MICROGEOGRAPHIC AND VERTICAL-DISTRIBUTION OF ACROMYRMEX LANDOLTI (HYMENOPTERA, FORMICIDAE) NESTS IN A NEOTROPICAL SAVANNA

Nests of the grass-cutting attine ant Acromyrmex landolti (Forel) were excavated in a lowland tropical savanna in eastern Colombia over a pe riod of 27 mo. Of 135 colonies, nests excavated during the dry season were deeper, had more chambers, and were approximate to 2.5 times larg er than those excavated in the same area of native savanna during the wet season. Chambers containing fungus gardens were found to a depth e xceeding 2 m during the dry season. During the rainy season, all chamb ers containing fungus gardens were located within 30 cm of the soil su rface. Individual chamber size did not vary seasonally. During the rai ny season, colonies relocated to superficial chambers and were smalles t in terms of total colony volume and number of chambers per colony. D uring the dry season, colonies excavated new chambers to depths where soil moisture asymptotically approached a maximum level. Maps of colon y location and microtopography of the savanna revealed an aggregated d istribution associated with microrelief. Colonies were aggregated on s lightly raised ridges that were most likely caused by surface water ru noff. Implications for sampling and control are discussed.