A global review on invasive traits of macrophytes and their link to invasion success

Abstract: Aim Biological invasions by exotic macrophytes represent one of the main reasons for biodiversity and ecosystem changes in aquatic ecosystems. The reasons for their ability to succeed in new environments have been of ecological interest in the last years. We made a global review, aiming to describe functional traits related with invasiveness of macrophytes. Methods Our search was performed using keywords regarding invasive macrophytes and functional traits. We related the group traits of invasive species with their probability of species invasion success in new localities (invasiveness). We also performed a nestedness analysis that helped us to see which species possessed the higher number of traits related to invasiveness, as well as which traits were more common among the invasive species. Results Traits most often related to invasiveness were those indicating growth (94.5%) and reproduction (90.1%). Nearly 70.4% of invasive macrophytes traits were related with the probability of invasion success. Invasive species had a higher number of morphological and biotic interaction traits related with invasiveness than native species. Our nestedness analysis indicated a low degree of nestedness, but showed us that Egeria densa, Elodea canadensis and Elodea nutalli were the species with a wider range of environmental tolerances, explaining their invasibility across ecosystems. Conclusions We summarized and complement existing reviews on the functional traits related to invasion success of macrophytes. We believe this review contributed to the identification of the most common set of traits related with invasiveness, helping to speculate on successful invaders in the future. Graphical Abstract


Introduction
Biological invasions are a major threat to natural environments, and therefore it would be useful to know if invasive species are better equipped by their functional traits to cope with the conditions of new environments, therefore aiming to prevent future invasions. Many works have tried to make a direct link between invasive macrophyte traits and the probability of invasion success (Alpert et al., 2000), however many results indicate that traits constantly associated with invasion are difficult to identify. On the other hand, there are other authors that claim that certain types of traits that related to invasion success are crucial for explaining and predicting invasion (Pyšek & Richardson, 2008). The number of articles working with invasive macrophyte traits is increasing over time, and there is relevant work being made. Recently, Hussner et al. (2021) reviewed traits associated with the most invasive macrophytes species and their relation to macrophytes' growth form.
It is often argued that some types of traits, such as the ability of vegetative reproduction, high growth rates, high seed production, and germination rates, short life cycles, and the production of allelopathic substances are usually linked to plant invasion success (Michelan et al., 2010;Hussner et al., 2021). Still, some native plants could possess such traits, and the question of what kind of traits differentiate successful invaders from natives or other unsuccessful invaders remains. The comparison between native and invasive species traits can shed some light on that matter, as has been shown in some previous works (Hamilton et al., 2005;Pyšek & Richardson, 2008), whereas invasion success may be influenced by differences in the functional traits of native communities, as already shown for fish (e.g. Azzurro et al., 2014;Skóra et al., 2015).
In this work, we reviewed the invasive macrophyte traits described in articles found in a global search and then related them with the possibility (or not) of invasion success (i.e. invasiveness). Also, as many articles compared invasive species traits with native traits, we added native species traits in our work as well, aiming to verify if there were any significant differences among traits between native and exotic species. Our main hypothesis here is that invasive species must have a higher number of traits associated with their invasiveness than native species, although it is very difficult to assume that one or more traits are exclusive to invaders. We must assume that when planning and executing this review, Hussner et al. (2021) published a very well-done review paper on invasive macrophytes and their traits of invasiveness. Here, we complement Hussner et al. (2021) work with a different approach. Indeed, we investigated a wider range of invasive species, comparing their traits with native species traits, and also investigated their success through different environmental conditions.

Methods
A scientometric analysis was also performed by measuring the number of articles resulting from the search, the increase in the number of articles involving invasive species and functional diversity over time, the location of the studies and the families of the species analyzed. The articles search was made through the ISI Web of Science database using the following keywords: (macrophyte* OR aquatic plant* OR aquatic vegetation* OR aquatic weed*) AND (biologic* invas* OR inva* OR introduced OR alien OR exotic OR non-native OR non-indigenous) AND (Function* Divers* OR function* trait* OR function* group* OR function* type*) in May of 2020. All articles obtained from this search were screened based on their titles and abstracts and later fully read. Only articles that studied (directly or indirectly) invasive macrophytes and related their functional traits with any ecological questions were selected. We reported all traits found in the articles and later related them with the possibility of invasion success. Invasion success traits were therefore any traits that would give an invasive species an advantage in new locations, such as high growth rates, allelopathy, or high competitive potential. The selected articles followed the PRISMA guidelines for reporting systematic reviews (Page et al., 2020) (Figure 1).
