Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Abstract:

In tropical forests, herbivorous arthropods remove between 7% up to 48% of leaf area, which has forced plants to evolve defense strategies. These strategies influence the palat- ability of leaves. Palatability, which reflects a syndrome of leaf traits, in turn influences both the abundance and the mean body mass not only of particular arthropod taxa but also of the total communities. In this study, we tested two hypotheses: (H1) The abundance of two important chewer guilds (‘leaf chewers’ and ‘rostrum chewers’), dominant components of arthropod communities, is positively related to the palatability of host trees. (H2) Lower pal- atability leads to an increased mean body mass of chewers (Jarman-Bell principle). Arthro- pods were collected by fogging the canopies of 90 tropical trees representing 31 species in three plots at 1000 m and three at 2000 m a.s.l. Palatability was assessed by measuring several ‘leaf traits’ of each host tree and by conducting a feeding trial with the generalist her- bivore Gryllus assimilis (Orthoptera, Gryllidae). Leaf traits provided partial support for H1, as abundance of leaf chewers but not of rostrum chewers was positively affected by the experi- mentally estimated palatability. There was no support for H2 as neither leaf traits nor experi- mentally estimated palatability affected the mean body mass of leaf chewers. The mean body mass of rostrum chewers was positively related to palatability. Thus, leaf traits and experimentally estimated palatability influenced the abundance and mean body mass of chewing arthropods on the community level. However, the data were not consistent with the Jarman-Bell principle. Overall, our results suggest that the palatability of leaves is not among the dominant factors influencing abundance and mean body mass of the community of chewing arthropod herbivores. If other factors, such as the microclimate, predation or fur- ther (a-)biotic interactions are more important has to be analyzed in refined studies.

Abstract:

Herbivores play a key role for ecosystem functioning, but the current destruction of late-successional primary forests might negatively affect herbivore-mediated ecosystem processes. Understanding herbivore patterns between different succession stages is crucial for setting conservation priorities. Here we examined insect herbivore biomass and functional community composition along a successional gradient in a tropical montane rainforest in southern Ecuador. The insects were sampled off 42 tree individuals via canopy fogging. The trees comprised three succession stages (9 early, 25 intermediate, 18 late) and 16 species. Insect herbivores were identified on order level and assigned to one of three feeding guilds (leaf chewers, rostrum chewers, sapsuckers). Overall, we collected almost 16 000 herbivore individuals. Herbivore dry mass was not significantly affected by tree succession stage for almost all groups surveyed, except for caterpillars which weighed less on late- than on early-successional tree species. Furthermore, herbivore community composition was neither explained by tree succession stage nor by tree phylogeny. We conclude that solely conserving forests of late succession is not always expedient and that factors need to be identified which predict herbivory and herbivore community composition in tropical montane rainforests.

Abstract:

Caterpillar ensembles were sampled on 16 species of shrubs from the family Asteraceae and the genus Piper (Piperaceae) in open and forest habitats in the Andean montane forest zone of southern Ecuador between August 2007 and May 2009. Trophic affiliations of caterpillars to the host plants were confirmed in feeding trials. Overall, species richness of herbivorous caterpillars was high (191 species across all plants), but varied strongly between ensembles associated with different plant species (2–96 lepidopteran species per shrub species). Ensembles on Piper species were characterized by low effective species numbers and high dominance of one or two species of the Geometridae genus Eois Hübner. Low species number and high dominance were also found on latex-bearing Erato polymnioides, whereas ensembles on two other Asteraceae species were far more diverse and less strongly shaped by a few dominant species. The observed diversity patterns fit well to the concept that anti-herbivore defenses of plants are the major factors regulating associated insect ensembles. Local abundance and geographic range of host plants appear to have less influence. Lepidopteran species feeding on Asteraceae were found to be more generalistic than those feeding on Piper species. We conclude that caterpillar ensembles on most, but not all, studied plant species are defined by a small number of dominant species, which usually are narrow host specialists. This pattern was more distinct on Piper shrubs in forest understory, whereas Asteraceae in disturbed habitats had more open caterpillar ensembles.