Rethinking how niches evolve

Are Darwin’s finches the only, or even the best, model of evolution among competitors?

Many people have heard of these Galapagos birds and how competing species that co-occur must evolve to use their own niche and compete less.  But, what if this simple model is wrong? We don’t really know, in part because evolution can be very difficult to follow in a natural, multispecies communities. Recently, Tom Miller’s lab at Florida State University has begun to use the small aquatic communities that live inside the leaves of the pitcher plant Sarracenia purpurea to ask how competing protozoan species evolve. These protozoan species are blown into leaves where they compete for bacteria and are, in turn, eaten by filter-feeding mosquito larvae. Casey terHorst, a former graduate student at Florida State now at California State University Northridge, first demonstrated that these protozoa can evolve within days in response to competition and predation. Miller, terHorst, and Emma Moran followed the protozoa living in newly colonized leaves and quantified their competitive abilities over 3 months in naturally occurring plants in North Florida. If the Darwin’s finch model is correct, the species should evolve to use unique resources through time and compete less, or else go extinct.

Instead, Miller’s group found a surprising pattern. As predicted, the species that were poor competitors evolved to become better when growing in competition with other species. However, protozoa species that were initially good competitors often evolve to become worse competitors against other species. Miller calls this the “Robin Hood” model, in which the poor get richer and the rich get poorer. His lab is now exploring one possible explanation for this result, which is that the species that are good competitors are mostly competing against themselves, while the poor competitors are competing against the good competitors. So, if there is a tradeoff between intra- and interspecific competitive abilities, the Robin Hood pattern might be exactly what we should expect when there are initially strong competitive rankings. However, Miller thinks that perhaps the most important message from the work is that other competition models exist and they can be tested in real communities. Perhaps the Darwin’s finch model is just for the birds.

The publication is available online HERE.


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