In the depths of a pitcher plant, competitors and predators cancel each other out

ResearchBlogging.orgSpecies interactions are probably pretty important, in the evolution of life. There are all sorts of studies showing that the fitness and evolutionary history of individual species depends upon interactions with pollinators, symbiotes, food plants, herbivores, parasites, predators, and competitors. Most of these studies focus in on a single interaction—but what living thing interacts with only one other organism? Coevolution, when it happens, happens in a community context.

Adding even a second interaction into the scientific picture can be difficult, but it may also dramatically change the evolutionary outcome, as seen in a new study of evolution in the protozoan communities living in purple pitcher plants. Individually, competitors and predators are significant agents of natural selection—but together, they seem to counterbalance each other [$a].

The purple pitcher plant, Sarracenia purpurea. Photo by petrichor.

Carnivorous pitcher plants grow funnel-shaped leaves that collect water to form a pitfall trap for hapless insects, which provide a source of nitrogen in swampy, nutrient-poor habitats. One species’ pitfall is another’s ideal habitat, however, and pitchers also play host to diverse micro-communities [PDF] of protozoans, bacteria, and even mosquito larvae. By recreating—and experimentally manipulating—these communities in the laboratory, the new study’s author, Casey terHorst, was able to disentangle the individual and combined effects of two different kinds of species interaction within pitcher plant pitfalls.

TerHorst focused on a protozoan species in the genus Colpoda, a widespread single-celled critter found in moist soil and standing water. In pitcher plants, Colpoda makes a living feeding on bacteria that break down insects trapped by the pitfall—and they themselves are prey for the larvae of the mosquito Wyeomyia smithii.

An example of genus Colpoda, the group of ciliates studied (but probably not the same species). Photo by PROYECTO AGUA** /** WATER PROJECT.

To determine the individual and combined effects of competition and predation on Colpoda, terHorst allowed experimental populations of the protozoan to evolve for 20 days (about 60-120 Colpoda generations) with either (1) no competitors or predators, (2) competition from another bacteria-eating protozoan, (3) predation by mosquito larvae, or (4) competition and predation. At the end of the experimental period, he sampled each evolved Colpoda population and measured a number of traits, including the size of Colpoda cells and their speed. Larger Colpoda cells are thought to be better competitors but more vulnerable to predators; faster ones should be better able to evade predation.

Individually, predators and competitors had significant effects on Colpoda evolution. In the presence of mosquito larvae, Colpoda evolved smaller, faster cells than it did alone. Unexpectedly, competitors also caused Colpoda to evolve smaller cells, though not faster ones. TerHorst suggests that this is because competition also favored more rapid reproduction by Colpoda, which meant that individual cells grew less before dividing.

Most interestingly, though, Colpoda evolving in the presence of both predators and competitors looked quite a lot like Colpoda that evolved alone. This is apparently because the mosquito larvae ate both Colpoda and its competitor—the mosquitoes acted to relieve some competitive pressure on Colpoda at the same time they ate fewer Colpoda because they had two prey species to pursue. In fact, the mosquitoes preferred to eat the competitor species, since it tended to hang out in the open while Colpoda hid among the plastic beads lining the base of the artificial habitat.

Thus the indirect effects of the predator offsetting competition, and of the competitor drawing away predation, canceled out the natural selection each imposed on Colpoda individually. Species interactions in a community context, even a simple one like this, are far from straightforward.

References

Buckley, H., Burns, J., Kneitel, J., Walters, E., Munguia, P., & Miller, E. (2004). Small-scale patterns in community structure of Sarracenia purpurea inquilines. Community Ecology, 5 (2), 181-8 DOI: 10.1556/ComEc.5.2004.2.6

terHorst, C. (2010). Evolution in response to direct and indirect ecological effects in pitcher plant inquiline communities. The American Naturalist, 176 (6), 675-85 DOI: 10.1086/657047

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Science online, miracle cure edition

Photo by rpongsaj.
  • Or, you know, the evolution of a super-cold. The discovery of a new way to stop viruses after they’ve already invaded cells could lead to an actual cure for the common cold. (The Independent)
  • Pleistocene Park, anyone? An extremely well-preserved mammoth skeleton unearthed near Denver, Colorado, may contain reasonably intact DNA. (The Denver Post)
  • Not just because of running to catch the bus. People who use public transit tend to be more active in general. (Obesity Panacea)
  • What the !?%$#**! do we know about human mutation rates, anyway? Less than you might think. (John Hawks Weblog)
  • This confirms what I already believe about both anti-vaxxers and corporate PowerPoint use. A GlaxoSmithKline presentation on the importance of vaccination leaves Jason Goldman pondering cognitive bias and the vital importance of good PowerPoint use. (The Thoughtful Animal)
  • All part of a conspiracy by socialist Radiolarians. Analysis of carbon isotopes in sediment cores suggest that a period of climatic warming in the middle Eocene was caused by increased atmospheric carbon dioxide. (Scientific American)
  • WTF is hepato-splen? That’s just one of many questions Scicurious can’t answer about a truly bizarre study investigating the effect of lunar phases on women’s menstrual cycles. (Neurotic Physiology)

Video this week, via io9: German researchers have determined that bats recognize bodies of water by echolocation because, when presented with a metal plate that reflects sound the way water does, they try to drink from it.

