Changes over 30 years in populations of the leafcutter ant Atta mexicana at Organ Pipe Cactus National Monument: An analysis of long-term environmental impacts on population size and survival (U.S. National Park Service) (2024)

Discussion and conclusions

The A. mexicana population at Organ Pipe Cactus National Monument does not exist in a steady state: 30 years of data suggest major population size fluctuations over time (table 1). The overall spatial pattern that emerged from the four arroyo transect surveys reveals several widely scattered local clusters (subpopulations or demes) of colonies, with isolated outlier colonies as remnants of old demes undergoing local extinction, and the formation of new demes under favorable conditions (Mintzer and Mintzer 1988; Mintzer 1997, 2006).

Mean colony lifespan estimates of 10–20 years proposed in transect resurvey reports (Mintzer 1997, 2006) are consistent with estimates for other Atta species in the field and in captivity (see Weber 1972). Several A. mexicana colonies have survived at the national monument for more than 20 years, but none has survived over the complete 30-year survey period from 1985 to 2016. The deme along the northern part of arroyo system (transect) 6 near park headquarters has been followed most closely since the first survey; all five colonies present in 1987 were still alive 10 years later, but only one survived until December 2005 and it died out by October 2007. Overall, only 9 of the 26 colonies found in 2005 were still alive in 2015. At Organ Pipe, colony migration (where a colony disappearance is associated with appearance of another colony 50–200 m away) is rare; I found colonies remained in the same place during visits between the decadal surveys.

A. mexicana clearly fits within the large and growing assemblage of Sonoran plant and animal species that maintain populations with successful reproduction only in certain favorable years, and little to no success in many or most other years. Although this ant’s reproductive success varies greatly from year to year, the species has responded very positively to conditions they encountered at Organ Pipe in recent years. Population recruitment since 2005 along arroyo systems 7, 9, 10, and 11 to the east of Highway 85 is striking (figs. 9 and 10). The majority of these colonies appear to be less than seven years old, and they occur in denser local aggregations than noted in the same areas in previous surveys. The population sizes along 8 of the 11 arroyo survey transects have increased over the past 10 years, and the short-term survival outlook for the Organ Pipe population has never been more favorable.

Management implications

To a great extent, Organ Pipe Cactus National Monument has been affected by external environmental factors over the past 30 years (NPS 2013, 2016). Agricultural development, road expansion, and urbanization in the area around Sonoyta, Mexico, immediately south of the park increased greatly between 1985 and 2005 and continue to this day, threatening monument ecosystems through habitat isolation, nighttime illumination, groundwater depletion, pesticide drift, woodcutting, and invasive species facilitation (NPS 2013). Major increases in cross-border violations during this period led to an equal interdiction response by law enforcement. This included major additions to border security physical infrastructure after 2006, such as a vehicle barrier along most of the border and a 5.2-mile- (8.4 km) long pedestrian fence (NPS 2013, 2016). Additionally, long-term climate change models forecast more frequent and intense droughts in the Sonoran Desert (NPS 2016), potentially having an impact on A. mexicana and its host plant community. These anthropogenic disturbances act in concert with other biotic and abiotic factors. All of these elements have concerned resource managers at the monument for decades and led to the selection of several long-term research sites adjacent to the international park boundary in order to measure ecological changes (Bennett and Kunzman 1987; Brown 1991; NPS 1995). How these factors affect population survival and recruitment of this leafcutter ant species at Organ Pipe and thereby drive long-term changes in population size is considered in the following paragraphs in order from most to least significant.

1. Year-to-year cycles in summer precipitation. Like other Atta species (Moser 1967; Weber 1972), A. mexicana colonies at Organ Pipe do not release all their alate ants on a single night of swarming; a large number are held back for one or more subsequent nights. After heavy (5 cm [2.0 in]) rains, swarming may occur over at least two nights consecutively; after lighter rains, colonies may swarm just once and then await additional rain events over the next few weeks. While distribution of reproductive activity over multiple nights of swarming may be adaptive in habitats receiving more regular and frequent summer rains (i.e., in southern Sonora and the rest of Mexico), it carries unique risks in the Sonoran Desert.

Although monsoonal storms occur every summer at Organ Pipe, their frequency and quality vary greatly from year to year and from place to place. Since the late 1980s, the national monument’s resource management staff has accumulated daily precipitation data records from three field stations most relevant to the ant population: Aguajita to the west, Senita Basin in the center, and Salsola to the east (fig. 5). Table 3 shows the number of significant (≥1 cm and ≥1 in) summer rainfall events (June through September) recorded at these weather stations from 1995 through 2014. During at least 2 of these 20 years at Aguajita (1998 and 2002) and Senita Basin (2002 and 2011) and 1 year (2007) at Salsola, inadequate summer monsoon activity (shown in red and so noted in table 3) almost certainly prevented any swarming. For the 11 years with complete and comparable data for all three stations, no geographic west-east trend is seen for 1-inch threshold events, but the number of 1 cm (0.4 in) threshold events increased 64% from the western station (Aguajita) to the eastern station (Salsola). Not every 1 cm rain is followed by swarming; early morning (midnight to 4 a.m.) rainfall or breezes will inhibit predawn swarming, and daytime heat after modest morning rainfall may dry the soil surface and prevent swarming the following night. Thus, tabulation of 1 cm events probably overestimates the number of reproductive episodes. The 1-inch (2.5 cm) threshold, however, indicates events of higher quality that are more likely to trigger swarming over the following night or two.

