Conflicting Selective Forces Underlying Seed Dispersal in the Endangered Plant Silene diclinis

Selective pressures underlying the distance of seed dispersal in Silene diclinis were evaluated in natural populations. We expected that high seed predation and high seedling density around mother plants would favor relatively long-distance dispersal and that microhabitat conditions might favor short-distance dispersal. We experimentally altered seed density and seed dispersal distance and found that seed predation by ants increased with density but not with distance. Observations of naturally occurring seedlings showed that their survival probability was higher at lower density and when closer to female plants. Overall, calculated cumulative survival probabilities (including both seed predation and seedling survival) favored limited dispersal. Observations of naturally occurring seedlings and adults showed a strongly clumped spatial population structure. Selection for reduced dispersal and the resulting reduction of colonization ability make S. diclinis populations extremely sensitive to environmental stochasticity, and in the long run, this could contribute to the extinction of this endangered species as a consequence of changing land-use patterns.