Associational resistance protects mangrove leaves from crab herbivory
Associational defenses are well documented in terrestrial plant and algal ecosystems but not in mangroves. Mangrove tree crab (Aratus pisonii) density and herbivory on the red mangrove (Rhizophora mangle) were documented in pure red versus mixed-species and predominantly non-red mangrove stands containing black (Avicennia germinans) and white (Laguncularia racemosa) mangroves in 1999-2000 in Tampa Bay, Florida. This study established that R. mangle is the focal species in the context of associational resistance because it is damaged more than other mangrove species. It was hypothesized that crab density and leaf damage on R. mangle would be lower in mixed-species and non-red versus red mangrove stands.
A non-significant trend suggested that crab density was lower in mixed-species and non-red stands, while crab damage on R. mangle leaves was significantly lower in both of them. Mechanisms to explain associational resistance were examined. Positive Pearson correlations between the percent of adult R. mangle in a stand and both crab density and leaf damage provided support for the resource concentration hypothesis. Limited support was found for the attractant–decoy hypothesis; the total amount of damaged leaves of all mangrove species combined typically differed among stands, suggesting that crabs were not shifting to alternative mangrove species to offset reduced availability of R. mangle leaves. Finally, intra-specific differences in R. mangle leaf chemistry and sclerophylly among stands failed to explain associational patterns. Experiments that elucidate mechanisms responsible for defensive plant associations in mangrove ecosystems and determine whether such associations could be used in mangrove restoration are needed.