The numerical and functional responses of a granivorous rodent and the fate of Neotropical tree seeds

Abstract

Despite their potential to provide mechanistic explanations of rates of seed dispersal and seed fate, the functional and numerical responses of seed predators have never been explicitly examined within this context. Therefore, we investigated the numerical response of a small-mammal seed predator, Heteromys desmarestianus, to disturbance-induced changes in food availability and evaluated the degree to which removal and fate of seeds of eight tree species in a lowland tropical forest in Belize were related to the functional response of H. desmarestianus to varying seed densities. Mark-recapture trapping was used to estimate abundance of H. desmarestianus in six 0.5-ha grids from July 2000 to September 2002. Fruit availability and seed fate were estimated in each grid, and two experiments nested within the grids were used to determine (1) the form of the functional response for nine levels of fruit density (2-32 fruits/m2), (2) the removal rate and handling times, and (3) the total proportion of fruits removed. The total proportion of fruits removed was determined primarily by the numerical response of H. desmarestianus to fruit availability, while removal rates and the proportion of seeds eaten or cached were related primarily to the form of the functional response. However, the numerical and functional responses interacted; H. desmarestianus showed strong spatial and temporal numerical responses to total fruit availability, and their density relative to fruit availability resulted in variation in the form of the functional response. Types I, II, and III functional responses were observed, as were density-independent responses, and these responses varied both among and within fruit species. The highest proportions of fruits were eaten when the Type III functional response was detected, which was when fruit availability was high relative to H. desmarestianus population density. Numerous idiosyncratic influences on seed fate have been documented, but our results indicate that shifts in the numerical and functional responses of seed predators to seasonal and interannual variation in seed availability potentially provide a general mechanistic explanation for patterns of removal and fate for vertebrate-dispersed seeds.

Long‐term seed survival and dispersal dynamics in a rodent‐dispersed tree: testing the predator satiation hypothesis and the predator dispersal hypothesis

“…Fixed factors in bold indicate significant differences capita proportion of seeds surviving and the per capita proportion of seeds dispersed at longer distances, which is counterbalanced by mast seeding. Our long-term data from oil tea further support this finding; the per capita proportion (up to 6%) of seeds surviving at caches was higher when seed availability was lower (2002)(2003)(2004)(2005), whereas the per capita proportion (up to 22% in 2007) of seeds surviving at source was higher when seed availability was higher (2006-2009Figs 4 and 6). If the dispersal/survival trade-off exists in predatordispersed plants, non-dispersed seeds near parent plants should also have the potential to establish and recruit as adults (e.g.…”

“…Kelly & Sork (2002) suggest that when testing the predator satiation hypothesis, it is essential to quantify how seed predation, seed dispersal and seed survival are related to per capita seed availability, which is determined by annual seed production and annual abundance of seed predators and seed dispersers. To our knowledge, however, few studies have quantified how per capita seed availability varies and the effect that it has on seed fates (but see Klinger & Rejm anek 2009, 2010.…”

A strong conditional mutualism limits and enhances seed dispersal and germination of a tropical palm

“…(Arecaceae) were determined by seed predation and seed dispersal, and whether the relative rates of seed predation and dispersal varied with alteration in species composition of the seed predator/disperser guild. In the course of previous investigations (Brewer and Rejmánek 1999; Brewer 2001; Klinger 2007; Klinger and Rejmánek 2009) we observed that a large proportion of A . mexicanum nuts were both preyed on and dispersed by the most abundant small mammal in the forest, Heteromys desmarestianus subsp.…”

Seed removal of Dipteryx alata Vog. (Leguminosae: Faboidae) in the edge and interior of Cerrado

“…These contrasting results reinforces that, besides the probability of discovery, the degree of exploitation need to be taken into account to understand seed removal patterns (Romo et al, 2004). If a seed pile is discovered by predators, it might incur in variable rates of removal according to predator species and its local density, besides the availability of alternative food resources Klinger and Rejmánek, 2009). I documented the lowest intensity of endocarp removal in the edge during the late dry season, a well known period of flesh fruit scarcity in tropical areas (Van Schaik et al, 1993).…”