Mangroves partner with rats in China

Many of us have seen firsthand the havoc that invasive plants can wreak on ecosystems.  We are accustomed to think of native plants as unable to defend themselves, much like a skinny little kid surrounded by a group of playground bullies. ‘Not so fast’ says Yihui Zhang.  As it turns out, many native plants can defend themselves against invasions, and they do so with the help of unlikely allies.

In southern China, mangrove marshes are being invaded by the salt marsh cordgrass, Spartina alterniflora, which is native to the eastern USA coastline. Cordgrass seeds can float into light gaps among the mangroves, and then germinate and choke out mangrove seedlings.  However, intact mangrove forests can resist cordgrass invasion.  Zhang and his colleagues wanted to know how they resist.

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Cordgrass (pale green) meets mangrove (bright green) as viewed from space. Credit: Yihui Zhang.

Cordgrass was introduced into China in 1979 to reduce coastal erosion.  It proved up to the task, quickly transforming mudflats into dense cordgrass stands, and choking out much of the native plant community.  Dense mangrove forests grow near river channels that enter the ocean, and are considerably taller than their cordgrass competitors.  The last player in this interaction is a native rat, Rattus losea, which often nests on mangrove canopies above the high tide level. At the research site (Yunxiao), many rat nests were built on mangroves, using cordgrass leaves and stems as the building material.

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Rat nest constructed from cordgrass shoots rests upon a mangrove tree.  Credit Yihui Zhang.

Zhang and his colleagues suspected that cordgrass invasion into the mangrove forest was prevented by both competition from mangroves and herbivory by rats on cordgrass.

Baby rat in the nest

Baby rats in their nest. Credit Yihui Zhang.

 

To test this hypothesis, they built cages to exclude rats from three different habitats: open mudflats (primarily pure stands of cordgrass), the forest edge, and the mangrove forest understory, (with almost no cordgrass). They set up control plots that also had cages, but that still allowed rats to enter.

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Arrow points to resprouting cordgrass. Credit Yihui Zhang.

The researchers planted 6 cordgrass ramets (genetically identical pieces of live plant) in each plot and then monitored rodent grazing, resprouting of original shoots following grazing, and shoot survival over the next 70 days.

They discovered that the cages worked; no rats grazed inside the cages.  But in the control plots, grazing was highest in the forest understory and lowest in the mudflats (Top figure below).  Most important, both habitat type and exposure to grazing influenced cordgrass survival.  In the understory, rodent grazing was very important; only one ramet survived in the control plots, while 46.7% of ramets survived if rats were excluded.  In the other two habitats, grazing did not affect ramet survival, which was very high with or without grazing (Middle figure). Rodent grazing effectively eliminated resprouting of ramets in the understory, but not in the other two habitats (Bottom figure).

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Impact of rat grazing on cordgrass in the field study in three different habitats.  Top figure is % of stems grazed, middle figure is transplant survival, and bottom figure is resprouting after grazing (there was no grazing in the rodent exclusion plots). Error bars are 1 standard error. Different letters above bars indicate significant differences between treatments.

The researchers suspected that low light levels in the understory were preventing cordgrass from resprouting after rat grazing. This was most easily tested in the greenhouse, where light conditions could be effectively controlled.  High light was 80% the intensity of outdoor sunlight, medium light was 33% (about what strikes the forest edge) and low light was 10% the intensity of outdoor sunlight (similar to mangrove understory light).  Rat grazing was simulated by cutting semi-circles on the stembase, pealing back the leaf sheath, and digging out the leaf tissue. Cordgrass ramets were planted in large pots, exposed to different light and grazing treatments, and monitored for survival, growth and resprouting following grazing.

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Cordgrass growing in greenhouse under different light treatments. Credit: Yihui Zhang.

Zhang and his colleagues found that simulated grazing sharply reduced cordgrass survival from 85% to 7% at low light intensity, but had no impact on survival at medium or high light intensities.  Cordgrass did not resprout after simulated grazing at low light intensity, in contrast to approximately 50% resprouting at medium and high light intensity.

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Survival (top) and resprouting (bottom) of cordgrass following simulated grazing in the greenhouse experiment.