By functional trait, we consider any anatomical, morphological, or physiological behavior trait that is related to how these macrophytes affect and are affected by the environment and other individuals (Capers et al., 2010;Cardinale et al., 2012;Tilman, 2001). Different groups of traits were obtained by the article's search, such as physical, morphological, physiological, and response traits to environmental filtering and traits related to biotic interaction. Response traits are traits that measure how an individual responds to changes in the environment (Lavorel & Garnier, 2002) and biotic interaction traits are traits reflecting the interaction with other macrophyte species. Continuous attributes were included in the review as response/biotic interaction traits in a qualitative manner, for comparison between species.
Native species traits were also included in our review for means of comparison since many articles found in the search made a comparison between the traits of invasive and native macrophytes. By comparing with natives, we went further: if invasive and native species indeed possess different traits, functional traits could be better related to invasion processes.
For both invasive and native groups, functional traits obtained in the articles were separated as traits related to growth (such as type of growth, clump-forming for example, fast colonization or high growth rates), reproduction, and life cycle (such as vegetative propagation, life cycles, sexual or asexual reproduction), physiological processes (such as photosynthesis efficiency or nutrient concentration in their tissue), morphology (leaf morphology, height, canopy structure for example), effect traits (traits that alter ecosystem functions), life form (emergent, floating, submerged or emergent), tradeoffs, competition potential, as response traits to environmental conditions (alterations in morphological, competition or any other traits that are affected by any type of environmental stress) and biotic interaction traits (any group traits that shows alterations in a scenario with the presence of new macrophyte species). Later, we related the reported traits to their indication of invasion success (invasiveness). Traits that did not indicate success are not necessarily a barrier to invasion; however, they cannot be immediately related to species' invasiveness. The data concerning invasion success by invasive species in different environmental conditions (i.e. only response traits related to success in new environments) were compiled and analyzed through a network analysis (nestedness). This kind of analysis usually is performed with the interaction of two different sets of species, however, this analysis proved to be useful to us, since it allowed us to see graphically and statistically which conditions were more related to the invasive species, showing us these species and also what species were related to which conditions (or more than one condition). We used the NODF method and the analysis was performed through R software and with "bipartite" package.  Page et al., 2020) illustrating the steps of our search and review through ISI Web of Science database.

Results
A total of 349 articles were found in the search. However, only 61 (17.47%) of them actually measured traits or made relationships between invasive macrophytes and functional traits. From those 61 articles, 31 (50.81%) compared invasive and native macrophyte traits. Invasive or native status depended on the studied region. Therefore, some species received both invasive and native status in our review, depending on the region where the study was performed. This was more clearly discussed after the comparison of native and invasive macrophyte traits in the 31 articles abovementioned.
There was a clear temporal increase in the interest of studies involving invasive macrophytes and functional diversity ( Figure 2), with articles ranging from 1996 (n = 1; 1.63%) to 2019 (n = 14; 22.94%). Although our review is global and articles from many countries were found, there was a bias towards articles published in the United States (n = 22; 36.06%) and France (n = 10; 16.39%) ( Figure 3).
The majority of traits reported for invasive species were related to macrophyte responses to environmental conditions and biotic interactions, followed by morphological, reproductive, and life form traits (Table 2). From the traits regarding responses to environmental conditions, 70.4% (n = 86) indicated success in terms of responses to environmental changes or stresses. Considering invasive biotic interaction traits, 88.6% (n = 141) were related to invasiveness. Growth and reproductive invasive traits were the most related to invasiveness (94.5%, n = 35; and 90.1%, n = 46, respectively). Within competitive traits, only 10 traits were reported, all of them positively related to species invasiveness. Invasiveness, or probability of invasion success, was related to patterns such as high reproductive traits, better competition abilities than other species, or higher tolerance to stress (see Methods section). Given that native species could share some of these characteristics, we also related them to invasiveness, even when they can or cannot be considered invasive in their native ecosystem.
