Community Ecology Papers
Weak trophic interactions and the balance of nature.
Kevin McCann, Alan Hastings and Gary
R. Huxel
Nature 395:794-798 (1998)
Ecological models find that food web complexity usually destabilizes food webs, predicting that food webs should not amass the vast numbers of interacting species which are found in nature. Here, using nonlinear models, we investigate the influence of interaction strength on nonequilibrium food web dynamics. Consistent with previous suggestions, our results show that weak to intermediate strength links play a significant role in promoting community persistence and stability. Weak links act to dampen oscillations in consumer-resource interactions. In turn, this tends to bound population densities further away from zero, which decreases the statistical chance that a population will go extinct (lower population densities are more prone to such chance). Interestingly, recent data on interaction strengths in natural food webs indicate that food web interaction strengths are indeed characterized by a large number of weak interactions and a few strong interactions.
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Food web stability: the influence of
trophic flows across habitats.
Gary R. Huxel
and Kevin McCann
American Naturalist 152:460-469 (1998)
ABSTRACT: In nature, fluxes across habitats often bring both nutrient and energetic resources into areas of low productivity from areas of higher productivity. These inputs can alter consumption rates of consumer and predator species in the recipient food webs thereby influencing food web stability. Starting froming a well-studied tritrophic food chain model, we investigated the impact of allochthonous inputs on the stability of a simple food web model. We considered the effects of allochthonous inputs on stability of the model using four sets of biologically plausible parameters that represent different dynamical outcomes. We found that low levels of allochthonous inputs stabilize food web dynamics when species preferentially feed on the autochthonous sources. While either increasing the input level, or changing the feeding preference to favor allochthonous inputs, or both led to a decoupling of the food chain that could result in the loss of one or all species. We argue that allochthonous inputs are important sources of productivity in many food webs and their influence needs to be studied further. This is especially important in the various systems, such as caves, headwater streams and some small marine islands, in which more energy enters the food web from allochthonous inputs than from autochthonous inputs.
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On the influence of food quality on
consumer-resource interactions
Gary R. Huxel
ECOLOGY Letters 2:256-261 (1999)
ABSTRACT: While nutrients are an important regulating factor in food webs, no theoretical studies have examined limits to consumer growth imposed by nutrient concentrations (i.e.; food quality) of their prey. Empirical studies have suggested that nutrients may play a role in limiting assimilation efficiencies of herbivores. Using a simple food chain model, I find that prey nutrient concentration does directly influence the growth rate of consumers and potentially increase the stability of consumer-resource interactions. This suggests that the strength of trophic cascades and the relative importance of top down versus bottom up control in food webs is significantly influenced by nutrient availability in food resources of consumers. Additionally, the results imply that increases in resource input may cause a change in which resource is limiting and thereby negate any potential "paradox of enrichment."
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The
effect of partitioning of allochthonous and autochthonous resources on
food web stability.
Huxel,
G. R., K. McCann
and G. A. Polis.
Ecological
Research.17:419-432.2002.
ABSTRACT--The
flux of energetic and nutrient resources across habitat boundaries can
exert major impacts on the dynamics of the recipient food web.Competition
for these resources can be a key factor structuring many ecological communities.Competition
theory suggests that competing species should exhibit some partitioning
to minimize competitive interactions.Species
should partition both in situ (autochthonous) resources and (allochthonous)
resources that enter the food web from outside sources.Allochthonous
resources are important sources of energy and nutrients in many low productivity
systems where they significantly influence community structure.The
focus of the paper is on (1) the influence of resource partitioning on
food web stability, but at the same time we examine the compound effects
of (2) the trophic level(s) that have access to allochthonous resources;
(3) the amount of allochthonous resource input; and (4) the strength of
consumer-resource interactions.We
start with a three trophic level food chain model (resource - consumer
- predator) and separate the higher two trophic levels into two trophospecies.In
the model allochthonous resources are either one type available to both
consumers and predators or two distinct types, one for consumers and one
for predators.The feeding preferences
of the consumer and predator trophospecies were varied so that they could
either be generalists or specialists on allochthonous and/or autochthonous
resources.The degree of specialization
(1) influenced system persistence by altering the structure and therefore
the indirect effects of the food web.In
regard to (2), we found that a single allochthonous resource available
to both consumers and predators is more unstable that two allochthonous
resources.The results demonstrate
that species populating food webs that experience low to moderate allochthonous
resources are more persistent (3).The
results also further the notion that strong links destabilize food web
dynamics but weak to moderate strength links stabilize (4).The
results are consistent with the idea that the particular structure, resource
availability, and relative strengths of links of food webs (such as degree
of specialization) can influence stability of communities.Given
that allochthonous resources are important resources in many ecosystems,
we argue that the influence of such resources on species’ and community
persistence needs to be examined more thoroughly to provide a clearer understanding
of food web dynamics.
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Polis, G. A., A. L. W. Sears, G. R. Huxel, D. R. Strong and J. Maron.2000.When is a trophic cascade a trophic cascade?Trends in Ecology and Evolution 15:473-475.
Trophic cascades are the time-honored focal point
of food-web dynamics. They are the best loved example of indirect effects
in undergraduate ecology textbooks and they represent a potentially useful
application of theory. Researchers have found them from the Arctic to the
tropics. But, can we agree on what they are? Here, we seek to clarify the
terminology of trophic cascades and call for a consensus on how to quantify
cascading effects in the future.
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