Introduction

Anthropogenic actions have caused environmental degradation, leading to global biodiversity loss and higher species extinctionrates1. These effects are visible in hotspots across Africa2, Australia3, Asia4, the America5, and Europe6, with more pronounced effects in tropical areas7,8. Ambitious conservation efforts have been implemented to tackle the global biodiversity crisis, including in situ, ex situ, and in vitro conservation of endangered species worldwide9,10,11. New global targets under the post-2020 Global Biodiversity Framework are aimed at conserving at least 30% of the surface of the Earth by 203012. As a cornerstone of biodiversity conservation, protected areas comprise a promising method for achieving this target13,14,15.

Protected areas play crucial roles in conserving biodiversity and maintaining ecosystem services16. They provide relatively undisturbed habitats for the protection of threatened and endangered species. Although more evidence is needed in order to understand the impact of protected areas on wildlife populations, land conservation practices through the establishment of protected areas have been shown to effectively reduce deforestation and buffer against the rapid process of agricultural and urban expansion17,18. In addition, protected areas could serve as climate change refugia for biodiversity19. In Great Britain, protected areas support more species than unprotected areas do20. For example, rare and declining bird species benefit most from the designation of protected areas for conservation in the UK21. Additionally, protected areas have a positive impact on the conservation of bird species in many of the world’s most diverse and threatened terrestrial ecosystems22. Research indicates that protected areas can effectively maintain populations of mammals within their boundaries through active management, overcoming the challenges of lower reproductive rates and more severe threats experienced by larger species23. Meanwhile, the nearby unprotected areas were also associated with substantially enhanced mammal diversity in the extensive reserves24. Equally as important, achieving the post-2020 Global Biodiversity Framework target for terrestrial biodiversity conservation would bring an additional ~ 2.8 million ha of habitat under protection, which would benefit 1,134 ± 175 vertebrate species whose habitats currently lack any form of protection16.

Accelerated human population growth at the edges of protected areas is a looming threat to protected area effectiveness and biodiversityconservation25,26. One-third of the global protected land is under intense human pressure27; at least 64 million square kilometers (44% of the terrestrial area) require conservation attention (the interventions ranging from protected areas to other land-use policies)16. Several studies have indicated that habitat fragmentation and loss, along with human disturbance, lead to biodiversity and species loss in protected areas. A synthesis of 35 years fragmentation experiments spanning multiple biomes and scales demonstrated that habitat fragmentation reduces biodiversity by 13–75% and undermined key ecosystem functions by decreasing biomass and altering nutrient cycles28.Large carnivores in Africa are suffering rapid declines due to human activities, rapid human population growth and land use patterns29.Apex and meso mammals populations were negatively affected by habitat fragmentation on Amazonian forest islands30.Furthermore, forest fragmentation and the loss of fauna also could trigger an atypical ecological chain reaction31.The ‘ecosystem decay’ hypothesis predicts the exacerbation of biodiversity loss with habitat loss, specifically, that ecological processes change in smaller and more-isolated habitats in such a way that more species are lost than would be expected simply through loss of habitat alone32.

In China, the protected area system has been evolving for nearly 70 years, since the first nature reserve was established in Dinghushan, Guangdong Province, in 1956. However, 98.2% of China’s nature reserves were established after 1980, with a boom in the number of protected areas over the past 40 years33. By 2017, 2,750 nature reserves formed the backbone of the country’s protected area network34.Additionally, more than 11,800 protected areas have been established, covering 18% of the land area and 4.6% of China’s sea area in 201935. Stopping global biodiversity loss requires policies and actions to address all major drivers and their interactions36. Since 2015, the central government started to redesignate some nature reserves as centrally controlled national parks in order to establish “one protected area with one title, one regulation, and one management agency” and to promote the development of a coherent national park management system37.

