All experimental studies and materials involved in this research fully comply with the IUCN Policy Statement on Research Involving Species at Risk of Extinction and the Convention on the Trade in Endangered Species of Wild Fauna and Flora (CITES), and are in full compliance with relevant institutional, national, and international guidelines and regulations.
This study evaluated the accuracy of various models, including ANN, CTA, FDA, GAM, GBM, GLM, MARS, MAXENT, MAXNET, RF, XGBOOST, and SRE, along with an ensemble model (EM) for W. carpophilus. The TSS values for CTA, FDA, GAM, GBM, GLM, MARS, MAXENT, MAXNET, RF, and XGBOOST models were all greater than 0.8, while the AUC values exceeded 0.9 (Fig. 4). Consequently, these ten high-accuracy models were selected to construct the EM using six ensemble approaches: EMmean, EMcv, EMci, EMmedian, EMca, and EMwmean.
The developed EM demonstrated the highest accuracy among all models evaluated in this study. Therefore, the EM derived from individual high-accuracy models significantly enhanced predictive precision while reducing uncertainty during the modeling process. These findings suggest that the predicted potential geographic distribution of W. carpophilus, as inferred from the EM, is highly reliable.
Based on the current potential geographic spread of W. carpophilus, the species distribution model (SDM) results indicate distinct gradients of habitat suitability globally. The current distribution is categorized into four classes: unsuitable habitat, low suitability habitat, moderate suitability habitat, and high suitability habitat (Fig. 5).
The unsuitable habitat represents the largest area, covering approximately 1.30 × 10km², characterized by regions where the climatic and environmental conditions are not conducive to the establishment or spread of W. carpophilus. These areas are primarily located in regions with climates that are either too extreme or do not provide the necessary environmental requirements for the pathogen's survival and proliferation.
Low suitability habitats account for an estimated 9.73 × 10 km². These regions are characterized by marginal conditions that may support the pathogen but are not optimal for widespread establishment. The low suitability areas are distributed predominantly across parts of North America, Eastern Europe, Central Asia, and Southern Australia. These areas reflect environments where some climate variables align with the known conditions suitable for W. carpophilus, but other factors may limit its establishment or survival.
Moderate suitability habitats are estimated at 5.71 × 10 km². These areas are considered suitable for W. carpophilus, with favorable climatic conditions that can facilitate the species' establishment and survival, albeit with some limitations. Moderate suitability habitats are particularly prevalent across parts of Europe, North America, and temperate regions of East Asia. The distribution pattern suggests that these regions may have a combination of moderate temperature and precipitation conditions that are favorable for the development of W. carpophilus but may still experience some constraints on optimal growth.
High suitability habitats, which represent areas most favorable for the establishment and spread of W. carpophilus, cover approximately 3.33 × 10 km². These regions are concentrated mainly across central Europe, extending into parts of Eastern Europe and Western Asia, as well as localized areas in North America. High suitability habitats provide the most conducive environmental conditions for the proliferation of W. carpophilus, with optimal temperature, precipitation, and seasonal variations that align well with the species' ecological requirements. This suggests that these regions are at the highest risk of invasion and potential establishment of the pathogen, necessitating targeted monitoring and management strategies.
Overall, the results indicate that W. carpophilus has a significant potential range under current climatic conditions, with particularly favorable conditions across temperate regions of Europe, parts of Asia, and North America. The identification of these suitability zones provides crucial insights for prioritizing areas for pathogen monitoring and developing appropriate management interventions.
Based on the results of the biomod2 model for the prediction of the potential habitable zones of Wilsonomyces carpophilus, its distribution in China showed significant spatial heterogeneity and was classified into four grades, namely, non-habitable zone, low-habitable zone, medium-habitable zone and high-habitable zone. The areas of each class of suitable areas were 7.88 × 10 km² for non-suitable areas, 1.25 × 10 km² for low suitable areas, 2.87 × 10 km² for medium suitable areas, and 1.67 × 10 km² for high suitable areas, which accounted for 84.70%, 13.40%, 3.10%, and 1.80% of the national land area, respectively (Fig. 5).
The largest area of the non-adapted zone covers most of the country, mainly in the north-central part of the three northeastern provinces (around the Daxinganling, Xiaoxinganling, and Zhang Guangcailing), as well as in the high-altitude and desert regions of the northwestern region (the Taklamakan Desert, the Qinghai-Tibetan Plateau, the Qilian Mountains, and the Tian Shan Mountain Range). In addition, the North China Plain, the middle and lower reaches of the Yangtze River Plain, and the southern tropical region (most of Hainan Island) also belong to the unsuitable areas. The climatic conditions in these regions are not suitable for the expansion of W. carpophilus.
