Exploring the interplay of biotic interactions and salinity stress in freshwater invertebrate assemblages: a response to Kefford et al. (2022)

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 (2022) dispute Chessman's (2021) findings by analysing previously unpublished data from the experiment, which they claim demonstrates that the experimental design was not confounded, and criticise Chessman's (2021) statistical analysis.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).

Automated summary

Controlled mesocosm experiments can partition the effects of different drivers and contribute to our understanding of the influence of environmental factors on freshwater assemblages. This article responds to a dispute over the results of such an experiment, in which one study concluded that the effects of salinity on stream invertebrates were modified by biotic interactions, while another study questioned this conclusion and conducted a statistical analysis.