Is it a top-down or bottom-up ecosystem?

Reintroducing a top predator like the wolf will not only affect prey populations, but it will influence the complexity of interacting species, including indirect effects. Food webs are an essential feature of every ecosystem and consumer-prey interactions are a fundamental linkage among species (Estes, 1996). Ecologists divide ecosystem interactions and predator-prey relationships into bottom-up and top-down processes (Hunter and Price, 1992 cited in Estes, 1996). A bottom-up system concentrates attention on how resources (space and nutrients) influence higher trophic forms. A top-down system focuses on interactions at top level consumers (predators) and their prey influence on lower trophic forms (Estes, 1996). In other words, the structure of bottom-up systems is food or resource limited, and the structure of top-down systems is driven by predation (Kay, 1998). McLaren (1994) states that a top-down trophic model predicts changes in density at one trophic level caused by opposite changes in the next higher trophic level, and such inverse correlations cascade down the food chain. In a forested ecosystem, top-down control means that plant growth rates are regulated by cycles in herbivore density and respond to increased potential for primary productivity only when released from herbivory by wolf predation (McLaren, 1994).

Through top-down control, wolves can indirectly affect plant communities. For example, if predators are missing, herbivore populations tend to overgraze their habitat with adverse consequences for the ecosystem (Primack, 1993 cited in Breitenmoser, 1998). Wolves can affect spatial organization and therefore browsing patterns of ungulates (Ripple and Larson, 2000). Browsing patterns and tendencies have a great impact on the vegetation and rate of regeneration. However, it is important to note that although overgrazing may shift ecosystems (e.g., short-grass prairie to desert), sterility is not the result. From the perspective of the new organisms, the shift in plant communities may be advantageous.

In North America there are also cases of strong interactions between herbivores and plants: deer and caribou on Alaskan islands, elk in Yellowstone (Servheen and Knight, 1993 cited in Estes, 1996), and large herbivores in North American grasslands (Mack and Thompson, 1882 cited in Estes, 1996). Skow (1989) writes that no wolves in Yellowstone meant too many elk and as a result, the elk were starving by the thousands in winter die-offs because of such high densities and the limited resources available. Understanding the structure of the ecosystem is critical in terms environmental management. Predators are seen as balance wheels in ecosystems. Therefore reintroduction is going to have profound effects on species at each following trophic level. The Nez Perce living near Yellowstone described the return of the wolf as "making the circle whole again (Chadwick, 1998)." The return of the wolf results in a cascade of effects within the circle.

The role of the wolf in its ecosystem in turn forces the question of whether the wolf should be reintroduced after the ecosystems have changed dynamically in the absence of wolves. In the early 1900's humans eliminated the wolf from its ecosystem, and the system was forced to adapt and change such that new carnivores and competitors dominated, ungulate population densities shifted, and food chain cycles adjusted. Now, over 60 years later, the wolf has been thrown back into the mix where it will naturally reclaim its top spot. The ecosystem will again change to support the wolf's return, but one can't help wondering how much can humans rightly interfere with nature, and is it too late to try and fix the natural ecosystem where we once stole a key member?

next to Trophic dynamics

return to Ecosystem Response

return home