The above- and below-ground legacies of ant colonies

Background/Question/Methods

Ants can be ecosystem engineers that modulate ecosystem processes in a diversity of ecosystems via bioturbating the soil, creating localized “islands” of soil nutrients, altering litter decomposition rates, decreasing soil humification, consuming other soil organisms, and shifting vegetation composition and productivity. While comparative studies of soils on and off of ant nests have demonstrated that nutrient cycling can differ on and off active nests, we do not know if these differences persist after the colony dies or relocates and if they lead to changes in ecosystem trajectory (i.e., plant colonization and associated activities vs abandonment following colony death). Formica obscuripes, is the most common thatching ant in the Western US. F. obscuripes constructs large, domed nests composed of thick masses of plant debris, fecal material, excavated soil, and other organic matter. Over the last three years we have monitored F. obscuripes colonies near Almont, CO (elev. 2720 m). We have tracked colony birth and death, colony size, soil properties, as well as the plant and soil communities on and off colonies of various ages (random open areas, dead <1 year, live <1 year, live >2 years).

Results/Conclusions

In 2011, out of 300 colonies located over 2 years, 50 were inactive (dead <1 year), 50 were new (<1 year), and 200 were ‘old’ (>2 years). Change in colony activity from 2010 to 2011 showed that, 14% of the prior year’s colonies were abandoned, but new colonies established at a rate of 24% resulting in net growth of 10%. Potential nitrogen mineralization rates were 187% higher on colonies relative to colony-free areas and soil nematode abundance, was significantly higher on, relative to off, ant colonies. Interestingly, this effect essentially disappears one year following colony death. In addition, plant community composition differed among colonies of different ages as well as on and off colonies: communities surrounding older colonies (>2 years) consisted of a greater proportion of woody and herbaceous species than those in colony free areas. Plant communities surrounding new colonies (<1 year) consisted of a higher percentage of woody species (40%) than those away from colonies, suggesting that ants may choose sites with woody species for colonization. Finally, plant species richness around colonies increased with colony age. Our results indicate that ants alter ecosystem properties and processes rates, both above and below-ground, but these effects dissipate quickly once a colony dies.