A little knowledge of natural history is a dangerous thing. This is, first, because knowing a few things — the names of some common wildflowers, or the songs of some neighborhood birds — begets curiosity about more things, and soon enough you’ve gone from installing an identification app on your smartphone, to ordering a field guide, to pricing binoculars. But, second, it’s dangerous because once you know a little natural history, you start to notice living things are often not where you expect them.
Should the violet-blue flowers of a jacaranda really be shading the upper slopes of the Griffith Park ridgeline, overlooking the urban sprawl of Los Angeles? Can that really be an eastern gray squirrel chattering at you from the bushes in suburban Seattle? Why on Earth is a honeybee visiting the flowers of a Joshua tree, which offer no nectar and very little pollen?
The most immediate answer for all of these is, because humans put them there. Jacarandas and eastern gray squirrels and honeybees are just some of the species we’ve carried to new habitats for our own purposes. But in all three cases these organisms are not going about human-directed business — they’ve gotten away from the places they were introduced, to make a living on their own terms. And the questions they prompt apply much more broadly.
The ecologist Dan Janzen was struck by those questions, not while walking through a city full of species from other continents, but while contemplating a Costa Rican forest. The hundred square kilometers of Santa Rosa National Park had, Janzen estimated, 650 species of plants, 250 species of birds, 58 species of mammals, and thousands of species of insects. All that tropical biodiversity worried him, he wrote in 1985, because he’d realized that his observations of those many and varied species might not tell him much about their evolutionary history, especially their shared evolutionary history.
The vast majority of species Janzen could observe at Santa Rosa had geographic ranges well beyond the park’s boundaries, and probably had their origins as distinguishable species elsewhere. Was it more likely, Janzen reasoned, that the predators and prey, or hosts and parasites, or plants and pollinators interacting at Santa Rosa had those interactions because they had adapted specifically to each other — or because, when they encountered each other at Santa Rosa, they already had traits that made their interactions possible?
Janzen called this second possibility ‘ecological fitting’, and he realized it would be the rule, rather than the exception, for any moderately widespread species. As a species disperses beyond its place of origin, it establishes local populations in the places where it finds resources, and perhaps an escape from predators or competitors. There might be many other places it could establish if not for a lack of suitable prey, or the presence of a particularly virulent parasite. For the most part, the species is not evolving to fit the places it establishes — it’s establishing where it already fits well enough.
Ecological fitting is a concept most natural historians now encounter at the very start of their careers, though we don’t usually call it that; we call it ‘invasive species.’ Any species humans have introduced into new habitats, either accidentally or on purpose, has done some ecological fitting to establish in those new habitats. The ones we identify as ‘invasive’ have done ecological fitting so well that they overrun the locals. Thinking about these species as examples of ecological fitting makes every hike and nature walk into a litany of the ecologically fitted, from European starlings schooling in the air above the cornfields around my childhood home in rural Pennsylvania, to California poppies growing among roadside wildflowers in Mediterranean France.
Ecological fitting is a problem for the ecologist or the evolutionary biologist, because it means that what we see in a single field trip, or even many field seasons, may not reflect the specific conditions that shaped the species we study. Janzen explicitly linked ecological fitting to the “punctuated equilibrium” model of evolution proposed by Stephen Jay Gould and Niles Eldredge, in which species spend long stretches of their existence with a fairly stable form, behavior, and habitat needs, with the kinds of changes that define new species occurring (relatively) quickly, in “punctuational” bursts. Those periods of stability between big changes are periods of ecological fitting, in which the species finds enough resources and environments to which it is already reasonably well adapted, and so does not experience strong natural selection, or dramatic changes in population size, that might facilitate big, species-redefining changes.
There’s a connection here to another of Gould’s big ideas, his extended argument against the “adaptationist programme” in evolutionary biology. Just as Gould and Richard Lewontin argued that we cannot assume any particular feature of an organism is specifically adapted to its present-day function — because organisms evolve via more processes than natural selection, and because the uses of body parts and behaviors can change with ecological and anatomical context — ecological fitting implies that, however well two species may work together in the present day, we cannot assume that the traits and behaviors necessary for the interaction are the result of that specific interaction.
We also cannot assume that they represent the best possible versions of those interactions. A hummingbird would probably do well enough drinking from any number of tubular red flowers, and a scarlet monkeyflower would probably be adequately pollinated by a Black-chinned hummingbird or an Anna’s hummingbird — or even, as a fallback, a visiting bee. In all likelihood the hummingbird’s traits and behaviors have adapted, not in response to its interaction with one species of monkeyflower, but in response to the general availability of red, tubular flowers full of nectar across the bird’s full geographic range. Or, vice-versa, the monkeyflower’s color and shape reflect the presence of hummingbirds and hummingbird-sized animals with hummingbird-like attraction to the color red, interest in nectar, and ability to drink from a tubular flower.
For ecologists and evolutionary biologists, ecological fitting is a reminder to humility in the face of the living world’s complexity. We can travel the globe, or spend years learning to know a single field site, and only see a little of it directly. Contemplating the ecological fitting of Santa Rosa National Park certainly seems to have put Janzen in such a mood: at the end of his essay he pivots, rather abruptly, to personal reflection:
Widespread species are not adapted to their habitats, they just are. In fact it can be argued that most members of most widespread species are quite maladapted to their habitats. As anyone knows who has suffered a setback in life, you don’t have to be well-adapted to survive. You just have to survive. We are all asymmetrical pegs in square holes.
It’s not possible or practical to expect we’ll achieve a perfect understanding of how the living world works, and how it came to be. But by the same token, perhaps with time and patience, we can understand it well enough.