Competition between Two Shrub Species: Dispersal Differences and Fire Promote Coexistence

Abstract

Coexistence mechanisms of two co-occurring species of Proteaceae, Protea lepidocarpodendron and Leucospermum conocarpodendron, were studied. Both are tall, arborescent shrubs forming the overstory of the fynbos communities in which they occur. Seeds of L. conocarpodendron are dispersed by ants while those of P. lepidocarpodendron are dispersed by wind The seedlings compete for space made available after periodic fires. Seedlings of the ant-dispersed species are scarce under the skeletons of the wind-dispersed species after a fire but relatively common in open sites. In contrast, seedlings of the wind-dispersed species are densest under their own skeletons and rare in open sites. Seedlings of the ant-dispersed species are outgrown by their wind-dispersed competitors and suffer reduced fecundity and increased mortality when they co-occur. The adult survival of the ant-dispersed species in light fires is also reduced when it establishes under or next to the wind-dispersed species. A Markovian model of the system suggests that ant dispersal of Leucosperrnum seed reduces the rate of competitive exclusion by Protea but is not sufficient to explain the persistence of the former in this system. To persist, Leucospermum requires open space, which is made available either by adult survival in light fires which set back its Protea competitor or by survival as seed banks through burns that destroy the seed banks of Protea.