How urban forests shape arthropod communities: Essential roles for abiotic conditions and forest composition

Urban forests provide essential ecosystem services, including stormwater interception, shading and cooling, mitigation of pollution, recreational opportunities, habitat for wildlife, and community and individual well-being. Understanding factors that influence forest health is essential to protecting and enhancing the green spaces where over half the human population now lives. In this review, we describe the composition of urban forests, tracing the historical developments of urban forests and the patterns of tree density and diversity that have resulted. We next describe how herbivorous arthropods have responded to those patterns and identify overall trends, commonly studied taxa, and gaps in the literature. In cities, abiotic conditions such as the urban heat island effect, altered water availability, and pollution can directly affect herbivore-plant interactions and indirectly affect these interactions via impacts on natural enemies. Additionally, exotic trees are far more prevalent in urban compared to rural forests. With concurrent introductions of exotic herbivores, cities now contain a mix of native and exotic plants and native and exotic herbivorous arthropods. Overlaying new combinations of native-native, native-exotic, and exotic-exotic plant-herbivore interactions on top of altered abiotic conditions leads to a wide variety of outcomes between plants and herbivores in cities. These outcomes run the gamut from herbivore outbreaks and severe damage to host plants, to robust herbivore and natural enemy communities, to urban plants that are nearly devoid of arthropod communities. We highlight conditions that may foster each of these outcomes and how abiotic and biotic aspects of urban forests may cascade to higher trophic levels.