Released from predators, guppies reshape themselves—and their environment

A (domestic) male guppy. Photo by gartenfreuden.

ResearchBlogging.orgConsider a population of guppies living in the Aripo River in Trinidad. They have a happy existence, as far as guppies can be happy, but their lives are shaped by the constant threat of larger, predatory fish. The river runs clear over a colorful gravel bed, and guppies who stand out against that background are eaten quickly. Even guppies whose coloration helps them blend in have to be ready to make a break for it if a predator shows up. All in all, a guppy’s chances of surviving to mate depends most on its ability to hide from bigger fish, and to swim quickly when it can’t hide.

Then one fine day a biologist comes along, scoops up a couple hundred guppies, and moves them to a pool in a tributary of the river. The pool is separated from the mainstream by a series of waterfalls, so larger fish can’t swim up—the guppies are now free from their most dangerous predators. They can be fruitful and multiply. In this new habitat, camouflage and evasive maneuvers don’t matter so much. What does matter is finding enough food to make some babies in the midst of a whole bunch of other guppies who are also not particularly worried about predators.

John Endler started the experiment I’ve just described back in 1976 to see whether guppies’ coloration helps them hide from predators [PDF]. The guppies he moved to a predator-free stream have continued to evolve, though, and three decades later, new studies are showing how release from predators changed the guppies—and how those changed guppies could be changing the living community around them.

Since the 1976 introduction, Endler and other biologists have tracked the Aripo River guppies’ response to the change in natural selection he created. Release from predators is considered one of the classic sources of ecological opportunity that can free a population to evolve new traits and behaviors, and explore new ways of making a living. At the same time, a sudden lack of predators means that competition within the population can become stronger.

Points of measurement for guppy body and head shape, illustrated on a stained specimen. Image from Palkovacs et al, fig. 1.

In one study just published by PLoS ONE, Eric Palkovacs and two colleagues compared the body shape of guppies from the experimental population with guppies from the source stream. (Endler had noted changes in body shape along with changes in coloration in his original paper.) First, Palkovacs and his coauthors gauged how rapidly female guppies taken from each site snapped up standardized food. Then they killed the test fish, treated them with stain, and measured their body and head shape. Fish from the site with lower predation ate faster, and they had bigger mouths and deeper bodies than fish from the site with more predators.

Palkovacs and his coauthors also observed that the guppy populations at the experimental site were denser—without predators thinning them out, the fish are probably most limited by their food supply. A study published last year in PNAS suggests that this denser guppy population might reshape its own environment. The paper’s authors created artificial ponds stocked with algae and small invertebrates, then introduced guppies from the high-predation source site or from the low-predation experimental site. They also controlled for the differences in guppy population density associated with predator pressure, maintaining the fish at either the high density observed with low predation, or the lower density observed with high predation.

Where the guppies came from made a significant difference in the artificial ecosystems, and these differences were in some cases exaggerated by the increased population density caused by predator release. Guppies from the “released” site ate less selectively than guppies from the site experiencing higher predation, who favored invertebrates over algae. As a result, guppies from the released site were associated with less algae growth, and higher invertebrate population density. Probably because they ate more plant matter, guppies from the released site also excreted less nitrogen, reducing the nutrient’s availability for plant growth.

These results echo a study I discussed last year, which used a very similar approach to show that speciating sticklebacks can change their environment. It’s another reminder that evolutionary change can feed back to change the environmental conditions that prompted change in the first place—that natural selection operates in the midst of continuous change.

References

Bassar, R., Marshall, M., Lopez-Sepulcre, A., Zandona, E., Auer, S., Travis, J., Pringle, C., Flecker, A., Thomas, S., Fraser, D., & Reznick, D. (2010). Local adaptation in Trinidadian guppies alters ecosystem processes. Proc. Nat. Acad. Sciences USA, 107 (8), 3616-21 DOI: 10.1073/pnas.0908023107

Endler, J. (1980). Natural selection on color patterns in Poecilia reticulata. Evolution, 34 (1), 76-91 DOI: 10.2307/2408316

Palkovacs, E., Wasserman, B., & Kinnison, M. (2011). Eco-evolutionary trophic dynamics: Loss of top predators drives trophic evolution and ecology of prey. PLoS ONE, 6 (4) DOI: 10.1371/journal.pone.0018879

