Changes over 30 years in populations of the leafcutter ant Atta mexicana at Organ Pipe Cactus National Monument

Mintzer, Alex

The leafcutter ant Atta mexicana lives at the extreme northern margin of its range in the Sonoran Desert of Organ Pipe Cactus National Monument. This ant has a fascinating natural history that has been described to thousands of park visitors over the last 30 years. Because the National Park Service had had no knowledge of ant colony distribution or population size in the backcountry of Organ Pipe, in 1985 resource managers solicited and supported the first systematic survey. I used high-resolution aerial photographs to identify 11 arroyo channel transects with suitable habitat in the southern third of the park along the international boundary. We walked these transects in December 1985 and again in 1995, 2005, and 2015, to determine the local colony abundance and distribution of this rare ant and study long-term changes in this park population. These decadal surveys revealed that the population size undergoes major fluctuations, and also showed that typical field colony lifespans are 10–20 years. Population decreases by late 2005 raised concerns for long-term population viability, in light of potential challenges presented by increased borderland agricultural development, increased cross-border activity including border security infrastructure development in their preferred habitat, and long-term climate change. However, the 2015 survey revealed major increases in population size to “record” levels, indicating that this ant population has not responded negatively to these anthropogenic challenges. Other factors such as year-to-year variation in summer rainfall and predation intensity probably control major long-term population size fluctuations at Organ Pipe.

Changes over 30 years in populations of the leafcutter ant Atta mexicana at Organ Pipe Cactus National Monument

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