Response to traits and stress keys to munity effects of low levels of toxicants in test systems by Liess and Beketov (2011).pdf

Response to ‘‘traits and stress: keys to munity effects of low levels of toxicants in test systems’’ by Liess and Beketov (2011) Paul J. Van den Brink ?Cajo J. F. Ter Braak Published online: 18 November 2011 ? The Author(s) 2011. This article is published with open access at To Editor, We read with great interest the paper entitled ‘‘Traits and stress: keys to munity effects of low levels of toxicants in test systems’’ by Matthias Liess and Mikhail Beketov (Liess and Beketov 2011 ). The paper presents a new way to analyse data from microcosms and mesocosms, which the authors claim that to be more sensitive than monly-used principal response curves (PRC) method (Van den Brink and Ter Braak 1999 ). Since PRC was developed more than a decade ago, new developments in the ?eld of munity responses to stress are very e. However, after reading the paper, we have concluded that their new method, . SPEAR mesocosm , may not offer the level of improvement suggested and in this letter to the editor we will brie?y explain why. Both PRC and SPEAR mesocosm display the time-depen- dent treatment effects of a toxicant. The fundamental dif- ference between the PRC and SPEAR mesocosm methods is that PRC is a multivariate statistical method and SPEAR mesocosm is a univariate method. For SPEAR mesocosm to work, a single index must be constructed on the basis of predictions derived from a priori