Forest canopy fixes nitrogen shortage

The two billion hectares of forest canopy remaining on our planet are ideal habitat for nitrogen fixing microorganisms that can convert N2 to ammonia.

StantonCanopy

View of the forest canopy at the research site. Credit: D. Stanton.

The forest canopy tends to be nutrient-poor because there is no access to nutrients that accumulate in the soils on the forest floor, and rainfall can leach away any nutrients that do accumulate in the canopy from atmospheric deposition. So if you are a microbe, and you want to enjoy the view from the canopy, it is to your advantage to be able to fix atmospheric nitrogen so you can build essential molecules such as proteins and ATP.

As I mentioned in a previous post (Nitrogen continues to confound convention) both phosphorus (P) and molybdenum (Mo) are essential nutrients for biological nitrogen fixation.  Daniel Stanton and his colleagues hypothesized that nitrogen fixation in the canopy might be limited by the availability of P and Mo, so they designed a series of experiments to explore the role of these nutrients at the San Lorenzo Canopy Crane in San Lorenzo National Park in the Republic of Panama.  The crane provides about 1 ha of canopy access to non-acrophobic ecologists.

Stantoncrane

The crane at the research site: Credit: D. Stanton.

In one experiment, Stanton and his colleagues filled thin nylon stockings with vermiculite to form 40 cm long cylinders of 4 cm diameter.  Each cylinder was then soaked in either pure water (control), a molybdenum (Mo) compound, a phosphorus (P) compound, or a combination of Mo and P,  thus establishing four treatments. They attached each of these stockings to five different trees and allowed them to reside in the canopy for six months, to be colonized by microorganisms.

Stantonstockings

Nylon stockings treated with nutrients (or untreated controls) and affixed to branches in the canopy. Credit: D. Stanton.

The researchers measured the rate of nitrogen fixation by cutting a 50 cm2 rectangle from the area of densest growth on each stocking, and incubating it (along with the colonizing microorganisms) in a closed bottle that they had inoculated with heavy nitrogen (15N).  They then measured how much 15N the colonizers took up during a 12 hr incubation period.

Stantonfixationlab

Samples incubating for 12 hr to measure the rate of nitrogen fixation. Credit: D. Stanton.

The most common colonizers were nitrogen fixing filamentous cyanobacteria. These cyanobacteria fixed nitrogen at a somewhat (but not statistically significant) higher rate with Mo addition and at a much higher rate with P addition, and even more so with Mo + P addition.

StantonFig1A

Nitrogen fixation rates for each experimental treatment. C = control.  Note that the y-axis is logarithmic, so these differences in fixation rates are substantial.  Non-overlapping lowercase letters above the bars indicate significant differences between the means.

Nitrogen fixation is complex and costly.  Part of the complexity arises because nitrogenase, the enzyme that catalyzes the reaction, cannot tolerate oxygen.  To deal with this problem, cyanobacteria have evolved heterocysts, which are specialized anaerobic cells where nitrogen fixation occurs.  How does nutrient addition influence heterocyst abundance and function?

There are actually two aspects to this story.  One finding is that Mo addition had no effect on heterocyst abundance, while P addition had a pronounced effect.

StantonFig1B

Heterocyst frequency for each experimental treatment.

A second aspect is that Mo addition had a pronounced effect on the efficiency of nitrogen fixation.  For one analysis the researchers compared the nitrogen fixation rate per heterocyst for the phosphorus addition treatments either without or with Mo addition (in other words, they compared the P added treatment to the Mo + P treatment). Nitrogen fixation rates were much higher in the Mo + P treatments.  So while Mo does not increase heterocyst abundance, it does dramatically increase heterocyst fixation efficiency.

StantonFig2B

Quantity of N fixed per heterocyst per day in relation to absence (left bar) or presence (right bar) of Mo.  P was added for both treatments.  Dark horizontal lines are the median values, quartile range is represented by top and bottom of each box, and the whiskers represent the range of values for each treatment.

Phosphorus acts by markedly increasing the overall cyanobacterial growth.  It increases the amount of cyanobacteria that colonizes the canopy and also increases heterocyst density per filament. In contrast molybdenum’s effect is more nuanced as it increases the efficiency of the nitrogen fixation reaction without having any (obvious) effect on cyanobacterial structure.

