Diversity, Environmental Drivers, and Niche Overlap of Native and Invasive Gastropods in Southern Iraq’s Freshwater Ecosystems

Abstract

This study assesses the diversity, distribution, and ecological interactions of freshwater gastropod communities across eight sites in southern Iraq, spanning marshes, rivers, and canal systems within the Tigris-Euphrates basin. Diversity indices (Shannon–Wiener H′ and Pielou’s evenness J) were calculated to evaluate community structure, and a revised stacked bar chart of relative abundances revealed widespread dominance by species such as Melanoides tuberculata and Physella acuta, which frequently exceeded 50% of local assemblages. While P. acuta is an established invasive species, M. tuberculata is now considered native or long-established in the region. Species interactions were examined using Pianka’s niche overlap index, and null model testing (999 permutations) revealed only a few statistically significant overlaps (p < 0.05), suggesting that species co-occurrence is shaped more by environmental filtering than direct competition. To directly examine the influence of environmental drivers on species composition, Multiple Factor Analysis (MFA) was performed. MFA revealed patterns of association between dissolved oxygen, salinity, and species assemblages, suggesting these abiotic factors may influence community structure. To our knowledge, this study is the first in Iraq to use null models, constrained ordination, and MFA to investigate community assembly of freshwater gastropods, ultimately producing novel insights regarding the interactions between environmental stressors and aquatic biodiversity patterns. The results of this study highlight the need for long-term ecological monitoring and conservation in marshland habitats important for the resiliency of native species.

1. Introduction

Freshwater gastropods play a crucial role within aquatic ecosystems; they participate in nutrient cycling, periphyton regulation, and energy transfer through food webs. Their functional diversity, ranging from detritivores and algivores to forage sources for invertebrates and vertebrates, demonstrates their integral ecological roles and their contributions to freshwater trophic dynamics [1,2]. While gastropods are taxonomically diverse and habitat-generalized at a global scale, many species exhibit habitat-specific responses to environmental variables such as salinity, oxygen, and substrate composition [3,4,5,6].

Globally, invasive gastropods have emerged as a major ecological concern in freshwater systems [7]. Among the most impactful is Physella acuta (Draparnaud, 1805), a North American species that has become widespread in lentic habitats globally, particularly in eutrophic or organically polluted waters [8]. Its rapid reproduction, generalist feeding, and tolerance to environmental stressors have contributed to its global expansion [9,10].

In contrast, Melanoides tuberculata (O. F. Müller, 1774), historically viewed as introduced in some regions, has been increasingly recognized as a native or long-established species in parts of the Middle East, including Iraq [11,12]. Though it possesses traits often associated with invasive potential, such as parthenogenetic reproduction and wide eco-physiological tolerance [13,14], its consistent presence in faunal assemblages across Iraq suggests it is ecologically integrated and not a recent arrival. Some earlier faunal assessments may have underreported this species due to methodological limitations or regional sampling gaps [15].

The Tigris-Euphrates basin is one of the most ecologically significant freshwater systems in the Middle East, supporting diverse molluscan assemblages across its rivers, marshes, and artificial canal systems [16]. However, anthropogenic pressures such as dam construction, salinization, nutrient pollution, and species introductions have altered these aquatic habitats dramatically [17,18,19,20,21]. Similar invasion-driven declines have already been reported among unionid mussels in the region, where Sinanodonta woodiana (I. Lea, 1834) has displaced the endemic Anodonta vescoiana (Bourguignat, 1856) across southern Iraq [21]. Despite the urgency of these changes, few quantitative assessments have evaluated how native and invasive gastropods interact across environmental gradients or what drives species coexistence in these dynamic systems.

With biological invasions continuing globally, it remains critical to differentiate the roles of environmental filtering versus biotic interactions such as competition or coexistence in shaping freshwater gastropod assemblages [22]. Recent taxonomic studies, e.g., [12,23,24] have expanded the regional checklist of gastropods in Iraq, but ecological analyses remain limited.

