Do metacommunity mass effects predict changes in species incidence and abundance

Metacommunity matrices contain data on species incidence or abundance across sites, compactly portraying community composition and how it varies over sites. We constructed models based on an initial metacommunity matrix of either species incidence or abundance to test whether such data suffice to predict subsequent changes in incidence or abundance at each site. The models reflect both species and site mass effects in using products of the row and column totals to predict the incidence or abundance of each species expected at each site. We tested these models against empirical data on vascular plant incidence and abundance collected from two sets of forested sites in both the 1950s and 2000s. Predictions from these models parallel observed changes in species incidence and abundance in these distinctly different metacommunities and differ greatly from null model predictions. The abundance model shows greater power than the incidence model reflecting its higher information content. Predictions were more accurate for the more diverse forests of southern Wisconsin that are changing quickly in response to succession and fragmentation. Simulations show that these results are robust but sensitive to sampling intensity. Because these models require no data on site conditions or species' characteristics, they provide a useful baseline to assess more complex models based on species' functional traits, local site conditions, or landscape context. They may also prove useful to conservation biologists seeking to predict local population declines and extinction risks. This article is protected by copyright. All rights reserved.