Eco-evolutionary semantics

What is in a name? Presumably a name should be clear and unambiguous, thereby nicely delineating what it does and does not include. After all, we should call a spade a spade, right? Or is it that simple? Perhaps a spade is instead a shovel or a trowel or a digger or a spud or a geotome. “Now wait a minute,” you might say, quoting Wikipedia: “the words spade and shovel should be held in contradistinction (piercing and digging [spade] versus scooping and moving [shovel]).” “Sure, but,” I might retort, quoting the next line in Wikipedia: “Natural language does not widely follow these prescriptions; it more often treats spade and scoop as contradistinguished subsets under shovel.” Thus, even if a shovel is not always a spade, a spade is often a shovel and, regardless, we can call either the other and still get the job of digging done.

What, then, are eco-evolutionary dynamics? I would venture the following definition: eco-evolutionary dynamics are interactions between ecology and evolution that play out over contemporary time scales, with “contemporary time scales” intended to represent time scales ranging from years to centuries. (See more details here.) This definition is intended to be inclusive, thus providing an umbrella framework for understanding how ecology and evolution interact on short time scales. But it doesn’t satisfy everyone – as I will elaborate below.

Cruising the Netherlands at dusk.

For the past week, I have been at a workshop on “Eco-evolutionary dynamics in a changing world” held in Leiden, Netherlands, at the Lorentz Centre. (Much to the surprise of almost everyone present, we learned on the first day that this was not the guy with the geese – that would be Lorenz.) Organized by Stephanie Jenouvrier, Thomas Reed, and Marcel Visser, the workshop had plenary talks, break-out discussion sessions, and group discussions. These activities were interspersed with ample time for social interaction – call them break-out drinking sessions if you will. Antics ensued.

At one dinner, a group of us decided to form an “Institute of This Table (ITT)” with each of us holding a research chair endowed by one of the others. I can’t remember all the resulting chairs, but I do recall that Richard Gomulkiewicz christened Nelson Hairston the “Dick Chair of Cryptic Dynamics” and Mike Kinnison christened himself the “Hendry Chair of Paleontological Genomics.” (I later gave him two Euros.) Later that night, we ended up at an English bar with a truly amazing collection of Scotch whisky. I tried these out on my palate while Dick tried out his Dutch pick-up lines on Mike Kinnison – stay tuned for the youtube video. On another night we took a cruise through the canals around Leiden. After much debate, Chris Thomas and I agreed to reunite for dinner 36 years hence so that he (I) could gloat over me (him) at having proven (disproven) the predictions from his 2004 Nature paper that “on the basis of mid-range climate-warming scenarios for 2050, that 15–37% of species in our sample of regions and taxa will be ‘committed to extinction’.”

The ITT.

For the first two days of the meeting, most of the arguments – and certainly the most animated ones – were about what eco-evolutionary dynamics are and are not. Here is a restrictive sampling of the many uncertainties.

1. If phenotypic differences have an effect on some ecological variable (such as population size or species richness or nutrient cycling), but you haven’t isolated or confirmed that these effects have a genetic basis, then are you studying eco-evolutionary dynamics? The excluders might argue that, if you haven’t demonstrated evolution, then you can’t say you are studying eco-evolutionary dynamics, and that you would be better off saying eco-phenotypic dynamics or some such. The includers might argue against this precision on several grounds. First, if we go down this slippery slope, then the field of evolution quickly gets much smaller. As Mike Kinnison pointed out, we would have to kick out all of the paleontologists given that none of them has yet proven that the changes they study have a genetic basis. Second, we often know that phenotypic differences have a strong genetic basis even if we haven’t proven it in a particular experiment. This is why the eco-evolutionary pantheon includes those great studies of the ecological effects of alewife divergence, guppy divergence, and stickleback divergence – even though all of those studies used wild-caught fish for the experiments. As some of the perpetrators of those studies were present (Ron Bassar and Mike Kinnison), I was tempted for a vote on whether or not we should – right then and there – drag them out of the room and throw them off the eco-evolutionary island – but then I realized I wouldn’t be far behind. Third, phenotypic plasticity evolves, and so plastic responses in a given generation are the product of evolutionary change in a past generation. In this sense, plasticity is the gift that evolution in one generation keeps giving to future generations. Fourth, it is phenotypes and not genotypes that interact with ecology; genotypes interact with ecology only indirectly through their effects on phenotypes. In short, phenotypes – even without information on genotypes – are an integral part of the eco-evolutionary dynamic framework. (Just as every modern evolution text book has a chapter or major section on plasticity, and numerous examples that focus only on phenotypes, so too should any eco-evolutionary textbook. Of course, investigators should make every attempt to explore the genetic basis underlying phenotypic effects and shouldn’t assert genetic change until they have explicitly confirmed it.)