How do these findings influence our understanding of tropical forests in the western hemisphere?  It turns out that episodes of nutrient addition actually happen in nature, courtesy of vast plumes of nutrient-rich rock-derived dust that periodically blow over the Atlantic Ocean from the Sahara desert in western Africa. Preliminary estimates by Stanton and his colleagues indicate that nutrient enrichment from these dust plumes is sufficient to  profoundly increase the rates of nitrogen fixation in tropical forests.  This may require us to reconsider our understanding of how nitrogen cycles within and between ecosystems.

note: the paper that describes this research is from the journal Ecology. The reference is Stanton, D. E., S. A. Batterman, J. C. Von Fischer, and L. O. Hedin. 2019. Rapid nitrogen fixation by canopy microbiome in tropical forest determined by both phosphorus and molybdenum. Ecology 100(9):e02795. 10.1002/ecy.2795. 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.

Nitrogen continues to confound convention

Ah nitrogen…  It is the most abundant molecule in the air that we breathe (close to 80%), yet plants always seem to be starving for it.  Annually, nitrogen fertilizers are a $75 billion dollar industry. The problem is that the nitrogen gas that we breathe (N2) is very nonreactive, because the two nitrogen atoms are held together by a massively powerful triple bond.  So N2 must be broken down to some other more usable form (such as ammonia) – a process we call nitrogen fixation.  Most nitrogen fixers are microorganisms that live in soils or symbiotically within plants.  Unfortunately, N-fixation is energetically very costly, so even organisms that can fix nitrogen will generally happily use nitrogen compounds from the soil or leaf litter (the layer of fallen leaves above the soil) if they are available, rather than expending enormous energy to fix it for themselves. The general formula for nitrogen fixation (ignoring protons, electrons and energy transfers) is…

DynarskiEquation

A few years ago Scott Morford, Benjamin Houlton and Randy Dahlgren (the first two are co-authors of the present study) stunned the ecological world by identifying a previously unsuspected source of nitrogen – weathering of bedrock such as the mica schist pictured below. This bedrock was formed from seabeds which were rich in organic matter and had a high concentration of nitrogen compounds When the rock breaks down, both carbon and nitrogen compounds leach into the soil. Katherine Dynarski became interested in nitrogen fixation as an undergrad at Villanova University, so it was natural for her to move to the University of California at Davis to begin her graduate work with Morford and Houlton on how nitrogen cycles through ecosystems.

20150905_162823

Nitrogen-rich mica schist bedrock. Credit: Katherine Dynarski.

Dynarski got involved in this specific project essentially by accident. She was helping a fellow graduate student collect rocks at adjacent forests on contrasting bedrock (one high-N mica schist, and one low-N basalt), and figured that while she was out there, she might as well measure some N-fixation rates. In leaf litter and the soil below, most N-fixation is done by free-living soil bacteria. Dynarski expected higher N-fixation rates in the litter collected above the N-poor bedrock, reasoning that the microorganisms would need to fix nitrogen from the air, because there was little present in the litter.  In contrast, she expected to find lower N-fixation rates in litter collected above the N-rich bedrock, reasoning that the micro-organisms could save considerable energy by using existing nitrogen that had leached into the soil and leaf litter layer. She was shocked when she ran the samples and found exactly the opposite of her expectation, which led her to develop a more substantial project looking at the relationship between bedrock and N fixing microbes.

20150626_080249

Katherine Dynarski conducting gas incubations to measure N-fixation rates in the field. Credit: Scott Mitchell.

Working in northern California and Western Oregon, Dynarski and her colleagues identified sites whose bedrock was low in nitrogen (below 500 parts per million N) or high in nitrogen (above 500 ppm N). The researchers used soil and leaf litter samples from 14 paired sites – high N bedrock with nearby low N bedrock. They analyzed soil and leaf litter samples from each plot for concentration of nitrogen, carbon (C), phosphorus (P) and molybdenum (Mo) – the latter two elements have been shown in other systems to limit the rate of N-fixation.  The researchers also collected samples of underlying bedrock and analyzed N and Mo content of these rocks.