The milu deer (also known as Père David’s deer, Elaphurusdavidianus, 1865), is a large herbivore endemic to China, typically inhabiting wetlands in warm and humid plains before its extirpation in the wild. The deer is currently listed on the IUCN Red List as ‘Extinct in the Wild’ and is also under the class I species in the List of Key Protected Wildlife in China. This species likely became extinct in the wild 1,500 years ago but was kept extant in captive holdings. Records and fossils show that milu were once widely distributed throughout the wetlands of the Yangtze River basin and the Yellow River basin, and the last captive population in China disappeared from Nanhaizi in Beijing in 1900 due to floods and war38, but not before a few individuals were sent to Europe. At that time, only 18 individuals survived in the world, all of which were gathered and protected by 11th Duke of Bedfordshire at the Woburn Abbey, Bedfordshire, England in the last decade of the 19th century38,39. The recovery of milu in China began when 77 individuals were reintroduced from England into Beijing and Dafeng, Jiangsu Province, between 1985 and 1987. Through nearly 40 years of effort in in situ and ex situ conservation of milu, especially through nature reserve conservation and management, the extinction of milu in China has been reversed through the efforts of scientists, nature conservation workers and the Chinese government40,41. Since its reintroduction in the wilds of China from captive European individuals, the population of milu has recovered rapidly. There were 671 milu in 199742, and ca. 9,062 milu in 2020 (including 2,825 wild individuals) distributed across 83 sites, with 7,380 individuals living in Beijing Milu Park, Jiangsu Dafeng Milu Nature Reserve, and Hubei Shishou Milu Nature Reserve40.

Although there have been recent renewed commitments to increase the extent of protected areas to combat the growing biodiversity crisis, the underpinning evidence for protected area effectiveness is rarely substantiated21,43, and similar studies on the impact of protected areas on wildlife populations are lacking44. Nonetheless, the population of Eurasian lynx (Lynx lynx) increased nearly twice from 2009 to 2018 in a strictly protected area in the Bohemian Forest Ecosystem45. Since 1974, many species distributions have declined.After analyzing the distribution changes of over 2,800 animal and plant species from 1974 to 2014, Cunningham et al. (2021)43found that protected areas had a small positive impact over time on landscape-level representation trends of declining species and priority species, although the declining distribution trends were similar (on average) in landscapes containing protected areas and unprotected areas. Moreover, monitoring focal species populations over time to understand population trends and evaluate the impact of management actions is necessary for sound wildlife management within and outside of protected areas18,21.

To assess the long-term contribution of protected areas to rare species population rejuvenation, we used China’s milu deer population – once extinct in the wild, but now recovering via rewilding schemes – as a case study. Through long-term monitoring, we collected data on free-ranging and wild milu populations from 1998 to 2020. We fitted their population growth curves using statistical methods to assess the wild population dynamics of milu in protected areas and unprotected areas in China. Our objectives were to: (1) assess the population growth trends of milu, (2) explore habitat changes in free-ranging and wild milu populations from 1998 to 2020, and (3) analyze the contribution of protected areas to the status of milu deer in China.

Materials and methods

Study area

The subjects of the study were six milu populations, including milu in Jiangsu Dafeng Milu National Nature Reserve (Dafeng Reserve) in Yancheng City, Hubei Shishou Milu National Nature Reserve (Shishou Reserve) in Shishou County, Yangbotan Wetland in Shishou County, Sanheyuan Wetland in Shishou County, Hunan East Dongting Lake National Nature Reserve (Dongting Reserve) in Dongting Lake, and Jiangxi Poyang Lake National Wetland Park (Poyang) in Poyang Lake (Table 1; Fig. 1). The wild population from Dafeng Reserve could migrate and diffuse beyond the nature reserve to areas within the protected areas of Jiangsu Yancheng Wetland National Nature Reserve in Yancheng City. The wild populations in Dongting Reserve and Poyang migrate and diffuse outside the nature reserve freely in the Dongting Lake basin and Poyang Lake basin respectively.

Table 1 Description of the main free-ranging and wild areas of milu deer in China.
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Fig. 1
figure 1

Study area: (A) Jiangsu Yancheng Wetland National Nature Reserve in Yancheng City; (B) Dafeng Reserve in Yancheng City; (C) Yangbotan wetland in Shishou County; (D) Sanheyuan wetland in Shishou County; (E) Shishou Reserve in Shishou County; (F) Protected areas in Dongting Lake; (G) Protected areas in Poyang Lake. The satellite data was sourced from Geographic remote sensing ecological network platform (www.gisrs.cn).

