Convergence versus Nonconvergence of Mediterranean Ecosystems

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Mediterranean-climate ecosystems have been often cited as classic examples of convergence in ecosystem structure and function owing to their similar environments. But comparative studies during the 1980s have demonstrated that this convergence pattern is too simplifying, and several divergences exist also.

Most discussions of the Mediterranean convergence hypothesis have focused on similarities among communities and on ecological similarity of distantly related taxa. Several cases of ecological convergence deserve attention. SW Australia and SW Cape are characterized by larger amounts of summer rain, lower soil fertility, and more frequent fires, compared to the three other Mediterranean regions. These two Southern Hemisphere Mediterranean ecosystems show remarkable convergence of plant traits and community structure on climatically and edaphically matched sites. Patterns of plant biodiversity are also similar on nutrient-poor soils in South African fynbos and Mediterranean heathlands of Spain. Striking convergences exist in the two Northern Hemisphere Mediterranean eco-regions, and several forest-types of the Mediterranean Basin are relatively similar from a biogeographic and ecological point of view, to those of the California Floristic Province. Ancient landmass connections which lasted until the Eocene (Madro-Tethysian flora) explain the existence of some common tree genera (Pinus, Quercus, Arbutus,

Cupressus, Juniperus, Platanus) between these regions. At low and medium altitudes, physiognomical similarities exist for sclerophyll oak forests. The existence of thermophilous coniferous forests at the thermo- and meso-Mediterranean (Pinus halepensis, P. brutia, P. pinaster) and the thermo- and meso-Californian (Pinus attenuata, P. sabiniana, Cupressus macrocarpa) levels, but also the presence of several meso-philous coniferous (Abies, Pinus, etc.) at higher altitude constitutes another major resemblance.

Life-history traits shared by many sclerophyllous woody plants of Mediterranean ecosystems represent an outstanding example of convergence, with evergreen, small, or even needle-like (ericoid) leaves, low specific leaf area (SLA: ratio of fresh leaf area to dry mass), strongly seasonal photosynthesis and growth, vigorous resprouting following fire or cutting. Sclerophylly (evergreen and thick leathery leaves) is the most common and widespread strategy of plants in these Mediterranean ecosystems.

Convergence can be explained by similar abiotic factors (climate and notably the intensity of summer drought, soil nutrient status fertility, fire regimes), and phylogenetic composition of lineages induced by common historical biogeography. Rainfall reliability was also recently suggested as another important factor; the two more ecologically similar regions (SW Australia and the SW Cape) have indeed significantly more reliable regimes than California and the Mediterranean Basin.

If we consider plant traits, and notably leaf evolution and resprouting capacity of evergreen woody plants, two hypotheses were formulated to explain the observed convergences: (1) evolutionary adaptation, that is, the production of new phenotypes by the action of natural selection; (2) niche conservatism, with a relative stasis in trait evolution, induced by the ecological match between organisms and their environment caused by the spatial and temporal sorting of existing lineages. Methods of phylogenetic comparative biology have recently demonstrated that similarity between traits of Mediterranean woody plants could be due more to phylogenetical inertia than to common adaptive strategies under Mediterranean climate. The absence of deep morphological changes in leaf size and SLA suggests that most of the ancestors of shrubland taxa had already acquired plant life-history traits that contributed to their success under Mediterranean climates. Thus, these subtropical 'phantoms' predate the onset of the mediterraneity during the mid-Pliocene.

More precisely, there exists a clear co-variation oflife-history traits with regards to the lineage age, and two groups with distinctive characters associations can be defined for Mediterranean angiosperms:

1. A pre-Pliocene group, consisting of mostly sclero-phyllous, vertebrate-dispersed, fleshy-fruited, and large-

seeded plants which resprout from stump after disturbance (fire, clearing) and are often late colonizers in successional stages of ecosystem dynamics (e.g., Arbutus unedo, Olea spp., Quercus spp.). This resprouting 'strategy' often considered as typically 'Mediterranean' represents in fact an ancient trait that emerged under a subtropical climatic regime, well before the advent of the Mediterranean climates.

2. A post-Pliocene group, including nonsclerophyllous plants which are obligate seeders after disturbance (e.g., Cistus spp., Lavandula spp.). These taxa successfully diversified and competed with taxa of the pre-Pliocene group due to their short life cycle, high seed production with small and dry-fruited anemochorous seeds. Seeders grow significantly faster and allocate more leaf biomass than resprouters. These set traits account for the important ecological plasticity of these plants which are associated with earlier successional stages.

Therefore, historical biogeography could largely explain the physiognomic similarities of some ecosystems or species found also in non-Mediterranean-type climate regions. This is the case of several modern matorral-type communities present in the American Southwest, Mexico, or eastern Australia, whose sclerophyllous vegetation is roughly similar from a structural and functional point of view to the true Mediterranean shrublands. Even in southwestern China, some sclerophyllous species possess striking morphological convergence with Mediterranean trees; for example, Quercus phillyroides in the Danxiashan escarpments (Guangdong Province) is astonishingly similar to the evergreen Mediterranean oaks (Q. ilex and Q. coccifera), whereas the matorrals with Olea and Pistacia recently discovered in some gorges of the Yunnan Province are globally alike to the Mediterranean ones. These similarities support the view that sclerophylly represents more a general response of semiarid conditions with a severe dry season, few intense frost episodes, and low-nutrient soils, rather than a specific adaptation to the summer-drought pattern per se.

Evolutionary convergence among bird communities was also tested between California, Chile, and the Mediterranean region, along matched habitat gradients ofincreasingly complex vegetation structure (from shrub-lands to forests), and compared with non-Mediterranean communities. Results of these ecomorphological comparisons indicate that the Mediterranean bird communities do not resemble each other any more than the non-Mediterranean ones. To summarize, convergence depends on the ecological compartments considered; Mediterranean convergence may exist for some community attributes (e.g., species-richness, community structure), and is more likely to occur among organisms such as plants, invertebrates, or reptiles that depend on seasonal patterns of climate and nutrient cycling, whereas homeotherm vertebrates are more deeply influenced by the structure of ecosystems.

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