People and Predators: The Disappearance of Large Carnivores

Currently, large carnivores throughout the world are facing massive declines in their numbers as well as their geographical ranges. According to a study looking at 31 different species of carnivores conducted by Ripple and colleagues (2014), over three quarters of these species are now in decline [1]. These animals have been widely considered to be both majestic and vicious, and comprise such beloved species as polar bears, lions, tigers, and wolves among others. The thought of these animals disappearing off the face of the earth is disquieting at the very least. These predators occupy a deceivingly comfortable position at the top of the food web– deceiving because this very position is precarious, making them even vulnerable to extinction. Humans have played a significant role in exacerbating this vulnerability. Large carnivores face enormous threats brought about by our species, including habitat loss and degradation, persecution, utilization, and depletion of prey [1]. The impact of their dwindling numbers has sweeping consequences for the ecosystems that they are members of. This issue is both consequential and pressing, yet it seems to be flying under the radar.

Carnivores share the common characteristic of subsisting largely on other animals and are rare, given their position at the top of the food web. Carnivores of various sizes play vital roles in regulating ecosystems. These roles differ because the impacts of these carnivorous species are influenced by factors such as their size, metabolic demands, density, sociality, and hunting tactics [1]. Larger carnivores tend to have large energetic constraints, slow life histories, low population densities and roam widely in search of larger prey when compared to smaller carnivores. This combination of low population densities, reproductive rates with high food requirements, and wide-ranging behavior in order to meet their energy demands is what makes large predators vulnerable and impedes them from being able to rapidly respond to threats [1].

Tiger

Most notably, the effects of large carnivores extend down the food web. It has been discovered that their cascading influences propagate broadly to other species mediated by their controlling effects on mesocarnivores, which are carnivorous animals that have a 50-70% meat diet. Hairston et al. (1960) initially introduced the green world hypothesis (GWH) suggesting that predators maintain global plant biomass at high levels by limiting herbivore densities [2]. Subsequently, Fretwell (1977) and Oksanen et al. (1981) refined and extended the GWH to what is known as the exploitation ecosystems hypothesis (EEH) [3, 4]. The EEH predicts stepwise trophic relationships among plants, herbivores, and predators along an increasing net primary productivity (NPP) gradient or increased plant contributions through the synthesis of organic compounds from carbon dioxide. With increasing primary productivity, the EEH predicts significant increases in plant and carnivore biomass, but little increases in herbivore biomass. Thus, carnivores are involved in limiting both large herbivores through predation, and mesocarnivores through direct competition for similar resources.  Together, these controls infer a role for large predators in structuring ecosystems along multiple interconnected food-web pathways that is of paramount importance [1].

Ripple et al. (2014) described several case studies of large carnivores that have been impacted by humans and subsequent impacts on their ecosystems. For instance, the case of gray wolves, one of the world’s most widely distributed mammals and the most studied large carnivore [1]. The gray wolf has been extirpated from much of Western Europe, the United States, and Mexico, and its overall range has been reduced by approximately one-third as a result of hunting by humans and habitat fragmentation [5]. In recent decades, wolf population declines have been halted with the implementation of enhanced legal protection, reintroduction programs, and natural re-colonization, resulting in population recoveries in portions of the Rocky Mountains, Great Lakes, and southwestern regions of North America, as well as in various parts of Europe [1]. By virtue of their widespread geographic distribution, group-hunting nature, and year-round activity, the gray wolf, aside from humans, is the most significant predator of cervids, or deer species, in the Northern Hemisphere [6]. Predation by wolves, in conjunction with bears occupying the same territory, generally limits cervid densities [6]. In North America and Eurasia, cervid densities were, on average, nearly six times higher in areas without wolves than in areas with wolves [7]. Fewer wolves results in less predation of cervids, which are herbivorous mammals whose expanded populations deplete vegetation. This then subsequently disrupts the habitat and food sources of birds and small mammals. This illustrates that, in addition to direct effects on herbivores, carnivores have numerous indirect effects on other organisms in the ecosystem, which also holds true for the majority of carnivores.

