Influence of Herbivory and Grazing

Herbivore pressures also constitute a crucial feature in some Mediterranean ecosystems, and grazing affects often seriously the structure and diversity of Mediterranean communities, notably grasslands. In the Mediterranean Basin, grasslands and shrublands have experienced frequent man-induced grazing for c. 9000 years and ungulate browsing. These ancient selective pressures explain the widespread existence of distinct plant species life attributes, which can contribute efficiently to reduce the cumulative damage induced by herbivores and linked to the long leaf life span of evergreen plants. Morphological changes following herbivory include the reduction of leaf size, low SLA, physical toughness, modification of branch density, developments of thorns and hairs, and increase of leaf chemical defenses by phenolic or tannin compounds. Variations of phenolic levels in relation to leaf age and season suggest an evolutionary adaptation of Mediterranean evergreen plants to high densities of large herbivores.

Without any disturbance for a few years, grasslands become dominated by a few species of perennial grasses, forbs, or tall large-seeded annuals, which form closed swards with high biomass, cover, and height. Thus, species regeneration will be only successful in gaps created by light grazing, and the observed peaks of species-richness with moderated grazing are consistent with the classic intermediate disturbance hypothesis. The barren areas between mature shrublands or grasslands are also possibly driven by rodent and rabbit activities, which control not only plant community boundaries and structure but also dynamics by influencing nutrient linkages between communities. The complex mosaic of microsites and the constant grazing pressure shift advantage to smaller plants that can occupy the microheterogeneity finely, and the diversities observed can be considerable with more than 50 plant species on 1 m plots in moderately grazed grasslands or steppes of the Mediterranean Basin.

Fire and grazing must be regarded as two disturbances with not always additive consequences, but often with distinct and interactive effects on structure and dynamics of communities. Moreover, the ability of Mediterranean woody species to resprout after fire seems not to originate from an adaptation to recurrent fires, but rather from an older adaptation to losses of aboveground biomass mainly induced by herbivory.

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