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Assessment of habitat suitability for waterbirds in the West Songnen Plain, China, using remote sensing and GIS
Ecological Engineering 55 (2013) 94–100
Contents lists available at SciVerse ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Assessment of habitat suitability for waterbirds in the West Songnen Plain, China, using remote sensing and GIS Zhangyu Dong a,b , Zongming Wang a,∗ , Dianwei Liu a , Lin Li c , Chunying Ren a , Xuguang Tang a,b , Mingming Jia a,b , Chunyue Liu a a b c
Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, China University of Chinese Academy of Sciences, Beijing 100049, China Department of Earth Sciences, Indiana University-Purdue University, Indianapolis, IN 46202, USA
a r t i c l e
i n f o
Article history: Received 20 November 2012 Received in revised form 2 February 2013 Accepted 3 February 2013 Available online 22 March 2013 Keywords: Habitat suitability Object-oriented classification method Analytic hierarchy process (AHP) Biodiversity conservation Remote Sensing and GIS The West Songnen Plain China
a b s t r a c t In the past five decades, waterbirds habitats have been seriously damaged in the West Songnen Plain due to the continuous deterioration of wetland ecosystems. Identifying suitable habitats is critical to wetland management and restoration. In this study, by developing an object-oriented segmentation approach in conjunction with geographic information system (GIS) spatial analysis and remote sensing image data, we analyzed and assessed the suitability of habitat for waterbirds in the West Songnen Plain, China. Nesting locations were used to validate the precision. The results show that the area of good, fair, poor, and not suitable habitat for waterbirds is 216 × 103 ha, 329 × 103 ha, 109 × 103 ha, and 283 × 103 ha, respectively. Approximately 23.48% of the total area of the West Songnen Plain is good suitable habitat. The results also indicate the great potential of this approach for objectively and effectively evaluating habitat suitability for waterbirds. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The habitat of a species is determined by the abiotic components of the environment necessary for its survival. However, in recent decades, a common problem threatening the ecological function of species habitats is the alternation, loss, and fragmentation of these habitats (Jacquin et al., 2005; Lee et al., 2010), which are induced by pollution, changes in land use, and other human activities (Ronka et al., 2008). The loss of habitat is a major factor contributing to the decline of waterbirds around the world, and it seriously threatens the health and integrity of waterbirds (Debeljak et al., 2001). Therefore, it is important to understand the habitat requirements of waterbirds and to effectively assess the waterbird habitat suitability. Different methods are available to assess the suitability of a habitat for a species. A straightforward approach is to use habitat suitability (HS) models (Hirzel and Guisan, 2002; Daniel et al., 2006). These models determine the different environmental factors (such as terrain morphology, land cover, meteorological conditions and distribution of human activities) significantly affecting the
∗ Corresponding author. Tel.: +86 43185542233; fax: +86 43185542298. E-mail address: [email protected] (Z. Wang). 0925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2013.02.006
presence, abundance and distribution of different species (Brotons et al., 2004) and produce the potential suitability of the geographic distribution of a species habitat in a territory. The results could have a wide range of applications in the management of species populations and conservation planning, design of habitat connection corridors, local landscape planning and so on. While improving the ecological environment and biodiversity conservation, most traditional methods have focused on the occurrence of species and have provided relatively qualitative information on habitat suitability (Fielding and Bell, 1997). Qualitative information depends upon a researcher’s experiences and descriptions, so it cannot provide accurate information. Quantitative analysis, however, provides accurate data. At present, some studies have attempted to conduct quantitative assessments of species habitats. Recent studies have shown that the object-oriented approach is necessary because habitat suitability is related to different factors (such as land cover, terrain morphology, water availability, habitat conditions, and so on) at different spatial scales (To et al., 2005; Stow et al., 2008). A number of studies have adopted the object-oriented approach to analyze habitat suitability. Bock et al. (2005) described the object-oriented approach for habitat mapping at local and regional scales. Tian et al. (2008) presented an objectedoriented method to analyze habitat suitability for migratory birds with multi-spectral images. The advantage of the object-oriented
Z. Dong et al. / Ecological Engineering 55 (2013) 94–100
approach is that features incorporating texture, spectral values, context relationship and shape are used to define an image object (Frohn et al., 2008). However, the application of this approach to assess waterbirds habitat suitability has yet to be further explored. The West Songnen Plain in Northeast China is an ideal area to conduct this research, as it has many lakes and wetlands with abundant water, biological and land resources and is one of the richest biodiversity regions (Ma et al., 2006). The region includes national nature reserves for wetlands. These nature reserves consist of one of the largest wetlands in China, providing an ideal habitat for many species of waterbirds. The marsh reserves also serve as a stopover and habitat for a large number of storks, swans, herons, grebes and other waterbird species (Wang et al., 2004). Most previous studies in the West Songnen Plain have focused on the significance and eco-environmental effects of wetland restoration and the preservation of waterbird habitats from a qualitative viewpoint (Liu et al., 2010). Limited attention has been given to quantitatively studying and spatially analyzing the waterbird habitats suitability in this region. The objectives of this study are as follows: (1) to select and analyze key factors that have direct impacts on the foraging and resting of waterbirds, (2) to determine habitat suitability grades for waterbirds in the West Songnen Plain, and (3) to analyze and assess the spatial characteristics of waterbird habitat suitability. The results of this study are intended to provide an approach for objectively and effectively evaluating waterbird habitat suitability and for serving as a tool for biodiversity conservation. 