BayCEER Workshop 2016

Contrasting patterns of insect herbivory and predation pressure across a rainfall gradient

Introduction 

Density-dependent insect herbivory, being highest on locally abundant tree species while granting rare species an advantage, has been suggested to maintain high tree diversity of tropical forests. Theory further suggests that higher insect herbivory in aseasonal, wet forests may drive the increase in tree diversity with rainfall. In contrast, intensified top-down control in wetter forests has been proposed to reduce the impact of herbivores on tree species.

Material and Methods

To investigate how herbivore imposed Janzen-Connell effects vary with rainfall, we quantified insect herbivory in six forests spanning a rainfall gradient across the Isthmus of Panama, and related herbivory to the local abundance of 56 tree species. To evaluate the impact of predation pressure on herbivory patterns, we measured predation on artificial caterpillars in two of the forests. 

Results

Abundant tree species were more often attacked by herbivores, but showed lower amounts of herbivore damage than rare species. Herbivory strongly decreased with increasing rainfall. Conversely, predation pressure was higher at the wet site. 

Conclusions

Insect herbivory unlikely drives increasing tree diversity with rainfall. The contrasting patterns of herbivory and predation support the need for studies on multitrophic interactions across environmental gradients to understand the processes contributing to tree diversity and predict ecosystem responses to future climatic changes.