Effects of Dispersal Pattern and Mammalian Herbivores on Seedling Recruitment for Virola michelii (Myristicaceae) in French Guiana¹

Abstract

The effects of dispersal pattern (seeds in small clumps vs. seeds scattered in pairs) and distance to the nearest Carapa procera (Meliaceae; a tree that produces seeds preferred by terrestrial vertebrates) on survival of seeds and seedlings were examined for the animal–dispersed tree species Virola michelii (Myristicaceae) in a mature forest‘at Paracou, French Guiana, in 1992 and 1993. We assessed the putative role of ground–dwelling mammalian herbivores, rodents, and ungulates that filter the seed shadow, acting either as dispersers or predators and thus modifying the original pattern of seed dispersal made by frugivores. We measured the effects of simulated seed burial by rodents using marked seeds and quantified the effect of protecting seeds and seedlings from ground–dwelling vertebrates on seedling germination and survival with fence exclosures in 1992. Dispersal pattern had short–term but no long–term effects on the proportion of V. michelii seeds that survived one year later as seedlings. In the short term, within six weeks, clumped seeds survived better than scattered seeds in both years. Marked seeds that were removed from their site of dispersal were eaten; rodents only rarely buried seeds of V. michelii, and seed burial reduced seed and seedling survivorship. The combined effect of the factors year and Carapa proximity significantly affected seed survival within six weeks. Although six–week seed survival was greater in 1993 than in 1992, seedling establishment was lower in 1993 than in 1992 following a lower rainfall regime during the key period of seed germination (February). One–year seed and seedling survivorship was similar between treatments and years. Seed survival and seedling establishment in V. michelii was dependent on vertebrates in the short term and on climate in the long term. Overall, seed and seedling survivorship depended on a combination of these factors.

On the density-dependence of seed predation in Dipteryx micrantha , a bat-dispersed rain forest tree

“…Forget et al (2000) found that in the short term (6 weeks), clumped seeds of Virola michelii had a higher probability of survival than did scattered seeds, but that no such difference remained after 1 year. Our results indicate that a similar process may be operating in Dipteryx.…”

Distance-dependence in two Amazonian palms: effects of spatial and temporal variation in seed predator communities

“…Due to their biomass in the ecosystem and dietary preferences, T. pecari are likely to have an essential, although poorly understood, role in altering the seed shadow of large seeded tree species. T. pecari are quickly eliminated in the presence of human inhabitants (Peres 1996) and most Amazonian seed predation studies have been conducted in areas with reduced T. pecari populations (Burkey 1994;Notman et al 1996;Forget et al 2000).…”

“…The Janzen-Connell model predicts that seed predators will exert greater predation pressures where dispersed seeds are densest or closest to a reproductive conspecific. Distance-or densitydependent mortality during recruitment leads to a decreased probability that a species will replace itself at a site and promotes maintenance of diversity (Janzen 1970;Augspurger 1984;Clark and Clark 1984;Howe 1989;Forget et al 2000). The Janzen-Connell predictions have been studied intensively, but most studies have been conducted in areas without a full complement of large mammals (Wright 2002).…”

Crop damage of Eriotheca gracilipes (Bombacaceae) by the Blue-Fronted Amazon (Amazona aestiva, Psittacidae), in the Brazilian Cerrado

“…Janzen's model predicts an enhancement of seed predation rate according to seed density or proximity to a reproductive tree. Distance-or density-dependent mortality during recruitment leads to a decreased probability that a species will replace itself at a site and promotes maintenance of diversity (Janzen, 1970;Clark and Clark, 1984;Forget et al, 2000). In fact, seed predation may be either positively or negatively densitydependent and the pattern can be expected to vary with the relative abundance of seeds and their predators.…”

“…In this respect, a canopy or a collection of neighboring canopies bearing enhanced fruit crop sizes tend to cause predator satiation, which consequence is an improved seed survival within a dense seed patch (Augspurger, 1981;Ghazoul and Satake, 2009). Therefore, seed success is strongly influenced by pre-dispersal seed predators, and negative density-dependent effect on seed predation or predator satiation will determine the fecundity level of individual plants (Forget et al 2000;Ghazoul and Satake, 2009).…”