2. Materials and methods 2.1. Study area The West Songnen Plain (121◦ 31 –126◦ 32 E, 43◦ 57 –48◦ 30 N) is located in the central part of Northeast China on a transitional belt between pastoral and agricultural regions. The study area is approximately 101,551 km2 (Fig. 1). The main rivers flowing through the study area are Tao’er, Luolin, Songnen, and Nenjiang. Three (Xianghai, Zhalong, and Momoge) wetland nature reserves exist on the West Songnen Plain. Two of them (Xianghai and Zhalong) have been listed as wetlands of international importance for water bird conservation by the Ramsar Convention Bureau. The West Songnen Plain has the greatest concentration of marsh wetlands in China and is noteworthy for its rich biodiversity. A more detailed description of this area can be found in Wang et al. (2009). 2.2. Data sources and processing Landsat Thematic Mapper (TM) remote sensing images, digital elevation model (DEM) data, and ancillary data were used in this research. Landsat TM remote sensing images and digital elevation model (DEM) data were obtained from the International Scientific Data Service Platform (http://datamiror.Csdb.cn/index.jsp) and the U.S. Geological Survey (http://glovis.usgs.gov). A Landsat TM image has a resolution of 30 m. The DEM data, from which slope was derived, have a 30-m resolution. The mainly ancillary data included road vector data, digital photographs and global positioning system (GPS) data. We collected the location data of nests and stopovers of waterbirds during the breeding season in late April 2010. The locations of all nests and stopover points were recorded using a handheld GPS unit. Land cover data of the study region for 2010 were obtained from Landsat Thematic Mapper (TM) images by object-oriented classification in conjunction with the manual interpretation method. Object-oriented image classification was performed using the
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eCongnition software (Kartikeyan et al., 1998). The classification accuracy was assessed through field investigation from July to August in 2011, and typical ground objects were verified using GPS, a measuring rope and a digital camera. The total accuracy of the land cover for 2010 was approximately 87%, meeting the needs of this study (Liu et al., 2009); NDVI data were obtained from Landsat TM 4 (near infrared) and 3 (red) images (Zhang et al., 2012). NDVI =
NIR − R NIR + R
(1)
where NIR and R are the reflectance in the near-infrared and red bands. 2.3. Waterbird habitat suitability analysis approach In this study, GIS was employed to analyze and assess waterbird habitat suitability in the West Songnen Plain. The assessment units were extracted from Landsat TM images acquired in 2010 and were derived by the object-oriented segmentation approach. By analyzing relationships between waterbirds with key habitat environmental factors, we selected such indices as disturbance degree, water situation, food abundance and shelter condition as affecting factors for waterbird habitats. The weight for each factor was assigned by the analytic hierarchy process approach. In this way, an assessment matrix of waterbirds and the key habitat factors and the index of habitat suitability for waterbirds could be established. Based on the establishment of a geo-referenced database, the assessment of habitat suitability could be performed using GIS spatial analysis. 2.3.1. Object-oriented image segmentation The TM images were used to extract polygonal assessment units that contained relatively uniform spectral and spatial characteristics through object-oriented segmentation for habitat suitability. The final segmentation parameters were as follows: color:shape = 8:2, compactness:smoothness = 2:4. The multi-resolution segmentation algorithm was performed using Landsat TM4 (near-infrared), TM3 (red), and TM2 (green) images at five segmentation scales: 5, 10, 20, 30, and 50. After comparing the results derived from the five segmentation scales, the finerscale parameter of 20 was used, which was deemed as the most acceptable. 2.3.2. Selection of suitability factors The selection of suitability factors is one of the important steps in the analysis of waterbird habitat suitability (Tian et al., 2008). Key factors directly affecting the foraging and resting of waterbirds were selected, including disturbance (the density of roads and residences), water availability (the density of lakes and rivers), food abundance (NDVI) and shelter condition (land cover and slope). The densities of roads and residences tend to have direct effects on waterbird habitat (Hirzel et al., 2001). The residence vector data were extracted from 2010 land cover data. The road data, including railway, highway, and provincial roads, were extracted from the National Road System Database and had been extensively validated (http://www.geodata.cn/Portal/?isCookieChecked=true). The water availability of the study area was an important habitat feature for waterbirds, and it depends on the densities of rivers and lakes (Soulliere et al., 2007) The extraction of river and lake vector data was based on the 2010 land cover data. NDVI (Zhang et al., 2012), an important index reflecting vegetation growth status and coverage, is a relative measure of forage for waterbirds (Tang et al., 2011). Land cover and slope can reflect the safety of shelters, which is a relative measure of resting for waterbirds (Brotons et al., 2004).
