Trophic cascades in the Cascade Mountains: top-down forcing and wolf reintroduction

Pleistocene North America was lorded over by megafauna such as the dire wolf, American lion and sabertooth cats, along with the more familiar grey wolf and cougar. These predators filled important, apex, roles in their ecosystems.

Top-down versus bottom-up control of ecosystems is an ongoing debate in ecology, although there is considerable empirical evidence for top-down control (Ripple and Valkenburgh, 2010). Ripple and Valkenburgh (2010) argue that this was the case in Pleistocene North America, operating in a trophic cascade system. The predators have effects that cascade to lower trophic levels and eventually effect primary producers.

Previously, it was put forward by Janzen (1983) that humans had help from these predators in causing extinctions. Ripple and Valkenburgh (2010) argue for the opposite, that predators, operating within a cascading top-down process, had help from humans.

“The top-down hypothesis is based on the premise that the arrival of the first humans as hunters and scavengers in the New World at the end of the Pleistocene triggered a sequential collapse of large herbivores and their predators.”

Ripple and Valkenburgh, 2010

Unlike other mammalian carnivore systems, in which interspecific competition (competition between species) is known to affect species densities, humans were omnivorous and probably less subject to 'intraguild predation' (being eaten by other predators), allowing their numbers to increase independently of large-carnivore densities (Ripple and Valkenburgh, 2010).

The trophic cascade with top-down control theory has been pointed to as a benefit of grey wolf reintroduction into areas such as the (aptly named) Cascade Mountains in the USA, Scottish Highlands and mountainous areas of Western Europe. The successful reintroduction of wolves to Yellowstone National Park, USA, has restored diversity, especially with regards to the primary producers which were previously overgrazed by high numbers of herbivores, particularly elk (Cervus elaphus) (Ripple and Beschta, 2012).

There are two main ways in which wolves (and predators in general) influence ecosystems through their effects on herbivores:

(1) Lethal (density-mediated) effects:

by killing herbivores and, therefore, reducing grazing pressure.

(2) Nonlethal (behaviourally-mediated) effects:

by altering foraging patterns and habitat use of herbivores under risk of predation (Ripple and Beschta, 2007). Also called landscapes of fear, (for example see  Laundré et al. (2010)) herbivores in fear of being eaten avoid certain areas where they are more likely to be eaten, affecting ecosystem structure, composition and function.

From Ripple and Beschta (2012):
'Comparison photographs taken in 1997, 2001, and 2010 near the confluence of Soda Butte Creek with the Lamar River illustrating the stature of willow plants during suppression (A) from long-term browsing and their release (B and C) following wolf reintroduction in the winters of 1995–1996. As of 2010, both willow height and canopy cover increased compared to the earlier dates.'

A long term study by Callan et al. (2013) of recolonizing wolves in Wisconsin, USA, found that forb and shrub species richness at local scales was significantly higher in high wolf areas. This is consistent with the predicted trophic effects on understory plant communities triggered by a keystone predator, recovering from regional extinction as the grey wolf currently is (Callan et al., 2013).