- by A Sunjian and Li H.
|Sunjian A and Li H. (2005) Atta mexicana in the resort community of Nuevo Vallarta, Mexico. Notes from Underground 11 (1)|
(revision date: August 8, 2004)
Map of Mexico with Nuevo Vallarta
Knowledge about the impact of human activity on the surrounding fauna and flora is of particular importance in rapidly developing regions. Deforestation, urbanization, and the inadvertant destruction of ecological niches might not only lead to a catastrophic loss in biological diversity, but may also inadvertantly cause the proliferation of pest species who benefit from such drastic changes in the environment.
Leafcutter ants of the genus Atta and Acromyrmex are one of the most economically important pests in Central and South America because of their ability to efficiently and rapidly defoliate crop plants and grasses. They are the dominant herbivores in the neotropics, with the amount of vegetable matter being cut estimated at an astonishing 12-17% of annual leaf production! Damage (indirect and direct) has been estimated to be in the billions (USA dollars) annually, and the grass cutting species reduce the carrying capacity of pasturelands in the area by as much as 10% (Fowler et al, 1986a).
The authors conducted a study of the status of the major leafcutter ant Atta mexicana in the rapidly developing town of Nuevo Vallarta, in the state of Nayarit, Mexico. Nuevo Vallarta is a gated 465 hectare residential-resort community north of Puerto Vallarta on the central western coast of the country. It was originally planned by the government as a modern adjunct to Puerto Vallarta, with tourism as its major function. However, major development of the area did not occur until the late 1990s. The community now has at least one 18-hole professional golf-course, a 300-slip marina, 4.6 km of white sandy beach, and 10 km of inland canals winding through the numerous five-star hotels, condominiums, and commercial zones.
Methods and Materials
Surveys of the surrounding area were accomplished by walks in the early morning and late afternoons. The diameter and heights of any separable nest mounds found were measured. A Palm Pilot III GPS unit provided accurate determinations of the exact locations of many of the colonies, and a compass was used to orient diagrams drawn to show the relative distances between nest mounds in a colony. Many of the mounds were filmed using a Canon Ultra miniDV camcorder for latter analysis. The authors also conducted impromptu and informal interviews with the local people in order to determine human relations with the leafcutter ants.
A survey of an approximately 2 hectare area in the community revealed a total of 12 colonies of Atta mexicana. Three had extensive nest mounds, while the rest were in various intermediate stages of growth. This is most likely an undersampling of the actual number of colonies, since portions of the surveyed area were not accessible because they lay in private enclosed land.
Reported Atta colony densities in the literature vary widely, most likely due to variations in the local habitat, but the number of A. mexicana colonies in Nuevo Vallarta is high for a dicot-specific Atta. For example, various studies on Atta sexdens in Paraguay and Brazil showed densities from 0.5-5 colonies/ha, while the densities of Atta cephalotes colonies in Trinidad, Guatemala, and Costa Rica varied from a low of 0.5 colonies/ha to a high of 5 colonies/ha (Fowler et al., 1986b).
Nest mounds erupt on sidewalk. Click thumbnail for detailed image.
Location of Nest Mounds
Nest mounds tended to be located in shaded areas, usually under a cover of trees, and were found both in sloped and flat land.
The authors were surprised to note the large nest mounds of two large colonies erupting from the sidewalks of one of the most heavily trafficked road (Paseo de las Palmas). In one case, the nearly half-meter high mounds had constricted the sidewalk to such a degree that passing bicycles had to thread carefully between the encroaching mounds. The islands of grass and palm trees that separated the two lanes of traffic on roads also seemed to be a favorite area of colonization for smaller colonies.
Physical characteristics of nest mounds
The nest mounds of Atta mexicana were easily distinguished from mounds made by other species in the area by size and texture. The leafcutters were by far the most prevalent large ant in this disturbed area, and the nest mounds of even the smallest colonies were as large or larger than the nest mounds of the smaller formicines and dolichoderines that dotted the landscape. In addition, the authors noticed that soil composing the nest mounds of Atta mexicana colonies tended to look slightly different than the surrounding soil, probably because these originated from deeper underground than the topsoil layer.
