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Generalists are more specialized in low-resource habitats, increasing stability of ecological network structure
Edited by Rodolfo Dirzo, Department of Biology, Stanford University, Stanford, CA, and approved December 12, 2019 (received for review November 29, 2018)

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Significance
The degree to which food webs are divided into “modules” of strongly interacting species is an important property conferring stability in ecological communities; however, the mechanisms causing variation in this emergent property are poorly understood in nature. Here, we document temporally consistent differences in plant-herbivore network modularity across two soil types. Next, we parse the mechanistic basis of this structural change between evolutionary constraint (e.g., prevalence of species with specialized versus generalized diet breadth) and interaction plasticity (e.g., shifts in “realized” diet breadth of generalist species). Our findings broaden understanding of abiotic context as a driver of food web structure, suggesting that higher modularity—and thus greater stability—may be a general network property in low-resource contexts.
Abstract
Linking mechanistic processes to the stability of ecological networks is a key frontier in ecology. In trophic networks, “modules”—groups of species that interact more with each other than with other members of the community—confer stability, mitigating effects of species loss or perturbation. Modularity, in turn, is shaped by the interplay between species’ diet breadth traits and environmental influences, which together dictate interaction structure. Despite the importance of network modularity, variation in this emergent property is poorly understood in complex natural systems. Using two years of field data, we quantified interactions between a rich community of lepidopteran herbivores and their host plants across a mosaic of low-resource serpentine and high-resource nonserpentine soils. We used literature and our own observations to categorize herbivore species as generalists (feeding on more than one plant family) or specialists (feeding on one plant family). In both years, the plant-herbivore network was more modular on serpentine than on nonserpentine soils—despite large differences in herbivore assemblage size across years. This structural outcome was primarily driven by reduction in the breadth of host plant use by generalist species, rather than by changes in the composition of species with different fundamental diet breadths. Greater modularity—and thus greater stability—reflects environmental conditions and plastic responses by generalist herbivores to low host plant quality. By considering the dual roles of species traits and ecological processes, we provide a deeper mechanistic understanding of network modularity, and suggest a role for resource availability in shaping network persistence.
- ecological networks
- plant-herbivore interactions
- network modularity
- resource availability hypothesis
- herbivore diet breadth
Footnotes
- ↵1To whom correspondence may be addressed. Email: mrobinson{at}ucdavis.edu.
Author contributions: M.L.R. and S.Y.S. designed research; M.L.R. performed research; M.L.R. analyzed data; and M.L.R. and S.Y.S. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
Data deposition: Data used in this paper can be found on Figshare, https://doi.org/10.6084/m9.figshare.11492343.v1. All code associated with these analyses can be found at GitHub, https://github.com/moria-robinson/PH-soil-networks-2019.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1820143117/-/DCSupplemental.
Published under the PNAS license.
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