1. Biodiversity data are needed for conservation and management of tropical habitats, but the high diversity of these ecosystems makes comprehensive surveys prohibitively expensive and indicator taxa reflecting the biodiversity patterns of other taxa are frequently used. Few studies have produced the necessary comprehensive data sets to assess the quality of the indicator groups, however, and only one previous study has considered the monetary costs involved in sampling them.
2. We surveyed four plant groups (herbs, liverworts, trees, lianas) and eight animal groups (ants, canopy and dung beetles, birds, butterflies, bees, wasps and the parasitoids of the latter two) in 15 plots of 50 × 50 m2 each, representing undisturbed rainforest and two types of cacao agroforest in Sulawesi, Indonesia. We calculated three biodiversity measures (α and β diversity; percentage of species indicative of habitat conditions), built simple and multiple regression models among species groups (single groups, combinations of 2–11 groups, averaged relative diversity of all 12 groups), and related these to three measures of survey cost (absolute costs and two approaches correcting for different sampling intensities).
3. Determination coefficients (R2 values) of diversity patterns between single study groups were generally low (<0·25), while the consideration of several study groups increased R2 values to up to 0·8 for combinations of four groups, and to almost 1·0 for combinations of 11 groups. Survey costs varied 10-fold between study groups, but their cost-effectiveness (indicator potential versus monetary cost) varied strongly depending on the biodiversity aspect taken into account (α or β diversity, single or multiple groups, etc.).
4. Synthesis and applications. We found that increasing the number of taxa resulted in best overall biodiversity indication. We thus propose that the most cost-efficient approach to general tropical biodiversity inventories is to increase taxonomic coverage by selecting taxa with the lowest survey costs.