Vegetative propagation (reproduction trait) was the most recorded trait for invasive species, followed by submersed life form (life form trait -other life forms were also often recorded, such as emergent or floating), perennial life cycle (reproduction), formation of dense mats (growth), high competitive potential (competitive potential) and high growth rate (growth) (Figure 4). Many of these traits are often related with macrophyte invasion success. Also, other traits usually linked to invasiveness were often reported only for invasive species, such as high seed persistence, fast reproduction, and large leaves. Life form, although often described, is a difficult Figure 2. Temporal trend of the number of articles measuring and/or relating invasive macrophytes and functional traits found in ISI database following the Boolean operators described in methods.

Figure 3.
The number of articles measuring and/or relating invasive macrophytes and functional traits per country of study, found in ISI database following the Boolean operators described in methods. Table 1. List of species studied in the articles analyzed (see methods for description of the search), and their respective status (native or invasive), family, region where the species is native or invasive and the number of articles that worked with the species. *To our knowledge, there is no species named 'Myriophyllum salicaria', this supposed species were cited in an article, but we do suspect that the correct species would be another one. **The authors refer to varieties of Nymphaea in their study, then we cited the genus author here. *To our knowledge, there is no species named 'Myriophyllum salicaria', this supposed species were cited in an article, but we do suspect that the correct species would be another one. **The authors refer to varieties of Nymphaea in their study, then we cited the genus author here. *To our knowledge, there is no species named 'Myriophyllum salicaria', this supposed species were cited in an article, but we do suspect that the correct species would be another one. **The authors refer to varieties of Nymphaea in their study, then we cited the genus author here. *To our knowledge, there is no species named 'Myriophyllum salicaria', this supposed species were cited in an article, but we do suspect that the correct species would be another one. **The authors refer to varieties of Nymphaea in their study, then we cited the genus author here. group of traits to directly relate to invasion success. Therefore, we did not assume the relationship between life form and the possibility of invasion success in any case, since a more detailed analysis would be necessary (see also Hussner et al., 2021). Biotic interaction traits were measured in different conditions and with the comparison of invasive macrophytes traits to natives or other invasive macrophytes traits. Most often for invasive species, traits were related to competition (n = 142; 89.3%) or tolerances (n = 4; 2.5%), and in general, 88.1% (n = 161) of traits indicated possible success in new environments.
The temperature increase was the most studied condition to measure response traits of invasive and native species, and for both invasives and natives, the success probability (invasiveness) was very high, indicating no particular advantage to invasive species. Desiccation and light limitations by shadow were also very studied, and for desiccation invasive macrophytes traits were mostly related to invasion success (70.5%), as for shadow this percentage was lower (40%, Figure 5).
When analyzing the 31 articles that compared invasive and native macrophytes traits, the main differences between the two groups can be seen in morphological, response and biotic interaction traits (Table 3). Invasive species had a higher proportion of morphological traits related to invasiveness (n = 11 out of 19; 57.8%), as well as biotic interaction traits (n = 120 out of 136; 88.2%). Native species had a lower proportion of morphological and biotic traits related to invasiveness, however, response traits were overall more related to invasiveness (n = 29 out of 37; 78.3%) than invasive species (n = 21 out of 39; 31.4%). Most traits recorded for native species involved macrophyte life form, mostly submersed, which as we pointed out, cannot be directly related to invasiveness without the understanding of the environment where the species are. Biotic interaction traits of native species were often related to competition (n = 24; 72.7%), followed by physiology (n = 6; 18.2%) and facilitation (n = 3; 9.1%).
The NODF analysis showed a low degree of nestedness (NODF = 19.77), and also the most  recorded tolerance traits for each invasive species ( Figure 6). E. densa, E. canadensis and Elodea nutalli (Planch.) H.St.J were the species connected to more environmental conditions, showing a wider range of environmental tolerances ( Figure 6). Also, most species seemed more connected to high temperatures and desiccation conditions, indicating a vast range of invasive species tolerant to such conditions.

Discussion
The interest in studies involving invasive macrophytes and functional traits is clearly increasing and has gained popularity, being thoroughly studied throughout the years (see Figure 1). There are studies that go way back and already analyzed species traits, even without calling it so. The opposite is also true, in the sense that many articles claimed to work with species traits, but only discussed some species characteristics, without linking them with any ecological question. This raises the question of whether or not the concept of functional traits is being correctly used in studies. The concept is already very broad, and so if one really thinks about it, any species' characteristics could be a functional trait. However, this can really be a challenge when working with functional traits: what traits do you consider or not in your work? In a review, what traits are included or not? Here, we propose that a functional trait should be considered only when this particular trait is indeed related to an ecological function in the study. Therefore, studies should state the contingencies that define the use of functional traits. In that way, broad and meaningless characteristics are ruled out and the concept is narrowed.
