A unified theory for macroecology based on spatial patterns of abundance

Macroecology proceeds by identifying patterns and then identifying processes that cause those patterns. Most of the processes that macroecologists study are local in nature and tend to involve species interactions and speciation and extinction processes. In contrast, we propose that several important macroecological patterns can be explained by very large-scale processes that are primarily spatial in nature. Specifically, we suggest that the structure of abundance across a species’ entire range combined with interspecific patterns in range location and global abundance can explain the well-known macroecological patterns of: (1) a positive correlation between range size and abundance, (2) the species–area relationship, (3) decay of species similarity with distance and (4) the species abundance distribution. We show that spatial processes produce these patterns through a combination of analytical and Monte Carlo analysis. We also show that the connection is robust (indifferent) to the precise mathematical assumptions. Such a theory might be called a unified theory, because it explains multiple patterns with a few processes. To differentiate among the growing number of unified theories, we suggest that testing additional predictions over and above producing curves of the correct shape is important. To this end, we present several novel, quantitative predictions and provide empirical tests. In short, we provide an empirically grounded and tested theory, which suggests that superimposing individual species ranges across space creates local community patterns.