions in their respective fields.  Restoration biologists wishing to return native species to previously occupied habitat might adopt different approaches based on a species life style. Species with fast life styles suffer from low survival, so restoration biologists should focus on promoting survival by controlling predators or provisioning extra food. Species with slow life styles suffer from low reproductive success, so conservation managers might consider providing extra nest boxes or other resources that promote successful breeding.

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A successful foraging event for an Atlantic puffin (Fratercula arctica), reintroduced in Maine, USA. Credit: Simon Ducatez.

This research informs invasion ecologists that the same trait can have opposite effects on the likelihood that a biological invasion will actually happen.  Thus a slow life style species is more likely to survive moving to a novel habitat, but is unlikely to breed successfully once it gets there.  In contrast a fast life style species is less likely to survive the move, but if it does survive, it may be more likely to successfully reproduce. How this plays out in actual biological invasions is yet to be determined, but at least we now have a better grasp on what factors we should be examining.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Ducatez, S. and Shine, R. (2019), Life‐history traits and the fate of translocated populations. Conservation Biology, 33: 853-860. doi:10.1111/cobi.13281. 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.

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.

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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.

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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).

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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.

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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)

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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.

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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.

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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.

Invasive crayfish hit the self-destruct button

One important feature of a biological invasion is that invaders can change an entire ecosystem in a substantial way.  A possible outcome of this change is that, in theory, an invasive species could inadvertently make an ecosystem less suitable as a habitat for itself.  Does this happen, and if so, under what circumstances?  One reason invasive species are so successful is that they usually can increase in population size very quickly.  Ecologists have discovered that species with the potential to increase very quickly may also have the potential to decline equally rapidly and then increase again, going through repeated boom-bust cycles of population size.  Thus if an invasive species starts to decline, it does not always mean that this decline will continue over time. Consequently, monitoring a biological invasion for only a few years may give a misleading picture of long-term prognosis for the invasive species and the ecosystem.

Eric Larson was able to address these problems when he began his postdoctoral research with David Lodge at the University of Notre Dame in 2014. Lodge (and John Magnuson before him) has studied the rusty crayfish (Faxonius rusticus) invasion in 17 northern Wisconsin lakes since the 1970s, using the same bait (beef liver) and the same traps on the same days each year.

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Crysta Gantz prepares to bait a trap with beef liver, which the crayfish love, but she – not so much! Credit: Eric R. Larson.

Three graduate students (the other co-authors of the paper) had continued data collection and done extensive mapping of the lake bottoms.  When Larson joined the research program he had about 40 years of data and 17 well-described lakes.  He knew that rusty crayfish were declining in some lakes and not others, and he and his colleagues were ready to explore whether these declines could be tied in to some environmental variable that the crayfish were influencing in some lakes, but not others.

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Allequash Lake. Credit Eric R. Larson

As an avid fisherman (more in my mind than in actuality), I have, on many occasions, caught a nice bass only to have it regurgitate the contents of its stomach, which usually includes bits of crayfish.  As it turns out, predacious fish such as bass love to eat crayfish, and crayfish are more likely to survive in environments that provide hiding places such as rocks or luxurious macroalgae that grow in sand or muck. The problem is that crayfish enjoy dining on macroalgae, so they can do themselves a disservice by eating their shelter from predators, effectively changing their environment so that their invasion is no longer sustainable.  Does this actually happen?

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Two rusty crayfish discuss the issues of the day. Credit: Eric R. Larson.

Larson and his colleagues continued collecting data on 17 lakes, and used their long-term data set to evaluate whether rusty crayfish populations were not declining (steady or increasing), declining or occupying an ambiguous gray zone where there was no clear trend in how the population was changing. The analysis showed that three lakes were not declining since the rusty crayfish invasion, eight lakes had declined substantially and six lakes were ambiguous.

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The researchers turned their attention to the lake-bottom substrate.  Were rusty crayfish more successful in rocky bottom lakes that gave them continuous predator protection?  Their analysis indicated that the three lakes where the invasion was going strong had the rockiest substrate, while the eight lakes experiencing population declines after the rust crayfish invasion were significantly less rocky.

