title: Predators modify biogeographic constraints on species distributions in an insect metacommunity.
abstract: Theory describing the positive effects of patch size and connectivity on diversity in fragmented systems has stimulated a large body of empirical work, yet predicting when and how local species interactions mediate these responses remains challenging. We used insects that specialize on milkweed plants as a model metacommunity to investigate how local predation alters the effects of biogeographic constraints on species distributions. Species-specific dispersal ability and susceptibility to predation were used to predict when patch size and connectivity should shape species distributions, and when these should be modified by local predator densities. We surveyed specialist herbivores and their predators in milkweed patches in two matrix types, a forest and an old field. Predator-resistant species showed the predicted direct positive effects of patch size and connectivity on occupancy rates. For predator-susceptible species, predators consistently altered the impact of biogeographic constraints, rather than acting independently. Finally, differences between matrix types in species' responses and overall occupancy rates indicate a potential role of the inter-patch environment in mediating the joint effects of predators and spatial drivers. Together, these results highlight the importance of local top-down pressure in mediating classic biogeographic relationships, and demonstrate how species-specific responses to local and regional constraints can be used to predict these effects.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada.
authors.forename: Tess Nahanni
authors.initials: TN
authors.lastname: Grainger
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada.
authors.forename: Rachel M
authors.initials: RM
authors.lastname: Germain
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada.
authors.forename: Natalie T
authors.initials: NT
authors.lastname: Jones
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S 3B2, Canada.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Gilbert
mesh_terms: D000818:Animals; D017753:Ecosystem; D007313:Insecta / Q000502:physiology*; D011157:Population Dynamics; D011235:Predatory Behavior; D013045:Species Specificity
keywords: 
  Asclepias syriaca
  ; Island Biogeography; biotic interactions; dispersal; matrix; metacommunity; metapopulation; milkweed; predation; species distributions; top-down