The researchers conclude that grazing by rats and shading by mangroves are two critical factors that make mangroves resistant to cordgrass invasion. Rats tend to build their nests near the mangrove forest edge, so it is not clear how far into the forest the rat effect extends. Rats do prefer to forage in the understory (rather than right along the edge), presumably because the understory helps to protect them from predators.  In essence, mangroves compete directly with cordgrass by shading them out, and also indirectly by attracting cordgrass-eating rats. Conservation biologists need to be aware of both direct and indirect effects when designing management programs for protecting endangered ecosystems such as mangrove forests.

note: the paper that describes this research is from the journal Ecology. The reference is Zhang, Y. , Meng, H. , Wang, Y. and He, Q. (2018), Herbivory enhances the resistance of mangrove forest to cordgrass invasion. Ecology. Accepted Author Manuscript. doi:10.1002/ecy.2233. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Plant communities bank against drought

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

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

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

 

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

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

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Plants germinating in the greenhouse. Credit Marina LaForgia.

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

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

LaForgiaFig1

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

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

LaForgiaFig2

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

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

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

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

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

Too much of a good thing is killing Monarch butterflies

There was a time in the mid-Pleisticine when a photo of an ecological event was an awesome novelty, and a movie of an ecological event even more so.  Dodderers of an ecological bent (myself included), can vividly recall viewing a series of photos or a movie, either in a seminar or in an ancient ecology text, of a blue jay consuming a monarch butterfly, Danaus plexippus.  Consumption is immediately followed by explosive vomiting, as the cardenolides within the monarch butterfly claim another victim.  The monarch sequesters these cardenolide toxins from its larval food (milkweed), and incorporates them into its tissues as a means of protecting itself from predators – presumably blue jays learn from this very aversive experience.  I should point out that the individual sacrificial butterfly enjoys no fitness from this learning event – which raises some evolutionary questions we will not explore at the present.

Karen Oberhauser

Five instars (stages of development) of monarch caterpillars on a milkweed leaf. Credit: Karen Oberhauser

Rather we turn our attention to the relationship between milkweed, monarchs, and climate change. In several places in this blog we’ve talked about how climate change has influenced the behavior or physiology of a single species. For example, my first blog (Jan 31, 2017) discusses how increasing temperatures create more females in a loggerhead turtle population. But there are fewer studies that explore how climate change influences the ecological landscape, ultimately affecting interactions between species.  Along these lines, Matt Faldyn wondered if increased air temperature would change the chemical constitution of milkweed in a way that might influence monarch populations.  As he describes, “With milkweed toxicity, there is a ‘goldilocks’ zone where monarchs prefer to feed on milkweed that produce enough toxins in order to sequester these (cardenolide) chemicals as an antipredator/antiparasite defense, while also avoiding reaching a tipping point of toxicity where feeding on very toxic milkweeds negatively impacts monarch fitness.” He expected that at higher temperatures, milkweed would become stressed, and be physiologically unable to sustain normal levels of cardenolide production.

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Monarch butterfly feeds on a native milkweed, Asclepias incarnata. Credit: Teune at the English Language Wikipedia.

For their research, Faldyn and his colleagues worked with two milkweed species.  Asclepias incarnata is a common, native milkweed found throughout the monarch butterfly’s range in the eastern and southeastern United States.  Asclepias curassavica is an exotic species that has become established in the southern United States.  In contrast to A. incarnata, A. curassavica does not die back over the winter months; consequently some monarch populations are no longer migratory, relying on A. curassavicato provide them with a year round food supply.

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The exotic milkweed, Asclepias curassavica. Credit: 2016 Jee & Rani Nature Photography (License: CC BY-SA 4.0)

To protect against herbivory, milkweeds have two primary chemical deterrants: (1) the already-mentioned cardenolides, which are toxic steroids that disrupt cell membrane function, and (2) release of sticky latex, which can gum up caterpillar mouthparts and actually trap young caterpillars.

field_noborderii.jpgThe researchers wanted to simulate climate change under field conditions, so they created open-top chambers with plexiglass plates that functioned much like mini-greenhouses, into which they placed one milkweed plant that was covered with butterfly netting.  This setup raised ambient temperatures by about 3°C during the day and 0.2°C at nighttime.  Control plots were single milkweed plants with butterfly netting. Half of the plants were native milkweed, and the other half were the exotic species.

