Category Archives: Paper of the Week

Extreme Climate Events: Is There A Silver Lining?

Image Credit: Wikipedia Commons, CC BY-SA 3.0, Image Cropped

The silver lining of extreme events (2018) Coleman & Wernberg, Trends in Ecology and Evolution, https://doi.org/10.1016/j.tree.2020.08.013

The Crux

We here at Ecology for the Masses recognize the harm of climate change and the danger that it poses to countless species the world over. Part of climate change involves extreme climate events such as floods, droughts, unusual cold spells, or cyclones, all of which can be devastating to natural systems. By and large these events are seen as negative, and rightfully so! But today’s paper offers another perspective on extreme climate events: their potential for driving evolution towards increased resilience.

Now, I’m not saying that these extreme climate events are good. I dislike them just as much as the next person with a shred of concern about the natural world. That being said, the authors raise some interesting points about the evidence that exists for these events being a positive force for evolution and adaptation. As such, I want to touch on a few of those points, address some issues with this ‘silver lining’, and talk about what it means going forward.

What Evidence Exists

Extreme climate events result in massive losses of organic life, local extinctions, and can drive range shifts. This is quite costly from not only an ecological point of view, but also a social and and an economic one. Due to these costs, a significant amount of effort and money has been dedicated to working on issues associated with these events. Interestingly enough, despite the negative connotations and costs associated with extreme climate events, there is emerging empirical evidence for a “benefit” in that they can cause non-random mortality (see Did You Know?), driving rapid evolution and adaptation.

Scientific theory has predicted that when extreme climate events occur in such a way that they select against weak individuals, but aren’t so extreme that “tougher” individuals cannot live, then these more tolerant and stronger individuals can persist in populations/areas undergoing extreme events. If these tougher individuals can pass on their genes, then a population can rapidly adapt to these extreme conditions. For example, a study showed that a severe cold snap selected for cold tolerance in green anoles (Anolis carolinensis), and similar work has shown that heatwaves selected for thermal tolerance in kelp. While plenty of the lizards/kelp didn’t have the proper traits to survive these extreme temperatures, some of them did. And because they passed on those genes to the next generation, the population is better-suited to survive future extreme temperatures.

Did You Know: Non-Random Mortality

Evolution is a fact of life, and the driving force behind the persistence of life on our planet. However, what you may not know is how evolution actually results in changes in a population/species over time. Individual organisms don’t evolve, species do. So how does that work? Well, it all has to do with how often certain individuals pass on their genes. “Survival of the fittest” refers to the biological concept of “fitness”, which is how good a given organism is at passing on its genes. So in order to be the most fit, you have to pass on the most genetic material, relative to other members of the population. This is where non-random mortality comes into play. Non-random mortality means that there is a pattern behind the death rates. Put into other words, the individuals that survived had something that the ones that died did not. This is how evolution works slowly over time, non-random mortality means that individuals with a given trait tend to die less often than those that don’t have that trait, which means that that trait gets passed on more often than others. Eventually, that trait will become the new normal for that population/species, and evolution has occurred.

What This Means

The potential for extreme events to select for resilience and drive rapid adaptation means that groups dedicated to conservation and preservation of species and ecosystems may be able to proactively anticipate future events. The authors highlight the difficulty inherent in studying non-model organisms for traits/genes that may promote persistence to future climate events, as it involves a LOT of background research to understand the mechanisms behind such persistence. However, to use the anoles from earlier as an example, there are better ways. If one was to go to an area that recently suffered a cold snap like those anoles did and collect the survivors, chances are that most of those survivors have the cold-tolerance trait. By selectively breeding/relocating those survivors conservation workers could prevent future die-offs due to cold snaps.

Problems With These Approaches

This all sounds great, right? No issue? Well, not quite. Just because a given trait may promote persistence to one stressor (the environment) does not mean that it promotes persistence to all others (like disease). Another issue with this silver-lining of adaptation and rapid evolution is the bottleneck effect: extreme events cause mass die-offs. Though the survivors may have a trait that allows them to persist in extreme events, the reduced population size of the survivors may result in such a marked decrease in genetic diversity that the population fails eventually anyway due to the issues associated with inbreeding.

