|I find the manuscript improved since the previous version, and more comprehensible, but significant problems remain. In particular, the authors are applying the concepts of “habitat”, “weighted usable area”, and “biological diversity” inappropriately.|
Please see references listed below regarding correct use of the word “habitat” and related terms. Habitat is organism-specific, and defined as the resources and conditions that enable a particular organism to occupy a particular place. It is not a term than can be applied to a community (e.g., revised manuscript, line 85) and the BMWP_PL index is not a measure of “habitat suitability” (lines 152-158). Indeed, phrases such as “suitable habitat” (e.g., line 28) and “usable habitat” (e.g., line 205) should not be used – the words “suitable” and “usable” are redundant because if an area is not suitable or usable for an organism, it is not part of its habitat. In short, “habitat” should be removed from the manuscript because the authors do not assess the extent or quality of habitat for any particular organism.
Similarly, the term “Weighted Usable Area (WUA)” (e.g., line 232) is meaningless without reference to the particular organisms the area is usable for. An area that is usable for one organism is not usable for some other organisms, because different species and life-history stages have different environmental requirements. And the BMWP_PL index is not a measure of the area that is usable for any organism. Therefore, the authors should also remove reference to WUA.
What the authors have done is not to assess ”habitat suitability” or “weighted usable area” but rather to model the discharge that maximises the value of the BMWP_PL index. However, they have not articulated a rationale for doing so. They state that the purpose of environmental flow is to “maintain biological diversity in the river ecosystem” (lines 46-47). But the concept of biodiversity is multi-scaled and multi-faceted - please see Rolls et al. (2018), referenced below, for an overview of the scales and facets of biodiversity relevant to environmental flows. Biodiversity is not equivalent to the value of a biotic index just because the index incorporates species with varied environmental preferences (lines 132-133). If maintaining biodiversity is the goal. the authors need to justify their index choice by explaining which scale(s) and facet(s) of biodiversity the BMWP_PL index predicts, on what evidence, and why BMWP_PL is a better indicator of these scales and facets of biodiversity than alternative indices or metrics.
I also note that while the authors have provided responses to my previous comments, in several cases they have not made corresponding changes to the manuscript. For example, the tautological phrase “multispecies communities” is still present (line 38), the definitions of “low low flow” and “mean low flow” (line 175) have not been incorporated, the procedure for random selection of the case-study site is still not stated (line 179). The authors should make changes in response to every previous comment, or else provide a reason for not making a change.
Bamford M, Calver M (2014). A precise definition of habitat is needed for effective conservation and communication. Australian Zoologist 37, 245-247.
Darracq AK, Tandy J (2019). Misuse of habitat terminology by wildlife educators, scientists, and organizations. Journal of Wildlife Management 83(4), 782–789.
Kirk DA, Park AC, Smith AC, Howes BJ, Prouse BK, Kyssa NG, Fairhurst EN, Prior KA (2018). Our use, misuse, and abandonment of a concept: whither habitat? Ecology and Evolution 8, 4197-4208.
Krausman, P. R., & Morrison, M. L. (2016). Another plea for standard terminology. Journal of Wildlife Management 80(7), 1143–1144.
Rolls RJ, Heino J, Ryder DS, Chessman BC, Growns IO, Thompson RM, Gido KB (2018). Scaling biodiversity responses to hydrological regimes. Biological Reviews 93, 971-995.