Wednesday, 19 Jun 2019

Wolf Community Ecology

Wolf Community Ecology

“The World of Wolves”

Edited by Marco Musiani, Luigi Boitani and Paul C. Paquet

The first comprehensive theory of food-web dynamics recognized the critical role for predation in structuring ecosystems, building on the seminal work of Hairston et al. (1960). In the green-world hypothesis of  Hairston et al. (1960), top-down control by predators on herbivore density results in increased plant biomass. Predictions of the trophic cascade hypothesis are alternating correlations in abundance between trophic levels, which were coined ‘trophic’ cascades. Figure 1.3.1a represents hypothesized conditions before wolf recovery, where herbivore density, released from predation by wolves, increases and herbivory subsequently reduces plant biomass.

Wolf Community Ecology - photo 1

Wolf Community Ecology

Wolf Community Ecology. Following wolf recolonization in Figure 1.3.1b, predation causes a top-down trophic cascade with declining herbivore density, releasing plant biomass from herbivory. Distinguishing between direct effects and indirect effects between species is important in this simplified food-web model. Direct effects occur when there are no intermediary species between two interacting species, for example, through predation (or herbivory). Indirect effects are mediated by an intermediate species, such as the indirect effect of wolves on plants mediated by wolf-caused changes to herbivore density in Figure 1.3.1b. Indirect effects of predation can also arise because of behavioural changes by herbivores in addition to direct effects of predation on herbivores.

Wolf Community Ecology - photo 2

Wolf Community Ecology

Wolf Community Ecology. In a well-known test of these ‘behaviourally-mediated’ indirect effects, grasshoppers showed behavioural avoidance of spiders that were rendered experimentally ineffectual predators. Yet presence of these ineffectual spiders still resulted in a 40% grasshopper population reduction, compared with 70% reduction in the control with effective spider predators (Schmitz et al. 2000). These exciting results prompted a host of studies examining behaviourally mediated indirect effects in terrestrial systems, including areas recently recolonized by wolves. Unfortunately, whether the results of small-scale experimental treatments can scale up to large-scale community dynamics remains unknown.

Wolf Community Ecology - photo 3

Wolf Community Ecology

Wolf Community Ecology. It seems certain, however, that indirect effects will interact following wolf recolonization to reshape terrestrial ecosystems. Effects of wolves will go far beyond trophic cascades, however . Perhaps the most important non-predation form of direct interaction is competition, where at least one of two (or more) interacting species has negative effects on the other species (Fig. 1.3.2, H1 and H2). Three main forms of competition are recognized in community ecology (Holt 1977): interference competition, where two species on the same trophic level kill, but do not consume, each other; exploitative direct food competition, where two species consume the same resources; and predatormediated apparent competition, whereby two prey species share a common predator species.

Wolf Community Ecology - photo 4

Wolf Community Ecology

Wolf Community Ecology. Note that only interference competition is a direct effect; exploitative and apparent competition are indirect effects mediated by a shared food or predator. Another form of indirect interaction is mutualism, where both species benefit from an interaction (e.g., cowbirds (Molothrus ater) on bison (Bison bison)). An even rarer form of species interaction is commensalism, whereby one species benefits without costing or benefiting another (Fig. 1.3.2). Scavenging is an important indirect interaction mediated by (in this case) wolf-killed ungulates.

Wolf Community Ecology - photo 5

Wolf Community Ecology

Wolf Community Ecology. Scavenging can either be a form of indirect competition or commensalism, based on the strength of the indirect interactions between the carnivore (+ effect on scavengers) and scavenger effect on predator, e.g. Recent reviews have reminded ecologists that direct effects are but a fraction of the potential species interactions possible in even a simple food-web. For example, in Fig. 1.3.2, the total number of direct interactions between the six species is 30, whereas the number of indirect species interactions is 1,920! (see (Estes et2004) for calculations). Thus, the focus to date on trophic cascades in wolf systems seems almost myopic, and will surely be eclipsed in the future by studies of the indirect effects of wolves.

Wolf Community Ecology - photo 6

Wolf Community Ecology

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