Marine mammals play a key role in maintaining the functioning and productivity of our aquatic ecosystems, and their importance takes shape in many different ways (Kiszka, Heithaus, & Wirsing, 2015). The polyphyletic group is diverse in size, diet, behavior, habitat range, and many other ways, making it difficult to make sweeping assumptions about the group (Kiszka et al., 2015). Our understanding of these species is far from complete due to complex deep diving patterns, exploited populations, and other anthropogenic and biological factors that serve as limitations to our knowledge (Rogan et al, 2017).
Due to the biomass necessary to sustain healthy marine mammal populations, they take on the role top predators, having an influential role on the structure and maintenance of their communities, oftentimes being keystone species (Heithaus, Frid, Wirsing, & Worm, 2007; Kiszka et al., 2015). The influence of marine mammals in their ecosystems can often be described in terms of ‘top-down’ and ‘bottom-up’ effects (Kiszka et al., 2015).
Knowing the diets of marine mammals allows us to understand their feeding habits and the role they fill in the food web, especially since they can be attributed to eating large amounts of prey within their communities (Fitch & Brownell, 1968; Kiszka, et al.
, 2015). In a healthy ecosystem there is a balance amongst trophic levels, but in cases where a top predator’s population is reintroduced or restored, the ecosystem in which they are apart of can be completely restructured, such as the case with sea otters, Enhydra lutris, in Alaska (Estes & Duggins, 1995). Sea urchins feed on kelp and algae, and E.
lutris prey on sea urchins, typically leading to a balance in that ecosystem, but in the case where the population of E. lutri was restored, sea urchin populations experienced a sharp decline and the kelp forests substantial regrowth (Estes & Duggins, 1995). While marine mammals such as E. lutris have played vital roles in the restoration of nearshore and intertidal ecosystems, drivers for ecosystem decline should not be assumed and anthropogenic and other influences should be taken into consideration (Estes & Palmisano, 1974).
Aside from direct predation influences, top predators, which marine mammals often are, drive foraging and migration behavior of prey species through the assumption of risk (Heithaus, et al., 2007). A great example of this is the daily migration many species make up to the surface at night and back down deep into the ocean during the day to avoid predation (Heithaus, et al., 2007). Vocalization has been found as an influence in the movement of prey, as this can be an indication of imminent predation (Kiszka et al., 2015). Amongst marine mammal prey, the reaction to risk effects can take form in greater group cooperation and stability, even between members of different species, in order to avoid predation (Kiszka et al., 2015).
Nutrient cycling is critical for the maintenance and productivity of our oceans, and marine mammals, especially large migratory ones like whales, drive a lot of this mixing (Roman et al., 2014). The sheer mass of some marine mammals contributes to the mechanical mixing of the water column, and this nutrient mixing process has been called the “whale pump” due to their significant role in this process, but seals have also been noted for their nutrient cycling contributions (Roman et al., 2014; Kiszka et al., 2015). After surfacing from feeding deeper in the ocean, cetaceans have been found to deliver large quantities of nitrogen, carbon, iron, and other limiting macronutrients up to the photic zone through defecation and urea excretion (Roman et al., 2014). By stimulating greater primary productivity, the energy in the increased plankton biomass then gets transferred back down to the deep sea once they die, continuing this process of nutrient cycling throughout the ocean (Roman et al., 2014). Processes like these are known as a ‘bottom-up’ effect and there is still so much to be learned about the contribution made by marine mammals (Kiszka et al., 2015).
Another key source of nutrients in marine ecosystems is transferred through the decaying carcasses of marine mammals (Roman et al., 2014). A single, forty-ton whale carcass can deliver the equivalent of 2000 years worth of natural carbon accumulation to nutrient devoid areas of the deep sea (Roman et al., 2014). These carcasses have helped develop diverse deep-sea communities, and many species have evolved out of this process, highlighting the threat commercial whaling poses to the functioning of this process (Roman et al., 2014). Marine mammals are not confined to the ocean, and many, like the polar bear, split their time between foraging in the ocean and living on land (Monnet & Gleason, 2006). This creates a mobile link between the two ecosystems, transferring energy and biomass between the ocean and land and freshwater habitats (Kiszka et al., 2015). As mentioned, fallen mammal carcasses bring nutrients to the deep sea, but deceased species need not sink every time (Roman et al., 2014). Stranded carcasses can provide nourishment to terrestrial animals, strengthening this biological connection between land and sea (Roman et al., 2014).
There is a high level of intelligence, social interaction, and cohesion observed in marine mammal species and this has allowed for greater survival of these species, solidifying their role as top predators (Lusseau, 2006). For example, vocalization within pinnipeds is critical for intra-group communication for mating and indicating danger (Schusterman, Southall, Kastak, & Reichmuth Kastak, 2001). Other vocalizations, like echolocation, aid in group cooperation as well as signaling their presence and possible motives to other species (Kiszka et al., 2015). There have been findings of social stratification and roles based on sex and age within species groups, leading to an increased complexity of the influence individual mammals has on their communities (Kiszka et al., 2015). Species like the bottlenose dolphin have complex seasonal and temporal migrations as groups, allowing for a larger range of species influence (Shane, Wells, & Würsig, 1986).
Being able to understand the impact of a decline of marine mammals and their current population numbers is crucial for guiding conservation management strategies (Hammond, et al., 2013). Though we have investigated the critical roles marine mammals serve in our ecosystems, fisheries often target them as nuisance species in an attempt to increase their catch (Yodzis, 1998). Whether it is cetaceans or fur seals, the accuracy at which we can assess the damage this causes is limited in some areas (Yodzis, 1998; Bearzi, Holcer, & Notarbartolo Di Sciara, 2004). While there is still a lot to learn about what exactly these impacts are, at a glance, studies have shown a logical impact on predator-prey interactions through predator decline and increase in their prey, leading to the decline of these species prey, having a top-down, cascading impact on the ecosystem (Heithaus, et al., 2007). Revisiting the case of the E. lutris, population declines in some areas has lead to the unchecked growth of sea urchins, decimating the kelp forests there, indicating a heavy top-down influence (Heithaus, et al., 2007). It is worthy to invest effort into understanding the impact of anthropogenic effects on marine mammals to be able to better predict future changes in ecosystem functioning (Kiszka et al., 2015).
There is so much diversity within the species of marine mammals, yet they all share similar roles and influences within the communities they reside in, in both obvious and nuanced ways (Kiszka, et al., 2015). Marine mammals can be fierce top predators in their communities, but some, like many whales transferring nutrients throughout the oceans, incidentally have crucial roles in the survival and maintenance of others in their communities (Heithaus et al., 2007; Roman et al., 2014). There is still so much to be understood of the role marine mammals play in our aquatic ecosystems, driving the importance of establishing baseline indicators, accurate population abundances, and global cooperation (Hammond et al., 2013).