Urban land use decouples plant-herbivore-parasitoid interactions at multiple spatial scales.

Amanda E Nelson,Andrew A Forbes,Robert B Srygley

doi:10.1371/journal.pone.0102127

Abstract

Intense urban and agricultural development alters habitats, increases fragmentation, and may decouple trophic interactions if plants or animals cannot disperse to needed resources. Specialist insects represent a substantial proportion of global biodiversity and their fidelity to discrete microhabitats provides a powerful framework for investigating organismal responses to human land use. We sampled site occupancy and densities for two plant-herbivore-parasitoid systems from 250 sites across a 360 km2 urban/agricultural landscape to ask whether and how human development decouples interactions between trophic levels. We compared patterns of site occupancy, host plant density, herbivory and parasitism rates of insects at two trophic levels with respect to landcover at multiple spatial scales. Geospatial analyses were used to identify landcover characters predictive of insect distributions. We found that herbivorous insect densities were decoupled from host tree densities in urban landcover types at several spatial scales. This effect was amplified for the third trophic level in one of the two insect systems: despite being abundant regionally, a parasitoid species was absent from all urban/suburban landcover even where its herbivore host was common. Our results indicate that human land use patterns limit distributions of specialist insects. Dispersal constraints associated with urban built development are specifically implicated as a limiting factor.

Highlights

Global intensification of urban and agricultural land use has critical implications for biological diversity
We focused on two tree species native to North America, each of which is the exclusive host of a Rhagoletis fruit fly species, and each of which is in turn attacked by specialist parasitoid wasp species
Distributions of walnut fly herbivory rates differed significantly with levels of subsoil phosphorous so that more phosphorous was associated with lower herbivory (Kruskal-Wallis test: p = 0.026), but phosphorous levels did not vary with coarse landcover (Fisher exact test: p = 0.078)

Summary

Introduction

Global intensification of urban and agricultural land use has critical implications for biological diversity. Insects provide ecosystem services (including pest control, pollination, waste decomposition, and a food source for other important species) worth an estimated $57 billion annually in the U.S [1]. As intensity of development increases on a gradient from natural systems to agricultural land to urban landcover, patches of suitable habitat tend to become smaller in area and increasingly isolated [7]. Agricultural landcover often contains inadequate spatial and temporal plant resource diversity (i.e. food, habitat) to sustain diversity at higher trophic levels [8]. Plant resources tend to be highly diverse but configured in complex mosaics of disparate habitat types often characterized by frequent disturbance as well as altered biogeochemical, hydrological, and temperature regimes [9]

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