For their experiments, Faldyn and his colleagues introduced 80 monarch caterpillars (one per plant) and allowed them to feed normally until they pupated.  Pupae were brought into the lab and allowed to metamorphose into adults.

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Matt Faldyn holds two monarch butterflies in the laboratory. Credit Matt Faldyn.

At normal (ambient) temperatures, monarchs survived somewhat better on exotic milkweed.  But at warmer temperatures, there is a strikingly different picture. Monarch survival is unaffected by warmer temperatures on native milkweed, but is sharply reduced by warmer temperatures on exotic milkweed (top graph below). The few that managed to survive warm temperatures on exotic milkweed grew much smaller, based on their body mass and forewing length (middle and bottom graph below)

FaldynFig1

Survival (top), adult mass (middle) and forewing length (bottom) of monarch butterflies raised under normal (ambient) and warmed temperatures.  Error bars are 95% confidence intervals.

Both milkweed species increased production of both types of chemicals over the course of the experiment. But by the end of the experiment, the exotic species released 3-times the quantity of latex and 13-times the quantity of cardenolides than did the native milkweed species.

FaldynFig2

Average amount of latex released at the beginning and end of the experiment.  Error bars are 95% confidence intervals.

FaldynFig2

Average cardenolide concentration at the beginning and end of the experiment.

The researchers argue that the exotic milkweed, Asclepias curassavica, may become an ecological trap for monarch butterflies, in that it attracts monarchs to feed on it, but will, under future warmer conditions, result in dramatically reduced monarch survival. Interestingly, these results are not what Faldyn originally expected; recall that he anticipated that temperature-stressed plants would reduce cardenolide production. The tremendous increase in cardenolide production in exotic milkweed at warmer temperatures may simply be too much toxin for the monarchs to process. The researchers predict that as climate warms, milkweed ranges will expand further north into Canada, and lead to northward shifts of monarch populations as well.  They urge nurseries to emphasize the distribution of native rather than exotic milkweed, so that monarchs will be less likely to become victims of this ecological trap.

note: the paper that describes this research is from the journal Ecology. The reference is Faldyn, M. J., Hunter, M. D. and Elderd, B. D. (2018), Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies. Ecology. doi:10.1002/ecy.2198. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Parrotfish put on their big boy pants

While it would be awesome if parrotfish were named for their conversational abilities, it turns out that they earn their moniker for their specialized teeth that are fused together for scraping algae from coral, thus resembling a parrot’s beak. Despite lacking verbal skills these fish are incredible. Approximately 100 species occupy reefs, rocky coastlines and eelgrass meadows in tropical and subtropical waters. Many species are sequential hermaphrodites, beginning life as females and then changing into males after reaching a certain size. While female reproductive success is limited by the number of eggs she can produce, male reproductive success can be much higher if he can fertilize the eggs of many females.  So if a parrotfish transitions into a large male, and can control access to numerous females, he will enjoy greater reproductive success than if he had remained a female.

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Two Chlorurus spilurus parrotfish show off their teeth and colors.  The large colorful fish on the right is a male, while the smaller darker fish to his left is a female. Credit: Brett Taylor.

Phenotypic plasticity describes the ability of an individual with a particular genetic makeup to vary in a variety of traits (such as what it looks like, or how it behaves) in response to different environmental conditions. About 15 years ago, Nick Gust’s PhD research on tropical reef fish revealed that tremendous variation in parrotfish traits existed over a distance of a few kilometers. But what causes this variation? When funding became available, Brett Taylor jumped at the opportunity to pinpoint the causes, focusing on the diverse parrotfish community in the Great Barrier Reef (GBR).

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Eastern slope of the Great Barrier Reef hosts a diversity of fish and coral species. Credit: Brett Taylor.

Taylor and his colleagues surveyed 82 sites within 31 reefs across 6 degrees of latitude in the northern GBR. To standardize data collection, divers, armed with a multitude of cameras and GPS devices, swam at a standardized rate (about 20 meters/minute) for 40 minutes per survey, recording each parrotfish along a 5 m wide swath. They collected data about the habitat and the environment, about the physical traits of each individual parrotfish (such as size and sex), and about the type and abundance of parrotfish and their predators present at each site.