The cheetah is an example of an organism that underwent a population bottleneck, and as such now suffers from very low levels of genetic diversity (Image credit: Ken Blum, CC BY-SA 3.0)

So What?

Extreme climate events are an unfortunate reality, and they are only predicted to get worse and become more frequent. Today’s paper offers a pleasant silver lining to that very grim reality, as it highlights the potential for these events to drive evolution and selection to extreme conditions. It may not be as good as not having these events in the first place, but the authors bring up an important point by drawing attention to the evidence that exists for populations adapting to these extreme conditions, many of which seem to be driven by human-induced climate change. I’ve recently re-read Michael Crichton’s Jurassic Park, and I can’t help but think of a quote from the character Dr. Ian Malcolm’s as I was reading this paper: “The planet has survived everything, in its time. It will certainly survive us”.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

On Dispersal, Connectivity and the Will of the Fish

Image Credit: Dennis Jarvis, CC BY-SA 2.0, Image Cropped

Integrating dispersal along freshwater systems in species distribution models (2020) Perrin et. al., Diversity & Distributions, https://doi.org/10.1111/ddi.13112

The Crux

Trying to figure out where a species can comfortably live is one thing, but figuring out which habitats they can actually access is another. I like to think most marsupials would do quite well in South America or Africa, but the fact is that they’re not dispersing across the Atlantic or Pacific anytime soon. However a Species Distribution Model (a statistical model that can be used to predict the likelihood of a species being found somewhere) often requires a more nuanced approach than “big ocean separating these two habitats”.

To integrate a species’ ability to actually access an area into a Species Distributions Model (SDM), we often use the concept of connectivity. Often, this means simply measuring the distance between two populations. But sometimes a species ability to disperse might not reflect something as simple as how far it needs to go. A perfectly good habitat might be only 100 metres away, but cut off by a raging great cliff. Or a road.

In this study, we wanted to see whether we could relate connectivity parameters used in an SDM to the actual ability of the species to disperse.

What We Did

We used two separate study systems here. One consisted of roughly 300 lakes within Northern Norway housed within a single catchment, or watershed, whereby a single path between each lake could be traced. Here we had presence-absence records for two species, the northern pike (Esox lucius) and the European perch (Perca fluviatilis). Both are native to the region, but they are starting to expand into more lakes and have a more severe effect as the climate warms. We used an SDM to investigate which factors determined species presence, including connectivity variables like the length of the rivers between each lake and a downstream population, and the average slope of those rivers.

The second ecosystem was a series of lakes in Sweden which pike and perch had previously occupied, but had been removed from in the 60s and 70s through the use of rotenone, a chemical dumped in small lakes which wipes out fish populations. These were useful, as we knew that the lakes were otherwise suitable for the species given their presence beforehand. As such, here we used a much simpler model to focus on dispersal ability, simply comparing whether or not the species were able to access and then recolonise the lakes from which they had been removed. We compared successful recolonisation from the nearest downstream lake to the same connectivity parameters as in the larger model.

Did You Know: Island Biogeography & Lakes

They obviously don’t look it, but when it comes to biogeography, lakes are essentially a special type of island. Most of the rules of island biogeography apply to them (for fish anyway); larger lakes are more likely to have more species, lakes close to the ocean or other large lakes (the ‘mainland’) are more likely to have those species as well. The big difference between regular islands and lakes is that we can mark pathways between them much more easily. You’d think that would make it easy for us to stop fish spreading into new lakes as the climate warms, but the problem is as always people – people often spread fish from lake to lake, and the rules of island biogeography don’t apply in quite the same way to someone with a car.

What We Found

The slope of the river was a much more important factor in determining a species presence than the actual distance between populations. This makes sense, as a steep slope could make it difficult for a fish to swim up, or could indicate the presence of a waterfall. Furthermore, adding connectivity parameters to our SDM in our first study system did improve our models, but did it represent dispersal accurately?

For pike, the effect of slope was pretty consistent across the two study systems, indicating that the effects of connectivity in a large SDM can mirror a species dispersal ability. However for perch there was some inconsistency across the two study systems, indicating that perhaps there was some other aspect of the rivers between populations that had a larger effect on dispersal.

