Seed Selection by the Harvester Ant Pogonomyrmex rugosus (Hymenoptera: Formicidae) in Coastal Sage Scrub: Interactions With Invasive Plant Species

Effects of Introduced Plants on Diet and Seed Preferences of Pogonomyrmex carbonarius (Hymenoptera: Formicidae) in the Patagonian Steppe

Invasive species may interact with resident species and disrupt previously established interactions, with effects on the whole community. If introduced seeds are selectively consumed by native granivores, this could limit the establishment or spread of invasive plants (biotic resistance), and reduce the predation pressure upon native seeds. We determined if the presence of introduced plants affects the diet and the seed preferences of the ant Pogonomyrmex carbonarius (Mayr) in a Patagonian steppe. We expected a higher proportion of introduced seeds in the diet of nests located in areas with a high abundance of introduced species (roadsides) than in nests located in low invaded areas. Diet composition was obtained by collecting items retrieved by workers to colonies and compared between areas of contrasting abundance of introduced species. Field-based choice experiments were performed to evaluate whether exotic seeds were preferred to native ones under a paired comparisons design. Native seeds predominated in the diet. A low proportion of introduced species were included only in colonies close to the road. Ants preferred native seeds to introduced ones, and showed a marked preference for seeds of the native grass Pappostipa speciosa (Trin. & Rupr.) Romasch. (Poaceae), typical of the Patagonian steppe. The presence of introduced plants had little influence on interactions of P. carbonarius with seeds in the Patagonian steppe. Therefore, this ant species would not exert a control on the studied introduced plants, illustrating a case of low biotic resistance to invasion by these species.

From dispersal to predation: A global synthesis of ant-seed interactions

Ant-seed interactions take several forms, including dispersal, predation, and parasitism, whereby ants consume seed appendages without dispersal of seeds. We hypothesized that these interaction outcomes could be predicted by ant and plant traits and habitat, with outcomes falling along a gradient of cost and benefit to the plant. To test this hypothesis, we conducted a global literature review and classified over 6,000 pairs of ant-seed interactions from 753 studies across six continents. Linear models showed that seed and ant size, habitat, and dispersal syndrome were the most consistent predictors. Predation was less likely than parasitism and seed dispersal among myrmecochorous plants. A classification tree of the predicted outcomes from linear models revealed that dispersal and predation formed distinct categories based on habitat, ant size, and dispersal mode, with parasitism outcomes forming a distinct subgroup of predation based on seed size and shape. Multiple correspondence analysis indicated some combinations of ant genera and plant families were strongly associated with particular outcomes, whereas other ant-seed combinations were much more variable. Taken together, these results demonstrate that ant and plant traits are important overall predictors of potential seed fates in different habitat types.