Founding, Foraging, and Fighting: Colony Size and the Spatial Distribution of Harvester Ant Nests

Abstract

This study examines how the spatial distribution of nests is related to the behavioral interactions of conspecific neighbors in a population of the seed-eating ant, Pogonomyrmex barbatus. Colonies live for 15-20 yr, reaching reproductive age and a stable size at =5 yr. Spatial distributions were measured for 6 yr (1988)(1989)(1990)(1991)(1992)(1993) in a population of =250 colonies of known age. The probability that a 1-yr-old colony occurs in a given location is related to the distance to, and ages of, its five nearest conspecific neighbors.One-year-old colonies are most likely to occur near small, 2-and 3-yr-old colonies.Neighboring colonies encounter each other when foragers of both colonies search the same area on the same day. The probability of an encounter between two colonies decreases with the distance between their nests. For colonies of all ages, encounters are most likely with their larger neighbors, ~5 yr old. Encounters are more likely if there was an encounter the previous day, and this effect can overwhelm the effect of distance. Two-yr-old colonies are more likely than colonies of other ages to lose ground in t;epeated encounters with a particular neighbor.Though encounters are frequent, their costs are low: few of the foragers on a trail that meets a neighbor's actually encounter an ant of the neighboring colony, and interaction with a neighboring colony does not increase the typical duration of a foraging trip. If foragers of two colonies do meet, most fights are brief, with few resulting in injury or death. These results suggest that the cost of conspecific neighbors searching the same ground for seeds may be greater than the cost of behavioral interaction itself. Exploitative competition may have more important effects than interference competition on founding colony survival and thus on the spatial distribution of nests.

When should a territory resident attack?

“…Davies & Houston 1983;Stamps & Tollestrup 1984;Gordon 1995). Furthermore, in any territorial species whose members establish stable home ranges in or near territorial neighbourhoods, individuals can repeatedly intrude into particular territories: examples include species in which neighbours regularly intrude into one another's territories (Temeles 1994;Gordon & Kulig 1996), or in which 'floaters' living in or near territorial neighbourhoods repeatedly intrude into nearby territories (e.g. Smith 1978;Arcese 1987;Stamps & Eason 1989;Rohner 1997).…”

Reclaiming Lost Territory: the Response of Owyhee Harvester Ants to Forager Intrusions by Neighboring Colonies

“…Whitford (1976) reported no cases of intercolony aggression in P. rugosus, in contrast to the observations of Hölldobler (1976). Gordon and Kulig (1996) report that encounters between members of the same neighboring P. barbatus colonies may lead to fights on one day but not on another. Such differences in the observed occurrence of fighting within and among species may reflect differences in the circumstances that promote aggression among neighbors rather than species' specific tendencies toward aggression.…”

“…Competition for foraging space between neighboring harvester ant colonies is often cited as an important influence on the spatial distributions of nests, at least at smaller spatial scales (Hölldobler 1976;De Vita 1979;Levings and Traniello 1981;Ryti andCase 1988, 1992;Wiernasz and Cole 1995;Crist and Wiens 1996, Gordon 1991, 1992, Gordon and Kulig 1996, 1998Adler and Gordon 2003). Individual foragers travel to and from their nest along habitual foraging trails that typically radiate up to 20 m away from the nest, sometimes farther, and gradually dissipate into resource patches where foragers search for food (Gordon 1991(Gordon , 1995MacMahon et al 2000).…”

“…Aggressive, and sometimes lethal contests have been reported in P. californicus (De Vita 1979), P. rugosus, P. maricopa (Hölldobler 1976), P. mayri (Kugler 1984), and P. barbatus (Hölldobler 1976;Gordon 1992Gordon , 1995Gordon and Kulig 1996). By contrast, Harrison and Gentry (1981) observed overlapping foraging ranges and no aggressive interactions among neighboring P. badius colonies; chance encounters between neighbors were brief and resulted in little more than mutual agitation (Harrison and Gentry 1981).…”

Digging beneath the surface: incipient nest characteristics across three species of harvester ant that differ in colony founding strategy

“…Thus, reproductive success is solely dependent upon the success of the foundress at initiating a nest. Harvester ant foundresses are subject to predation and desiccation risks (Hölldobler and Wilson, 1990), resulting in a high mortality rate (Hölldobler and Wilson, 1990;Wiernasz and Cole, 2003) that may reach 99% (Gordon and Kulig, 1996;Johnson, 2001;Pfennig, 1995). This indicates that there is likely high selective pressure on foundresses during colony initiation.…”