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Z. Dong et al. / Ecological Engineering 55 (2013) 94–100
Fig. 1. Location of the West Songnen Plain, China.
2.3.3. Calculation of the suitability index We assessed waterbird habitat suitability in the West Songnen Plain by overlaying factor layers using ArcGIS 9.3 software. These data layers were translated into numerical data before the index of habitat suitability was calculated, and all data were standardized. The waterbird habitat suitability was graded as good (75–100), fair (50–75), poor (25–50), and not suitable (0–25). The AHP method was adopted to determine reliable and objective weights for the factors. This method combines the relationships between habitat suitability factors (David and Ludwig, 2006). The main steps of the AHP to determine the weights of each factor are as follows (Dolan, 2008): (1) define the decision elements (habitat suitability factors); (2) construct the decision model, with a linear function adopted to calculate the integrated index; (3) decompose and make pairwise comparisons to determine local priorities; (4) combine the scales created through the pairwise comparisons to determine how well the options can be expected to meet the goal; (5) perform a sensitivity analysis, if
desired, to allow a range of sensitivity analyses to determine how different judgments or assumptions affect the analysis; (6) either make a decision or go back and refine the analysis until everyone is satisfied that a decision can be made.
3. Results 3.1. Indices of suitability factor The values of density of roads and residences (Fig. 2a and b) obtained from the vector data based on the spatial analysis function of the Arcgis9.3 software ranged from 0.086 to 0.978 and 0 to 0.4442, respectively. The data were divided into four classes. The distribution maps for the density of rivers and lakes, ranging from 0 to 0.258 and 0 to 0.396, respectively, are presented in Fig. 2c and d. The NDVI value was extracted by computing the mean value from the Landsat TM image, ranging from 0.63 to −0.38, as presented in
Table 1 The results of AHP to determine the weights of each factor for the waterbirds in the West Songnen Plain. Objective level
Criterion level Factors
Habitat suitability analysis in the West Songnen Plain
Index level Weights
Water situation
0.3
Disturbance degree
0.2
Shelter condition Food abundance
0.2 0.3
Factors
Weights
Density of lakes Density of rivers Density of roads Density of residences Land cover Slope NDVI
0.35 0.65 0.45 0.55 0.75 0.25 –
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Fig. 2. The waterbirds habitat suitability factors of the West Songnen Plain, (a–d) density of roads, and residences, rivers, lakes, (e) NDVI, (f) land cover type, and (g) slope.
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Z. Dong et al. / Ecological Engineering 55 (2013) 94–100
Fig. 3. The result of habitat suitability in the West Songnen Plain for waterbird.
Fig. 2e. The 2010 Land cover data of the study area are presented in Fig. 2f. The distribution map for the slope, ranging from 0 to 71.9268, is presented in Fig. 2g. Table 1 presents the AHP results to determine the weights of each factor for the waterbird habitat.