The area of low habitat is 1.25 × 10 km², mainly distributed in the transition area with lower climate suitability. Typical distribution areas include the edge of the the valleys of Ili, the Turpan Basin and the northern foothills of the Tianshan Mountains in Xinjiang, the Jiayuguan, Jiuquan and Zhangye areas of the Hexi Corridor in Gansu, parts of the Yunnan-Guizhou Plateau (Dali and Kunming in Yunnan, and Guiyang in Guizhou), and parts of the Loess Plateau (the central part of Shanxi and the eastern part of the Guanzhong Plain in Shaanxi). The low-fitness areas usually show a banded or blocky distribution with weak fitness.
The mesophilic zone covers an area of 2.87 × 10 km², and is mainly concentrated in mountainous and hilly areas with more favorable climatic conditions. The edge of the core area of the valleys of Ili in Xinjiang, the low-altitude basins on the north and south sides of the Tianshan Mountains, and the area to the east of the Qilian Mountains in Gansu are the main distribution ranges of the mesophilic zone. In addition, the central part of the Yunnan-Guizhou Plateau (Yunnan Dianzhong Plateau) and the edge of the Sichuan Basin (south of the Chengdu Plain) are also moderately hospitable.
The area of highly suitable zone is the smallest, only 1.67 × 10 km², but the distribution is concentrated, mainly in the climatic conditions that are most suitable for the growth of W. carpophilus. In Xinjiang, the core area of the valleys of Ili and the northern foot of Tianshan Mountain are the distribution hotspots of highly suitable areas. In addition, the west-central part of the Hexi Corridor in Gansu (near Zhangye, Jiuquan and Dunhuang), part of the Sichuan Basin (around Panzhihua), and part of the Yunnan-Guizhou Plateau (Dianzhong Plateau) also belong to highly suitable areas. These areas are characterized by moderate temperatures, adequate and stable precipitation, and provide ideal environments for the survival of W. carpophilus. W. carpophilus in China is concentrated in the Tianshan Mountains and the Hexi Corridor in the northwest, the Yunnan-Guizhou Plateau and the southern edge of the Loess Plateau. The suitable areas are mainly in the low suitable areas, while the medium and high suitable areas are small in size but concentrated in distribution. The non-viable areas occupied most of the country, indicating that the distribution of W. carpophilus is significantly limited by climatic conditions.
The results of the analysis of the potential fitness zones of Wilsonomyces carpophilus in Xinjiang based on the biomod2 model showed that the fitness zones of this pathogen have significant heterogeneity in spatial distribution, the area of each grade of suitable zone is 1.17 × 10 km² in non-suitable zone, 2.37 × 10 km² in low suitable zone, 1.83 × 10 km² in medium suitable zone and 1.48 × 10 km² in high suitable zone, accounting for 31.50%, 63.70%, 4.90% and 4.00% of the total area of Xinjiang, respectively (Fig. 5).(https://desktop.arcgis.com).
The non-viable areas are mainly located in the Taklamakan Desert and the Kunlun and Altun mountain ranges in the southwestern part of the southern border, as well as in the northern Altai Mountains in the northern border, and other high-altitude or extremely arid areas. These areas are characterized by harsh climatic conditions, low temperatures and scanty precipitation, which are unsuitable for the growth and spread of W. carpophilus.
The low-fitness zone is the most widely distributed area, covering the oasis agricultural areas of Xinjiang and some of the mountainous fringe areas. It mainly includes Urumqi, Changji periphery and Shihezi in the northern foothills of the Tianshan Mountains, Korla and Yuli areas in the southern foothills of the Tianshan Mountains, the periphery of the valleys of Ili, and low altitude areas such as Burqin, Fuhai and Jimune counties in the southern foothills of the Altai Mountains. These areas have suitable hydrothermal conditions, but local limiting climatic factors reduce their habitability.
The medium-fitness zone is smaller in size and is concentrated in areas with better climatic conditions, including Changji and Fukang in the middle and low elevation areas of the Tianshan Mountains, Yining City, Huocheng County, and Nilek County in the core area of the valleys of Ili, and part of Hami City in the extended area of the Hexi Corridor. These areas are transitional zones from low to high habitability zones, with climatic conditions in between.
Highly suitable areas are the smallest in size and highly concentrated in distribution, mainly located in the core area of the valleys of Ili in the counties of Tekes, Gongliu and Zhaosu, as well as in the oasis areas at the northern foothills of the Tianshan Mountains. These areas have warm and humid climate, sufficient precipitation, and frequent agricultural activities, which provide the most suitable environment for W. carpophilus to survive.
