Plant Diversity in Mediterranean-Climate Regions

REVIEWS Plant diversity in mediterranean-climateregions Richard M. Cowling, Philip W. Rundel, Byron B. Lamont, Mary Kalin Arroyo and Margarita Arianoutsou he five mediterranean- The high plant diversity of mediterranean- regions except those in Australia climate regions of the world climate regions has attracted much and South Africa, where only a few (Fig. 1, Box 1) occupy less attention over the past few years. such species appear in the im- Tthan 5% of the Earth’s This review discusses patterns and mediate post-fire periodislr. The surface yet harbour about 48250 determinants of local, differential and average diversity of fynbos, kwon- known vascular plant species regional plant diversity in ail five regions. gan and mallee communities in the (Table l), almost 20% of the world Local diversity shows great variation within western regions, which are more total. These regions also have and between regions and explanations for nutrient-impoverished and have exceptionally high numbers of rare these patterns invoke a wide range of high winter rainfall, is similar to that and locally endemic plant+5 and hypotheses. Patterns of regional diversity in the less-seasonal and more-fertile include two recognized species are the result of differential speciation eastern regions of mediterranean flocks, one in southwestern Aus- and extinction rates during the South Africa and AustraliasJ8. tralia and the other in the south- Quaternary. These rates have been western Cape, South Africas. These influenced more by the incidence of Determinants patterns are of interest because fire and the severity of climate change A wide range of equilibrium they refute predictions on the rela- than by environmental heterogeneity. and non-equilibrium theories have tionships between diversity and Ail regions have a high number of rare been invoked to explain local di- area, productivity and latitude3a416. and locally endemic taxa that survlve as versity and species coexistence in There has been much interest small populations, many of which are mediterraneanclimate shrublands. recently in the patterns, determi- threatened by habltat transformation. These include differentiation along nants and function of biodiversity structural or growth-form niche in mediterranean-climate ecosys- axes3Jg, spatial variation in re- tems (e.g. Refs 6-8). Here, we re- Richard Cowling is at the Institute for Plant source availabilii$20, disturbances view patterns and determinants of Conservation, University of Cape Town, Private Bag, such as firen and grazing22, Rondebosch 7700, South Africa; Phil Rundel is at plant species diversity and rarity neighbourhood effectsigZ23,lottery the Laboratory of Biomedical and Environmental at various spatial scales. We also Sciences, University of California, Los Angeles, processes iiJ4, and the influence of highlight aspects from research in CA 90024-1786, USA; Byron Lamont is at the School regional processe@. mediterraneanclimate regions that of Environmental Biology, Cur-tin University of Hustonr interprets the relative are salient to the evolution and con- Technology, PO Box U1987, Perth, WA 6001, diversity of mediterranean plant servation of plant biodiversity in Australia; Mary Arroyo is at the Dept of Biology, communities in terms of a general general. Faculty of Science, University of Chile, Santiago, model where low rates of competi- Chile; Margarita Arianoutsou is at the Dept of Ecology tive displacement result from low Local diversity and Systematics, Faculty of Biology, University of soil nutrients and summer drought, Patterns Athens, 15784 Athens, Greece. coupled with a moderately high At the local scale (0.1 ha or less), frequency of disturbance in the the vegetation of mediterranean- form of fire and grazing. There is climate regions is moderately species-rich by global stand- good evidence from several mediterranean-climate regions ards, on average less than half that recorded for tropical that plant diversity is a unimodai function of productivity or rainforests, but much richer than most other temperate other measures of nutrient supply9JiJ6. Most studies on communitieG. Within each region, there is great variation within-guild coexistence are for communities of proteoid in local diversity. Highest diversity occurs in frequently (Proteaceae) shrubs in fynbos and kwongan. Cody19 found burnt open heath and scrub on nutrient-poor soils clear patterns of segregation into leaf-dimension niches, (Australian kwongan and South African fynbos), and the and evidence for character displacement, for proteoid heavily grazed shrublands and woodlands of the eastern shrubs in fynbos (see also Ref. 11). Using a null model to Mediterranean Basin (Table 2, see Box 1 for description of generate a relationship between local and regional