Effects of area, environmental status and environmental variation on species richness per unit area in Mediterranean wetlands

We propose a mechanistic model to relate alpha- and gamma-diversity to area per se, moisture status and environmental variation (local and total), and explored the effects these abiotic variables have on species richness per unit area (alpha-diversity) for plant communities in a network of wetland h abitats located in a Mediterranean mountainous region of central Spain. In this study, environmental status is measured as actual evapotranspiration ( as an expression of energy), slope and soil wetness, and environmental vari ation refers to slope variation and soil wetness variation. Species richnes s per unit area was related to soil wetness, soil wetness variation, ground slope and ground slope variation. There were also positive correlations am ong moisture status and environmental variation variables. There is a joint effect of slope and soil wetness variation in explaining species richness per unit area of these wetland habitats, but area effects and energy are re latively unimportant. We conclude that species richness per unit area of we tland vegetation can be explained by moisture status and local environmenta l variation, and that habitat area may not have an important effect. Area c ould affect gamma-diversity directly through random sampling and/or indirec tly through increasing beta-diversity, and energy may be important in areas with larger energy ranges. Complete surveys of environmental status, local and total environmental variation, and their associated species assemblage s are needed to explain the processes that give rise to the rule that large r areas have larger species richness.