Invertebrate community organization across spatial scales in a wetland complex

Background/Question/Methods

We investigated how dispersal ability affected the relative importance of habitat and sampling site in an invertebrate metacommunity in a Lake Michigan tributary wetland complex by using a hierarchical diversity partitioning approach.  Three sites were sampled in a hierarchical sampling regime in May, June, and August of 2004.  At each site invertebrate communities were sampled at 3 locations in 3 different vegetation types (SAV, emergent, lilies).  At each location invertebrates were sampled separately from three microhabitats (water column, stem, and sediment), identified to lowest taxonomic group, and enumerated (72 samples total per month).  We applied four hierarchical levels (1. all samples, 2. site-specific habitat type, 3a. site-vegetation type, 3b. site-microhabitat type, 4a. site, 4b. micro-habitat type, 4c. vegetation type) to the data.  β-richness was measured for all hierarchies each month by calculating the average of pair-wise comparisons using both the Jaccards and the Bray-Curtis index of similarity.  Permutation tests were used to determine whether β-richness was significantly different from randomly assembled communities.  Analyses were conducted first on the whole community and then on sub-sets of the community based on dispersal mode (flying, swimming) and ability (non, poor, good).

Results/Conclusions

Looking at the entire invertebrate community, we generally found that samples that were grouped by vegetation type (within sites, level 3a) had higher β-richness and higher α-richness than sites grouped by micro-habitat type (within sites, level 3b).  This suggests that vegetation type was more important to invertebrate community structure than the microhabitat within the vegetation.  At level 4 we found that α-richness  was highest between sites (4a) followed by microhabitat (4b) then vegetation type (4c), however, β-richness was highly variable between sampling months.  The Jaccards and Bray Curtis approaches returned similar results earlier in the summer but diverged strongly by August as common species experienced large population increases.  Subset communities returned different results than the whole community.  Communities composed of strong dispersers showed weaker effects of microhabitat on community structure.  Communities composed of stronger dispersers also tended to have higher β-richness across sites regardless of the spatial scale of the observations.