Figure 1.

A graphical representation of the intermediate difference hypothesis described in the text. The broken line represents the rate of species invasions and the dotted line the success rate of those invaders, both expressed as functions of the similarity between adjacent habitat patches. The precise shapes of the curves are arbitrary, but should not matter so long as both are monotonic in the indicated directions. The strength of mass effects (represented by the solid line) is a product of the two. Where neighboring plots are very similar, there are likely to be few species that are not present in both, and thus, little potential for mass effects. Where they are extremely dissimilar, species moving between plots should have little chance of survival. The effects of mass effects should therefore be most pronounced where plot differences are of intermediate strength.