Centrifugal Community Organization

Pimm has invented a useful classification of community organizations based upon ecological preferences. We add the category of centrifugal structure to it. If the primary preference of all species is shared, but their secondary preferences are distinct, they are said to be organized centrifugally. The biological basis for centrifugal structure may be that habitats represent combinations of different environmental variables and, although the ideal combination is the same for many species in a guild, each species is adapted to tolerate relative deprivation of a different component of the mixture. We have modelled the isolegs of centrifugal organization. (Isolegs are the lines of equal optimal behavior drawn in a state space.) Their principal distinguishing feature is their negative slope. Because of this, niche shifts in such communities should occur backwards: removing a competitor produces narrower niches in those left. Another feature which distinguishes it is its inability to generate a 'ghost of competition past': competition should always be a strong dynamical influence. We examined the relationship of two species of gerbils in the Negev Desert, Israel, and determined that they both exhibit negative isolegs. Other aspects of their habitat selection are consonant with a picture of centrifugal organization. But it remains unknown how general or important is this pattern of community organization.