Patterns and Determinants of Mediterranean Biodiversity

To compare the biodiversity of Mediterranean-type ecosystems, it is useful to partition species-richness and diversity into three main types of spatial scales, and to consider successively regional diversity, differentiation diversity, and local diversity (Table 4). Regional diversity is the product of local richness and turnover along habitat and geographic gradients.

The five Mediterranean-climate regions harbor a remarkable and huge regional biodiversity, among the highest in the world. With only 2% of the world's terrestrial surface, the Mediterranean biome contains nearly 20% of the Earth's total plant diversity, making very significant biodiversity hot spots, second only after tropical ones. The Mediterranean Basin exhibits the greatest diversity of plant species, both general and endemic, but with the much greater surface area (84% of the total of Mediterranean ecoregions). This ecoregion possess a higher tree richness (290 indigenous trees with 201 endemics) than the California Floristic Province (173 trees with 77 endemics), although its surface is seven times larger. In fact, the latter ecoregion has more or less the same surface area and plant biodiversity as Morocco, but California is four times richer in strictly local endemics. The case of the Cape Floristic Region is even more remarkable, since the endemic plant richness reaches close to 70% and the total plant species reaches 9090 taxa, which makes it one of the world's richest areas. The interplay between diverse processes of historical biogeography and heterogeneous environmental conditions has promoted these considerable species-richness and ende-mism levels in the different Mediterranean ecoregions. Regional diversity peaks generally in areas with high topographical and climatic heterogeneity. However, the two highest plant species-rich regions of the SW Cape and SW Australia are characterized by global topographically and climatically uniform lowlands. Edaphic complexity, and more recently rainfall reliability (measured as interannual variation in seasonal and monthly rainfall and as the

Table 4 Main biodiversity components of the five Mediterranean ecoregions

North Hemisphere

South Hemisphere

Mediterranean

Biodiversity components

Basin

California

Central Chile

SW Australia

Cape Region

Local diversity

Low-very high

Low-moderate

Low-?high

Low-high

Moderate-high

Differentiation diversity

Moderate

Moderate

Low-moderate?

High

High

Regional diversity

Moderate

Moderate

Low

High

High

Plant richness/endemism

c. 25 000/12 500

3488/2128

3539/1769

5710/3000 (52.5%)

9086/6226 (68.5%)

(50%)

(61%)

(50%)

Mammal richness/

224/25 (11%)

151/18(12%)

65/14 (22%)

57/12 (21%)

90/4 (4%)

endemism

Bird richness/endemism

497/32 (6%)

341/8 (2%)

226/12 (5%)

285/10 (4%)

324/6 (2%)

Reptile richness/endemism

228/77 (34%)

69/4 (6%)

41/27 (66%)

177/27 (15%)

100/22 (22%)

Amphibian richness/

86/27 (31%)

54/25 (46%)

43/29 (67%)

33/19 (58%)

51/16(31%)

endemism

Freshwater fish richness/

216/63 (29%)

73/15(21%)

43/24 (56%)

20/10 (50%)

Data from Cowling RM, Rundel PW, Lamont BB, Arroyo MK, and Arianoutsou M (1996) Plant diversity in Mediterranean-climate region. Trends in Ecology and Evolution 11: 362-366; and Mittermeier RA, Robles Gil, Hoffmann M, et al. (2004) Hotspots Revisited: Earth's Biologically Richest and Most Endangered Terrestrial Ecoregions. Monterrey: CEMEX, Washington: Conservation International, Mexico: Agrupacion Sierra Madre.

endemism

Data from Cowling RM, Rundel PW, Lamont BB, Arroyo MK, and Arianoutsou M (1996) Plant diversity in Mediterranean-climate region. Trends in Ecology and Evolution 11: 362-366; and Mittermeier RA, Robles Gil, Hoffmann M, et al. (2004) Hotspots Revisited: Earth's Biologically Richest and Most Endangered Terrestrial Ecoregions. Monterrey: CEMEX, Washington: Conservation International, Mexico: Agrupacion Sierra Madre.

frequency of different-sized rainfall events), have been invoked as main determinants of this exceptionally high biodiversity. The regional-scale plant richness is indeed twofold higher in the western Cape Region, which receives reliable winter rainfall than the less reliable and nonseasonal zone in the eastern Cape. Reliable rainfall regimes are argued to promote higher and rapid speciation and lower extinction rates, and this pattern could partly explain the overall highest plant diversity of the SW Cape and SW Australia which have significantly more reliable regime than the other three Mediterranean ecoregions. If we consider diverse groups of vertebrates (Table 4), biodiversity patterns are more contrasted, and species-richness and endemism are often attenuated compared to plants, notably for birds. Nevertheless, for reptiles, amphibians, and freshwater fishes, the uniqueness of Mediterranean biotas is again noteworthy since the endemism rate is generally comprised between 30% and 60% (Table 4).

The differentiation diversity refers to changes of species composition along habitat gradients (beta diversity) or geographical gradients (gamma diversity). Highest levels of differentiation diversity are recorded for plant species in the winter rainfall zones of SW Cape and SW Australia, with a high turnover for fire-killed shrub lineages. Disproportionate radiation of several shrub genera (e.g., 667 species of Erica in the Cape Floristic Region with 96.5% of endemic heaths) explains the strong dissimilarities in species composition between morphologically close communities. A similar, but attenuated, pattern is found in California and in the Mediterranean Basin for relatively recent shrub lineages and annual herbs, which are respectively the keystone species of matorrals and grasslands.

At the local scale, that is, less than 0.1 ha (alpha diversity), Mediterranean biodiversity is two times lower than that of tropical regions. However, a great variation exists within each ecoregion and between different habitats. Open and frequently burned shrublands and heaths on nutrient-poor soils, in particular, fynbos in SW Cape and kwongan in SW Australia, xerophytic rocky grasslands, and temporary pools encompass the highest plant diversity. Postfires communities in dense shrublands, notably chaparral in California (see Chaparral), and maquis in the Mediterranean Basin, are also characterized by a rich fire-ephemeral flora with numerous annual plants. Mean local plant richness of Mediterranean forests is comprised between 10 species perm2 and 25-110 species per 1000m2. At this spatial scale, woody plant communities of the Mediterranean Basin are both very heterogeneous, but also among the richest types, ahead of the alpha diversity found in the SW Cape. Several nonexclusive determinants have been invoked to explain this high local diversity and species coexistence: an important regional species pool linked to complex historical biogeography, differentiation, and character displacement along structural niche axes, spatiotemporal variations in resource availability, recurrent disturbances (fire, grazing), neighborhood effects, and lottery processes. Finally, few generalizations are available from the numerous studies on local plant diversity in the Mediterranean vegetation, and the strongest evidence is that diversity represents an unimodal function of productivity or nutrient supply of soils. Species-area relationships fit a power function model for the majority of Mediterranean plant communities, but communities with a preponderance of perennials and paucity of annuals (e.g., Australian heathlands, mature Californian matorrals) are fitted by the exponential species-area model.

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