Aggregation and Coexistence. I. Theory and Analysis

The aggregation model of coexistence states that intraspecific aggregation over patches stabilizes species coexistence and allows high diversity in patchy systems.The model is reviewed, with emphasis on the methods of analysis of aggregation data. Essential improvements in existing methods of analysis are presented. The density, rather than the number, of competitors is important when patches differ in size. Measures of intraspecific and interspecific aggregation, J and C, are modified to take account of variable patches. The discrete negative binomial distribution is not suitable to quantify crowding on variable patches. A new criterion for coexistence in a patchy environment is developed that is a necessary and sufficient condition for the persistence of a specified species in the presence of other species. A previous criterion, the 'relative strength of intraspecific aggregation', A, represents only a necessary condition for the coexistence of two species. The stability of coexistence is affected by clutch sizes. Existing methods to estimate clutch sizes are not satisfactory. The analyses are most sensitive to errors in the estimated clutch size when clutches are large relative to the number of individuals per patch.