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The founder populations of milu in Sanheyuan, Yangbotan, and Dongting Reserve were initially from Shishou Reserve in 1998. That year, the flooding of the Yangtze River allowed some animals to swim outside the fence of the reserve, thus spreading to other areas. In the Yancheng Wetland National Nature Reserve of Yancheng City, Jiangsu Province, there was also a small population of rewilded milu, but we excluded this population due to the lack of long-term continuous monitoring.

Data collection

For free-ranging and wild populations in five of the six areas studied (Dafeng Reserve, Shishou Reserve, Dongting Reserve, Yangbotan wetland and Sanheyuan wetland), we conducted specialized surveys on wild deer during winter and early spring (when the herbaceous vegetation turned yellow and became relatively short, or after the reeds had been harvested)and in summer after the calving period or during the mating season. We also conducted monthly surveys on the deer in Shishou Reserve, Yangbotan wetland and Sanheyuan wetland.During the epidemic time of hemorrhagic enteritis for milu in 2010 spring in Shishou Reserve, we conducted our count based on the observed number of deaths. The population size is based on the maximum number observed during the same survey period. Because of the difficulties of performing a synchronized survey for milu in Poyang, where the main body area and water body area of Poyang Lake is nearly 5000 km2, the annual population number of milu in this region was obtained by ‘Observed number of birth’ minus ‘Observed number of death’. The population number in the six areas were all based on the maximum value in the same period, avoiding duplicating individuals in time and space.

The data collection periods were showed in Table 1. Since 2017, hundreds of milu have been released from Dafeng Reserve into the wild areas surrounding Yancheng City. The time period used for the wild population in Dafeng Reserve was, therefore, from 1998 to 2016. Additional reintroduction events of milu into the main free-ranging and wild areas were also recorded(Table 2).

Table 2 Additional reintroduction events of milu intothe main free-ranging and wild areas in China.
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Data analysis

All statistical analyses were performed using SPSS 22 (IBM Corp., Armonk, NY, USA). We used a curvilinear regression analysis to fit the curves of the population dynamics in these populations, including eight different models (Linear, Quadratic, Growth, Cubic, S, Logistic, Power, and Exponential). We chose the model with the largest R2 value and a significance result of less than 0.0001, also referring to their scatter plots.In the wild, it is difficult for us to accurately count the number of births and deaths, so we primarily use the population growth rate.

Population growth rate = (Nt-Nt−1-Rt)/Nt−1.

(Nt is the population number in the end of year t, Rt is the number of reintroduction individuals in year t, Nt−1 is the population number in the end of year t-1)

Satellite datasets were obtained from the Landsat TM image of the United States Land Resources Satellite with a spatial resolution of 30 m and processed using the software ERDAS Image 9.0 remote sensing processing and ArcGIS 9.3.Data of human population density was based on the WorldPop dataset.

Results

Population dynamics of milu in protected areas and unprotected areas

There were over 3201 survey days on the free-ranging and wild milu deer during 1998 to 2022. The statistical results showed that (1) in the coastal marshy wetland of Dafeng Reserve, the population growth had a S-curve (with an initial phase accompanied by slow population growth, but it was about to enter a period of rapid growth)(R2 = 0.936, p < 0.001) (Fig. 2). (2) In the riverine wetlands of the outer nature reserve in Shishou Reserve, which included Sanheyuan(with an initial phase accompanied by slow population growth) (R2 = 0.499, p < 0.001) and Yangbotan(with an initial phase accompanied by slow population growth, but it was about to enter a period of rapid growth) (R2 = 0.980, p < 0.001), and in Shishou Reserve(with an acceleration phase, although there was a cliff-like drop in 2010) (R2 = 0.797, p < 0.001), the population growth curves were all S-curve.3) In the lake wetlands, the population growth in Dongting Reserve had an S-curve (with an initial phase accompanied by slow population growth, but it was about to enter a period of rapid growth)(R2 = 0.979, p < 0.001), while that in Poyang was linear (R2 = 0.950, p < 0.001).