2. If the ecological variable of interest does not change, then are eco-evolutionary dynamics occurring? The excluders might argue that if no change has occurred, then neither have any dynamics – by definition. The includers might reply that stability itself is likely the product of ongoing evolution. Such cryptic eco-evolutionary dynamics could occur in several basic ways: (a) changes in genotypes cause changes in phenotypes that cause stability in ecological variables, (b) changes in genotypes cause stability in phenotypes that cause stability in ecological variables, and (c) selection causes stability of genotypes that cause stability in phenotypes that cause stability in ecological variables. An example of (a) comes from the work of Nelson Hairston’s group, where changes in the frequencies of two different clones of algae that are differentially susceptible to a predator are necessary to maintain algae population size. An example of (b) comes from a number of studies showing “counter-gradient variation”, such as when environmental conditions (e.g., low food) that decrease growth are offset by evolutionary changes that maintain growth. The phenotype will here remain unchanged only because of evolution. Support for (c) comes from the realization that selection is constantly weeding out maladaptive genetic variation that arises owing to mutation, gene flow, and genetic drift. Without this selection, fitness would decline and the population would soon decline too. Stability is thus driven by the continual action of selection. For all of these reasons, it seems clear that the absence of ecological – or even phenotypic or genetic change – does not mean that eco-evolutionary dynamics are absent.

3. If you haven’t demonstrated a FEEDBACK (e.g., a trait influences an ecological variable that then influences selection on that trait), then are you studying ecological evolutionary dynamics? The excluders might argue that only true feedbacks should be considered dynamics but the includers feel this criterion as too restrictive. First, it is very hard to demonstrate feedbacks even when they are present; doing so usually requires manipulative experiments. Second, feedbacks of one sort or another likely occur in nearly all instances – at least in the broad sense. By this I mean that changes in any trait in some focal organism will likely influence some other organism in the ecosystem and thus have ecological effects. Overall, I would argue that eco-evolutionary dynamics should include all interactions between ecology and evolution, even one-way effects, whereas the term eco-evolutionary feedbacks should be reserved for situations where a feedback (ecology to evolution to ecology; or evolution to ecology to evolution) has been explicitly demonstrated or is explicitly hypothesized.  

4. If the interactions between ecology and evolution are happening only on long scales (e.g., millennia) are they eco-evolutionary dynamics? Hanna Kokko argued that such interactions often occur on quite long time scales, and this is certainly the case. Think of all the ecological and evolutionary changes that depended on the much earlier evolution of photosynthesis.  Although I tend to be an includer in all of the above debates, I will caucus with the excluders. The Oxford Dictionary states that dynamics is “a process or system characterized by constant change, activity, or progress”. This would seem to me to exclude long time scales but, oops, this definition is for its use as an adjective. As a noun, the definition is “a force that stimulates change or progress within a system or process.”  This one seems harder to invoke in order to exclude long term effects. And – after all – the Oxford Dictionary clearly calls only a spade a spade: “a tool with a sharp-edged, typically rectangular, metal blade and a long handle, used for digging or cutting earth, sand, turf, etc.” So do I really have any solid reason to exclude longer time scales from the umbrella eco-evolutionary dynamics? I guess I would hand-wave that interactions between ecology and evolution on long time scales have long been recognized but it is only recently that they have been appreciated on short time scales – and it is this latter appreciation that has driven the emerging field of eco-evolutionary dynamics. This is why definitions of eco-evolutionary dynamics have tended to focus on short time scales and this distinction currently delineates the field. And, after all, who wants to have to say “contemporary eco-evolutionary dynamics” (although CEED sounds cool) or, Darwin forbid, “rapid eco-evolutionary dynamics” (REED?).

