The Importance of Predators
Photo of gray wolf by Tracy Brooks
As we hike through the woods, cast into the river, or drive over the mountain pass, all around us the environment’s resident organisms work to eke out a living. Though people don’t experience it this way, our environment – any “ecosystem” or natural area – is an intricate system of relationships and interactions based on the need for every organism or animal to gain nourishment and reproduce. Scientists sometimes refer to these complex tangles of interactions as food webs.1 Predators are just one – albeit an important one – of many parts to living, breathing nature out there.
Predator species greatly impact their environments, whether in an urban park or large wilderness complex. Because they are animals that survive by preying on other organisms, they send ripples throughout the food web, regulating the effects other animals have on that ecosystem. This cause and effect process is called a “trophic cascade,” or the progression of direct (predation-driven) and indirect (fear-driven) effects predators have across lower nutritional (trophic) levels in a food chain. 2
One of the clearest examples of trophic cascades occurs when wolves, mountain lions, or bears prey on ungulates (elk or deer), which keeps the ungulates moving around and their populations at lower numbers or densities. This limits the impacts ungulates have on plant biomass – thus more trees, bushes, and grasses can grow – which then preserves or creates habitat for many other species, from insects and reptiles to beavers and birds, especially around riparian (stream or river) areas, preventing soil erosion at the same time (which has an entire other set of positive consequences). The importance of this predator impact cannot be overstated, because the composition of plant species in any natural area is an important regulating factor of the ecosystem’s functioning. 3 These impacts are occurring around the clock in every terrestrial and marine ecosystem.
Predators also have sideways and circular impacts throughout their communities because they affect the behavior of competitor predators, or animals that eat the same prey they do. In some cases, dominant predators even kill and eat their competitors (termed “intraguild predation”). 4 These effects in turn impact the animals in the competitor’s food chain, which on the whole is an overlapping yet different set of animals than in the dominant predator’s food chain. One of the most frequently studied dynamics between predators involves wolves and coyotes. Wolves regulate the numbers, movement, and distribution of coyote populations because wolves are dominant. When coyote populations are held in check, animals lower down in the coyote’s food chain tend to have higher survival rates, as has been found with pronghorn 5 and sage grouse.6 Thus, top predators support prey species at lower levels, again in turn triggering a whole host of other effects that contribute to the healthy functioning of a natural area.
In a decisive study on the effects of predators on the environment, William Ripple and Robert Beschta (2009) of Oregon State University found that the presence of large predators is important in sustaining native plant communities in both upland and riparian (stream or river) settings because plants contribute to a wide range of “ecosystem services” such as floodplain functioning, soil development, and stream bank/channel stability. Riparian areas are biodiversity “hotspots” because the diversity of native plants commonly present provides habitat and food web support for a large number of terrestrial and aquatic species, from amphibians and birds to fish and insects. 7 By contrast, “in areas where livestock foraging is the dominant land use, simplification of plant communities, reduced ecosystem services, impacts to wildlife, and a shift towards alternative states are common.” 8
Predators are important not only because they create biodiversity, but also because they indicate biodiversity. 9 In addition to regulating natural systems as described above, predators (especially large predators) serve as a measure of the health of communities around them. Top predators are associated with high biodiversity value because:10
- They are sensitive to ecosystem dysfunctions, such as pollution, habitat fragmentation, and other human impacts that would affect many species.
- They select sites with productive habitat and vegetational complexity.
- Most have diets dominated by a few main prey species but a large number of secondary prey species. Communities with many prey species are richer and allow for prey-switching if necessary, which helps prey populations persist yet still allows for top-down regulation of the area by the predator.
- Studies have shown that carnivore density is correlated with ecosystem productivity and services like the health of soil, water, and vegetation.
Top predators typically need large areas for foraging and breeding and are thus considered umbrella species; that is, if an area supports a large predator, it will encompass the requirements of less demanding species. 11 Finally, today, because of their large area requirements, many predator species serve as a draw for tourists, encouraging the protection of wild spaces while generating income and employment. 12
Why should we care about healthy ecosystems? What are ecosystem services?
Of course, humans preside over the entire food web and sit at the top of every pillar of trophic cascades.13 Unfortunately, as we eliminate and fragment natural areas (habitat) and kill predator populations for the benefit of livestock production or to inflate game herds for human sport hunting, in a twist of irony we affect our own health. Ecosystems need to be healthy in order to maintain the health of all living things within and around them, including humans. Through their intact ecological processes, healthy, functioning natural areas provide people with several benefits, also known as ecosystem services – from clean water to spiritual and recreational havens.
The concept of ecosystem services is receiving increased attention as we approach a critical mass of environmental degradation, extinctions, and biodiversity loss on all scales – regional, national, global. The connections are subtle and often invisible to us, the effects sometimes long-term; thus, we tend not to pay attention to how our activities can be harmful to nature, wildlife, domestic animals, and ourselves. But human survival relies on ecosystem services, including production of clean water, trees/forests and timber, seed dispersal, natural pest control, climate regulation, healthy and sufficient amounts of vegetation, pollination, soil fertility, regulation of disease, regulation of animal species whose populations may otherwise become out of balance, and many more. Ecosystem services such as these are directly connected to biodiversity. Predators have direct impacts on the maintenance of biodiversity and thus many of these goods and services derived by humans from intact natural processes.14
Perhaps most importantly, all of these processes are part of the larger natural world, which operates on intricate fundamental laws that cannot be replicated by humans. Because these processes are bigger than us, they affect not only the lives of animal and plant species but our lives as well. We would argue that they not only provide us with trees and water but also contribute to important intangible aspects of human health such as our spiritual, cultural, ethical, and moral wellbeing. Preserving large predator species as a part of maintaining intact natural systems is a profound act of wisdom and integrity.
In the end, our success in rehabilitating an ecologically degraded world will be judged more on the persistence of interspecies interactions than on the geographically
limited persistence of populations based only on causing the least economic burden and ensuring only symbolic survival. Both the science of ecology and our obligations
to minimize harm to nature require that land use and conservation policies reflect this higher standard.15
For definitions of predator-related terms, see our Predator Glossary.
NOTE: Additional information on ecosystem services, including economic benefits, is coming soon.
Sources* Jamison, M. Tracking science: Biologist’s findings show forest diversity, health influenced by wolves, The Missoulian, October 25, 2009.
1 O’Gorman, E. J., and M. C. Emmerson. 2009. Perturbations to trophic interactions and the stability of complex food webs. PNAS, August 11, 2009, vol. 106, no. 32, 13393–13398.
9 Sergio, F., T. Caro, D. Brown, B. Clucas, J. Hunter, J. Ketchum, K. McHugh, F. Hiraldo. 2008. Top Predators as Conservation Tools: Ecological Rationale, Assumptions, and Efficacy. Annu. Rev. Ecol. Evol. Syst. 2008. 39:1-19.
14 Chapin, F. S., E. S. Zavaleta, V. T. Eviner, R. L. Naylor, P. M. Vitousek, H. L. Reynolds, D. U. Hooper, S. Lavorel, O. E. Sala, S. E. Hobbie, M. C. Mack, S. Díaz. 2000. Consequences of changing biodiversity. NATURE, Vol. 405, May 11, 2000, pp. 234-242.