Ants as Predators

Ants are one of the most important predators of other organisms in many ecosystems. There is also a clear latitudinal effect on the prevalence of ant predation, i.e., it decreases with increasing latitude. Bob Jeanne (1979) made this point in a simple yet elegant observational experiment using wasp larvae baits.

Abstract Predation by ants on wasp larva baits was assessed at five latitudes from 43°N to 2°S. Rates of predation were significantly greater in the tropics; that is, a food item was exploited by ants in significantly less time in the tropics than in the temperate zone. It was also found that predation rates were higher in low second-growth vegetation than in forests and higher on the ground than on vegetation, and that these differences were more pronounced in the temperate zone than in the tropics. Rates of predation occurring on buildings were lower than in natural situations at every latitude. The number of ant species taken at the baits increased from 22 at the northernmost locality to 74 at the southernmost. In all localities but the northernmost the forest habitat produced a greater diversity of species than the field habitat. There was a greater degree of microhabitat specificity among ants in the tropics than in the temperate zone. The proportion of forest canopy specialists also increased toward the tropics. Overall predation rate and ant species richness were found to be positively correlated on a latitudinal scale, but this relationship broke down at the level of habitats and microhabitats.

A more recent study, of predation on baits along an elevation gradient in the Andes, found a similar result (Camacho and Aviles 2019). Ants were the main predators of an live fly baits, and predation overall decreased with increasing altitude.

Abstract: Predation, which is a fundamental force in ecosystems, has been found to decrease in intensity with elevation and latitude. The mechanisms behind this pattern, however, remain unaddressed. Using visual sampling of potential predators and live flies as baits, we assessed predation patterns along 4,000-m elevation transects on either side of the equatorial Andes. At the lower elevations, we found that around 80% of predation events on our insect baits were due to ants. The decline in predation with elevation was driven mainly by a decline in the abundance of ants, whose importance relative to other predators also declined. We show that both predator density and activity (predation rate per individual predator) decreased with elevation, thus ascribing specific mechanisms to known predation patterns. We suggest that changes in these two mechanisms may reflect changes in primary productivity and metabolic rate with temperature, factors of potential relevance across latitudinal and other macroecological gradients, particularly for ectotherm predators and prey.