So where do these semantics leave us? I would like to suggest that eco-evolutionary dynamics is a FRAMEWORK for understanding interactions between ecology and evolution on contemporary time scales, and it therefore needs to be inclusive. Or, as Hal Caswell pointed out, eco-evolutionary dynamics can be considered a social identifier that places one comfortably into the group of researchers applying evolutionary thinking in ecological studies. I would suggest that this group is inclusive with respect to the first three debates (and many others) but is (arguably) exclusive for the fourth.

These semantic issues largely subsided toward the end of the meeting given that we really could communicate quite clearly without going to further trouble. This left the rest of the meeting to argue over more substantive issues, like whether or not evolutionary (or even ecological) models can reasonably predict what will happen in the future, how important is evolution in nature, can Lande Land accommodate density- and frequency-dependent selection , which deep-fried Dutch croquette was the best (or worst), and how could Heineken simultaneously be the worst beer and the best wine in the same restaurant.

The rise and fall of parallel evolution: dispatches from Belgium and Texas.

Evolutionary biology has traditionally sought support for the power of natural selection through evidence that similar phenotypes evolve in similar environments: flying animals all have wings, cave organisms predictably lose their eyes and pigments, and birds on small islands often lose their wings! If similar traits evolve in similar environments despite independent origins, then surely natural selection is overwhelmingly powerful in shaping the deterministic evolution of organisms. This pattern is generally called “parallel” when it occurs from similar ancestors and “convergent” when it occurs from different ancestors. (Although some interpretations focus more on whether similar [parallel] or different [convergent] genetic changes are involved.) In this vein, countless studies have reported parallel or convergent evolution and thereby provided overwhelming evidence for the power of natural selection.

I will here argue that parallel evolution is – or should be – on the wane. I don’t mean this in the simple semantic sense that evolution is never parallel but is instead convergent; as has been recently argued by a number of authors. I instead mean it in the more subtle, but probably more important, sense that parallel (and convergent) evolution is much less parallel (and convergent) than is normally promulgated – and that evolutionary biologists are increasingly realizing this fact. My inspiration to revisit this topic (see the earlier post) stems from meetings I recently attended in Leuven, Belgium, and Austin, Texas.

The typical approach in studies of parallel or convergent evolution is to test whether independent populations that have evolved in similar environments (or habitats or food types) are more similar than independent populations that have evolved in different environments. If a statistically significant effect of “environment type” is found across such populations, then parallel evolution is invoked. The trouble with using this result to invoke parallel evolution is that independent populations in similar environments, although often superficially similar, do show many subtle (or even obvious) differences. Wings look extremely different for pterosaurs, bats, birds, and insects! Not all cave organisms lose their eyes!  Not all birds on small islands lose their wings! Has our need to invoke the power of selection blinded us to the fact that evolution is not very predictable? Or, in short, how parallel (or convergent) is parallel (or convergent) evolution, really?

Several talks at the Leuven meeting invoked parallelism (or convergence): spiders in Hawaii, spiders in Galapagos, beetles in salt marshes, stickleback in freshwater, and many others. In most cases, however, examination of the data made clear that although all evolution generally occurred in roughly similar directions, the independent populations colonizing similar environments were never the same either morphologically or genetically. That is, they might share some phenotypic or genetic similarity but the differences among different populations in similar habitats were often, to my eye, as striking as the similarities. A couple of talks near the end of the meeting brought this to the idea to the front of my brain and thus precipitated this post. First, Joop Ouborg showed that the genetic basis for inbreeding depression was almost entirely non-overlapping between different plant species. Second, Freddy Chain showed that lake-stream stickleback divergence in five independent watersheds was almost entirely non-parallel at the genetic level. In short, the fact that evolution is generally non-parallel (or non-convergent) is remarkably parallel (or convergent).

