Niche Models Differentiate Potential Impacts of Two Aquatic Invasive Plant Species on Native Macrophytes
Round 1
Reviewer 1 Report
The manuscript written by Verhoeven et al. dealt with niche model differentiation for two invasive alien aquatic plants, Potamogeton crispus and Myriophyllum spicatum, and a set of native species. The authors used three parameters for their niche models, water depth, light availability and phenology.
In general, using a niche model approach to identify potential reasons for the invasive success of invasive alien aquatic plants is interesting, even though two already very well studied were used.
However, the chosen parameters water depth and light are strongly related to each other and should not be analysed separately. I would recommend to use only light as a parameter in this study. Unfortunately, the way how light availability has been calculated is somehow not explained in detail based on secchi depth values. The presented values of “Proportion of surface irradiance remaining at substrate” are very high and I have some doubts if these calculated values are correct. The light availability strongly decrease within the water column (see e.g. Hofmann et al., 2008, Wave-induced variability of the underwater light climate in the littoral zone. Verh. Internat. Verein. Limnol. 30 (4): 627-632.), and at the substrate usually less than 10 % of the light availability of the incoming irradiance at the water surface level is available. Particularly submerged plants should be considered as plants acclimated to low light conditions of 50 – 200 µE, which are about 2.5 – 10 % of the incoming light (in summer about 2000 µE during full sunshine). I hardly recommend to reconsider the recalculate these data.
The use of “growing degree days” is interesting, even though it might be hardly to follow for some readers. I recommend to simplify some statements, as the major outcome is that (i) Pot crispus have a biomass peak in early season (would be good to indicate at least a range of the period, e.g. end of May until mid of July) and thus cover more or less an empty niche, (ii) Myr spicatum peaks biomass later during the year with a strong overlap with native species.
Specific comments
p4, line 120: please be more detailed in explaining the macrophyte sampling
p5, line 168: please explain in detail the calculation of light availability based on secchi data. Please consider to use another way to get better and more plausible data
p5, lines 177 ff: how can be ruled out, that the used weather data math the conditions at the lake, if the mean distance between weather station and lake is 64 +- 31 km?
p5, lines 181: I am wondering, if 4°C (which was used for a charophyte study, which usually grow better at cooler temperatures than vascular macrophytes) is appropriate for Pot crispus and Myr spicatum; is it not better to check the literature about minimum temperature requirements for growth of these species (probably about 10 °C) and use this value?
p6, line 240: figure 3? Not figure 4?
P 8, figure 4: please give the units for the y scale
p11, line 328: please be more specific in your statement. Why is the water clarity in early season in general higher than later in the year?
Data and Figure S1 and S2: Why are water depth and light availability relevant for free floating (e.g. Lemna minor, SPirodela polyrhiza) or rooted floating leaved (Nuphar) or emerged species (Typha?)?
I recommend to consider relevant published articles on light and temperature preferences of Pot crispus and Myr spic. (see e.g. older work by Barko)
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript Diversity-747172 presents a descriptive analysis of the niches for two invasive species and compare them with the niches of native species. The dataset is quite impressive and all analyses are properly applied. In general, the manuscript is clear and goes to the point, which is something that I highly appreciate. Finally, supplementary material is outstanding and I hope everyone would prepare a supplementary like this. Congratulations! However, there are some concerns that I want authors to address before publication.
MAJOR COMMENTS
1- I think authors need to provide more information about the vegetation survey. They only mention point-intercept but we do not know how many points per lake, if all lakes have the same number of points, the distance between points, if more than one species can be surveyed per point etc. This information does not change the results of the study but helps a lot to understand the data and frame the discussion. For example, authors talk a lot about competitive exclusion and even in the discussion speak about the relation between co-occurrences and competition. The connection between co-occurrences and competition only happen at very local scales (Pearson & Dawson 2003); thus, if we know the scale of the survey will allow a better linkage between results and discussion.
2- I miss a lot the information about the abundance of the species in the main text. I think that will provide an important perspective about the dominance or not of the invaders. For example, P. crispus is the 3rd most abundant species in the survey, but it is said that will not displace others because non overlapping niches. M. spicatum is abundant but less. This difference in abundances can be explained by competing with native species due to similar environmental niches. The discussion suggests this, but by looking at the abundance data authors can provide further support to this point.
3- Sometimes authors compare invaders and natives for significant differences, but they do not mention any correction for multiple testing. If that is the case, a correction (Bonferroni etc) should be applied.
SPECIFIC COMMENTS
Introduction
L46, modern coexistence theory posits that coexistence depends on the relation between niche differences and fitness differences, but here authors only speak about niche differences. As authors speak so much about competitive exclusion, a comment about fitness differences would be great.
L52, what means ‘negative drivers of change’? Please clarify.
L76, as written, this sentence suggests that species have broad tolerance to phenology, which does not make sense to me. Rewrite for clarity.
