The Effect of Seed Predators on Plant Distributions: Is There a General Pattern in Mangroves?

Abstract

Selective crab predation has been hypothesized to maintain mangrove forest zonation by preventing the establishment of certain species' seedlings at particular tidal elevations. Support for this hypothesis consists of observations that the propagules of some mangrove species are consumed at a rate that is inversely related to the species' relative abundance in the adult canopy. In addition, one previous study found that rates of crab predation were lower in large light gaps than in small ones. We conducted a mensurative field experiment to assay rates of crab predation on dispersing propagules of the dominant mangrove species (Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle) in our study forests at Punta Galeta on the Caribbean coast of Panama. The experiment specifically examined whether rates of crab predation varied 1) with species of propagule, 2) with changes in stand composition along the tidal gradient, and 3) inside versus outside of light gaps. Lightning-created canopy gaps are the primary sites of tree regeneration in these and other tall-stature mangrove forests. Propagules of the three mangrove species were tethered inside and in the understory surrounding nine light gaps, with representatives in each of the major stand types that occur along the intertidal gradient. Fates of propagules were monitored over a four-wk period. Rates of propagule predation varied spatially, with highest rates of removal occurring in lower intertidal R. mangle/L. racemosa stands, where the large herbivorous crabs, Ucides cordatus and Goniopsis cruentata are common. In mid- and upper intertidal stands, dominated by A. germinans or L. racemosa, where detritivorous Uca spp. are abundant but larger herbivorous crabs are rare, few propagules were consumed. The propagules of A. germinans were preyed upon more heavily than those of the other two species; rates of predation on R. mangle and L. racemosa propagules did not differ. Light environment (i.e. inside versus outside a gap) had no significant effect on the rate of propagule predation. The enhanced survival of juvenile mangroves in light gaps that we have observed is probably due to other factors such as higher light and nutrient availability, and in some situations, lower insect herbivory. Only the propagules of A. germinans experienced a pattern of predation consistent with the dominance-predation model: they were consumed at higher rates in the low intertidal where adults of the species are rare, and at lower rates in high intertidal areas where the species dominates the canopy. This suggests that predation on mangrove propagules by herbivorous crabs could influence the species composition of low intertidal forests by differentially reducing the recruitment of A. germinan On the other hand, rates of crab predation on A. germinans propagules seem insufficient to prevent A. germinans from establishing in low intertidal areas. In opposition to the dominance-predation hypothesis, crab predation cannot account for the distribution patterns of R. mangle or L. racemosa whose propagules were preyed on most heavily in lower intertidal areas where these species are very abundant. Our results, together with those of two earlier studies, indicate that predation by crabs on dispersing propagules is not a general explanation for the zonation of tree species along tidal gradients in mangrove forests.