Evaluation of urban ecological carrying capacity: a case study of Beijing, China

1878-0296 © 2010 Published by Elsevier doi:10.1016/j.proenv.2010.10.199

Procedia Environmental Sciences 2 (2010) 1873–1880

International Society for Environmental Information Sciences 2010 Annual Conference (ISEIS)

Evaluation of urban ecological carrying capacity: a case study of

Beijing, China

Linyu X8

_{*, Peng KANG}

_{, Jinjin WEI}

a_{State Key Laboratory of Water Environment Simulation; School of Environment, Beijing Normal University, Beijing 100875, China}

Abstract

In process of global rapid urbanization, it becomes a hot issue that whether urban ecosystems can sustain the high intensity of the human activities and eliminate impact that socio-economic development induced or not, therefore the evaluation of urban ecosystem carrying capacity has become a significant point of urban ecology. Currently the theory and method of urban ecological carrying capacity can be categorized into three main patterns by represented characteristics: population carrying capacity, ecological footprint and relative carrying capacity. The relative carrying capacity in urban ecosystem can be defined as a potential ability, which can show the sustainable developing trend via comparison between ecological pressures and supportive ability. In this paper, an evaluation method of urban relative carrying capacity based on grey relevant degree is proposed and applied with Tongzhou district of Beijing, China as case study. The results indicated that accompanied with the improvement of urbanization ratio, the region comprehensive carrying capacity had been enhanced from 2000 to 2004 year, but its development in planning year is not sustainable and might create some detrimental impacts on urban ecosystem if it keeps the current developing mode.

© 2009 Published by Elsevier Ltd.

Keywords:Urban ecosystem; carrying capacity, gray relevant degree; Beijing

1. Introduction

As a socio-economic–natural complex ecosystem grounded on biophysical environment, urban ecosystem is confronted with various problems when have created the society prosperity and economy boom. The typical issues of large city include segregation, neighbourhood degradation, serious traffic jam, socio-economic deprivation and inequities in health, well-being and health-care accessibility˄Kamp et al,2003; Burak and Karen, 2008˅.

Therefore cities have particularly dramatic impact upon natural ecosystems, and these impacts have become increasingly apparent because the size and number of city are growing and their negative impacts on ecosystems are also increasing. However, it is widely accepted that the ability of the contamination elimination and resource

* Corresponding author. Tel.: 010-58800397; fax: 010-58893228.

E-mail address: xly@bnu.edu.cn.

Open access under CC BY-NC-ND license.

provided by cities is limited. In another words, the urban ecosystem have to provide the maximum carrying capacity to support urban sustainable development, which is the hot research topic in urban ecology research field presently.

The concept of ‘carrying capacity’ originated from ecology. It usually refers to the maximum population that can be carried for an organism, given a certain quantity of food, habitat, water and other life infrastructure present (Rees, 1996). Accompanied with the development and enlarging range of human activities, the connotation of carrying capacity has undergone the process from initial population carrying capacity, environmental or resources carrying capacity, to ecological carrying capacity. Ecological carrying capacity is the overall level of characterization of the ecosystem. City, as a natural socio-economic complex, needs lots of material and energy to support its development, simultaneously generated impact on ecological environment. So the concept of urban ecosystem carrying capacity can be regarded as applying ecological carrying capacity into urban agglomeration of human activity.

On the basis that synthesizing and comparing massive researches about urban ecological carrying capacity, we can categorized methods referring to this field into three main forms: population carrying capacity, ecological footprint, relative carrying capacity. Relative carrying capacity method mainly transforms the calculation of urban ecosystem carrying capacity into non-dimensional unit, in order to compare its respective component. It can be defined as the potential ability to maintain urban ecosystem health, which includes the ability to develop under normal conditions, and the ability of resilience under stress conditions (Xu et al., 2008). Here, the stress conditions refer to the normal status without fight or natural disaster. Urban ecological carrying capacity is a relative value comparing with the pressure to urban ecosystem in this paper. These pressures include the human activities, population, land use, physical development. Compared with traditional concepts, the urban ecological carrying capacity has covered the ability to develop, and it can provide a framework for integrating physical, socioeconomic, and environmental systems into planning for a sustainable environment (Bernadette et al, 2009). Moreover, this carrying capacity approach is very useful to identify the thresholds ahead of time. The determination of the capacity of a system is straightforward when managing such urban facilities as water supply, sewage treatment, and transportation (Roberta and Alessandra, 2002). In valuation process, the method adequately put the urban ecosystem social, economic and environmental complex relationship in consideration. So far, methods which put urban ecosystem carrying capacity into dimensionless units include analytic hierarchy process, synthetical index, sustainable development degree approach (Gao, 2001; Xu, et al, 2003; Yang and Sui, 2005;) .