3.2. Analysis of habitat suitability for waterbirds The integrated index of habitat suitability was calculated for waterbirds according to the grades good, fair, poor and not suitable, as shown in Fig. 3. Most of good habitat suitability sites were located in the nature reserves or were concentrated on both sides of the river and around the lake. The area of these sites was 216 × 103 ha, accounting for 23.48% of the total study area. The fair grade had the largest area (319 × 106 ha), and accounted for 33.33% of the total area. The second-largest was the not suitable grade, with an area of 283 × 106 ha, or 30.35% of the study area. The poor grade was the smallest, only accounting for 12.84% of the total study area. The distribution of habitat suitability of each grade shows significant spatial differences. The good grade was scattered in the wetlands and water-bodies, and most of this grade occurred in the middle of the Songnen Plain, with smaller areas in the northern and southern parts. The fair grade was primarily located adjacent to the good grade. Compared with the good grades, the poor and
not suitable grades mainly existed in broader areas and were distributed far from wetlands, rivers and farmland. In total, 28 nesting locations in the Zhalong National Nature Reserve were used to validate the accuracy of the waterbird habitat suitability (Fig. 3). In all, 20 waterbird nesting locations were contained in the good grade. The other 8 points were all contained in the fair grade. To a certain extent, these data verified the results of our evaluation.
4. Discussion 4.1. The approach used to assess waterbird habitat suitability Many of the studies assessing waterbird habitat have focused on qualitative analysis (e.g., Wilsey et al., 2012). In contrast, we quantitatively assessed waterbird habitat using remote sensing and GIS. Our efforts were not intended to replace field studies for selecting habitat sites but rather to improve field efficiency by analyzing good and fair habitat suitability. We believe that greater success in assessing waterbirds habitat will be achieved if additional factors are considered. For example, the landscape, fresh water sources, ecological niche, and other relevant biophysical factors also influence waterbird habitat. However, it is impossible to include all the factors affecting waterbird habitat.
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The choice of factors is always a compromise between feasibility and accuracy. One of the major contributions of this study is the development and testing of an object-oriented approach to produce the calculation units, which are represented by meaningful and homogeneous segments instead of standard-shaped units. Most traditional raster map layers are made up of square grid pixels or regular rectangles, with grid pixel values for the factors and row-column values for the geographic location (Im et al., 2011). The factors of habitat suitability are then synthesized for each grid pixel. The limitations of these methods are that they involve a large amount of computation and that they lack flexibility (Store and Jokimäki, 2003). Compared with these traditional methods, the object-oriented approach involves less computation, works more efficiently, and can be more easily adjusted because the calculation units are the polygonal segments. In addition to the unique methods and application in this study, the 2010 land cover data of the West Songnen Plain provide more accurate data for assessing waterbird habitat. These data have a number of advantages over conventional data: (1) they provide classification results with higher accuracy; (2) they incorporate spectral, contextual, textural, and shape information; (3) they present classification results in a form that is immediately useable in a geographic information system; and (4) they reduce local spectral variation.
4.2. Implications for biodiversity conservation and management The results from this study indicate that the quantity and spatial distribution of waterbird habitat suitability for the four grades varied greatly at the West Songnen Plain and are closely related to the wetland area and sites. Some studies have indicated that wetlands were degraded because of agricultural activities in the West Songnen Plain (Wang et al., 2008). It can be predicted that the loss of wetland will pose a great threat to the waterbird habitat suitability. Thus, policies and measures should be taken to manage and control the existing waterbird habitat. Evidence shows that habitat loss and destruction are an important cause of biodiversity decrease and species extinction (Roddriguez and Delibes, 2003; Riley and William, 2005). This study showed that the area of good suitable habitats for waterbirds is 216 × 103 ha in the West Songnen Plain, but most of these areas are isolated into many patches that are out of reach of each other. To reverse this trend, the government should give great importance to restoring and reconstructing waterbird habitats. National and provincial ecological restoration projects should be adopted, and channels and corridors formed for waterbirds to communicate with each other.
5. Conclusions Waterbirds are endangered species with a narrow breeding distribution and habitat requirements restricted by vegetation species composition and water distribution. The combination of remote sensing, GIS, AHP and object-oriented segmentation is an objective and effective way to analyze and assess waterbird habitat suitability, and it presents a sensible starting point for the assessment of waterbird habitat suitability. The results of this study show that 23.48% of the total area of the West Songnen Plain is good suitable habitat for waterbirds. The method and results drawn from this study may help assess habitat suitability in other regions in China and even in other countries. These conclusions are useful for biodiversity conservation and proper ecosystem management under increased pressure from the population increase.
