With their dense tangle of roots, trees and shrubs in mangrove forests help to stabilize coastlines all across the equator region by reducing erosion from daily tides and major storm surges alike. But they’re disappearing faster than tropical rainforests, and according to a recent Hydrobiologia study, efforts to restore native mangrove forests are being gobbled up by seed-eating crabs.

To study the importance of seed predation by crabs on mangroves, UC Berkeley’s Emily Dangremond conducted an experiment with propagules (or seedlings) of the rare neotropical mangrove Pelliciera rhizophorae, which occur in freshwater-influenced sites. The propagules were placed in forests that were either dominated by P. rhizophorae or low-salinity Rhizophora mangle on the Caribbean and Pacific coasts of Panama.

On the Caribbean coast, crabs ate 86.7% of the propagules in the R. mangle forest but only 3.3% in the P. rhizophorae area. Meanwhile, on the Pacific side, crabs ate 90% of propagules in the R. mangle area and 66.7% in the P. rhizophorae forest. But when Dangremond kept out crabs using “exclosures” made of nylon mesh, there wasn’t much difference in seedling survival between the different forest types. This means that rather than affecting seedling survival rates, predation by the crabs limits P. rhizophorae populations after they have been established.

"People often attempt restoration projects by planting mangrove propagules," Dangremond tells New Scientist. "In those cases, it might be useful to exclude crabs." R. mangle produces many, many more propagules than P. rhizophorae, and these may keep the crustacean satisfied and provide a refuge from crab predation for the rarer mangrove.

Some experts disagree, however, noting that crabs and mangroves have evolved together for millions of years. In addition, some locals think that the crab harvest is a key reason to protect mangroves in the first place. 

[Via New Scientist]