Mechanisms of evolutionary change can affect community structure by acting on heritable traits within a single population if those traits are important for species interactions within the community. Here we ask whether gene flow between phenotypically divergent populations of a single species has consequences for the structure of communities of which they are apart.

Timema cristinae is a univoltine, flightless, stick-insect (Order: Phasmatodea) resident to the southern California chaparral and has two distinct ecotypes. One ecotype has a conspicuous, white, dorsal stripe and feeds on Adenostoma fasciulatum, while the other lacks a dorsal stripe and feeds on Ceonothus spinosus.

We take advantage of the low dispersal ability of T. cristinae individuals (~12m per year) as well as a variable geographic arrangement of A. fasciculatum and C. spinosus plants in nature such that we can compare the structure of arthropod communities inhabiting bushes, host to T. cristinae, for which there is gene flow between ecotypes (parapatric arrangement) to those for which there is not gene flow between ecotypes (allopatric arrangement). We compare these differences to those between parapatric and allopatric bushes naturally lacking T. cristinae to help isolate gene flow between ecotypes as a determinant of community structure.

Results/Conclusions

We report differences between parapatric and allopatric T. cristinae populations in the relative species abundances of arthropod species inhabiting C. spinosus and A. fasciculatum that are not found between parapatric and allopatric bushes lacking T. cristinae.

These results suggest that gene flow between locally adapted ecotypes in T. cristinae can have cascading effects on the whole arthropod community of which they are apart. These results will inform the design of future manipulative experiments to more strongly identify gene flow between T. cristinae ecotypes as the cause of the observed differences, and further to determine the mechanism underlying these effects.