Spatiotemporal patterns of insect diversity and multitrophic interactions across a tree diversity gradient

Abstract

Throughout the last decade, linking biodiversity with ecosystem functioning and services, such as pollination and biocontrol of pest species, has gained major attention in ecological research. In particular biodiversity loss due to human land use and disturbance has been determined as a major threat, eventually leading to a decline in ecosystem services and functions. The major proportion of global biodiversity can be found in forest ecosystems, and especially in tropical regions pristine forest habitats become more and more scarce. In temperate regions the situation is hardly any better: even though in Germany 30 % of land surface is covered by forest, the proportion of old-growth natural forest equals zero. Hence it is of immediate importance to identify the contribution of the remaining semi-natural forests to biodiversity and ecosystem functioning. Investigations in tropical rainforests have shown that insects in the forest canopy contribute highly to global biodiversity, but data on canopy fauna in temperate deciduous forests are scarce. Even though plant diversity in agricultural or experimental grassland habitats has often been related to insect diversity, no information is available whether the same relationship exists in forest habitats. In the study on hand, we related diversity and function of canopy and understory insect communities to a natural tree diversity gradient ranging from simple beech to mixed deciduous forest stands. The scope of our investigation was to analyse effects of canopy tree diversity on insect species richness, community structure and multitrophic interactions across various spatiotemporal scales in a real-world ecosystem, based on two major hypotheses: (1) insect diversity increases with increased canopy tree diversity, (2) herbivory decreases with increased canopy tree diversity. The study was conducted using 12 forest plots in the Hainich National Park, Germany s largest remaining semi-natural deciduous forest. A variety of methods was applied to sample the resident insect fauna, such as flight-interception traps, beating samples, and trap nests for cavity-nesting hymenopterans. Furthermore, herbivory of tree saplings was estimated (leaf area loss, abundance of galls and mines). This approach ensured that a variety of taxa (beetles, true bugs, bees, wasps and their natural enemies) and functions (herbivory, predation, parasitism) was included in the analyses. Total species richness (γ-diversity) of beetles and true bugs increased across the tree diversity gradient as hypothesised, and the same pattern was found for most functional groups (e.g forest specialists, predators etc.). Species richness of cavity-nesting bees and wasps did not respond to tree diversity. For beetles and true bugs, species turnover (β-diversity) in space and time contributed most (~90 %) to overall γ-diversity. Turnover between tree individuals was high if a sufficient number of tree replicates was analysed, even among trees of the same species in simple beech stands. Besides stand-level heterogeneity, between-tree heterogeneity apparently plays a major role in determining insect species richness in deciduous forests. Furthermore, the forest stands showed a distinct stratification of bee and wasp abundance, community composition, and parasitism rates. Enhanced resource availability (e.g. nesting sites, host and prey abundance) might be one reason why increased tree diversity also controls species richness, but not function of higher trophic levels. Concerning herbivory damage on tree saplings in the forest understory, leaf area loss was generally higher in maple compared to beech saplings, but only beech showed a decline in damage across the tree diversity gradient. No pattern was found for galls and mines. Relative abundance of predators on beech showed a seasonal response and increased on species-rich plots in June, suggesting higher biological control. Rather than tree diversity per se, the identity and abundance of the sampled tree species in most cases was a better predictor for the observed responses, and effects can be explained by mechanism of resource concentration and abundance of natural enemies (enemies hypothesis). In conclusion, our study gives new insights into the effects of tree diversity on insect species richness and multitrophic interactions. Although planted experiments are of great value for determining fundamental mechanisms behind observed effects, these effects might not occur or run differently in undisturbed, semi-naturally established forests, since planted experiments are insufficient in imitating the age structure and spatial heterogeneity of natural forests. By studying one of Europe s largest semi-natural deciduous forests we could demonstrate that complex biodiversity-functioning relationships are effective in real-world forest ecosystems, and that temporal and spatial species turnover is the key to understanding biodiversity patterns. It became evident that simple beech stands alone are unsatisfactory in conserving the full set of regional insect species richness. We suggest that sustainable forest management should aim at maintaining a diverse mix of structurally different tree species and individuals, thereby enhancing habitat heterogeneity and providing resources for a diverse and highly functional insect community.