Species ranges as a quintessential biological problem

A question that vexes me greatly is why scrub oak (or Gambel oak, Quercus gambelii) lives some places and not others. Why, for instance, does the species almost completely disappear north of Boulder, Colorado, but extend into the Sierra Madre Mountains of Wyoming west of the Continental Divide? Why does it carpet the lower slopes of the Wasatch, but barely occur at a similar elevation and latitude near Fort Collins?

If you squint at a map of observations of scrub oak—which, to be clear, doesn’t do much to capture its abundance, or the degree to which it dominates a landscape and gives it flavor—you might be able to convince yourself you understand. Maybe temperature is key: east-facing slopes on the Front Range don’t reap the thermal benefits of warm afternoon sunshine the way west-facing slopes in Utah do. But then again, the east-facing slopes of the Jemez Mountains are covered in scrub oak, while the west-facing slopes of the Sangre de Cristo Mountains near Santa Fe are relatively scrub oak-poor. Of course, the fire-scarred eastern Jemez no longer has much in the way of mature ponderosa pine or mixed conifer forest, which could be shading out scrub oak in the mostly unburned Sangres. So maybe competition has something to do with it—or at least an interaction between competition and temperature. And so on, and so on.

Trying to figure out why species live some places and not others is a significant part of my day job, and over the last few months I’ve started to consider it what you might call a quintessential biological problem: something that reflects the unique inferential challenges that make the scientific process different in biology than say, chemistry or physics. What makes something a “quintessential biological problem”? I can think of five interrelated criteria that should be relatively uncontroversial (at least among organismal biologists / ecologists / evolutionary biologists):

  • Variation: There are always exceptions to biological patterns, and any particular explanation for a biological phenomenon probably only does an O.K. job of explaining things (at best!). Maybe you conduct a study in a greenhouse and find 70 out of 100 scrub oak acorns fail to germinate when temperatures drop below -15° F—how do you make sense of the 30 that did?

  • Multicausality: Any given biological phenomenon probably has multiple causes. The geographic distribution of scrub oak may be partly limited by cold tolerance—but also by soil type, and competition from other plants, and sun exposure, and many other variables.

  • Scale dependency: Any explanation is going to depend on the spatial and temporal scale of the question you ask. It’s trivial to come up with explanations for why you don’t find scrub oak above 13,000 feet, or in temperate rainforests—it’s a lot harder to explain why it is or isn’t in a particular drainage in the Sandias.

  • Correlation / causation issues: Without well-designed studies and a lot of work, it’s tough to distinguish whether you’ve actually figured out what’s responsible for a given biological pattern, or some other factor that is only incidentally correlated with it. Is it really the case that mature conifer forest shades out scrub oak thickets? Or do the two habitat types simply require different amounts of precipitation?

  • Historical contingency: Any given answer is going to be dependent on evolutionary and biogeographic history. There are no scrub oak in Spain for reasons entirely different than why there is no scrub oak in the Wind River Range: it didn’t evolve there, and it hasn’t managed to disperse there, even though the climate and soil conditions might be entirely appropriate.

Reading over these five criteria, you’d be forgiven for thinking the entire field is a fool’s errand. But it’s also why biological theories and frameworks with true explanatory power—population genetics, evolution by natural selection, island biogeography—are so impressive, and why adding grains of sand to the rambling castle of biological knowledge is so rewarding.

Still, thinking about species ranges as a quintessential biological problem has some weighty implications. If uncertainty compounds uncertainty, understanding what determines the current distribution of species is very difficult; predicting how these distributions will change as the planet warms is even harder. Which means that we’re far better served trying to preserve biological processes than static polaroids of the habitats and ecosystems we know today—and also by learning to love landscapes as they are becoming, not as they once were. Both of these things are easier said than done.