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Researcher takes notes while conducting a dive.  Credit Kendra Taylor.

The researchers wanted to identify what factors influenced growth rate, maximum body size, and the size at sex change, and how these factors related to the parrotfish mating system. Four species of parrotfish were sufficiently abundant across the GBR to allow researchers to do this type of analysis.

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Four parrotfish species  abundant along exposed outer shelf (yellow sites) and protected inner shelf (blue) regions of the Great Barrier Reef. Males are larger and more colorful.

The GBR varies structurally across a relatively small spatial scale of 40 – 100 km, with outer shelf regions (eastern) exposed to wave action, and inner shelf regions (western) relatively protected. All four species tended to change sex at a larger size in protected sites than they did at exposed sites. However, the differences are only compelling for two of the species: C. spilurus and S. frenatus. There were fewer data points for the other two species, so it is possible (but unknown) that they too would show a more pronounced trend if more data were available.

TaylorFig1bottom

Proportion terminal phase (sex-changed males) in relation to body size (measured to the fork of the tail) in exposed (yellow) and sheltered (blue) sites.

Not surprisingly, parrotfish grew larger in protected areas. Presumably, less wave action provided a more benign environment for rapid growth, both of parrotfish and their preferred food items (algae growing on rocks and coral).

TaylorFig2I

Standardized maximum size (Lmax) attained by parrotfish in sheltered vs. exposed sites.

The researchers were somewhat surprised that most other factors, such as latitude, coral cover, sea surface temperature, and predator abundance, had very little effect on the size at sex change. Rather, the size at sex change appears to be strongly influenced by the local size distribution. In protected habitats, parrotfish grow large and change sex at a large size, while in exposed habitats, parrotfish are smaller, and change sex at a smaller size.

But sex is never simple. Nick Gust’s PhD research showed that C. spilurus had different patterns of sexual allocation in protected vs. exposed areas. In protected areas, the mating system is haremic, with a large male defending a territory and servicing a harem of females. In exposed areas, the mating system is mixed; there still are large territorial males with their harems, but they compete with many more small males, and group spawning is much more prevalent. Theoretically, the presence of these small males may make it less worthwhile for a female to transition into a male, and may influence the optimal size for transitioning in exposed reefs. Given that we still don’t know the mating system details of the other parrotfish in this study, it will be fascinating to see if they too show similar patterns of haremic vs. mixed mating systems in relation to habitat structure.

note: the paper that describes this research is from the journal Ecology. The reference is Taylor, B. M., Brandl, S. J., Kapur, M., Robbins, W. D., Johnson, G., Huveneers, C., Renaud, P. and Choat, J. H. (2018), Bottom-up processes mediated by social systems drive demographic traits of coral-reef fishes. Ecology 99(3): 642-651. doi:10.1002/ecy.2127. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Snails grow large to fight fear

In a recent post (Jan 12), I discussed research showing that song sparrow parents reduce provisioning to their offspring when threatened by predators, ultimately reducing offspring survival rates.  But in a turnabout that highlights the natural world’s dazzling diversity, a recent study by Sarah Donelan and Geoffrey Trussell revealed a very different impact of fear on the development of snail offspring. Donelan had worked as Trussell’s laboratory technician for two years and became fascinated by the egg capsules laid by the carnivorous snail Nucella lapillus, an ecologically important species in rocky intertidal communities. Earlier work had shown that predator-induced fear reduced snail feeding and growth rates, so Donelan decided that for her PhD work she would see how predator-induced fear influenced offspring development.

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Adult Nucella alongside ca. 100 egg capsules. Credit: Sarah Donelan.

The researchers recognized that the fear environment experienced by parents before or during reproduction, and by the embryos during early development, could influence growth and development of those embryos. At their research site along the Massachusetts, USA coast, the predatory green crab, Carcinus maenas, can be a source of fear for these adult and embryonic snails. Donelan and Trussell exposed snails to fear by housing separately one male and one female snail in adjacent protected perforated containers (with six blue mussels in each container to feed them) that were set within a large plastic bucket. This bucket also had a somewhat larger perforated container (the risk chamber) containing the dreaded green crab (and two snails to feed it). The control risk chamber had two snails, but no crab.

