The Effect of Foraging Army Ants on Leaf-Litter Arthropods

Abstract

The effect of Eciton burchelli army ants (Hymenoptera: Formicidae) on leaf-litter arthropods was studied at Finca La Selva, Costa Rica. Analyses of leaf-litter samples indicated that both arthropod abundance and the number of taxa were reduced following the passage of foraging army ant swarms. Many of the arthropods in the path of an ant swarm escape; the escape tactics that we observed are presented. Full evaluation of the impact of army ant swarms on the arthropod community requires quantification of the rates of capture by both the army ants and the associated swarm-following birds, and of the rate of parasitization by flies that follow the ant swarms. ECITON BURCHELLI (FORMICIDAE: ECITONINAE) IS A COMMON, swarm-raiding army ant of the neotropical forests; its foraging swarms cover broad area and capture wide diversity of arthropods. Because colonies of E. burchelli have large daily intake of food, they presumably have substantial impact on litter arthropods in the areas in which they forage (Wilson 1971, Rettenmeyer et al. 1983). Their effectiveness in prey capture is not clear, however. In an early publication, Schneirla commented that they clear insect life rather completely from terrain over which they pass (1934, based partly on observations of Bates 1863 and Belt 1874). More recently, Schneirla (1949, 1971) and Rettenmeyer (1963) estimated that 30-50 percent of the litter arthropods escape E. burchelli swarms. In study of tropical leaf-litter arthropods, Williams (1941) analyzed the fauna from two plots the day after they were raided by army ants Labidus praedator and indicated that arthropod densities were reduced to a minimum. Franks (1982a, b; Franks & Bossert 1983; Franks & Fletcher 1983) has studied the effects of E. burchelli ants on leaf-litter communities in detail; his results will be referenced below when appropriate. In this study, we investigated the impact of foraging E. burchelli by measuring the densities of leaf-litter arthropods before and after the passage of army ant