Covariation between local and landscape factors influences the structure of ground-active arthropod communities in fragmented metropolitan woodlands

Abstract

Context: The world is becoming increasingly urbanized, with more than half of the global population now living in cities. Understanding the factors impacting natural communities in fragmented landscapes is therefore crucial for predicting how the remaining ecosystems will respond to global change. Ground-active arthropods, which are important in nutrient cycling, are likely sensitive to habitat changes resulting from urbanization. Objectives: We addressed two questions: (1) What is the relative importance of local and landscape factors in shaping ground-active arthropod communities in urban woodlands? (2) How does body size (as a surrogate for dispersal ability) affect sensitivity to landscape-level factors? Methods: In the summers of 2010 and 2011, we sampled ground-active arthropod communities in 19 woodlands in the Chicago metropolitan region using pitfall traps. We also assessed local plant and soil characteristics, as well as landscape-level variables using GIS. Results: Redundancy analyses and variation partitioning revealed that local factors, particularly invasive woody-plant cover and soil nitrate, had the most influence on arthropod communities, explaining 12% of the total variation. Of the landscape-level variables, landscape richness, which is one measure of landscape fragmentation, explained the most variation; however, the shared variance between landscape and local variables was responsible for half (16%) of the total explained variation (32%). Landscape factors alone explained only 4% of variation. No relationship between arthropod body size and landscape variables was observed, but several groups (e.g. ants and ground beetles) were correlated with landscape-level factors. Conclusions: Our research shows that both local and landscape variables are important in influencing ground-active arthropods, but the majority of explained variance is attributed to the covariation between landscape richness, invasive woody-plant cover, and soil nitrate. We therefore conclude that landscape fragmentation is likely affecting the ground-active arthropods through its positive influence on invasive woody plants and soil nitrogen.