Trophic cascades, burrowing badgers and otter latrines – or, why we like predators
Posted December 4, 2009
Earlier this week we ran a post about Wildlife Services and the number of predators that they eliminate each year through various means. At NRDC we have a number of concerns about Wildlife Services’ predator control practices from their use of poisons on public lands to the use of lethal over non-lethal methods for addressing predator/livestock conflicts. But just as important to us is the fact that the removal of predators can have broader consequences that are reflected at the ecosystem level. By killing over a hundred thousand predators every year, Wildlife Services is not just removing “problem” animals, they are likely to be altering the ecological balance of entire landscapes.
Many people are aware, for example, of the dramatic changes that wolves have brought to places like Yellowstone National Park which had become overgrazed by deer and elk in the wolves’ absence. Through a phenomenon known as a “trophic cascade,” their reintroduction to the northern Rocky Mountains has led to the re-growth of streamside vegetation which in turn provides more habitat for birds and small mammals. In addition, wolves have reduced coyote populations leading to an increase in pronghorn antelope. Wolves also provide additional food sources in the form of carrion for scavengers including bears and raptors. In short, wolves benefit not only riparian habitat, but overall species diversity.
Comparison photos taken in 1997 and 2001 in Yellowstone National Park illustrating the stature of willow plants during suppression (left photo of A) from long-term browsing and their release (right photo of A) following wolf reintroductions that began in the winter of 1995–1996. With permission from Ripple and Beschta 2003.
Recent studies have shown that in some places mountain lions have had similar effects on riparian areas with their presence being linked to a higher species abundance of plants, frogs, lizards and butterflies.
Less known perhaps are the effects that smaller predators have on their ecosystems. In areas that lack wolves like Texas and southern California, coyotes often serve as a top predator. When their numbers are reduced though control methods, the result has sometimes been an increase in other small to medium sized predators – such as foxes, badgers, raccoons and skunks. Without other top predators to keep their numbers in check, this increase in “mesopredators” has then been shown to decrease overall species diversity and density of smaller prey such as bird and rodent populations. Because birds and rodents are often either seed dispersers or seed predators, fluctuations in their abundance can have a corresponding effect on the surrounding plant community.
Other small predators that are killed by Wildlife Services include the river otter which preys on a wide variety of animals including fish and crayfish, but also frogs, insects and birds. Their presence is thought to regulate the interactions among fish and invertebrates in freshwater systems. Additionally, the presence of otter ‘latrines’ along streamsides has been shown to increase the nitrogen content and growth rate of some plant species. By contributing to primary production, river otters increase the prevalence and growth of the plant community in the surrounding riparian habitat.
Finally, another target of Wildlife Services is the badger. Badgers prey on animals that mostly live underground by locating them with their sense of smell and then burrowing into the ground. This method of foraging contributes to soil conditions through aeration, nutrient mixing and by maintaining moisture – all of which aid in the recruitment of native plant species. Badgers also sometimes cache their food which can additionally distribute nutrients to the soil from decomposing prey.
The point is, each of these predators plays an important role in their ecosystem whether by controlling the distribution, abundance and diversity of their prey or influencing the structure of the ecosystem through their physical presence by burrowing, dispersing seeds or distributing rich nutrients after they forage. The importance of these documented ecological roles and the consequences of their loss are why we are concerned about the continued large scale removal of predators from our landscapes.
River otter image shared by Juliana Klose via Flick'r