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Scientific American guest blog: Ecological opportunity is all around us

ResearchBlogging.orgThe latest entry in the wide-ranging Guest Blog at Scientific American is a post by yours truly, about a subject I’ve discussed before:

Since the Origin was first published, biologists have come to use the phrase ecological opportunity to describe the processes that can produce a diverse group of species from a single colonizing ancestor. Islands provide colonizing species with new food resources and an escape from predators and competitors. Under these highly favorable conditions, island species can live at much higher population densities than possible on the mainland—a phenomenon called density compensation. This increase in population size is often accompanied by increased variation among individuals, and greater competition from crowding neighbors creates strong benefits for individuals that try new ways to make a living.

Given enough time, one big, variable population will begin to fracture into smaller populations with different lifestyles. Given even more time, those smaller populations will stop interbreeding, and become different enough to call separate species. If that seems like a stretch of the imagination, consider that the processes of ecological opportunity are occurring all around us—as invasive species spread across the landscape, and viruses multiply in a new victim’s bloodstream.

To learn how ecological opportunity really is all around us, you’ll have to go check out the whole post.

Kudzu, taking advantage of ecological opportunity. Photo by Suzie T.

Reference

Yoder, J.B., S. Des Roches, J.M. Eastman, L. Gentry, W.K.W. Godsoe, T. Hagey, D. Jochimsen, B.P. Oswald, J. Robertson, B.A.J. Sarver, J.J. Schenk, S.F. Spear, & L.J. Harmon. (2010). Ecological opportunity and the origin of adaptive radiations. Journal of Evolutionary Biology, 23, 1581-96 DOI: 10.1111/j.1420-9101.2010.02029.x

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On competition, ecological opportunity, and Sahney et al.

ResearchBlogging.orgThere’s already been a lot of blogospheric discussion of the BBC’s recent declaration that “Darwin may have been wrong” based on a recently-published paleontology paper. I hadn’t paid it much attention, because while sloppy science journalism irritates me, it’s not quite in my wheelhouse, expertise-wise. Then I actually got around to reading the paper, and it turns out that it’s directly related to some of my own work—and the conclusion that led to the sensationalistic sub-headline doesn’t make any sense.

Coauthors Sahney, Benton, and Ferry analyze the fossil record of four-limbed vertebrates—tetrapods—to show that in general, as more species evolve, they also evolve to fill a wider variety of ecological roles [$a]. Ecological roles are here defined by combinations of body size, diet, and habitat. (Sahney et al estimate there are 207 such combinations possible, though only 75 are “occupied.”) That’s a straightforward and mostly unsurprising result—the number of tetrapod species increases as tetrapods evolve new ways to make a living. But then we get to the conclusions of the paper, and things get weird.

Sahney et al. conclude that because diversification is associated with finding unoccupied ecological roles, competition is mostly unimportant in the diversification of tetrapods: “Given the unrestricted access tetrapods have to ecospace, perhaps there is little need for competitive interactions to shape diversification.” In other words, if diversification happens by finding ways to make a living that aren’t already occupied, competition isn’t important.

Except that the very reason species diversify following an ecological opportunity like the development of a new ecological role is the lack of competition the new role provides. As my coauthors and I documented in a recently published literature review, competition shapes the kind of diversification documented by Sahney et al. in two ways: first, by its absence following the evolution of a new lifestyle; then in spurring an adaptive radiation as new species evolve to partition up the newly-available “ecospace.”

What makes this doubly odd is that Sahney et al. refer to another kind of ecological opportunity, the extinction of competitors, as a good example of competition-driven diversification. But a central insight of the literature on ecological opportunity is that diversifying because a whole bunch of ecological roles have just opened up is not fundamentally different from diversifying after a new mutation makes a never-before-seen ecological role possible. Think of it like starting a new business: to avoid competition, you could either sell an existing product in a place where no one else sells that product, or you can invent a product no one else offers. Both approaches give you a market all to yourself, and both are defined by competition.

It’s hard for me to understand why Sahney et al. don’t make this conceptual connection—which, for what it’s worth, has its roots in The Origin of Species.