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Proportion rocky substrate in lakes whose rusty crayfish populations are in decline (red), have an ambiguous trend (black) or are not in decline (blue). The horizontal line within each box is the median value, box bottom and top are 25th and 75th percentile, and whiskers are the 10th and 90th percentile. Non-overlapping letters above the bars (a and b) indicate significant differences between the groups.

The researchers conclude that in the absence of rocky substrate, the rusty crayfish is eating the aquatic macrophytes that grow from the sandy lake bottom, thereby exposing itself to predators.  Larson and his colleagues recommend simultaneous surveys of crayfish populations and density of aquatic macrophytes to see whether lakes may oscillate between states dominated by one or the other.

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Captured crayfish. Photo Eric R. Larson

Researchers want to know how commonly invasive species modify habitat in a self-destructive way.  A literature review of invasive species declines failed to find much evidence, but there are not enough long-term data sets to get a sense of how frequently this occurs. The problem is that researchers need to monitor the invasive species population and the relevant habitat variables for an extended time period.  The jury is still out on this question and only time (and careful data collection) will tell.

note: the paper that describes this research is from the journal Ecology. The reference is Larson, E. R.,  Kreps, T. A.,  Peters, B.,  Peters, J. A., and  Lodge, D. M.  2019.  Habitat explains patterns of population decline for an invasive crayfish. Ecology  100( 5):e02659. 10.1002/ecy.2659. 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

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.

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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.

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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.

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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.

LindsayFig2

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).

LindsayFig3

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.

Mating brine shrimpHans Hillewaert

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.

Fewer infections found in forest fragments

As human populations expand, we are converting ecosystems from one state to another.  In the case of tropical forests, conversion of forest to cropland may leave behind fragments of relatively undisturbed forest surrounded by a matrix of cropland or other forms of development.  Conservation ecologists are exploring whether ecological processes and ecosystem structure in these fragments work pretty much like normal forested regions, or whether fragments behave differently.  To do this, in a few locations around the world such as the Wog Wog Fragmentation Experiment in New South Wales, Australia, researchers have systematically created forest fragments of various sizes.  They can then ask a variety of questions comparing fragments vs. intact forest. For example,  how does species diversity, or how do processes such as competition, predation and mutualism differ in the two landscapes?

aerial photo2

Aerial photo of Wog Wog Fragmentation Experiment at the time the experiment began in 1987. Credit: Chris Margules.

Julian Resasco was working as a postdoctoral associate in Kendi Davies’ lab at the University of Colorado on a study that looked at changes in invertebrate communities in response to fragmentation at Wog Wog. Beginning in 1985, researchers had set up a network of pitfall traps, which are cups that are buried with their tops level to the ground, so that any careless organism that wanders in will be trapped in the cup.  Some pale-flecked garden skinks, Lampropholis guichenoti, also had the misfortune to become entrapped and became subjects for the study. The invertebrates, and the 186 unfortunate skinks were preserved in alcohol and stored as part of the Australian National Wildlife Collection.

Museumspec

Skink museum specimens at the Australian National Wildlife Collection. Credit: Julian Resasco.

Much later, Resasco arrived and began dissecting skink guts to analyze the prey items for a study that looked at how the skinks shifted prey consumption (their feeding niche) in response to fragmentation. While dissecting the skink guts, he noticed that some of the skinks had worms (nematodes) inside their guts.  These nematodes were relatively common among skinks from continuous eucalypt forests, rare among skinks from eucalypt fragments, and absent from skinks in the cleared, pine plantation matrix.