This study addresses that gap by combining niche-based interaction models (Pianka’s niche overlap index and null models) with multivariate ordination techniques (MFA) to assess community structure, niche overlap, and environmental correlates of gastropod distributions in southern Iraq. While such frameworks have been widely applied in Europe, Asia, and the Americas [24,25,26,27], this is the first application in Iraq’s Mesopotamian wetlands. Importantly, the integration of deterministic models (e.g., RDA) with stochastic null models [28,29] enables a robust evaluation of how gastropod communities respond to both environmental stressors and biotic interactions.

By doing so, this study offers new ecological insights into the drivers of community assembly in Iraq’s freshwater ecosystems and contributes a quantitative baseline to guide conservation planning and long-term biodiversity monitoring in one of the world’s most ecologically vulnerable wetland systems.

2. Materials and Methods

2.1. Study Area

This study was conducted in 8 sites throughout southern Iraq with a variety of freshwater types, including marshes and rivers (Figure 1). The chosen sites cover a wide spectrum of ecological conditions (lentic and lotic as well) and human impact. The study site is the lower Tigris-Euphrates basin, an ecologically distinguished area as a rich zone for aquatic biodiversity [19]. The arid to semi-arid southern Iraq has hot summers, warm winters, and low annual rainfall, all of which greatly affect hydrological cycles and the dynamics of freshwater ecosystems.

2.2. Gastropod Collection and Sampling Design

Gastropods were sampled in spring (April 2024) to match the time of highest activity and fecundity. The latter was conducted according to a standard transect-based methodology at each site. Three replicate quadrats (1 m²) were randomly placed in typical microhabitats (e.g., submerged vegetation, muddy banks, rocky substrate), and snails were manually collected by hand after a careful visual inspection of substrates, aquatic plants, shallow sediments. Collected specimens were placed in labeled plastic containers, transported to the laboratory, and preserved in 70% ethanol for further examination.

2.3. Identification and Taxonomy

Gastropods were identified to species level using morphological characteristics, including shell shape, aperture, coloration, and ornamentation. Identification was carried out using regional keys and taxonomic guides, particularly those provided by [23,24,30,31,32,33,34,35].

2.4. Data Analysis

2.4.1. Diversity and Evenness

The Shannon-Wiener diversity index (H′) was calculated to assess species diversity at each site the formula based on [36]:

$$H^{\prime }=-{\sum }_{i=1}^{s}pi\mathrm{l}\mathrm{n}\left(\mathrm{p}\mathrm{i}\right)$$

(1)

$$pi={\displaystyle \frac{\mathrm{n}\mathrm{i}}{N}}$$

(2)

Here, pi corresponds to the proportion of individuals (ni) belonging to species i from the total number of individuals found in the sample (N). Then, S corresponds to the total number of species found.

Pielou’s evenness index (J′) was computed to assess the uniformity of species distribution based on [37]:

$$J^{\prime } = {\displaystyle \frac{H^{\prime }}{InS}}$$

(3)

where the Shannon-Wiener diversity index (H′) is divided by the natural logarithm of species richness (lnS).

All diversity and evenness indices were calculated in R (version 4.5.0) using base functions and the vegan package.

2.4.2. Relative Abundance

Relative abundance (%) was calculated for each species at each site by dividing the number of individuals of a species by the total number of gastropods collected at that site, multiplied by 100. This provided a measure of dominance and frequency of occurrence.

2.4.3. Niche Overlap

Pianka’s niche overlap index [38] was used to quantify the degree of overlap in habitat use, specifically, site co-occurrence patterns between pairs of gastropod species. The index is based on the proportional use of shared resources (in this case, sampling sites), and is calculated as:

$$Ojk= {\displaystyle \frac{{\sum }_{i=1}^{n}pij\cdot pik}{\sqrt{{\sum }_{i=1}^{n}p2ij\cdot {\sum }_{i=1}^{n}p2ik}}}$$

(4)

where p_ij and p_ik represent the relative abundances of species j and k at site i, normalized to the total abundance across all sites for each species. In this study, resource use is interpreted as the proportional abundance of each species at each site, reflecting the degree to which species utilize the same habitats.