The relative carrying capacity can be applied to evaluate the carrying capacity of diverse complex ecosystems such as river valleys, regions, cities, regions, etc. Considering urban dimension, accompanied with technological progress and improvement of living standards, the material flows, energy exchange and information flow which has an intimate linkage with lifestyles and urban form, always varies by a series of time. Therefore the urban ecosystem carrying capacity can be views as retaining in the status of adjustment and dynamic change in constant. Currently, some researchers have keep track of the dynamic variation in urban ecosystem carrying capacity through calculating long time series of ecological footprint values (Wackernagel, et al., 2005; Huang et al., 2005). However, researches in majority can simply compute data of each year and then put them into combination, inadequately further explore the inherent principles which can be gotten in the development trend of urban ecosystem carrying capacity. In conclusion, during a series of developing stages in city, there exists relative theoretical threshold of the urban ecosystem carrying capacity, whose value to some extent can be determined by the strength and rate of ecological stress, so it is appropriate to select the relative capacity method in consideration.

After analyzing the characteristics of urban ecological carrying capacity, we propose a new method to evaluate relative carrying capacity based on grey theory, and choose TongZhou District of Beijing, China as case study to analyze its change of carrying capacity from 2001s to 2004s using this new method, and predict its development trend in 2020.

2. Methodology

According to the concept of relative carrying capacity, grey relevant degree method was selected as the foundation to construct the evaluating model of relative carrying capacity. Grey relevant degree method acts as an efficient mathematical method, whose theory has been widely applied and developed in relevant field of society, economy, industry, ecology and so on (He and Guo, 1999). It can be used to analyze and evaluate effectively that kind of system whose information is incomplete or date lack accuracy, through making a comprehensive or measurable comparison among evaluating factors. Therefore the grey relevant degree method is applicable to assess urban complex ecosystem. The procedure of evaluation of urban ecosystem carrying capacity can be departed into different steps. Firstly, the actual value should be compared with the reference value. Then the divergence degree or

) ( ) ( ) ( k M k N k W i i i

similarity between ecological carrying capacity and the pressure difference should be analyzed. When we select the indicators of relative carrying capacity, the selecting principle should satisfy the sequence of comparability (Comparison), proximity (Closing) and the polar consistency (Consistency) and so on. Through calculating the system-related factors variable data sequence (ie evaluation of series) and system characteristics of variable data sequence (the referring sequence), we used the gray relational degree of advantage analysis to obtain evaluating results.

It is essential to establish indicator system to complete urban ecosystem carrying capacity assessment. Due to the characteristics of urban ecological systems, the configuration of indicator system should be abided by the principle such as comprehensiveness, dynamic property, scientific, sustainability. The index system of carrying capacity predominantly taking into account the three aspect elements which can reflect the relevant attribute in economy, environment, society, therein supportive indicators and pressure indicators was included in economy and society in urban ecosystem. Here pressure indicators refer to the impact on urban ecological system because of human activity such as self-development of the human and acquirement of social welfare. On the other hand, supportive indicators primarily reflect urban form status and developing prospect, such as the major city ownership of natural resources. Consequently, due to properties of indicators, supportive indicators is a positive indicator (maximum polarity indicators), and the pressure index is a kind of negative indicators (minimum polarity indicators).

The relative value of urban ecosystem carrying capacity is based on the theory of grey relevant degree method. The procedure includes five steps, the concrete process as follows:

2.1. The normalization of urban ecosystem carrying capacity indicators

There need to cope with original data and then take all the original variables into dimensionless as result that the original data sources in indicators’ factors are derived from different types and cannot be directly compared. If the polarity in the target sequence of indicators is inconsistency, the polarity can be transferred based on concrete circumstance.

