Promising yet variable performance of cross-taxon biodiversity surrogates: a test in two marine habitats at multiple times

Surrogates are a potential solution to the often-cited problem of there being insufficient information for biodiversity assessments or conservation planning. Cross-taxon surrogacy is the ability of a group of well-known taxa to represent variation in other poorly known taxa. To date, tests of the effectiveness of cross-taxon surrogacy in marine environments have yielded variable results and a significant qualification to the outcomes of tests that have demonstrated surrogacy is the near absence of tests for its persistence through time. This study tested for cross-taxon surrogacy and its persistence through time for three surrogates (crustaceans, molluscs, polychaetes). We used data on biodiversity variables and species assemblages of the surrogates and their targets that had been collected during multiple sampling events over 2.5 years in two habitats (seagrass, unvegetated sediment) in a large bay in south-eastern Australia. We tested surrogacy by fitting a series of linear models using generalized least squares for biodiversity variables and by Mantel tests of dissimilarity matrices of species assemblages. We also tested whether the type of data transformation affected Mantel tests. We found that each of the groups were effective surrogates for some but not all biodiversity variables (with molluscs or polychaetes being effective surrogates for species richness in both habitats), that none of the groups were effective surrogates for species assemblages, and that the outcomes of Mantel tests of dissimilarity matrices of surrogates and their targets were unaffected by the data transformation used. We conclude that while our results for surrogacy for biodiversity variables are promising the inconsistent results from other studies argues for caution about their application beyond the area and context in which they were assessed. The lack of evidence that we found for surrogates of species assemblages, and similar lack of evidence from other studies, suggests that assemblage-level surrogates are unlikely to be useful in biodiversity surveys.