1. Introduction
The Earth is currently undergoing its sixth mass extinction, primarily driven by habitat fragmentation due to degradation and loss, with climate change being a major factor [
1,
2,
3]. The Intergovernmental Panel on Climate Change (IPCC) reports a significant increase of 1.1 °C in average global surface temperature from 2011 to 2020 compared with pre-industrial levels [
4]. This increase, which is linked to escalating greenhouse gas emissions from human activities, has exacerbated global warming. The combined effects of global warming and other stressors significantly impact terrestrial ecosystems, altering their structure and function and causing shifts in species habitats and distributions [
5,
6]. Habitat fragmentation further disrupts interspecies interactions, increases vulnerability, and leads to widespread extinction and extirpation [
7,
8,
9].
Vegetation is crucial as both an indicator and component of climate response, with species survival depending on genetic diversity, adaptability to environmental changes, and migration to suitable climates [
10,
11]. Species in fragmented or rare habitats face higher risks of migration difficulties or extinction [
12,
13]. Climate is a key determinant of species distribution, and changes in distribution patterns are clear indicators of climate shifts [
14,
15]. The complex relationships among climate change, human disturbances, and biodiversity conservation present a multifaceted challenge [
16,
17]. Forests cover a third of the Earth’s land area and are vital for global carbon sequestration, holding 56% of terrestrial ecosystem carbon [
18]. Trees absorb significant carbon dioxide through photosynthesis and store it in biomass, litter, deadwood, and soil organic matter. Natural forests, unlike simpler artificial forests, are more effective in biodiversity conservation and provide key ecosystem services, such as carbon storage, soil retention, and water conservation [
19].
The wild fruit forest region, identified as one of the key biodiversity hotspots worldwide, has garnered widespread academic attention owing to its abundant biological resources and unique ecosystem structure. These forests, which have survived geological changes and climatic variation since the Tertiary Period, serve as sanctuaries for thermophilic mesophytic broadleaf trees. Their existence is a result of their unique geological history and local climate conditions of warmth and humidity, in contrast to early desert climates [
20,
21]. However, climate change, overgrazing, tourism, and unsustainable harvesting have severely affected these habitats, causing a drastic reduction in their area and number [
22,
23]. Key species such as
Prunus armeniaca L.,
Malus sieversii, and
Prunus ledebouriana (Schltdl.) Y.Y.Yao, all Tertiary relicts, are the dominant species in these wild fruit forests [
24,
25,
26,
27,
28].
M. sieversii, the ancestor of modern apples, is native to the Tianshan Mountains, including the regions within China, Kazakhstan, and Kyrgyzstan [
20].
P. armeniaca, from the Rosaceae family, has played a crucial role in the history of cultivated apricots and is prominent in Xinjiang’s wild fruit forests [
20,
26].
P. ledebouriana, also in the Rosaceae family, now exists only in relict distributions in Kazakhstan and Xinjiang, China, with notable fossilisation in Europe [
29,
30,
31]. The loss of habitats for these three keystone species will directly result in a reduction in biodiversity in the wild fruit forests, which, in turn, will have a negative impact on ecosystem functions and stability. It also affects the continuation of breeding and conservation programs. Understanding the potential overlapping geographical distributions of these species, along with their ecological niche dynamics and conservation area planning, is essential for their utilisation and protection against habitat loss due to climate change.
Species Distribution Models (SDMs) are crucial for predicting how climate change affects important species by integrating environmental data and species occurrences to forecast their potential geographic distributions [
32,
33,
34,
35,
36,
37]. Common SDMs, such as the maximum entropy model (Maxent), CLIMEX, generalised linear model (GLM), and genetic algorithm for rule-set prediction (GARP), are widely used to predict the distribution of important species, weeds, biocontrol agents, disease vectors, and pathogens [
38,
39]. Biomod, an R-based SDM platform developed in 2003 and now updated to biomod2 [
40], provides a range of algorithms, including artificial neural networks (ANNs), classification tree analysis (CTA), flexible discriminant analysis (FDA), generalized additive models (GAMs), gradient boosting machines (GBMs), generalized linear models (GLMs), multivariate adaptive regression splines (MARSs), maximum entropy (MAXENT), MAXNET, random forest (RF), species range envelope (SRE), and eXtreme Gradient Boosting (XGBOOST) [
41], for predicting species distributions, while ensemble models (EMs) are increasingly favoured for their ability to separate signal from noise in individual SDMs, avoid exposing vulnerabilities to species limitations, enhancing the reliability of geographical distribution predictions [
42,
43,
44,
45,
46]. The ‘ecospat’ package effectively integrates environmental principal component analysis (PCA-env) to streamline ecological niche dynamic analysis. This integration ensures a more coherent and efficient workflow [
47,
48]. Introduced in 2014, the COUE (centroid shift, overlap, unfilling, and expansion) framework has gained widespread application in examining the dynamics of species’ ecological niches. This approach is notably applied in research focused on
Ambrosia artemisiifolia L., commonly known as common ragweed, and
Agastache rugosa, a significant cash crop [
49,
50,
51].