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Between two host plants: The middle road doesn’t work for hybrid butterflies

This post was chosen as an Editor's Selection for ResearchBlogging.orgNew species form when separate populations of related organisms are no longer able to interbreed. Reproductive isolation can arise if two populations evolve different mating behaviors, or lifestyles so different that individuals from different populations don’t even encounter each other—but it need not mean that matings between the two populations never occur. In fact, speciation can arise in the face of quite a lot of interbreeding, so long as the hybrids produced by interbreeding are less fit than “purebred” individuals.

Edith’s checkerspot in Mount Diablo State Park, California. Photo by davidhoffman08.

This is what seems to be occurring in populations of Edith’s checkerspot, a small butterfly native to Western North America. Checkerspot populations in California use a wide variety of different host plants, and a recent study has shown that the offspring of parents from different host plants are maladapted in the wild.

In the Sierra Nevada mountains, logging has created a new kind of habitat for Edith’s checkerspot [PDF]—patches of cleared forest where the butterfly’s locally preferred host plant, Pedicularis semibarbata, is rare or nonexistent, but an alternative host plant, Collinsia torreyi, is plentiful. In the transition between clearings and less-disturbed forest, the two plants may often grow side by side.


A tale of two host plants: Pedicularis semibarbata and Collinsia torreyi. Photos by Wayfinder_73.

Examination of checkerspot populations that have access to only one of the two host plants suggests that each plant is best used in rather different ways. For instance, Pedicularis-using checkerspot females lay lots of eggs on a few plants, while Collinsia-using females lay a few eggs on each of a large number of plants. Once they hatch, larvae from Pedicularis populations feed on leaves closer to the ground than larvae from Collinsia populations, which makes sense since Pedicularis grows lower in general.

If these differences have a genetic basis, then hybrid checkerspots might exhibit intermediate behaviors, which might not work so well on either host plant. To test for this “hybrid inviability,” the new study’s authors crossed checkerspots from populations encountering only one host plant or the other, and then tested the hybrids’ performance in the field—and what they found confirms those predictions.

The Goldilocks principle—intermediate is better–doesn’t apply to hybrid checkerspots. Hybrid caterpillars foraged on leaves at an intermediate height on both host plants, and grew more slowly than purebred caterpillars. Hybrid females laid an intermediate number of eggs on both host plants, and laid them at an intermediate height. This left their offspring in a poor position for foraging after they hatched—and indeed, they grew more slowly than larvae hatched from eggs that were laid at the “traditional” heights on the host plants.

So it looks as though natural selection for better performance on Collinsia has led to the evolution of checkerspots that are at a disadvantage using Pedicularis (and vice versa). This even to the point that hybrids, which feed and oviposit in ways that are only somewhat different from the optimum, pay performance costs.

What’s interesting, though, is that this hasn’t led to greater genetic differentiation of checkerspot populations using different host plants; as assessed using randomly-selected genetic markers, there is an isolation-by-distance effect, but no effect of host plant use. (The authors cite a previous study using about 400 AFLP loci [PDF].) That suggests that only a few genes are responsible for the observed adaptive differences, and that natural hybridization between checkerspot populations using different hosts may be mixing together the rest of the genome.

References

McBride, C., & Singer, M. (2010). Field studies reveal strong postmating isolation between ecologically divergent butterfly populations. PLoS Biology, 8 (10) DOI: 10.1371/journal.pbio.1000529

Singer, M.C., & Wee, B. (2005). Spatial pattern in checkerspot butterfly-host plant association at local, metapopulation and regional scales. Annales Zoologici Fennici, 42, 347-61

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Election night

With all due respect to Mr. Stewart, the Python lads nailed election night coverage to the wall decades ago.

If you haven’t already done so, fellow U.S. citizens, please hurry up and find your fucking polling location so you can vote for the Sensible candidate. Need help remembering what the fuck a Sensible government did for you in the last two years? There is also a site for that—more detail, with less swearing, is here.

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The grad student’s cookbook

Scicurious has officially posted her epic compilation of recipes by and for graduate students, i.e., compiled with budget and preparation time in mind. (I put my contribution online early.) They’re neatly sorted by meal—breakfast, lunch and snacks, dinner, desserts, and Ramen.

If you don’t understand why Ramen is a meal in itself, well, consider yourself lucky. Otherwise the range, which runs from pot roast to Pad Thai, is impressive and delicious. The lesson I take away is that (surprise!) it’s entirely possible to cook well and simply on a budget—you only need to decide to do it.

Really, my fellow graduate students, you can do better than this. Photo by pinprick.
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