While European perch might be native to parts of Scandinavia, it is alien to others. If it’s able to freely disperse between lakes, it could be a serious problem as the climate warms (Image Credit: Christa Rohrbach, CC BY-SA 2.0)

Problems?

This study suffers from the same “lab vs. field” pitfalls as any other experiment that compares a complex study system to a smaller, ‘simpler’ one. Here, time is a factor. Our first study system looks at populations that have had centuries, in some cases millenia, to establish, whereas the second one looks at short-term re-establishments. It’s possible that given enough time, pike or perch could have eventually recolonised some of those lakes.

So What?

Having an idea of the effect of how different slope measurements can affect the dispersal of species is a great help, as it lets us know which lakes are protected by natural dispersal barriers, and which are likely to be invaded by species moving from downstream. However the fact that for perch, slope parameters varied in their effects across the study systems is a stern reminder that we need to always be mindful of how connectivity parameters actually relate to dispersal ability.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who is now completely done with this paper and never wants to look at it again. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

Form Versus Function

Image Credit: Graham Wise, CC BY 2.0, Image Cropped

Sexual differences in weaponry and defensive behavior in a neotropical harvestman (2018) Segovia et al., Current Zoology, https://doi.org/10.1093/cz/zoy073

QUICK NOTE: Harvestmen (aka Daddy Long Legs in North America) are NOT spiders! Despite the false myth that they can’t bite you due to short fangs, harvestmen aren’t even venomous. They can’t hurt you! There, now that I got that off my chest…

The Crux

Sexual dimorphism is a common phenomenon in nature whereby male and female members of a given species differ from one another physically. Think of the large bull moose or elk with its antlers, peacocks and their colorful tails, or the larger horns of male stag beetles. Because of these differences, natural selection is able to act on both their behavioral and functional differences. That is to say, differences in performance and morphology mean that males and females of the same species may experience differential selection pressures. As a result, males and females could be expected to react differently to the same challenge, such as a predator.

Harvestmen (known in North America as Daddy-Long-Legs) are a group of arachnids that, although bearing a resemblance to and being commonly mistaken for spiders, are not actually spiders. They belong to a group called Opiliones. Some males of this group have thicker legs with pronounced spines, used in male-to-male competition and anti-predator defenses. In addition to using these spines against predators, these arachnids also engage in thanatosis (“playing dead”, see Did You Know?) and use chemical defenses. Due to these morphological differences, the authors hypothesized that males and females would differ in their response to predators.

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Cool As A Moose?

Guest post by Katariina Vuorinen

Cool as a moose: How can browsing counteract climate warming effects across boreal forest ecosystems? (2020) Vuorinen et al., Ecology, https://doi.org/10.1002/ecy.3159

The Crux

When temperatures increase, trees grow more. When a moose struts in and eats the twigs, trees grow less. So, if we just have enough moose around, climate warming won’t be able to increase the growth rate of trees. This is what we call the “cooling” effect. Rather simple – and cool – story, right?

However, every ecologist knows that the biological theatre is more complex than this. What if snow protects saplings against browsing? What if changes in temperature affect moose in such a way that they will not feed on trees in the same way as they used to? What if trees’ response to moose is actually different depending on whether it is warm or cold? In complex ecological systems, tree growth is determined in an intricate network of interactions, where the story line is so mind-bogglingly complicated that it seems almost impossible to say what is actually going on.

Luckily, it’s not quite impossible. In this paper, we set out to model those intricate networks, taking into account everything from the climate, the tree species, the effect of time, to the presence of herbivores and their browsing intensity, in an attempt to disentangle that complex biological theatre.

What We Did

To get a baseline impression of what happens to trees when moose aren’t around, we set up fences to keep the voracious ungulates away. Originally, this fencing was started by NTNU University Museum in Trøndelag, later expanding to 62 sites scattered across Norway and eastern Canada. As moose can only browse on relatively small trees, fences were placed in clear-cut areas where we could monitor the growth of the trees from the sapling phase.

After keeping track of the height of hundreds of trees inside and outside of the fences for more than a decade, we had assembled over 16 000 growth measurements. These were accompanied by annual estimations of the proportion of browsed twigs. Based on existing knowledge about which plant species the moose preferred, we also estimated the amount of food available for moose at each site. The tree growth and browsing data were complemented with data on three climatic variables, namely, growth period temperature, precipitation and winter snow-water equivalent, as well as data on regional moose density.

