Soil Ecology Restoration Group

last update October 29, 2000

The interaction of harvester ants and vesicular-arbuscular mycorrhizal fungi in a patchy semi-arid environment: the effects of mound structure on fungal dispersion and establishment



1. Disc areas created by the western harvester ant, Pogonomyrmex occidentalis, around their mound nests can compromise over 10% of the land surface in semi-arid shrub steppes of the Great Basin, USA. The harvester ant mounds are abandoned after 12-20 years and the mycorrhizal fungal inoculum dispersion patterns within these patches may affect subsequent plant establishment.

2. It was hypothesized that harvester ant activity reduces mycorrhizal inoculum levels within the disc, and that mycorrhizal fungi must re-establish in these patches from surface-deposited, immigrating inoculum.

3. To test this, we first excavated two ant mounds, which revealed a chambered core consisting of a mat of densely packed, clipped roots with 2000-5000 times the vesicular-arbuscular mycorrhizal fungal spore density found in the surrounding undisturbed vegetation. To assess the effectiveness of these enriched patches of inoculum, non-mycorrhizal seedlings of Artemisia tridentata (sagebrush) and Oryzopsis hymenoides (Indian ricegrass) were planted around the perimeter of nine mycorrhiza-enriched ant mounds and compared with surface inoculation of an exotic mycorrhizal fungus, Gigaspora margarita, capable of being tracked through immunofluorescence.

4. Results from these field bioassays demonstrated that surface-deposited mycorrhizal inoculum did not expand downward into the soil and buried inoculum resulted in high levels of endophyte root infection.

5. It was concluded that harvester ant mounds exhibit nest microbial enrichment, and that the ants leave high densities of vesicular-arbuscular mycorrhizal fungi in locations suitable for assisting in the establishment of mutualistic mycorrhizal associations after the mounds are abandoned.