Effects of dung and seed size on secondary dispersal, seed predation, and seedling establishment of rain forest trees

Abstract

Seeds dispersed by tropical, arboreal mammals are usually deposited singly and without dung or in clumps of fecal material. After dispersal through defecation by mammals, most seeds are secondarily dispersed by dung beetles or consumed by rodents. These post-dispersal, plant-animal interactions are likely to interact themselves, as seeds buried by dung beetles are less likely to be found by rodents than unburied seeds. In a series of three experiments with seeds of 15 species in central Amazonia (Brazil), we determined (1) how presence and amount of dung associated with seeds influences long-term seed fate and seedling establishment, (2) how deeply dung beetles bury seeds and how burial depth affects seedling establishment, and (3) how seed size affects the interaction between seeds, dung beetles, and rodents. Our overall goal was to understand how post-dispersal plant-animal interactions determine the link between primary seed dispersal and seedling establishment. On average, 43% of seeds surrounded by dung were buried by dung beetles, compared to 0% of seeds not surrounded by dung ( n=2,156). Seeds in dung, however, tended to be more prone than bare seeds to predation by rodents. Of seeds in dung, probability of burial was negatively related to seed size and positively related to amount of dung. Burial of seeds decreased the probability of seed predation by rodents three-fold, and increased the probability of seedling establishment two-fold. Mean burial depth was 4 cm (0.5-20 cm) and was not related to seed size, contrary to previous studies. Probability of seedling establishment was negatively correlated with burial depth and not related to seed size at 5 or 10 cm depths. These results illustrate a complex web of interactions among dung beetles, rodents, and dispersed seeds. These interactions affect the probability of seedling establishment and are themselves strongly tied to how seeds are deposited by primary dispersers. More generally, our results emphasize the importance of looking beyond a single type of plant-animal interaction (e.g., seed dispersal or seed predation) to incorporate potential effects of interacting interactions.

The value of trophic interactions for ecosystem function: dung beetle communities influence seed burial and seedling recruitment in tropical forests

“…Secondary dispersal by dung beetles has been demonstrated on a number of occasions to be beneficial to buried seeds [27,28,50]. However, contrary to our predictions, we show that functional richness, species richness and total biomass of beetle communities are negatively correlated to the emergence success of seedlings, suggesting that dung beetle activity may be detrimental for some species.…”

“…Seed predator escape is a key mechanism underpinning the increased germination success observed in seeds secondarily dispersed by dung beetles in tropical forests [27,28]. We found no evidence for this process in this investigation.…”

“…We predicted that highest germination would occur in microsites near the surface (from 1 cm to 4 cm), deep enough to reduce predation, yet shallow enough to avoid soil depth preventing emergence following germination (cf. [27,28]). …”

Dung beetles as secondary seed dispersers in a temperate grassland

“…Nonetheless, on the short-term scale, our experimental results clearly contrast the positive outcome found by Andresen and Levey (2004), who conducted the only field study following dung beetle mediated dispersal up to the point of seedling establishment until now. Evidently, differences in the dung beetle community, predator guild and/or plant species involved led to deviant patterns.…”

“…Since rollers are abundant in tropical regions (cf. Andresen & Levey, 2004) but absent from typical assemblages of northern temperate regions (Hanski, 1991), this presumed positive effect does not occur in the latter.…”