Large carnivores also help augment ecosystem carbon storage by suppressing herbivores, thereby allowing for plants to flourish [7]. Because plants are the trophic foundation of all ecosystems, these vegetation changes, as demonstrated in the case of gray wolves, can be expected to have wide-ranging influences on a multitude of other species in an ecosystem. Furthermore, these cascades originating from the loss of large carnivores can be expected to influence numerous other ecological processes, including disease dynamics, wildfire, and carbon sequestration [8, 9, 10]. There is now a substantial body of research demonstrating that, alongside climate change, eliminating large carnivores is one of the most significant anthropogenic impacts on nature [11].

During the previous two centuries, carnivores have experienced geographic range contractions, and fragmentation of their habitats. Human threats, including habitat destruction, hunting, utilization (such as for traditional medicine, trophy hunting, or furs), and depletion of prey are key amongst other forces conspiring against many endangered carnivore species [1]. Because of the high metabolic demands that accompany endothermy and a large body size, these carnivores often require large prey and expansive habitats, leading to wide-ranging behavior, which further brings them into conflict with humans and livestock [1]. Adding to the example of wolves, Mexican gray wolves in southwestern North America have not yet been restored to an ecologically effective density in relation to that of their main prey, elk, because of ongoing conflicts with livestock grazing [12]. Likewise, recent wide-scale hunting of recovering gray wolf populations in parts of the Great Lakes region and the western United States may reduce wolf populations below sizes at which they are able to exert their effects on ecosystems [13, 14]. Furthermore, wolves and other carnivores may have little influence on other species in areas where human hunters have disproportionate effects on prey densities [15].

One of the most potent threats to carnivores is the ever expanding human population and its associated resource consumption, which is expected to continue rising significantly through to at least 2050 [1]. Increased human population size has and continues to lead to an increased demand for meat. Interestingly, meat consumption by humans significantly rivals that of carnivores. The need for humans to produce meat puts extra pressure on large carnivores in many ways, including ongoing habitat loss from land conversion, depletion of prey, and direct eradication due to conflicts with livestock [1]. Increases in both human population and meat consumption can also affect biodiversity, greenhouse gas emissions, food security, deforestation, desertification, and water quality and quantity, further impacting these creatures [16, 17].

As alluded to previously, climate change precipitated by greenhouse gas emissions is another mechanism whereby we are impacting wild animals around the world. This phenomenon has already caused geographic range shifts that have proceeded to disrupt existing species interactions [1]. The recently documented change in hunting locations and food habits among polar bears is a prime example of this. With receding ice in the Hudson Bay, polar bears have found it increasingly more difficult to hunt seals, their traditional prey. However, it has now been shown that the nutritional stress placed on their populations is resulting in a shifting of their diet. Polar bears are now exploiting more abundant resources such as caribou and snow geese as well as newly available resources such as eggs of migratory waterfowl [18]. The downstream effects of this remain to be elucidated. However, new combinations of predator and prey species are likely to result in restructuring of communities [1].

PolarBear

Currently, the IUCN Species Survival Commission (SSC), a special commission operated by the International Union for Conservation of Nature (IUCN), represents perhaps the most comprehensive attempt to establish priorities for individual species or taxa [1]. These action plans not only provide assessments of threats but also recommend conservation monitoring and actions for each large-carnivore species. More research on the trophic factors of large carnivores is needed as well as the minimum required densities for large carnivores to maintain trophic cascades in different ecosystems, and the strength of those effects based on various factors such as location [1]. It is also important to further research the impacts of human activities on carnivores, and which of these activities are most harmful. If the world continues to transition into one that replaces top carnivores with livestock and mesopredators, it is vital to understand more about the ecological ramifications [1]. More protected areas alone will not be sufficient, so one of the most important paths moving forward will be to outline strategies needed to facilitate human coexistence with these animals [1]. Cultural, ideological, and territorial concerns must be balanced with ecological considerations. Biodiversity conservation programs intended to reintroduce large carnivores must ultimately address all of these issues. Finally, as an appeal to human moralities, large-carnivore conservation can also be seen as an obligation in which we recognize the intrinsic value of all species [1].

