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Abstract Many critical drivers of ecological systems exhibit regular scaling relationships, yet the underlying mechanisms explaining these relationships are often unknown. Trophic interaction strengths, which underpin ecosystem stability and dynamics, are no exception, exhibiting statistical scaling relationships with predator and prey traits that lack causal, evolutionary explanations. Here we propose two universal rules to explain the scaling of trophic interaction strengths through the relationship between a predator’s feeding rate and its prey’s density --- the so-called predator functional response. First, functional responses must allow predators to meet their energetic demands when prey are rare. Second, functional responses should approach their maxima near the highest prey densities that predators experience. We show that independently parameterized mathematical equations derived from these two rules predict functional response parameters across over 2,100 functional response experiments. The rules further predict consistent patterns of feeding rate saturation among predators, a slow-fast continuum among functional response parameters, and the allometric scaling of those parameters. The two rules thereby offer a potential ultimate explanation for the determinants of trophic interaction strengths and their scaling, revealing the importance of ecologically realized constraints to the complex, adaptive nature of functional response evolution.
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In defense of the original Type I functional response: The frequency and population-dynamic effects of feeding on multiple prey at a time
Abstract Ecologists differ in the degree to which they consider the linear Type I functional response to be an unrealistic versus sufficient representation of predator feeding rates. Empiricists tend to consider it unsuitably non-mechanistic and theoreticians tend to consider it necessarily simple. Holling’s original rectilinear Type I response is dismissed by satisfying neither desire, with most compromising on the smoothly saturating Type II response for which searching and handling are assumed to be mutually exclusive activities. We derive a “multiple-prey-at-a-time” response and a generalization that includes the Type III to reflect predators that can continue to search when handling an arbitrary number of already-captured prey. The multi-prey model clarifies the empirical relevance of the linear and rectilinear models and the conditions under which linearity can be a mechanistically-reasoned description of predator feeding rates, even when handling times are long. We find support for linearity in 35% of 2,591 compiled empirical datasets and support for the hypothesis that larger predator-prey body-mass ratios permit predators to search while handling greater numbers of prey. Incorporating the multi-prey response into the Rosenzweig-MacArthur population-dynamics model reveals that a non-exclusivity of searching and handling can lead to coexistence states and dynamics that are not anticipated by theory built on the Type I, II, or III response models. In particular, it can lead to bistable fixed-point and limit-cycle dynamics with long-term crawl-by transients between them under conditions where abundance ratios reflect top-heavy food webs and the functional response is linear. We conclude that functional response linearity should not be considered empirically unrealistic but also that more cautious inferences should be drawn in theory presuming the linear Type I to be appropriate.
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- Award ID(s):
- 2129758
- PAR ID:
- 10578135
- Publisher / Repository:
- bioRxiv
- Date Published:
- Format(s):
- Medium: X
- Institution:
- bioRxiv
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Posted Content:
- https://doi.org/10.1101/2024.05.14.594210