Figure 1, b and c also include a? indices suggest that but the figure does not. Please clarify that info.
Figure 1, what is the point of including both shared and non-shared dissimilarity? By the formulas it seems that shared = 1 – non-shared. Authors do not use both in the main text; thus, I suggest deleting.
Methods
L150, an explanation about what lake-year means would be convenient.
L157-159, as you are using a probabilistic model, if you fill the gaps with the expected values based on the distribution, when you calculate the distribution you are including data that was derived from the same distribution. Thus, you are calculating one distribution based on that same distribution. Theoretically, if you do not use that data you should get the same final distribution. I recommend to do the analysis without estimating any data and see how different results are (it should not be different). Maybe authors lose some species but it is not a big issue for the study. This will be the more convenient outcome of the analyses.
Equation 1, check if ‘2 – base temperature’ goes within parentheses.
L213, Kernel Density Estimation methods require to define a bandwidth. Provide that info and why that value was chosen.
L224, although commonly applied, the average value of a similarity matrix has several problems (Baselga 2013). I recommend revisiting this analysis with the reference provided.
Results
Figure 3, this figure is totally unnecessary, manuscript does not loss any information if removed. I suggest removing.
Figure 5 and 6 are tables. Should not be named like that?
Discussion
L278-285, I do not agree with this part. Authors compare the variation of one species in communities with different PFG richness with the variation of four species in PFG treatment. A fair comparison would compare variation in same scale (variation of one species in PFG treatment). I think that to show the importance of intra vs interspecific variability, the standard error (standardized by number of observation) instead of range will be better.
L315-137, if I understood it properly, higher biodiversity -> lower litter quality -> lower nutrient accumulation. Then, it will not be a negative effect of biodiversity?
L318-332, I have one question on this part. The efficiency is based on a ratio, so authors observe a more efficient use of resources in high biodiversity. However, there is a decrease in
L323, there is a double space after ‘(Vergutz et al., 2012).’
L339-342, include a reference for this sentence.
L379-381, it seems to me that this sentence is disconnected from the next one. I would like some sentence explaining why ruderals have this particular response.
Tables
Why use ‘ns’ in table 1 and not table 2? I suggest removing ‘ns’.
In Table 2 I recommend using the common ‘.’ instead of ‘^’ to indicate marginal significance.
Figures
In all figures is possible to see that PFG richness = 1 and 4 only have one value, which can influence very importantly the regression.
In all figures the R2 and p-value are based on the linear regression with all data or only in the mean values for each treatment? If authors insist on using PFG richness instead of functional diversity, please specify in the caption.
In figure 3 (s.g.) I found very weird the difference in R2 between green and senescence leaves. Even data seems more disperse for green leaves. I suggest checking it.
Bibliography
Laliberté, E., & Legendre, P. (2010). A distance‐based framework for measuring functional diversity from multiple traits. Ecology, 91(1), 299-305.
Wu, J., Naeem, S., Elser, J., Bai, Y., Huang, J., Kang, L., ... & Han, X. (2015). Testing biodiversity-ecosystem functioning relationship in the world’s largest grassland: overview of the IMGRE project. Landscape ecology, 30(9), 1723-1736.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 3 Report
This is an excellent mansucript that takes a novel approach to quantify the impact of two invasive alien aquatic plant species and thus developing a tool for managers to prioritize which species should be targetted for control. The manuscript is well written, the problem well conceptualized, the methods are thorough, results well presented and the discussion a good mix of interpretaion, integration and reflection.
I was a bit surprised that water nutrient status was not used as one of the environmental gradients as many invasive aquaitc weed species have been shown to respond well to pollution. This is perhaps something the authors might want to mention in the discussion.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Comments to the revised version of the manuscript entitled <<Niche models differentiate potential impacts of two aquatic invasive plant species on native macrophytes>>.
The authors responded very comprehensive to comments made by three reviewers. They added / changed figures and references and rewrote some parts of the manuscript. This strongly improved the overall quality and the precisement of the statements and the whole manuscript.
I still have some few comments:
Figure 2: This figure is very helpful to get an impression about the study area (particularly for readers from outside the USA). Obviously, most of the weather stations are outside the main study region and as I not familiar with the geographic and climatic conditions within this part of the USA I am wondering, if this could have any effect on the outcome? If not (what I guess), this should be at least mentioned.
Line 273: The water temperature will somehow “follow” the air temperature (the water need so time to heat up) and thus there will be a small difference in the air GDD and water GDD. This should be mentioned.
Figure S1: Even though the authors argued, that including free floating species is necessary at some point, I recommend to exclude free floating species from this figure, as water depth will definitely have no impact on the species. The fact, that these species occurred only at low water depth is only caused by the fact, that these plants were just accumulated at shore of the lakes due to wind or water drift.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
