One paradox in the recent flurry of papers reporting insect declines is that insects—ectotherms that rely on external sources of heat—are often predicted to benefit as their environment warms. In an open access paper accepted as a Report in the journal Ecology our team of ecologists—including Michael Weiser, Jelena Bujan, Karl Roeder, and Kirsten deBeurs—all from the University of Oklahoma, help resolve that paradox.
In a resurvey of 34 North American ecosystems after 20 years—both surveys funded by the National Science Foundation and that contribute to the NEON continental observatory—ant communities from deserts to forest have increased their abundance and diversity, but only up to a point.
To get these ecological “before” and “after” pictures, our team of post docs, graduate students, and undergraduates, traveled across North America in 2016 and 2017. We used the same methods of sampling ants from the same field stations, parks, and forest service districts that Weiser and I, along with Leanne and Alfonso Alonso, had sampled back in the mid 90s.
Twenty years later, these sites were different in important ways. First, they averaged 1°C warmer, with some much warmer than that. This evidence of global heating, differentially expressed, allowed us to evaluate a key theory underlying global change. that systems where ecotherms run the show should increase their activity and production—in fact accelerate it—before they suddenly crash. Second, they tended to be more productive–that is plants were producing more food, in the form of sugars, than before.
This second snapshot of the ant communities revealed that ant abundance at most sites had increased, and increases in the number of ant species had increased even more consistently.
Ant colonies tended to be more abundant in ecosystems that had warmed by a degree or so. However, those few that had warmed more—and some forest sites were up to 2.5°C hotter—suffered big declines in colony numbers. This result supports an important ecological theory: that when you warm a system composed of ectotherms it often accelerates. But beyond a certain temperature, it crashes.
The number of species found in each community—a measure of its biodiversity—was in turn sensitive to these changes in abundance. The communities that increased their abundance were able to support more biodiversity, on average 2 more species on our sample transects after 20 years.
Such data on insect changes—collected all in the same way and over a large geographic area—are still relatively rare, making generalizing about insect declines difficult, and contentious.
But ants are an important test case. They are ecologically dominant and widespread. Their apparent increases—when studies of say, butterflies often show decreases–also likely arise because ants tolerate higher temperatures than other insect groups. Moreover, ants tend to be diet generalists, eating plants, animals, and carrion, and many colonies can seek respite underground from the hottest parts of the day. All of these traits combine to make them relatively resistant to moderate increases in temperature.
However, the precipitous decline of ant communities at temperature increases > 1.5° C is a cause for concern. We will all be watching how this unfolds over the next 10 years.