*3.4. Ecological Niche Modeling*

To study the distribution of alien fish species in the present and in the future, we created two models (Maxent, CliMond 1975 and 2090) for each species (Figures 4 and 5). As a result, it was shown that taking into account tendencies of global warming till 2090, the range will increase by 1.2 times for guppies and by 1.07 times for mosquitofish. Despite the fact that these American viviparous fish species are thermophilic, they occupy different ecological niches (Figures 2 and 3), which certainly do not coincide with native northern fish species. To compare the SDM models (Maxent, CliMond 1975) of two species of American fish we used the regression module in SagaGis; the resulting coefficient of correlation was only r = 0.18, with global warming by 2090, the rate increases: r = 0.33. The similarity will become greater over time, thus suggesting that with global warming these

fish species can occupy similar habitats in Eastern Europe. Nevertheless, the tendencies for the expansion of the range of these two invasive alien species in Eastern Europe (as well as in the south of Ukraine, the Caucasus) are quite realistic in the future (by 2090). Evaluating the performance of the SDM by the threshold-independent receiver operating characteristic (ROC) approach, where the calculated area under the ROC curve (AUC) is considered as a measure of prediction success, we came to the conclusion that the indicators are quite high—AUC > 0.95, TSS > 0.62.

To calculate the significance of the model, we used all the American fish meeting points (see Materials and Methods, [23,24]) and the Maxent model (CliMond 2090, Figure 6 [30]). The check was done using a binomial test using the cumulative binomial probability of success of predicting correctly an occurrence given the validation data and the proportional area predicted as present in the niche model. Therefore, we can conclude that the mosquitofish has great prospects for distribution in the south of Eastern Europe, since it is less demanding on temperature factors. (Figures 2 and 6).

**Figure 3.** Niche clustering (Geographic space, CliMond 1975 (1970–2000)) from: (**A**)—*P. reticulata*; (**B**)—*G. holbrooki,* Differently coloured spots represents biotopes where the fish species was registered according to data collected from GBIF [23,24], where the species was registered and which has combination of climatic factors different from those in their native distribution area (marked with red circles).

**Figure 4.** Potential (probabilistic) model of *P. reticulata* expansion built in the Maxent program based on the CliMond: (**A**)—1975 (1970–2000); (**B**)—2090 (2081–2100)) climatic data and GBIF data (2021). Areas of the highest habitat suitability (r > 0.3–0.5) are colored in red and areas of the lowest (r < 0.2)—in blue (SagaGis).

**Figure 5.** Potential (probabilistic) model of *G. holbrooki* world expansion built in the Maxent program based on the CliMond: (**A**)—1975 (1970–2000); (**B**)—2090 (2081–2100)) climatic data and GBIF data (2021). Areas of the highest habitat suitability (r > 0.3–0.5) are colored in red and areas of the lowest (r < 0.2)—in blue (SagaGis).

**Figure 6.** Result of the analysis of binomial tests (CliMond 2090 (2081–2100) [30]): (**A**)—*P. reticulata*; (**B**)—*G. holbrooki*.

### **4. Discussion**

As a result of GIS modeling, climatic factors have been identified that allow new invasive alien species to inhabit southern Europe in the future. The new perspectives in Eastern Europe are very different from the conditions of the ecological niche of the native range of these fish species (Figure 3). However, due to increasing effects of climate change (to 2090; Figures 4B and 5B) and the emergence of "synanthropic ecological niches" in anthropogenic territories [36], these new perspectives provide great opportunities for the naturalization of invasive alien thermophilic poikilothermic animals with their own viviparity strategy. Under such conditions, not all native species will be able to survive in

