1. Introduction
Plant interspecific interactions (competition or collaboration) have important implications for species composition and structure and the maintenance of their diversity in ecosystems [
1]. Competition is an important phenomenon in the growth process of terrestrial plants [
2]; it is a dominant factor in the formation of a community’s structure, and an important factor in determining the evolutionary pattern of species [
3]. Environmental changes and human activities can lead to a more hostile environment for plant species to survive in [
4,
5,
6]. For endangered plants, the spatial distribution of competing species and the impact of environmental changes on their distribution patterns are directly related to a plant population’s ability to reproduce [
7]. Therefore, analyzing changes in the geographic distribution patterns of endangered plants and their competing species under current and future environmental patterns, and spatially exploring the impacts of competing species on endangered plants, will contribute to the conservation and utilization of endangered species.
The Species Distribution Model (SDM) is an important modeling tool based on the ecological niche theory which is to predict the fitness zones of species under current or future climatic conditions based on the known distribution points of the species and relevant environmental factors [
8,
9,
10]. Among the models, the Maximum Entropy Model (MaxEnt) is more widely used [
11]. MaxEnt has the advantages of a low sample size requirement [
12], flexible variable handling [
13], good noise reduction, and high budget accuracy [
14]. The Geographical and Temporally Weighted Regression (GTWR) model intuitively detects the non-stationarity of spatial relationships [
15], and takes geographic location and time into account in the regression model to more accurately predict and explain the relationships between variables [
16,
17,
18].
Heptacodium miconioides is an endemic and endangered plant in China which is perennially distributed in hilly and low mountainous areas, which are cool and foggy and have a relative humidity that is often around 90% in summer; it is mainly found in southern China, usually grows to maturity over spring and summer, can flower profusely, and reproduces by seed.
H. miconioides is listed as an endangered species (EN) by the World Conservation Union (IUCN) [
19]. As an excellent ornamental tree,
H. miconioides is scientifically valuable for studying the phylogeny of the
Caprifoliaceae Juss. Some scholars have found that the main reasons for the endangerment of plant species are the negative effects of competing species and environmental changes. The main competing species within the vicinity of
H. miconioides is the
F. insularis [
20,
21].
Fraxinus insularis likes warm and humid climates, is not strict on the soil requirements, although well-drained, humus-rich slopes and woodlands are preferable; the range of
F. insularis’ distribution is wide, and it basically occupies the same areas as
H. miconioides [
22]. Previous studies have not explored the distribution patterns of plants and competing species in large-scale spaces. Competing species can inhibit
H. miconioides’ growth space and resource utilization, leading to changes in the spatial distribution pattern of the species;
F. insularis may occupy more space in a given area, while
H. miconioides retreats to smaller areas. Species competition prevents us from further improving the conservation of rare and endangered species. Based on this, in our study, we applied MaxEnt to analyze the potential distribution areas of
H. miconioides and its competitor species under the current environmental model and their responses to the future environmental model. The results obtained from the MaxEnt model were also coupled with the GTWR model to explore the interactions between
H. miconioides and competing species in the current and future environments to better protect the survival space of the rare and endangered species,
H. miconioides.
4. Discussion
The MaxEnt model’s prediction accuracy is mainly determined by a combination of species distribution data and environmental factors involved in model construction [
44]. Since
H. miconioides mainly grows on cliffs ranging from 600 to 1000 m, not only were factors such as temperature and precipitation taken into account, but three topographic factors were also selected. It can be seen that the environmental factors selected for this study have a strong coupling with the species’ distribution points and could be better applied in a habitat suitability study of the two species.
According to the results for the contribution rate of the evaluation factors, the main factors affecting the living environment of
H. miconioides and
F. insularis are temperature and precipitation. The study by Rawat et al. [
45] similarly found that the environmental factors most affecting endangered species in hilly terrain are temperature and precipitation, which is in agreement with the present study. The distribution of
H. miconioides and
F. insularis is more dependent on precipitation than factors such as temperature and topography. Precipitation in Driest Quarter (Bio17), a factor contributing up to 52.8% and 49.8%, was the most critical factor influencing the distribution of the fitness zones of
H. miconioides and
F. insularis. This is in line with Yi’s study [
46], which examined the environmental influences on the endangered plant
Homonoia riparia in southern China and found that the amount of Precipitation in Driest Quarter (Bio17) was likewise the most critical factor affecting the plant species. When the Precipitation in Driest Quarter (Bio17) exceeded 90 mm, the probability of fitness for
H. miconioides increased with the increase in rainfall, and after exceeding 150 mm, the probability of fitness was practically unchanged, which is consistent with the characteristics of
H. miconioides as it prefers wet climate in the dry period and is intolerant to flooding. The impact factor curve for
F. insularis is similar to that of
H. miconioides, and again Precipitation in Driest Quarter (Bio17) shows a decrease in the probability of fitness when it reaches 150 mm.
