Aquatic Mammals 2018, 44(5), 580-594, DOI 10.1578/AM.44.5.2018.580 Historical Perspectives James A. Estes (Born 2 October 1945) After receiving his doctorate from the University Pew Fellow in marine conservation, a Fellow of the of Arizona in 1974, Jim Estes worked as a research California Academy of Sciences, and a member of scientist for the U.S. Department of the Interior and the U.S. National Academy of Sciences. He received later as a professor in the Department of Ecology and the Western Society of Naturalists’ Lifetime Evolutionary Biology at the University of California Achievement Award in 2011 and the American at Santa Cruz. His interests are in the ecology and Society of Mammalogists’ C. Hart Merriam Award conservation of large-bodied vertebrates. Jim is a for excellence in research in 2012. Historical Perspectives 581 The History and Future of Marine Mammal Ecology James A. Estes Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, CA 95073, USA In autumn of 1976, less than two years after com- mink, and beaver) or are expatriates from lineages pleting my doctorate, I spoke to a class taught by that arose in the sea (e.g., river dolphins and sire- Roger Gentry at the University of Washington enti- nians). All extant marine mammals arose on land tled “The Marine Mammal Phenomenon.” The U.S. and radiated into the sea via fresh water or across Marine Mammal Protection Act had been signed the land–sea interface. The remarkable taxonomic into law a few years earlier, and Roger’s purpose and functional diversity of marine mammals has was to explore rapidly growing societal interests been molded largely by life in the sea. in marine mammal science and policy. While my Much of the strong human interest in marine interests and background occurred within the realm mammals is the product of this evolution and of what might have been viewed as marine mammal adaptation. Large body size engenders an intrin- science, the species (sea otter), system (shallow sic sense of excitement and awe, which is why we coastal ocean), and process (predation) I had stud- keep records of the biggest but almost never the ied and knew best seemed only marginally relevant smallest. Our own modest abilities to swim and to and perhaps even idiosyncratic with the larger dive make us marvel over the ability of air-breath- questions of marine mammal science and policy. ing homeotherms to live in stormy, frigid waters I see it differently now. Sea otters have taught while at the same time diving for long durations us much that is germane to other species of marine to great depths, all with apparent normalcy and mammals; the coastal ocean has been a productive disregard for conditions that to humans would arena for understanding ecological processes that be instantly fatal. We are intrigued by marine are difficult to study and understand in other ocean mammal intelligence, social interactions, modes ecosystems; and the effects of sundry predators of communication, and any number of features are known to influence the structure and function- of form and function that bear on their enthrall- ing of diverse ecosystems. The former assertions ing natural histories. These and other features of all occur within the prefecture of ecology, defined marine mammals motivate human interest in these broadly as a search to understand the distribution creatures, and human interest, in turn, motivates and abundance of species. science. I think of myself first and foremost as an ecolo- Research on marine mammals has left a distinct gist. For these reasons and because aquatic mam- mark on many divisions of organismal biology. mals are the unifying subject of this journal, my aim The study of morphology has been enriched by in writing this Historical Perspectives essay is to the immense size of marine mammals and their explore the intersection between aquatic mammals structural adaptations for swimming and feeding and ecology. I’ll begin with a discussion of science (Goldbogen, 2010); physiology and biochemistry and aquatic mammals, the purposes of which are to have benefitted from the study of marine mammal juxtapose the intersection of aquatic mammals and diving (Kooyman et al., 1980); the understand- ecology with similar intersections of aquatic mam- ing of mating systems and social behavior has mals and other scientific disciplines, and to remind been informed by work on pinnipeds (Le Boeuf readers of the diversity of aquatic mammal species & Peterson, 1969; Bartholomew, 1970); studies of and the environments in which they live. bottlenose dolphins (Tursiops truncatus) and sea otters (Enhydra lutris) have added data-rich anal- Science and Aquatic Mammals yses and novel perspectives to the understanding of prey selection and foraging behavior (Smolker Most aquatic mammals are marine-living. My et al., 1997; Tinker et al., 2008); and the inter- focus from here forward will thus be on marine section of biogeography, the