As temperatures increase with climate change, species are expected to expand their ranges to higher latitudes, where it will be warm enough for them to survive. Similarly, many species are predicted to move up in elevation as higher altitudes experience warmer temperatures—we know that some have already started to do so. But we also know that some species can shift their ranges in response to temperature change more quickly than others (i.e. the tiny and winged vs. the large and rooted). So what happens when one relatively mobile species moves up in elevation before the relatively stationary species with which it co-evolved?
In a recent paper in PNAS, Raffa et al. tackled this question in a really interesting system: bark beetles and pines. In the Rocky Mountains, bark beetles (Dendroctonus ponderosae) use lodgepole pines (Pinus contorta) as a host. Both species occur at low elevations. Adult beetles bore into a tree, mate and lay their eggs, and larvae feed on the tree until it dies. In response, lodgepole pines have evolved physiological defenses against the beetles: trees secrete resin to push the beetles out of tree tissue, and produce chemicals to kill the beetles. As temperatures have warmed, the beetles have moved up in elevation and started attacking whitebark pines (Pinus albicaulis), the dominant species at high elevations. Little is known about whitebark response to bark beetles because these species have rarely come into contact before.
To get a better understanding of the susceptibility of whitebark pines to beetle attack, Raffa et al. first compared the defense physiology of whitebark and lodgepole pines. To do this, they sampled levels of defense chemicals in un-attacked individuals of both species. They then induced defense production in each of the sampled trees by mimicking a bark beetle attack (basically, they poked a hole in the tree and inserted some compounds produced by beetles and their fungal symbionts). They found that lodgepole pines produced more defense chemicals in response to a simulated attack than whitebark pines did. By surveying other insects in the pine stands, they also found that, once attacked, lodgepole pines attracted more bark beetle-enemies than whitebarks did. This is another defense strategy for lodgepoles, in an ‘the enemy of my enemy is my friend’ kind of way. These results make sense from an evolutionary perspective—lodgepoles have evolved with beetles for a really long time and are therefore harder for beetles to attack successfully. Whitebark pines are naïve to bark beetles and are much easier to attack.
Although whitebark pines appear to be more vulnerable to beetle attack, tree-defense is only one part of tree-susceptibility—beetles must choose to attack a tree in the first place. To determine beetle preferences, Raffa et al. recorded attack rate in mixed stands of whitebark and lodgepole. They found that beetles overwhelmingly preferred to attack lodgepole pines, despite the abundance of defense chemicals! But lab tests showed that beetles wouldn’t turn down whitebark pines when lodgepoles weren’t available. This suggests that even though beetles prefer lodgepole pines, they will still attack stands of higher-elevation whitebark pines where there aren’t any lodgepoles. So unless lodgepole pines also expand their range to higher elevations, whitebark pines are likely to suffer more and more beetle attacks.
Why this paper is cool:
This study suggests that as bark beetles shift their range in response to warming temperatures, they will encounter trees that are less-defended against their attack. However, the results also highlight the importance of coevolution—even though whitebark pines appear to be a better host for bark beetles, the beetles still preferentially attack lodgepole pines, presumably because they’ve adapted to focus on cues from lodgepole pines.