Evidence for the spatial segregation hypothesis: a test with nine-year survivorship data in a Mediterranean shrubland

A current focus of ecology is the investigation of spatial effects on population and community dynamics; however, spatiotemporal theory remains largely untested by empirical observations or experimental studies. For example, the segregation hypothesis predicts that intraspecific aggregation should increase the importance of intraspecific competition relative to interspecific competition, thereby enhancing local coexistence in plant communities. We applied recent methods of point pattern analysis to analyze a unique long-term data set on fully mapped seedling emergence and subsequent survival in a Mediterranean gorse shrubland after experimental fires and simulated torrential rainfall events. Our overall aim was to test if the observed spatial patterns were consistent with the segregation hypothesis during the entire community dynamics from early seedling emergence to the establishment of a mature community, i.e., we explored if the observed initial segregation did indeed prevent interspecific competition from becoming dominant. We used random labeling as the null model and specific test statistics to evaluate different biological effects of the spatial interactions that determine mortality. We found that mortality was clearly not random. Comparison of the probability of mortality in dependence on the distance to conspecific and to heterospecific plants showed that mortality was controlled almost entirely by intraspecific interactions, which is consistent with the segregation hypothesis. Dead plants were aggregated and segregated from surviving plants, indicating two-sided scramble competition. Spatial interactions were density dependent and changed their sign over the course of time from positive to negative when plants grew to maturity. The simulated torrential rainfall events and subsequent erosion caused nonspecific mortality of seedlings but did not reduce the prevalence of intraspecific competition. Our results provide support for the hypothesis that the spatial distribution of plants may profoundly affect competition and can be an important determinant in the coexistence of species and biodiversity.