16.1: Defining an Organism’s Habitat

Like teenagers, mobile organisms­—the nekton—tend to congregate in locations best suited to their lifestyle and temperament. They have their preferred place to chill, their favorite fast food spot, their ideal spot to spend time with their special someone. Indeed, the whole advantage of being mobile is the ability to go where you want. Isn’t this why teens can’t wait to learn how to drive? The places where an organism spends time to feed, rest, hide, find mates, and reproduce define its habitat (e.g., Hall et al. 1997; Fraschetti et al. 2008; Bamford and Calver 2014; Boero et al. 2019).

There’s a tendency in general textbooks to equate habitat with “the environment where an animal lives” (Castro and Huber 2019). But this definition contributes to an impression that habitat and environment are synonymous (e.g., Fraschetti et al. 2008; Costello 2009; Bamford and Calver 2014). In fact, an organism’s various activities may occur in different environments, the geological, chemical, physical, and biological conditions at a given time and place. In many cases, organisms inhabit a range of environments during their lives, and their habitat includes all these multiple environments. Thus, I define habitat as the range of environments that support the survival and reproduction of a species on a permanent or temporary basis (e.g., Lack 1933; Morris 2005; Bamford and Calver 2014). This definition especially applies to marine organisms.

Consider benthic species that produce planktic larvae. The larvae live in the water column for an extended period of time. A deep-sea benthic larva that travels to the surface will cross a range of environments twice, once on the way up and once on the way down. Salmon offer another example. They’re born in freshwater, migrate to saltwater to feed until adulthood, and then return to freshwater to spawn. Their habitat includes the rivers, estuaries, and oceanic regions where they spend at least part of their life cycle.

We also have to be careful to avoid equating habitats with specific geographic locations. Free-swimming organisms follow a specific set of geological, chemical, physical, and biological conditions wherever they occur. Changes in ocean climate—natural and human-caused—lead to changes in ocean conditions across multiple timescales. As a result of such changes, organisms may shift their location. For example, during periods of El Niño, when warmer-than-average water temperatures occur along the California coast, we often find tropical species—red crabs, mahi mahi, swordfish, and marlin—inhabiting our temperate shores (e.g., Lluch-Belda et al. 2005; Cimino et al. 2021; Broughton et al. 2022). Ocean heat waves can cause similar displacements of organisms and disruptions of regional food webs (e.g., Cavole et al. 2016 ; Arimitsu et al. 2021).

This distinction between habitat and environment proves critically important for the protection of species whose life cycles include multiple environments. Efforts to restore commercially important fish populations, for example, must recognize the importance of the different habitats that affect their birth, growth, survival, and successful reproduction (e.g., Lowerre-Barbieri et al. 2019). Such efforts have necessitated conversations between people not typically accustomed to talking to each other: commercial and recreational fishermen, loggers, tribal councils, water and hydroelectric managers, and city, state, and federal government officials (e.g., Heikkila and Gerlak 2005; Cote et al. 2021). A complete understanding of an organism’s habitat necessarily takes into account the range of environments required by that organism to survive and reproduce.