Our hypotheses focused on the direct impacts of climate change, specifically, the effects of rising temperatures and altered precipitation patterns on the habitats of three important species: P. armeniaca, M. sieversii, and P. ledebouriana. We predicted that increasing global temperatures and more frequent extreme climatic events will significantly reduce suitable habitats for these species, compress their ecological niches, and decrease the area and biodiversity of wild fruit forests. To predict the future geographic distribution of these important species, we employed SDMs, integrating existing distribution data with future climate projections to estimate likely survival areas. We chose climatic factors—temperature, precipitation, and seasonal variation—and non-climatic factors like soil type and topography to define current ecological niches and project how future climate changes might alter species distributions. Our methodology included the following: (1) selecting high-precision individual models for each species in biomod2 after simulation testing; (2) EM application to forecast possible distribution zones and intersecting areas in both current and future climate conditions, specifically targeting the 2050s and 2090s periods; (3) analysis of ecological niche overlap and dynamics among these species, particularly between native and habitat loss areas; (4) identifying environmental variables that significantly impact their potential distributions; and (5) using the Marxan model to identify priority conservation areas and suitable growth environments. The study primarily aimed to provide a scientific basis for conserving wild populations and germplasm resources of these species.
3. Results
3.1. Model Precision Assessment
We assessed the accuracy of various models, including ANN, CTA, FDA, GAM, GBM, GLM, MARS, MAXENT, MAXNET, RF, XGBOOST, and SRE, and the EM for
P. armeniaca,
M. sieversii, and
P. ledebouriana. For these species, the TSS values of the CTA, FDA, GAM, GBM, GLM, MARS, MAXENT, MAXNET, RF, and XGBOOST models were >0.8, and their AUC values were >0.9 (
Figure 2). Consequently, 10 models with high accuracy were chosen to construct EMs using six different integration methods: EMmean, EMcv, EMci, EMmedian, EMca, and Emwmean.
The developed EMs demonstrated the highest accuracy in this assessment. For
P. armeniaca,
M. sieversii,
and P. ledebouriana, the TSS values of the EMs were 0.92, and the AUC values were up to 0.98 (
Figure 3). Hence, the EMs developed from singular high-precision models significantly enhanced the fit accuracy while diminishing uncertainties in the fitting process. This suggests a high reliability in the predicted potential geographic distributions of the three important species, as inferred through the use of EMs.
3.2. Present Potential Geographic Spread
The EM projections of the current (1979–2013) potential geographic ranges for
P. armeniaca,
M. sieversii, and
P. ledebouriana are shown in
Figure 4.
P. armeniaca and
M. sieversii share common distribution zones in Armenia, Azerbaijan, China, Iran, Italy, Kazakhstan, Turkey, and Kyrgyzstan, while
P. ledebouriana is primarily native and endemic to Kazakhstan. Notably, the shared areas in China, Kazakhstan, and Turkey represent 73.38% of the total joint distribution for
P. armeniaca and
M. sieversii. China has the largest shared area, at 8.76 × 10
6 ha, accounting for 37.32% of the total, followed by Kazakhstan (27%, 6.34 × 10
6 ha) and Turkey (9.05%, 2.12 × 10
6 ha) (
Table 2). The largest global habitat suitability area for
P. ledebouriana is significantly concentrated in Kazakhstan (979.17 × 10
6 ha), followed by the global habitat suitability areas for
M. sieversii (468.20 × 10
6 ha) and
P. armeniaca (445.51 × 10
6 ha).
