My brother Ian was a picky eater. Breakfast was always a bowl of Crispex. For lunch, he ate a PB&J and refused to eat the crusts. I was the opposite. Even as a young child, I loved proverbially “gross” foods like mushrooms and started drinking coffee when I was twelve.
Turns out that some animals are like Ian and some are like me. For example, monarch caterpillars only eat milkweed. We call animals like the monarch specialists. Conversely, some animals will eat, well, just about anything. Raccoons, for example, are equally happy eating crayfish from the creek or scraps from your garbage can. We call such species generalists.
Diet isn’t the only thing to be picky about! Some species exhibit preferences for precise habitat types. For example, the Kirtland’s Warbler breeds only in young jack pine barrens, primarily in Michigan, but also occasionally in Wisconsin. On the other hand, some species are ubiquitous. The coyote is an exemplar habitat generalist—you might spot one in the wilds of the Chequamegon-Nicolet National Forest or in a suburb of Milwaukee.
Taken together, diet and habitat comprise what we call the ecological niche of a species. You can think of a niche as the “cubbyhole” that a species occupies within the broader tapestry of its environment. The breadth of a niche is a continuum from extreme specialists (like Kirtland’s Warblers) to extreme generalists (like raccoons). Some species fall between those extremes; deer are a great example. Deer are strict herbivores, but they can be found in many different habits, from forests to farmlands. So, not every species can be neatly classified as a generalist or a specialist.
Scientists are interested in generalists and specialists because they exhibit different responses to change. Like a trained craftsman whose job is replaced by a machine, the specialist has nowhere to go when the environment changes. Generalists, on the other hand, can capitalize on the vacant niche space and colonize altered landscapes. Given the widespread changes humans are exerting on the earth, we are seeing global proliferation of generalists while many specialists are disappearing, a process known as biotic homogenization.
This may seem dire, but the more we learn about generalists and specialists, the more we’ll be able to do to maintain biodiversity and lose fewer specialists. In the meantime, I encourage you to think about the animals you see on a regular basis. Is that squirrel outside your window an ecological jack-of-all-trades? Are there any habitat specialists that live on your property? And maybe even think about your own niche—are you a generalist, a specialist, or somewhere in between?