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- Chase, J. M., et al.
(författare)
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The interaction between predation and competition : a review and synthesis
- 2002
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Ingår i: Ecology Letters. ; 5:2, s. 302-315
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Tidskriftsartikel (refereegranskat)abstract
- This review discusses the interface between two of the most important types of interactions between species, interspecific competition and predation. Predation has been claimed to increase, decrease, or have little effect on, the strength, impact or importance of interspecific competition. There is confusion about both the meaning these terms and the likelihood of, and conditions required for, each of these outcomes. In this article we distinguish among three measures of the influence of predation on competitive outcomes: short-term per capita consumption or growth rates, long-term changes in density and the probability of competitive coexistence. We then outline various theoretical mechanisms that can lead to qualitatively, distinct effects. of predators,. The qualitative effect of predators can depend both on the mechanism of competition and on the definition of competitive strength/impact. In assessing the empirical literature, we ask: (1) What definitions of competitive strength/impact have been assumed? (2) Does strong evidence exist to support one or more of the possible mechanisms that can produce a given outcome? (3) Do biases in the choice of organism or manipulation exist, and are they, likely, to have influenced the conclusions reached? We conclude by discussing several unanswered questions, and espouse a stronger interchange between empirical and theoretical approaches to this important question.
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- Diehl, S., et al.
(författare)
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Effects of multiple, predator-induced behaviors on short-term producer-grazer dynamics in open systems
- 2000
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Ingår i: American Naturalist. ; 156:3, s. 293-313
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the population consequences of multiple behavioral responses of grazers to a foraging return-predation risk trade-off in an open system consisting of primary producers, grazers, and predators. Using a dynamical model where grazers adjust their foraging activity and emigration rate to the densities of predators and producers, we explored how changes in control variables (predator density, grazer immigration, and producer immigration and carrying capacity) affect the dynamics of producers and grazers at temporal scales shorter than consumer and predator reproduction. The model predicts that producer biomass increases and that both the density of foraging grazers and the feeding rate of predators decrease with predator density. These predictions hold although total (foraging + nonforaging) grazer density may actually increase with predator density. The latter will occur whenever the benefit of higher resource density outweighs the increased risk of predation. In this case. per capita grazer emigration decreases with predator density, which might be misinterpreted as a direct "freezing" response to predators. Increased grazer immigration is predicted to result in decreased producer density and increased densities of both foraging and total grazers, as well as increased grazer emigration and predator feeding rates. Increased producer immigration or carrying capacity should increase producer and grazer densities and predator feeding rate but decrease per capita grazer emigration. Manipulation of predator (trout) densities in a set of nine large (50 m(2)) stream channels produced results in broad agreement with model predictions. Most notably, a positive effect of trout on benthic algal biomass was mainly mediated through grazer behavior (changes in the use of epibenthic surfaces and in emigration rare) rather than through consumptive reductions of grazer numbers by trout.
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- Diehl, S., et al.
(författare)
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Effects of enrichment on three-level food chains with omnivory
- 2000
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Ingår i: American Naturalist. ; 155:2, s. 200-218
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Tidskriftsartikel (refereegranskat)abstract
- Although omnivory (the consumption of resources from more than one trophic level) is widespread, this fundamental limitation to the applicability of food chain theory to real communities has received only limited treatment. We investigated effects of enrichment (increasing carrying capacity, K, of the resource) on a system consisting of a resource (R), an intermediate consumer (N), and an omnivore (P) using a general mathematical model and tested the relevance of some of its predictions to a laboratory system of mixed bacteria (=R) and the ciliates Tetrahymena (=N) and Blepharisma (=P). The model produced six major predictions. First, N may facilitate or inhibit P. Enrichment may revert the net effect of N on P from facilitation to inhibition. Second, along a gradient of K, up to four regions of invasibility and stable coexistence of N and P may exist. At the lowest K, only R is present. At somewhat higher K, N can coexist with R. At intermediate it, either N and P coexist, or either consumer excludes the other clef ending on initial conditions. At the highest K, N may be excluded through apparent competition and only R and P can coexist. The pattern of persistence of Tetrahymena and Blepharisma along an enrichment gradient conformed fairly well to the scenario allowing coexistence at intermediate K. Third, fur stable equilibria of the omnivory system, R always increases and N always decreases with R. The abundances of bacteria and Tetrahymena were suggestive of such a pattern but did not allow a strict test because coexistence occurred at only one level of enrichment. Fourth, an omnivore can invade an R-N system at a lower K than an otherwise identical specialist predator of N. Fifth, an omnivore can always invade a food chain with such a specialist predator. Sixth, over ranges of K where both omnivory systems and otherwise identical three-level food chains are feasible, N is always less abundant in the omnivory system, whereas the relative abundances of R and P in omnivory systems compared to food chains may change with K. It is thus possible that total community biomass at a given It is lower in an omnivory system than in a food chain. Both the model and the experimental results caution that patterns of trophic-level abundances in response to enrichment predicted by food chain theory are not to be expected in systems with significant omnivory.
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