Gall-forming and free-feeding herbivory along vertical gradients in a lowland tropical rainforest: the importance of leaf sclerophylly
Ribeiro S; Basset Y
In contrast to most insect guilds gall-forming insects are thought to reach highest diversity on sclerophyllous vegetation such as Neotropical savannas and Mediterranean vegetation types. The water and nutrient stress endured by meristems of canopy trees in tall wet tropical rainforests may cause leaf sclerophylly. Hence the upper canopies of such ecosystems may represent a suitable habitat for gall-forming insects. At the San Lorenzo Protected Area Panama we estimated free-feeding herbivory and gall densities within five sites in 2003 and 2004 by surveying leaves in vertical and horizontal transects. In each sample we recorded leaf density (mature and young foliage) free-feeding herbivore damage and number of galls including the presence of live larvae parasitoids or fungi. We surveyed 43994 leaves including 231 plants and 73 tree and liana species. We collected 5014 galls from 17 host-plant species including 32 gall species of which 59% were restricted to the canopy (overall infestation rates: 2.4% in 2003 5.5% in 2004). In 2003 16% of the galls were occupied by live larvae against 5% in 2004. About 17-20% of leaves surveyed suffered from free-feeding herbivory. Leaf sclerophylly increased significantly with sampling height while free-feeding herbivory decreased inversely. Conversely the number of live galls collected in the canopy was 13-16 times higher than in the understorey a pattern consistent across sites and years. Hence the probability of gall survivorship increased with increasing leaf sclerophylly as death by fungi parasitoids or accidental chewing were greater in the understorey. Increasing harsh ecophysiological conditions towards the upper canopy appear favourable to galls-forming population maintenance in support of the hypothesis of harsh environment. Hence gall diversity and abundance in the upper canopy of tall tropical forests are perhaps among the highest in the world.