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Experimental setup with buckets containing egg capsules in perforated cages experiencing different exposure to fear. Credit: Sarah Donelan.

In late spring of 2015 and 2016, field-collected female and male snails were matched to create a total of 80 parental pairs. Donelan and Trussell set up experiments to explore the effects of parental experience with predation risk, embryonic experience with predation risk, and duration of embryonic experience.

Parent snails were exposed to a risk chamber (with a crab in the experimental group, and without a crab in the control group) for three days, and then placed together for four days (without risk) to mate. If an egg capsule was laid, the researchers removed it, and immediately exposed it to an experimental or control risk chamber for a week. Embryonic risk duration was further manipulated by continuing to expose half of the egg capsules to risk for a total of six weeks. The table below summarizes the treatments received by parents and offspring.

DonelanTableGood

DonelinFig1

Mean (+ standard error) shell length (top graph) and tissue mass (bottom graph) of snail embryos exposed to predation risk. Parents were either exposed (solid circles) or not exposed (open circles) to risk before mating.

 

When parents were not exposed to risk, but their offspring were exposed, these offspring had shorter shells and reduced tissue mass compared to all other groups. When both parents and offspring were exposed to risk, offspring shell length increased by 8% and offspring mass increased by a whopping 40% over risk-exposed offspring whose parents were not exposed to risk (left data points in figures a and b). If embryos were not exposed to risk, parental exposure had no significant impact on embryonic development (right data points on figures a and b). Embryonic risk duration had no impact on development.

 

In addition, risk-exposed offspring of risk-exposed parents emerged from their egg capsules an average of 4.1 days sooner than other offspring.

Donelanfig4

Mean (+standard error) number of days until emergence of snail offspring that experienced the presence or absence of predation risk during early development.  Their parents were exposed to risk (solid circle) or no risk (open circle) before mating.

What could be causing these differences in size and rate of development? Donelan and Trussell hypothesized that embryonic snails could grow larger and more quickly if they were somehow able to reduce their metabolic rate. With a reduction in metabolic rate, more energy could be diverted to growth and development, resulting in larger and faster-growing snails. The researchers used an oxygen meter to measure oxygen consumption rates of individual egg capsules (from the eight different treatments in the first experiment) six weeks after deposition, about a week before embryos would begin to emerge. They exposed some of the capsules to predation risk during the experiment (current risk graph below), and left other capsules unexposed. When tested under risky conditions, capsules from parents who were exposed to risk, and that experienced risk as embryos during early development, had 56% lower metabolic rates than the other three groups (left graph), and similarly low metabolic rates as capsules tested without risk (right graph).

Donelanfig2

Mean (+ standard error) respiration rate of egg capsules that were (left graph) or were not (right graph) exposed to current predation risk.  During early development, the embryos in these capsules experienced risk or no risk, and were produced by parents exposed to risk (solid circles) or no risk (open circles) before mating.

Overall, parental experience with predation risk enhances offspring growth and development in the presence of risk. If the parents lack this exposure, risk-exposed offspring suffer the costs associated with small size and slower development. Currently Donelan and Trussell are trying to figure out what these costs are. Smaller snails have less energy reserves, may feed on a less diverse group of prey, and are less likely to remain in safer habitats than are larger juveniles. But we still don’t know whether these effects on early stages of life have lasting impacts as a snail gets older and larger. More generally, we don’t know whether there are similar types of interactions between parental and embryonic experiences of other stressors, most notably environmental stresses that are already being imposed by climate change.

note: the paper that describes this research is from the journal Ecology. The reference is Donelan, S. C. and Trussell, G. C. (2018), Synergistic effects of parental and embryonic exposure to predation risk on prey offspring size at emergence. Ecology, 99: 68–78. doi:10.1002/ecy.2067. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Field gentian – when it’s good to be eaten

We tend to think of plants as victims – after all any interested herbivore can simply walk, fly or crawl over to its favorite plant, and begin munching. But not so fast! In reality, plants have a variety of ways they can make life difficult for potential herbivores. Plants can escape herbivores by simply growing in places that are not easily accessible (such as in cracks, or high enough to be out of a herbivore’s reach) or by growing at a time of year when herbivores are away from the plant’s habitat. Plants also use mechanical defenses such as thorns or a diverse array of chemical defenses to thwart overzealous herbivores. A third approach – tolerance – can take many forms. For example, following attack by a herbivore some plants can increase photosynthetic rates or reduce the time until seed production . Tommy Lennartsson and his colleagues were interested in a particular form of tolerance that ecologists call overcompensation, in which damaged plants produce more seeds than undamaged plants.

