Light pollution: a landscape-scale issue requiring cross-realm consideration

This paper discusses the importance of managing harmful light pollution in a manner that recognises “cross-realm” impacts. The paper is important in raising cross-realm issues for the ecosystems and organisms that form part of the case studies and the use of night-time light pollution (referred to as ALAN in the paper) as the stressor.

The paper provides a review and some synthesis about (a) material that provides the framing for the topic, including the multi-disciplinary elements and the role of influencing/regulatory/governance actors (b) material that provides a realms context for this specific paper and (c) material linked to evidence that ALAN needs to be managed from a cross-realm perspective and finally (d) material linked to future management approaches.

The paper relies on Threlfall (2021) for the basis of the cross-realm perspective and on work by Gaston and his colleagues and others for the ecological basis of the impacts of light.

General comments

Overall, the paper sets out interesting perspectives on the issues it addresses (a to d above) but it appears repetitive or discursive. A strong edit would give the paper more focus on its unique contribution (case studies of cross-realm issues linked to ALAN; management approaches).

The paper does not present any new data and the new perspectives it offers can become lost in the more discursive text. Tabulating the main issues based on the published literature and providing some indication of the quality of the literature considered would be helpful. It is appreciated that it would be inappropriate to conduct a systematic review of the literature as no cross-realm specific interventions appear to exist, but the paper could be structured more as a themed or systematised review or more centred on the case studies as a source of evidence. For example, if the material was tabulated in some way to illustrate commonalities between different species or realm combinations this would help the reader understand how strong the evidence base was. Giving slightly more detail about how organisms are affected might provide more for a reader to bite on. Given many of the examples appear to be aquatic this should be possible – an additional unifying diagram may also help. If this is not possible then that should be stated and a reason given.

The Introduction should set out more clearly what the paper covers (e.g. as indicated in a to d above).

The paper tends to argue from a perspective that says this is what could be the case in  cross-realm way of viewing ALAN impacts and here is the evidence for that. I would normally expect the evidence to be set out first. If the authors prefer to take the first approach it would help greatly to set that out as the approach used in the review. There may be intellectual reasons for doing that or even the practical one that the evidence base for cross-realm approaches are currently very limited.

Specific suggestions

Line 81: More text needed to define “realm”, as the current text is unclear and might lead a reader to think that all the factors covered after “i.e.” are one realm. Double brackets need to be avoided if at all possible.

Line 84 to 88: Maybe this text should appear slightly earlier so that any mention of management practices follows material on realms. This would help the narrative flow of the paper.

Lime 89: It cannot yet be the case that “are” is correct. There may be some evidence here but surely at present it should be “may”. Even if it is “may” at present that is good enough.

Around line 100: The Introduction needs an additional line or two to distinguish the paper from Threlfall so that it is clearer how this paper goes beyond the work in Threlfall. Some of the text in the covering letter expresses the unique nature of the paper more clearly than either the Abstract or Introduction do. Re-use of that text might be helpful.

Line 105: insert “appropriate” before “conservation actions” as the actions are those to with ALAN not that set of actions in general.

Line 105 to 107: This reads awkwardly. Rephrase, perhaps such that Taylor’s work comes earlier.

Line 107 to line 175: This section seems more like a discussion of possibilities – the word likely is used several times – rather than a section that lays out the facts of what is known about ALAN in the realms context and this kind of issues may be why the general point was made about repetition/discursive. The authors should consider how the facts about ALAN can be separated from any discussion on its potential seriousness from the realms perspective and revise text accordingly.

Case Studies: These are valuable but some indication of the magnitude of impact on the organisms or systems needs to be supplied. It also needs to be clear if these case studies are actually more like thought experiments. If this paper is to make a case that a cross-realm approach is required then the effects seen within each realm and what the overall significance of the cross- or multi-realm effect is needs to be set out else the paper risks failing to convince. For example, biomass might be reduced or predation increased – but by how much and what is the combined impact of those losses? …………

…….. And in line 201-202: what is the alteration in migratory behaviour? And, also, what is the nature and magnitude of the effect that the Oppendal paper describes?

In each case study the authors should present the evidence of effects of ALAN on the case study organism before discussing what that might mean in a cross-realm context. This may shorten the text substantially or at least increase its impact. As written the paper leads the reader to question (say on line 235) the argument proposing a bioindicator/biomonitor even before the evidence is reached.

Fig 1: Again, how big are these effects?

Line 244 – “infer” can only be done soundly if more quantitative information were to be supplied in the paper. If the effects were all very small inference would be awkward. Maybe more deduction is required? That might be possible if the evidence came to the readers attention and then the argument for the importance of cross-realm approaches?

Line 278: why is “assuming” used here? Rephrase needed.

