On the mechanisms of coexistence among annual-plant species, using neighbourhood techniques and simulation models

Many studies have investigated the density-dependent regulation of annual-plant populations on coastal sand dunes, but few have explored the consequences of competition for the coexistence of plants in these simple communities. We used neighbourhood techniques to parameterize competition and dispersal functions from field data collected for two species of dune annual (Aira praecox and Erodium cicutarium) over three successive years, and then combined these functions into spatially explicit simulation models. The population size of Aira varied enormously between years, while Erodium remained steady. Competition with neighbours reduced the spike length of Aira plants only in one of the three years (when its population density was highest), while competition with neighbouring Erodium plants appeared to result in the local death of Aira plants. However, these density-dependent effects were far too weak to generate the observed changes in the population size of Aira among years, or to maintain populations below the upper limits observed. The large-seeded Erodium was affected by intraspecific competition but was unaffected by small-seeded Aira plants. Therefore, the larger-seeded species was competitively superior to the smaller-seeded species, an affect that could promote coexistence (albeit weakly) by a competition-colonisation trade-off. Modal dispersion distances of Aira and Erodium were 45 and 60 mm respectively, greater than the radius within which competitive interactions occurred (40 mm). Theoretical studies suggest that under these conditions the spatial arrangement of plants should be nearly random. In fact Aira was spatially aggregated, especially when rare, suggesting that patchy mortality across the dunes was important in generating spatial structure. The study suggests that density dependence only weakly regulates dune annual communities, while year-to-year environmental variation exert major influences on population sizes and spatial structures.