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Herbivores in action. Notice the difference in vegetation height inside and outside the pasture. Credit: Tommy Lennartsson.

Overcompensation is an evolutionary puzzle, because undisturbed plants produce fewer offspring than partially eaten plants. That outcome seems to fly in the face of the scientific principle that natural selection favors individuals with traits that promote reproductive success. Lennartsson and his colleagues investigated this evolutionary puzzle by comparing two subspecies of the herbaceous field gentian Gentianella campestris. The first subspecies, Gentianella campestris campestris (which we’ll just call campestris), has relatively unbranched shoot architecture when intact, growing to about 20 cm tall, but produces multiple fruiting branches when the dominant apical meristem is eaten. The second subspecies, Gentianella campestris islandica (which we’ll call islandica), is much shorter (about 5-10 cm tall), and always has a multi-branched architecture.

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Two subspecies of field gentian – campestris (left) and islandica (right).

Environmental conditions and soils can vary dramatically, even on a small spatial scale. The field site was a gently-sloped grassland in Sweden that had coarser, dryer soil on the ridge, and finer, wetter and richer soil in the valley. This created a productivity gradient, with taller vegetation in the valley. The average  height of all the vegetation was 15 cm in the high-productivity valley, 10 cm on the medium-productivity slope and 5 cm on the low-productivity ridge.

The researchers used this natural variation to set up an experiment that would allow them to explore hypotheses about why an undisturbed campestris is less successful than one that is partially-eaten. One hypothesis (the overcompensation hypothesis) is that campestris restrains branching to conserve resources, so that when it is grazed it has plenty of resources in reserve to be used for regrowth and the production of prolific branches, flowers and seeds. Limited branching and limited seed production of ungrazed campestris are simply a cost of tolerance, while overcompensation after damage maximizes reproductive success. A second hypothesis (the competition hypothesis) is that restrained branching allows the plant to grow tall, so it can compete better in ungrazed pastures than can the much shorter islandica. These two hypotheses are not mutually exclusive.

To test these two hypotheses, the researchers set up 2 X 2 meter experimental plots in the valley (18 plots), slope (12 plots) and the ridge (6 plots). They planted 2000 seeds per subspecies in each plot, which ultimately yielded about 20 plants of each subspecies per plot. Of course there were many other neighboring plant species in these plots. In the high productivity plots (valley), the neighboring plants in six plots were clipped to a height of 12 cm, six plots to 8 cm and six plots to 4 cm. In the medium productivity plots (which naturally only grew to 10 cm), the researchers cut neighboring plants to 8 cm in 6 plots and 4 cm in six plots. Finally, in the low productivity plots, the researchers cut neighboring plants to 4 cm in all six plots. In mid July, half of the gentian plants in each plot were clipped to the same height as the surrounding vegetation, while the remainder were not clipped.

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Experimental plots from the valley (left), slope (middle) and ridge (right).  Black squares represent plots where neighboring plants were clipped to 12 cm, grey squares to 8 cm, and clear squares to 4 cm. Squares with slashes through them (left)  represent plots that were used for a different purpose.

The beauty of this experimental design, is that by counting seeds, the researchers could assess the reproductive success of both subspecies under conditions of high competition (when surrounded by tall neighbors) and low competition (when surrounded by shorter neighbors). At the same time, clipping the two subspecies allowed the researchers to simulate grazing in these different competitive environments. Lennartsson and his colleagues found that unclipped islandica did better than unclipped campestris when surrounded by short or medium height neighbors, but that islandica success plummeted when the neighbors were very tall (see the left graph below). Campestris reproductive success also dropped when surrounded by tall competitors, but not as much as did islandica, so that campestris produced twice as many seeds than islandica in the high competition environment (also the left graph).

When plants were clipped to simulate grazing, campestris outperformed islandica in all three competitive environments. Campestris actually produced more seeds when it was clipped than when it was not clipped in the low and medium competition environments. Thus campestris overcompensated for grazing under conditions of low and moderate competition (see the right graph below).

