Interspecific spatial patterns support indirect facilitation of harvester ants by kangaroo rats

1. Interspecific spatial patterns support indirect facilitation of harvester ants by kangaroo rats Andrew J. Edelman Dept. of Biology, University of New Mexico, Albuquerque, New Mexico 87131 (andrewe@unm.edu) • Background Information • Heteromyid rodents (e.g. kangaroo rats) are hypothesized to indirectly facilitate granivorous ants in the Chihuahuan Desert by influencing ant food resources. • Heteromyid rodents forage selectively on large-seeded winter annuals, whereas granivorous ants primarily forage on small-seeded winter annuals. • Long-term removal experiments have shown that Heteromyid rodents reduce the abundance of large-seeded winter annuals, which competitively releases small-seeded winter annuals. • However, increases in small-seeded winter annual abundance does not lead to large increases in granivorous ant abundance as predicted. • I am examining interspecific spatial patterns of kangaroo rats and harvester ants for evidence of indirect facilitation effects. • Study Organisms • These species are the largest and most dominant granivores in rodent and ant communities of the Chihuahuan Desert. • Banner-tailed kangaroo rat (Dipodomys spectabilis) • Builds large mounds (4-m diameter) that contain burrows and seed caches. One adult lives at each active mound. • Rough harvester ant (Pogonomyrmex rugosus) • Builds large underground colonies with cleared surface disc (1-m diameter). Each colony has 1 queen and several hundred workers. • Are spatial associations influenced by recruitment of ant colonies? • No • Kangaroo Rat Mounds vs. Young Ant Colonies • Research Questions • Do kangaroo rat mounds and harvester ant colonies exhibit positive spatial association as predicted by the indirect facilitation hypothesis? • Are spatial associations influenced by recruitment or mortality patterns of ant colonies? • Do fitted spatial models support the indirect facilitation hypothesis? • Do fitted spatial models support the indirect facilitation hypothesis? • Yes, the best fit model includes a positive spatial interaction between species at scales ≤ 10 m. ↓ large-seeded winter annuals Table 1. Summary of multi-type Strauss hard-core models fitted to locations of kangaroo rat mounds and older ant colonies. The full model was a significantly better fit than the null model (P < 0.01). ↑ ant abundance ↑ small-seeded winter annuals • Methods • Study area: 8.5-ha grassland site at the Sevilleta National Wildlife Refuge, New Mexico. • All occupied mounds (n = 48) and discs (n = 209) were mapped to sub-meter accuracy on the study area during Summer 2007 using a GPS unit. The study area was resurveyed during Summer 2008 and new colonies (n =166) were mapped. • Ant colonies were checked for activity and diameter of disc was measured (index of colony age). • Occupancy of kangaroo rat mounds was monitored monthly since March 2005 by live-trapping and marking individuals. • All spatial analyses and model fitting were performed using the R package spatstat. • Mean intensity of opposing species sites (transformed bivariate Ripley’s K with edge correction) was calculated for a range of distances. Test significance was constructed using a Monte Carlo procedure (1000 simulations). Rejection limits (P < 0.05) based on spatial independence were used to determined at what distances the observed intensity differed from expected. • Multi-type Strauss hard-core models were fitted to spatial locations of kangaroo rat mounds and older ant colonies. A Monte Carlo test (100 simulations) was used to determine the best fit model. Fig. 3. Spatial association of kangaroo rat mounds and young ant colonies (n = 53, disc < 64 cm) during 2007. Kangaroo Rat Mounds vs. New Ant Colonies • Discussion • Kangaroo rat mounds and harvester ant discs were positively associated at fine scales (< 12 m) as predicted by the indirect facilitation hypothesis. • Recruitment of harvester ant colonies was spatially random with respect to mounds. • Positive spatial association between mounds and older colonies may indicate that survivorship of colonies near mounds is higher than away from mounds. • The best fit spatial model includes a positive spatial interaction between kangaroo rat mounds and ant colonies supporting the indirect facilitation hypothesis. • Kangaroo rats may indirectly facilitate harvester ants by increasing the seed abundance around their mounds. • Colonies near mounds have shorter foraging times and greater seed availability. Fig. 4. Spatial association of kangaroo rat mounds and new ant colonies (n = 166) during 2008. Results Do kangaroo rat mounds and harvester ant colonies exhibit positive spatial association? Yes, at scales < 12 m Kangaroo Rat Mounds vs. Ant Colonies • Are spatial associations influenced by ant colony mortality. • Yes, at scales < 12 m • Kangaroo Rat Mounds vs. Older Ant Colonies • Future Research • Monitor survival and recruitment of harvester ant colonies in 2009. Acknowledgments Funding for this project was provided by grants from Sevilleta LTER, NSF, ASM,UNM RPT, UNM SRAC, and UNM BGSA. I thank J. Brown, M. Friggens, S. Johnson, A. Kodric-Brown, F. Smith, E. Tuttle, and the personnel of the Sevilleta LTER and NWR for their assistance. Fig. 1. Study area with mapped locations of kangaroo rat mounds (○) and harvester ant colonies (∆) Fig. 2. Spatial association of kangaroo rat mounds (n = 48) and harvester ant colonies (n = 209) during 2007. Fig. 5. Spatial association of kangaroo rat mounds and older ant colonies (n = 156, disc > 64 cm) during 2007.