The poor birds at the edges of these resource clusters are called: edge species (yellow cylinders).  The edge species have to deal with highly variable resource availability, so they have evolved a couple of tricks.  They become nomadic or migratory, they constantly search for available resources when the resources they are using run out.  Some species though can't handle being an edge species and their populations simply decline. 
    So how can I figure out which species are going to be nomadic, migratory, or declining with just a bunch of body mass data?  If I know the body masses of all the species in an environment, all I have to do is look for gaps in the distribution of species' body masses; and at the edges of the gaps lay the nomadic, migratory, and declining species. 
    That's pretty much the Textural Discontinuity Hypothesis in a nut shell.