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Acknowledgments This study was supported by the National Basic Research Program of China (No. 2013CB430401) and the Key Deployment Project of Chinese Academy of Sciences (No. KZZD-EW-08-02).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ecoleng.2013. 02.006.
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Contents lists available at SciVerse ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Assessment of habitat suitability for waterbirds in the West Songnen Plain, China, using remote sensing and GIS Zhangyu Dong a,b , Zongming Wang a,∗ , Dianwei Liu a , Lin Li c , Chunying Ren a , Xuguang Tang a,b , Mingming Jia a,b , Chunyue Liu a a b c
Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, China University of Chinese Academy of Sciences, Beijing 100049, China Department of Earth Sciences, Indiana University-Purdue University, Indianapolis, IN 46202, USA
a r t i c l e
i n f o
Article history: Received 20 November 2012 Received in revised form 2 February 2013 Accepted 3 February 2013 Available online 22 March 2013 Keywords: Habitat suitability Object-oriented classification method Analytic hierarchy process (AHP) Biodiversity conservation Remote Sensing and GIS The West Songnen Plain China
a b s t r a c t In the past five decades, waterbirds habitats have been seriously damaged in the West Songnen Plain due to the continuous deterioration of wetland ecosystems. Identifying suitable habitats is critical to wetland management and restoration. In this study, by developing an object-oriented segmentation approach in conjunction with geographic information system (GIS) spatial analysis and remote sensing image data, we analyzed and assessed the suitability of habitat for waterbirds in the West Songnen Plain, China. Nesting locations were used to validate the precision. The results show that the area of good, fair, poor, and not suitable habitat for waterbirds is 216 × 103 ha, 329 × 103 ha, 109 × 103 ha, and 283 × 103 ha, respectively. Approximately 23.48% of the total area of the West Songnen Plain is good suitable habitat. The results also indicate the great potential of this approach for objectively and effectively evaluating habitat suitability for waterbirds. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The habitat of a species is determined by the abiotic components of the environment necessary for its survival. However, in recent decades, a common problem threatening the ecological function of species habitats is the alternation, loss, and fragmentation of these habitats (Jacquin et al., 2005; Lee et al., 2010), which are induced by pollution, changes in land use, and other human activities (Ronka et al., 2008). The loss of habitat is a major factor contributing to the decline of waterbirds around the world, and it seriously threatens the health and integrity of waterbirds (Debeljak et al., 2001). Therefore, it is important to understand the habitat requirements of waterbirds and to effectively assess the waterbird habitat suitability. Different methods are available to assess the suitability of a habitat for a species. A straightforward approach is to use habitat suitability (HS) models (Hirzel and Guisan, 2002; Daniel et al., 2006). These models determine the different environmental factors (such as terrain morphology, land cover, meteorological conditions and distribution of human activities) significantly affecting the
∗ Corresponding author. Tel.: +86 43185542233; fax: +86 43185542298. E-mail address: [email protected] (Z. Wang). 0925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2013.02.006
presence, abundance and distribution of different species (Brotons et al., 2004) and produce the potential suitability of the geographic distribution of a species habitat in a territory. The results could have a wide range of applications in the management of species populations and conservation planning, design of habitat connection corridors, local landscape planning and so on. While improving the ecological environment and biodiversity conservation, most traditional methods have focused on the occurrence of species and have provided relatively qualitative information on habitat suitability (Fielding and Bell, 1997). Qualitative information depends upon a researcher’s experiences and descriptions, so it cannot provide accurate information. Quantitative analysis, however, provides accurate data. At present, some studies have attempted to conduct quantitative assessments of species habitats. Recent studies have shown that the object-oriented approach is necessary because habitat suitability is related to different factors (such as land cover, terrain morphology, water availability, habitat conditions, and so on) at different spatial scales (To et al., 2005; Stow et al., 2008). A number of studies have adopted the object-oriented approach to analyze habitat suitability. Bock et al. (2005) described the object-oriented approach for habitat mapping at local and regional scales. Tian et al. (2008) presented an objectedoriented method to analyze habitat suitability for migratory birds with multi-spectral images. The advantage of the object-oriented
Z. Dong et al. / Ecological Engineering 55 (2013) 94–100
approach is that features incorporating texture, spectral values, context relationship and shape are used to define an image object (Frohn et al., 2008). However, the application of this approach to assess waterbirds habitat suitability has yet to be further explored. The West Songnen Plain in Northeast China is an ideal area to conduct this research, as it has many lakes and wetlands with abundant water, biological and land resources and is one of the richest biodiversity regions (Ma et al., 2006). The region includes national nature reserves for wetlands. These nature reserves consist of one of the largest wetlands in China, providing an ideal habitat for many species of waterbirds. The marsh reserves also serve as a stopover and habitat for a large number of storks, swans, herons, grebes and other waterbird species (Wang et al., 2004). Most previous studies in the West Songnen Plain have focused on the significance and eco-environmental effects of wetland restoration and the preservation of waterbird habitats from a qualitative viewpoint (Liu et al., 2010). Limited attention has been given to quantitatively studying and spatially analyzing the waterbird habitats suitability in this region. The objectives of this study are as follows: (1) to select and analyze key factors that have direct impacts on the foraging and resting of waterbirds, (2) to determine habitat suitability grades for waterbirds in the West Songnen Plain, and (3) to analyze and assess the spatial characteristics of waterbird habitat suitability. The results of this study are intended to provide an approach for objectively and effectively evaluating waterbird habitat suitability and for serving as a tool for biodiversity conservation. 