Of course, these are just my interpretations and I have to confess that I have been wrong before – even quite recently. Take grappa, for instance; that crazy Italian drink made by fermenting the leftovers from the pressing of grapes to make wine. Distinctively musty, this drink is quintessentially Italian and is wonderful when drunk for the first time late at night in downtown Napoli surrounded by locals cheering their football team against Barcelona. Or is grappa really that good? Maybe I just liked it because of the context and novelty. Indeed, subsequent consumptions of grappa have further separated the drink from the context and somewhat decreased my enthusiasm. Well, I told the above sad tale at dinner to Nelson Hairson, Luc De Meester, and Isabelle Olivieri right after Nelson ordered grappa. Bah, says Nelson, grappa is good even outside of the context. Ok, then, I challenged him, name three mainstream drinks worse than grappa. And then Isabelle had a brilliant idea – Nelson should pick one drink on the menu that was worse that grappa. That will get him, I figured. So Nelson takes a careful academic look at the menu and pronounces that “Spiriteux de poivre” would be worse. So we ordered this drink and passed it around the other end of the table, where they had not heard our argument and didn’t know what the drinks were: a single-blind experiment if you will. And the verdict: grappa was not very good – but Spiriteux de poivre was infinitely worse.

The Texas gunslinger re-envisioned

Back to parallel evolution. The idea that (even) parallel evolution is often non-parallel was the focus of the meeting that I had in Austin with Dan Bolnick, Yoel Stuart, Dieta Hanson, Rowan Barrett, and Katie Peichel. We had recently received money from the NSF to quantify the non-parallel and parallel contributions to lake-stream stickleback divergence. We will use 16 independent lake-stream pairs to quantify parallelism at the ecological level, the morphological level, and the genetic level. We will then ask to what extent parallelism and non-parallelism at each level can be explained by parallelism and non-parallelism at the other levels. This meeting was set up to plan our first real field season. But, in reality, that was really just the excuse because my main memories involve eating outstanding tacos and barbeque, drinking copious amounts of beverages from Texas, California, and Scotland, and climbing, both in Dan’s backyard “cave” and at Reimer’s Ranch. (If you are at the NSF, I am just kidding.) To push a metaphor perhaps too far, all climbing routes go in an overall parallel direction (up!) but each one follows its own idiosyncratic route and often ends up in a different place.

Parallel?

Multitasking par excellence- or "why McGill hired Dr. Barrett"

Carnival of Evolution #56: World Travel Edition!

  Carnival of Evolution #56 is now posted.  This month’s contribution from eco-evo evo-eco is a great post by Victor Frankel (who also did our featured post last month!) on how the rise of the Isthmus of Panama has affected evolution of the local species.  Check out all the other cool posts at the Carnival for the latest from the evolution blogosphere.

  The theme of this Carnival is world travel.  But why stop with just one planet’s worth of eco-evolutionary dynamics?  In the spirit of Andrew’s recent post asking whether eco-evolutionary dynamics are stronger in the tropics: should we expect eco-evolutionary dynamics to be stronger or weaker on other planets?  Perhaps it depends on how tropical their climates are.  I'll leave you to ponder that, inspired by this astrobiology-themed image (from NASA!).

Evolution of life-history traits in sister species across the Isthmus of Panama

    The rise of the Isthmus of Panama created the conditions with which to test hypotheses considering the causes and consequences of adaptive radiation and speciation of terrestrial, freshwater and marine species. The rise of the isthmus first led to an important paleozoogeographic event called the “Great American Interchange” characterized by the migration of terrestrial and freshwater fauna between South American and North American ecozones. In addition, the Isthmus of Panama isolated reproductive populations of marine species across taxa that allowed the speciation and adaptive radiation of geminate, or sister, species on either side of the isthmus to different biotic and abiotic conditions in the western Atlantic and eastern Pacific environments.

