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Apex predator

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The lion is one of Africa's apex land predators.
The saltwater crocodile is the largest living reptile[1] and the dominant predator throughout its range.[2]

An apex predator, also known as an alpha predator or top predator, is a predator at the top of a food chain, with no natural predators.[a][4][5]

Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic levels. Food chains are often far shorter on land, usually limited to being secondary consumers – for example, wolves prey mostly upon large herbivores (primary consumers), which eat plants (primary producers). The apex predator concept is applied in wildlife management, conservation and ecotourism.

Ecological roles

Effects on community

Apex predators affect prey species' population dynamics and populations of other predators, both in aquatic and in terrestrial ecosystems. Non-native predatory fish, for instance, have sometimes devastated formerly dominant predators. A lake manipulation study found that when the non-native smallmouth bass was removed, lake trout, the suppressed native apex predator, diversified its prey selection and increased its trophic level.[6] As a terrestrial example, the badger, an apex predator, predates on and also competes with the hedgehog, a mesopredator, for food such as insects, small mammals, reptiles, amphibians and ground-nesting bird's eggs. Removal of badgers (in a trial investigating bovine tuberculosis) caused hedgehog densities to more than double.[7] Predators that exert a top-down control on organisms in their community are often considered keystone species.[8]

Effects on ecosystem

Apex predators can have profound effects on ecosystems, as the consequences of both controlling prey density and restricting smaller predators, and may be capable of self-regulation.[9] They are central to the functioning of ecosystems, the regulation of disease, and the maintenance of biodiversity.[10] When introduced to subarctic islands, for example, Arctic foxes' predation of seabirds has been shown to turn grassland into tundra.[11] Such wide-ranging effects on lower levels of an ecosystem are termed trophic cascades. The removal of top-level predators, often through human agency, can cause or disrupt trophic cascades.[12][13][14] For example, reduction in the population of sperm whales, apex predators with a fractional trophic level of 4.7, by hunting has caused an increase in the population of large squid, trophic level over 4 (carnivores that eat other carnivores).[15] This effect, called mesopredator release,[16] occurs in terrestrial and marine ecosystems; for instance, in North America, the ranges of all apex carnivores have contracted whereas those of 60% of mesopredators have grown in the past two centuries.[17]

Conservation

The gray wolf is both an apex predator and a keystone species, affecting its prey's behaviour and the wider ecosystem.

Because apex predators have powerful effects on other predators, on herbivores, and on plants, they can be important in nature conservation.[18] Humans have hunted many apex predators close to extinction, but in some parts of the world these predators are now returning.[19] Dramatic changes in the Greater Yellowstone Ecosystem were recorded after the gray wolf, both an apex predator and a keystone species (one with a large effect on its ecosystem), was reintroduced to Yellowstone National Park in 1995 as a conservation measure. Elk, the wolves' primary prey, became less abundant and changed their behavior, freeing riparian zones from constant grazing and allowing willows, aspens and cottonwoods to flourish, creating habitats for beaver, moose and scores of other species.[20] In addition to their effect on prey species, the wolves' presence also affected one of the park's vulnerable species, the grizzly bear: emerging from hibernation, having fasted for months, the bears chose to scavenge wolf kills,[21] especially during the autumn as they prepared to hibernate once again.[22] The grizzly bear gives birth during hibernation, so the increased food supply is expected to produce an increase in the numbers of cubs observed.[23] Dozens of other species, including eagles, ravens, magpies, coyotes and black bears have also been documented as scavenging from wolf kills within the park.[24]

Human trophic level

Humans hunt other animals for food, fur, and recreation.

