Spatial extent is inherently related to the potential roles of the main mechanisms structuring metacommunities. We examined the effects of varying spatial extent (ecological province, region and subregion) on the environmental and spatial components of variation in lake macrophyte communities. We also studied these effects separately for three macrophyte functional groups.
The US state of Minnesota.
We examined average and heterogeneity differences in macrophyte community composition and environmental variation among the subregions of Minnesota using canonical analysis of principal coordinates (CAP) and homogeneity of multivariate dispersion (PERMDISP), respectively. We further used partial redundancy analysis (pRDA) to decompose variation in macrophyte community composition between environmental variables and spatial location at each spatial extent and geographical region. Spatial variables were derived using principal coordinates of neighbour matrices (PCNM) analysis.
CAP and PERMDISP analyses showed that the subregions differed both in average community composition and in the heterogeneity of community composition for all macrophyte taxa, for emergent and submerged macrophytes, but not for non-rooted macrophytes. We did not, however, find significant differences in overall environmental heterogeneity among the subregions. Variation partitioning using pRDAs showed that species sorting is more important than spatial processes for macrophytes, although these patterns were relatively weak. There was, however, much regional specificity, with the environmental and spatial fractions of community composition varying widely at different spatial extents, among different geographical regions and among functional groups. Contrary to our initial expectations, we did not find increasing spatial structuring and decreasing environmental control with increasing spatial extent.
Our findings indicate that, in macrophyte metacommunities, the relative contribution of spatial processes and environmental control varies rather unpredictably with spatial extent and geographical region. Our findings are thus of importance in advancing metacommunity ecology by showing that drawing wide-ranging conclusions based on a single spatial extent or a single geographical region may be unwise.