The effects of size structure for a top predator cascade down trophic levels in a pitcher plant community.

Background/Question/Methods

Populations with overlapping generations consist of individuals of different size classes.  This widespread source of intraspecific variation can alter a range of ecological phenomena from conspecific interactions to ecosystem dynamics. We investigated the population and community level effects of size structure in a top predator within the inquiline communities of pitcher plants (Sarracenia purpurea).  A response surface design was used to manipulate the densities of both small and large larvae (corresponding approximately to 2nd and 4th instars) of the mosquito (Wyeomyia smithii [Coq.]) in naturally occurring leaves.  This allowed us to quantify changes in mosquito development and the microbial community.  We addressed the following questions: (1) how does size structure influence cohort growth rates (i.e. what are the relative strengths of intra v intercohort interactions) and (2) does top predator size structure influence the protozoa community and (3) can these effects cascade down to the bacterial community?

Results/Conclusions

We found little evidence for intra or inter size class interaction in development rates among mosquito larvae, in part because the development rates differed greatly among replicates, precluding the distinction between small and large sized larvae at the end of the experiment.  However, the consequences of size structure were seen throughout the lower two trophic levels, as size classes had interactive effects on protozoa richness and community composition. In addition, mosquito size classes had significant and independent effects on bacterial community composition.  Our results reinforce the idea that stage structure in predators can play some role in shaping community structure, with effects that can cascade down multiple trophic levels.