Drivers of Biodiversity

Old soils and a frequent fire cycle encourage plant biodiversity in Western Australia. Image credit: Peggy L. Fiedler

Mediterranean-type climate areas are renowned for their tremendous vascular plant diversity. Several factors promoting the coexistence of seemingly ecologically equivalent plant species have been identified, and appear to be associated with fire and geologic stability. Strangely, the factors most commonly thought to correlate with biodiversity among the five MTEs, regional topographic and climatic heterogeneity, are poor predictors of diversity.

Central Chile, California, and the Mediterranean Basin experience relatively long intervals between fires on the order of decades to over a century. Their matorral, chaparral, or maquis communities grow on soils that have been renewed by seismic or glacial activity. In these areas, short-lived species are excluded by rapidly growing shrubs. Open woodlands in the Mediterranean Basin and California, however, may have very high local diversity among annuals and short-lived perennials. This is especially true where grazing maintains open habitat for species establishment. Yet Chile, California, and the Mediterranean Basin also have lower biodiversity than their Southern Hemisphere counterparts.

Natural selection apparently permits fine-scale discrimination of habitats and niches under the selective pressures of stable climates and predictably frequent fires. Neither South Africa’s Cape Region nor Southwestern Australia has experienced volcanism, seismic activity, and glaciers. In the absence of these geologic upheavals, their soils have become leached of nutrients. Geologic stability has also kept extinction rates low, encouraging greater biological diversity over time. Rainfall levels are highly predictable from year to year, while relatively limited summer drought periods may contribute further to the low level of extinctions.

South Africa’s fynbos shrublands and Australia’s kwongan shrublands also burn frequently. Their vegetation grows slowly, has diverse postfire regeneration strategies, and exhibits a wide variety of postfire reestablishment strategies. Plants that dominate these areas, which include shrubs, geophytes (plants with bulbs, corms, and similar underground storage structures), and the Restionaceae grasses, all exhibit these characteristics.

South Africa’s Cape Region and South and Southwestern Australia also experience periodic drought. Drought promotes community turnover and diversification by killing existing vegetation. For all of these reasons, fynbos and kwongan communities have evolved species-rich landscapes in topographically homogeneous areas.