Diversity is an important characteristic of communities, with paramount influences on ecosystem properties . A wide range of measures have been applied for quantifying diversity, the simplest of which is species richness (SR): the number of species in a community . SR assumes species as comparable, distinct entities of similar ecological importance. However, differences between species regarding ecological traits may range from almost ecologically similar to very different. Therefore, in recent years the focus has turned from SR towards functional diversity, which considers components that influence ecosystem function rather than taxonomic units . The general concept of species function being more important than species richness has been shown in several studies, e.g. in predicting plant community resistance  and plant biomass accumulation .
A common approach in measuring functional diversity is classification of functional species groups [3, 6, 7]. This requires an a priori classification of species resulting in a discontinuous and, therefore, less accurate measure of functional diversity  than a continuous measure (FD) defined by Petchey & Gaston [9, 10]. Additionally, it can be difficult to fit species varying ecomorphologically in a complex multidimensional space into predefined groups defined by a limited number of characters [e.g. ]. Alternatively, FD compiles a variable range of ecological characteristics of species and is regarded as a very powerful measure of functional diversity .
Patterns between changes in functional diversity and SR provide information on the relative contribution of a species' ecological function to the sum of ecological functions of the community. Therefore, if functional diversity and SR show a one by one relationship, all species are different and contribute equally. Deviations from this pattern occur with differences in species contribution, e.g., if SR changes but functional diversity remains constant, the additional or diminished species do not exhibit unique ecological traits and can be considered as functional redundant . Patterns of functional redundancy were identified using FD in mammal, bird , and amphibian communities [15, 16]. However, these findings of functional redundancy are so far only related to anthropogenically disturbed landscapes. Comparing FD of observed and random assemblages can be used to test for non-randomness, which can highlight general processes of community assembly , such as competition or environmental filtering . Communities harbouring a large number of species are likely to contain species that are redundant in their ecological traits. The question of functional diversity and redundancy in species communities is therefore of particular interest when facing taxonomic groups that are rich in species. Tropical anuran communities represent an appropriate model as they are known to be remarkably rich but still taxonomically ascertainable. Studies on tropical frogs often focus on the ecology of adults [19–23], and have shown that SR can be predicted by environmental variables [21, 24], and that species specific habitat requirements may be overlaid by stochastic processes [23, 25]. Minor attention has focused on functional diversity and functional redundancy in tropical amphibians, although Ernst et al. [15, 16] showed that functional redundancy can be found in disturbed tropical frog communities and the classical measure of species richness fails to reflect the real dimensions of biodiversity.
Of the available amphibian community studies, only a few included larval stages [26–28]. Even less attention was given to diversity patterns of the tadpole communities themselves [29, 30], although in pioneering studies different habitat variables were found to be possibly related to SR of tadpole communities [22, 27]. There are no published data on functional diversity in tadpole communities and the validity of SR as an adequate measure of diversity remains to be verified.
This is especially true as there are several ways tadpole communities might influence ecosystem function. There is evidence that e.g. by moving sediment and feeding on primary algae producers, tadpoles can alter algae abundance, composition, and chlorophyll a level and therefore net primary production in stream ecosystems . Furthermore, due to their influence on basal resources e.g. removing sediments and exposing periphyton, they affect other primary consumers . Tadpoles can therefore affect stream ecosystem structure and function [31, 32] depending on where they live in the stream and how they forage. This might be especially true if some higher trophic levels are missing in the ecosystem.
The remarkable backlog of tadpole community studies may have been caused by identification difficulties, especially in species-rich tropical communities where the ecological importance of tadpole communities may be paramount [32–35]. Madagascar, regarded to be one of the most important hotspots for biodiversity conservation  harbours over 400 fully endemic frog species [37, 38]. Even if many of these species are yet undescribed scientifically, a near-complete database of genetic markers exists . This allows application of molecular identification methods to identify tadpoles to species , and allows community studies of tadpoles in an area known to harbour rich frog communities [37, 40].
Here, we report on the SR and functional diversity of stream communities of tadpoles in Ranomafana National Park (RNP) in eastern Madagascar as determined by DNA barcoding, and on the environmental variables that might influence these measures of diversity. We addressed three main questions: (1) are stream tadpole communities in Madagascar as rich as expected given the highly diverse amphibian communities, (2) is SR predictable by either adult or tadpole related environmental variables, and (3) does the functional diversity measure expose patterns of diversity that are not detectable by SR and point to general rules of species' trait assemblage?