Fig. 2
figure 2

Population growth curves for Jiangsu Dafeng Milu National Nature Reserve (Dafeng Reserve), Hubei Shishou Milu National Nature Reserve (Shishou Reserve), Sanheyuan, Yangbotan, Hunan East Dongting Lake National Nature Reserve (Dongting Reserve), and Jiangxi Poyang Lake National Wetland Park (Poyang).

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However, the average population growth rate sat Dafeng Reserve, Sanheyuan, Yangbotan, Dongting Reserve, and Shishou Reserve were 0.151, 0.135, 0.183, 0.132, and 0.185, respectively. There were no statistically significant differences in population growth rates among Dafeng Reserve, Sanheyuan, Yangbotan, Dongting Reserve, and Shishou Reserve, with the only exception being that the growth rate of the population of Yangbotan was significantly higher than that of Dongting Reserve (p < 0.05). The founder population of Poyang was 47 individuals (23♂, 24♀) and the average annual population growth rate was 0.107.

Habitat fragmentation in protected and unprotected areas

According to the land use map – based on the distribution of farmland, water, construction land and wetlands – Fig. 3 maps the land use of Yancheng City, Jiangsu Province, including in Dafeng Reserve and Yancheng Wetland Nature Reserve in 1998 and 2016. The figure shows that the habitat began recovering after the protected area was established, owing to the transformation of construction land to wetland (Fig. 3). Figure 4 shows the land-use map of the Dongting Lake area in 1998 and 2020, which shows that the wetland habitats (water and swamp) of the protected area of Dongting Lake (including Dongting Reserve, Hunan South Dongting Lake Provincial Nature Reserve, Hunan West Dongting Lake National Nature Reserve, and Jicheng Milu Provincial Nature Reserve of Hunan Province) were well maintained and did not fragment. Owing to management and the presence of a fence, the habitat of Shishou Reserve was also well maintained (Fig. 5). However, Fig. 5 shows that the habitat of the unprotected area of Sanheyuan and Yangbotan were invaded and occupied by anthropogenic activities such as the construction of a fishpond, planting of economic forest, and farming.

Fig. 3
figure 3

Land-use map of Yancheng City, Jiangsu Province, including Jiangsu Dafeng Milu National Nature Reserve (Dafeng Reserve) and Yancheng Wetland National Nature Reserve in 1998 (left) and 2016 (right). (A) construction land; (B) wetlands. The satellite data was sourced from Geographic remote sensing ecological network platform (www.gisrs.cn).

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Fig. 4
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Land-use map of the Dongting Lake area in 1998 (left) and 2020 (right). (A) Hunan East Dongting Lake National Nature Reserve (Dongting Reserve); (B) Hunan South Dongting Lake Provincial Nature Reserve; (C) Hunan West Dongting Lake National Nature Reserve; (D) Jicheng Milu Provincial Nature Reserve of Hunan Province. The satellite data was sourced from Geographic remote sensing ecological network platform (www.gisrs.cn).

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Fig. 5
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Satellite images of Hubei Shishou Milu National Nature Reserve (Shishou Reserve), Sanheyuan, and Yangbotan in 2014 (left) and 2023 (right) (A)Fishpond; (B) plantation; (C) farmland. The solid red line represents the PA boundary, whereas the dotted red line represents the unprotected area boundary. Satellite images are from https://livingatlas.arcgis.com/and processed using Photoshop CS6.

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Human population density and floods in protected areas

No human beings have been living in Shishou Reserve since its establishment in 1993. The human density was highest in Jicheng Milu Provincial Nature Reserve in the protected areas of the Dongting Lake area, and the human population density in Hunan South Dongting Lake Provincial Nature Reserve was higher than that in the other two National Nature Reserves of Dongting Lake (Fig. 6 (a)). Figure 6 (b) shows the concentration of the human population at the edge of the protected area of Yancheng City. There was a higher human density in the east and west of the Poyang Lake area than in the protected areas in its middle part (Fig. 6 (c)).

