Potamopyrgus antipodarum (Gray, 1843) has been considered one of the most successful and widespread invasive freshwater molluscs worldwide. This snail has been present in mainland Portugal since the 1960s, but there is only scarce information regarding its presence in Madeira or Azores archipelagos. In this work, macroinvertebrate communities were sampled in six Madeira Island streams. P. antipodarum was found in all sampled sites, with abundances ranging from 3 to 3528 individuals per sampling area, indicating that the species arrived at Madeira recently and is now in a clear spreading phase. Our results indicate no relationship between the abundance of P. antipodarum and macroinvertebrates’ biodiversity indices. Nevertheless, the abundance of P. antipodarum was extremely high in some locations, representing more than 30% of all benthic invertebrates, which triggered alterations in the food web and community structure. The future impacts of this invasion are for now unpredictable, but given P. antipodarum parthenogenetic reproduction and fast spread, its presence can potentially affect Madeira Island’s aquatic ecosystems’ ecological status and services. These results can inform environmental agencies on the importance of monitoring the presence and spread of this invasive species and call for management strategies focused on early detection and control measures. Given the endemicity and insularity of Madeira macroinvertebrate communities, the spread of P. antipodarum should be limited to mitigate its ecological impacts and biodiversity loss.

Freshwater macroinvertebrates, Invasion ecology, Island ecosystems, New Zealand mud snail

Invasive species are a significant component of human-caused global environmental change (Brook et al. 2008), and freshwater invasions are of particular concern since freshwater ecosystems are among the most diverse and endangered in the world (Dudgeon and Strayer 2024). Alien invasive species are a major driving force behind species extinctions (Pyšek et al. 2020) and a major threat to ecosystem services, biodiversity and genetic diversity of native species (Higgins and Zanden 2010). Becoming invasive is a complex process, and the impacts of alien species on native communities will depend on the characteristics of the invaded community and environmental conditions (Gherardi 2007). As climatic, hydrological, and other environmental conditions will vary across regions, the success and impacts of potentially invasive species are also likely to diverge across regions (Alonso and Castro-Díez 2008). Moreover, global trading and transport have led to a collapse of biogeographical barriers, and human-aided transport has increased the frequency of introduction events of alien species without precedent (Capinha et al. 2017; Moser et al. 2018; Pyšek et al. 2010). In islands, given their isolation, fewer intentional and accidental alien arrivals (i.e., lower number of arriving and colonization pressures) are to be expected and consequently, lower colonization rates are usually registered. Nevertheless, human activities drastically changed the invasive patterns and the invasion rate in island ecosystems. The number of arrivals is now dependent more on the size of the island economy than on the distance to mainland (Deidda 2016; Russell and Kueffer 2019). Also, isolated island native species usually exhibit reduced phylogenetic diversity, which can make them more vulnerable to biological invasion (Weigelt et al. 2015). Consequently, geographical isolation of remote oceanic islands can often increase the successful establishment of alien species and the impacts caused by the colonization of alien species in island ecosystems (Fridley and Sax 2014; Moser et al. 2018).

The New Zealand mud snail, Potamopyrgus antipodarum (Gray, 1843) (hereafter NZMS), is an ovoviviparous and parthenogenic freshwater Tateidae snail native to New Zealand (Alonso et al. 2019). NZMS is primarily a scraper/grazer species that feeds mostly on diatoms, but also organic detritus and macrophytes (James et al. 2000). NZMS possesses internal gills for aquatic respiration and an operculum to seal itself tightly into the shell (Richards et al. 2001). This snail species can survive unfavorable conditions such as high salinity levels, desiccation, temperature variations and predation, including surviving many fishes’ digestive systems (Bennett et al. 2015; Haynes et al. 1985). This species also exhibits a high capacity to establish itself in human-altered water bodies and streams that experience continuous pressure from anthropogenic activities (Vermonden et al. 2010; Spyra et al. 2015). Invasive populations of NZMS are composed entirely of parthenogenetic females (Butkus et al. 2012), allowing the species to colonize rapidly and extend to new ecological niches (Poirier 2013), reaching incredibly high densities (more than 500 000 individuals per m²) (Hall et al. 2006).

