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

. We propose a mechanistic model to relate α‐ and γ‐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 (α‐diversity) for plant communities in a network of wetland habitats 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 variation refers to slope variation and soil wetness variation. Species richness per unit area was related to soil wetness, soil wetness variation, ground slope and ground slope variation. There were also positive correlations among 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 relatively unimportant. We conclude that species richness per unit area of wetland vegetation can be explained by moisture status and local environmental variation, and that habitat area may not have an important effect. Area could affect γ‐diversity directly through random sampling and/or indirectly through increasing β‐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 assemblages are needed to explain the processes that give rise to the rule that larger areas have larger species richness.