Latitudinal Patterns of Herbivory in Mangrove Forests: Consequences of Nutrient Over-Enrichment

Ecosystems in the tropics are predicted to have stronger responses to nutrient enrichment, greater diversity, and more intense biotic interactions than in temperate areas. Mangrove forests, which occur across a broad biogeographic range from warm temperate to tropical, provide a unique opportunity to test these hypotheses by investigating the responses of herbivores to nutrient enrichment in temperate versus tropical latitudes. Mangroves are complex intertidal ecosystems with spatial differences in structure and diversity along tidal gradients and are threatened globally by human activities including nutrient over-enrichment. In this study, we used long-term fertilization experiments at the Indian River Lagoon, FL; Twin Cays, Belize; and Bocas del Toro, Panama to determine how increased nutrients impact herbivore abundance and herbivory of Rhizophora mangle at the tree, forest, and regional scales. At these locations, which span approximately 2185 km and 18.4A(0) of latitude, we fertilized individual trees with one of three treatments (Control, +N, +P) in two zones (fringe, scrub) along transects perpendicular to the shoreline and measured their responses for 4 years. Herbivory was measured as folivory, loss of yield, and tissue mining. Although nutrient enrichment altered plant growth, leaf traits, and nutrient dynamics, these variables had little effect on folivory at any location. Our results did not support the prediction that herbivory and per capita consumption are greatest at the most tropical location. Instead, folivory was highest at the most temperate location and lowest at the intermediate location. Folivory was generally higher in the fringe than in the scrub zone, but the pattern varied by location, herbivore, and nutrient treatment. Folivory by a dominant herbivore, Aratus pisonii, decreased from the highest to the lowest latitude. Our data suggest that factors controlling population dynamics of A. pisonii cascade to the mangrove canopy, linking herbivory to crab densities.