2. Materials and methods 2.1. Study area The West Songnen Plain (121◦ 31 –126◦ 32 E, 43◦ 57 –48◦ 30 N) is located in the central part of Northeast China on a transitional belt between pastoral and agricultural regions. The study area is approximately 101,551 km2 (Fig. 1). The main rivers flowing through the study area are Tao’er, Luolin, Songnen, and Nenjiang. Three (Xianghai, Zhalong, and Momoge) wetland nature reserves exist on the West Songnen Plain. Two of them (Xianghai and Zhalong) have been listed as wetlands of international importance for water bird conservation by the Ramsar Convention Bureau. The West Songnen Plain has the greatest concentration of marsh wetlands in China and is noteworthy for its rich biodiversity. A more detailed description of this area can be found in Wang et al. (2009). 2.2. Data sources and processing Landsat Thematic Mapper (TM) remote sensing images, digital elevation model (DEM) data, and ancillary data were used in this research. Landsat TM remote sensing images and digital elevation model (DEM) data were obtained from the International Scientific Data Service Platform (http://datamiror.Csdb.cn/index.jsp) and the U.S. Geological Survey (http://glovis.usgs.gov). A Landsat TM image has a resolution of 30 m. The DEM data, from which slope was derived, have a 30-m resolution. The mainly ancillary data included road vector data, digital photographs and global positioning system (GPS) data. We collected the location data of nests and stopovers of waterbirds during the breeding season in late April 2010. The locations of all nests and stopover points were recorded using a handheld GPS unit. Land cover data of the study region for 2010 were obtained from Landsat Thematic Mapper (TM) images by object-oriented classification in conjunction with the manual interpretation method. Object-oriented image classification was performed using the
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eCongnition software (Kartikeyan et al., 1998). The classification accuracy was assessed through field investigation from July to August in 2011, and typical ground objects were verified using GPS, a measuring rope and a digital camera. The total accuracy of the land cover for 2010 was approximately 87%, meeting the needs of this study (Liu et al., 2009); NDVI data were obtained from Landsat TM 4 (near infrared) and 3 (red) images (Zhang et al., 2012). NDVI =
NIR − R NIR + R
(1)
where NIR and R are the reflectance in the near-infrared and red bands. 2.3. Waterbird habitat suitability analysis approach In this study, GIS was employed to analyze and assess waterbird habitat suitability in the West Songnen Plain. The assessment units were extracted from Landsat TM images acquired in 2010 and were derived by the object-oriented segmentation approach. By analyzing relationships between waterbirds with key habitat environmental factors, we selected such indices as disturbance degree, water situation, food abundance and shelter condition as affecting factors for waterbird habitats. The weight for each factor was assigned by the analytic hierarchy process approach. In this way, an assessment matrix of waterbirds and the key habitat factors and the index of habitat suitability for waterbirds could be established. Based on the establishment of a geo-referenced database, the assessment of habitat suitability could be performed using GIS spatial analysis. 2.3.1. Object-oriented image segmentation The TM images were used to extract polygonal assessment units that contained relatively uniform spectral and spatial characteristics through object-oriented segmentation for habitat suitability. The final segmentation parameters were as follows: color:shape = 8:2, compactness:smoothness = 2:4. The multi-resolution segmentation algorithm was performed using Landsat TM4 (near-infrared), TM3 (red), and TM2 (green) images at five segmentation scales: 5, 10, 20, 30, and 50. After comparing the results derived from the five segmentation scales, the finerscale parameter of 20 was used, which was deemed as the most acceptable. 2.3.2. Selection of suitability factors The selection of suitability factors is one of the important steps in the analysis of waterbird habitat suitability (Tian et al., 2008). Key factors directly affecting the foraging and resting of waterbirds were selected, including disturbance (the density of roads and residences), water availability (the density of lakes and rivers), food abundance (NDVI) and shelter condition (land cover and slope). The densities of roads and residences tend to have direct effects on waterbird habitat (Hirzel et al., 2001). The residence vector data were extracted from 2010 land cover data. The road data, including railway, highway, and provincial roads, were extracted from the National Road System Database and had been extensively validated (http://www.geodata.cn/Portal/?isCookieChecked=true). The water availability of the study area was an important habitat feature for waterbirds, and it depends on the densities of rivers and lakes (Soulliere et al., 2007) The extraction of river and lake vector data was based on the 2010 land cover data. NDVI (Zhang et al., 2012), an important index reflecting vegetation growth status and coverage, is a relative measure of forage for waterbirds (Tang et al., 2011). Land cover and slope can reflect the safety of shelters, which is a relative measure of resting for waterbirds (Brotons et al., 2004).