To grow or not to grow: for a rowan or a birch, a protective fence (on the left side in the photo) might be a matter of life and dead (Image credit: Katariina E. M. Vuorinen, CC BY 2.0)

We analyzed the data with structural equation models (SEMs) that combine multiple predictors and response variables into one big model network. A SEM allows you to treat an environmental variable both as an explanatory variable and a response variable simultaneously. For instance, the amount of competing trees could be explained by moose presence, but it itself could explain tree growth.

Did You Know

Herbivore cooling effects are better documented in arctic and alpine systems, where smaller woody plants namely shrubs, play the role of the trees. Empirical studies have shown that for example reindeer can slow down climate-change driven shrubification that would otherwise result in loss of open tundra. However, also in the arctic, herbivore effects take multiple forms: sheep effect seem to be modified by climate, potentially via plant-plant competition.

What We Found

Three of the studied tree species played along with the simple cooling story: Canadian rowan and birch and Norwegian birch. They benefited from higher temperatures and suffered from moose. However, most of the tree species wrote their own, more nuanced narratives. Canadian fir responded more weakly to temperature when moose were missing. Norwegian rowan flipped its temperature response curve around if moose were present. Norwegian pine responded negatively to temperature, but did not seem to be bothered by moose. This is understandable, as heat turns into an enemy when it gets too hot. The soft, palatable species took more damage from moose than spiky spruce and pine.

From a tree’s point of view, the role of a moose can change from a foe to a friend if the moose browses on a neighbouring competitor tree. Canadian fir and rowan and Norwegian birch and pine benefited from the fact that moose lowered the growth of competing trees. Snow complicated the story even further. Norwegian rowan benefited from increasing winter precipitation, but only outside of the fences, suggesting that individual trees were indeed protected from browsing by a snow layer. This is potentially a result of snow lowering the proportion of browsed twigs. Interestingly, also temperature affected browsing intensity, but the effect size and direction varied between different tree species.

The bottom line given by these results is clear: the moose cooling effect exists, but how important it is really depends on ecological context.

Problems

We always need to careful when assessing results obtained by using datasets of different accuracies. Where locally estimated browsing data was highly precise, regional moose density and climate data were less so. Thus, the effect strengths may partly reflect differences in data quality rather than true differences between explanatory variables. Overcoming these weaknesses might reveal side-plots yet to be unravelled.

So What?

So, if the pathways of climate effects are this complex, what is actually going to happen in the boreal forests when temperatures rise? Some tree species may benefit from increase of temperature just to end up on the moose dinner menu. Less tasty ones may thrive, or suffer from excess heat and increased competition. If global warming brings us snowless springs, cooling potential of browsing may increase. In contrast, if we get more snow with increasing precipitation, moose may turn into a trivial side-character.

In the complex interplay of biotic and abiotic actors, only one thing is certain: that we do not know what will happen outside the observed variable boundaries. Interactions and non-linearities make any future predictions highly uncertain. If we are to place hope on herbivory as a cooler of climate change impacts, constraints imposed by species differences, snow, competition, as well as climate effects on browsing must be acknowledged – not so neat of a story, and perhaps less cool, but nearer the ecological reality.

Katariina E. M. Vuorinen is a PhD candidate at the Norwegian University of Science and Technology. She studies the effects of climate and large herbivores on plants by using data from across boreal and arctic biomes. You can read more about her work at this link.

Title Image Credit: James D. M. Speed, NTNU University Museum, CC BY 2.0, Image Cropped

How Well Do Biodiversity Experiments Represent The Real World?

The results of biodiversity–ecosystem functioning experiments are realistic (2020) Jochum et al., Nature Ecology and Evolution, https://doi.org/10.1038/s41559-020-1280-9

The Crux

Testing how different measures of biodiversity contribute to important ecosystem functions, like carbon cycling or tree decomposition, are crucial to our understanding of how the loss of species will impact both local and global ecosystems. Yet these studies are hard to undertake in the real world, since species come and go all the time, and constantly accounting for important environmental factors like temperature or sunshine can be near impossible. It makes understanding exactly what is driving those important ecosystem functions difficult.

