The world's richest tadpole communities show functional redundancy and low functional diversity: ecological data on Madagascar's stream-dwelling amphibian larvae

Abstract

BackgroundFunctional diversity illustrates the range of ecological functions in a community. It allows revealing the appearance of functional redundancy in communities and processes of community assembly. Functional redundancy illustrates the overlap in ecological functions of community members which may be an indicator of community resilience. We evaluated patterns of species richness, functional diversity and functional redundancy on tadpole communities in rainforest streams in Madagascar. This habitat harbours the world's most species-rich stream tadpole communities which are due to their occurrence in primary habitat of particular interest for functional diversity studies.ResultsSpecies richness of tadpole communities is largely determined by characteristics of the larval habitat (stream structure), not by adult habitat (forest structure). Species richness is positively correlated with a size-velocity gradient of the streams, i.e. communities follow a classical species-area relationship. While widely observed for other taxa, this is an unusual pattern for anuran larvae which usually is expected to be hump-shaped. Along the species richness gradient, we quantified functional diversity of all communities considering the similarity and dissimilarity of species in 18 traits related to habitat use and foraging. Especially species-rich communities were characterised by an overlap of species function, i.e. by functional redundancy. By comparing the functional diversity of the observed communities with functional diversity of random assemblages, we found no differences at low species richness level, whereas observed species-rich communities have lower functional diversity than respective random assemblages.ConclusionsWe found functional redundancy being a feature of communities also in primary habitat, what has not been shown before using such a continuous measure. The observed species richness dependent pattern of low functional diversity indicates that communities with low species richness accumulate functional traits randomly, whereas species in species-rich communities are more similar to each other than predicted by random assemblages and therefore exhibit an accumulation of stream-specific functional traits. Beyond a certain species richness level, therefore, stream-specific environmental filters exert influence whereas interspecific competition between species does not influence trait assemblage at any species richness level.

Species distribution and assembly patterns of frog larvae in rainforest streams of Madagascar

“…Within the assemblages and within a stream, tadpoles of some species show selectivity for particular microhabitats (Eterovick & Barata, 2006;Grosjean et al, 2011a, b;Randrianiaina et al, 2011a). However, these preferences are not necessarily rigid (Eterovick & Barros, 2003), and there can be a high level of spatial niche overlap among species (Eterovick & Barros, 2003) indicating that in natural stream assemblages competition might be of minor importance (Inger et al, 1986;Eterovick & Barata, 2006;Strauß et al, 2010).…”

“…ecomorphological groups; Altig & Johnston, 1989;Raharivololoniaina et al, 2006;Randrianiaina et al, 2009b). They were defined based on body and oral disc morphology (Altig & McDiarmid, 1999; for an overview of morphological characters and their potential ecological relevance, see Strauß et al, 2010).…”

“…So far the most species rich stream tadpole assemblages with up to 25 species per stream site are found on Madagascar (Vences et al, 2008;Strauß et al, 2010). Other assemblages in Brazil contain up to ten species per site (Hero et al, 2001;Eterovick & Barata, 2006;Afonso & Eterovick, 2007b).…”

“…Tadpoles of all morphological clusters can co-occur in the same streams, and it has been shown that the morphological parameters present in an assemblage contain information of functional relevance (Strauß et al, 2010). Whether these morphological clusters therefore also represent distinct functional groups has not been shown so far.…”

“…After some pioneering studies (Inger et al, 1986), more recent works have shown that, in tropical streams, the species richness of tadpoles is to a varying degree (but sometimes not) influenced by environmental factors, such as stream size, velocity, light regime, temperature, aquatic predator abundance or stream vegetation cover (Parris & McCarthy, 1999;Eterovick & Barata, 2006;Afonso & Eterovick, 2007b;Both et al, 2008;Strauß et al, 2010). Characteristics of adult habitat (e.g., forest structure) may play a minor role (Strauß et al, 2010). In general, the composition of a tadpole assemblage is primarily influenced by breeding site selection of adults and less by tadpole success (e.g., Inger et al, 1986;Alford, 1999).…”