There’s an old saying that when the going gets tough, the tough get going. And for many species, climate change is set to make things a whole lot tougher. The question on ecologists’ minds is, which species will be able to get going?

Plants are of particular concern, given that they tend to lack legs, making rapid migrations problematic. (There are a few notable exceptions, but they make tortoises appear speedy.) Ecologists have speculated that weedy species—which reproduce quickly and disperse widely—may not suffer as many consequences, but trees are not likely to be as fortunate. Two new studies in the journal Ecology Letters set out to test these hunches, and their results largely confirm what many had suspected.

Going to the weeds

Many species are already on the move as a result of climate change, and these migrations have a way of upending the previously established order. Species that both inhabit and define stable environments tend to be long-lived, large, and produce fewer, more successful offspring. Think trees, great apes, elephants, tigers, and so on. But when things go topsy-turvy, fast-growing, short-living, mass-reproducing species gain the upper hand. Weeds win, in other words.

And that’s just what the first study predicts. The researchers modeled a simple environment occupied by one hypothetical species with three very broad but heritable traits—dispersal propensity, reproductive capability, and competitive ability. As the researchers shifted the species’ suitable environment from one end of the modeled landscape to the other, individuals with the traits that allowed them to reproduce abundantly and disperse broadly did best on the expanding front. Those with the competitive trait did not fare as well.

These results not only suggest that weedy species will be favored in the future, but that individuals members of non-weedy species will be more successful if the have some weedy traits.

Long-lived species, such as trees, may not be coaxed north by the heat, but rather muscled out by scrappier types. The second study seems to confirm this notion. The authors chose not to forecast how species’ ranges will shift as the climate changes (that's notoriously difficult) or how they moved in response to past warming (not necessarily informative, given the rapid pace of current climate change). Instead, they closely analyzed the current geographic distribution of 102 eastern North American tree species.

Heading north

Using records on 3 million trees from the US Forest Service, they calculated how much each species’ north-south distribution deviated from a bell-shaped, normal distribution using a special set of five statistical tools known as Huisman-Olff-Fresco models. The technique allowed the researchers to tailor their analyses to the peculiarities of each species’ geographic distribution. For example, if a species is more numerous in the north, the model will select a skewed distribution as the best fit.

As expected, the northern extents of most species’ ranges appear to be expanding, albeit in a restrained fashion. More dramatic was the shift north in abundance—within most species’ ranges, more trees were concentrated toward the northern end of the range. It’s a response that appears remarkably anthropomorphic. Imagine a hurricane evacuation: the majority of people will evacuate ahead of the storm, but will only travel as far enough to be safe. A minority will either remain until the last minute or choose to ride out the storm.

The results also imply many species’ southern extents are eroding, though the changing climate may not be to blame, at least directly. Rather, ecologists think plant ranges are limited at their southern extent by competition—species from northern regions simply cannot hold their ground against faster growing plants. Thus, the warmer temperatures are probably drawing better competitors northwards, and these will eventually crowd the existing tree species out of the area.

The end result will be smaller ranges for many tree species compared with today, according to the study. Trees that are numerous in the northern part of their range, such as American beech, red maple, and cork-bark oak, have the greatest potential to push north. But parts of the American Southeast will probably see some species, such as American beech, river birch, and shagbark hickory, disappear.

Ecology Letters, 2010. DOI: 10.1111/j.1461-0248.2010.01505.x and 10.1111/j.1461-0248.2010.01526.x (About DOIs).

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