The index ranges from 0 (no overlap) to 1 (complete overlap), where higher values indicate that two species tend to co-occur at the same sites and use similar habitats. In contrast, lower values suggest segregation or distinct habitat preferences.

Niche overlap was calculated using the niche.overlap function from the spaa R package. To test the significance of observed overlap values, a null model analysis was performed using 999 permutations of the species-by-site matrix, randomly shuffling species occurrences to generate expected overlap distributions under the assumption of no structured habitat use.

2.4.4. Niche Overlap Significance Testing

To assess whether the observed niche overlap values between gastropod species were greater than expected by chance, we applied a null model analysis using Pianka’s index of niche overlap [38]. Observed overlaps were calculated based on the relative abundance of species across the eight sampling sites. Each pairwise comparison was then tested against a null distribution generated through 999 random permutations of species abundances across sites.

For each species pair, we independently randomized the relative abundance vectors and recalculated Pianka’s index for each permutation. The resulting null distribution was used to estimate a p-value by calculating the proportion of randomized values that were greater than or equal to the observed value:

$$p={\displaystyle \frac{Number of null values \ge observed value}{999}}$$

(5)

p-values lower than 0.05 were considered statistically significant, indicating that the observed overlap between two species was unlikely to be due to random chance.

2.4.5. Environmental Data Collection and Analysis

At each sampling station, determination of some environmental factors was made with a multi-parameter probe (U-10 water quality checker; Horiba, Kyoto, Japan). These parameters were the water temperature (°C), pH, dissolved oxygen (DO; mg/L) and salinity (ppt). These variables were selected due to their known influence on gastropod physiology, reproduction, and species distribution patterns in freshwater ecosystems [4].

To explore the shared structure between gastropod species abundances and environmental variables, we performed a Multiple Factor Analysis (MFA) using the FactoMineR package in R version 4.5.0 [39], within the RStudio environment (version 2025.05.0 Build 496) [40]. This multivariate technique allows joint ordination of multiple blocks of variables measured on the same sampling units, in this case, species abundances and environmental gradients across sites. The species matrix (15 species) and environmental matrix (temperature, pH, dissolved oxygen, salinity) were treated as separate groups in the analysis. MFA outputs included a joint biplot of species and environmental vectors, enabling interpretation of co-variation among blocks and ordination of sites based on their shared structure.

3. Results

3.1. Species and Taxonomic Composition

A total of 15 freshwater gastropod species were reported for southern Iraq, indicating the rich fauna of the lower Tigris-Euphrates basin as well as the mixing impact among Palearctic, Afro-Asian, and Mesopotamian zoogeographical elements. The vast majority of species were native, such as Melanoides tuberculata, Gyraulus huwaizahensis, Gyraulus convexiusculus, Lymnaea auricularia, Theodoxus jordani, Neritina violacea, Bithynia hareerensis and various Melanopsis spp., which have been part of the regional fauna for a long time [12,16]. Five alien species (Ferrissia fragilis, Ecrobia grimmi, Filopaludina bengalensis, Physella acuta and Potamopyrgus antipodarum) were recognized; non-indigenous taxa tend to display larger distributions and higher dominance than do indigenous species [12]. Indigenous gastropods usually exhibit more niche-specific distributions, and introduced species often tend to dominate several habitats through their plasticity [12]. The freshwater molluscan fauna in the Mesopotamian area is threatened by invasive species and environmental perturbations caused by water abstraction and habitat alteration [12]. This new taxonomic context is, however, a basis for investigating diversity patterns and ecological interactions in the region’s freshwater environments [12,16].

3.2. Diversity and Evenness

The Shannon-Wiener index (H′) values indicated moderate gastropod diversity throughout the study area, ranging from 1.196642 (Gharraf River) to 1.726206 (Auda marsh). Diversity values were also high at Ali Al-Garbi (1.706264) and Al-Majer Al-Kaber (1.631852). Gastropod species richness (S) varied from 5 species at Gharraf River to 9 species at Auda marsh and Al-Majer Al-Kaber, which may indicate differences in habitat complexity or ecological suitability for gastropods. Species evenness (J) ranged from 0.743515 at Gharraf River to 0.820539 at Ali Al-Garbi (

Table 1. Shannon-Wiener diversity index (H′), species richness (S), and equitability evenness (J) of gastropod communities across eight freshwater sites in southern Iraq.