Fig. 6
figure 6

Human population density in Protected areas. (a) Dongting Lake, Hunan Province. (A) Hunan East Dongting Lake National Nature Reserve (Dongting Reserve); (B) Hunan South Dongting Lake Provincial Nature Reserve; (C) Hunan West Dongting Lake National Nature Reserve; (D) Jicheng Milu Provincial Nature Reserve of Hunan Province. (b) Yancheng City, Jiangsu Province. (c) Poyang Lake, Jiangxi Province. The satellite data was sourced from Geographic remote sensing ecological network platform (www.gisrs.cn). Data of human population density was based on the WorldPop dataset (https://hub.worldpop.org/). Processing: with a spatial resolution of 30 m and processed using the software ERDAS Image 9.0 remote sensing processing, Envi 5.3 and ArcGIS 9.3.

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Because Shishou Reserve, Sanheyuan, and Yangbotan are on the old route of the Yangtze River, and Dafeng Reserve is on the Yellow Sea mudflat, their protected areas are not influenced by floods. However, Poyang Lake and Dongting Lake are the two biggest freshwater lakes in China; therefore, the protected areas in Poyang and Dongting Reserve are influenced by summer floods every year. The satellite image of the Poyang Lake area showed that the water area was 4,819.48 km2in the flood period (July 2020) and less than 600 km2 in an extreme dry season (December 2022), showing that the flood influenced the milu habitat area in the wild wetlands of the Poyang Lake area (Fig. 7).

Fig. 7
figure 7

Satellite image of the Poyang Lake area during the flood season (July 2020) and extreme dry season (December 2022). The satellite data was sourced from Geographic remote sensing ecological network platform (www.gisrs.cn).

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Discussion

The IUCN World Conservation Congress in 2020 called for ‘Extinct in the Wild’ (EW) species to be re-established in the wild by 2030.However, this requires that the conservation community, legislators and the public have a collective realization in the minds of the existence and plight of these EW species and spare no effort to protect them11. Our study highlights the important role that protected areas can play in the conservation of endangered species (especially EW species), through protecting them and their habitat, reducing human disturbances and preventing humans from changing the landscape and documents the successful recovery of one EW species so that their wild populations are now rebounding.

Conservation policy in protected areas

Some mammals have experienced massive contractions or total extinction in their geographic ranges during historical times46. As we face a global extinction crisis, international commitments promise to prevent the extinction of threatened species. However, some of the most threatened species that became extinct in the wild have often been overlooked. In some cases, rebuilding species in the wild has been successful, but only about a quarter of EW species have been attempted to be rewilded at the moment11. The introduction of rare species into protected areas has proven to be one of the most effective ways to conserve and restore these species40.

Habitat conservation is essential for rare species, and with the country’s reform and opening up, China has established many nature reserves for wildlife and their habitats33. Indeed, many protected areas were established primarily because they targeted specific rare and endangered species, such as Zhalong National Nature Reserve of Heilongjiang for the red-crowned crane (Grus japonensis) in 1979, Bawangling National Nature Reserve of Heilongjiang for the Hainan gibbon (Nomascus hainanus) in 1980, Chinese Alligator National Nature Reserve of Anhui for the Chinese alligator (Alligator sinensis) in 1982, Kekexili Reserve of Qinghai for the Tibetan antelope (Pantholops hodgsonii) in 1995, and Hunchun Siberian Tiger National Nature Reserve of Jilin for the Siberian tiger (Panthera tigrisaltaica) in 2001.

For milu, a species that has been extinct in the wild for approximately 1,500 years, there was one breeding base and three nature reserves established by the government. Beijing Milu Park and Jiangsu Dafeng Milu National Nature Reserve were established specifically for the purpose of milu reintroduction in 1985 and 1986, respectively, and Shishou Reserve in Hubei Province was established in 1992 specifically for rewilding and releasing milu40. Jicheng Milu Provincial Nature Reserve in Hunan Province was established in 2000. To enlarge the area of milu habitat, the area of Dafeng Reserve was increased from 10 km2to 26.67 km2in 1996. After two expansions, the area of Shishou Reserve had been expanded from the initial 0.75 km2to 10 km2and then to 15.33 km2. To implement stringent actions to save wildlife and its habitat, Dafeng Reserve and Shishou Reserve were raised from provincial nature reserves to become national nature reserves in 1997 and 1998, respectively.