The present and future suitable areas are distributed in the subtropical climate zone within the range of 20°~40° N in China, and generally, the lower the latitude, the higher the temperature [
47].
The prediction results from this study showed that the area which was suitable for
H. miconioides was much smaller than that for
F. insularis, and the distribution area of
F. insularis was wide, but the highly suitable area was mainly concentrated in 25°~30° N, which indicated that the latitude might have some limiting effect on the distribution of
F. insularis. Under the 2050 (2040–2060) climate, the potential areas that
H. miconioides and
F. insularis inhabit would expand, but human impacts on the habitats of both could not be ruled out. Under the future environmental change, the average annual precipitation in China will increase by 0 to 20%, especially in southwest and south China, and the surface temperature in China will increase by 2.7 to 2.9 °C [
48]. The area of
H. miconioides expansion has shifted significantly towards the west, which may be related to precipitation [
49]. In contrast, the lower expansion of
F. insularis is most likely related to local socioeconomic development [
50]. Yang et al. [
51] have studied
Magnolia wilsonii, an endangered tree native to China. It has severely declined and become critically endangered in the last few years due to habitat loss and fragmentation. The future environment will be equally favorable for
Magnolia wilsonii. The study by Huang et al. [
52] Found, that for tall tree species, the future environment will be suitable for their survival and show a trend of expansion in suitable growth areas. These findings are also consistent with the results of this study.
The ecological niche overlap usually reflects the phenomenon that two or more species with similar ecological niches share or competition for common resources when they live in the same space [
53]. Due to limited habitat resources, there may be an ecological niche overlap between species in the same geographical area, leading to increased competition among species [
54]. The finite nature of resources and the non-isolated existence of individual organisms make competition an inherent part of biology that cannot be eliminated [
55]. What should be of concern is not whether competition occurs or not, but rather how the different outcomes of the competition will affect the ecosystem as a whole, and in particular, the impact on species’ coexistence [
56]. Mario et al. [
57] concluded that the higher the value of ecological niche overlap between plant species and the stronger the response to the same environmental factors, the more intense the competition between species will be. Kermavnar et al. found that the greater the ecological niche overlap, the more intense the interspecific competition in types of forest vegetation [
58]. As an endemic and endangered plant in China, the
H. miconioides’ survival environment is severe, and in addition to the influence of environmental factors and anthropogenic activities, the influence of competition within its geographical distribution should also be considered [
59,
60].
The spatial distribution of the fitness Index of
F. insularis, the main competitive species of
H. miconioides, was highly and negatively correlated with
H. miconioides. It was found that the negative effect of
F. insulari’ fitness index on
H. miconioides gradually increased, and the area of influence was gradually shifted to the east, and the shift in the area of competition was mostly caused by environmental and anthropogenic disturbances [
61]. Future environments will result in an eastward shift in
F. insularis’ habitat and a westward shift oinf
H. miconioides’ habitat, thus slowing the pressure on western
H. miconioides’ survival. In the Çoban study [
62], a model of the current and potential future distribution of
Quercus libani Olivier, a tree species in Turkey, was created to predict changes in its geographical distribution under different climate change scenarios,
Quercus libani Olivier will also migrate westward in the future. For plant interspecific competitive relationships, Ma et al. [
63]. argued that competitive suppression in plants leads to the severe impairment of nutritional and reproductive growth in vulnerable species and a significant reduction in their survival. Forest management practices can have a significant impact on woody plants in forests, and over-harvesting has led to the destruction of forest habitats, thus causing damage to the balance of forest ecosystems [
64,
65]. Environmental changes will lead to an expansion in the growing ranges of
H. miconioides and
F. insularis, which will cause an increase in the ecological niche overlap, and competition is likewise likely to be more intense. For other competing species, competition may become more intense under future environmental conditions. Through our research, we found that the future survival of Hepatica is also not optimistic. Therefore, we suggest 1. establishing nature reserves 2. promoting ecological restoration and habitat protection, and 3. conducting scientific research and monitoring.