Recall that the conventional paradigm is that microorganisms should have lower N-fixation rates in N-rich environments.  There was negligible N-fixation occurring in the soil, but considerable N-fixation in the leaf litter above.  Thus the conventional prediction was that N-fixation rates would be higher in leaf litter above low-N bedrock. As I mentioned previously, Dynarski found the exact opposite to be true in one site; would this unconventional finding be confirmed by the 14 sites explored in this study?

The answer is yes!  Considerably more N-fixation was detected in leaf litter above high N bedrock than in leaf litter above low N bedrock.

DynarskiFig3

Mean leaf litter N-fixation rates and low-N and High_N bedrock sites.  Error bars are one standard deviation. P = 0.014.

You will notice the large error bars above the graph.  As it turns out, N-fixation rates vary dramatically – even on a very small spatial scale, which is why the researchers took multiple samples from each site. Some sample sites (hotspots) have unusually high rates of N-fixation.  These hotspots are also strongly correlated with high carbon concentration, with greater C in the leaf litter associated with much higher rates of N-fixation.

DynarskiFig4

Litter N-fixation rates in relation to % soil carbon at N-fixation hotspots. Hotspots are defined as having fixation rates greater than 1 kg N per hectare per year.

Dynarski and her colleagues also discovered that, in general, leaf litter above high-N bedrock tended to have more C and P than did leaf litter above low-N bedrock.  Given this finding (along with the hotspot finding) we are now ready to explore the question of why microbes are expending more energy to fix nitrogen in regions where more nitrogen is naturally available.

The researchers considered two hypotheses.  First, it takes N to make N.  N-fixation is catalyzed by N-rich enzymes. It may be that leaf litter above low-N bedrock is too N-poor to provide microbes with enough nitrogen make these enzymes. So the additional nitrogen from high-N bedrock is just enough to allow microbes to produce the N-fixation enzymes.

The second hypothesis is that the litter above low-N bedrock is also low in C, P and Mo, all of which are required for N-fixation. Thus the positive effect of these nutrients overwhelms the negative effect of additional nitrogen on the rate of nitrogen fixation.  According to this hypothesis, the conventional paradigm of high nitrogen availability reducing the rate of N-fixation is correct, but other factors may be equally or more important in natural ecosystems.

Fortunately, this conundrum is easily resolved.  Dynarski and her colleagues took some leaf litter samples and added a small amount of nitrogen to them.  These N-additions significantly reduced N-fixation rates at both low and high bedrock N sites.  Thus environmental N does reduce biological N-fixation, but other factors, such as the availability of other essential nutrients, can overwhelm the inhibitory effect of environmental nitrogen in natural ecosystems

20151121_094514

A Douglas fir forest in the Oregon Coast Range, where some of this research was conducted.  Credit: Katherine Dynarski.

The researchers conclude that nitrogen input from bedrock weathering leads to increased C storage and P retention, ultimately enhancing rates of N-fixation. About 75% of Earth’s surface is underlain by rocks with substantial N reservoirs, so we need to continue exploring the effects of weathering bedrock on ecosystem processes and functioning.

note: the paper that describes this research is from the journal Ecology. The reference is Dynarski, K. A., S. L. Morford, S. A. Mitchell, and B. Z. Houlton. 2019. Bedrock nitrogen weathering stimulates biological nitrogen fixation. Ecology 100(8):e02741. 10.1002/ ecy.2741. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Tropical trophic cascade slows decomposers

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

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

stream

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

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

killifish

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

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

guppies

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

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

SimonFig1

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

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

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

simonexperiment

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

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

SimonFig3

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

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

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

SimonFig4

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

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

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

SimonFig6

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

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

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

 

Mystifying trophic cascades

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

Maldonado Halo

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

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

DCIM112GOPRO

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

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

Urchinfig1

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

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

UrchinFig3a

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

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

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

UrchinFig4

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

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

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

UrchinFig5bc

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

UrchinFig6a

Summary of interactions.  Arrow width indicates relative importance.

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

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

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

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.

Gantz_BeefLiver

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.

AllequashLake

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?

rusty_crayfish

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.

LarsenFig1

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.

LarsenFig2

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.

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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).

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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.

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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?

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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.

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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.

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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.

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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.

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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.

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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.