Ecological distribution of four co-occurring Mediterranean heath species

Erica australis, E. scoparia, E. arborea and Calluna 6ulgaris are the most abundant heath species on acid, sandstone-derived soils of the Strait of Gibraltar region (southern Spain and northern Morocco). Despite their apparently similar ecological requirements, these four species are somewhat ecologically segregated. Erica australis is abundant only on poor, shallow soils, with a high content in soluble aluminium, generally on mountain ridges and summits. Erica scoparia becomes dominant on deeper sandstone soils with lower aluminium. Calluna 6ulgaris coexists with these Erica species in communities under low or no tree cover. In the Spanish side of the Strait (Algeciras), Erica arborea tends to be relegated to communities under moderate to dense tree cover, whereas this species is more abundant and widespread in the Moroccan side (Tangier). Tolerance to extreme physical conditions – high aluminium and dense tree cover – and interspecific competition seem to explain the ecological distribution of these four heath species in the Strait of Gibraltar region. The more fragmented pattern of sandstone patches and higher disturbance levels in Tangier might account for the differences in the patterns of ecological distribution of these four heath species between both sides of the Strait of Gibraltar.