Most articles were published in the United States, which makes sense since this is a developed country with many funding opportunities and researchers. France also published many articles, and even though France is geographically smaller than the United States, functional diversity research is very strong there (see CEFE, 2023).
The invasive species most recorded were E. densa and E. canadensis, both wide world invaders, and therefore very studied macrophytes. It is worth mentioning that both of them have a wide environmental tolerance ( Figure 6) and both belong to the Hydrocharitaceae family, which was the most recorded family of invasive species in this study. Indeed, Hydrocharitaceae is a family with a high number of invasives, mostly submerged species, and the reasoning behind that fact is of interest in invasion studies. One of the traits that could help the members of this family to become invasive is allelopathy, a functional trait very present among the Hydrocharitaceae (Hussner et al., 2021). Through allelopathy, macrophytes species release chemical substances in the sediment or the water column, such as phenolics, which could hinder the development of other species, such as natives in new locations, and therefore may allow invasive colonization (Thiébaut et al., 2018). For instance, E. nuttallii and E. canadensis can produce flavonoids, which can affect other macrophytes and has shown to hinder cyanobacteria and herbivores' larvae growth as well (Erhard & Gross, 2005). As far as environmental tolerances, many invasive species usually show tolerance to stressful conditions (Bora et al., 2020;Michelan et al., 2010, etc), however, it is curious that E. densa and E. canadensis were the species most related to a wider range of environmental tolerances. This could be due to the number of articles working with these particular species. Still, within Hydrocharitaceae, there are other invasives with wide environmental tolerances, such as Hydrilla verticillata and Thalassia testudinum (Benzecry & Brack-Hanes, 2016), which may indicate a pattern inside the family.
The most studied invasive species group traits were morphological, response and biotic interaction traits. Morphological traits most often recorded were large leaves (n=6) and the formation of a canopy outside of the water surface (n=8). This group of traits is often measured and analyzed aiming to generate knowledge about the physiology and anatomy of the invasive species (Riis et al., 2012). When a given nuisance species is well studied the chances of a successful management action against it are higher. Response traits often are measured in ecological studies not only to study possible management actions but also to evaluate possible new environments where species could invade, therefore is a way to prevent invasion (Bora et al., 2020). Furthermore, biotic interaction traits often are measured in competition studies. Competition is the interaction and interference in performance between two or more individuals, affecting species abundance (Thouvenot et al., 2013). Therefore, it makes sense that there are many studies within this theme since it can measure invasive species' impacts in a given community (Carniatto et al., 2013) and also investigate native species that can act as a barrier against invasives establishment, preventing invasion from happening (Levine et al., 2004). It is interesting though, that biotic interaction traits were all very related to species invasiveness, and it raises the question of whether or not traits related to invasiveness are inherent in invasive species (Hussner et al., 2021). Even if traits related to invasiveness are a result of plastic phenotypic responses after an environmental selective pressure event, if phenotypic plasticity is considered a trait, then this hypothesis could be corroborated. Indeed, phenotypic plasticity is often recorded for invasive species, which could be responsible for their success in the colonization of new environments (Fleming & Dibble, 2015).
Overall, most invasive macrophyte traits were related to invasiveness as well, such as high seed persistence, high growth and reproductive rates, and vegetative reproduction. Vegetative reproduction was the trait most reported in the articles from the search and is very related to invasiveness since asexual clonal reproduction is very fast and aids aquatic plant species invasion (Fleming & Dibble, Figure 6. A plot of interactions between invasive macrophyte species and their tolerances to different environmental conditions. Each line corresponds to one connection (tolerance). Species subtitles (see authors names in Table 1 2015). Within response traits, high temperature, desiccation, and shadow were the environmental conditions more studied. Simulations of higher temperatures within ecological studies are driven by the current climate change scenario. Niche modeling approaches are very useful for invasion studies since they can estimate possible new locations of invasive species occurrence in the future. Many studies claim that warmer temperatures can enhance macrophytes invasions (Calvo et al., 2019), however, in the articles found from the search, both invasive and native species possessed traits related to high-temperature tolerance, such as higher growth or biomass in this condition (Stephens et al., 2019). Macrophytes are by definition aquatic plants that need water for their survival or reproduction (Barrat-Segretain & Cellot, 2007;Silveira et al., 2009). However, there are many studies that recorded macrophytes relatively tolerant to this condition (Michelan et al., 2010). This is what we also observed, since most invasive macrophyte traits in desiccation regimes were related to invasiveness, that is, traits that indicated tolerance within this condition as well. Shadow conditions had not the same invasiveness trait rate recorded: invasives presented fewer traits related to shadow tolerance. Some studies have already demonstrated that shadow provided by native riparian vegetation could indeed prevent exotic macrophyte invasions (Evangelista et al., 2017).