the changing climatic conditions under the influence of anthropogenic pressure [37]. In the more northern regions of Eastern Europe in a changing climate such invasive alien thermophilic fish species will not compete directly with the native ichthyofauna in the nearest future, as in very warm drains in anthropogenic territories or during mass summer fish kills, most of the native fish species die [1]. Instead, they can occupy their niche in new habitats, having an important role in ecosystems and for biological control (especially in anthropogenic areas) in a changing climate. It should be noted that in the so-called "warm countries" of Europe and other parts of the world where these two species are invasive, they can pose a threat, particularly mosquitofish, by consuming eggs of fish and amphibians of the 'Critically Endangered (CR)' yellow-spotted tree frog (see *Litoria castanea* in IUCN Red List of Threatened Species); the 'Endangered (EN)' green and gold frog (see *Litoria raniformis* in IUCN Red List of Threatened Species); and the 'Vulnerable (VU)' golden bell frog (see *Litoria aurea* in IUCN Red List of Threatened Species) [38]. Taking into account mouth size and ability to switch to a more zooplanktivorous diet under certain conditions [12,39–41], it is quite likely that in the northern regions such species can possibly become an additional threat for amphibian species native for Northern Europe, feeding on eggs and tadpoles/larvae of oriental tree frogs *Hyla orientalis* (Bedriaga, 1890), fire-bellied toads *Bombina bombina* (Linnaeus, 1761), crested newts *Triturus cristatus* (Laurenti, 1768) and common newts *Lissotriton vulgaris* (Linnaeus, 1758). It was confirmed by some studies [42] that in countries with warmer climate these alien invasive fish species can become a threat to native ichthyofauna, competing with aboriginal species for food resources, feeding on phyto- and zooplankton (cladocerans, copepods, ostracods, rotifers and insects) decreasing feeding base for native species and damaging already established food chains. Therefore, their potential impact on aboriginal biota in terms of changing climate conditions should not be underestimated.

#### **5. Conclusions**

We determined the bioclimatic variables influencing the expansion of the area of these two invasive alien fish species, which will allow them to occupy their special niche in anthropogenic territories of Southern Europe. As a result of GIS modeling of collected data, it was found out that by 2090 expansion of these species to Eastern Europe is highly likely. The appearance of guppies in the south of Ukraine is possible (the estuary of the Danube river, as well as in several parts of the Caucasus, Turkey, lower part of the Don river (>0.3–0.5, Figure 4). *G. holbrooki* will also slightly expand its range in Europe, the Caucasus as well as in Ukraine, namely in the Crimean peninsula, western part of the Black Sea region and the Danube estuary (Figure 5). The results obtained showed that *P. reticulata* are more attached to warm drains than *G. holbrooki* and the quality of water seems not affecting its distribution.

**Author Contributions:** Conceptualization, O.N., M.P. and V.T.; data curation, O.N., M.P., A.S. and A.C.; Formal analysis, O.N., V.T. and O.M.; funding acquisition, M.P., A.S. and A. ˇ C.; investigation, ˇ O.N., M.P., O.M. and A.C.; methodology, O.N. and V.T.; project administration, A.S., O.N., A. ˇ C. ˇ and M.P.; resources, O.N., M.P. and A.C.; Software, O.N. and V.T.; supervision, O.N., V.T. and M.P.; ˇ validation, O.N., V.T., A.C. and M.P.; visualization, O.N., V.T. and M.P.; writing—original draft and ˇ writing—review and editing, author: O.N., V.T., M.P., A.C., O.M. and A.S. All authors have read and ˇ agreed to the published version of the manuscript.

**Funding:** This research was funded by the project "Pond aquaculture production and ecosystem service innovative research with modeling of the climate impact to tackle horizontal challenges and improve aquaculture sustainability governance in Latvia" (lzp–2020/2–0070) financed by Fundamental and Applied Research Projects (FLPP).

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are openly available in [*Gambusia holbrooki* Girard, 1859 in GBIF Secretariat (2021); GBIF.org (2 April 2021) GBIF Occurrence Download at https://doi.org/10.15468/dl.d5e6w9, accessed on 2 April 2021], reference number [23] and *Poecilia reticulata* Peters, 1859 in GBIF Secretariat (2021); GBIF.org (16 June 2021) GBIF Occurrence Download at https://doi.org/10.15468/dl.tuwazq, accessed on 16 June 2021, reference number [24].

**Acknowledgments:** Special thanks are due to Alex Volkov, freelance translator, for editing the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
