Organism size determines the strength of environmental filtering

Background/Question/Methods

Environmental variables can have a strong filtering effect on local species
composition; however these effects are not equally strong for all groups of
organisms.  This variation may be related to body size: larger bodied organisms
show a greater degree of habitat filtering.  Observational studies have shown an
increase in environmental structuring as body size increases from bacteria
(weak), zooplankton (intermediate) and insects (strong). This differential
environmental filtering can also occur on very small spatial scales; indicating
that, for some groups, environmental effects outweigh those of dispersal.

We tested the hypotheses that bacteria, zooplankton and insects respond
differently to environmental filtering by homogenizing aquatic communities among
three bromeliad species with different habitat preferences: Aechmea
nudicaulis, Neoregelia cruenta, and Vriesea neoglutinosa.  We hypothesized
that as habitat filtering occurs, via mortality and -- for non-insects --
reduced fecundity, we will observe a 'relaxation' of the community towards the
orginal species composition. We hypothesize that this relaxation will be
greatest for insects, less for zooplankton and nearly zero for bacteria.

Results/Conclusions

Insect community composition changed most dramatically over the course of the
experiment, while bacteria showed large variation both before and afterwards.
Interestingly, insect community change resulted in decreased variation among
replicates for some bromeliad species, indicating a filtering effect.  In other
bromeliads, large changes in community composition had neither strong direction,
nor did they tend to change variation in community composition. Bromeliads had
distinct bacteria communities before the experiment; after homogenization and
environmental filtering these differences were no longer present.  Bacteria
communities varied widely after the experiment, indicating that bacterial
communities are less sensitive to strong habitat differences than insects.
Zooplankton abundance and diversity was extremely low, limiting the power of
multivariate analyses -- however, as our experiment proceeded we found a
decrease in species richness of zooplankton in V. neoglutinosa.  Our results
offer insight into how the strength of environmental filtering changes with
organism body size.