     The evolution of Atlantic and Pacific sister species has also been the subject of decades worth of research that has measured rates of evolutionary change on the molecular level by calibrating the molecular clock with the geologic data on the dates of the rise of the isthmus of Panama, which is thought to have occurred about 3.5 million years ago, although this hypothesis is now contested, as an alternative, but not conclusive, date of approximately 18-20 million years has been proposed for the rise of the isthmus. Nevertheless, the rise of the isthmus has permitted one of the most important and extensively studied natural experiments in evolution, a natural experiment that now allows scientists to test specific predictions considering the ecological and evolutionary dynamics that have allowed species to adapt to the biotic and abiotic conditions in different environments.

     Adaptation to different environment conditions can impose differential selection pressures on reproductively isolated populations. Therefore, we should predict that physiological and behavioral traits should confer adaptive value to the different conditions found in the coastal marine environments of Panama. We have recently tested the effects of adaptive radiation to different environmental conditions of the Pacific and Atlantic Oceans on the strategies of larval development in two geminate species of coastal mud snails (Potamididae) across the Isthmus of Panama, Cerithideopsis californica on the Pacific coast and C. pliculosa on the Atlantic. Differences in the productivity of western Atlantic and eastern Pacific coastal environments in this region allowed us to predict that in an environment of relatively low productivity (the Atlantic), natural selection will favor the evolution of larval developmental strategies that allow it to survive in these environments of low productivity through increased maternal investment and reduced larval duration in development.

     This pattern of larval development is consistent with models of life-history evolution in which increased maternal investment is selected for in low-productivity environments. In this case, we observed larger size and reduced planktonic duration of larvae in the Atlantic than in a relatively highly productive environment (the Pacific).  While geminate sister species pairs across taxa that inhabit the Pacific and Atlantic coasts of Panama are excellent systems with with to study adaptive radiation to different environments, this pair of snail species is an especially important system with chih to study the drivers of life-history evolution because both species have an exceptionally broad biogeographical distribution, spanning over 30 degrees latitude on both the Atlantic and Pacific coasts. Both of these species are found in habitats with different abiotic and biotic conditions, such as food availability, salinity and temperature that change with increasing latitude that can thus impose differential selection on larval and adult snails that can drive the evolution of important life-history traits, as was observed in the study considering larval developmental strategies across the Atlantic and Pacific coasts of Panama. Testing whether differences in life history traits of Atlantic and Pacific snails across their latitudinal range is an evolutionary or plastic response to different environments, however, is a whole different story. Write on!

Paper can be found here.  http://mollus.oxfordjournals.org/content/77/3/255.short

Email me at victor.frankel@mail.mcgill.ca for a copy of the PDF.

Are eco-evolutionary dynamics stronger in the tropics?

Biodiversity is higher in the tropics. Terrestrial productivity is higher in the tropics. The pace of life is faster in the tropics. Mountain passes are higher in the tropics. The tropics are just bigger, faster, and stronger. So what about eco-evolutionary dynamics? Are they stronger in the tropics? A recent trip to Panama provided the motivation to speculate on this possibility.

Coati

Early in the trip, I visited the famous research site of Barro Colorado Island (BCI), where I was able – with my family – to see howler monkeys and all sorts of other wonders. Back at the town of Gamboa a few days later, I was called on to give a lecture to the “tropical boot camp” class that included graduate students from the McGill-STRI NEO program, the STRI-Indiana IGERT program, and Arizona State University. I decided to give my boiler-plate talk outlining a conceptual framework for eco-evolutionary dynamics because I figured most of the students would be ecologists and it would perhaps be worthwhile to encourage them to include an evolutionary perspective into their ecological thinking. 

A leaf cutter ant in the clutches of an ant lion (zoom in for a better view of the lion).

