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Loss of Top Predators in the Ocean

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Published: Tue, 12 Sep 2017

It is becoming increasingly difficult to ignore the sign of deterioration within the population dynamics of marine apex predators. Sharks, mammals, and large teleost fish amongst others species, play a critical role in maintaining a stable and balanced marine ecosystem by regulating species abundance, diversity, and distribution (Stier et al., 2016). Recently, researchers have shown an interest in the increasing densities of medium size predators known as the mesopredators due to rapid decline in apex predators (Brook et al., 2008). However, these rapid changes are having a serious adverse effect on the marine community, thereby leading to a top-down trophic cascade due to the removal of a top predator (Shackell et al., 2010). To be more specific, the loss of top predators affects the aquatic community on a number of levels: trophic, behavioral, and populations. This essay will provide an overview and highlight the importance of top predators in marine ecosystem as well as change in trophic levels and behavioral patterns that affect the aquatic food web.

Apex predators have been victim to human imposed activities such as overexploitation through fishing industries, habitats destruction, introduction of invasive species and increased anthropogenic pollutants creating unfavorable conditions to thrive (Madin et al., 2016). The major impact relating to loss of top predators manifests in the trophic aspect following in a close correlation between the decline of the predators and preys (Baum and Worm, 2009).

Extensive research indicates that the existences of the invasive species has a negative contribution towards the top predators (De Poorter et al., 2010). For example, in 1980s the comb jelly fish (Ctenophora) originally from North America was introduced in the black sea, which lead to a dramatic alteration within the food chain, thus releasing toxin causing the death a total of 14 humpback whales (De Poorter et al., 2010). On the other hand, overfishing across the globe has led to 90% decrease in large shark biomass regionally (Heupel et al., 2014). Due to the decline of larger sharks it has led to the inflexion of a particular ray species which diet constitute about 70% of scallops thus contributed towards the drop of scallop fishery causing ecological and economic losses in the North Atlantic Ocean (Ferretti et al., 2010) (Grubbs et al., 2013). For instance, a research conducted on the U.S eastern seaboard indicates that a decrease in 11 types of large sharks results in the addition of 12 to 14 small mesoconsumers (Madin et al., 2016). Additionally, a decline in catch rates of 13 large pelagic predators results in an upsurge catch of pelagic stingrays and other small animals (Madin et al., 2016). Furthermore, studies showed that decline in the Canadian cod (Gadus morhua), led to the upsurge in the number of small pelagic and marine invertebrates. As a result, this has led to the cascading alterations in the copepod in addition to the phytoplankton communities (Ara├║jo and Bundy, 2012).

Studies have also shown that shifts in the diet by destroyer whales, which move along the Aleutian island have resulted from a reduction in the number of sea otters present in the island. In effect, urchins are released from the predations thereby causing the reduction of kelp forest due to their overgrazing (Heithaus et al., 2008). Therefore, it is evident that there is a close connection between the decline of the predators and the prey in the ecosystems. Another aspect that should be closely examined is the behavioral one.

The scholars believe that aquatic life uses the behavioral responses to different risk in efforts to avoid encountering predators. For instance, dolphins and other animals forage in productive shallow seagrass during a time when the number of sharks is reduced. However, they move to less industrious but harmless surroundings when the population of sharks in the ecosystem is high (Griffin et al., 2008). Additionally, when top predator disappear the mesoconsumer population increases and may shift their behavior in foraging pattern, which affect the ecosystem. For instance, shark and killer whale help maintain the seal population but due to loss in numbers the fur seals (Arctocephalus forsteri) in New Zealand thrives causing the temperate reef fish (morwong Chelilodactulus nigripes) to reduce foraging effort, leading to reduced grazing on turf algae (Heithaus et al., 2008).

The impacts of behavior response to predation risks among the prey may include experiencing energetic cost and the lack of adequate utilization of resources (Langerhans, 2007). As such, their growth and reproductive outputs are limited in the ecosystems by the predation risks (Abdulla, 2004). In other cases, the risks of tiger sharks limit the number of prey and activate trophic cascades. Tiger sharks ultimately decrease browsing on the sea grasses in risky places but escalating in harmless environments that are portrayed in longitudinal processes of seagrass nutritional components (Griffin et al., 2008).

