The authors of this manuscript have put together a novel meta-analysis dataset, evaluating the influence of parasites on components of host fitness and host species interactions. The authors explored the overall influence of parasites on host response, and then evaluated additional questions by sub-setting the data. The additional subsets evaluated parasite effects on species interactions, components of host fecundity, parasite evolutionary strategy, host interaction role (predator or prey), and host habitat. The results were predominantly variable – in most cases there were examples of parasite influence that ranges from positive to neutral to negative. Notable exceptions were that trophically-transmitted parasites on average had a negative effect on host response, parasitoids had a positive effect on host response. They also found that parasites had a negative influence on interspecific competition. Interestingly, the effects of parasites on species interactions varied by broad habitat, with a positive influence in terrestrial systems and negative in marine and freshwater.

I think the variance found in this study is interesting, and the authors do a good job of both exploring it and discussing how to interpret this high level of variance. I appreciated that the discussion raised the point that the broad range of effects that have been observed suggests that the potential to misconstrue the influence of infection on ecological data is actually higher than if there were a clear directional influence.

Overall, the manuscript is well written, the figures are clear, and the results are important and worthy of a broad audience. Please see the line edits for minor points below.

Line edits:

Line 162-63 – This sentence states that the goal was to study species interactions within food webs, but the discussion has very little mention of food webs. I think it would be more accurate to say you focused on species interactions, removing the ‘food web’ context from this point.

Line 394, 396, 410, 411 – I think that you could remove or tone down the superlative words describing the responses throughout, for example; ‘severely’ (394), ‘strongly’ (396), ‘large’ (410), and ‘severe’ (411). The numbers and figures already indicate the level of the response and the application of the different descriptive terms seems somewhat arbitrary and in several of the cases a bit of an overstatement. If you’d like to keep some level of language around the magnitude of the change in the results, maybe set up a defined language that is consistent between all of the responses.

Line 432-433 – I am a bit confused by this summary statement. From this description I would expect a bimodal response, not the continuous response from negative to positive.

Line 531-553 – I think this section could use some restructuring – a lot of content is in this paragraph and it comes across feeling a bit like a list of options.

Line 557-560 – I am a bit confused by these sentences and what results they are referring to. Do you mean ‘fitness’ here? I don’t see results where fitness is broken down by trophically-transmitted parasites. Is this referring to the result in 2a and 2c? I am also confused by the reference to specifically trophically-transmitted parasites in intermediate hosts. Is the trophically-transmitted category in 2c only in intermediate hosts?

Line 577-581 – I am not sure I understand this point and the supporting examples. Do you mean that the difference in the patterns is driven by the different types of parasites found in terrestrial, marine and freshwater systems? If that is the case, it might make sense to have a supplemental figure/table that shows how parasite types (e.g. nematode, acanthocephalan, etc) are distributed between the habitat types.

Line 1036 – effects of parasites on what mean response? Host mean response?