September 25, 2008
Assessing Egret Ecological Safety in the Urban Environment: A Case Study in Xiamen, China
By Lin, Tao Xue, Xiong-zhi; Huang, Jing; Cui, Sheng-hui
Key words: Egrets, ecological safety, assessment, urban environment, Xiamen SUMMARY
In general, wildlife is facing increasing threats from rapid urbanisation in China. Ecological safety for wildlife requires an ecosystem that provides the conditions necessary for sustaining life. In this paper, two major breeding habitats and ten typical foraging habitats of egrets living in the urban environment were selected to assess the extent to which the needs of egrets can be satisfied and maintained. Ecological suitability and impacts from humans are the two major factors affecting egret ecological safety. An ecological indicator system was adopted to assess the ecological suitability of areas. Different land-use types were used to represent different degrees of human impact on the egret habitats, by employing a geographic information system. The results show that the two breeding habitats are at a safe level, while the ten foraging habitats are on the whole marginally safe. Full attention should be given to the conservation of wildlife foraging habitats in Xiamen Island.
Habitat conservation is important to protect biodiversity (Noss 1990; Franklin 1993; Dobson et al. 1997; Myers et al. 2000), and the survival of wildlife depends on safe habitats. Ecological safety means that the ecosystem in which an organism lives provides the conditions necessary for sustaining life. The extent to which a habitat is considered safe depends on two major factors, ecological suitability and human influence upon it. Currently, wildlife is facing increasing threats from the rapid urbanisation in China (You et al. 2005; Wang 2005; Li et al. 2006; Zhao et al. 2006; Weng 2007). Egrets are a common wading bird around coastal areas and have a high sensitivity to changes in local ecosystems (Ruan et al. 2003). With rapid economic development and intense expansion of human activities since Xiamen's transformation into one of China's earliest special economic zones in 1980, the egret habitats are increasingly being invaded and turned into new urban or industrial areas (Xue et al. 2004; Lin et al. 2007). At the same time, egret breeding and foraging conditions have been disturbed directly and indirectly by human activities (Chen and Song 1999; Wu et al. 2001; Fang et al. 2004). The ecological balance between egrets and humans seems to be intensely disturbed, and the egrets are suffering from growing anthropogenic pressures. Here, we examine the pressures that egrets face in habitats in the Xiamen area.
Xiamen City on the southeast coast of China has a coastline of 234 km, an urbanisation rate of 80.1% in 2006 and is one of the five fastest growing Special Economic Zones in China, having a terrestrial area of 1565.09 km^sup 2^ and a marine area of 390 km^sup 2^. Xiamen's coastal wetlands comprise areas of tidal wetlands, shallow sea areas, lagoons and mangroves, which provide suitable natural habitats for egret nesting, breeding and foraging. Xiamen Island, which is known as 'egret island', is downtown of the Xiamen City. Six of the seven egret species in China (Egretta alba, E. intermedia, E. garzetta, E. sacra, E. eulophotes, E. picata and E. novaehollandiea) live on Xiamen Island (Chen et al. 2000). In this study, two major breeding habitats and ten typical foraging habitats of egrets along the Xiamen coast were selected to assess egret ecological safety in the urban environment. Their breeding habitats cover the entire land and inter-tidal area of Dayu Islet and Jiyu Islet, which have been designated as the Xiamen National Nature Reserve. Ten representative foraging locations cover all wetland types used by egrets for foraging, including mudflats (Xiangshan, Liushan, Zhongzhai, Shihushan, Aotou and Gaopu), estuary (Haicang), mangrove (Dongyu), reservoir (Xinglin) and semi-salt lake (Yundang), which are mostly close to or part of Xiamen urban area (Figure 1).