References

  1. Ripple, W. J., J. A. Estes, R. L. Beschta, C. C. Wilmers, E. G. Ritchie, M. Hebblewhite, J. Berger, et al. 2014. Status and ecological effects of the world’s largest carnivores. Science 343 (6167): 1241484–1241484.
  2. Hairston, N.G., Smith, F.E., Slobodkin, L.B. 1960. Community structure, population control, and competition. The American Naturalist 94:421–425.
  3. Fretwell, S.D. 1977. The regulation of plant communities by food chains exploiting them. Perspectives in Biology and Medicine 20:169–185
  4. Oksanen, L., Fretwell, S.D., Arruda, J., Niemela, P. 1981. Exploitation ecosystems in gradients of primary productivity. The American Naturalist 118:240–261
  5. Hunter, Luke. 2011. Carnivores of the World. Princeton, NJ: Princeton Univ. Press.
  6. Peterson, R. O., Vucetich, J. A., Page, R. E., Chouinard, A. 2003. Temporal and spatial aspects of predator–prey dynamics. Alces 39, 215–232.
  7. Ripple, W.J., and Robert L.B. 2012. Large Predators Limit Herbivore Densities in Northern Forest Ecosystems. European Journal of Wildlife Research 58 (4): 733–42. doi:10.1007/s10344-012-0623-5.
  8. Levi, T., Kilpatrick, A.M, Mangel, M., Wilmers, C.C. 2012. Deer, predators, and the emergence of Lyme disease. Proceedings of the National Academy of Sciences of the United States of America 109 (27): 10942–47.
  9. Holdo, R.M., Anthony, R.E.S., Andrew P.D., Kristine L.M, Benjamin M.B., Mark E.R., and Robert D.H. 2009. A disease-mediated trophic cascade in the Serengeti and its implications for ecosystem. PLoS Biology 7 (9): e1000210.
  10. Schmitz, O.J., Peter, A.R., James, A.E., Werner, A.K., Gordon, W.H., Mark, E.R., Daniel, E.S., et al. 2013. Animating the carbon cycle. Ecosystems 17 (2): 344–59.
  11. Estes, J.A., John T., Justin S.B., Mary, E.P., Joel, B., William, J.B., Stephen, R.C., et al. 2011. Trophic downgrading of planet Earth. Science 333 (6040): 301–6.
  12. Beschta, R.L., and William J.R. 2010. “Mexican Wolves, Elk, and Aspen in Arizona: Is There a Trophic Cascade?” Forest Ecology and Management 260 (5): 915–22.
  13. Levi, T. and Christopher C.W. 2012. Wolves–coyotes–foxes: A cascade among carnivores. Ecology 93 (4): 921–29.
  14. Callan, R., Nibbelink, N.P., Rooney, T.P., Wiedenhoeft, J.E., Wydeven, A.P. 2013. Recolonizing wolves trigger a trophic cascade in Wisconsin (USA). Journal of Ecology 101, 837–845.
  15. Wikenros, C. 2011 Thesis, Swedish University of Agricultural Sciences. Uppsala, Sweden.
  16. Smith, P., Helmut, H., Alexander, P., Karl-heinz, E., Christian, L., Richard, H., Francesco N.T, et al. 2013. How much land-based greenhouse gas mitigation can be achieved without compromising food security and environmental goals? Global Change Biology 19 (8): 2285–2302.
  17. Ripple, W.J., Pete, S., Helmut, H., Stephen, A.M., Clive M., Douglas H.B. 2014. Ruminants, climate change, and climate policy. Nature Climate Change 4 (1): 2–5.
  18. Gormezano, Linda J, and Robert F Rockwell. 2013. What to eat now? Shifts in polar bear diet during the ice-free season in western Hudson Bay. Ecology and Evolution 3 (10): 3509–23.

Image Credit: USFWS Endangered Species. 2010. Endangered, threatened gray wolf (Canis lupus). Flickr. http://www.flickr.com/photos/usfwsendsp/5039508124/in/photostream/ (Accessed February 21, 2014)

Alan D. Wilson. 2007. Polar Bear ANWR 1. Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Polar_Bear_ANWR_1.jpg (Accessed February 21, 2014)

Gavin Bell. 2005. Tiger Snarl. Flickr. http://www.flickr.com/photos/gavinbell/34918312/in/photostream/ (Accessed February 21, 2014)

 

Ikreet Cheema is an undergraduate student at the University of Calgary majoring in neuroscience. Follow The Triple Helix Online on Twitter and join us on Facebook.

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