Influence of human disturbance on patterns of leaf herbivory at Gazi Bay mangrove forest, Kenya

Abstract

Mangrove herbivores cause leaf serration, perforation and galls prior to leaf abscission. This study compared damage by herbivores on leaves of four mangrove species at sites under different levels of human physical disturbance, and provides further evidence of the indirect effects of man on these valuable habitats. From 2001 to 2003 leaves collected fortnightly using litter traps were examined for evidence of herbivore damage, e.g. holes, margin damage, and a mix of both holes and margin damage on the leaf, and compared using ANOVA. Of 10 600 leaves examined, 3 604 had evidence of herbivore attack, with margin damage being most prevalent (60%). On Rhizophora mucronata, holes and mixed damage occurred on over 30% of damaged leaves, while margin damage was common to all other species examined. Leaf attack intensity was 14% lower at undisturbed sites, with margin damage being most common. Comparison between corresponding mangrove forest sites in Kenya showed significant increase in margin damage and concurrent decline in holes and mixed damage at disturbed sites. Variation in leaf damage occurrence was attributed to changes in forest structural and biological complexity owing to selective harvesting of flora and fauna that influenced the competitive ability and prevalence of aerial herbivorous guild.