Research Article |
Corresponding author: Emanuele Pelella ( emanuele.pelella@uniroma3.it ) Academic editor: Carla Lambertini
© 2024 Emanuele Pelella, Flaminia Mariani, Beatrice Questino, Simona Ceschin.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Pelella E, Mariani F, Questino B, Ceschin S (2024) Environmental conditions influencing the early colonization stage of Ludwigia hexapetala, an aquatic plant recently invasive in Italy. Aquatic Invasions 19(2): 137-152. https://doi.org/10.3391/ai.2024.19.2.117212
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Freshwater ecosystems are among the most susceptible to biological invasions. The South American Ludwigia hexapetala is an aquatic plant that is becoming an increasing threat in many European waterbodies, recently including Italy. This study aimed to define the main parameters influencing the early colonization stage of L. hexapetala by overlapping the percentage cover of this species with environmental parameter data collected at 24 aquatic sites from six waterbodies in north-central Italy. At each site, chemical and physical characteristics of the water (temperature, pH, dissolved oxygen, conductivity, nitrates, phosphates, ammonia, depth, transparency), grain size of the substrate and level of anthropogenic disturbance were evaluated. The results showed that although L. hexapetala prefers shallow, warm, alkaline, moderately rich in ions and nutrients (especially phosphates) and oxygen-poor waters, it can grow in a wide range of environmental conditions. Moreover, as a typical invasive alien species, it spreads opportunistically in disturbed, unstable sites. Thus, L. hexapetala can invade freshwater habitats with different environmental conditions and subjected to anthropogenic disturbance. However, the results suggest that water depth may be a limiting factor in the early colonization stage of this species, which does not seem to be able to colonise waters deeper than 1 m in investigated sites, while it has been observed in significantly deeper waters in other European countries with a longer invasion history. Detecting the environmental parameters that most influence the growth of L. hexapetala becomes crucial both to identify the sites most at-risk of invasion in which to initiate timely monitoring actions for the species, and to be able to develop better management and control actions for this alien species in sites that have already been invaded.
aquatic invasion, autoecology, biological pollution, freshwater ecosystem, invasive alien species, non-native macrophyte, water primrose
Biological invasions, meaning colonization of alien species to areas outside of their home range, are a major cause of biodiversity loss and functional and structural alteration of ecosystems worldwide (
Invasive alien plants are adaptable species, with high reproductive capacity, that often outcompete native species, causing deterioration of local biodiversity and alteration of plant communities in colonized habitats (
A highly invasive aquatic plant in several European countries, recently including Italy, is Ludwigia hexapetala (Hook. & Arn.) Zardini, H.Y. Gu and P.H. Raven (Onagraceae), which is native to South America (
The aim of the present study has been to define the main environmental parameters that influence the colonization and abundance of L. hexapetala by collecting field data in aquatic habitats in north-central Italy recently invaded by this alien species. Particular attention was paid to the environmental characteristics of aquatic habitats, which represent the first “front” of invasion of this species, in the early colonization stage of a new area. Detecting the environmental parameters that most influence the growth of L. hexapetala becomes crucial, both to identify the sites most at-risk of invasion in which to initiate timely monitoring actions for the species, and to be able to develop better management and control actions for the alien species in sites that have already been invaded.
A total of 24 relevés were performed (one in each sampling site) in six waterbodies in north-central Italy (Fig.
The study was performed between late June and early September 2022, the most favourable months for the growth of L. hexapetala (
To identify the quantitative and qualitative parameters most influencing the distribution of L. hexapetala, environmental data collected in the field were overlapped with percentage of species cover. This was done by analysing quantitative parameters (temperature, pH, dissolved oxygen, conductivity, ammonia, nitrates, phosphates, nitrate/phosphate ratio, depth) separately from qualitative environmental parameters (water transparency, substrate grain size, site disturbance level), which were evaluated using nominal categories. To investigate the relationship between abundance of the alien species and environmental conditions as a whole, ordination analysis was performed on environmental data. In order to do so, assumptions of linearity were checked, and an unconstrained linear method (Principal Component Analysis, PCA) was chosen. All variables were standardized setting scale = TRUE in the “rda” function while performing the ordination. Subsequently, L. hexapetala cover was considered as a continuous response variable and it was fit over the unconstrained ordination results using a post-hoc test (envfit) to find out if there was a correlation with the PCA axes, in order to identify which of the environmental parameters most influenced the alien species distribution. The post-hoc test was performed using the “envfit” function from package vegan (
As for the quantitative chemical and physical water data, the first two PCA axes were chosen, together explaining over 55% of the total variance (Fig.
