As ecosystems heat and green, ant abundance and diversity increases; but too much heat and these communities lose colonies and species.

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.

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Ants are diverse and abundant in terrestrial ecosystems. They act as nature’s cleanup squad, and are threaded throughout her food webs. Understanding how their abundance and diversity is changing as Earth heats up is a major challenge to ecologists. Images used by permission of Alex Wild @Myrmecos.

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.

Map

The ants living in 34 ecosystems across North America were first surveyed in the 1990s, and then again 20 years later.

KarlRoederPlotDiversity

The desert, tundra, grassland and forests resampled by the Kaspari lab as we explored 20-year changes in the ant communities. Image by Karl Roeder.

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.

Temperature and NAP change

The 34 sites were on average 1°C warmer with higher estimates of plant production. Both trends are predicted to increase the numbers of ant colonies, and numbers of ant species in the ecosystem.

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.

TPT

A graphical prediction from Thermal Performance Theory, that systems accelerate before they crash. In this case, it predicts that smaller increases in temperature are likely to enhance ant abundance, but bigger temperature increases will cause abundance to suddenly decline.

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.

Community Change

The distribution of observed 20-year changes in North American ant communities. Left of the blue line, communities are losing colonies and species. To the right, they are gaining.

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.

Jelena

Dr. Jelena Bujan sampling a one m2 plot for ant colonies at the Jornada field station in southern New Mexico. Photograph by Josh Kouri.

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.

MikeK

Dr. Michael Kaspari using an aspirator, or “pooter” to suck up ants at the Ordway Swisher field station in northern Florida. Photograph by Deborah Kaspari.

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.

MikeW

Dr. Michael Weiser, inspects the ants active at night from a plot at the Sevilleta National Wildlife Refuge. Photo by Josh Kouri.

 

One Comment on “As ecosystems heat and green, ant abundance and diversity increases; but too much heat and these communities lose colonies and species.

  1. Pingback: NEW PAPER OUT! Species Energy and Thermal Performance Theory predict 20‐year changes in ant community abundance and richness – Karl A. Roeder

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