Line 284 to 286: The Holker et al (2015) reference seems very odd in this place. This is a clear example of where more detail needs to be given about what the paper referred to is dealing with. Giving this greater level of detail would provide an internal check for the authors of this paper so that their points and references are more closely tied together. The authors might want to do a similar exercise with respect to the other references as an internal QA exercise. That could be described as part of the systematised review and would increase readers confidence.

Line 286 to 291: Complete revision of this text is needed. Some of it is more than difficult to understand and seems highly speculative as written.

Line 369: Are these spatial or temporal zones? The text seems to refer to both. Please define.

Line 415: The use of a bold upper case heading here is the first in the paper since line 54. Apologies if I have missed another main heading such as “Evidence review”.

Fig 3: What do the symbolic representations of: ?, an ecosystem, a group of people and a map mean to convey. A more extensive legend is needed.

Line 445: What is n here? Is it the total number of respondents or the number that represents 90% of the total?

Line 445 to 449: If 17% is equivalent to over 60 respondents it is difficult to see that 89 respondents is 90% (??)

Line 477 to over 550 et seq on integrating knowledge and action (I think) needs careful re-examination along with some structural realignment of the paper to make section headings more consistent and meaningful. This material is important to the paper but seems to be less well articulated than it could be.

Section beginning on line 554 and that beginning on Line 587: These two sections need revisiting as the first seems to suggest that cross-realm issues can most easily be dealt with by single species interventions (which theoretically could be subject to systematic review) and the final section seems to push forward network approaches on cross-realm issues that are perhaps rendered needless if the first of these two sections is the case. There is a potential inconsistency here that needs resolving.

Mayer-Pinto and colleagues submitted a review paper entitled « Light pollution: A landscape-scale issue requiring cross-realm consideration ». This work deals with a subject of great interest, the impacts of artificial light at night (ALAN) on organisms and ecosystems. This subject is the matter of intensive research and several original and review papers have been published the last years on this topic, as a result from an increasing awareness that ALAN induces dramatic negative effects on species, populations and ecosystems, and that politics of mitigation must be implemented in order to reduce the effects of ALAN. The originality of this submission lies in the fact that it considers mainly the effects resulting from the interactions in between aquatic and terrestrial realms, with the help of case studies.

The paper is well written and of interest for a general audience. I believe however that some aspects could be treated more in depth, in the light of recent publications on the matter. My comments follow.

-L60. Isn’t it, at the start, a key driver of biological (molecular and cellular) processes?

-L86-88. Most of the studies today indicate that on the long term ALAN affects dramatically and durably population abundance and richness, and ultimately ecosystems. Thus the term “might” (L86) sounds weak to me. Same comment for the use of “may” L136.

-L111. Since daily and seasonal are terms that refer to cyclic/periodic variations, please specify what “… or cyclic movement of …” refers to.

-L176-180. Is this useful?

-L194. I do not understand the meaning of “… associated with human populations …” here.

-L294-296. This has already been said before.

-The paper is based on the choice of 4 case studies. Since the salmon is one of these cases, perhaps the authors could refer to a previous recent review in which a whole section is dedicated to the impact of ALAN and other factors on the migrating Atlantic salmon. In this review paper examples of ALAN impacts on riparian systems are also provided and discussed in light of the literature available (see: Falcón et al., Frontiers in Neuroscience 2020; 10.3389/fnins2020.602796).

-In the review paper cited above, it is also emphasized that ALAN is not the only factor affecting populations and ecosystems and that the addition of anthropogenic threats, including ALAN, should also be considered when studying solutions to mitigate the effects of ALAN. Perhaps this could also be discussed in this MS, particularly at the end, in the part dealing with the challenges and practical solutions? On site studies prospective studies should consider the existence of 'multifactotial effects' whether positive (sometimes) or negative (most of the times).

-L320. Vertical migration refers to the LD synchronized movement, up and down the water column. Thus here I would write "vertical movement" or more simply "migration to the surface".

-L391-393. I do not see why they should be more sensitive; isn’t this the case for all the living? Or perhaps I am missing something here?

-L435-440. Indeed. Starting from the observation that ALAN is not “a natural environment” it would seem reasonable to believe it is per se a polluting factor; not mentioning that the absence of effect on the short term does not mean there will be no effect on the long term (as previous studies have shown). And not mentioning that the presence or absence of other harmful factors (depending on the area investigated) might interact with ALAN to promote effects not observable with ALAN alone.

L451-453. I disagree with this assumption; many studies now include this aspect; see the review paper referenced above (which discusses this extensively); and, the authors write later (L513-514) “…any light that is not natural in its origin is likely to interfere with ecological process”. I agree 100% with this.

-L493-496. I do not understand this sentence here. Aren’t all species important for the maintenance of an ecosystem?

-L500. What is an acceptable threshold?

-L547. There is more information available than suggested from the sentence. See Falcón et al, 2020, in which this is discussed.

-The reference list should be checked, as some references appear incomplete ( e.g ., lines 847, 883, 893, 967, 989, 1067, 1078,…).