Predictability of species diversity by family diversity across global terrestrial animal taxa

Aim: Although biodiversity is in sharp decline around the globe, collectiing precise information on changes in overall species richness remains extremely challenging. Efficient and reliable proxy methods are therefore needed, with the diversity of higher taxa representing one such potential proxy for species-level diversity. Nonetheless, the stability of using this measure across different regions and animal taxa at the global scale has never been investigated thoroughly. Location: Global. Time period: Up to 2016. Major taxa studied: Animalia. Methods: We used a large global dataset containing published studies on diversity in the terrestrial Animalia to analyse the relationship between diversity at the family, genus and species level across different orders. Results: Family and species diversity were positively correlated, with the strongest correlations in Diptera, Hemiptera and Coleoptera. Correlations were slightly weaker in family–species than in genus–species relationships, whereas differences were stronger in observed richness than in diversity indices. Across all taxa, family–species correlations of Shannon diversity index values were independent of sample size, and they showed limited variation across biomes for the three orders containing sufficient case studies for this analysis. Based on the Shannon diversity index, the species diversity per site increased linearly with the increase in family diversity, with an average species : family diversity index ratio of 2.5, slightly lower than the ratio of 2.7 for observed species and family richness values. Main conclusions: Our study confirmed that recording family-level diversity can be a meaningful proxy for determining species-level diversity patterns in biodiversity studies, and trade-offs between identification costs and retained information content need to be considered when using higher taxon surrogacy.