Effects of native and exotic congeners on diversity of invertebrate natural enemies, available spider biomass, and pest control services in residential landscapes

Exotic plants are common in urban landscapes and are often planted by landscape managers in an effort to reduce herbivory damage and improve landscape aesthetics. However, exotic plants may be less palatable to many native insects and reduce herbivore biomass that may fuel higher trophic levels. Furthermore, a loss of herbivores in exotic ornamental landscapes may reduce top-down control by natural enemies. In this study, we compare herbivory in native and exotic congener ornamental landscapes. We also explore if caterpillar abundance, natural enemy abundance, diversity, community composition, spider biomass, and egg predation differ between the two landscape types. We predicted that herbivory, as well as natural enemy abundance and predation would be greater in native landscapes. Although we found that leaf area lost to herbivory was greater in native plots in one of the collection years, this relationship was weak. Natural enemy diversity differed between landscape types, but depended on plant genus. The relationship between plant origin and natural enemy diversity was also weak. Caterpillar abundance, natural enemy community composition, spider biomass, and predation services did not differ between treatments. Overall, our results suggest that ornamental landscapes planted in native plants may not differ from landscapes planted in exotic congeners in the pest management and conservation services they provide, particularly with regard to invertebrate natural enemies. However, our findings cannot be used to make more general claims about plant origin, especially with regard to native plants and non-congeners, as we only compared ornamental landscapes with native plants and their exotic congeners in this study. We conclude that for optimizing natural enemy diversity and biomass on city landscapes, plant choice and other plant traits may be as important as plant origin to consider. Our work demonstrates that both native and exotic congener ornamental landscapes provide valuable ecosystem services and will help guide landscape design that serves both the people and wildlife that use them.