P. armeniaca has a highly suitable habitat spread over 151.14 × 10
6 ha, predominantly in Europe (Ukraine, Romania, Russia), Central Asia (Kazakhstan, Turkmenistan, Pakistan, northwestern China), and North America (United States, southern Canada), with sparse distribution in Oceania (Australia). Eastern Europe is the main area of the most suitable habitat for
P. armeniaca, accounting for 70.4% of the total suitable area, followed by Central Asia and North America with 21.89% and 0.44%, respectively (
Table 3). For
M. sieversii, highly suitable habitats, covering 121.99 × 10
6 ha, are mainly in Central Asia (Mongolia, Iran, Turkey, Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, northwestern China), Europe (Georgia, Spain), Africa (Morocco, Algeria, Swaziland), South America (Chile, Argentina, Trinidad, and Tobago), and North America (United States, southern Canada). The highly suitable habitat of
M. sieversii is concentrated in Central Asia with 56.05%, followed by North America (16.39%), Africa (10.32%), Europe (6.31%), and South America (6.31%) (
Table 4). For
P. ledebouriana, highly suitable habitats span 468.86 × 10
6 ha, predominantly in Europe (Azerbaijan, Ukraine, Russia, Romania, Bulgaria, Germany), Central Asia (Kyrgyzstan, Kazakhstan, Iran, Turkey, northwestern China), and North America (United States, southern Canada). The main habitat of
P. ledebouriana is located in Europe, which accounts for 60.51% of the total area, while Central Asia and North America account for 19.66% and 11.89%, respectively (
Table 5).
3.3. Comparative Analysis of Ecological Niches
The geographic ranges of these three important species partially mirror their fundamental niches, defined as the array of environmental conditions allowing their survival. The ecological niche dynamics of
M. sieversii,
P. armeniaca, and
P. ledebouriana, based on a comparison of the climate niche space between native and lost habitats, are presented in
Figure 5. Based on occurrence data and bioclimatic variables for native and habitat loss, the ecological niche overlaps of
P. armeniaca,
M. sieversii, and
P. ledebouriana revealed Schoener’s D values of 0.83, 0.74, and 0.76, respectively, indicating that the ecological niche overlap is relatively high between
P. armeniaca,
M. sieversii, and
P. ledebouriana (
Figure 5). The ecological niches of
P. armeniaca,
M. sieversii, and
P. ledebouriana in their extirpated habitats were smaller than those in their native habitats. This indicates a decrease in resources that can be jointly utilised in both the current and future periods. In terms of habitat loss, the ecological niches of
P. armeniaca,
M. sieversii, and
P. ledebouriana did not occupy all the ecological niches of their native areas. The null hypothesis of the ecological niche equivalence of
P. armeniaca,
M. sieversii, and
P. ledebouriana based on the bioclimatic variables of the native and invasive areas was not rejected (
p = 0.1908, 0.9523, and 0.5238, respectively).
The niche widths of
P. armeniaca,
M. sieversii, and
P. ledebouriana were calculated using the ENMTools (Version 5.26) software package, focusing on various climate conditions. As shown in
Table 6, the B1 and B2 values of
P. armeniaca,
M. sieversii, and
P. ledebouriana were all >0.7 and >0.9, respectively. Hence, each period showed no marked distinction between B1 and B2, suggesting that
P. armeniaca,
M. sieversii, and
P. ledebouriana tend towards being generalist species. Furthermore, B1 and B2 exhibited an upward trend in alternative climate scenarios compared with the present period. This trend implies an expansion in the range of resources utilised by these species in forthcoming climate conditions, demonstrating their extensive distribution and robust environmental adaptability.
3.4. Projected Future Geographic Ranges
The potential global geographical distributions of
P. armeniaca,
M. sieversii,
and P. ledebouriana under SSP126, SSP245, and SSP585 during the 2050s and 2090s are shown in
Figure 6,
Figure 7 and
Figure 8 and
Table 7.
