Trophic Cascade Essay

A trophic cascade is defined as ecological changes such as alterations in species abundance or composition, nutrient concentrations of plants, and the physical structure of the vegetation and environment. After wolves were eradicated from Yellow Stone National Park it caused a collapse in the tri-trophic cascade of predators, prey, and vegetation. They were reintroduced to restore the cascade that was present in the past to the park. The reintroduction of the wolves resulted in a major decline in the overall populations throughout the park. They also exhibited altered behavior by changing the intensity and location when feeding. This resulted in increased plant biomass and recruitment of a variety of woody plants. The increased forest cover and forage results in increased species diversity
and overall health of the ecosystem. In the absence of wolves as predators the elk caused the trophic to cascade to collapse. It had a major effect on the wildlife, woody plants, and soils. This was a product of intensive browsing of several woody plant species which prohibited them from growing into the understory. However the aspen were still able to grow in these areas in fenced off enclosures that prohibited elk from eating them.
After the wolves were reintroduced it began to restore the trophic cascade. The reduced browsing because of the altered behavior allowed increase in the recruitment of woody plants like Aspen (Populus tremuloides) and willow (Salix spp.) (Ripple and Beschta 2012). Without the elk decimating woody plants populations they were able to regenerate. This resulted in an increase in multiple species native to the park. These species included bison (Bison bison), beaver (Castor canadensis), and bears (Ursus arctos). The relative abundance and richness of 6/7 species of songbirds also increased because of the increased recruitment (Ripple and Beschta 2012). The wolves also have a positive effect on the age/sex structure. By concentrating on calves and older females they consistently eliminate the older sicker elk that are less suitable to survive. The wolves rarely prey on females in their reproductive prime or full grown males. The reintroduction has also had a positive effect on the bison population. The elk and bison share 80% of the same browsing food source. As elk are driven from their preferred browsing areas the bison are able to exploit the areas with the most nutritional
value. It may also contribute to a reduction in brucellosis that is contracted by the bison from elk fluids due to decreased interaction. The wolves also caused a decrease in the coyote population through competition and hunting. This allows small herbivore populations that are used as a food source by badgers and foxes to increase as well. The decreased browsing allows for greater berry production which is eaten by birds and bears throughout the park. The reintroduction has been especially beneficial for the beaver populations. Beavers are main drivers of the trophic cascade because they create wetlands, reduce erosion, modify nutrient cycling, and increase overall abundance and diversity of plant and animal species. Waterfowl are 75 times more abundant in areas also inhabited by beavers (Ripple and Beschta 2012).