Context

According to the trophic-rank hypothesis, species may be differentially affected by habitat isolation due to their trophic position in the food chain, i.e. high-order trophic levels may be more negatively affected than low-order levels.

Objectives

The aim of this paper is to study how species richness, abundance and composition of saproxylic beetle communities are affected by patch (=tree) quality and small-scale patch connectivity. Following the trophic-rank hypothesis, we expected predators to be more negatively affected by patch isolation than wood-feeding beetles.

Methods

We studied the beetle community, patch connectivity and patch quality on 28 large oaks. Different connectivity measures were calculated using 50 m-buffers around trees and using distances to the five nearest trees.

Results

Beetle species richness increased with the diameter of oaks, i.e. patch quality. No evidence of the trophic-rank hypothesis was found for species richness patterns. In accordance with the trophic-rank hypothesis, abundance of predatory beetles increased with patch connectivity but lower trophic levels were unaffected or even decreased with patch connectivity.

Conclusions

The structure of invertebrate communities on trees changes with small-scale patch connectivity due to a differential response of low-order and high-order trophic levels. Isolated trees are more exposed to the sun than the more connected trees, which may affect the beetles; however, it was impossible to distinguish the microclimatic from the spatial effects. Although scattered trees generally have a higher conservation value than trees in forests, we conclude that forest trees may be more important for certain trophic levels.