No	Site	Shannon (H′)	Species_Richness (S)	Evenness (J)
1	Al-Huwaizah marshes	1.565557	8	0.752874
2	Al-Majer Al-Kaber	1.631852	9	0.742688
3	Ali Al-Garbi	1.706264	8	0.820539
4	Auda marsh	1.726206	9	0.78563
5	Gharraf River	1.196642	5	0.743515
6	Nasiriyah	1.682072	8	0.808906
7	Ptera dam	1.562939	7	0.803192
8	Ukaikah	1.704646	7	0.876015

).

3.3. Relative Abundance Patterns

The relative abundance patterns of gastropod species varied substantially across the eight sites (Figure 2). Melanoides tuberculata, a native and ecologically dominant thiarid species, was the most abundant gastropod, contributing up to 56.6% of the total gastropod abundance at Gharraf River and 44.5% at Al-Huwaizah marshes. Physella acuta, an invasive species, was also dominant at several sites, particularly at Auda marsh (46.7%) and Ali Al-Garbi (32.2%).

Other invasive species, such as Filopaludina bengalensis (Lamarck, 1822) and Potamopyrgus antipodarum (J. E. Gray, 1843), occurred in lower proportions, with F. bengalensis reaching 10.3% at Gharraf River and P. antipodarum peaking at 5.1% at Al-Majer Al-Kaber. Native species like Gyraulus huwaizahensis (Glôer & Naser, 2007) and Lymnaea auricularia (Linnaeus, 1758) were generally less abundant, each contributing less than 10% at most sites (Figure 2).

3.4. Spatial Distribution and Niche Overlap

The spatial distribution of gastropod species revealed clear patterns across different freshwater habitats. Melanoides tuberculata was the most widespread species, occurring at all eight sites with high abundance, indicating its broad ecological tolerance and long-established presence across both marshes and river systems. Physella acuta also exhibited a wide distribution and high relative abundance, particularly in lentic environments such as Auda marsh, suggesting a preference for slower-flowing or stagnant waters (Figure 2).

Several species showed more restricted distributions. For example, Ecrobia grimmi (Clessin, 1887) was only found in the Euphrates at Nasiriyah and Gharraf River, while Potamopyrgus antipodarum was detected in both the Tigris and Euphrates and at Auda marsh. The co-occurrence of invasive species such as Physella acuta and Filopaludina bengalensis at several sites suggests potential niche overlap, particularly in resource use and habitat preference (Figure 3).

3.5. Niche Overlap and Statistical Significance

From the 105 species pairs evaluated, only four pairs showed statistically significant overlap (p < 0.05;

Table 2. Gastropod species pairs showing statistically significant niche overlap (p < 0.05) based on Pianka’s index and null model testing (999 permutations). The table reports observed niche overlap values and associated p-values.

Species 1	Species 2	Observed Pianka’s Index	p-Value	Pair	Significance
F. bengalensis	E. grimmi	0.776873	0.045045	F. bengalensis vs. E. grimmi	*
M. costata	M. subtingitana	0.791084	0.043043	M. costata vs. M. subtingitana	*
N. violacea	T. jordani	0.866409	0.021021	N. violacea vs. T. jordani	*
P. acuta	P. antipodarum	0.763579	0.023023	P. acuta vs. P. antipodarum	*

* refers to statistically significant niche overlap (p < 0.05).

), suggesting that most co-occurrence patterns are likely driven by shared habitat use or stochastic processes. While significant overlap may indicate potential ecological interactions, such patterns should be interpreted cautiously, as co-occurrence alone does not confirm biotic interactions.

