Understanding salt-tolerance and biota–stressor interactions in freshwater invertebrate communities

Abstract

Salinity is increasing in many naturally fresh waters because of human activities, and there are concerns about the ecological effects of these increases. Salinity, as with any stressor, can affect organisms both directly and indirectly. In a previous study (Bray et al. 2019), we evaluated the relative importance of direct and indirect effects of increased salinity on stream invertebrates. Chessman (2021) criticised that study, claiming that the biotic treatments were confounded and did not directly test the hypotheses. Chessman (2021) also conducted a reanalysis of the data. We show through the analysis of new data that our biotic treatments were not confounded and that the conclusions made by Chessman (2021) were probably a consequence of the low statistical power of his analysis. Consequently, we argue that Chessman’s (2021) comments do not substantively alter the conclusions of our study, and we provide more evidence to support the conclusions of our previous publication. The study of biota–stressor interactions is increasingly relevant to a wide range of global ecosystems. There is a need to develop tractable experimental and survey designs that address these problems, and we identify further avenues for study of these complex issues.

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

“…The additional data and analyses presented by Kefford et al (2022) compare the putatively salt-sensitive faunal component in the salinity-control mesocosms between the pure and mixed assemblages ~1 week into the experiment. They report results of univariate analyses (method not stated) of taxon richness, total abundance, EPT (Ephemeroptera, Plecoptera, and Trichoptera) abundance and OCH (Odonata, Coleoptera and Hemiptera) abundance, as well as multivariate analyses (permutational multivariate analysis of variance and analysis of similarities) of assemblage proportional composition.…”

“…These analyses likely had low statistical power, because they incorporated only the salinity-control mesocosms, there were only four such mesocosms for each assemblage, and variability in the response variables was high. Moreover, Kefford et al (2022) offer no explanation of why the saltsensitive faunal component would be the same in the pure and mixed assemblages a week into the experiment when it must have differed between the two assemblages at the start of the experiment because the pure assemblage received an extra kick-net sample from the Cotter River. Conceivably, something caused greater mortality of salt-sensitive invertebrates in the pure assemblage than in the mixed assemblage during the first week, evening out the numbers.…”

“…If so, the two assemblages differed in sensitive-sensitive biotic interactions as well as potentially in sensitive-tolerant biotic interactions, and the experimental design was indeed confounded. Kefford et al (2022) state that Bray et al (2019) did not statistically test end-of-experiment differences in density-EC relationships of individual taxa between the pure and mixed assemblages because the focus of their study was on community-level responses. However, the proposition that individual salt-sensitive taxa demonstrated a variety of effects of sensitive-tolerant biotic interactions on salinity responses is central to their conclusions.…”

“…Bray et al (2019) devote two of their five Results paragraphs, four of their six Discussion paragraphs, and three of their five figures either entirely or primarily to responses of single taxa. Kefford et al (2022) also state that Bray et al (2019) acknowledged uncertainty in their findings for individual taxa by using phrases such as 'appeared to show' and 'appeared to depend'. While such phrases are often used, Bray et al (2019) also make unqualified claims such as that 'at the population level, however, salinity and tolerantsensitive taxa interactions caused a range of species-specific and context-dependent responses' and that their 'results reinforced [that] interspecific biological interactions both mediated salinity effects and were important on their own, irrespective of salinity toxicity, influencing taxa and community responses.'…”

“…Chessman (2021) also conducted a statistical analysis of Bray et al's (2019) publicly available data, which found no support for their hypothesis. 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, as well as by criticising 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).…”