Kentucky yellowwood, Cladrastis kentukea, is a small tree in the legume family, with smooth bark and white flowers that dangle like bunches of grapes. It is rare. Small, scattered populations occur in Ohio, North Carolina, Tennessee, Missouri, Oklahoma, Georgia, and Alabama, along with its namesake Kentucky. For ecologists and conservationists, this type of rarity raises a question: Why does the tree occur naturally in those places and those places only? More broadly, what is it that keeps rare, isolated species rare and isolated? 

For a long time, scientists assumed that the answer, mostly, was climate. The present range of a species, by this view, was largely a reflection of where that species could stand the weather. Rare species were simply pickier. But a growing body of evidence suggests that, for plants at least, climate might be less of a limiting factor than once thought. 

A study published late last year in the Journal of Biogeography supports this conclusion. A group of researchers led by ecologist Ben Seliger found that the ranges of hundreds of American tree and shrub species, including Kentucky yellowwood, are in only some of the areas with suitable climate, meaning that something else constrains them. The findings add fuel to an already heated debate among conservationists over whether people should help rare, isolated species reach places where the current and future climate will better suit them. 

It’s always been obvious that different plants occur in different climatic settings—that, as Carl Linnaeus and Isaac Biberg wrote in the mid-1700s, “some of them can bear an intense cold, others an equal degree of heat; some delight in dry ground, others in moist.” What’s less obvious is where these generalities give way to absolutes—that is, where, exactly, does dry ground becomes too dry for a given species or moist ground too moist?  

In the mid-1900s, the British ecologist G. Evelyn Hutchinson proposed the “fundamental niche” as a way of describing those limits. A species’ fundamental niche, he wrote, was composed of all the combinations of heat, cold, moisture, and other physical conditions in which that species can survive, grow, and reproduce. Hutchinson also proposed a “realized niche,” which was all those suitable combinations of physical conditions minus all the sets of conditions that the species was unlikely to actually occupy due to competition, predation, or other interactions with other living things. The realized niche would likely be smaller than the fundamental niche. 

Hutchinson’s niche concepts provided a good way of thinking about the limits, but they didn’t translate well onto an actual map. As Jorge Soberón and A. Townsend Peterson pointed out in a 2005 paper, there is a third factor at play in the real-world ranges of species: distribution. The actual range of a species is all of the places with suitable physical conditions, minus places that the species can’t occupy due to other living things, minus places that the species didn’t manage to reach. The actual ranges of many species are therefore likely to be far smaller than the space with suitable climates. 

Hints abound that this is the case. Farmers and horticulturalists have proved thousands of times over that species often thrive in places that are unlike where they are found in the wild. The runaway success of invasive organisms further suggests that, for many species, the problem isn’t that no more suitable habitat exists, but simply that they couldn’t get there on their own.

Despite these hints, says Jens-Christian Svenning, a macroecologist and a coauthor of the Journal of Biogeography paper, at the turn of the 21st century, the prevailing view among ecologists was that ecosystems, broadly speaking, were in equilibrium with climate—species had mostly reached the places where the climate was suitable to them. In a 2004 study, Svenning and another researcher, Flemming Skov, modeled the range of some 55 species of European trees. For each of those species, they used range maps and climatic data to compare the number of growing-degree days (days warm enough for plants to grow), minimum yearly temperature, and water balance (precipitation minus evaporation) in the places where the species did occur with places where the species did not occur.  

In essence, the resulting model was a version of Evelyn Hutchinson’s fundamental niche, showing where survivable sets of conditions turned to unsurvivable sets of conditions. Svenning and Skov then ran these models, allowing them to project where the species could occur. What they found was striking: On average, European tree species were occupying only slightly more than a third of the places with suitable climates. The picture was not of species perfectly matched with climate but of pervasive disequilibrium. 

The new Journal of Biogeography study suggests the same may be true in North America. Following Svenning and Skov’s rough outline, Seliger and his colleagues created niche models for 455 North American trees and shrubs. They found that, on average, those species had filled less than half of the space climatically suitable to them. They found that this gap was smaller with wide-ranging species and far greater for rare, isolated species—including the Kentucky yellowwood, which filled just 8 percent of the space with apparently suitable climate. “We cannot find climate variables that explain these small-ranged species well at all,” Seliger says. “We think it’s evidence that they’re limited by other factors, like dispersal.” 

The question of what is constraining species’ ranges is at the heart of the debate over whether people should move species to suitable places, a conservation method known as “assisted migration.” Since the 1980s, botanists have worried that many plants would be unable to keep pace with the rate of modern climate change. For small, isolated species, this could mean extinction. While people have often moved species from one part of the world to another for more mundane reasons, the idea of doing so as a conservation method has proven controversial. Skeptics worry about inadvertently creating new invasive species, and about disrupting the existing ecosystems that would receive the assisted migrants.  

But there is also the question of whether species actually need the help. As ecologist Mark Schwartz wrote in a 2004 paper arguing against assisted migration, “the arguments about range and climate rely on very important assumptions that are not well justified. We usually do not have empirical data from which to judge whether narrowly distributed species are, as assumed, limited by climate and not by other environmental factors.… As a consequence, I believe that we should exercise caution.” 

On the one hand, the Biogeography study, which shows species widely in disequilibrium with climate, seems to reinforce Schwartz’s point. “People advocating that we need to assist movement should think twice,” says Carsten Rahbek, a macroecologist who was not involved in the Journal of Biogeography study. “It looks like these small-ranged species are not determined by contemporary climate, and that kind of undermines the necessity of people to go in and help them.” 

Svenning draws a different conclusion. Tree species, he says, have had more than 10,000 years of relatively stable climatic conditions since the end of the last ice age and still haven’t reached equilibrium with the climate. “We’re now expecting strong climate change in the next decades and centuries,” he says. “It’s super unrealistic to expect many species to track this. They couldn’t do it across these 10,000 years. They can’t do it in this short time frame.”  

Seliger says that, in an odd way, both perspectives can be true. The current distributions of many tree species might have little to do with the current climate, and, for many of those species, climate change could be the biggest threat in the future. “Unfortunately,” he says, “the debate is going to go on.”  

Whether people take the possibility that many species are out of sync with the current climate as evidence for or against assisted migration, Svenning says he hopes it will lead them to consider a slightly broader view of whether species are “native” to a particular place. The places we find species today, he says, represent only where they’ve managed to reach.  

It seems to be true, at least, of Kentucky yellowwood. For decades, people have planted it in yards and along streets across eastern North America. Today, the species inhabits an area far greater than it ever managed to reach on its own.