The highest significant overlap was observed between Neritina violacea (Gmelin, 1791) and Theodoxus jordani (G. B. Sowerby I, 1836) (Pianka’s index = 0.87, p = 0.021), suggesting similar ecological preferences or possible resource sharing. Significant overlap was also found between P. acuta and P. antipodarum (0.76, p = 0.023), Melanopsis costata (Olivier, 1804) and Melanopsis subtingitana (Annandale, 1918) (0.79, p = 0.043), and F. bengalensis and E. grimmi (0.78, p = 0.045). These results highlight a limited number of non-random potential interactions, primarily among species that are Melanopsis costata (Olivier, 1804) and either invasive or habitat generalists.

3.6. Environmental Patterns and Ecological Correlations

Environmental conditions varied notably across the eight freshwater study sites. Temperature ranged from 23.3 °C to 25.0 °C, pH from 7.6 to 8.3, dissolved oxygen (DO) from 7.2 to 8.0 mg/L, and salinity from 0.5 to 2.0 ppt.

MFA jointly analyzed gastropod species composition and environmental data across the eight study sites. The first two dimensions explained 44.2% of the total variance (Dim1 = 26.9%, Dim2 = 17.3%). The MFA biplot (Figure 4) reveals that Physella acuta, M. tuberculata, and P. antipodarum are positioned opposite to the direction of the salinity and DO vectors, suggesting a negative association with these environmental gradients. In contrast, E. grimmi aligns more closely with higher salinity and DO values, while F. bengalensis and T. jordani appear associated with pH and temperature.

4. Discussion

4.1. Diversity and Community Composition

The variability in gastropod diversity between the sampled sites signifies the multitude of interacting environmental factors, habitat heterogeneity, and anthropogenic pressures in the Tigris-Euphrates basin. Our marsh sites (e.g., Auda Marsh and a branch of the Tigris, Al-Majer Al-Kaber) with higher Shannon-Wiener diversity indices and species richness probably benefitted from structural complexity, hydrological stability, or sufficiently low-flow conditions. Marsh ecosystems are unique in their combination of microhabitats, including vegetated zones, submerged subsurfaces, and soft sediments, which provide ecological niches and ecological coexistence for gastropods [41,42]. The vegetated and muddy regions within marshes create favorable Pogonatum (P. alopecuros) food offers, as well as shelter from predation or flow, both factors associated with increased mollusk density and diversity [43,44].

Similar reports exist about habitat complexity in other freshwater systems across the world, where habitat complexity (as above) is consistently positively correlated with mollusk diversity and density [45]. For example, studies in mountain streams and lowland wetlands suggest that habitat heterogeneity and frequencies of substrate types and aquatic vegetation are important drivers of gastropod assemblages [46]. Conversely, habitats experiencing hydrological instability or simplification (e.g., dam-regulated rivers or highly urbanized waterways) support fewer species from simplifications associated with decreased niche availability and greater environmental stress [47].

The Tigris-Euphrates Basin has seen many anthropogenic changes in the past few decades, including dam building, pollution, and marshland drainage, all of which modified primary habitat conditions and altered aquatic community composition [48,49,50]. Such changes can lead to biotic homogenization that promotes biodiversity to become concentrated into a few tolerant or invasive species while reducing the biodiversity of native taxa [51]. This was especially clear at some of the riverine sampling sites of this study that had low diversity, suggesting some level of habitat degradation or decreased variability in the environment. The high diversity of gastropods encountered from the marsh sites is additional evidence for the significance of structurally diverse and ecologically stable habitats to support freshwater mollusk communities.

In contrast, the lower diversity in the Gharraf River could result from habitat simplification, pollution, or altered hydrology that reduced the habitat suitability for multiple taxa. Disturbances induced by anthropogenic actions often result in reduced biodiversity of rivers from a loss of habitat quality and an increase in environmental stressors [52].

4.2. Dominance of Invasive Species

Disturbance-tolerant gastropod species, such as Melanoides tuberculata and the invasive Physella acuta, were recorded from multiple freshwater sites in southern Iraq, raising ecological concern. Melanoides tuberculata, considered native or long-established in Iraq and other parts of the Middle East [11,12], is notable for its high ecological adaptability. Its success is attributed to traits such as parthenogenetic reproduction, high fecundity, and broad environmental tolerance, which enable effective occupation of both natural and human-modified freshwater systems with minimal competition [53]. This species exhibits tolerance to salinity and pollution, as well as to changes in water chemistry traits that reinforce its dominance in disturbed environments [54].

