3.6 Niche differentiation analysis and priority reserve planning
In Figure 6, the green and red represent the niche spaces of the future
and the current periods, respectively, and the blue represents the
overlapping space. The figure shows that the degree of niche
differentiation in different periods of the 2050s and 2090s showed a
downward trend. The niche distribution degree of population distribution
in the same period of the 2050s and the 2090s showed an increasing
trend. The size of niche overlap reflects the similarity of plant
utilization resources. Large niche overlap indicates that they have
similar ecological adaptability, resource utilization, and biological
characteristics under certain environmental conditions (Yuan et al.,
2021). Compared with the population distribution in the current period,
the D value of the population distribution in the future period is
greater than 0.6; the degree of niche overlap is large; the adaptability
to habitat resources is similar, and the resource competition is fierce,
so there is no significant niche differentiation. By calculating the
niche breadth of Leonurus japonicus in each period via the
ENMTools software package, the results showed that the niche breadth
values of Leonurus japonicus in each period in the future, i.e.,
B1 (minimum value is 0.581) and B2 (minimum value is 0.967), are larger
than those of Leonurus japonicus in the current period (i.e., B1
(0.543) and B2 (0.961)). This indicates that the niche breadth ofLeonurus japonicus in each period in the future will be larger
than that in the current period, and its distribution will be more
extensive. There is a greater opportunity for niche overlap with the
current species, and more intense competition is likely to occur in the
case of limited available resources. The results are consistent with the
above ecospat package analysis.
The planning of Leonurus japonicus priority protection areas was
proposed according to the principle of maintaining ecosystem integrity
and species habitat connectivity and ensuring consistent protection and
management. The priority protected areas are shown in Figure 7. The map
shows that priority protected areas included southeastern Anhui,
southwestern Hubei, southeastern and northeastern Chongqing,
southeastern Sichuan, eastern Yunnan, Guizhou, Fujian, Guangxi,
southwestern Hubei, southeastern and northwestern Zhejiang, northeastern
Guangdong, a small part of western Jiangxi, and a small part of the
south. These places are highly coincident with the high and middle
suitable areas, indicating that the modeling prediction results were
accurate.
4. Discussion
4.1. Optimization of the model for predicting the current suitable area
of Leonurus japonicus
This study used the ENMeval data package to optimize the model based on
climate, topography, and soil factors. This method limits the background
data to the area corresponding to the calibration location, so that the
potential geographical distribution area simulated by Maxent covers the
current distribution points. This method allows the adjustment of model
parameters to improve the performance of the Maxent model, and then an
adjustment test is performed by changing the regularization level,
thereby reducing the complexity of the model. Finally, the accuracy of
the model is measured by improving the fitting degree between the
predicted results and the actual distribution area and by visual
inspection of the geographic prediction map (Phillips et al., 2017). The
Maxent model with optimized parameters can efficientlyreduce the
complexity of the model, improve the fitting degree between the
predicted results and the actual results, and predict the distribution
of species well. The response curve becomes smooth and approach the
normal distribution curve, conforming to the Shelford tolerance rule
(Ouyang et al., 2019; Li et al., 2018; Phillips et al., 2017).
Leonurus japonicus grows in various habitats, mostly in areas
with the sun exposure, and it can grow in general soils. It is
distributed in Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan,
Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin,
Liaoning, Inner Mongolia, Shaanxi, Shandong, Shanxi, Sichuan, Yunnan,
Zhejiang, and other places
(http://www.iplant.cn/info/Leonurus%20japonicus?t=f). In this research,
based on the optimized maximum entropy model, the current suitable areas
of Leonurus japonicus were predicted, and the results were
consistent with the actual distribution, indicating that the Maxent
model was reliable and accurate for predicting the distribution ofLeonurus japonicus .
4.2. Restriction of Environmental Variables on Geographical Distribution
of Leonurus japonicus
The highest temperature in the hottest month, the lowest temperature in
the coldest month, precipitation in the wettest month, altitude, and
precipitation in the driest month were the main environmental factors
affecting the distribution of L. japonicus . To understand the
influence of main environmental factors on the distribution ofLeonurus japonicus , the relationship between distribution
probability and ecological factors was judged according to the response
curve. It was assumed that when the distribution probability ofLeonurus japonicus was greater than 0.5, the corresponding
environmental factors were suitable for plant growth (Guo et al., 2017;
Jia et al., 2019). In this study, when the probability was greater than
0.5, the highest temperature range of the hottest month was 26.8–33°C,
and the highest temperature of the hottest month most suitable for the
growth of Leonurus japonicus was about 31°C. When the probability
was greater than 0.5, the range of the lowest temperature in the coldest
month was −8–11.5°C; the lowest temperature in the coldest month that
was most suitable for the growth of Leonurus japonicus was about
8.3°C, a result that was consistent with the research of Wang and
colleagues (Wang et al., 2015) suggesting that the suitable temperature
for the growth of Leonurus japonicus is 22–30°C, at high
temperatures above 35°C the plant grows well, and the growth is slow
below 15°C. In this forecast, when the probability was greater than 0.5,
the wettest monthly precipitation was in the range of 178–370 mm, and
the wettest monthly precipitation that was most suitable for the growth
of L. japonicus was about 240 mm. The most suitable altitude for
the growth of L. japonicus was about 200 m, which fully reflects
the warm and humid climate preference of L. japonicus and is
suitable for growth below a 1000-m altitude (Wang et al., 2015).
This study shows that precipitation, topography, and temperature
determine the potential geographical distribution of Leonurus
japonicus on a certain scale. Although precipitation play an important
role in the potential distribution pattern of L. japonicus,topography and temperature factors will also redistribute precipitation
resources to a certain extent. Therefore, the effects of temperature,
precipitation, and topography on the potential distribution pattern ofL. japonicus cannot be ignored, and the interactions between
these factors affect the potential distribution pattern of L.
japonicus .
4.3. Effect of Climate Warming on Geographical Distribution of