Abstract
Small cetaceans (dolphins, porpoises, and small toothed whales) occur from the poles to the tropics, and from freshwater habitats to the open ocean. Most ecological research has focused on the influence of abiotic factors on the abundance, distribution, and behavior of these species. The ecological impacts of small cetaceans on communities and ecosystems remain poorly quantified. Their movement patterns, often high local and regional abundances across a range of ecosystems, and high metabolic rates suggest that small cetaceans could have large effects on ecosystem structure, dynamics, and function through a variety of mechanisms. These include top-down (e.g., direct predation and risk effects) and bottom-up effects (e.g., translocation of nutrients within and across ecosystems), but also behavior-mediated processes where these predators can facilitate access to resources to other predators or modify the physical properties of habitat (e.g., bioturbation). Most small cetaceans can be consumed by other marine predators, particularly killer whales (Orcinus orca) and large sharks. Although consumption rates of small cetaceans can be high, there is a paucity of information on their effects on population sizes or behaviors of their prey. Mass-balance ecosystem models suggest that small cetaceans may impact the populations of short-lived prey species (particularly fish and cephalopods), but other factors (e.g., eutrophication and fisheries) also affect ecosystem functioning and population trends. Delphinids can also mediate the translocation and recycling of limiting nutrients between spatially distinct ecosystems on a diel basis. Despite intriguing possibilities, large gaps remain in our understanding of the roles and importance of small cetaceans in aquatic ecosystems, both marine and freshwater.
Highlights
While there are increasing efforts to improve our understanding of the ecological roles and importance of small cetaceans, these efforts lag behind those focused on large whales (e.g., Roman et al., 2014; Doughty et al., 2016)
With their high abundance in some ecosystems and widespread distribution, small cetaceans could have significant impact on the structure and function of aquatic ecosystems. These effects may be important in tropical and warm temperate ecosystems that are oligotrophic or where interactions are spatially confined and there may be limited dilution of their ecological impacts. These latter systems represent an important opportunity for quantifying effects of small cetaceans because they would theoretically be easier to detect and measure
Efforts to elucidate the ecological importance of small cetaceans, need to be focused largely on the behavior and population dynamics of their prey which remain poorly understood in many systems
Summary
Understanding the ecological roles and importance of large-bodied aquatic consumers ( high trophic level predators) has been the focus of an increasing number of studies over the past two decades, including marine mammals, sharks, sea turtles, seabirds, and crocodilians (e.g., Katona and Whitehead, 1988; Bowen, 1997; Heithaus et al, 2008; Heithaus, 2013; Roman et al, 2014; Kiszka et al, 2015; Estes et al, 2016; Roff et al, 2016; Somaweera et al, 2020) While these studies and reviews have demonstrated that species within each of these taxa may play important roles, both as Ecological Impact of Small Cetaceans predators and prey, it is becoming increasingly apparent that there is a need to better understand the diversity of functional roles predators have in aquatic ecosystems, the pathways through which they might affect the structure and function of biological communities and ecosystems, and the contexts in which species may be more or less important. The ecological importance of species refers to the community and ecosystem consequences of changes in their abundance
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