Trophic dynamics in the ecosystem in regard to the presence of wolves

photograph from www.iup.eduThere is a great deal of evidence supporting the changes in trophic cascades in regard to the presence of wolves in an ecosystem. Introducing a top predator into a top-down ecosystem will have a cascading influence down ecosystem levels. Moose populations are influenced by the number of wolves, and therefore the intensity of wolf predation is evidence that trophic cascades occur in terrestrial ecosystems where top predators are the dominant high level consumers (Estes, 1996). McLaren (1994) studied wolves on Isle Royale and found that during 1988-1991, the wolf population had reached an unprecedented low, which was correlated with moose populations reaching a new high level, accompanied by strong suppression of balsam fir (Abies balsamea) growth. This is a 3-trophic level system dominated by top-down control. Moose (Alces alces) herbivory suppressed foliar biomass and annual wood accrual on the firs (Ripple and Larson, 2000). The growth of trees in this study was determined using tree-ring analysis. This was concluded after observations confirmed that fir growth rates decreased when wolf density decreased, due to an increase in moose density.

photograph from www.mindspring.eduOn Isle Royale, balsam fir is 59% of the moose's diet in winter (McLaren, 1994). Over the course of time, the sensitivity of the balsam fir population to moose numbers became obvious; in 1848 there was 46% abundance of over-story, 1978 it had dropped to 13% over-story, and by 1994 it was down to approximately 5% over-story. On the Island there were other forage species responding to increasing herbivory similar to the response of fir. In neighboring islands where moose numbers were much lower by comparison, there was greater over-story fir growth (McLaren, 1994). On Isle Royale, big herbivores, moose, are limited in number not by the wolf, but by the primary supply of vegetation (Zimen, 1981). As mentioned earlier, ungulates are poor self-regulators. Although there are alternative means of self-regulation, possibly disease or parasites, without predation ungulate populations would significantly expand and deplete much of their own resources and damage vegetation. From this, one might conclude that reintroducing a top predator like the wolf is bound to benefit the entire ecosystem in that it reestablishes balance and natural cycles.

A trophic structure involving wolves, elk, and aspen trees was discovered in Yellowstone. The presence of the wolf may increase aspen growth indirectly, through trophic structure interactions in which predation regulates terrestrial systems (Ripple and Larson, 2000). Kay (1990 cited in Ripple and Larson, 2000) and Wagner et al. (1995 cited in Ripple and Larson, 2000) stated that the decline of over-story aspen is due primarily to over browsing caused by an over abundance of elk. This is because in the winter, elk browse the leaders off aspen suckers and this prevents growth. Thus the aspen population in the strong presence of elk has limited growth. Aspen growth was examined by looking at the ages and sizes of aspen trees. Data from the study found that approximately 10% of the current over-story of aspen originated before 1871, 85% of the current over-story is grew between 1871 and 1920, and only 5% of the over-story is from after 1921 (Ripple and Larson, 2000).

The percentage of current overstory aspen stems established by decade on YNP's northern range.  Ripple and Larsen, 2000.

The years 1872-1886 were the market hunting era in YNP, where large animal populations were being decimated, including bison, elk, wolves and other large animals. During this time browsing pressure on aspen was minimal. Populations were allowed to reestablish after this era, including wolves, which were increased in numbers by 1912. Then came concerns for protecting ungulate populations, and thus the effective removal of the wolf in YNP from 1914-1926 (Ripple and Larson, 2000). The study found that aspen over-story recruitment ceased during the same years that wolves were removed from YNP.

Canada's Jasper National Park (Dekker, 1985; Dekker et al., 1996 cited in Ripple and Larson, 2000) noted an increase in aspen over-story recruitment in areas frequented by wolves. Furthermore, White et al. (1998 cited in Ripple and Larson, 2000) suggested that aspen might be regenerating in areas avoided by elk, because of a predator avoidance strategy. The trophic connection observed in YNP was supported by observed inverse relationships between wolf populations and aspen recruitment in Sweden, Finland, and northern Russia (Angelstam, 1998 cited in Ripple and Larson, 2000). Brandli (1995 cited in Breitenmoser, 1998) found in Switzerland a clear correlation between estimated ungulate density, damaged trees, and lack of natural regeneration. Substantial evidence suggests that wolves may have a positive impact on aspen over-story through a trophic cascade involving elk populations, movements, and browsing patterns.

One proposed possible trophic relationship was that with wolves present, elk would be killed, the carcasses would benefit the grizzly bear and its population would expand, in turn increasing predation on elk, which would contribute to a change towards a decline in elk herbivory effects (Ripple and Larson, 2000). This grizzly reinforcement hypothesis supports the return of the wolf where competition between large carnivores creates coexistence beneficial to each species as well as the environment. Another study on wolf-bear interactions uncovered the importance of their coexistence, but was discovered through different observations of grizzly populations. Grizzly bears in Yellowstone are fairly small and low in density (Estes, 1996). This might be explainable through a trophic relationship. The bears have a limited availability of fruits at their disposal, which is caused by elk overgrazing as a result of the absence of wolves in this ecosystem (Estes, 1996).

Boulder-White Cloud Mountains, Idaho.  photograph from www.wolf.orgIn Mount McKinley National Park, biologists predict that wolves serve to keep the caribou population in check, and without wolves limiting caribou numbers the caribou might populate so extensively that vast areas of lichen range would be severely damaged. Trophic cascades are prevalent in all ecosystems and the importance of which are often only identified after a key element has been removed.

The eradication of wolves in the northwest United States disrupted many natural functions and pathways that were one a key part of the ecosystem. Now this top predator is being put back into the ecosystem and the wolf will find its role and its importance in the community and ecosystem functions. The definition of an ecosystem means that no single component is independent of all other components. Each organism, species, and process are linked somehow. Many connections are unlikely and surprising, as is described above, but all are critical to a balanced productive functioning ecosystem.

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