Designing nature reserves: traditional criteria may act as misleading indicators of quality

Traditionally, numerous criteria have been used for selecting valuable areas for the establishment of nature reserves. In most cases these criteria are applied indiscriminately with the assumption that their generalized use gives them universal validity. In this study we test the value of different variables (area and shape of patches of natural vegetation, degree of internal fragmentation, and diversity of habitats at the periphery of patches) at the landscape level as indicators of plant richness and diversity, and relative abundance of native plants, in a group of natural vegetation relicts ranging from 0.004 km² to 1.027 km² and surrounded by sub-urban and industrial settings. Species richness, diversity, and number of native and exotic species increased with the area of the patches. The shape of the patch was the second most important variable to influence species richness and diversity. The number of exotic species increased with increasing numbers of native plants. Thus, patch size and plant richness should be carefully used for selecting conservation areas because it could result in choosing places threatened by the presence of exotics. Many of the other variables analysed showed no effects on biodiversity at the temporal and geographic scales considered. Ignoring these outcomes could result in choosing sub-optimal areas. We recommend the critical use of general criteria considering the selection process as an opportunity to evaluate the relevance of each criterion at the local level.