Rebuttal related to “Traits and Stress: Keys to identify community effects of low levels of toxicants in test systems” by Liess and Beketov (2011)

Identifying long-term community effects of low toxicant concentrations is one of the major challenges in ecotoxicology at present. Mesocosm investigations, which are one valuable tool to identify such long-term community effects, face the problem that results are obscured by confounding factors and high variability between replicates (Sanderson et al. 2009). This problem is even greater when assessing long-term effects of toxicants because inter-replicate variation increases with time. Under such conditions, multivariate analyses on species data such as usually conducted with PRCs dramatically lose power to identify statistical links. With the SPEARmesocosm approach based on the investigations of Liess and von der Ohe (2005), we are aiming at resolving these shortcomings. We suggested a trait based aggregation of species that reduces the variability of community measures to reveal causality between exposure and effect for mesocosm experiments. We also show that this approach offers advantages in detecting long-term community effects compared to the traditional species-based PRC approach of (Van den Brink and Ter Braak 1999). Van den Brink and Ter Braak stated that the SPEARmesocosm approach (Liess and Beketov 2011) ‘‘may not offer the level of improvement suggested’’, is not ‘‘an entirely appropriate approach for the evaluation mesocosm studies’’, and ‘‘the generality of the proposed method is at best rather questionable’’. We had to come to the conclusion that most of their critics are based on a lack of understanding of the SPEARmesocosm approach. We are happy to assist in gaining further insights to our approach with comments targeted directly to the critics of Van den Brink and Ter Braak.