Echoes of a Glacial Past

Climate change is often talked about in the context of direct effects on species. However, as John Muir so eloquently put it, "When we try to pick out anything by itself, we find it hitched to everything else in the Universe." In essence, nothing is ever black and white and a recent publication by Dr. Robert Warren and Dr. Mark Bradford illustrates this fact quite well.

Ants and plants have some very intricate interactions. A multitude of plant species rely on ants as their seed dispersers. Many of these plant species are spring ephemerals that take advantage of the fact that there is little else for ants to eat in the early spring by attaching fatty capsules to their seeds that are very attractive to foraging ant species. There are two big players in the foraging ant communities of eastern North America, the warm adapted Aphaenogaster rudis and the cold adapted Aphaenogaster picea. The cold adapted A. picea emerges from winter dormancy early in the spring while the warm adapted species emerges from dormancy much later in the spring. In the southern portions of their range, A. rudis outcompetes A. picea.

What is the big deal? Well, the researchers looked at two plant species that rely on these ants for seed dispersal, Hepatica nobilis and Hexastylis arifolia. Hepatica nobilis sets seed early in the spring, relying on ant species like A. picea to disperse its seed whereas Hexastylis arifolia sets seed late in spring, which is prime time for A. rudis. They noticed that, in the southern portions of their range where A. picea had been displaced, Hepatica has a very clumped and patchy growth habit where farther north it did not. Hexastylis on the other hand seemed to have a more normal growth pattern in the south.

By performing some transplanting and examining foraging and seed dispersal, they found that the absence of A. picea in the south spelled ecological disaster for Hepatica. It continues to set seed but because A. rudis emerges long after seed set, it is not filling the gap left by the missing A. picea. Hexastylis, which only grows in the south and sets seed much later, does just fine with the warm adapted A. rudis. Farther north where A. picea still rules, Hepatica has no trouble with seed dispersal but Hexastylis drops out of the ecosystem entirely. In essence, because of warming climate trends since the end of the Pleistocene, Hepatica is falling out of sync with its mutualistic ant partner in the southern portions of its range and, in time, may become extirpated.

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