1. Introduction
Global climate change alters the temperature and precipitation patterns on the Earth’s surface [
1,
2,
3,
4,
5,
6], which in turn affects plant growth and development patterns, morphological structures, and geographic distribution patterns[
7,
8,
9,
10,
11,
12] , ultimately endangering the diversity and stability of global ecosystems[
13,
14,
15] . Global climate change, dominated by climate warming[
16,
17] , is one of the greatest threats to, and challenges for, ecosystem balance[
18,
19,
20,
21]. The Sixth Assessment Report on Climate Change published by the Intergovernmental Panel on Climate Change (IPCC) in 2021 states that the global surface temperature increased by 0.99°C from 2001 to 2020 compared to the previous century [
22,
23], and global warming is expected to continue to 1.5°C by 2030—2052[
24]. Thomas et al. (2004) found that approximately 37% of species globally will be at risk of extinction and habitat loss by 2050 as a result of climate warming[
25], while Urban (2015) found that the risk of species extinction accelerates with increasing temperatures[
26], especially for some rare and endangered species with small population sizes, high habitat specificity, and severe habitat fragmentation[
27,
28]. Daskalova et al. (2020) found that rare and endangered species with small population sizes and limited geographic distributions will be the first to go extinct[
29]. In addition, mountain ranges are crucial for global sustainable development because of their specific geographic features and diverse climates[
30,
31]. Global climate change has caused serious harm to species living in mountainous regions[
32,
33,
34,
35,
36], and such species are more sensitive to climate change and at greater risk of species extinction[
37,
38,
39,
40]. This is especially true for the native plants of low-altitude mountain areas, which will be limited in terms of their vertical dispersal, leading to a greater risk of extinction[
41]. Therefore, analyzing the response of rare and critically endangered species in low-altitude mountain areas to climate change can not only clarify the dominant climatic factors affecting the geographical distributions of species, but also be crucial for vegetation restoration and ecological conservation efforts.
Species distribution models (SDMs) are widely used in the fields of endangered species conservation and species invasion[
42,
43,
44,
45], and can efficiently and accurately analyze the effects of global changes on species distributions and diversity patterns[
46]. SDMs are effective tools for predicting the effects of climate change on the distributions of endangered species in the context of ecological conservation and management[
47]. Several widely used SDMs emerged in the late 20th century, among which the MaxEnt (Maximum Entropy) model not only has stable predictive performance, but also has higher accuracy[
48,
49]. By studying the distribution data of 226 species, Elith and Graham et al. (2006) concluded that the MaxEnt model simulated the best results[
50]. Meanwhile, Prevéy, J. S. et al. (2020) used the MaxEnt model to predict the potential fitness zone and phenology of
Vaccinium membranaceum (American lingonberry) and found that the simulated results were in good agreement with observations[
51]. The MaxEnt model is an ecological niche model based on the theory of maximum entropy[
52], and is the preferred model for predicting the potential distributions of critically endangered species because of its simplicity, high degree of accuracy, low sample size requirement, and high stability compared to other modeling methods [
53,
54,
55].
Ilex nanchuanensis is a species of holly tree endemic to China and is classified as critically endangered in the IUCN’s Red List of Threatened Species , standard conservation level. It is mainly found on Jinfo Mountain (the highest peak of the Dalou Mountains in Nanchuan District, Chongqing, China) at altitudes of 600–800 m. It has high economic and ecological value and plays an important role in the ecological balance and water conservation of the region; plus, it is an important indicator species for the state of the ecological environment[
56,
57]. To date, little research has been reported on
I. nanchuanensis, and in particular, its response to climate change remains unclear. Therefore, using the MaxEnt model to simulate the response of the geographic distribution of
I. nanchuanensis to climate change will not only help to clarify the causes of habitat loss and potential species extinction, but also the effects of strengthened climate change on its potential geographic distribution, thus providing a theoretical and practical basis for conservation management of this species as well as other rare and endangered species.
In this study, based on current distribution data for I. nanchuanensis and climatic factors in different periods, the MaxEnt model, combined with ArcGIS software, was used to simulate the potential habitat of the I. nanchuanensis during the Last Interglacial (LIG), the Last Glacial Maximum (LGM), the current period, and two future periods (2050s and 2070s), to clarify the response of this cherished and critically endangered species to climate change, as well as provide a theoretical and practical basis for conservation management of not only this species but also other rare and endangered species throughout the world living in fragile habitats.
5. Conclusions
This study simulated the geographical distribution of
I. nanchuanensis using the MaxEnt model, and the following conclusions were obtained: (1) The climatic factors affecting the geographical distribution of
I. nanchuanensis are Bio4, Bio6, Bio7, Bio13, Bio15, and Bio17 (see
Table 1 for definitions). (2)
Ilex nanchuanensis migrated to the northeast in historical periods but is projected to migrate northwest in the future. (3) The total suitable area for
I. nanchuanensis was projected to reduce the most under RCP6 in the 2070s (17.779519×10
4 km
2), and the least (3.937494×10
4 km
2) under RCP4.5. To address the threat of global climate change to
I. nanchuanensis, a protected area centered on Nanchuan District in Chongqing, China, should be established, and germplasm resources protected and cultivated, with core, buffer, and experimental areas scientifically and rationally delineated. In addition, greenhouse gas emissions need to be reduced, and future greenhouse gas emissions need to be controlled within the scenario of RCP4.5.