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Abstract Ecological theory predicts that herbivory should be weaker on islands than on mainland based on the assumption that islands have lower herbivore abundance and diversity. However, empirical tests of this prediction are rare, especially for insect herbivores, and those few tests often fail to address the mechanisms behind island–mainland divergence in herbivory. In particular, past studies have not addressed the relative contribution of top‐down (i.e. predator‐driven) and bottom‐up (i.e. plant‐driven) factors to these dynamics.To address this, we experimentally excluded insectivorous vertebrate predators (e.g. birds, bats) and measured leaf traits associated with herbivory in 52 populations of 12 oak (Quercus) species in three island–mainland sites: The Channel Islands of California vs. mainland California, Balearic Islands vs. mainland Spain, and the island Bornholm vs. mainland Sweden (N = 204 trees). In each site, at the end of the growing season, we measured leaf damage by insect herbivores on control vs. predator‐excluded branches and measured leaf traits, namely: phenolic compounds, specific leaf area, and nitrogen and phosphorous content. In addition, we obtained climatic and soil data for island and mainland populations using global databases. Specifically, we tested for island–mainland differences in herbivory, and whether differences in vertebrate predator effects or leaf traits between islands and mainland contributed to explaining the observed herbivory patterns.Supporting predictions, herbivory was lower on islands than on mainland, but only in the case of Mediterranean sites (California and Spain). We found no evidence for vertebrate predator effects on herbivory on either islands or mainland in any study site. In addition, while insularity affected leaf traits in some of the study sites (Sweden‐Bornholm and California), these effects were seemingly unrelated to differences in herbivory.Synthesis. Our results suggest that vertebrate predation and the studied leaf traits did not contribute to island–mainland variation patterns in herbivory, calling for more nuanced and comprehensive investigations of predator and plant trait effects, including measurements of other plant traits and assessments of predation by different groups of natural enemies.
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Synergistic effects of canopy chemistry and autogenic soil biota on a global invader
Abstract Soil biota have strong effects on plants, but we have a poor understanding of how plant chemistry might modify these effects. We examined the effect of soil biota associated with an exotic invasive tree,Prosopis juliflora, versus that associated with native species, from seven sites across India on conspecifics and two other plant species. We then measured changes in species‐specific soil biota effects (identified as plant–soil feedbacks, PSFs) when leaf leachate fromP. julifloraor from native plant species was added to soil containing respective live and sterile soil inoculum.We quantified the amino acid L‐tryptophan from leaf leachate ofP. juliflora,Leucaena leucocephala(another invader), and two native species. We also tested effects ofP. julifloraor native species soil inoculum amendment of tryptophan onP. juliflora,P. cinerariaandL. leucocephalaacross seven sites. We then quantified the microbially metabolized derivatives of tryptophan, phytohormone indole‐3‐acetic acid (IAA) and intermediates after adding tryptophan intoP. julifloraand native soils.Soil biota associated withP. julifloragenerated positive effects on conspecifics andL. leucocephala, but negative effects on the native congenerP. cineraria. WhenP. julifloraleaf leachate was added to soil with liveP. juliflorainoculum, PSFs became more positive forP. julifloraand other species, compared to leaf leachate amended with sterile soil inoculum. Native leaf leachate interacted weakly with soil biota to impact biomass of conspecifics and heterospecifics.There was roughly 10× more tryptophan in the leaf leachate ofP. juliflorathan in the leaf leachate of other species. Tryptophan generally increased positive PSFs associated withP. juliflorarelative to soil biota associated with other plant species. When tryptophan was added to liveP. juliflorasoil, IAA and its intermediates were produced at five of seven sites, and at four of these sites soil biota fromP. juliflorahad positive PSFs.Synthesis. These results provide the first experimental evidence that a chemical leached from the leaves of an invader regulates PSFs. Our results indicate that canopy effects and PSFs, which are usually studied independently, can interact in ways that strongly affect conspecifics and neighbouring species.
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- Award ID(s):
- 1757351
- PAR ID:
- 10485214
- Publisher / Repository:
- British Ecological Society
- Date Published:
- Journal Name:
- Journal of Ecology
- Volume:
- 111
- Issue:
- 7
- ISSN:
- 0022-0477
- Page Range / eLocation ID:
- 1497 to 1513
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1111/1365-2745.14113
