Habitat segregation based on soil texture and body size in the seed-harvester ants Pogonomyrmex rugosus and P-barbatus

1. The seed-harvester ants Pogonomyrmex rugosus and P. barbatus are ecologi cally equivalent sister species that have broadly overlapping distributions in the south-western U.S.A.; however the two species are only sympatric in localised contact zones. 2. Soil regimes at 25-50 cm below the surface were quantified across contac t zones to assess abiotic habitat factors related to distribution pattern. Physiological parameters related to foundress survival were also measured i n order to test for a correlation between these parameters and distribution pattern. 3. The two species segregated among microhabitats based on soil texture; P. barbatus occurred alone in soils with a higher clay content and/or higher moisture retention. In areas of sympatry, soil texture was similar for both species but was intermediate to that in areas where the two species occurr ed allopatrically. The pattern of microhabitat segregation was similar acro ss three sites that encompassed a broad range of soil regimes. 4. The only measure of foundress survival correlated with microhabitat diff erences was an approximate to 8% greater dry mass for alate females of P. r ugosus. This resulted in their surviving significantly longer than did alat e females of P. barbatus under desiccating conditions. 5. This microdistribution pattern may be caused indirectly by soil texture affecting plant species distribution and hence the seeds available to ants. A companion laboratory experiment demonstrated, however, that soils could also cause this distribution pattern of both ant species directly via effec ts on foundress wet mass.