Assessment of ecological suitability for egrets
Ecological factors affecting wildlife habitats consist of biotic and abiotic elements (Ouyang et al. 2001). The former include climate, topography, geology and height above sea level, which will remain stable over long time periods. The latter include vegetation, nesting plant species, foraging areas, habitat ecosystem stability and inter-specific or con-specific competition, which are prone to disturbance by human activities. Focusing on biotic element changes in the Xiamen urban environment, an ecological suitability indicator system was built (Table 1), mainly derived from relevant ecological research on egrets in Xiamen (Wu et al. 2001; Chen and Song 1999; Fang et al. 2002, 2004). Among the indicators, vegetation cover, nesting vegetation species, ecosystem stability and distance to foraging sites were used to assess ecological suitability of egret breeding habitats. Wetland type, water quality, distance to breeding habitats and aquaculture density were used to assess egret foraging habitats. All indicators were assigned assessment values and calculated quantitatively from:
where S is the ecological suitability of egret habitats; s^sup i^ and w^sup i^ are the evaluation value and weight of each ecological suitability indicator, respectively.
Assessment of human impact on egrets
Human disturbances to egrets in Xiamen mostly stem from intense human activities during urbanisation. The interaction between human behaviour and egret safety is complex, consisting of direct impact factors such as artificial light, noise and human intrusion, and indirect factors such as air and water pollution and habitat degradation. The level of intensive human activities has a tight relationship with, and is represented by, land-use type and building density. Referring to previous studies on bird behaviours and tolerance to human intrusion (Stolen 2003; Wang et al. 2004; Li et al. 2002), remote sensing, global positioning systems and geographical information systems (3S), and buffer analysis were used to assess human impacts on egrets. Within an inside buffer zone of 0- 600 m and an outside buffer zone of 600-1200 m, different land-use types around the egret habitats were assigned assessment values (Table 2) and calculated from:
where D is human impact on egrets in their habitats; A^sub 1^ is the area of the inside buffer zone, and a^sub 1i^ is the area of different land-use types within this zone; A^sub 2^ is the area of the outside buffer zone, and a^sub 2i^ is the area of different land- use types within this zone; d^sub i^ is the weighting of each different land-use type as a human impact indicator. It was assumed that human impacts from the outside buffer zone are half of those from the inside buffer zone.
Ecological suitability for egrets
Through quantitatively assessing the value of each indicator and calculating the assessment function, the ecological suitability of the two breeding habitats and ten representative foraging habitats were obtained (Figure 2). In terms of ecologically suitable egret breeding habitats, Jiyu Islet was slightly better than Dayu Islet. Ecosystem stability was the critical factor affecting the breeding habitat ecological suitability in Xiamen. For egret foraging habitats, the average ecological suitability was marginally suitable overall, and Yundang, Xinglin Bay and Haicang were the most suitable habitats from a natural ecological perspective. Water quality was the critical factor affecting ecological suitability of foraging habitats, and was weakly suitable in Dongyu, Shihushan and Gaopu.
Human impacts on egrets
The breeding habitats on Dayu Islet and Jiyu Islet were at a weak level in terms of the human impact, and the impact from the outside buffer zone was stronger than that of the inside. Dayu Islet was subject to higher human disturbance than Jiyu Islet. The foraging habitats were generally at a weak level because of human impact, and those from the outside buffer zone were stronger than those on the inside. The human impacts on the ten representative foraging sites differed remarkably. Yundang and Shihushan are the two areas facing strong human disturbance, whereas Liushan and Xinglin Bay face slight human disturbance. The other six areas had weak to moderate levels. Comparing foraging habitats with breeding habitats, human disturbance on foraging habitats was more severe than on breeding habitats.
Ecological safety of egrets
The ecological safety of egrets can be understood as the maintenance of ecological suitability in spite of anthropogenic impacts, which can be presented by the following assessment function that combines results from the ecological suitability assessment and human impact assessment:
ES = S - D x W^sub D^
where ES is the ecological safety of egrets along the Xiamen coast; Sis the ecological suitability of egret habitats; D is the human impact on the ecological suitability for egrets; and w^sub D^ is the weight of the human impact on breeding and foraging habitats, since egrets would suffer more from human disturbance in foraging habitats than from that in breeding habitats.
