Lifespan, age, size-specific mortality and dispersion of colonies of the Florida harvester ant, Pogonomyrmex badius

Abstract

Colonies of social insects such as ants have species-typical life history attributes that include mature colony size, mortality rate, size at first reproduction, lifespan, reproductive investment and many others. A population of over 400 colonies of the Florida harvester ant, Pogonomyrmex badius, was repeatedly resurveyed over 6 years, producing a record of deaths, births, colony sizes and relocations. From these data the life span, growth rate and colony-size-specific mortality rates were determined. Colonies averaged about 2500 workers and ranged from less than 500 up to 10,000. The annual mortality rate of colonies decreased from about 25% for the smallest colonies, to about 6% for mid-sized colonies. Under steady-state assumptions, these extrapolate to lifespans of about 4 and 17 year, respectively. Over 90% of the largest colonies were still alive after 6 years, so that their lifespan could not be reliably determined, but probably exceeded 30 years or more. As new colonies grew, their probability of surviving increased, but many colonies stabilized at less than maximum size, thus remaining subject to the mortality rates characteristic of their size, not age. At the level of the whole population, colonies were significantly clumped, probably as the result of habitat heterogeneity. Large colonies were associated with more open areas. Colonies with more and/or larger neighbors had moderately higher mortality rates. This rate increased as size asymmetry increased. The need for demographic data on ant species with a range of mature colony sizes is discussed.