In the last century, NZMS has invaded temperate and subtropical aquatic ecosystems worldwide (Ponder 1988; Naser and Son 2009; Collado 2014; Taybi et al. 2021; Geist et al. 2022). The presence of NZMS in mainland Portugal has been reported since 1960 (Heuss 1961). Still, there is limited information on Portuguese island ecosystems: in Madeira Island, the species was recently reported (Órfão et al. 2024), and in the Azores archipelago, its presence was not yet confirmed. Faunal surveys and monitoring programs conducted on Madeira until 2015 did not find evidence of the occurrence of NZMS (Hughes 2003; Borges et al. 2008; Raposeiro et al. 2009; Raposeiro et al. 2022) suggesting a recent invasion whose impacts on native communities remain unknown.

As in other oceanic islands, species richness in Madeira’s streams is low, in comparison with mainland lotic ecosystems (Malmqvist 1988; Raposeiro et al. 2012). Freshwater benthic invertebrate communities are characterized by low diversity, with some taxonomic groups absent (e.g., Plecoptera and Amphipoda) (Raposeiro et al. 2022) and many families are usually represented by few genera with a few or even single species (Stauder 1991). Insect orders dominate benthic communities, particularly Diptera (Hughes 2005) and non-insect orders are poorly represented and tend to be introduced by more passive dispersal methods such as attachment to birds or ships (Zaranko et al. 1997; Hughes 2005). On the other hand, Madeira Island freshwater habitats harbour many endemic taxa (circa 30% of the species, including Trichoptera, Coleoptera, Hydracarina and some Dipteran families), most of them being restricted to areas of indigenous vegetation located in the upper part of the rivers (>500 m above sea level) (Hughes et al. 1998; Hughes 2003). Generally, island native communities around the globe are suffering significant negative impacts caused by invasive species, primarily leading to the loss of biodiversity but also impacting community composition and ecosystem services (Russell et al. 2017).

Given the invasive potential of NZMS and the inherent conservation importance of Madeira’s freshwaters, the aim of this study was to investigate the distribution and possible impacts of this invasive snail species by correlating its occurrence with macroinvertebrate native community composition and structure.

The streams sampled were characterized by low and medium water velocities, low depth (between less than 0.25 and 0.5 meters), temperature varying between 13 and 21 °C, conductivity ranging between 63 and 226 μs/cm, dissolved oxygen between 7.5 and 10 mg/L O₂ and pH between 7.2 and 8.2 (Suppl. material 1: table S1). NZMS was found in all streams and in 9 of the 13 sampling sites, namely in Socorridos upstream, Brava upstream and downstream, São Vicente downstream, Tábua upstream and downstream, Faial downstream and Janela midstream and upstream. The abundance of NZMS per stream ranged from 3 to 3528 organisms per sample (Table 1). No relationship was found between the abundance of NZMS and the measured abiotic parameters (temperature, dissolved oxygen, conductivity or pH, Suppl. material 1: fig. S1).

Looking at community descriptors of all sampled streams, Community Richness ranged between 11 and 21 taxa; Abundance in each sampled area ranged between 202 and 6295 organisms; Shannon-Wiener’s diversity ranged between 0.82 and 1.64 (Table 1); Pielou’s species evenness index ranged between 0.29 and 0.61; and Simpson’s specific dominance index ranged between 0.39 and 0.73 (Suppl. material 1: table S2). The correlation between NZMS abundance and species richness showed an unexpected and moderate positive correlation, showing that where biodiversity is greatest is where NZMS abundance is higher (R² = 0.36; Figure 2a). Regarding the other community descriptors, no correlation was found between the abundance of NZMS and Shannon-Wienner diversity index (R² = 0.11), Simpson’s diversity (R² = 0.09) and Pielou’s species evenness (R² = 0,009), showing that the structure of the macroinvertebrate communities was not explained by variation in NZMS abundance values (Figure 2). Regarding the percentage of EPT taxa in the benthic communities, it was very low in all sampled sites, as expected, ranging between 0,08 and 0,78%. No correlation with the abundance of NZMS was found (R² = 0,026, Figure 3).