The egrets' ecological safety can be divided into five levels: safe (0.8-1.0), marginally safe (0.6-0.8), moderate (0.4-0.6), weakly safe (0.2-0.4) and unsafe (0-0.2). Jiyu and Dayu Islets were both at the safe level as egret breeding habitats, and the former was slightly safer than the latter. None of the ten representative foraging habitats was at the safe level. The average ecological safety of foraging habitats of egrets in Xiamen was marginally safe on the whole, with six foraging sites marginally safe, and four moderately safe. Among the three most ecologically suitable foraging habitats, two (Xinglin Bay and Haicang) were marginally safe and one (Yundang) was moderately safe. DISCUSSION
Egret ecological safety assessment and field survey verification
Safe habitats make it possible to maintain a stable or even increasing wildlife population and biodiversity. However, habitat safety is a rather complex problem related to wildlife living environment, behaviour, physiology and genetics. Human disturbance makes it more complex because of direct effects of habitat destruction, intrusion and pollution. Population growth and economic development drive rapid urban sprawl towards rural areas and wildlife habitats. Many of the original wildlife habitats in China are close to or now part of urban areas. The challenge is to understand the wildlife requirements during rapid urbanisation and to undertake conservation as soon as possible. Our study summarised the current research results relevant to egret ecology in Xiamen and applied them to egret ecological safety assessment. Egret population dynamics in two breeding habitats and their distribution in ten foraging habitats were used to verify the ecological safety assessment results. From 1996 to 2004, the egret population stability in Jiyu Islet has been significantly better than in Dayu Islet, with population variances of 0.39 and 0.64, respectively. Afield survey (Fang et al. 2002) on foraging sites on the Xiamen coast found that the habitats with most foraging egrets were Xinglin Bay, Haicang and Liushan, which were also the safest foraging habitats in our assessment, thus verifying our synthesis assessment method. This method can be used as an efficient tool for bird conservation and habitat management in the urban environment.
Urban development and egret conservation
The constant influence of human activity and inappropriate exploitation has considerably changed Xiamen coastal wetlands and the egret habitat. Rapid population growth and economic development are the two major drivers of urbanisation, leading to coastal reclamation and sprawling built-up areas in Xiamen Island (Figure 3). From 1952 to 2006, 90.13 km^sup 2^ of coastal wetland was reclaimed into urban, industrial or aquaculture areas to meet the city's development requirements (Lin et al. 2007). Mangroves, the most suitable habitat for egrets, have disappeared from Xiamen Island (Lin et al. 2005). The demand for water from factories and houses has increased, directly causing increasing industrial and municipal sewage discharged into coastal waters, leading to degradation of coastal seawater and surface water quality. Such factors suggest that there are no safe foraging habitats in Xiamen Island, the hub of Xiamen City. On the other hand, two breeding habitats - Dayu islet and Jiyu islet - were designed as a natural reserve for egrets in 1994 and strict protection has been undertaken to control and avoid direct human impacts, e.g. fishing near the islands was forbidden. Results from the current study prove the success of the two nature reserves. However, safe breeding habitats alone cannot support sustainable egret populations without also providing safe foraging habitats. According to our results, foraging habitat conservation in Xiamen Island should be prioritised in the near future.
The authors gratefully acknowledge funding from the National Science Foundation of China and the Chinese Academy of Sciences through research programmes: NSFC 40701059, KZCX2-YW-422, and 0714081d10.
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Tao Lin1, Xiong-zhi Xue2, Jing Huang1 and Sheng-hui Cui1
1 Institute of Urban Environment, Chinese Academy of Sciences, Xiamen
2 Coastal and Ocean Management Institute, Xiamen University, Xiamen
Correspondence: Xiong-zhi Xue, Coastal and Ocean Management Institute, Xiamen University, Xiamen 361005, China. Email: [email protected]
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