Global patterns of pre-dispersal propagule predation in mangrove forests

Mangroves in disparate families produce viviparous seedlings (propagules) that are attacked by many crab and insect predators both before and after dispersal. While post-dispersal predation is viewed as an important factor in structuring many mangrove communities, pre-dispersal predation rates and agents have been characterized for few species. Ten species of mangrove and 3299 propagules were surveyed for pre-dispersal propagule predation at 42 sites around the world. Pre-dispersal predation rates were variable among sites and species, ranging from 0 to 93 percent within stands, with a global total predation rate of 23.3 percent (across all propagules examined) and a mean level of 28.3 percent across sites. Grapsid crabs, Coleoptera and Lepidoptera were the primary predators identified. Forests near human population centers and stands occurring at high intertidal sites exhibited higher levels of propagule predation than those in unpopulated or low-intertidal sires. Predation rates on a species were weakly, negatively correlated with conspecific seedling density at a sire. To explore temporal variation in, and ramifications of pre-dispersal predation for propagule growth and abscission dynamics, Rhizophora mangle propagules were monitored over two years at three sires in Belize, Central America. Predation did not significantly reduce hypocotylar growth of germinated propagules on the parent tree, but nearly doubled the abscission race of premature propagules. Pre-dispersal propagule predation is a ubiquitous feature of mangrove forests world-wide, and must be accounted for in estimates of reproductive output, stand health, and propagule availability for forestry and restoration efforts.