To get around this, researchers often set up more controlled experiments, filled with different plots containing random assemblages of species often found in the wild. Since there are different communities in each plot, but each is subject to similar environmental conditions, they can examine the different levels of ecosystem functioning within the different plots and start to understand the differences. But since they’re taking random species of plants, is this even useful as an indicator of what’s going on in the ‘real world’? That’s what today’s researchers tested.

What They Did

The authors looked at two long-term grassland experiments, one based in Jena Germany, the other in Cedar Creek, USA. They compared different metrics of biodiversity (like species richness and taxonomic diversity) of the plots to similar areas in the nearby region. They used these comparisons to determine which of the plots in the controlled experiments were ‘realistic’.

Additionally, they compared whether the relationships between the biodiversity of the controlled plots and some of the key ecosystem functions remained the same when the unrealistic plots were removed from the analysis.

Did You Know: The Cedar Creek Experiment

The Cedar Creek experiment mentioned here is actually a smaller experiment taking place at the Cedar Creek Ecosystem Science Reserve. The Reserve has been a massive undertaking, first established in 1942 by the university of Minnesota. It includes literally thousands of long-term experimental plots set up by different researchers, and has contributed an immeasurable amount to our understanding of plant community ecology.

What They Found

The experimental plots showed a wider variety of communities than the real-world plots, but nestled within that variety were a large number of communities very similar to the real-world plots. Experimental plots tended to be much more similar to the real-world plots when they were not weeded, suggesting that human interference could create key differences between the two, as opposed to surrounding environmental conditions.

The researchers classed 28% and 77% of the Jena and Cedar Creek experiments as realistic, respectively. The relationships between biodiversity and ecosystem functioning remained relatively similar when removing the 23% of unrealistic Cedar Creek plots from analysis, however there was some variation in the relationship when removing the unrealistic plots from the German analysis (though many relationships remained similar).

Problems

The scope of this paper is massive, but it’s important to remember that the scale of these experiments were fairly local, and only dealt with one habitat type. That’s not to downplay the results, since this sort of experiment can of course be scaled up and repeated in other ecosystems. However a lot of the communities studied here both in the real-world and the experiments were quite species poor, so it would be interesting to see how similar research coped with more diverse ecosystems.

So What?

This research is tremendously encouraging (and probably let some researchers breathe a sigh of relief), as it validates the work that both the Cedar Creek and Jena teams have been doing to decades now. And whilst only a subset of their plots might be ‘realistic’, those unrealistic plots still tell us a great deal about potential future scenarios that could come about as a result of climate change or species migrations. Even knowing which plots are realistic will probably be very helpful for experiments going forward.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who is not fan of botany but concedes that it must have place somewhere in science. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

Title Image Credit: Обновить фото обложки სანდრო აბაშიშვილი, CC0 1.0

It’s EVERYWHERE: The true extent of microplastics

Image Credit: Oregon State University, CC BY-SA 2.0

Quantitative analysis of selected plastics in high-commercial-value Australian seafood by pyrolysis gas chromatography mass spectrometry (2020) Ribeiro et al., Environmental Science & Technology, https://doi.org/10.1021/acs.est.0c02337

The Crux

Plastic is one of those things that we hear about all the time these days. More specifically, we hear about how there is an absolute ton of it in the environment thanks to human negligence and the lack of concern that a large amount of people have for where their plastic goes when they are finished with it. Plastic isn’t like paper or metal, it takes a long, LONG time for it to break down. Plastic bags take anywhere from 10-20 years, but the normal time it takes for most plastic waste to decompose is about 1000 years. To put that into perspective, Leif Erikson led an expedition from Greenland to the coast of what is now North America in the year 1002. If his crew had some plastic with them and left it in the places they visited (typical tourists) there’s a good chance that it would STILL be there today.

I hope I’ve convinced you why plastic is bad, but another danger that plastics pose are microplastics, small bits of plastic that have come from a larger piece, all of which are less than 5mm in size. Our environment is full of them, and the ocean in particular has been saturated with microplastics. In 2014 a research expedition sailed from Bermuda to Iceland (a trip of 2500 miles/4023 km) and found microplastics in every single sample they took. And that was just plastic in the environmental samples they took, the real threat to marine life comes from what happens to all of that microplastic.

