In the Midwestern US, forested and other woody plant habitats are embedded in a matrix of agricultural and urban landcover that alters configurations of “natural” habitats and creates novel habitat types. Variation in the type and juxtaposition of landcover in the matrix between habitats can profoundly impact the spatial and temporal distributions of insects. 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. Specialist herbivores and parasitoids that depend on discrete plant habitats simplify assessment of how trophic interactions, local demographic traits, and dispersal processes affect responses to landcover heterogeneity. Herbivore responses to landcover change are highly idiosyncratic and not well characterized. Parasitoid wasps are predicted to be more prone than their herbivore hosts to local extinction in response to increased habitat fragmentation, but often respond differently to similar landcover contexts. Understanding and predicting idiosyncratic spatial population dynamics of simple host-parasitoid communities and other insect systems requires integration of metacommunity-level ecological paradigms with spatial analyses across multiple spatial scales.
We sampled site occupancy and densities for two plant-herbivore-parasitoid systems from 250 sites across a 360 km2 urban/ agricultural landscape across three study years to ask whether and how human development decouples interactions between trophic levels. We first performed a single year analysis to investigate broad scale patterns. 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.
Our multi-year analysis of trophic interactions in urban versus agricultural landcover showed that important results from our single-year study are consistent over time and provided useful insights into the factors mediating spatial distributions of specialist insects in altered landscapes. While we observed that insect species responded to landcover at consistent local- and landscape-scale spatial extents, we observed that coarse grain landcover categories (i.e. urban versus agricultural) at low spatial resolution yielded the most consistent patterns of organismal response. Our results indicate that agricultural versus urban landcover contexts can mediate distinct spatial population structuring across linked trophic levels. This finding has important implications for conservation and pest management strategies in heterogeneous landscapes and is an important consideration when translating heuristics regarding metacommunity dynamics from one broad spatial context to another.