Effects of Mammalian Herbivores on Plant Recruitment in Two Neotropical Forests

Abstract

Major changes in seed or seedling survivorship could alter the recruitment dynamics of tropical forest trees. One mechanism that could cause altered survival is modified herbivore pressure, in our context, by mammals. Clear differences in seed and seedling survivorship result when human hunters and/or habitat fragmentation extirpate mammalian herbivores or greatly reduce their abundances (DeSteven and Putz 1984, Sork 1987, Dirzo and Miranda 1991, Leigh et al. 1993). Increases in mammalian herbivore abundances may also influence tree recruitment. In particular, Terborgh (1992) hypothesized that absent carnivores contribute to high abundances of medium-sized mammalian herbivores on Barro Colorado Island (BCI), Panama and that this may in turn alter tree recruitment. We tested the second part of this hypothesis. We compared seed and seedling dynamics for BCI and for Cocha Cashu (CC), Peru, where the apparent population densities of medium-sized mammalian herbivores are much lower. We studied trees in the genus Dipteryx (Leguminosae, Papilionoidae) because their seeds are consumed by terrestrial mammals, closely related species producing almost identical seeds occur in Panama and Peru, and the seed and seedling demography of D. panamensis have been studied in Panama and Costa Rica (DeSteven and Putz 1984, Clark and Clark 1987, DeSteven 1988). To examine the implications of high vs. low numbers of mammalian herbivores, we followed the fates of Dipteryx seeds and seedlings in impermeable and semipermeable mammal exclosures and in open controls at both sites. We also monitored the survival and recruitment of all plants (mostly tree seedlings) growing in the experimental and control plots.