Using the guild ratio to characterize pelagic rotifer communities

Ecological research is moving from a species-based to a functional-based approach to better understand the underlying principles that govern community dynamics. Studies of functional-based ecology, however, have been limited for zooplankton and particularly for rotifers. While rotifers show a variety of trophi types and coronal shapes, suggesting the importance of niche differentiation in their feeding strategy, relatively little is known of how this relates to rotifer dynamics. We used the guild ratio (GR0, a ratio of raptorial to microphagous species), an index based on a functional trait (i.e. feeding strategy), as a novel approach to rotifer dynamics. We extracted the seasonal GR0 by using seasonal trend decomposition and investigated similarities between study sites (Lake Washington, USA and Lake Caldonazzo, Italy) and its relation to cladocerans by cross-correlation analysis. Our study indicated that (i) raptorial and microphagous rotifers showed alternating dominance, and that raptorial rotifers and cladocerans had a synchronous pattern, (ii) the seasonal pattern of the GR0 was consistent across different sampling frequencies, and (iii) the GR0 was similar in both lakes. We interpreted these patterns as the general strength of the GR0: discernment of species–environment relationships and robustness across sampling regimes. The limitations of the GR0 (i.e. species identity is neglected, simplification of food preferences) can also be seen as its strong point: synthesis of multi-species patterns. In addition, the independence of GR0 from species-level identification and its potential to make use of datasets with infrequent sampling intervals and low taxon resolution could further support its innovative aspect.