Ordination plot of quantitative environmental parameters. Each dot represents a sampled site. The size of the dots increases with L. hexapetala cover, considered as a continuous variable; reference measurements are provided in the legend. The colour of the dots also gets darker with increasing L. hexapetala % cover. Black crosses represent uninvaded sites where L. hexapetala cover = 0. The black arrow shows the post-hoc analysis result, indicating the direction in which L. hexapetala cover increases. The blue arrows indicate the direction of each environmental parameter. Acronyms: C = conductivity; D = depth; DO = dissolved oxygen; pH = pH value; A = ammonia; N = nitrates; P = phosphates; N:P = N:P ratio; T = temperature; N:P = N:P ratio.
In ordination analysis of environmental qualitative parameters, the first two axes explained about 50% of the total variance (Fig.
Ordination plot of qualitative environmental parameters. Each dot represents a sampled site. The size of the dots increases with L. hexapetala cover, considered as a continuous variable; reference measurements are provided in the legend. The colour of the dots also gets darker with increasing L. hexapetala % cover. Black crosses represent uninvaded sites where L. hexapetala cover = 0. The black arrow shows the post-hoc analysis result, indicating the direction in which L. hexapetala cover increases. The blue arrows indicate the direction of each environmental parameter. Acronyms: G = grain size; T = transparency; D = site disturbance level. For the category explanation of transparency and site disturbance level, see Suppl. material
Water chemical and physical data collected in all sampled sites (see Suppl. material
Variations in chemical and physical water parameters in all sampled sites. Boxplots show the median (line across the box), upper and lower quartiles (the upper and lower parts of the box), and values outside the quartiles (the whiskers). The dots represent each sampled site and their size increases proportionally with the percentage cover of L. hexapetala; reference measurements are provided in the legend. The colour of the dots also gets darker with increasing cover of the alien species. Red asterisks show uninvaded sites where L. hexapetala cover = 0.
The analysis of variance (ANOVA) on environmental qualitative parameters pointed out that L. hexapetala cover was not significantly correlated with water transparency (p > 0.05), since the species showed the same cover values in both transparent and turbid waters (Fig.
We found that certain environmental conditions influence the growth of L. hexapetala, with shallow and poorly oxygenated waters favouring high abundance of the species. In particular, at investigated sites, L. hexapetala showed a marked preference for shallow waters (i.e., less than 50 cm depth) where its aquatic morphotype, characterized by short vertical stems, can better root on the bottom and emerge at the surface. However, established populations of the species have been found in its native range (
Taking the other water parameters into consideration, in the study area L. hexapetala showed a tendency to prefer alkaline and moderately ion-rich waters, although it has been found in waters with a wide range of conductivity. It should be noted that the positive relationship between L. hexapetala cover and water conductivity could also be a consequence of the presence of the alien species rather than a driving factor, since L. hexapetala has been found to increase water conductivity (
According to our data, L. hexapetala grows mainly in sites with a substrate characterized by fine grain size (i.e., silt and sand), which allows for better rooting of the aquatic morphotype on the substrate. In addition, the alien species cover did not vary significantly in different water transparency conditions, underlining its tolerance for a wide variety of conditions. Furthermore, it is noteworthy that the percentage cover of this species increased significantly in sites with a high anthropogenic disturbance level. This supports the idea that L. hexapetala, as a typical invasive alien plant, can successfully and opportunistically colonise disturbed, altered, and unstable sites, which are known in literature to be more susceptible to biological invasions (
Based on the results that emerged from this field study, pioneer L. hexapetala populations were able to grow in different environmental conditions. This wide ecological breadth is later confirmed in established populations, when L. hexapetala can produce extensive stands in both oxygenated and poorly oxygenated waters, in clear and turbid waters, in light and shaded conditions, in oligo-mesotrophic and eutrophic waters, although its growth increases with nutrient enrichment regardless of light regime (Hussner et al. 2010;
The authors have no specific funding to report.
Conceptualization: E.P., S.C.; Y; Methodology: E.P., S.C.; Formal analysis: E.P.; Investigation: E.P., F.M., B.Q., S.C.; Resources: S.C.; Data Curation E.P., B.Q.; Writing - Original draft E.P., B.Q; Writing - Review and Editing E.P., F.M., S.C.; Visualization E.P., S.C.; Supervision: S.C.; Project administration: S.C.; Funding Acquisition: S.C.
The authors thank M. Carboni and N.T.W. Ellwood for their help with statistical analyses and B. Luzi for her help during field data collection. We greatly appreciate the valuable comments of the independent reviewers and editors that improved our article. The authors also acknowledge the support of NBFC to Department of Sci-ence/University of Roma Tre, funded by the Italian Ministry of University and Research, PNRR, Missione 4 Componente 2, “Dalla Ricerca all’Impresa”, Investimento 1.4, Project CN00000033. We also thank the Department of Science - University of Roma Tre for providing the necessary funding for the article processing charge.
List of sampled sites with coordinates
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Category-explanation tables for water transparency (a) and disturbance level (b)
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Loadings from PCA regarding quantitative environmental data
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Loadings from PCA regarding qualitative environmental data
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Environmental parameters in invaded (a) and uninvaded (b) sites
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