HOW SPECIES WITH DIFFERENT REGENERATION NICHES COEXIST IN PATCHY HABITATS WITH LOCAL DISTURBANCES

We used a two-species simulation model to study mechanisms of coexiste nce of annual plants in patchy habitats with local disturbances. In ha bitats with nested scales of patchiness, short dispersal is advantageo us because favorable habitat tends to be aggregated. The invasion of a resident population with short dispersal distance by a species with l onger-range dispersal was simulated for combinations of habitat patter n, disturbance frequency and germination strategies. A germination str ategy was defined by the type of response to disturbance (''disturbanc e-broken'' when disturbances trigger germination, ''risk-spreading'' w hen germination is insensitive to disturbance) and the dormancy fracti on at dispersal. Simulations estimated the long-term low-density growt h rate of the invader, the mean local crowding (number of competing se eds per invader seed at each site) and the effective fecundity of each species (the mean number of seeds successfully dispersed per adult pl ant). Crowding increased with habitat suitability and decreased with i ncreasing dormancy fractions for the resident. Effective fecundity in a landscape can be taken as a measure of competitive ability. The shor t-dispersing resident invariably had higher effective fecundity, but t his difference decreased with increasing suitability, i.e. competitive differences decreased. Coexistence depended on both habitat suitabili ty and disturbance frequency. Maximum coexistence was obtained for hab itats of intermediate suitability with moderately frequent disturbance s. General linear modelling of the long-term low-density growth rate s howed that coexistence results from a reduction in local crowding. Thi s growth rate also increased for increasing habitat suitability and co nnectivity, and for a higher dormancy fraction of the resident species . The effects of disturbance frequency and of invader's dormancy fract ion depended on the type of dormancy of the resident species. The anal ysis showed that 2 different mechanisms are involved in the coexistenc e of species with different niches. Differences in regeneration niches permit coexistence through competitive equivalency with trade-offs be tween dispersal and germination traits, but for a limited range of hab itat pattern and disturbance conditions. On the other hand, coexistenc e through density fluctuations of a disturbance-broken species and sto rage effects can be achieved for a broad range of environmental condit ions and species germination strategies. Species coexistence thus resu lts from the combination of two mechanisms. Evidence from natural comm unities is discussed. Our results also demonstrate the importance of d etailed attention to spatial patterns and dispersal because of the com plexity of spatial effects. Further, spatial pattern and disturbance f requencies need to be considered jointly to understand the dynamics of diversity.