Changes over 20 years in populations of the Mexican leafcutting ant, Atta mexicana, at Organ Pipe Cactus National Monument, Arizona

Mintzer, Alex

The leafcutting ant Atta mexicana reaches its northern range limit in North America at Organ Pipe Cactus National Monument (ORPI), and provides a spectacular case of marginal rarity. The main range of this ant species lies in Mexico, with a southern limit in Guatemala and El Salvador. A. mexicana was first collected at Organ Pipe Cactus National Monument by R.E. Gregg in 1948 and L.F. Byars in 1949, who collected them near a gravel pit 1 km north of current park headquarters (Byars, 1949). W.S. Creighton visited Organ Pipe in 1952, but did not encounter these ants; they were not listed in his comprehensive monograph, The Ants of North America (Creighton, 1950). At Organ Pipe, the large nests of A. mexicana are occupied by colonies that access an annual foraging area that may be >2 ha, using a system of lateral tunnels that may extend >130 m from the nest (Mintzer 1989, 1994). The queen of A. mexicana is the largest known North American ant, and daytime foraging behavior of worker ants draws attention from park visitors in the winter. At Organ Pipe, colonies of A. mexicana near monument headquarters have been a dependable target for interpretive walks led by ranger-naturalists. In the course of a year, established colonies may collect about 200 kg (dry weight) of leaf and flower material (Mintzer, 1994). All of this forage becomes substrate for the growth of their unique symbiotic fungus, which is cultured in large chambers, probably 5-15 m underground. Although National Park Service staff at Organ Pipe have been aware of these ants since the 1970s, knowledge about colonies in the backcountry was nonexistent until December 1985, when I conducted a systematic survey in the park for this ant to assess the population size, distribution, and status. In the Sonoran desert, A. mexicana inhabits riparian (arroyo channel drainage) and floodplain habitats with fine soils. Supported by the Southwest Parks and Monuments Association and several field assistants, I conducted a ground search along 11 arroyo channels in the southern one-third of the park. Aerial photographs were used to identify arroyo channel systems of sufficiently large size with abundant vegetation to provide suitable habitat for the ants. The aerial photographs also proved invaluable in the field to pinpoint and map colony locations within the twisting and braided channel systems. A total of 38 colonies were found during the 1985 search, which revealed several widely scattered local clusters or demes of colonies, with several outlier colonies (Mintzer and Mintzer, 1988). Approximate age of colonies was inferred from the size and condition of superficial accumulations of excavated soil above the central nest (Fig. 1,2). Colonies of all ages were found, indicating a history of successful reproduction and population recruitment. At ORPI, a few A. mexicana colonies have been found beyond these arroyo survey transects: in Lukeville (1996), on the west (2001) and east (1989) park boundaries, and within 2 miles of the north park boundaries (1987). No colonies have been reported from the Tohono O'odham lands immediately to the east of ORPI; this ant species is not known to occur elsewhere in the United States. Resurvey of the 11 arroyo transects in January 1996 revealed 46 colonies (Mintzer, 1997). GPS coordinate data was recorded during this ground search. Substantial colony death and recruitment occurred in the 10 years between the first and second surveys. All five arroyo channel systems that contained colonies in the December 1985 survey still had colonies in January 1996; we found colonies in two additional arroyo systems that lacked colonies in 1985. Of the 46 living colonies found in January 1996, 15 were new since the first survey. Nine colonies located in the 1985 survey had died by January 1996. Five colonies appeared to be new recruits within local populations identified in the first survey.

Changes over 20 years in populations of the Mexican leafcutting ant, Atta mexicana, at Organ Pipe Cactus National Monument, Arizona

Collections