Predicting effects of ecosystem engineering on species richness along primary productivity gradients

Physical ecosystem engineering is the process by which some species change the distribution of materials and energy in ecosystems. Although several Studies have shown that this process is a driver of local species diversity, the current challenge is predicting when and where ecosystem engineering will have large or small impacts on communities, while also explaining why impacts vary in magnitude across engineer species and environments. This study addresses this issue and proposes a series of predictions for these effects at the three spatial scales (the patch, the habitat and the landscape) along environmental gradients of physical stress. The integrative prediction of this study was that the difference in species diversity between engineered and unmodified Situations (patches, habitats or landscapes) will increase as the difference in physical stress between engineered and unmodified patches becomes larger. To test the prediction, the effects of two well known high-Andean ecosystem engineers, the cushion plants Azorella madreporica and Laretia acaulis, were assessed on plant species richness in central Chile. The results support the main prediction, showing that ecosystem engineers have negative effects on species diversity at sites when the environmental modifications they perform increase physical stress for other species, while they have positive effects at sites where these habitat changes mitigate physical stress. Then, the effects of the ecosystem engineers on species diversity seem to depend on the environmental context, where larger environmental modifications are reflected in greater impacts, either positive or negative, on species diversity. (C) 2009 Elsevier Masson SAS. All rights reserved.