For P. armeniaca, the total area of suitable habitat is projected to decrease under most future climate scenarios in the 2050s and 2090s. The most significant reduction occurs in the 2090s under the SSP585 scenario, with the suitable habitat area decreasing to 885 × 106 ha, representing a 32.21% reduction from current conditions. Habitat losses are primarily noted in Romania, Ukraine, southern Russia, and sporadically across Central Asia, although some expansions are noted in Central Asia and southern Europe.
For M. sieversii, the total area of suitable habitat is projected to decrease under all climate scenarios in the 2050s and 2090s compared with current conditions. The most significant loss occurs in the 2090s under the SSP585 scenario, with the suitable habitat area decreasing to 1389 × 106 ha, a 3.71% reduction. Habitat losses are mainly in Romania, Ukraine, southern Russia, and sporadically in Central Asia, although some habitat expansions are observed mainly in Central Asia, with a few in southern Europe. Other notable reductions include the SSP126 scenario in the 2090s with a 3.34% reduction, followed by the SSP245 scenario in the 2050s and the SSP126 scenario in the 2050s with smaller decreases.
For P. ledebouriana, the total area of suitable habitat is expected to decrease under all future climate scenarios in the 2050s and 2090s compared with current conditions. The most significant loss is forecasted for the 2050s under the SSP585 scenario, with the suitable habitat area reducing to 517.95 million ha, a 47.1% decrease. The primary habitat losses are noted along the borders of Ukraine, Russia, and Georgia, with smaller affected areas in Kazakhstan, Mongolia, and northwestern China. Some habitat expansions are observed in Moldova and limited areas of southwestern Russia. Other significant reductions are projected under the SSP126 scenario in the 2090s with a 46.68% decrease, the SSP585 scenario in the 2090s with a 45.13% decrease, and the SSP126 scenario in the 2050s with a 43.99% decrease.
3.5. Overlapping Geographical Distributions under Climate Change
In the future, the primary regions experiencing a loss of overlapping geographic distribution areas for the three important species will mainly be located in Azerbaijan, Georgia, Kazakhstan, Tajikistan, Kyrgyzstan, Iran, and northwestern China, as illustrated in
Figure 9. The area of lost overlapping geographic distributions for the three important species was largest for the 2090s under the SSP245 scenario, followed by the 2050s under SSP126, 2050s under SSP245, 2050s under SSP585, 2090s under SSP585, and 2090s under SSP126.
In the 2050s, under the SSP126 scenario, the greatest loss in overlapping geographic distribution areas occurred in Iran, amounting to 108.8 × 104 ha, which represents 31.85% of the total loss in overlapping geographic distribution. This was followed by Kazakhstan (85 × 104 ha, 27.02%) and Georgia (72.25 × 104 ha, 22.97%). Under the SSP245 scenario, Kazakhstan experienced the most significant loss, totalling 67.15 × 104 ha, accounting for 31.71% of the total loss in overlapping areas, followed by Iran (51 × 104 ha, 24.09%) and Kyrgyzstan (46.75 × 104 ha, 22.08%). Under the SSP585 scenario, Iran again suffered the most, with a loss of 54.4 × 104 ha, representing 34.04% of the total, followed by Kyrgyzstan (47.6 × 104 ha, 29.78%) and Azerbaijan (45.05 × 104 ha, 28.19%).
In the 2090s, under the SSP126 scenario, Azerbaijan saw the most significant loss in overlapping geographic distribution areas, with 54.4 × 10
4 ha lost, accounting for 87.67% of the total loss. This was followed by Kyrgyzstan (38.25 × 10
4 ha, 61.64%) and Kazakhstan (25.5 × 10
4 ha, 41.09%). Under the SSP245 scenario, the largest loss occurred in Kazakhstan, with 124.95 × 10
4 ha, representing 31.68% of the total loss, followed by Iran (11.9 × 10
4 ha, 30.17%) and Kyrgyzstan (70.55 × 10
4 ha, 17.88%). Under the SSP585 scenario, Iran again experienced the largest loss, with 50.15 × 10
4 ha, constituting 43.06% of the total, followed by Kazakhstan (45.9 × 10
4 ha, 39.41%) and Azerbaijan (36.55 × 10
4 ha, 31.38%) (
Table 8).
