Kathleen H. Keeler

In The Insect Societies (Harvard, 1971) E. O. Wilson wrote compiled a table (Table 21-4) of Longevities of established colonies of social insects. There he listed the only data known to him for ants:

I read this in 1975 and seeing an opportunity in the distinct colonies of the western harvester ant (Pogonomyrmex occidentalis), in August 1976 I marked (with an aluminum tag on a nail) 57 colonies in the grazed short grass prairie above Cedar Point Biological Station, Ogallala, NE (Keith Co.)

I annually revisited those colonies through the 1980's, publishing the survivorship results in:

Keeler, K.H. 1981. Preliminary report of colony survivorship in the western harvester ant (Pogonomyrmex occidentalis) in western Nebraska. Southwestern Naturalist 27: 245-246.

Keeler, K.H. 1988. Colony survivorship in the western harvester ant (Pogonomyrmex occidentalis). The Southwestern Naturalist. 33: 480-482.

Keeler, K.H. 1993. Fifteen years of colony survivorship in the western harvester ant Pogonomyrmex occidentalis. The Southwestern Naturalist. 38: 286-289.

My observations lapsed in the 90's, but since 2002 I have been checking them annually. Of the colonies (of unknown age) marked in 1976, there are 5 survivors (Fig.1).

Fig. 1. Survivorship of Colonies marked in 1976, years are 1976 to 2007


Of the 14 colonies added in 1978, there was 1 survivors in 2007 (Fig.2).

Fig. 2. Colonies added to the study in 1978. Years are 1978-2007.

Five, 18.6%, of the 58 harvester ant colonies I observed at the beginning of August, 1976 were still alive July 31, 2007, 31 years later.

For some years I followed the entire population. After 1990 I stopped adding following new colonies. The peak population was 85, in all 104 colonies were marked. Of those, 12 (11.5%) survive. The youngest cannot be under 18 years old.

Two sites appear to have been reoccupied, with a new colony moving into the bare zone left by a colony that died. In the others, when the colony died, plants invaded. Colonies are overdispersed (allowing for rock outcrops that make for unsuitable nest sites) and gaps between colonies left by deaths have in many cases been occupied by a new colony at a different location.

Knowing what I know now, I’d replicate this study, to follow more than one population. Despite its weakness, since direct data on long lived species is so difficult to collect, it provides difficult to obtain information on the dynamics of long-enduring colonies.

As far as is known, Pogonomyrmex occidentalis has single queen colonies, so this study tracks the survival of individual insects (the queen). These queens have lived a minimum of 31 years. This is consistent with the projection I made from the survivorship curves in 1993 (Keeler 1993).