Ants in arid and semiarid ecosystems are one of the most important seed predators. Their behavior is complex, depending both on individual decisions and on collective responses such as trail formation or recruitment of new individuals to foraging tasks. Behavior also changes over the year depending on seed availability. Understanding ant foraging is crucial for a better comprehension of the consequences of granivory in nature, where different resources co-occur in space often with a patchy distribution. In these study we aimed to disentangle how seed density and presence of co-occurring seed species interact with individual and collective behaviors to determine seed removal by ants of the genus Pheidole. Seeds of three species in different densities and combinations were offered to ants in two seasons contrasting in seed availability in a semiarid grassland in Southern Mexico. Removal rates were recorded every two hours. Using these data we fitted a model that disentangles the individual (probability pi that an ant removes a seed of species i) and collective (number of active ants h) components of foraging.
Results/Conclusions
Seed removal increased with seed density due to increased h and pi. This suggests that ants have the capacity to evaluate resource density, collecting seeds only from dense patches and ignoring those with low-seed density. Seeds of companion species had nearly no effect on removal probabilities, but increased the number of ants active. This would be expected if ants have preferences for certain seed species and ignored the rest, so pi values were not affected by seeds of companion species. Recruitment was more likely in mixed-seed patches seemingly because of a higher chance that individuals with different preferences find their preferred resource. In the low seed-availability season, the number of active ants increased, ants traveled longer distances, and foraging was directed to the most profitable seed-patches indicating that ants were more selective. This seems to be at odds with optimal foraging theory. However, the reduction in individual acquisition rates caused whenever a low-density patch is ignored may be compensate by increasing collective gains because ants are recruited only to dense patches, reducing the whole colony energy investment and preventing overshooting (driving ants towards resource patches that are likely to be depleted by the time they arrive).