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Fig. 1. Location of the West Songnen Plain, China.
2.3.3. Calculation of the suitability index We assessed waterbird habitat suitability in the West Songnen Plain by overlaying factor layers using ArcGIS 9.3 software. These data layers were translated into numerical data before the index of habitat suitability was calculated, and all data were standardized. The waterbird habitat suitability was graded as good (75–100), fair (50–75), poor (25–50), and not suitable (0–25). The AHP method was adopted to determine reliable and objective weights for the factors. This method combines the relationships between habitat suitability factors (David and Ludwig, 2006). The main steps of the AHP to determine the weights of each factor are as follows (Dolan, 2008): (1) define the decision elements (habitat suitability factors); (2) construct the decision model, with a linear function adopted to calculate the integrated index; (3) decompose and make pairwise comparisons to determine local priorities; (4) combine the scales created through the pairwise comparisons to determine how well the options can be expected to meet the goal; (5) perform a sensitivity analysis, if
desired, to allow a range of sensitivity analyses to determine how different judgments or assumptions affect the analysis; (6) either make a decision or go back and refine the analysis until everyone is satisfied that a decision can be made.
3. Results 3.1. Indices of suitability factor The values of density of roads and residences (Fig. 2a and b) obtained from the vector data based on the spatial analysis function of the Arcgis9.3 software ranged from 0.086 to 0.978 and 0 to 0.4442, respectively. The data were divided into four classes. The distribution maps for the density of rivers and lakes, ranging from 0 to 0.258 and 0 to 0.396, respectively, are presented in Fig. 2c and d. The NDVI value was extracted by computing the mean value from the Landsat TM image, ranging from 0.63 to −0.38, as presented in
Table 1 The results of AHP to determine the weights of each factor for the waterbirds in the West Songnen Plain. Objective level
Criterion level Factors
Habitat suitability analysis in the West Songnen Plain
Index level Weights
Water situation
0.3
Disturbance degree
0.2
Shelter condition Food abundance
0.2 0.3
Factors
Weights
Density of lakes Density of rivers Density of roads Density of residences Land cover Slope NDVI
0.35 0.65 0.45 0.55 0.75 0.25 –
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Fig. 2. The waterbirds habitat suitability factors of the West Songnen Plain, (a–d) density of roads, and residences, rivers, lakes, (e) NDVI, (f) land cover type, and (g) slope.
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Fig. 3. The result of habitat suitability in the West Songnen Plain for waterbird.
Fig. 2e. The 2010 Land cover data of the study area are presented in Fig. 2f. The distribution map for the slope, ranging from 0 to 71.9268, is presented in Fig. 2g. Table 1 presents the AHP results to determine the weights of each factor for the waterbird habitat.