Having examined all the points, that were mentioned in the paragraphs above, one is able to come to the following conclusion: top predators are important as they regulate the species abundance, diversity, and distribution that contribute to a healthy marine ecosystem. The significance of the risk effects as well as individual predator species depends on the community diversity, habitat heterogeneity, life history features of mesoconsumers and predators. A decrease in the predator diversity in some occasions leads to positive impacts on the mesoconsumers (Baum and Worm, 2009). The comparative impacts of specific predator type elimination can decrease per an upsurge in variety, which relies on the pursuing strategies utilized by the alienated slayers in the population. The environmental structures have an influence on the ecological effects of top predators declines as well as the relative strengths of risk effects. Furthermore, the decreased number of predators results in the increased prey populations and other adverse effects such as shifting coral reefs to algae dominated habitats, a decline of seagrass among others. Resent research states the predators play a major role in carbon cycling within the ocean (Atwood et al., 2015). Therefore, there is the need to protect these predators from future declines through establishing some effective strategies. The strategies may include decreasing the demand for shark products, reducing the number of top predators reared for commercial fisheries and ensuring improved management.

References

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Atwood, T.B., Connolly, R.M., Ritchie, E.G., Lovelock, C.E., Heithaus, M.R., Hays, G.C., Fourgurean, J.W and Macreadie, P.I., 2015. Predators help protect carbon stocks in blue carbon ecosystem. Nature Climate Change, 5(12), pp. 1038-1045.

Araujo, J.N. and Bundy, A., 2012. Effects of environmental change, fisheries and trophodynamics on the ecosystem of the western Scotian Shelf, Canada. Marine Ecology Progress Series, 464, pp.51-67.

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Edwards, H., 2016. When Predators Become Prey: The Need For International Shark Conservation. Ocean and Coastal Law Journal, 12(2), p.5.

Ferretti, F., Worm, B., Britten, G.L., Heithaus, M.R. and Lotze, H.K., 2010. Patterns and ecosystem consequences of shark declines in the ocean. Ecology letters, 13(8), pp.1055-1071.

Griffin, E., Miller, K., Freitas, B. and Hirshfield, M., 2008. Oceana: Predators as Prey: Why Healthy Oceans Need Sharks.

Grubbs, R.D., Carlson, J.K., Romine, J.G., Curtis, T. and McElroy, D., 2013. Save the bay, eat a ray: a purported trophic cascade mediated by declines in large shark populations and the consequences of applying simplistic models to complex ecosystems. Conference abstract. In 141st American Fisheries Society meeting. American Fisheries Society, CITY, Maryland. Abstract_Book_ (pp. 9-20).

Heithaus, M.R., Frid, A., Wirsing, A.J. and Worm, B., 2008. Predicting ecological consequences of marine top predator declines. Trends in Ecology & Evolution, 23(4), pp.202-210

Heupel, M.R., Knip, D.M., Simpfendorfer, C.A. and Dulvy, N.K., 2014. Sizing up the ecological role of sharks as predators. Marine Ecology Progress Series, 495, pp.291-298.

Langerhans, B.R., 2007. Evolutionary consequences of predation: avoidance, escape, reproduction, and diversification. In Predation in Organisms. Springer Nature, pp. 177-220.

Madin, E.M., Dill, L.M., Ridlon, A.D., Heithaus, M.R. and Warner, R.R., 2016. Human activities change marine ecosystems by altering predation risk. Global change biology, 22(1), pp.44-60.

Myers, R.A., Baum, J.K., Shepherd, T.D., Powers, S.P. and Peterson, C.H., 2007. Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science, 315(5820), pp.1846-1850.

Shackell, N.L., Frank, K.T., Fisher, J.A.D., Petrie, B. and Leggett, W.C. 2010. Decline in top predator body size and changing climate alter trophic structure in an oceanic ecosystem’, Proceedings of the Royal Society B: Biological Sciences, 277(1686), pp. 1353-1360.

Stier, A.C., Samhouri, J.F., Novak, M., Marshall, K.N., Ward, E.J., Holt, R.D. and Levin, P.S. 2016. Ecosystem context and historical contingency in apex predator recoveries, 2(5).

Thomsen, S.K. and Green, D.J., 2016. Cascading effects of predation risk determine how marine predators become terrestrial prey on an oceanic island. Ecology, 97(12), pp.3530-3537.


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