Figure 2. 

Pearson Correlation between NZMS abundance and community descriptors: Species richness (a), Pielou’s evenness (b), Shannon-Winner diversity index (c) and Simpson’s diversity index (d). NZMS abundances went through a logarithmic transformation. 95% confidence bands of the best-fit line are represented with dashed lines.

Figure 3. 

Pearson Correlation between the NZMS abundance and the ratio of EPT taxa in the community. NZMS abundances went through a logarithmic transformation. 95% confidence bands of the best-fit line are represented with dashed lines.

According to the MBS II index, the biotic index for Madeira Island, we observed that the sampled streams of Madeira Island varied between sites classified as Poor and Good. Streams of good quality typically were found in forested areas, while streams of poor quality are mainly associated with urban and rural areas where human activity is more prevalent (see Table 1). No correlation was found between the NZMS abundances and the Madeira biotic index, meaning that the presence of the invasive species seems to be independent of the stream’s ecological status (Figure 4). Regarding feeding modalities, Madeira macroinvertebrate communities are characterized by being almost entirely composed of filterer and scraper/grazer species, with a low number of predators, fine sediment feeders/collectors and shredders (Suppl. material 1: table S3). As the abundance of NZMS (that presents a predominantly grazer feeding behavior) increases in a community, the relative abundances of other scrapers/grazers and filter feeders decrease. In sampling sites without or with low abundance of NZMS other grazers constituted, on average, around 60% of the total abundance, while in communities where NZMS was abundant, the ratio of scrapers/grazers dropped to less than 50%. For instance, at Tábua downstream and Janela midstream, where the highest abundance of NZMS was recorded, the percentage of other grazers was only 33 and 28%, respectively (Figure 5).

Figure 4. 

Pearson Correlation NZMS abundance and MBS II index (Madeiran Biotic Score II). NZMS abundances went through a logarithmic transformation. 95% confidence bands of the best-fit line are represented with dashed lines.

Figure 5. 

Graphical representation of the percentage of macroinvertebrate feeding group modalities (shredder, scraper/grazer, predator, piecer, fine sediments, filterer) for all sampling sites. Sites are ordered from left to right by the abundance of NZMS. The circles next to the streams names indicates Madeiran Biotic Score II classification: Poor (yellow), Fair (green), Good (blue).

The current study revealed the occurrence of the highly invasive New Zealand mud snail in different catchments in Madeira Island. Freshwater alien snail species are often introduced inadvertently by fish-farming and commercial trade of aquaculture products, attached to fish or aquatic plants, or because of human and cargo transportation, for example, within ballast waters (Zaranko et al. 1997; Cowie 2000). According to Raposeiro et al. (2022) NZMS was not present in Madeira Island until 2015, and the first records of the species date from 2017 (Órfão et al. 2024). Our results thus show that within the last seven years NZMS has become common in freshwater systems across the island, and illustrate the incredible invasive potential of this species, supported by its fast and asexual reproduction, broad diet and lack of predators on the island. According to Órfão et al. (2024), NZMS arrival may be related to the introduction of Rainbow trout (Oncorhynchus mykiss) in Madeira for recreational fishing purposes. Previous data supports the hypothesis of the Rainbow trout as a vector since it can transport alive snails in its digestive tract (Vinson and Baker 2008). Nevertheless, further investigation is needed to clarify the origin of the invasion. Like most invasive species, NZMS can probably be accidentally transported between basins within the island through natural vectors, such as birds, or by human activities, such as hiking or canyoning.