diversity vegetation types). Denser shrublands (e.g. chaparral) and among southwestern Australian Banksiu (Proteaceae) woodlands (e.g. eucalyptus forest) have lower diversity. species, Richardson et al.25 recently showed that local di- However, diversity in the immediate post-fire year in chap- versity of real assemblages was rapidly saturated, suggest- arral, when a rich fire-ephemeral flora coexists with the re- ing that local-scale processes limit diversity. However, they sprouts and seedlings of long-lived shrubs, approximates could find no conclusive evidence that local assemblages that of mature fynbos and kwonganio. are consistently structured by growth form or regeneration Growth form diversity is low in fynbos and kwongan, niche differentiation. instead, they argued that habitat spe- which include many structurally and functionally similar spe cialization of bank&s23 causes lower diversity than would ties, often belonging to the same genusii-13, but it is high in be predicted from niche differentiation and from the size of vegetation that seldom burns, for example, South African the regional species pool. scrub-forest13 and Chilean matorrali4. Annuals comprise 15% Few generalizations emerge from the many studies or more of the local floras in all the mediterranean-climate on local diversity in mediterranean vegetation. It appears, 362 0 1996, Elsevkr Science Ltd PII: SOlSS-5347(96)10044-6 TREE uof. I I, no. 9 September 1996 REVIEWS however, that in frequently burnt shrublands on nutrient- poor soils (fynbos and kwongan), relatively low growth rates and the reshuffling of competitive hierarchies owing to differential post-fire regeneration, allows the coexistence of many seemingly equivalent shrubs and long-lived graminoidsl3Js. On more fertile soils, where fire-free inter- vals are longer (chaparral, garrigue, phrygana), shorter- lived species are rapidly excluded by the developing shrub canopyiOJ7. In Chilean matorral and grassy shrublands and woodlands in the eastern Mediterranean Basin, both of which seldom burn, grazing by livestock permits the coexistence of both short-lived and long-lived speciesi4J7J2. Differentiation diversity Differentiation diversity refers to compositional change along habitat gradients (beta diversity) and along geo- Fig. 1.The five mediterranean-climate regions of the world. graphical gradients (gamma diversity)z. High differentiation diversity is largely the product of the evolution of habitat specialists and geographical vicariants2a3. Comparable and exceptionally high levels of differen- western Australiad. A similar explanation has been invoked tiation diversity, amounting to almost complete turnover of for the species-rich but topographically and climatically fire-killed shrub species along edaphic and geographical uniform lowlands of the southwestern Capes. gradients, have been recorded in the strongly winter rainfall Cowling et al.3 have suggested that regional diversity in zones of southwestern Australia and the southwestern mediterranean-climate regions is the product of local diver- Cape394Js.In the southeastern zones of these regions, which sity and differentiation diversity in relation to environmen- experience more summer rain, differentiation diversity is tal heterogeneity. Thus, regions with high local diversity much lower3Js. In California, high turnover is associated and high turnover associated with long habitat and geo- mainly with predominantly fire-killed shrub lineages (Arcto- graphical gradients will support species-rich landscapes and and short-lived herbs or annu- sfaphylos Ceanothus) (Table 3). The driving force is rapid speciation and/or low al&21.29.A similar pattern occurs in the Mediterranean Basin extinction rates of habitat specialists and geographical where the shrub lineages include and Astragalus, Genista vicariant@. This would explain why regional diversity and the herbs are mainly annual members of the Cistus, converges in small areas of the southwestern Cape and Leguminosae, Compositae and Gramineae’7JO.In Chile, high southwestern Australia, which have similar heterogeneity and turnover is concentrated in suffrutescent shrubs and per- ennial herbs belonging to many genera including Senecio, Adesmia, Oxafis and Cafceolarial4J5. The tall shrub and tree floras that have persisted since the Tertiary in all regions ex- Box 1. Plant life in mediterranean-climate regions cept Australia, and that regenerate mainly from vertebrate- dispersed propagules in the absence of fire, have wide gee- Five geographically remote regions (Pig. 1, Table 1) have mediterranean-type cli- mates with warm, dry summers and cool, wet winters. By agricultural standards, all graphical distributions and contribute little to turnover3J5Jg. these regions have infertile

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