References

Sahney, S., Benton, M., & Ferry, P. (2010). Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land. Biology Letters, 6 (4), 544-7 DOI: 10.1098/rsbl.2009.1024

Yoder, J.B., Des Roches, S., Eastman, J.M., Gentry, L., Godsoe, W.K.W., Hagey, T., Jochimsen, D., Oswald, B.P., Robertson, J., Sarver, B.A.J., Schenk, J.J., Spear, S.F., & Harmon, L.J. (2010). Ecological opportunity and the origin of adaptive radiations Journal of Evolutionary Biology, 23 (8), 1581-96 DOI: 10.1111/j.1420-9101.2010.02029.x

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Getting away from it all: Why are invasive species invasive?

ResearchBlogging.orgWhen humans move from place to place, we almost always bring other organisms with us. Sometimes it’s intentional — domestic animals carried along with Polynesian colonists, for instance. Just as often, it’s accidental, as with mice stowing away on Viking longships. A lot of these introduced species have done so well in their new habitats that they become invasive, outcompeting natives and disrupting local ecosystem processes. But the species that go crazy-invasive — the cane toads and the purple loosestrife — are probably only the very successful subset of the species that hitch rides in cargo holds and ballast tanks. What sets the dangerously successful invasive species apart from others?


Purple loosestrife, introduced
from Europe, now common in East
Coast wetlands.
Photo by Muffet.

A new dataset published in last week’s PNAS suggests that it may be an interaction between available resources in a new habitat and a lack of compatible pathogens [$-a]. This is an amalgam of two hypothesized causes for successful invasion: access to new resources, and escape from antagonistic species. Focusing on European plant species that have successfully invaded North America, the authors, Blumenthal et al., assembled records of viral and fungal infections on each plant species in its native range, and in North America. They classified the plant species based on the habitats each occupies — wet vs. dry, nitrogen-rich vs. -poor — and on whether the plants tended to grow slowly or rapidly. This is because plant species adapted to rich, wet environments are generally thought to evolve fewer defenses against infection and herbivores; they can “afford” to grow new tissue instead of fight to keep it.


Garlic mustard, another invasive
from Europe.
Photo by clspeace.

If resource availability interacts with freedom from infectious agents to spur a successful invasion, then invasive plants adapted to rich conditions should tend to host more pathogens in their home ranges than they do in their introduced range; and this difference should be less pronounced in invasive plants adapted to dry, resource-poor conditions. This is exactly what the analysis found — plants adapted to richer habitats saw a larger reduction in the number of pathogen species attacking them in their new ranges than plants adapted to less-productive conditions.

This is a valuable result for its basic application — helping to predict which introduced species are likely to become invasive, and target them for eradication efforts before they become well-established. But it also provides us with an insight into how evolution works. Many authors, particularly G.G. Simpson and Dolph Schluter, have described ecological conditions that set the stage for adaptive radiation — the rapid diversification of a lineage into many species — which sound a lot like the “ecological release” that invasive species seem to experience.

Rapid evolutionary diversification may be triggered by the evolution of a key innovation; by colonization of a new, empty habitat; or the removal of antagonistic species (usually by their extinction). These three classes of conditions are closely related, and they can be mimicked, or even replicated, when humans move species to new habitats [$-a]. Blumenthal et al. suggest, for instance, that species invasions entail both colonization of a new habitat and escape from pathogens. This is a broad observation; a good next step would be to directly compare natural selection acting on invasive plants in their native and introduced ranges. Through day-to-day processes like this, the specific ecology of a species can ultimately shape its evolutionary fate.

Reference

Blumenthal, D., Mitchell, C., Pysek, P., & Jarosik, V. (2009). Synergy between pathogen release and resource availability in plant invasion. Proc.Nat. Acad. Sci. USA, 106 (19), 7899-904 DOI: 10.1073/pnas.0812607106

Vellend, M., Harmon, L., Lockwood, J., Mayfield, M., Hughes, A., Wares, J., & Sax, D. (2007). Effects of exotic species on evolutionary diversification. Trends Ecol. & Evol., 22 (9), 481-8 DOI: 10.1016/j.tree.2007.02.017

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