Spatio-Temporal Dispersal Strategies and Annual Plant Species Coexistence in a Structured Landscape

Abstract

Field observations and models have shown: (1) differences in dispersal and dormancy strategies could account for species coexistence in disturbed environments, (2) small scale disturbances contribute to species coexistence in plant communities, and (3) recolonization success may depend on the spatial pattern of suitable sites for establishment. A spatially explicit, two-species, simulation model was developed to examine the interaction between dispersal strategies, dormancy strategies and small-scale disturbances on the pattern of coexistence of two annual plant species in patchy landscapes. Patchy landscapes with three hierarchical levels of structure were randomly generated and the dynamics for species differing in mean dispersal distance and dormancy strategy was simulated. The results showed that when species differed only in mean dispersal distance, the species with the shorter range dispersal would always displace the other. When disturbances prevented reproduction and activated dormant seeds, then differences in dormancy strategy could result in coexistence of species with different dispersal capabilities. When species differed only in their dormancy rate (i.e., the proportion of dormant seeds produced per year), the range of disturbance intensities allowing coexistence was rather narrow, but differences in the response of species to disturbance (i.e., disturbance-broken seed dormancy versus gradual germination of dormant seeds) allowed coexistence under a wider range of disturbance intensities. These results suggest that differences in the ways species exploit the resources in space and time can mediate coexistence through storage effects. The patterns of coexistence obtained in the simulations were consistent with conclusions of a field study, in species-rich Mediterranean old-fields, about the role for coexistence of the interactions among dispersal and dormancy strategies, spatial heterogeneity, and small-scale disturbances. Coexistence patterns depended on the degree of suitability and the patchiness of the landscapes, mostly in relation to the interactions between landscape structure and mean dispersal distance, but also through complex interactions with dormancy characteristics. Therefore, disturbance regimes are not sufficient for predicting patterns of coexistence, and the unique spatial features of the landscapes need to be taken into account.