In contrast, Physella acuta, a North American native, is a well-documented global invader. It has become one of the most widely distributed freshwater gastropods worldwide due to its rapid reproductive capacity, high tolerance to organic pollution and eutrophication, and generalist habitat preferences across lentic and lotic systems [55]. Its prevalence in anthropogenically altered environments such as irrigation canals, urban streams, and nutrient-rich wetlands enables it to outcompete native species, often leading to local extirpations [56].

The considerable relative abundance and widespread distribution of disturbance-tolerant gastropod species such as Melanoides tuberculata and the invasive Physella acuta suggest that they may be competitively excluding more sensitive native taxa. In many freshwater systems, dominant species like P. acuta in Iraq and native species such as M. tuberculatus can shape community structure, potentially reducing overall diversity and altering key ecosystem functions such as grazing pressure and nutrient cycling [53]. Additionally, M. tuberculatus is known to serve as an intermediate host for several trematode parasites, including Centrocestus formosanus, thereby posing potential risks to native fish populations, aquaculture operations, and even human health [57].

4.3. Impact on Native Species and Ecosystem Dynamics

The observed low relative abundance and limited distribution of the native gastropods Gyraulus huwaizahensis and Lymnaea auricularia indicate that conservation actions may be needed in southern Iraq, particularly to ensure the long-term persistence of these species.

If their apparent decline is indeed stemming from competitive replacement by invasive species along with increasing anthropogenic pressures (e.g., water pollution, habitat fragmentation, and hydrologic alterations), then it raises serious concerns about their conservation potential in southern Iraq. In similar systems, invasive gastropods have also diminished the richness and abundance of native mollusks through direct competition and habitat deterioration [58].

Disturbance-tolerant species, such as the native Melanoides tuberculata and the invasive Physella acuta, typically exhibit broad environmental tolerances, rapid reproductive rates, and strong colonization abilities. These traits allow them to become dominant taxa in disturbed or modified freshwater systems [53,59,60].

As invasive species expand their ranges, they often reduce the availability of food, space, and shelter for native gastropods, leading to competitive exclusion and local declines of native populations, patterns widely reported across freshwater ecosystems worldwide [61,62,63,64].

The loss of native gastropods has cascading consequences for aquatic ecosystem functioning. Native snails like L. auricularia and G. huwaizahensis are integral to benthic food webs, playing key roles in algae grazing, detritus processing, and nutrient cycling [65]. Their decline can therefore affect water clarity, sediment stability, and energy transfer to higher trophic levels such as insect larvae, fish, and waterfowl.

4.4. Niche Overlap Patterns and Null Model Interpretation

Overall, most high observed overlap values were not statistically significant, supporting the conclusion that broad habitat tolerance and stochastic distribution patterns play a larger role in shaping gastropod community structure than direct competitive exclusion.

Although several gastropod species in this study exhibited moderate to high Pianka’s niche overlap values, particularly between Melanoides tuberculata, Physella acuta, and Filopaludina bengalensis, our null model analysis revealed that only four species pairs demonstrated statistically significant overlap (p < 0.05; see Table 2). These results suggest that broad habitat tolerance and environmental filtering may play a prominent role in shaping community composition in these freshwater ecosystems, although this interpretation should be viewed cautiously, given limitations in replication and significance.

Pianka’s index, while useful for identifying shared resource use, can overestimate ecological interactions if not assessed against randomized expectations [66]. For example, the observed overlap between Melanoides tuberculata and Physella acuta was high (0.80), yet statistically insignificant (p = 0.999), suggesting that their frequent co-occurrence reflects shared tolerance of disturbed, eutrophic environments. Rather than competing with each other, these disturbance-tolerant species may be displacing more sensitive native gastropods through environmental filtering and dominance in degraded habitats.

