Exploring the interplay of biotic interactions and salinity stress in freshwater invertebrate assemblages: a response to Kefford

Abstract

Controlled mesocosm experiments can add substantially to our knowledge of the influence of environmental factors on freshwater assemblages by partitioning the possible effects of different drivers. Reporting results of such an experiment, Bray et al. (2019) concluded that effects of salinity on salt-sensitive stream invertebrates were substantially modified by interspecific biotic interactions with salt-tolerant invertebrates from a high-salinity stream. Chessman (2021) questioned this conclusion on three grounds: (1) confounding of the experimental design, (2) lack of evidence that purported diverse effects of biotic interactions were beyond mere stochastic variation, and (3) absence of mechanistic explanations for supposed effects grounded in organism biology and ecology. Chessman (2021) also conducted an independent statistical analysis of publicly available data from the experiment, which did not support the study’s conclusions. Kefford et al. (2022) dispute Chessman’s (2021) findings by analysing previously unpublished data from the experiment, which they claim demonstrates that the experimental design was not confounded, and criticise Chessman’s (2021) statistical analysis. Here, I respond to their new analysis and criticisms, explaining why they do not dispel any of the concerns expressed by Chessman (2021).