The abundance of NZMS in Madeira streams is not related to any of the measured abiotic parameters (temperature, dissolved oxygen, conductivity or pH), which contrasts with previous findings for this species. Research indicates that low conductivity levels can limit the growth and survival of NZMS (Herbst et al. 2008; Larson et al. 2020). Additionally, field studies suggest that NZMS is more likely to occur in water bodies with low velocity and pH levels, along with high conductivity (Spyra et al. 2015; Larson et al. 2023). Also NZMS showed no preference for specific sections of the stream and it was found at both upstream and downstream sections. NZMS may establish itself in the upper portion of streams, mainly by spreading through vegetation and edges of the faster-flowing waters and then moving into new habitats, particularly unoccupied vegetation habitats (Richards et al. 2001). The dispersion along streams shows that, despite the recent invasion, NZMS is already in a spreading phase. Given its high fecundity and generalist food and habitat requirements, NZMS can achieve extremely high densities in invaded ecosystems and out-compete native species (Kerans et al. 2005; Hall et al. 2006; Riley et al. 2008). Nevertheless, given the short time since the arrival of NZMS on Madeira Island, significant impacts were not expected. Our study found no correlation between the abundance of NZMS and most community descriptors. Instead, native communities’ richness and diversity are related to the ecological status of streams and not to the presence of invasive species (Borges et al. 2008). Also, riparian vegetation and land use adjacent to the streams varied among the different sampling sites: sites with viniculture, or areas susceptible to urban and agricultural runoff showed lower scores on the regional ecology status index and less biodiversity (Hughes and Furse 2001; Hughes 2005). The absence of a relationship between the number of EPT taxa and the abundance of NZMS indicates that there is currently no evidence suggesting that the invasion is impacting the most sensitive species. For instance, Kerans et al. (2005) found a negative correlation between the densities of NZMS and the densities of taxa competing with the snails for food and/or space such as Ephemerellidae and Brachycentridae. Thus, further studies are needed to ascertain the effects of the mud snails on native EPT species such as the mayfly nymphs Baetis sp. or caddisfly larvae such as Tinodes sp. or the endemic Hydropshyche maderensis. Capinha et al. (2020) noted that the presence of exotic species on islands has a stronger influence on community composition than on species richness. The presence of exotic species often increases the number of species locally but also increases homogenization of macroinvertebrate communities due to better dispersion ability of invasive taxa. This trend may predict future changes in Madeiran macroinvertebrate communities, and it is already possible to observe the absence of some Trichoptera species, such as Tinodes or Polycentropus species, in catchments with NZMS. Madeira macroinvertebrate community composition and structure should, therefore, be carefully monitored to determine whether there is a temporal trend towards homogenization.

Regarding feeding group diversity, some differences between heavily invaded and non-invaded sites were recorded. In the highly invaded sites, the proportion between the different feeding groups was altered by the presence of NZMS, especially the significant reduction of the relative abundance of native grazers and filter feeders. Previous studies already demonstrated that NZMS can compete with other grazer species for resources, reducing the biomass of food available for other herbivores (Riley et al. 2008; Moore et al. 2012). In Ribeira da Janela (intermediate), the sampling site where the highest abundance of NZMS was recorded, other detritivores species (such as Asellus sp., relatively common in other sampling sites) are completely absent, suggesting that competition with these species may also be occurring. These results show that changes in the community structure may be happening due to NZMS presence, which can lead to effects for insular freshwater food webs and ecosystem functions. Given the natural low diversity and abundance of Madeira’s benthic macroinvertebrates, the presence and spread of NZMS over time could lead to a decline of the functional integrity of the invaded ecosystems, leading to habitat degradation or modification (Arango et al. 2009; Hall et al. 2006). The alteration of food webs by the presence of NZMS has already been described by several authors, showing that the species can consume most of the primary production, heavily influencing nutrient fluxes (Arango et al. 2009; Hall et al. 2006). The depletion of native grazers can also lead to imbalances in the concentration and distribution of phytoplankton and alterations in periphyton community structure and biomass (Bennett et al. 2015; Grant et al. 2008). NZMS, when present in high densities, can alter algae communities, reducing the abundances of medium-sized and large diatoms and several filamentous cyanobacteria and chlorophytes, thus increasing the relative abundances of tough filamentous chlorophytes (Krist and Charles 2012; Bennett et al. 2015).