3.2. Analysis of habitat suitability for waterbirds The integrated index of habitat suitability was calculated for waterbirds according to the grades good, fair, poor and not suitable, as shown in Fig. 3. Most of good habitat suitability sites were located in the nature reserves or were concentrated on both sides of the river and around the lake. The area of these sites was 216 × 103 ha, accounting for 23.48% of the total study area. The fair grade had the largest area (319 × 106 ha), and accounted for 33.33% of the total area. The second-largest was the not suitable grade, with an area of 283 × 106 ha, or 30.35% of the study area. The poor grade was the smallest, only accounting for 12.84% of the total study area. The distribution of habitat suitability of each grade shows significant spatial differences. The good grade was scattered in the wetlands and water-bodies, and most of this grade occurred in the middle of the Songnen Plain, with smaller areas in the northern and southern parts. The fair grade was primarily located adjacent to the good grade. Compared with the good grades, the poor and
not suitable grades mainly existed in broader areas and were distributed far from wetlands, rivers and farmland. In total, 28 nesting locations in the Zhalong National Nature Reserve were used to validate the accuracy of the waterbird habitat suitability (Fig. 3). In all, 20 waterbird nesting locations were contained in the good grade. The other 8 points were all contained in the fair grade. To a certain extent, these data verified the results of our evaluation.
4. Discussion 4.1. The approach used to assess waterbird habitat suitability Many of the studies assessing waterbird habitat have focused on qualitative analysis (e.g., Wilsey et al., 2012). In contrast, we quantitatively assessed waterbird habitat using remote sensing and GIS. Our efforts were not intended to replace field studies for selecting habitat sites but rather to improve field efficiency by analyzing good and fair habitat suitability. We believe that greater success in assessing waterbirds habitat will be achieved if additional factors are considered. For example, the landscape, fresh water sources, ecological niche, and other relevant biophysical factors also influence waterbird habitat. However, it is impossible to include all the factors affecting waterbird habitat.
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The choice of factors is always a compromise between feasibility and accuracy. One of the major contributions of this study is the development and testing of an object-oriented approach to produce the calculation units, which are represented by meaningful and homogeneous segments instead of standard-shaped units. Most traditional raster map layers are made up of square grid pixels or regular rectangles, with grid pixel values for the factors and row-column values for the geographic location (Im et al., 2011). The factors of habitat suitability are then synthesized for each grid pixel. The limitations of these methods are that they involve a large amount of computation and that they lack flexibility (Store and Jokimäki, 2003). Compared with these traditional methods, the object-oriented approach involves less computation, works more efficiently, and can be more easily adjusted because the calculation units are the polygonal segments. In addition to the unique methods and application in this study, the 2010 land cover data of the West Songnen Plain provide more accurate data for assessing waterbird habitat. These data have a number of advantages over conventional data: (1) they provide classification results with higher accuracy; (2) they incorporate spectral, contextual, textural, and shape information; (3) they present classification results in a form that is immediately useable in a geographic information system; and (4) they reduce local spectral variation.
4.2. Implications for biodiversity conservation and management The results from this study indicate that the quantity and spatial distribution of waterbird habitat suitability for the four grades varied greatly at the West Songnen Plain and are closely related to the wetland area and sites. Some studies have indicated that wetlands were degraded because of agricultural activities in the West Songnen Plain (Wang et al., 2008). It can be predicted that the loss of wetland will pose a great threat to the waterbird habitat suitability. Thus, policies and measures should be taken to manage and control the existing waterbird habitat. Evidence shows that habitat loss and destruction are an important cause of biodiversity decrease and species extinction (Roddriguez and Delibes, 2003; Riley and William, 2005). This study showed that the area of good suitable habitats for waterbirds is 216 × 103 ha in the West Songnen Plain, but most of these areas are isolated into many patches that are out of reach of each other. To reverse this trend, the government should give great importance to restoring and reconstructing waterbird habitats. National and provincial ecological restoration projects should be adopted, and channels and corridors formed for waterbirds to communicate with each other.
5. Conclusions Waterbirds are endangered species with a narrow breeding distribution and habitat requirements restricted by vegetation species composition and water distribution. The combination of remote sensing, GIS, AHP and object-oriented segmentation is an objective and effective way to analyze and assess waterbird habitat suitability, and it presents a sensible starting point for the assessment of waterbird habitat suitability. The results of this study show that 23.48% of the total area of the West Songnen Plain is good suitable habitat for waterbirds. The method and results drawn from this study may help assess habitat suitability in other regions in China and even in other countries. These conclusions are useful for biodiversity conservation and proper ecosystem management under increased pressure from the population increase.
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Acknowledgments This study was supported by the National Basic Research Program of China (No. 2013CB430401) and the Key Deployment Project of Chinese Academy of Sciences (No. KZZD-EW-08-02).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ecoleng.2013. 02.006.
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