The abundance of ectotherms should ultimately be limited by both plant productivity and the radiant energy that makes that productivity accessible. We analyzed a 21-yr data set on ant colony abundance from a North American arid grassland site. We tested the hypothesis that seed production and temperature co-limit granivorous ant abun- dance. We contrasted the responses of the four most common granivorous species and two omnivorous species to rainfall (summer vs. winter) and to an 8-yr seed addition experiment (summer vs. year-round seed additions). The abundance of granivorous ants was positively correlated with summer rainfall, but not winter rainfall, and was 30% higher on plots with summer vs. year-round seed additions. In contrast, the abundance of omnivorous ants was unaffected by either rainfall or the timing of seed additions. These data suggest that both the sign and size of the covariance between monthly temperature and precipitation may affect bottom-up regulation in ectotherms.