Overall, the potential fitness zones of W. carpophilus in Xinjiang were mainly distributed in the Tianshan Mountains and its surrounding oases, the valleys of Ili and the southern foothills of the Altai Mountains, with the low fitness zones covering the widest areas, while the high fitness zones were concentrated in the areas with optimal climatic conditions, suggesting that the hydrothermal conditions and the topographic features were the key influencing factors for the distribution of the pathogen's fitness zones.
Based on the projections under different Shared Socioeconomic Pathways (SSPs), the future potential global geographic distribution of W. carpophilus reveals substantial shifts across different suitability levels under various climate scenarios (Fig. 6). The results are presented for two future periods (2050s and 2090 s) under two contrasting climate scenarios, SSP126 and SSP585.
In the 2050 s under SSP126, the unsuitable habitat constitutes approximately 1.26 × 10 km², which is slightly reduced compared to the current distribution, suggesting minor shifts in habitat suitability. The low suitability habitat is projected to expand to 1.14 × 10 km², indicating an increase in marginally favorable conditions for the pathogen's establishment. Moderate suitability habitats are projected to cover 6.88 × 10 km², and high suitability areas are expected to occupy 4.49 × 10 km², with notable expansion in regions such as Europe, North America, and parts of Asia, indicating more favorable climatic conditions for W. carpophilus compared to the present.
Under SSP585 in the 2050 s, the unsuitable habitat area slightly increases to 1.27 × 10 km². The low suitability habitat is projected to decrease to 1.09 × 10 km² compared to SSP126, reflecting slight variability in areas with marginal suitability due to more extreme climatic conditions. Moderate suitability areas are expected to shrink to 6.33 × 10 km², whereas the high suitability habitat expands slightly to 4.66 × 10 km². These trends suggest a shift in habitat quality with increased concentrations of suitable areas under more severe climate conditions, particularly in central Europe and North America.
In the 2090 s, under SSP126, the unsuitable habitat decreases slightly to 1.25 × 10 km², indicating some areas transitioning to more favorable conditions for pathogen establishment. The low suitability habitat expands further to 1.23 × 10 km², suggesting a gradual broadening of marginal habitats. Moderate suitability areas increase to 6.99 × 10 km², and high suitability regions also expand to 5.13 × 10 km². These findings indicate a trend towards increased suitability under the less severe climate scenarios, particularly in regions like Europe, which remain consistently suitable for the pathogen.
In contrast, the 2090 s under SSP585 reveal notable changes, with the unsuitable habitat slightly decreasing to 1.24 × 10 km², similar to SSP126. The low suitability areas are projected to occupy 1.23 × 10 km², indicating stability in areas with marginal conditions. Moderate suitability habitats expand further to 7.25 × 10 km², and high suitability regions increase significantly to 5.76 × 10 km². This trend highlights a notable expansion of highly suitable areas under extreme climate conditions, suggesting a considerable increase in favorable environments for W. carpophilus, particularly across Europe, parts of Asia, and North America, making these regions potential hotspots for the pathogen's future distribution.
Overall, the future projections indicate a general trend of increasing habitat suitability for W. carpophilus, particularly under more extreme climate scenarios (SSP585). High suitability habitats are projected to expand significantly by the 2090 s, suggesting that climate change may create increasingly favorable conditions for the spread and establishment of this pathogen. These insights underscore the need for targeted monitoring and proactive management strategies, especially in regions projected to transition to higher suitability classes.
The Multivariate Environmental Similarity Surface (MESS) analysis for W. carpophilus under future climate scenarios indicates notable shifts in environmental similarity across the pathogen's current and projected distribution range. The mean multivariate environmental similarity values for the 302 known occurrence sites under SSP126 and SSP585 scenarios were calculated as 26.83 and 26.89, respectively, during the 2050 s, and 27.22 and 23.51, respectively, during the 2090 s (Fig. 7). These values reflect variations in the degree to which future environmental conditions resemble the baseline conditions used to train the model.
The analysis highlights significant climatic anomalies, particularly in the northern parts of the current habitat range, where environmental conditions are projected to diverge considerably from the baseline. The climate scenarios ranked by decreasing degrees of climatic anomalies are as follows: SSP585 during the 2090 s, SSP585 during the 2050 s, SSP126 during the 2090 s, and SSP126 during the 2050s. This order indicates that the severity of environmental changes, which affect the suitability of habitats for W. carpophilus, is most pronounced under the more extreme emissions scenario (SSP585), especially towards the end of the century.
