Collapse of an Ant‐Plant Mutalism: The Argentine Ant (Iridomyrmex Humilis) and Myrmecochorous Proteaceae

Many plant species in Cape fynbos shrublands have seeds dispersed by ants. Iridomyrmex humilis, the Argentine ant, has recently invaded areas of fynbos and displaced the dominant native ants. The consequences of the invasion for seed dispersal were studied. Iridomyrmex differed from native ants in being slower to discover the seeds (morphologically achenes) of ant—dispersed Mimete cucullatus (Proteaceae), in moving them shorter distances, and in failing to store them in nests elow the soil. Seeds left on the soil surface were eaten by vertebrate and invertebrate predators. To study seedling emergence, seed depots were established and then the area was burned. Emergence was 35.3% in sites not infested by Iridomyrmex and 0.7% in infested sites. Seedlings in naturally occurring stands of M. cucullatus burnt in the same fire were widely distributed in noninfested sites, but were fewer and were confined within the canopy radius in infested sites. We suggest that continued invasion of fynbos by Iridomyrmex may eventually lead to extinction of many rare, endemic Cape Proteaceae by slow and subtle attrition of seed reserves.