LennartssonFig2

Mean (+ standard error) seed production for unclipped (left graph) and clipped (right graph) field gentian subspecies in relation to surrounding vegetation height.  Sample sizes are in bars.

The researchers collected data on growth rates, development, survival probabilities and reproductive success for both species under conditions of being clipped or unclipped at different levels of competition. They then used these data to create a population growth model in relation to the percentage of grazing (damage risk) at different levels of productivity. In these graphs, a stochastic growth rate of 1.0 (on the y-axis) indicates that the population is stable, above 1.0 indicates it will increase and below 1.0 indicates a declining population.

LennartssonFig4

Population growth rate of both subspecies in relation to damage risk at different levels of productivity.  These models predict that the population will increase at growth rates above the dotted line (growth rate = 1.0) and decline below the dotted line.

This model shows that in high productivity environments, campestris always does better than islandica (top graph). However, the model predicts that islandica will decline at any damage level (note in the top graph that all islandica damage values yield a growth rate below 1.0), while campestris will also decline except for very high damage risks. In medium and low productivity populations (middle and bottom graphs), islandica does better than campestris when damage risk is low, but the reverse is true at high damage risk.

So how do these results relate to the two hypotheses for why an undisturbed campestris is less successful than one that is partially-eaten. Campestris overcompensated for damage by producing more seeds and having positive population growth under most levels of productivity. In contrast, islandica undercompensated when damaged, but produced more seeds than campestris when ungrazed, except for in the high productivity environment. These differences in responses support the hypothesis that restrained branching is favored by natural selection in environments where damage from grazing is common (the overcompensation hypothesis). But, the superior performance by campestris in productive ungrazed environments supports the competition hypothesis.

Can we generalize these findings to other plants? Lennartsson and his colleagues point out that many short-lived grassland plants can’t grow tall enough to be effective competitors for light. These plants are thus restricted to environments where the surrounding plants are not very tall. Two factors commonly create conditions where there are short neighboring plants: grazing and unproductive (low nutrient) soils. When grazing is widespread, tolerance mechanisms such as overcompensation are favored by natural selection. When soils are unproductive, unrestrained branching is favored. Therefore, Gentianella campestris provides us with a natural experiment for testing hypotheses about how natural selection acts on plants to promote their reproductive success in a variable environment.

note: the paper that describes this research is from the journal Ecology. The reference is Lennartsson, T., Ramula, S. and Tuomi, J. (2018), Growing competitive or tolerant? Significance of apical dominance in the overcompensating herb Gentianella campestris. Ecology, 99: 259–269. doi:10.1002/ecy.2101. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

 

Homing in on the micro range

I’ve always been fascinated by geography. As a child, I memorized the heights of mountains, the populations of cities, and the areas encompassed by various states and countries. I can still recite from memory many of these numbers – at least based on the 1960 Rand McNally World Atlas. Part of my fondness for geography is no doubt based on my brain’s ability to recall numbers but very little else.

Most geographic ecologists are fond of numbers, exploring numerical questions such as how many organisms or species are there in a given area, or how large an area does a particular species occupy? They then look for factors that influence the distribution and abundance of species or groups of species. Given that biologists estimate there may be up to 100 million species, geographic ecologists have their work cut out for them.

As it turns out, most geographic ecologists have worked on plants, animals or fungi, while relatively few have worked on bacteria and archaeans (a very diverse group of microorganisms that is ancestral to eukaryotes).

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Two petri plates with pigmented Actinobacteria. Credit: Mallory Choudoir.

Until recently, bacteria and archaeans were challenging subjects because they were so small and difficult to tell apart. But now, molecular/microbial biology techniques allow us to distinguish between closely related bacteria based on the sequence of bases (adenine, cytosine, guanine, and uracil) in their ribosomal RNA. Bacteria which are identical in more than 97% of their base sequence are described as being in the same phylotype, which is roughly analogous to being in the same species.

As a postdoctoral researcher working in Noah Fierer’s laboratory with several other researchers, Mallory Choudoir wanted to understand the geographic ecology of microorganisms. To do so, they and their collaborators collected dust samples from the trim above an exterior door at 1065 locations across the United States (USA).

