Disentangling habitat quality from spatial effects in the analysis of stream insect community assembly in multiple watersheds

The relative effects of in-stream environmental characteristics and stream network on macroinvertebrate community assembly are assessed for several watersheds in Chester County, PA between 1970 and 1997.  Three separate approaches are employed.  First, repeated measures ANOVA is used to test if streams become more similar to one another within watersheds versus between watersheds and to test whether streams become more similar to one another within stream types (types determined by stream order and habitat characteristics) versus between stream types.  Second, ordination is used to determine how distance to and genera richness of other stream sites in the hydrologic network in the preceding year, as well as current stream size, in-stream water quality, and catchment land use explain the current in stream genera richness of each site.  Third, graph theoretic measures are used to determine the functional diversity of individual stream sites each year and compare those values to the expected functional diversity based on random communities generated from the regional species pool for the observed genera richness.  We find that both habitat quality and spatial effects affect community assembly.  The results are discussed in the theoretical context of random versus deterministic processes in community assembly.