Ecologists have debated whether humans are apex predators. Sylvain Bonhommeau and colleagues argued in 2013 that across the global food web, a fractional human trophic level (HTL) can be calculated as the mean trophic level of every species in the human diet, weighted by the proportion which that species forms in the diet. This analysis gives an average HTL of 2.21, varying between 2.04 (for Burundi, with a 96.7% plant-based diet) and 2.57 (for Iceland, with 50% meat and fish, 50% plants). These values are comparable to those of non-apex predators like anchovy or pig.[25] Peter D. Roopnarine criticised Bonhommeau's approach, arguing that humans are apex predators, and that the HTL was based on terrestrial farming where indeed humans have a low trophic level, mainly eating producers (crop plants at level 1) or primary consumers (herbivores at level 2), which as expected places humans at a level slightly above 2. Roopnarine instead calculated the position of humans in two marine ecosystems, a Caribbean coral reef and the Benguela system near South Africa. In these systems, humans mainly eat predatory fish and have a fractional trophic level of 4.65 and 4.5 respectively, making them "apex predators in those systems."[b][26]

Interactions with humans

Tiger shark with human diver for scale

In ecotourism

Ecotourism sometimes relies on apex predators to attract business.[27][28] Tour operators may in consequence decide to intervene in ecosystems, for example by providing food to attract predators to areas that can conveniently be visited.[27] This in turn can have effects on predator population and therefore on the wider ecosystem.[27] As a result, provisioning of species such as the tiger shark is controversial, but its effects are not well established by empirical evidence.[27] Other affected apex predators include big cats and crocodiles.[28]

In populated areas

The reintroduction of predators like the lynx is attractive to conservationists, but alarming to farmers.

In some densely populated areas like the British Isles, all the large native predators like the wolf, bear, wolverine and lynx have become locally extinct, allowing herbivores such as deer to multiply unchecked except by hunting.[29] In 2015, plans were made to reintroduce lynx to the counties of Norfolk, Cumbria, and Northumberland in England, and Aberdeenshire in Scotland as part of the rewilding movement.[30] The reintroduction of large predators is controversial, not least because of concern among farmers for their livestock.[30] Conservationists such as Paul Lister propose instead to allow wolves and bears to hunt their prey in a "managed environment" on large fenced reserves.[30]

See also

Notes

  1. ^ Zoologists generally exclude parasites from tropic levels as they are (often much) smaller than their hosts, and individual species with multiple hosts at different life-cycle stages would occupy multiple levels. Otherwise they would often be at the top level, above apex predators.[3]
  2. ^ However, humans had a network trophic level (NTL) of 4.27 in the coral reef system, compared to an NTL of 4.8 for the blacktip shark in the same system. Therefore, humans were not the topmost apex predator there.[26]