The dominant environmental factor influencing habitat suitability was identified as Precipitation Seasonality (bio-15), which exhibited drastic changes beginning with SSP126 in the 2050 s and continued to dominate as the primary driver of habitat shifts. This suggests that fluctuations in the distribution of precipitation throughout the year are likely to have a substantial impact on the future distribution of W. carpophilus. Alongside Precipitation Seasonality, other climatic variables such as Mean Diurnal Range (bio-2), Isothermality (bio-3), and Precipitation of the Wettest Month (bio-13) also contributed to the overall suitability. However, their influence on the pathogen's potential range gradually decreased over time. As a result, the combination of increased variability in precipitation and a reduction in the stabilizing influence of other climatic factors is expected to negatively affect the fitness and potential range expansion of W. carpophilus.
The spatial analysis presented in Fig. 8 shows that the most dissimilar variables under future scenarios are concentrated in northern regions of the current habitat, primarily in parts of North America, Europe, and Asia. This highlights regions where environmental conditions will diverge most significantly from those suitable for the current distribution, introducing greater uncertainty in model predictions for these areas. Notably, under SSP585 in the 2090 s, the extent of regions characterized by high levels of environmental dissimilarity is substantially increased, suggesting that climate extremes will play a critical role in altering habitat suitability for the pathogen.
Overall, the results show underscore the impact of projected climate change on the distribution of W. carpophilus, with Precipitation Seasonality emerging as the key factor driving habitat shifts. The increasing divergence of environmental conditions from the present indicates the potential challenges for the pathogen's survival in certain regions, while also highlighting the areas that may become more susceptible to its spread. These insights are crucial for developing adaptive management strategies aimed at mitigating the risks posed by W. carpophilus under future climate scenarios.
The environmental factor response curves for W. carpophilus illustrate the relationship between key climatic variables and the predicted probability of the species' occurrence. These response curves provide insights into the optimal environmental conditions for the pathogen's survival and proliferation (Fig. 9). For each climatic variable, the range corresponding to a predicted occurrence probability greater than 0.5 is considered as the optimal habitat for W. carpophilus.
The analysis of Mean Diurnal Range (bio-2) suggests that W. carpophilus is most likely to occur in areas where the mean diurnal temperature range is between 9.97 °C and 13.65 °C. This indicates that moderate daily temperature fluctuations are favorable for the establishment of the pathogen. The response curve shows a sharp decline beyond these values, suggesting that significant deviations in temperature variability negatively affect habitat suitability.
For Isothermality (bio-3), which measures the ratio of diurnal to annual temperature range, the optimal range for W. carpophilus lies between 25.83% and 40.58%. This indicates that environments with moderate stability between daily and annual temperature variability are conducive to the pathogen. Beyond this range, particularly at higher values of isothermality, the probability of occurrence decreases significantly, suggesting that highly stable temperature environments may not favor the pathogen's establishment.
The Mean Temperature of the Wettest Quarter (bio-8) is another key factor influencing W. carpophilus distribution. The optimal range for bio-8 is between 8.54 °C and 22.17 °C, indicating that the pathogen prefers moderate temperatures during the wettest part of the year. Lower or higher temperatures during this critical growth phase are likely to limit its ability to thrive.
For Mean Temperature of the Driest Quarter (bio-9), the optimal range falls between 0.051 °C and 18.34 °C. This suggests that the pathogen can tolerate cool conditions during the driest quarter but prefers areas where temperatures are not excessively high. A gradual decline in suitability is observed with increasing temperatures beyond this range, highlighting the importance of cool, stable conditions during drier periods.
Precipitation of the Wettest Month (bio-13) shows an optimal range from 29.87 mm to 141.90 mm, indicating that W. carpophilus favors environments where precipitation during the wettest month is moderate. Extremely high levels of precipitation beyond this range are associated with a rapid decline in habitat suitability, possibly due to adverse effects on the pathogen's ability to establish under excessively wet conditions.
Precipitation Seasonality (bio-15), which measures the variation in monthly precipitation over the year, shows an optimal range of 3.72-51.45%. This suggests that W. carpophilus prefers areas with low to moderate variation in precipitation. Higher variability in precipitation throughout the year appears to be detrimental to its habitat suitability, indicating that stable moisture availability is crucial for the pathogen's growth.
In summary, the environmental response curves provide a detailed understanding of the optimal climatic conditions for W. carpophilus. Moderate temperature ranges, stable precipitation patterns, and specific thresholds of seasonal variability are all crucial for the pathogen's establishment and spread. Understanding these preferences can inform management strategies by identifying areas at greatest risk of invasion under both current and future climate scenarios, helping to mitigate the impact of this pathogen on susceptible agricultural and forestry systems.