Note: Rainfall events are from daily records over four months from June through September. Maximum daily rainfall is shown during these periods. Cumulative counts (last row) only include 11 years when comparable complete records are available for all three stations.

ND indicates station records with missing data.

^aGreen denotes wetter years.

^bBrown signifies drier years.

^cRed indicates the driest years.

Successful reproduction and population recruitment of new colonies appear to be associated with summers with more rain events, heavier maximum rain events, or both. Such summers provide leafcutter ant colonies with ample opportunities to deploy all or nearly all of their alates and maximize their reproductive success. Newly founded nests may survive the late summer and fall best in wet years. The Salsola weather station is located within arroyo transect 10, which had more Atta colonies than any other transect during three of the four surveys (and tied with transect 7 for most colonies in the 2005–2006 survey; see table 1). Salsola also provides an uninterrupted daily rainfall record over 20 years (1995–2014; table 3). Over the 10-year period 2005–2014, six years (2006, 2008, 2011, 2012, 2013, and 2014) delivered above-average frequency of 1 cm threshold events, heavier maximum rainfall events, or both. The age data (table 2) for transect 10 show a diverse range of colony ages, indicating successful reproduction during multiple “good” years spread throughout the decade, although it is not possible to ascribe origins of any specific colony to a particular rain event.

2. Anthropogenic climate change probably is an important factor, especially for the long-term future. These ants are living at their limits of tolerance for arid conditions. The American Southwest in general has experienced more frequent drought conditions in recent decades. Consensus climate change models forecast intensification of this trend this century (NPS 2016). The timing and quantity of summer and winter rainfall determine the health of host plants of the ant and the availability of foliage, flowers, and fruits for harvest. In the Sonoran Desert, A. mexicana colonies along arroyo channel margins have access to a mix of important host plants: larger trees such as palo verde (Cercidium spp. and Parkinsonia spp.), ironwood (Olneya spp.), mesquite (Prosopis spp.), and catclaw (Acacia spp.) along the channels, and creosotebush (Larrea spp.) in the open desert away from these channels. The ants depend upon these plants throughout the year. In addition, increased shade and soil moisture along arroyo channels is probably vital for new colony foundation and population recruitment. Along the western transects, diversity and coverage of larger trees (except Prosopis) is reduced, and abundant saltbush/bursage (Franseria spp./Ambrosia spp.) populations reduce the density of Larrea in the open desert. A. mexicana populations are smaller and more isolated in the western transects at Organ Pipe, which experience lower mean annual rainfall. For the 11 most recent years with complete and comparable data, mean annual rainfall was 6.09 ± 2.51 inches (15.47 ± 6.38 cm) at Aguajita (at the western edge of the survey area), 7.22 ± 2.82 inches (18.34 ± 7.16 cm) at Senita Basin, and 9.02 ± 2.70 inches (22.91 ± 6.85 cm) at Salsola along densely populated arroyo transect 10 to the east. Extended drought conditions that eliminate threshold (1 cm [0.4 in]) monsoonal rainfall events over a decade or more would prevent swarming activity, prevent successful colony establishment, and lead to decline or extinction of local populations. Sadly, A. mexicana should be a good indicator species for effects of sustained severe drought at Organ Pipe.

3. Inferred potential decreases in local populations or effectiveness of natural alate ant predators, particularly bats, nighthawks, and Colorado River toads. The large size and innocuous nature of alate A. mexicana males and females make them attractive food items for local nocturnal and crepuscular (dawn-active) insectivores, and it was easy to observe predation by bats and toads associated with predawn swarming activity in 1986, 1987, 1989, and 1997 at Organ Pipe. Even though Atta alates are a rare food resource, available in abundance only a few nights a year and in very limited areas, bats were seen circling and gleaning alates on the ground and those taking flight from swarming colonies. Nighthawks are also common at Organ Pipe and are a known predator of alates of other Atta species (Moser 1967). Other opportunistic mammalian predators that may seek alates and young colonies at Organ Pipe include the American badger and spotted skunks; armadillos, observed to feed on A. texana in Louisiana (Moser 1967), are absent in Arizona.

