The relationship between diversity and productivity of ecosystems remains a controversial issue, and there are still few experimental evidences for Mediterranean ecosystems. Nevertheless, some results related to grasslands and shrublands of the Mediterranean Basin suggest that plant diversity is positively correlated with ecosystem function, notably with primary production. However, in mixed grassland communities, where several growth forms coexist, one of a few dominant species (keystone species) may hide or reverse the observed diversity-biomass production relationship: the inclusion of a dominant grass in experimental low-diversity plots can produce a constant level of productivity along the gradient of species diversity. Thus, the species composition appears to be the main determinant of the productivity performance at each diversity level. Shrubs, grasses, and geophytes have different attributes in these communities and they can significantly affect the value of the ecosystem functioning. Each growth form or functional type contributes significantly to the overall productivity performance of the studied Mediterranean ecosystems.
Several mechanisms were proposed to explain these patterns: (1) the 'sampling effect', related to the higher probability to include highly productive species in species-rich communities; (2) niche complementarity between species, leading to more complete utilization of resource in intact ecosystems relative to depauperate ones: space filling, for example, by herbaceous plants occupying the gaps in shrub canopies, favors the exploitation of different soil layers and resource supply by different root systems and this phenomenon increases productivity; (3) the increase of positive interactions between species in complex assemblages.
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