ResascoFig1

Top. The study area in southeast Australia, showing location of continuous forest, forest fragments and surrounding matrix.  Dots indicate locations of pitfall traps. The matrix was planted in pine seedlings soon after fragmentation.  Bottom. The pale-flecked garden sunskink Lampropholis guichenotti. Credit: Jules Farquar

As it turned out, the nematode was a new species, which Resasco and a colleague (Hugh Jones) named Hedruris wogwogensis. Nematodes in the genus Hedruris use crustaceans as intermediate hosts, which alerted Resasco and his colleagues that the terrestrial amphipod Arcitalitrus sylvaticus, which was very common in the pitfall traps, was probably an important intermediate host.  When amphipods from pitfall traps were examined microscopically, a small portion of them were infected with Hedruris wogwogensis. The researchers concluded that amphipods became infected when they ate plants that harbored nematode eggs or young nematodes, which then developed in amphipod guts, and were passed on to skinks that ate the amphipods.  Thus somewhat inadvertently, one aspect of the study transitioned into the question of how fragmentation can influence the transmission of parasites.

After concluding their skink dissections, Resasco and his colleagues discovered that skinks in continuous forest had five times the infection rate as did skinks in fragmented forest.  In addition, no skinks collected in the matrix were infected. Infected skinks harbored a similar number of nematodes, whether they lived in continuous forest or fragments (see the Table below). Lastly, amphipods were considerably more common in skink guts and pitfall traps from continuous forest, less so in fragments, and least in the matrix.

ResascoTab1good

Summary of data collected by Resasco and his colleagues. Nematode intensity is the mean number of nematodes per infected skink. Nematode abundance is the mean number of nematodes per skink (infected and uninfected). 

The researchers put these findings together in a structural equation model.  The boxes in the model below represent the variables, while the numbers in smaller boxes over the arrows are the regression coefficients, with larger positive numbers (in black) indicating stronger positive effects, and larger negative numbers (in red) indicating stronger negative effects.  The model revealed three important findings.  First, habitat fragmentation strongly reduced amphipod abundance.  High amphipod abundance was associated with high nematode abundance (that is the +0.20 in the model), so lower amphipod abundance from fragmentation reduced nematode abundance. Second, habitat fragmentation positively affected skink abundance – more skinks were captured in fragments than in intact forest, but this increase had no effect on nematode abundance in skinks.  Finally, note the direct arrows connecting “Fragmentation” to “Log nematode abundance in skinks”.  This indicates that other variables (beside amphipod abundance) are reducing infection rates in skinks that live in fragments and the matrix.

ResascoFig3

Structural equation model showing effects of fragmentation on nematode infection in skinks. Amphipods are the intermediate host.  Black arrows indicate significant positive effects of one variable on the other, while red arrows indicate significant negative effects. Solid lines represent fragments compared to controls and dashed lines represent the matrix compared to controls. Thicker lines are stronger effects.

At this point, we still have an incomplete understanding of the system.  We know that fragmentation reduces amphipods, which require a moist and shaded environment to thrive.  Reduced amphipod abundance leads to lower nematode infection rates in skinks.  But we know that other variables are important as well; perhaps nematodes survive more poorly in fragment and matrix soils. Interestingly, pine trees were planted in the matrix and are beginning to mature and shade out the matrix environment. Amphipod abundances are on the rise, so the researchers predict that nematode infection rates will begin to increase accordingly.  Those studies have begun.

IMG_2646

Eucalypt forest canopy at Wog Wog. Credit: Julian Resasco.

Looking at the bigger picture, it is clear that fragmentation may decrease (as in this study) or increase the abundance of an intermediate host. As an example of fragmentation increasing intermediate host abundance, the researchers describe a study in which fragmentation increased the abundance of the white footed mouse, an intermediate host for black-legged ticks (that host the bacteria that causes Lyme disease). We need to unravel the connections between landscape factors and the various species they influence, so we can begin to understand how human changes to the landscape can influence the transmission of diseases.

note: the paper that describes this research is from the journal Ecology. The reference is Resasco, J.,  Bitters, M. E.,  Cunningham, S. A.,  Jones, H. I.,  McKenzie, V. J., and  Davies, K. F..  2019. Experimental habitat fragmentation disrupts nematode infections in Australian skinks. Ecology  100( 1):e02547. 10.1002/ecy.2547. 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.