Quantifying the impact of competition and spatial heterogeneity on the structure and dynamics of a four-species guild of winter annuals

We develop statistical methods appropriate for the analysis of spatially structured population data. The methods are used to study the structure and dynamics of a four-species annual plant guild recorded in 1,000 permanent squares over a 10-yr period. We parameterize models that predict population density from one year to the next. In agreement with theoretical expectation all the models have locally stable equilibria, and overcompensation is rare. We demonstrate that interspecific interactions are extremely weak, relative to intraspecific ones, and that the spatial arrangement of species and individuals within them is critical to the observed dynamics. The impact of spatial density-dependent population growth on observed densities was calculated. In 52% of the cases population size would have been increased by at least a factor of 1.5 had there been no interactions between individuals, and in 9% of these it would have increased by a factor of four or more. This effect is shown to be largely a result of intraspecific interactions. We discuss possible explanations for the weakness of interspecific interactions.