Of all articles found and kept for analysis after the search, nearly half of them compared the traits of invasive and native species. This is probably due to studies aiming to find competition results between the species, which could be very useful for strategies in invasion management actions (Richter & Gross, 2013). Also, the biotic resistance hypothesis is very strong in invasion ecology, and some authors question the hypothesis's effectiveness (Fridley et al., 2007;Jeschke et al., 2012). It is well-established that biotic resistance (i.e., the high number of native species preventing invasive species establishment) is effective (Elton, 1958;Beaury et al., 2020). Other than preventing species invasion, studies that confirm cases of biotic resistance are very good for conservation strategies since native biodiversity conservation is encouraged.
In the 31 articles that compared invasive and native macrophyte traits, the most interesting differences between native and invasive traits can be seen in morphological, biotic interaction, and response traits. Morphological traits and biotic interaction traits of invasives were more related to invasiveness (57.89 and 88.23%, respectively), however, response traits of natives were more often related to invasiveness than invasive traits. This is curious since it is often reported that invasives are tolerant to a wide range of environmental factors, which indeed is the case when we look at all the articles from the search, where biotic interaction traits are related to invasiveness. This could be due to the fact that articles comparing invasives and native macrophyte traits in a given condition could be made aiming to find some biological control or environmental filter for invasives (Mouton et al., 2019). Also, competition studies often use phylogenetically similar species and this could also be responsible for the similar traits with invasive and native species. Overall, almost all groups of traits of invasives were more related to invasiveness, with the exception of response and growth traits. However, the growth traits of natives had a low number of reports, which makes this comparison more difficult.
Here, we reported all functional traits of invasive macrophytes found in a global search and related them with species invasiveness. All group traits of invasive species found in the article's search were related to a high invasiveness rate, with the exception of group trait "ecosystems effects". Ecosystem effects traits were related to invasiveness when species' presence modified the environment in a way that could facilitate their establishment in the new environment. However, only 3 traits were registered in this group category, and therefore it is difficult to state that this group was indeed not related to invasive species invasiveness. In the articles comparing invasive and native macrophyte traits, almost all trait groups of invasives were more related to invasiveness than natives' traits, with the exception of response traits.
We do believe that our work complements the recent work of Hussner et al. (2021), as well as their work complements ours. After all, they related invasiveness traits with macrophytes' life form, which we did not here. On the other hand, our work considered another perspective, with the analysis of a wider range and a different set of traits of macrophyte species. We also performed a quantitative register of species traits as well as their percentage of relation with invasiveness. Furthermore, we also compared all traits of native and invasive species reported in the articles found in the search, aiming to find if invasive traits were more related to their probability of establishment in new environments. Finally, we show species with a higher range of environmental tolerances, as well as the conditions more related to macrophytes invasion success.
Since there was a very high rate of invasive macrophyte traits that were related to their probability of succeeding in new environments, and also invasive traits were more related to invasiveness than native traits, we can say that our review is useful to understand macrophyte invasiveness. That can mean that, although it is difficult to say that there are traits exclusive to invasive macrophytes (likely due to context dependencies), they indeed possess a higher number of invasiveness-related traits in general that aid their establishment in new locations. Although no specific traits can be generalized to all environments, a higher proportion of traits of invasive species related to invasiveness indicate that invasive species present 'novel weapons' (see Callaway & Ridenour, 2004) to deal with ecological trade-offs, explaining their invasion successes. Also, we found that of all the groups of traits analyzed, the traits related to growth and reproduction are more associated with invasive success. In that matter, we hope that our review can be used as a baseline for aquatic management strategies since it can be used to elucidate how macrophyte traits can predict invasiveness.