The biodiversity crisis is nowhere more evident and demanding of attention than on island ecosystems (Djamali 2013). Recent studies show that around 73% of threatened species on the IUCN Red List found only on islands are being threatened by invasive species (Russell and Kueffer 2019). Madeira Island is no exception since endemic species are usually limited to restricted habitats and are often already under serious threat due to agropastoral activities, urbanization, and biological invasions (Hughes 2003). The total number of invasive species on Madeira is nearly 400 (Região Autónoma da Madeira - Assembleia Legislativa 2023), while endemic species and subspecies from the Madeira archipelago are about 1,419 (1,286 species and 182 subspecies), representing 19% of the overall species richness (Borges et al. 2008). Invertebrates are the most diverse group within endemic taxa (210 Mollusca and 979 Arthropoda) with 71% of all mollusks endemic (Borges et al. 2008). The Laurisilva forest within Madeira Natural Park is an outstanding relict of a previously widespread laurel forest. It is the largest area of laurel forest, with an estimated 90% primary forest containing a unique suite of plants and animals, including a high proportion of endemic species (UNESCO 2009). Janela Stream is the main watercourse within the protected area harboring Laurisilva forest, but it was also the stream where the highest NZMS abundance was recorded. Given the importance of the area for conservation and its inherent fragility, this location is of concern and where significant impacts can be expected. To better monitor and evaluate possible impacts of the invasion by NZMS, it would be advisable to assess its influence on ecosystem functions, such as primary production and organic matter degradation and also to evaluate the association between the presence and abundance of NZMS and water quality parameters. In addition, a genetic characterization of the Madeiran NZMS populations may clarify possible invasion pathways, as well as the origin of the population(s) now present in the island. In addition, public awareness about the topic of biological invasions needs to be raised to prevent the risk of NZMS spreading or new invasions.

A recent and fast colonization of NZMS has been observed in Madeira Island streams. Some potential effects on native ecosystems were identified, namely possible shifts in food webs promoted by the presence of NZMS and possible depletion of native shredders and grazers. Our results also show that catchments located in Laurisilva forest, representing the top conservation sites of Madeira Island, are at a greater risk since they have high NZMS abundances. Attention should be directed at studying the possible effects of NZMS on native grazer and detritivorous species, but also other taxonomic groups, especially in algae and fish communities. Also, and since the spread and impacts of NZMS may be facilitated by different factors related to human activities, more in-depth studies are needed to clarify the role of each type of disturbance on these benthic communities and evaluate the impacts on ecosystem services. In addition, climate change is happening at an unexpected pace; therefore, studying the relationship between invasive success and climate variables, especially within the island context, should also be pursued. As far as is known, NZMS is not present in the rest of the islands in the Macaronesia group. Since the eradication of this species is challenging, the further spread of NZMS must be minimized, and prevention of future invasions is a top priority. For that, quarantine regulations could be applied to sterilize equipment used in freshwater-related activities. Such measures can be far more cost-effective than attempting to address the problems once the snails have invaded new streams. A crucial aspect involves raising public awareness to the intrinsic value of island freshwaters. Success in the long-term conservation of Madeira’s unique biodiversity including freshwater macroinvertebrates can only be achieved through public cooperation and understanding of the potential ecological problems caused by invasive species.

MD: Sample processing, data analysis and interpretation, writing - original draft and writing - review & editing. JP: research conceptualization, data interpretation, writing - review & editing. LM: research conceptualization, funding provision, sample design and methodology, data interpretation, writing - review & editing.

The present work was supported by the project GoBig - GlObal change and BioloGical Invasions: Potamopyrgus antipodarum as a case study (https://doi.org/10.54499/PTDC/CTA-AMB/3702/2020), financed by FCT through Portuguese national funding.

We also acknowledge financial support to CESAM by FCT/MCTES (UIDP/50017/2020+UIDB/50017/2020+LA/P/0094/2020), through national funds.

The authors would like to thank António Serafim for the sampling carried out in Madeira Island and Sofia Ramalho, for her help preparing the map.

We thank the anonymous reviewers for their comments and suggestions.