Ant-mediated seed dispersal does not facilitate niche expansion

1. Whereas classic niche theory is based on the contraction of the niche via negative interactions, facilitative niche theory suggests that mutualisms can expand the niche via positive interactions. Specifically, animal-mediated seed dispersal can expand the utilization of physical space by plants and allow greater access to resources and other environmental requirements. Ant-mediated dispersal of plant propagules (myrmecochory) is a common mutualism throughout the world, particularly in the deciduous forests of the eastern United States where this research is conducted. 2. We examine two facets of niche expansion via ant-mediated seed dispersal: (1) increased utilization of resources along resource gradients and (2) escape from unfavourable density-dependent conditions. 3. We test these assumptions by introducing Hexastylis arifolia seeds in cafeteria-style bait stations along abiotic gradients (moisture, temperature, light) for removal by key seed dispersers from the ant genus Aphaenogaster. We also examine plant aggregation along the same gradients. 4. Ant-mediated dispersal services decrease significantly with increasing soil moisture and ultimately fail at levels that are demonstratively within the plant's niche optima; further, the decline in dispersal services is correlated with increasing plant aggregation, suggesting that enemy escape also falters at relatively high levels of soil moisture. 5. Synthesis. Facilitated propagule dispersal fails to expand the Hexastylis arifolia niche in either enhanced resource utilization or decreased density dependence as the niche requirements for the ant disperser are nested within those for the plant. The strength of this interaction varies across space and time, and in doing so may undermine attempts to predict future distributions. Further, given that myrmecochores are typically poor dispersers, the incomplete niche overlap between the plant and its facilitator makes this plant guild particularly susceptible to climatic change if each participant responds individually to shifting environmental conditions.