Mangrove Leaf Herbivory along a Hydrocarbon Pollution Gradient in a Mangrove Forest (Rhizophora racemosa) in the Niger River Delta, Nigeria

This study was done to test the palatability of leaves of mangroves growing in a highly and lowly polluted environment. We hypothesized that bottom-up transfer of pollutants from soil to leaves would influence herbivory due to the toxic effect of pollution. Exclusion experiment was done to test for leave herbivory in lowly and highly polluted plots while cafeteria experiment was done to determine the feeding preference of 20 West African red mangrove crabs (Goniopsis pelii). In the first experiment a total of 453 mangrove leaves were sampled for six months, one year and two years. Results indicate that there was significant difference in herbivory between highly and lowly polluted treatment in the six months (F2, 160=3.33, P=0.04), one year (F2, 184=1.90, P=0.02) and two years (F1, 169=7.58, P=0.01) samples. There was more leave herbivory in highly than in lowly polluted plot. The pattern of incisions indicates that crab herbivory was the highest (53%) followed by insects (25%) and undetermined (22%). In the laboratory-based experiment, there was significant difference in leave incisions from both plots (P=0.01) with more leaves from highly polluted plot consumed (4, 521.69 cm2) than leaves from lowly polluted plot (2, 769.83 cm2). This implies that hydrocarbon pollution influenced leaf herbivory.