Luke Frishkoff
second year graduate student at Stanford University
Dear Scifund Supporters,
I have finished the lab's first foray into understanding how human agricultural expansion in southern Costa Rica has impacted the genetic diversity of tropical frogs. The study is comprised of a genetic comparison of populations of two frog species, one found only in forest and small forest fragments, the other occupying human altered habitats such as coffee plantations and pastures. In ancient times, this landscape was presumably covered primarily with forest (though it is quite possible that agricultural plots established by indigenous groups existed in the landscape prior to Columbus landing in the New World. However, after European colonization of Central America new diseases seem to have decimated the indigenous population, and any large agricultral plots in the area would have been recolonzied by forest). This forest was largely cleared in the mid twentieth century as the region was settled by italian immigrants. We set out to understand two things i) how has genetic diversity been influenced by recent human alteration of this landscape? and ii) what was the frog that currently lives in agriculture doing before the advent of widespread agricultural fields?
We found that despite extensive landscape conversion and forest fragmentation, the forest dependent species still has a great deal of genetic diversity, built up over thousands of years of plentiful habitat. All these frogs are closely related genetically, signaling that prior to the expansion of agriculture these forest denizens acted as one large population. However, the imprint of human landscape modification is present in their genome: Frogs in small forest patches have much less genetic diversity than do frogs in large contiguous forests, signaling that any small degree of migration between these forest fragments is insufficient to prevent inbreeding.
In contrast, the agricultural species as a whole had much lower genetic diversity across the landscape, meaning that in ancient times these frogs were much less common. This makes sense, given that expansion of agricultural habitat that these frogs use occurred relatively recently. Also, unlike the forest species, not all
agriculturally affiliated frogs were closely related, instead two very distantly related groups emerged, and these genetic groups occupy different areas on the landscape. This suggests that over evolutionary time scales these frogs were cut off from one another by some sort of barrier to dispersal, and, interestingly, this barrier was not experienced by the forest dwelling frog.
Luke