Mitigating human disturbances, such as fishing, farming, and harvesting reeds, have been an important factor in the conservation of milu in Dafeng Reserve and Dongting Reserve47,48. With the support of local governments and good management of the environment, the Migratory Bird Sanctuaries along the coast of the Yellow Sea-Bohai Gulf of China, which mostly comprise the Dafeng Reserve and Jiangsu Yancheng Wetland National Nature Reserve, where milu can freely forage, were inscribed as World Natural Heritage Sites in 2019. Similarly, the ten-year fishing ban that was implemented in the Yangtze River basin, including in Poyang Lake and Doting Lake in January 2020, also improved the habitat for wild milu populations in the Yangtze River basin wetlands, particularly the Dongting Lake wetlands and Poyang Lake wetlands. The reduced disturbance by humans in this area further aid milu conservation efforts40.

To protect the wetlands and migrating waterbirds of Poyang Lake, many nature reserves have been established, forming a small network of protected areas. Thus, through our monitoring, milu could freely forage across the entire southern part of Poyang Lake.

Population dynamics in protected areas and unprotected areas

Population dynamics are an important indicator for measuring and testing the success of the conservation of rare species and of protected area establishment19,21. Among the protected and unprotected areas investigated in the present study, there was a significant annual increase in population growth (Fig. 2). The average milu population growth rate was the highest in Shishou Reserve.This is most likely because there was a fence around this protected area, which could have isolated the deer from human activities, and also because the founder population had 64 individuals, which was 1.5times to 11.8 times higher than that of the populations in protected areas other than Poyang. Although there was an epidemic of hemorrhagic enteritis for milu in 2010 spring in Shishou Reserve, that resulted in the deaths of over 300individuals, since 2015, the milu population entered another period of rapid growth (Fig. 2)49. Despite having a large founding population, the average population growth rate of milu in Poyang was the lowest, likely owing to the flooding every summer that caused the deer to migrate away from Poyang Lake district.High rates of human-caused mortality outside protected areas and the increasing popularity of recreational activities can negatively affect wildlife populations50. For example, abandoned fishnets are one of the negative factors affecting population increases of milu in Poyang and Dongting Reserve40. Though the human-mediated introductions could unduly affect the population growth rate in Dafeng Reserve, there was no statistically significant difference between its growth rate and that of Sanheyuan, Yangbotan, Dongting Reserve.

For unprotected areas in Shishou County, the habitat types were all riverine wetlands in Shishou Reserve, Sanheyuan, and Yangbotan, and the population of Sanheyuan grew more slowly than that of Yangbotan and Shishou Reserve because there was an epidemic of hemorrhagic enteritis which affected the population of Sanheyuan and Shishou Reserve in 201049. According to our monitoring and observations, factors such as abandoned fishing nets, floods, anthropogenic causes of reed habitat loss, and artificial harvesting of wild vegetables could lead to a higher mortality rate of adults and a lower survival rate of fawn in the Sanheyuan milu population as compared to the Shishou Reserve milu population.

Land use change threats in unprotected areas and human pressure

Research has shown that human population size and current growth rates have largely contributed to the loss of biodiversity51. The destruction and degradation of natural ecosystems are the primary causes of decline in global biodiversity. It is generally believed that protected areas can effectively reduce habitat fragmentation and thus promote species conservation52. Mammal occurrence dynamics are affected by anthropogenic stressors acting at multiple scales, including outside protected areas.The survival probability of generalists (such as Leopardus pardalis and Papio anubis) is higher near the edge than in the core of protected areas when landscape-scale human population density is low; whilst the opposite is true when human population density is high46. However, many protected areas worldwide have suffered from habitat destruction. For example, analyses of remote sensing data from pre- and post-establishment periods indicate that protected areas have become more fragmented and less suitable for giant pandas (Ailuropoda melanoleuca) inhabiting the Wolong Nature Reserve (southwestern China). About twenty years ago, after the creation of the reserve, in stark contrast to the situation before the establishment of protected areas, the rates of habitat loss and fragmentation inside protected areas have unexpectedly risen to levels similar to or higher than those outside protected areas53.