3.6. Centres of Potential Geographical Distributions
The potential geographic distribution centres of
P. armeniaca,
M. sieversii, and
P. ledebouriana are shown in
Figure 10. The centres of potential geographical distributions of
P. armeniaca are Friuli-Venezia Giulia, Italy, under current climate scenarios; the centres of potential geographical distributions of
M. sieversii are Karaman, Turkey, under current climate scenarios; and the centres of potential geographical distributions of
P. ledebouriana are Vasvar, Vas, Hungary, under current climate scenarios. Under the three scenarios from the present to the 2050s and 2090s,
P. armeniaca showed an overall trend of moving northward and from northeastern Europe to Central Asia. For
M. sieversii, from the present to the 2050s and 2090s, the potential geographic distribution centres generally shifted northward and northwestward in Europe, with the centre oscillating under the SSP245 scenario. For
P. ledebouriana, from the current period to the 2050s and 2090s, the potential geographic distribution centres tended to move northward from northeastern Europe to Central Asia, exhibiting a swaying trend in Central Asia. Overall, the potential geographic distribution centres of these three important species predominantly trend northward under the three scenarios projected for the 2050s and 2090s (
Table 9).
3.7. Influence of Environmental Variables on Predicted Geographic Distributions
We utilised the EMca integrated model to evaluate the impact of each environmental variable on the potential geographic distributions of
P. armeniaca,
M. sieversii, and
P. ledebouriana, as detailed in
Figure 11. The analysis revealed that for
P. armeniaca, annual mean temperature (Bio1), isothermality (Bio3), and precipitation in the wettest month (Bio13) had the most significant cumulative contributions. For
M. sieversii, the key bioclimatic variables were precipitation in the wettest month (Bio13), driest month (Bio14), warmest quarter (Bio18), and coldest quarter (Bio19). For
P. ledebouriana, the major contributing variables for
P. ledebouriana were isothermality (Bio3), precipitation in the wettest month (Bio13), and temperature seasonality (Bio4). These variables predominantly define the multidimensional ecological niche of each species; however, other climatic, soil, and topographic factors play smaller roles.
3.8. Priority Protection Areas under Current Climate Conditions
The Marxan model was employed to identify priority conservation areas for the three important species, with the findings imported into ArcGIS to develop a focused conservation strategy. As depicted in
Figure 12, these priority areas are predominantly located in Armenia, Azerbaijan, China, Iran, Italy, Kazakhstan, Turkey, and Kyrgyzstan. This distribution aligns with the highly suitable habitats for these species as forecasted by the biomod2 model, affirming the precision of these predictions. Furthermore, these crucial conservation zones cover a relatively small portion of the land, exhibiting a compact distribution. This is beneficial for establishing specific conservation and management plans.
5. Conclusions
The potential geographical distributions of P. armeniaca, M. sieversii, and P. ledebouriana are shaped by bioclimatic variables, the HII, soil attributes, and topography, with global climate change significantly impacting their distribution and overlapping areas. Our EM outperformed individual models such as ANN, CTA, FDA, GAM, GBM, GLM, MARS, MAXENT, MAXNET, RF, and XGBOOST, showing that it is more reliable for predicting the geographical distributions of these important species.
The EM predictions showed that these species are primarily distributed in Central Asia and Europe and undergo ecological niche changes during invasion. Under climate scenarios SSP126, SSP245, and SSP585, suitable habitats are projected to decrease by the 2050s and 2090s. In the 2050s, the overlapping geographic distribution areas will mainly be in Azerbaijan, China, Iran, Kazakhstan, and Georgia. By the 2090s, these areas are expected to shift predominantly to China, Kazakhstan, Kyrgyzstan, and Georgia, with a general trend of moving northward.
Bioclimatic variables and elevation are significant influencers of their distribution, whereas the cumulative impact of topography and soil properties is low. The potential loss of distribution areas for these species is a serious concern, threatening the genetic diversity and adaptability of wild fruit forest ecosystems to environmental change.
In conclusion, the protection of P. armeniaca, M. sieversii, and P. ledebouriana extends beyond biodiversity conservation, and is essential for ecosystem vitality, scientific advancement, and economic resilience. A comprehensive conservation strategy that includes habitat protection, natural reproduction support, and management plans is crucial. This approach, guided by EMs and ecological studies, is the key to preserving biodiversity and ensuring sustainable coexistence in the face of climate change, habitat loss, and carbon sequestration.