Ecological Roles and Importance of Marine Mammals in Aquatic Ecosystems

Introduction
Marine mammals play an essential role in maintaining the aquatic ecosystems. They are found in ocean and freshwater ecosystems in almost all the latitudes (Blanchard et al, 2017). Owing to their abundance, high rate of metabolism and large scale movements they have a significant effect on the functioning of marine ecosystems. It is important to understand their role in the marine ecosystem in order to manage the effects of the changes that are occurring to their population and the population of their prey. Marine mammals influence the marine ecosystem through their ability to eliminate other marine animals through direct predation. They affect the marine ecosystem through diverse pathways some of which are functions of the behavior of the marine mammals. The effect of their behavior on the other marine animals is essential in sustaining the marine ecosystem (Bonar et al, 2015). The behavioral effects (also known as the non-consumptive effects) include the risk effects such as influencing the behavior of preys in ways that influence their population size and spatial distribution. The population of prey and their distribution influence the population and distribution of other marine animals as well thereby affecting the whole marine ecosystem (Hays et al, 2016 and Navarro et al, 2016). This illustrates that marine mammals influence the function and structure of many marine animals and their communities. Understanding the effects of marine mammals in the ecosystem is essential in evaluating their impact on the population of prey, their community structure and variations in their population. In the ecological sense, the term role refers to the functional significance of a given taxon or species. All the marine mammals influence the marine ecosystem in one way or the other and as such they influence the population and distribution of the other marine animals.

Effect of Consumption on the Marine Ecosystem

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Marine mammals are found in a wide range of ecological niches and in all oceans from the Polar Regions to the tropical zones. Some are even found in fresh water lakes and rivers. Their tolerance for varied conditions and high movement capacity implies that they significantly influence many ecosystems across a wide spatial area (Hocking et al, 2017). The ecological role of a species in the ecosystem is often analyzed based on the feeding and non-feeding effects on the habitat. According to Young et al (2015), many of the marine mammals belong to the upper tropic level of consumption. This means that they are at the apex of the food chain. By virtue of their position at the food chain, they have a significant influence on the population of their prey and the marine ecosystem. They can consume or alter traits of the other marine species (Apprill et al, 2014 and Doughty et al, 2016). Nonetheless, most of the marine mammals are also prey to other species such as the polar bear, crocodiles, birds, sharks and killer whales. In this role as mesoconsumers or mesopredators, the marine mammals influence or affect the population of their predators.

The influence of marine mammals is not limited to the predator and prey relationship. For instance cetaceans can influence foraging of other species such as the seabirds. In addition to this, the marine mammals can stimulate primary production through facilitating nutrient cycling. They also act as mobile vectors for transporting nutrients across ecosystems and microhabitats. It is important to distinguish the role of a species from its importance in the ecosystem (Kortsch et al, 2015 and Roman et al, 2014). The importance of a species in the ecosystem refers to the consequences of its substantial change in population to the ecosystem.

Although the most noticeable role of predators in the ecosystem revolves around their interactions with their prey, they often facilitate the movement of nutrients between or within habitats and strata thereby facilitating production in the ecosystem and influencing the population size of the animals in the lower trophic levels (Bogstad et al, 2015 and Pikitch et al, 2014). The marine mammals can transport nutrients from the oceans to the fresh water habitats or from the deep seas to the terrestrial environments. The nutrients are found in their carcasses, regurgitation and feces. For instance the marine mammals can move from the marine to the fresh water sources thereby transporting nutrients through defecation. However, not all transport of nutrient occur from the deep waters to the surface waters, because the carcasses of large whales that fall to the floor of the oceans often provide essential nutrients that sustain deep sea communities (Woinarski et al, 2015). Furthermore, such carcasses transport marine energy to the terrestrial habitats. For instance bears rely on whale carcasses for a significant portion of their diet.

Behavioral Effects Marine Mammals
The importance of marine mammals in the ecosystem can be analyzed using bioenergetic models to estimate the food requirements and potential biomass. There is some overlap between fish and the marine mammals but the effect can be quantified and eliminated (Piroddi et al, 2015). The importance of both marine animals and their prey to the ecosystem system remain understudied because of the absence of efficient models for estimation (Nelson et al, 2015). There are various behavioral decisions that can affect the role and importance of a species. For instance food selection, foraging techniques, habitat patterns and grouping can all influence spatial distribution and magnitude of ecological interactions. Factors that influence behavioral patterns can be used to elucidate the causal relationship between ecological dynamics and the behavior of marine mammals.

Marine mammals can influence the marine ecosystem through diverse pathways, some of which are driven by their behavior. The behavioral or non consumptive effects of the marine mammals on the ecosystem are essential in investigating their importance in the ecosystem (Bik et al, 2016). The behavioral effects include the risk effects; the predators often influence the behavior of their prey, their population size and spatial distribution (McCauley et al, 2015 and Yurkowski et al, 2016). For instance a change in the population of large animals found at the apex of the food chain can influence the behavior of marine mammals and their prey. Moreover, foraging tactics of marine mammals often facilitate the foraging of other species such as the seabirds.
According to Kelley & Pyenson (2015), although marine mammals at times share prey and compete with other marine animals, their presence may actually improve the size of population and foraging success of these taxa. For example seabirds and cetaceans forage closely in many regions. In most of the cases, seabirds often appear to benefit from these associations (Kiszka et al, 2015 and Young et al, 2016). Sardines and herrings are often forced to form tight aggregations close to the surface when under attack from predators such as cetaceans. As a consequence, they often become more accessible to the seabirds. This clearly illustrates the association between marine mammals, sardines (and other fish species) and the seabirds.

Conclusion
Most of the studies on the ecological role and importance of marine mammals have often focused on the causal association between the predator and prey. This study has illustrated that the behavioral patterns of marine mammals have an effect on the marine ecosystems. Predators can induce drastic changes to the behavior of their prey thereby altering their population size and spatial distribution within the wider ecosystem. An increase in the population of marine mammals can affect the behavior of other marine animals because of the increase in predator risk that may force them to cascade into the wider ecosystem. In addition to this, most marine mammals are mesopredators and as such they also experience risk effects. Marine mammals also facilitate the foraging of other marine animals and nutrient transport.

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