PARENT SESSION
Oral Session #28: Plant-Animal Interactions. Presiding: S. Louda.
Tuesday, August 7, 2001. 8:00 AM to 12:00 PM. Hall of Ideas J.

Fitness and population consequences of a seed dispersal mutualism.

CHRISTIAN, CAROLINE¹, ¹

ABSTRACT- Seed dispersers play fundamental roles in shaping plant populations by defining the environment in which seeds germinate, establish and grow. In the fire-prone shrublands of South Africa, the exotic Argentine ant alters seed dispersal mutualisms by eliminating the native ant species most efficient at dispersing and burying seeds. My previous work on this system has demonstrated that fewer seeds are buried and seed burial depths are shallower in areas invaded by Argentine ants compared to uninvaded areas. Here I present results from field experiments and matrix-model simulations that assess how a decline in seed dispersal affects plant fitness and population dynamics of an ant-dispersed shrub (Leucospermum truncatulum). Experiments reveal that seeds buried by ants had a significantly higher chance of escaping rodent seed predators than unburied seeds, and predation rates decreased with soil depth. However, germination after fire decreased with increasing soil depth, suggesting a cost of deep seed burial by ants. Further, seedling emergence was delayed, and seedling size and survival decreased with emergence depth. Seed dormancy rates also increased significantly with soil depth. Simulations showed that seed survivorship and dormancy play a crucial role in reducing the risk of population extinction in the face of uncertainty due to fire. Reductions in seed survivorship and dormancy rates in the absence of key native seed dispersers may explain observed population declines of ant-dispersed plants in South African shrublands invaded by Argentine ants.

KEY WORDS: Seed dispersal , Invasions, Mutualism, Population dynamics