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Big Fish, Small Fish, and How Body Size Decides Between Life And Death Around Humans

Guest post by Chloé Nater

Size‐ and stage‐dependence in cause‐specific mortality of migratory brown trout (2020) Nater at al., Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.13269

The Crux

When it comes to dying, not everyone is equal.

The corona-virus pandemic has reminded us of that over the last months: the same disease that passes often without any symptoms in young children is life-threatening for the elderly. Age, in this case, seems to influence how likely someone is to die from the disease. For other risks of death – take, for example, car accidents – age is not that important, but location may be: the chance of dying in a car accident is higher for someone who spends two hours per day commuting by car on a busy highway, than for someone who only needs to walk across one car-free road to get to work.

For animals, this is very much the same. They can die from a variety of causes (starving, predation, disease, hunting, etc.), and an individual animal’s risks of dying from any of these depend on characteristics like age, size, or colour, and on location. How many and which animals die from different causes then has consequences for the size of populations, and sometimes also for other species in the area, including humans.

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Forecasting Europe’s (Very Specific) Rodent Problem

Image Credit: Birgit, Pixabay Licence, Image Cropped

The potential current distribution of the coypu (Myocastor coypus) in Europe and climate change induced shifts in the near future (2020) Schertler et al., NeoBiota, https://doi.org/10.3897/neobiota.58.33118

The Crux

For all my talk on not immediately demonising alien species, there are a plethora of annoying little critters who the label ‘invasive’ was made for. This is the case with the Coypu, an annoying beaver-like rodent initially from South America who has since spread through other parts of the world, including large swathes of Europe.

The Coypu is a textbook invader – it reproduces quickly, and though individuals don’t stray far from rivers, as a species they are capable of expanding their range very quickly. They destabilise riverbanks through their burrowing, which can lead to severe ecological and economical damage.

They are, however, quite sensitive to temperature, and as such it’s important to know what the effects of climate change will have on their distribution. Today’s authors set out to come up with a habitat suitability map for the Coypu in light of the rise in temperature we’re expecting in the coming decades.

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Bad Neighbors

Image Credit: ksblack99, Public Domain, Image Cropped

Exposure to potentially cannibalistic conspecifics induces an increased immune response (2020) Murray et al., Ecological Entomology, https://doi.org/10.1111/een.12806

The Crux

Plasticity is a powerful force in nature that allows organisms to change the way they look, the way they act, and even their own physiological processes. Prey species commonly exhibit plastic responses when they are exposed to predators, and recent studies have shown that these predator-induced effects can affect the immune function of the prey species. Because of this, predators have the potential to modify disease dynamics, either increasing disease/parasite infection by reducing the prey’s immune function, or decreasing disease by increasing immune function.

Interestingly, predators are not the only organisms that consume prey species. Some prey species eat both members of their own trophic level (an intraguild predator, see Did You Know) and members of their own species (a cannibal). Because they act like a predator (by eating a prey organism), there’s a possibility that these cannibalistic individuals may have the same effect on their potential victims. Today’s authors used larval dragonflies to investigate that exact question.

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The Healthy Male Wins the Mate

Guest post by Miguel Gómez-Llano (Image Credit: Sharp Photography, CC BY-SA, Image Cropped)

Male-Male Competition Causes Parasite-Mediated Sexual Selection for Local Adaptation (2020) Gómez-Llano et al., The American Naturalist, https://doi.org/10.5061/dryad.cjsxksn35

The Crux

The natural world changes constantly: temperatures fluctuate, predators and parasites enter into the ecosystem, and the landscape itself could change (looking at you, Yellowstone). These changes mean that organisms are under a constant pressure to adapt to local conditions. Due to this pressure, one of the biggest questions for conservation biology is if species are able to adapt fast enough to keep up with environmental changes. Sexual selection is thought to promote rapid adaptation to such environmental changes, but most of the evidence comes from laboratory studies.

Our study looked at adaptation to one of nature’s ubiquitous pressures: parasitism. We were interested in the strength of selection by parasites and if there was subsequent adaptation by the host in a wild population.

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