Résumé
Biodiversity is not completely known anywhere, so conservation planning is always based on surrogates for which data are available and, hence, assumed effective for the conservation of unknown biodiversity. We review the literature on the effectiveness of surrogates for conservation planning based on complementary representation. We apply a standardized approach based on a Species Accumulation Index of surrogate effectiveness to compare results from 575 tests in 27 studies. Overall, we find positive, but relatively weak, surrogacy power. Cross-taxon surrogates are substantially more effective than surrogates based on environmental data. Within cross-taxon tests, surrogacy was higher for tests within the same realm (terrestrial, marine, freshwater). Surrogacy was higher when extrapolated (rather than field) data were used. Our results suggest that practical conservation planning based on data for well-known taxonomic groups can cautiously proceed under the assumption that it captures species in less well-known taxa, at least within the same realm.