The foraging holes of larger colonies frequently opened far (> 5 meters) from the nest mounds. Many foraging holes were to the sides of sidewalks, and oblong or oval in shape.
Colony orientation along North-South. Click thumbnail for detailed image.
Orientation of Nest Mounds
There was a surprising tendency for colonies to have their nest mounds distributed in such a manner that their long axis were oriented from North to South. Nearly all colonies whose nest mounds were measured manifested this trait to varying degrees. This occurred not only in some of largest colonies, but even in very young colonies, where the waste "dump" completed the long axis. Indeed, two of the largest colonies sprawled along the sidewalks on streets that run North-South.
The authors have no current hypothesis to account for this finding, nor are there clues as to whether this is an artifact or mere coincidence that will wash away when confronted by future surveys that examine many more colonies in the area.
Relations with Local Peoples
All interviewed locals were familiar with the "hormigas arrieras", although a few could not say whether they existed locally. This is similar to the authors' experiences in other areas, where the large colony populations of leafcutters, their unique behavior, and their economic impact, have made them almost a basic facet of everyday life in this region.
Not surprisingly, the immediate hotel grounds where the authors stayed (Paradise Village Resort) were seemingly free of any colonies. The grounds manager revealed that constant insecticide spraying kept the larger ants out of the hotel (although smaller ants created numerous small mounds on the hotel walkways), and that any small leafcutter colonies found were killed or driven away by pouring poison into the nest holes. Some condominium owners in the area also expressed dislike for the ants and admitted that they killed foragers whenever possible.
Finally, the lower trunks of many trees in the area were sprayed or painted with a white substance that contained lime, and which supposedly prevented the leafcutter ants from harvesting the tree's leaves. Observations suggests that the ants indeed are slightly thwarted by the substance, although the authors still observed laden foragers gingerly crossing the painted areas.
Comparison to Puerto Vallarta
A less exhaustive survey of nearby Puerto Vallarta revealed at least four large colonies around the town center (Isla Cuale), but no small nest mounds were discovered. Unlike Nuevo Vallarta, which is characterized by extensive open areas, the older city is crowded, much more heavily populated, and has much less ongoing construction and development activities. The seeming lack of small nest mounds when compared to Nuevo Vallarta may indicate that extensive human activity can lead to an increase in the survivability or creation of new and young A. mexicana colonies, probably by providing cleared areas for the ants. In addition, the presence of large mounds in Puerto Vallarta may also indicate that leafcutters in this area are at some steady-state level, with a low turnover of mature colonies.
Far from being threatened by the rapid pace of development occurring in Nuevo Vallarta, Atta mexicana colonies in fact seem to be proliferating at a very healthy rate. Certainly, the local people expressed no strong interest or love for the "hormigas arrieras", and did their best to try and destroy colonies that had managed to gain a foothold inside one of the many high-class resort hotels and condominiums in the area. Atta leafcutters seem to be more adapted to "open" areas (e.g. densities are higher in secondary forests than primary forests), so the large cleared areas in rapidly-developing Nuevo Vallarta may represent a benign and optimum environment for these ants. Indeed, various studies have shown that colony densities of some ants can rapidly increase in man-simplified habitats (Fowler et al, 1986b).
Future studies might focus on several questions raised by this survey, including:
Fowler GH, LC Forri, V Pereira-da-Silva, and NB Saes (1986a) Economics of Grass-cutting Ants. In: Fire Ants and Leaf Cutting Ants: Biology and Management (eds Lofgren CS, Vander Meer RK), pp. 123–145. Westview Press, Boulder
Fowler GH, V Pereira-da-Silva V, Forti LC, and NB Saes (1986b) Population dynamics of leaf-cutting ants. A brief review. In:
Fire Ants and Leaf Cutting Ants: Biology and Management (eds Lofgren CS, Vander Meer RK), pp. 123–145. Westview Press, Boulder