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Dr. Val McKenzie collects a dust sample from the top of a door sill. Credit: Dr. Noah Fierer.

The researchers sequenced the ribosomal RNA from each sample to determine the bacterial and archaeal diversity at each location. Overall they identified 74,134 gene sequence phyloypes in these samples – that took some work.

On average, each phylotype was found at 70 sites across the USA, but there was enormous variation. By mapping the phylotypes at each of the 1065 locations, the researchers were able to estimate the range size of each phylotyope. They discovered a highly skewed distribution of range sizes, with most phylotypes having relatively small ranges, while only a very few had large ranges (see the graph below). As it turns out, we observe this pattern when analyzing range sizes of plant and animal species as well.

Choudoir1C

Mean geographic range (Area of occupancy) for each phylotype in the study.  The y-axis (Density) indicates the probability that a given phylotype will occupy a range of a particular size (if you draw a straight line down from the peak to the x-axis, you will note that most phylotypes had an AOO of less than 3000 km2

Taxonomists use the term phylum (plural phyla) to indicate a broad grouping of similar organisms. Just to give you a feel for how broad a phylum is, humans and fish belong to the same phylum. Some microbial phyla had much larger geographic ranges than others. Interestingly, it was not always the case that the phylum with the greatest phylotype diversity had the largest range. For example, phylum Chrenarchaeota had the greatest median geographic range (see the graph below), but ranked only 19 (out of 50 phyla) in number of phylotypes (remember that a phylotype is kind of like a species in this study).

Choudoir3

Box plots showing range size distribution for individual phyla. Middle black line within each box is the median value; box edges are the 25th and 75th percentile values (1st and 3rd quartiles).  Points are outlier phylotypes. Notice that the y-axis is logarithmic.

With this background, Choudoir and her colleagues were prepared to investigate whether there were any characteristics that might influence how large a range would be occupied by a particular phylotype. We could imagine, for example, that a phyloype able to withstand different types of environments would have a greater geographic range than a phylotype that was limited to living in thermal pools. Similarly, a phylotype that dispersed very effectively might have a greater geographic range than a poor disperser.

The researchers expected that aerobic microorganisms (that use oxygen for their metabolism) would have larger geographic ranges than nonaerobic microorganisms, which are actually poisoned by oxygen. The data below support this prediction quite nicely.

Choudoir4a

Geographic range size in relation to oxygen tolerance.  In this graph, and the graphs below, the points have been jittered to the right and left of their bar for ease of viewing (otherwise even more of the points would be on top of each other).

Some bacterial species form spores that protect them against unfavorable environmental conditions. The researchers expected that spore-forming bacteria would have larger geographic ranges than non-spore-forming bacteria.

Choudoir4BC

Geographic range in relation to spore formation (left graph) and pigmentation (right graph).

Choudoir and her colleagues were surprised to discover exactly the opposite; the spore forming bacteria had, on average, slightly smaller geographic ranges. Choudoir and her colleagues also expected that phylotypes that are protected from harsh UV radiation by pigmentation would have larger geographic ranges than unpigmented phylotypes – this time the data confirmed their expectations.

The researchers identified several other factors associated with range size. For example, bacteria with more guanine and cytosine in their DNA or RNA tend to have larger geographic ranges. Some previous studies have shown that a higher proportion of guanine and cytosine is associated with greater thermal tolerance, which should translate to a greater geographic range. Choudoir and her colleagues also discovered that microorganisms with larger genomes (longer DNA or RNA sequences) also had larger ranges. They reason that larger genomes (thus more genes) should correspond to greater physiological versatility and the ability to survive variable environments.

This study opens up the door to further studies of microbial geographic ecology. Some patterns were expected, while others were surprising and beg for more research. Many of these microorganisms are important medically, ecologically or agriculturally, so there are very good reasons to figure out why they live where they do, and how they get from one place to another.

note: the paper that describes this research is from the journal Ecology. The reference is Choudoir, M. J., Barberán, A., Menninger, H. L., Dunn, R. R. and Fierer, N. (2018), Variation in range size and dispersal capabilities of microbial taxa. Ecology, 99: 322–334. doi:10.1002/ecy.2094. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.