Cooperation between organisms may have as much influence on shaping species niches as competition between organisms. Ant-dispersal of seeds and fruits (myrmecochory) is a common mutualism throughout the world and occurs in more than 3,000 plant species, including 30-40% of herbaceous species in the deciduous forests of the eastern United States. The interaction between plants and ants varies with geographic and environmental gradients that appear to reflect the distribution of the ants rather than requirements of the plants. We ask whether the distribution of two understory herbaceous myrmecochores – Hexastylis arifolia and Hepatica nobilis – is better explained by the response of ants or plants to soil moisture gradients. We expect seed dispersal services to decrease and plant aggregation to increase as a function of soil moisture beneath the deciduous forests of North Carolina and Georgia, USA. Seeds (n=2,000) were presented in bait station (n=100) transects (n=10) located along soil moisture gradients that bisected plant populations. Plant and seedling abundance and aggregation, volumetric soil moisture and percent diffuse light were measured along the transects, as well as, the removal rate of introduced myrmecochore seeds. Generalized linear models were used to analyze plant abundance, seedling abundance, seed removal and clumping as a function of site, soil moisture and light assuming Poisson error distributions.

Results/Conclusions

Approximately 22% of the seeds were removed, and Aphaenogaster sp. (Myrmicinae) were the dominant dispersers. Declines in the presence and abundance of H. nobilis and H. arifolia plants and seedlings, as well as, seed removal from bait stations, corresponded with increased soil moisture and was unrelated to site or light levels. Plant aggregation, consistent with undispersed seeds, also increased along the soil moisture gradients. As previous experimental research indicates that both H. arifolia and H. nobilis respond positively to increased soil moisture, the decline and aggregation of plants along soil moisture gradients, in synchrony with declines in seed removal by ants, suggests that the failure of seed dispersal services plays a crucial role in the plant distributions. Connell (1961) demonstrated that two barnacle species were limited by competition (and predation) at the lower limits of sea tides and by desiccation tolerance at the higher limits. This indicated tidewater niches bounded by physiological tolerances and biological interactions rather than an optimal resource. The niches of the myrmecochorous plants may be similarly delimited except that failed facilitative rather the competitive interactions are invoked at one environmental extreme.