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Anthropogenic fragmentation of landscapes: mechanisms for eroding the specificity of plant–herbivore interactions

  • Special Topic: From Plants to Herbivores
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Reduced ecological specialization is an emerging, general pattern of ecological networks in fragmented landscapes. In plant–herbivore interactions, reductions in dietary specialization of herbivore communities are consistently associated with fragmented landscapes, but the causes remain poorly understood. We propose several hypothetical bottom–up and top–down mechanisms that may reduce the specificity of plant–herbivore interactions. These include empirically plausible applications and extensions of theory based on reduced habitat patch size and isolation (considered jointly), and habitat edge effects. Bottom–up effects in small, isolated habitat patches may limit availability of suitable hostplants, a constraint that increases with dietary specialization. Poor hostplant quality due to inbreeding in such fragments may especially disadvantage dietary specialist herbivores even when their hostplants are present. Size and isolation of habitat patches may change patterns of predation of herbivores, but whether such putative changes are associated with herbivore dietary specialization should depend on the mobility, size, and diet breadth of predators. Bottom–up edge effects may favor dietary generalist herbivores, yet top–down edge effects may favor dietary specialists owing to reduced predation. An increasingly supported edge effect is trophic ricochets generated by large grazers/browsers, which remove key hostplant species of specialist herbivores. We present empirical evidence that greater deer browsing in small forest fragments disproportionately reduces specialist abundances in lepidopteran assemblages in northeastern USA. Despite indirect evidence for these mechanisms, they have received scant direct testing with experimental approaches at a landscape scale. Identifying their relative contributions to reduced specificity of plant–herbivore interactions in fragmented landscapes is an important research goal.

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We thank Miranda Davis, Howard Kilpatrick, Sabrina Strom, Crystal Wright, Quinn Brencher, and Daniel Grogan for their contributions to the ideas and collection of data presented in this paper. This work was funded by the National Science Foundation (Grant Number DEB-1557086). We are grateful to Colin Orians for inviting this paper. The paper was substantially improved by suggestions from Dr. Orians and three anonymous reviewers. The authors declare that they have no conflict of interest.

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RB, MSS, DLW, and CSE conceived the idea and obtained funding; RB and MSS drafted the initial manuscript, and LMB, CSE, and DLW contributed sections and revisions; and RB performed statistical analysis and RB and MSS developed the figures.

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Correspondence to Robert Bagchi.

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Communicated by Colin Mark Orians.

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Bagchi, R., Brown, L.M., Elphick, C.S. et al. Anthropogenic fragmentation of landscapes: mechanisms for eroding the specificity of plant–herbivore interactions. Oecologia 187, 521–533 (2018).

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