Enrichment scale determines herbivore control of primary producers
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Anthropogenic nutrient enrichment stimulates primary production and threatens natural communities worldwide. Herbivores may counteract deleterious effects of enrichment by increasing their consumption of primary producers. However, field tests of herbivore control are often done by adding nutrients at small (e.g., sub-meter) scales, while enrichment in real systems often occurs at much larger scales (e.g., kilometers). Therefore, experimental results may be driven by processes that are not relevant at larger scales. Using a mathematical model, we show that herbivores can control primary producer biomass in experiments by concentrating their foraging in small enriched plots; however, at larger, realistic scales, the same mechanism may not lead to herbivore control of primary producers. Instead, other demographic mechanisms are required, but these are not examined in most field studies (and may not operate in many systems). This mismatch between experiments and natural processes suggests that many ecosystems may be less resilient to degradation via enrichment than previously believed.
KeywordsTop-down versus bottom-up Eutrophication Consumer–resource dynamics Experimental bias Ideal free distribution Ecological resilience
Support was provided by a National Science Foundation (NSF) Graduate Research Fellowship (DGE-0802270), a Florida Sea Grant Fellowship, and NSF Grant OCE-1130359. We thank B. R. Silliman, T. Frazer, R. Fletcher, and anonymous reviewers for constructive comments on previous versions of this manuscript, and N. Hackney for assistance with our literature review.
Author contribution statement
M. A. G. and C. W. O. conceptualized the study; M. A. G., C. W. O. and J. J. developed the model; M. A. G. drafted the manuscript, and all authors revised the text.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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