fossil record, and mammals. Mammals arose and initially diversi- genetics of marine mammals has led to significant fied on land from which several lineages radiated advances in the study of systematics and evolu- into aquatic habitats. All extant freshwater-living tion (Hoelzel, 2002). mammals are either semi-aquatic (i.e., they have In contrast, marine mammals have played a minor an obligate association with land; e.g., otters, role in the growth of ecology. Although substantial 582 Estes effort has gone into monitoring, obtaining reliable summarize what I see as the highpoints of marine information on the distribution and abundance of mammal ecology, discuss how these highpoints marine mammals is difficult. This is because most map onto the foundations of modern ecology, and species range over vast areas that change season- look forward to the needs and opportunities for ally, are difficult to see or detect on the ocean’s sur- advance in marine mammal ecology. face, are undetectable when they are submerged in a dive, aggregate in ways that create high sampling A Brief History of Ecology variance, and are difficult and costly to mark or tag. Population estimates for marine mammals are noto- The formal study of ecology began in the late riously imprecise (Estes & Gilbert, 1978; Taylor 19th century in an effort to better understand the et al., 2007), the upshot being that any question distribution and abundance of plants. Attention to whose answer requires a measure of distribution and animals and their roles in the dynamics of nature abundance is beset by challenges from the outset. gained force with people like Sir Arthur George Marine mammals also have upsides that expe- Tansley (1871-1955; Figure 1), who developed the dite thinking about their distribution and abun- ecosystem concept, and Charles Sutherland Elton dance. In contrast with air, water is buffered (1900-1991; Figure 2), who conceived of what against rapid temperature change by the capac- has come to be known as the Eltonian Pyramid—a ity to store heat, creating an ocean environment characterization of ecosystems as having reduced in which neither temperature nor the availability biomass and production across successively of water (the two most important physically lim- higher trophic levels and the associated ideas of iting factors for many terrestrial organisms) vary food chains (consumer–prey linkages across mul- rapidly or extensively in space or time. By virtue tiple trophic levels) and food webs (consumer– of their large body size, endothermic metabolism, prey networks). Elton’s Pyramid was especially and the high thermal conductivity of water (the influential because it implied that the distribution medium in which all marine mammals live), the and abundance of species is controlled by just two distribution and abundance of marine mammals key processes: (1) net primary production (NPP) thus may be influenced more strongly by their and (2) the efficiency of energy and material biological surroundings than by direct influences transfer from prey to consumer. George Evelyn of their physical–chemical environment. Hutchinson’s (1903-1991; Figure 3) concurrent It therefore strikes me that two simple questions interest in diversity led to such groundbreaking define the backbone of marine mammal ecology: advances as the species/area function (Connor & (1) How do other species influence the distribution McCoy, 1997), the theory of island biogeography and abundance of marine mammals? and (2) How (MacArthur & Wilson, 1967), latitudinal patterns do marine mammals influence the distribution and of species diversity (Pianka, 1966), and the local abundance of other species? Answers to these ques- control of species diversity (Paine, 1966; Connell, tions depend on the ways in which species inter- 1978). act with one another, which, in turn, are limited to Three of Hutchinson’s protégées, Robert just a few possibilities. The influence of a species Helmer MacArthur (his graduate student; 1930- on some other species can be positive, negative, 1972; Figure 4), Frederick Edward Smith (another or neutral. When the influences of two interact- of his graduate students; 1920-2012; Figure 5), ing species on one another are both negative, we and Raymond Laurel Lindeman (a post doc with call this competition; when the influences are both Hutchinson; 1915-1942; Figure 6) set the course for positive, we call this mutualism; and when the much of what followed. influence of species 1 on species 2 (or 2 on 1) is Much of ecology’s trajectory from the early positive and the influence of species 2 on 1 (or 1 on 1960s to the mid-1980s grew from MacArthur’s 2) is negative, we call these agonistic interactions. belief that competition ruled the world (MacArthur, Agonistic interactions most commonly occur when 1972; Salt, 1983). one of the two species is a predator and the other is Lindeman’s (1942) 10% rule, the proposal that its prey.