During the lecture, I set up several key questions facing the study of eco-evolutionary dynamics, one being the importance of evolution (e.g., changes in phenotypic traits) relative to other non-evolutionary ecological forces (e.g., precipitation, temperature, flooding) in shaping ecological dynamics at the population, community, and ecosystem levels. The standard work addressing this question at the population level is that comparing the effects of phenotypic traits (e.g., body mass) on the population growth rate of ungulates in Canada and Scotland in comparison to climate variables (e.g., rainfall, Pacific Decadal Oscillation). Remarkably, effects of the two causal forces (evolution versus ecology) are roughly the same in each study population, suggesting that phenotypic change is incredibly important to ecological dynamics. Halfway through explaining all of this, it began to strike me as silly to use an example from a temperature vertebrate while lecturing in a building situated on the borders of a verdant tropical rainforest. Why not use a tropical example – even if just for hypothetical illustration. 

Lounging capybaras

For some reason, my mind hit on howler monkeys. What effect, I posed as an example, would evolutionary changes in the phenotypes of howler monkeys have on the productivity or diversity of the BCI forest relative to the amount of rainfall. I had no idea of the answer, of course, which got me to wondering. Would eco-evolutionary dynamics be stronger or weaker in the tropics? It seems like an opportune time for some speculation. 

Millipede delight

Several properties might increase the strength of eco-evolutionary dynamics. (1) Faster rates of phenotypic change. Perhaps the tropics have faster rates owing to the more rapid pace of life, or perhaps not given their more stable environment. (2) When the species causing the ecological effects have large effects as individuals, such as in the case of keystone species. Perhaps the tropics have more of these (elephants!), or perhaps not given that many more species are present and so the effect of any single species (besides elephants) might be weaker. (3) When the species causing the ecological effects are very numerous (bacteria, viruses, and some insects and plants). The tropics likely have more such organisms given the overall greater productivity, or perhaps not given that so many species are present the effects of any one species – however numerous – might be swamped by all the other numerous species. (4) When feedbacks between ecology and evolution are stronger, such as when trait changes causes an ecological change that promotes (through selection) further changes in that trait. Perhaps the tropics have more feedbacks of this sort because the environment is not reset each year by winter and because  so many cool mutualisms are present, or perhaps not because the system can be reset by dry and wet seasons and because mutualisms in temperate regions might have stronger effects given the relative paucity of other species. So would we expect eco-evolutionary dynamics to be stronger or weaker in the tropics than in temperature regions given that effects seem to point in both directions in each case? 

Yummy dung

I suggest that evolutionary dynamics on the part of single species (i.e., effects of the evolution of a focal species on aggregate ecological variables) might be weaker in the tropics – simply because the countless other species dilute the effects of any one species. However, I also suggest that eco-evolutionary dynamics in aggregate (i.e., across all species) will be stronger in the tropics – because there are so many other species and interactions, because they have been for around longer, and because the environment is somewhat more stable. I also suggest that eco-evolutionary dynamics associated with phenotypic CHANGE might weaker in the tropics given that organisms have had more time to stabilize their adaptations and so might be less subject to contemporary phenotypic change. However, I also suggest that eco-evolutionary dynamics based on phenotypic STABILITY might be stronger in the tropics. By this I mean that the very stability seen in (some) tropical ecosystems is likely the result of continual ongoing evolutionary change. That is, so many interacting species are present that extinction and extirpation would be common were it not for constant, ongoing eco-evolutionary dynamics that maintain and improve adaptations and thereby stabilize population sizes. 

Spooning is universal

 Of course, this is all speculation provided in fun and on the fly but how can one not be motivated to think about the uniqueness of the tropics when watching your children feed leaf cutter ants to ant lions, go all warm and fuzzy over a howler monkey mom and baby spooning, marvel at a massive tarantula, try to get close to a capybara, watch dung beetles fight over a prime piece of monkey stink, sneak up on a group of foraging coatis, chase a praying mantis around and around a tree, and try to find toads that look like leaves.

Toads all over the place

Praying mantis

A very big tarantula at our door asking to come in.

Industrious wasps

More nature photos from the Panama trip are here: http://www.flickr.com/photos/andrew_hendry/sets/72157632543426268/

Guppies, body condition and parasitism

(This post is by Christina Tadiri; I am just posting it for her.  -B.)


As many of you may know, a lot of research is done using guppies and their ectoparasite Gyrodactylus turnbulli as a model system for ecological and epidemic dynamics:

Gyrodactylus on a fish.  Video credit: Christina Gheorghiu.

