The population dynamics of invasive plants are influenced by positive and negative associations formed with members of the fauna present in the introduced range. For example, mutualistic associations formed with pollinators or seed dispersers may facilitate invasion, but reduced fitness from attack by native herbivores can also suppress it. Since population expansion depends on effective seed dispersal, interactions with seed dispersers and predators in a plant species introduced range may be of particular importance. We explored the relative contributions of potential seed dispersers (ants) and vertebrate predators (rodents and birds) to seed removal of two diplochorous (i.e., wind-and antdispersed), invasive thistles, Cirsium arvense and Carduus nutans, in Colorado, USA. We also conducted behavior trials to explore the potential of different ant species to disperse seeds, and we quantified which potential ant dispersers were prevalent at our study locations. Both ants and vertebrate predators removed significant amounts of C. arvense and C. nutans seed, with the relative proportion of seed removed by each guild varying by location. The behavior trials revealed clear seed preferences among three ant species as well as differences in the foragers' abilities to move seeds. In addition, two ant species that acted as potential dispersal agents were dominant at the study locations. Since local conditions in part determined whether dispersers or predators removed more seed, it is possible that some thistle populations benefit from a net dispersal effect, while others suffer proportionally more predation. Additionally, because the effectiveness of potential ant dispersers is taxonspecific, changes in ant community composition could affect the seed-dispersal dynamics of these thistles. Until now, most studies describing dispersal dynamics in C. arvense and C. nutans have focused on primary dispersal by wind or pre-dispersal seed predation by insects. Our findings suggest that animal-mediated dispersal and post-dispersal seed predation deserve further consideration.

Disturbances such as fire, grazing, and soil mixing by animals interact to shape vegetation in grassland ecosystems. Animal-generated disturbances are unique in that they arise from a suite of behaviors that are themselves subject to modification by external factors. The manner in which co-occurring animal taxa interact to alter vegetation is a function of their respective behaviors, which shape the characteristics (e.g., the magnitude or extent) of their disturbances. To determine whether prairie dogs (Cynomys ludovicianus) and harvester ants (Pogonomyrmex occidentalis) interactively alter vegetation structure and heterogeneity on the Colorado shortgrass steppe, we characterized the size, dispersion, and vegetation of prairie dog burrow mounds and ant nests (located on and off prairie dog colonies) and vegetation growing beyond mound and nest perimeters. Ants located on prairie dog colonies engineered significantly larger nests and disturbed nearly twice as much total soil area as their off-colony counterparts. Ant nests were overdispersed both on and off prairie dog colonies, while prairie dog mounds were randomly dispersed. Where harvester ants and prairie dogs co-occur, the overdispersed pattern of on-colony ant nests is in effect "overlaid" onto the random pattern of prairie dog mounds, resulting in a unique, aggregated pattern of soil disturbance. Ant nests on prairie dog colonies had significantly less vegetation and lower plant species diversity than did prairie dog mounds, while off-colony nests were similar to mounds. These results suggest that ant nests are more highly disturbed when located on prairie dog colonies. Beyond nests proper, ants did not appear to alter vegetation in a manner distinct from prairie dogs. As such, the interactive effects of prairie dogs and ants on vegetation arise mainly from the disturbance characteristics of mounds and nests proper.