Although birds and mammals potentially play important roles in several mechanisms hypothesized to maintain diversity in species-rich habitats, there have been few community-level tests of their importance. We excluded terrestrial birds and mammals from 14 6 x 7.5m plots in Australian primary tropical rain forest and followed recruitment and survival of marked seedlings over the subsequent seven years and for another six years for a subset of seven of those plots.
Results/Conclusions
Because terrestrial birds and mammals on our plots acted primarily as agents of seedling mortality rather than seed dispersal, open plots experienced significantly lower rates of seedling recruitment and higher seedling mortality. Higher seedling abundances on exclosure plots resulted in significantly higher species richness. Individual plot responses to vertebrate exclusion were variable and indicated that vertebrates could increase seedling community diversity only when recruitment was dominated by a seedling species sensitive to vertebrate disturbance that recruited in large numbers and at regular enough intervals to maintain dominance through time. Only one of 14 plot pairs showed this response. The significant reduction in seedling density in the presence of vertebrates we documented is consistent with the hypothesis that vertebrates could maintain tree diversity over longer time periods by reducing the likelihood of competitive exclusion, either by maintaining low seedling densities or increasing recruitment limitation.