In China, ecological red lines initiated by the State Council of China in 2011 were of vital importance for enhancing integrated ecosystem management and ecological protection in the country54. China had launched the construction of national parks at the national level, starting with the first proposal to establish a national park system in 201355. Many rare and endangered species such as the giant panda56, crested ibis57 (Nipponia nippon), Tibetan antelope58and snow leopard59 (Panthera uncia) have clearly recovered after these initiatives. Our study results show that the milu habitat has recovered after the protected areas of Dongting Lake (including Dongting Reserve, Hunan South Dongting Lake Provincial Nature Reserve, Hunan West Dongting Lake National Nature Reserve, and Jicheng Milu Provincial Nature Reserve of Hunan Province) were established because in the process of establishing the protected areas, construction land was transformed back into wetlands and wetland habitat (water and swamp). These reserves have also been well maintained and did not fragment. Thus, the milu in these protected areas were effectively protected, and their populations increased annually. Although the populations of milu in Sanheyuan and Yangbotan have grown, these unprotected areas have been invaded and occupied by anthropogenic activities such as the building of fishponds, planting of economic forests, and farming since 2017. Milu in these unprotected areas could be under human pressure (there has been no new data available since2020 because of the COVID-19 pandemic).

As the population increases, milu have begun to disperse randomly because of population density, resource competition, floods, and human disturbances60. To increase the wild population sizes, there were several occasions during which hundreds of milu were released into the wild habitat around Yancheng City from Dafeng Reserve since 2017. We assume that there are now more than 3,300 individuals in the Yellow Sea mudflat north of Dafeng Port and south of Chongming Island, Shanghai, owing to the lower human population density and the long Yellow Sea mudflat.

Owing to floods and human interferences, the milu population of Dongting Reserve grew slowly, and the deer migrated to Hunan South Dongting Lake Provincial Nature Reserve, Hunan West Dongting Lake National Nature Reserve, and Jicheng Milu Provincial Nature Reserve60. Apparently, we assume that the population of milu in Poyang was growing slowest due to flooding and higher population density, and when flooding hits and human activity increases, they had to move out of protected areas.

Conservation implications

Given the importance placed on protected areas, determining their effectiveness for rejuvenating and maintaining biodiversity is a core issue in conservation biology61. The results of this study are consistent with those of previous studies showing that protected areas provide a habitat with less human interference for endangered species and allow their populations to increase. Artificial rearing and breeding have provided the basic population for milu rewilding. Continuous supplementation of the population of free-ranging and wild milu is a good measure to restore milu to the wild. In addition, a larger founding population is more conducive to the rapid recovery of reintroduced endangered species than a smaller founding population.Finally, a strict and interconnected system of protected areas increases the total area of protected areas and promotes the recovery of rare species.

Our results indicate that milu have recovered very well in the wild since rewilding has started, especially in the protected areas, despite the small size of the founding populations. Moreover, the genetic diversity of milu in the rewilded populations seemed to improve, with the heterozygosity values and allelic richness in the Shishou Reserve population being higher than those of Dafeng Reserve62. Milu deer rewilding demonstrates a powerful case for rewilding efforts, being consistent with the specific targets related to genetic diversity in the Kunming-Montreal Global Biodiversity Framework (Goal A, Target 4), namely to maintain and restore the genetic diversity within and between populations of native, wild and domesticated threatened species to maintain their adaptive potential, including through in situ and ex situ conservation and sustainable management practices63. In addition, with significant disturbances, suitable habitats for milu deer are limited outside protected areas. However, these areas are crucial for milu deer and require effective conservation measures, such as establishing Other Effective area-based Conservation Measures (OECMs).