Introduction
At various trophic levels, including primary production, predatory fish, and other marine mammals including pinnipeds, killer whales, and polar bears, marine mammals operate as greater consumers of production (Basset, 2010). Marine mammals are thought to have a substantial impact on the structure and function of many aquatic communities due to their abundance and enormous body sizes (Basset, 2010). The long-running dispute over the ecological linkages between fisheries and marine mammals is supported by the idea that fisheries may have an impact on marine mammals and that marine mammals may have significant effects on populations of prey that are of commercial interest to humans (Ecott, 2007). A comprehension of the ecological roles and importance of marine mammals in aquatic ecosystems is essential since it gives a context for evaluating the potential effect of their predation on community structure and prey population, as well as the impact of environmental variation on the marine mammals’ dynamics (Gentry, 2010). This paper explores the various ecological roles and importance of marine mammals in aquatic ecosystems. The terms ‘role’ and ‘importance’ are used interchangeably in this paper and in an ecological sense to imply the functional significance of marine mammals.
Trophic Interactions
All marine mammals, in one way or another, influence various aquatic ecosystems of which they form a part. In the top-down view of different ecological interactions, research shows that marine mammals have a trophodynamic function in aquatic systems as consumers and their consumption has a significant effect on community structure and species interactions (Geist & Hawkins, 2016; Malakoff, 2001). The energy flow through the marine ecosystems’ cetacean component is considered as evidence of their ecological importance. According to research, cetaceans, such as sperm whales might consume a significant prey quantity than all human fisheries collectively (Reyes, 2006). Besides, research shows that marine mammals form the second-largest consumers of fish biomass and that their predation on other aquatic populations such as fish is significantly high, mainly due to whales (Wursig, 2006). Research evidence further indicates that marine mammals, especially whales, are essential predators of fish and that aquatic mammals, in general, are important prey consumers in several aquatic ecosystems (Reyes, 2006).
Impact on Community Structure and Species Interactions
Additionally, various alterations in the patterns of marine mammals’ food consumption resulting from their overexploitation in the Southern Ocean have undergone extensive legal analysis as evidence of the large cetaceans’ role (Basset, 2010; Sousa-Lima, 2013). The large cetaceans’ food consumption patterns, both in the Bering Sea and the Southern Ocean, have been found to have significant effects on community structure (Sousa-Lima, 2013). Also, pinnipeds, although having significantly smaller body sizes than the cetaceans, have a high likelihood of being ecologically important consumers in certain marine ecosystems. However, in all such cases, there is a need for caution in interpreting the marine mammals’ food consumption, especially at larger spatial scales, as a measure of their ecological importance (Kisz, Heithaus, & Wirsing, 2015).
Role in Nutrient Cycling and Habitat Modification
Marine mammals also play an essential role in shaping the life history traits and behavior of various prey species and their competitors in the context of modifying benthic habitat and nutrient recycling and storage (Irvine, 2012). Some cetaceans have been found to have an essential ecological role in aquatic ecosystems' nutrient recycling through feeding in the deep waters and then defecating in the marine ecosystems' euphotic zones (Irvine, 2012). However, research also shows that in the marine mammals’ role of recycling nutrients may be less significant in the open ocean. Large cetaceans may also continue playing an essential ecological role in marine ecosystems’ nutrient recycling, even after they die, through the downward nutrients transfer to benthic communities. In fact, according to recent research, large cetaceans may form essential agents for the dispersal of various deep-sea chemosynthetic communities over extensive areas (Geist & Hawkins, 2016).
Physical Restructuring of Benthos
Certain marine mammals may also play an essential role in the benthos' physical restructuring. Research shows that grey whales in the northern Bering Sea can turn over about 9 to 27 percent of the ecosystem’s benthic substrate annually (Nachtigall, 2009). Such disturbance can be helpful in maintaining early colonizing aquatic species at higher abundance, as well as providing habitat for the young ones of the whales’ primary amphipod prey that would otherwise be the case (Nachtigall, 2009). Besides, the feeding disturbances by the grey whales assist in maintaining the ocean’s sand substrate through the suspension of fine sand sediments. Such sandy sediments, in turn, help in supporting the amphipods’ dense beds that are consumed by gray whales. Therefore, the gray whales’ feeding disturbances can lead to higher prey densities over a larger spatial scale (Osinga & Hart, 2008).
Impact on Benthic Fauna and Community Structure
Additionally, walruses have been found to be highly specialized faunal bivalves’ consumers. In their feeding process, walruses produce several furrows and pits in soft sediments. As a result, they may structure the benthic fauna through selective feeding on various older bivalve mollusk species (Bowen, 1997). The walruses' ingestion and defecation may cause tremendous sediment redistribution, which may enhance the colonization of particular species as opposed to others (Bowen, 1997; Irvine, 2012). Walruses can, therefore, impact community structures in three primary ways. That may be through providing certain scavengers such as brittle stars with food, offering habitat for other aquatic species under bivalve shells, as well as lowering the population of macroinvertebrates in the feeding pits to numbers lower than the surrounding sediments (Bowen, 1997; Irvine, 2012).
Role in Nearshore Community Structure
Marine mammals such as the sea otter also play an important role in structuring various nearshore communities. The sea otters predate on a broad range of aquatic invertebrates, including crabs, clams, and urchins that inhabit the nearshore community (Abiati & Basset, 2001; Carlberg, 2015). The sea urchins, on the other hand, graze on kelp, which forms a primary component of the nearshore ecosystems (Abiati & Basset, 2001; Reyes, 2006). The ecological interactions among the kelp, urchins, and sea otters have been studied extensively, and research shows that the presence of sea otters allows for the development of kelp forests since the otters will predate on urchins, thereby reducing their population. That allows other aquatic species, like various fish species, to increase, thus creating a more diverse community (Abiati & Basset, 2001).
Conclusion
In conclusion, marine mammals have significant roles and importance in the aquatic ecosystems. It is evident that the distribution and abundance of marine mammals can create important impacts on the function and structure of some aquatic ecosystems. Besides, developing a better understanding of the role and importance of marine mammals in aquatic ecosystems is of great importance, although it also has various challenges since the temporal and spatial scales of marine mammal predation can vary depending on their preys. However, the study of different roles and importance of marine mammals in aquatic ecosystems remains vital despite the challenges involved and regardless of the approach used.
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