This finding aligns with broader ecological theory, which emphasizes that stochasticity, trait convergence, and environmental filtering often override competitive interactions in structuring assemblages, especially under anthropogenic stress [29,67]. In mollusk communities, co-occurrence of generalist invaders often reflects tolerance of degraded habitats rather than niche partitioning [68].

Importantly, the application of null model testing using 999 permutations proved essential for distinguishing true ecological signals from background overlap. This approach offers a robust framework for guiding conservation strategies, particularly in dynamic and invasion-prone ecosystems like the Tigris-Euphrates basin [69,70].

It is important to note that the relatively low number of significant species pairs detected in the null model (only 4 out of 105) may reflect limitations in statistical power rather than the absence of ecological interactions. The analysis was performed on site-level averages, which could mask finer-scale co-occurrence patterns detectable at the replicate (quadrat) level. Future analyses incorporating replicate-level data may increase sensitivity and allow for a more detailed understanding of species interactions in these communities.

4.5. Environmental Filtering and Community Structuring

The Multiple Factor Analysis (MFA) provided a multiblock ordination framework that revealed how environmental gradients jointly structure gastropod species composition across the eight freshwater sites. Unlike PCA or CCA, Multiple Factor Analysis (MFA) allows simultaneous interpretation of both species and environmental blocks, enabling a more integrative view of species–environment relationships [71,72,73,74,75]. The first two MFA dimensions explained 44.2% of the total variance (Dim1 = 26.9%, Dim2 = 17.3%). The biplot (Figure 4) showed distinct gradients in environmental variables: salinity and dissolved oxygen (DO) were oriented along the positive side of Dim1, while temperature and pH were aligned primarily with Dim2.

Gastropod species exhibited varied ecological associations. E. grimmi, for example, was strongly aligned with the vectors for salinity and DO, suggesting a preference for or tolerance of more mineralized and oxygenated conditions. In contrast, species such as P. acuta, M. tuberculata, and P. antipodarum were positioned opposite to these vectors, indicating a negative association with salinity and DO, potentially favoring more stagnant, low-flow or nutrient-enriched environments.

Species like T. jordani and F. bengalensis were more closely aligned with temperature and pH vectors, suggesting that thermal and pH gradients may influence their spatial occurrence. The close clustering of certain invasive species (e.g., P. acuta, M. tuberculata) implies ecological convergence in disturbed habitats, but their contrasting responses to abiotic conditions indicate that different environmental filters may govern their success across sites.

Overall, the MFA results support the conclusion that environmental filtering, particularly along axes of salinity, dissolved oxygen, and temperature, plays a central role in shaping gastropod community structure in these freshwater systems. This is supported by studies showing that gastropod tolerance to salinity, oxygen, and temperature significantly affects their survival and distribution, especially under stress or pollution conditions [73,76]. The divergence in species-environment associations highlights the complexity of assemblage patterns, where even disturbance-tolerant taxa exhibit nuanced ecological preferences [77,78]. These findings align with trait-based invasion theory and suggest that successful colonization by generalist species is mediated by multidimensional environmental gradients, not solely competitive exclusion [79,80].

5. Conclusions

This study of freshwater gastropod assemblages in southern Iraq combines diversity indices, null models, and multivariate ordination to examine species distributions and environmental associations. Both native (Melanoides tuberculata) and invasive (Physella acuta) species were common, leading both to homogenization of communities and possibly suppressing specialized indigenous fauna among sites. Based on the Pianka’s niche overlap index and null models, species co-occurrence patterns are more likely to be driven by common environmental requirements rather than direct competition. Dissolved oxygen and salinity were identified as the main abiotic factors influencing community composition in a multivariate analysis, consistent with strong environmental filtering, which, however, needs to be confirmed by experimental studies. It is highlighted that disturbance-tolerant invasive species replace sensitive native ones in response to environmental pressure, hence the critical need to preserve stable marshland habitats for native gastropod conservation. Long-term and active monitoring is essential to reduce the effects of the two sources (degradation and invasions) of pressure on the Mesopotamian basin [3,81].