Comparison of Salsola rainfall data (1995–2004 versus 2005–2014 in table 3) does not show a convincing decadal trend to explain overall increased reproductive success since 2005, so we must consider alternative explanations. The more recent surge in number of colonies in transects 9–11 near Salsola suggests that predator pressure on this local ant subpopulation may have decreased. This could be due to reduced predator population sizes, changes in predator foraging behavior, or temporary satiation of local predators associated with dense Atta swarms.

4. Increased human population and agricultural land use in Sonoyta, Mexico, adjacent to the national monument could isolate Organ Pipe populations of A. mexicana from source populations to the south, but this did not prevent vigorous colony reproduction and population recruitment over the last four years. These numerous younger colonies were almost certainly founded by females originating locally from the few surviving mature colonies in Organ Pipe or from colonies within a few kilometers of the international boundary. Clearance of land for agriculture that isolates park populations of A. mexicana from source populations in Mexico would have long-term effects on genetic diversity. The alates are not strong fliers and only swarm on calm nights; dispersal distances of winged reproductive Atta females are probably less than 5 km (3.1 mi) (Moser 1967). Very slight airflow from the southeast associated with monsoonal conditions may have had a role in moving some alate females from borderland Sonora into Organ Pipe.

There is no evidence so far suggesting that pesticide drift and groundwater depletion associated with intensive agriculture have affected host plants and ant populations near the international boundary. Cutting of Olneya and Prosopis trees in riparian habitats near the boundary could also have an effect. Because alate ants are attracted to lights, nighttime illumination will affect dispersal of some winged reproductive ants during predawn hours. However, these ants continue to thrive in well-lighted, urbanized areas in central and coastal Mexico, so this also may be a minor concern.

5. Increased border security infrastructure has the potential to amplify effects outlined in the factor above. Construction of a vehicle barrier along the international boundary began in 2003, but the limited vegetation clearance and 2-meter- (6.6 ft) deep posthole excavation associated with it have not had a major impact on colonies along the southern monument boundary. However, construction of the 18-foot pedestrian barrier (fence), widening of roads, and installation of high-intensity nighttime illumination along limited areas of the border began in 2007, and this infrastructure could have a negative effect on A. mexicana populations at Organ Pipe. Also, the use of all-terrain vehicles to interdict cross-border violators has produced visible local degradation of near-border plant communities (NPS 2013), although the effects on colony survival may be minimal for reasons outlined in the fourth factor above.

6. Cross-border activity is another factor. This activity increased significantly between 1985 and 2005 at Organ Pipe (NPS 2013), and was often concentrated along the large arroyo channel systems that are preferred habitat of A. mexicana in the Sonoran Desert. Since 2008, cross-border violations have decreased significantly, though certain routes, like arroyos, tend to be used with some frequency and routes change often in response to interdiction (NPS, T. Tibbitts, resource management specialist, personal conversation, April 2012). Transient pedestrian traffic probably has negligible effects on ant foraging behavior, and accumulation of trash probably has no effect on established colonies. Cross-border violators often break off lower branches to open up sheltering spaces under some larger trees along arroyos, but as long as these large host trees (Parkinsonia, Cercidium, Olneya, Prosopis, Acacia) are not seriously damaged by transient activity, this impact will be minor; in any case, the deep nests are highly resistant to local disturbance.

Conclusion

Atta mexicana is a charismatic invertebrate with complex social behavior and fascinating natural history, living at the limits of its tolerance for aridity and temperature extremes. Like other such species surviving at the margins of their range, it should be a sensitive indicator for long-term environmental impacts. Although Resource Management has not yet included this ant in its long-term monitoring program, its support for decadal surveys is likely to continue. Finally, an effort should be made to integrate molecular genetics technology with future study of this park population to assess inbreeding, genetic diversity levels (always a concern in small, isolated populations), gene flow (effective dispersal), colony genetic structure (number of queens per colony), and to estimate effective population size.

Acknowledgments

Many individuals assisted with field surveys: my spouse Carole; National Park Service staff Terry Peters, Bill Mikus, Dick and Margaret Anderson, Charles Conner, Tim Tibbitts, Ami Pate, Nancy Reid, Lara Dickson, and Willie Rohde; volunteers Ruth and “Freddie” Fredericks, Steve Prchal, Chip Hedgcock, Vaughn Tippit, Steve Birt, and Elna Otter; Terry Marsh and his students; student assistants Kevin Haack, Ahmar Hashmi, Rebecca Clark, Tate Holbrook, Steven Koteras, and Andrew Alton. Ami Pate prepared the transect maps. This research was supported by grants from Southwest Parks and Monuments Association (Tucson, Arizona) and Western National Parks Association (Tucson, Arizona).

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About the author

Alex C. Mintzer (amintzer@cypresscollege.edu or alexmintzer@socal.rr.com) is a professor of biology at Cypress College, Cypress, California.