The really cool thing about these parasites is that they’re easy to observe and to quantify, so we can collect data on their presence and growth-rates over time without having to sacrifice the host (guppy). Also they reproduce exponentially and are transmitted directly via host-host contact thus causing epidemics in the wild, a characteristic atypical of most macroparasites. These two characteristics combined make G. turnbulli a very convenient model parasite for studying epidemic dynamics, as has been done here, here, and here. Epidemics and high parasite loads (sometimes reaching well over 300 parasites per fish) lead to high levels of guppy mortality, making this system also well suited for the study of host-parasite coevolution.

Body condition (a metric of individual weight to length ratio) is used as a common proxy for health or well being. While research on how an individual’s food intake can affect their overall body condition and disease resistance is abundant, the question of how these relationships might translate to the population scale remained unanswered. Thus we decided to investigate how food availability in the environment and host body condition relative to others in the population affected the overall outcomes of an epidemic, using the guppy-Gyrodactylus system as a model.  To answer this question, we set up laboratory populations of guppies and subjected them to different levels of food availability and measured their relative body conditions (i.e. how much better or worse-off were they relative to all other experimental fish). Then we introduced parasites to the populations by infecting one randomly chosen fish from each group and monitored the epidemics over time. Pretty simple and it gives a huge load of information.

If you want to read more about our methods, measurements and analyses, you can do so here. Below we provide an overview of our most interesting results.

The first cool thing that we found is that in general, host relative body condition was positively associated with parasitism. This would in principle mean that the healthier a fish is, the more parasites it will have; nonetheless it can also mean that the “fatter” a fish is for its length, the more likely it is to have parasites, and when it is infected, the more likely it is to have more parasites. Specifically, we found that the incidence of parasitism (the number of fish that became infected over the course of our experiment) was greater in populations with a higher average condition index. This is pretty counter-intuitive, and it was therefore assumed that those with a high relative condition index would be more resistant to disease.

Another interesting finding was that the relative condition of the fish we used to introduce the disease to the population (the “source” fish) mattered a lot. Almost all of our epidemic variables were significantly affected by this factor. Specifically we found that the peak burden of parasites in the populations was significantly positively impacted by the relative condition index of the “source” fish.  Moreover, we found that parasites were more aggregated (crowded on one particular fish) when the condition of the source fish was high, and that this crowding usually occurred on high condition source fish, particularly when the average condition of the population was low. This is exciting because these results indicate that the way in which a disease is introduced to a population, or rather, the characteristics of the host through which it is introduced, can have significant impacts on the course of the epidemic (both the burden and distribution of parasites) in the population as a whole. To our knowledge, this is a novel result that could inspire further investigation.

So why would higher condition fish have more parasites? One theory is that larger fish are so because they invest more energy into growth  rather than into defense against disease, making them more susceptible to parasite infection. It is also possible that larger fish simply make better hosts because they provide more resources, allowing the parasites to rapidly grow and reproduce while making them less likely to transfer to a host of lower quality. This idea seems to fit our result that parasites were strongly aggregated on hosts of high condition.

But what about food availability? While our results did not indicate any significant impacts of food availability alone, we did find that the interaction of food availability with the condition of the source fish negatively impacted our epidemic parameters. What this means is that the positive relationship we found between peak parasite burden/aggregation and source condition was dampened (the slope of the regression is not as steep) as food availability increased. We think this means that in populations with high food availability, fish may be able to consume more resources and dedicate that energy towards resistance, rather than fat storage or growth, thus decreasing their “quality” as a host causing parasites to grow at a slower rate and forcing them to disperse throughout the population in search of a better host.

Overall, our results present some new and interesting ideas which we hope to follow-up on in future investigations.

Carnival of Evolution #55

Carnival of Evolution #55 is up!  Our contribution to it is a recent post by Victor Frankel on indirect effects of parasites in invasions, with lots of eco-evolutionary goodness.  Check it out!

This Carnival's theme is New Year's Resolutions.  Wait, that rings a bell.

Happy New Year!