References

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  2. ^ Whiting, Frances. "Terri fights to halt croc eggs harvest." Archived 2010-10-28 at the Wayback Machine. Australia Zoo. 2007-06-11. Retrieved 2010-01-25.
  3. ^ Sukhdeo, Michael VK (2012). "Where are the parasites in food webs?". Parasites & Vectors. Springer Nature. 5 (1): 239. doi:10.1186/1756-3305-5-239. 
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  5. ^ "apex predator". PBS. Archived from the original on 2009-07-22. Retrieved 2010-01-25. 
  6. ^ Lepak, Jesse M.; Kraft, Clifford E.; Weidel, Brian C. (March 2006). "Rapid food web recovery in response to removal of an introduced apex predator" (PDF). Canadian Journal of Fisheries and Aquatic Sciences. 63 (3): 569–575. Archived from the original (PDF) on 11 September 2008. 
  7. ^ Trewby, Iain D.; Young, Richard; McDonald, Robbie A.; Wilson, Gavin J.; Davison, John; Walker, Neil; Robertson, Andrew; Doncaster, C. Patrick; Delahay, Richard J. (April 2014). Criscuolo, François, ed. "Impacts of Removing Badgers on Localised Counts of Hedgehogs". PLoS ONE. 9 (4): e95477. doi:10.1371/journal.pone.0095477. 
  8. ^ Davic, Robert D. (2003). "Linking Keystone Species and Functional Groups: A New Operational Definition of the Keystone Species Concept". Conservation Ecology. Retrieved 2011-02-03. 
  9. ^ Wallach, Arian D.; Izhaki, Ido; Toms, Judith D.; Ripple, William J.; Shanas, Uri (2015). "What is an apex predator?". Oikos. 124 (11): 1453–1461. doi:10.1111/oik.01977. 
  10. ^ Stier, A. C.; Samhouri, J. F.; Novak, M.; Marshall, K. N.; Ward, E. J.; Holt, R. D.; Levin, P. S. (May 2016). "Ecosystem context and historical contingency in apex predator recoveries". Science Advances. 2 (5): e1501769–e1501769. doi:10.1126/sciadv.1501769. 
  11. ^ Croll, D. A.; Maron, J. L.; et al. (March 2005). "Introduced predators transform subarctic islands from grassland to tundra". Science. 307 (5717): 1959–1961. doi:10.1126/science.1108485. PMID 15790855. Archived from the original on 2009-10-28. Retrieved 2010-01-25. 
  12. ^ Egan, Logan Zane; Téllez, Jesús Javier (June 2005). "Effects of preferential primary consumer fishing on lower trophic level herbivores in the Line Islands" (PDF). Stanford at Sea. Stanford University. Archived (PDF) from the original on 2010-07-12. Retrieved 2010-01-25. 
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  14. ^ Berger, Joel (1999). "Anthropogenic extinction of top carnivores and interspecific animal behaviour: implications of the rapid decoupling of a web involving wolves, bears, moose and ravens" (PDF). Proceedings of the Royal Society B: Biological Sciences. 266 (1435): 2261–2267. doi:10.1098/rspb.1999.0917. 
  15. ^ Baum, Julia K.; Worm, Boris (2009). "Cascading top-down effects of changing oceanic predator abundances". Journal of Animal Ecology. 78 (4): 699–714. doi:10.1111/j.1365-2656.2009.01531.x. 
  16. ^ Soulé, Michael E.; Bolger, Douglas T.; Alberts, Allison C.; Wright, John; Sorice, Marina; Hill, Scott (March 1988). "Reconstructed Dynamics of Rapid Extinctions of Chaparral-Requiring Birds in Urban Habitat Islands" (PDF). Conservation Biology. 2 (1): 75–92. 
  17. ^ Prugh, Laura R.; Stoner, Chantal J.; Epps, Clinton W.; Bean, William T.; Ripple, William J.; Laliberte, Andrea S.; Brashares, Justin S. (2009). "The Rise of the Mesopredator". BioScience. 59 (9): 779–791. doi:10.1525/bio.2009.59.9.9. 
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  19. ^ Silliman, Brian R.; Hughes, Brent B.; Gaskins, Lindsay C.; He, Qiang; Tinker, M. Tim; Read, Andrew; Nifong, James; Stepp, Rick (2018). "Are the ghosts of nature's past haunting ecology today?". Current Biology. 28 (9): R532–R537. doi:10.1016/j.cub.2018.04.002. 
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  21. ^ Levy, Sharon (November 2002). "Top Dogs". Archived from the original on 2009-06-06. Retrieved 2010-01-25. 
  22. ^ Wilmers, Christopher C. (2004). "The gray wolf – scavenger complex in Yellowstone National Park" (PDF). pp. 56, 90 and throughout. Archived (PDF) from the original on 2010-07-12. Retrieved 2010-01-25. 
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  26. ^ a b Roopnarine, Peter D. (2014). "Humans are apex predators". Proceedings of the National Academy of Sciences. 111 (9): E796–E796. doi:10.1073/pnas.1323645111. PMC 3948303Freely accessible. 
  27. ^ a b c d Hammerschlag, Neil; Gallagher, Austin J.; Wester, Julia; Luo, Jiangang; Ault, Jerald S. (2012). "Don't bite the hand that feeds: assessing ecological impacts of provisioning ecotourism on an apex marine predator". Functional Ecology. 26 (3): 567–576. doi:10.1111/j.1365-2435.2012.01973.x. 
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  29. ^ Jones, Lucy. "The rewilding plan that would return Britain to nature". BBC. Retrieved 6 June 2018. wolves, bears and lynx roamed the land. ... Humans chopped down the trees to make space for farms, and hunted the large animals to extinction, leaving plant-eaters to decimate the country's flora. Britain is now one of the few countries in the world that doesn't have top predators. 
  30. ^ a b c Lister, Paul (28 April 2015). "Bring on a few more apex predators". The Daily Telegraph. Retrieved 14 March 2018. 
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