(1) in formula, W_i(k)—— i sequence k indicator of standardizing value;

) (k

M_i —— I sequence k indicator of referring value;

) (k

N_i ——-I sequence k indicator of actual value the set of Indicator factors in urban ecosystem carrying capacity is: W {wi|i 0,1,,m} which w0

^

`

for the reference sequence indicator; w_i

^

`

In order that making indicators to meet accessibility to magnitude, indicators should be serialized, and then build the set of gray factors˄@GRF˅ˈ@GRF {xi|i 0,1,,m}DŽ

If x is translating sequence ofj w , the process as follows: j

) ) ( ) 2 ( ) 1 ( (w

䋬

䋬

2.3. The differences information space of gray relation in urban ecosystem carrying capacity.

Established the differences information space of gray relation ˄'_GR˅formulated fundament for the calculation ¸ ¸ ¹ · ¨ ¨ © § ) 1 ( ) ( ) 1 ( ) 2 ( ) 1 ( ) 1 ( j j j j j j j _w n w 䋬 w w 䋬 w w x _

of urban ecosystem carrying capacity. (min)) (max) ( 0t 0i GR '䋬 䋬 ' 䋬 ' ' U (4) In this, '_0i(k) x₀(k)x_i(k) ˈkK {1䋬 2䋬 䋬 n}, '_0i is the differences information of indicator sequence, which represents the divergence between the comparing sequence and reference sequence; U is the coefficient of resolution, U(0,f), the taking value interval of U is from 0 to 1 , the smaller that the value of U , the ability of solution stronger, inhere U often takes 0.5.

2.4. Gray relevant coefficient of urban ecosystem carrying capacity

The point-to-point of comparative measurement between evaluating sequence and reference sequence is called the gray relational coefficient and the calculation of gray relational coefficient is the premise of gray. The formula for gray relational coefficient between evaluating indicators and reference indicators in urban ecosystem carrying capacity expressed as follow:

(5) 2.5. The gray relevant degree in urban ecosystem of carrying capacity

Based on aforementioned analysis, the comparative measurement between evaluating sequence and reference sequence was called gray relevant degree. The calculation in evaluating the gay relevant degree of a certain sequence was as follows:

(6) The total gray relevant degree is configured by all values of the Grey relevant degree in urban ecosystem carrying capacity. In total gray relevant degree, the higher the value of gray relevant degree in certain sequence indicated that the valuating indicators are closer to reference indicators in this sequence. In paper, through make direct comparison that the sequence of pressure indicators and the sequence of reference indicators, the sequence of supportive indicators and sequence of reference indicators, and then make indirect comparison between the sequence of pressure indicators and the sequence of supportive index, urban ecological carrying capacity trends can be analyzed by a series of comparisons, it is beneficial for analyzing the trend to get the status of sustainable development in urban and improving the performance of urban management.

3. Case study

3.1. Study Area

Tongzhou District is located southeast of Beijing, China, acts as one of Beijing's satellite towns. According to "Beijing City Master Plan (2004 -2020years)", Tongzhou District is planned as the main node of the eastern development belts in Beijing. The feature of the region includes terrain flat and sufficient land resources, especially the convenient transport links to the city centre, so Tongzhou district can be oriented as Beijing's new city zone and comprehensive service centres, which mainly play key role in adjustment and optimization of the central city, to undertake the population of central city and the new industrial clustering region. However, there exist so much disadvantages and unbalance in development of Tongzhou district, which contains such as a clear rural-urban complex, deficiency of water volume, serious water pollution and the unhealthy socio-economic development impeded by poor mineral resources. Therefore it is imperative for actualizing sustainable development in region to judge whether urban ecological system can sustain the rapid social- economic development and eliminate the impact that growing population and socio-economic activities brought about. In addition, it is beneficial for provision of scientific evidence to formulate or revise urban development of planning based on urban ecosystem carrying capacity assessment results.

) ( max max ) ( ) ( max max ) ( min min ) ( 0 0 0 0 k k k k k i k i i i k i i k i i _{'  '} '  ' U U H

¦

3.2. Evaluation of relative carrying capacity evaluation

In process of calculating the relative carrying capacity in Tongzhou, the scientific and reasonable index system was established, based on criterion of indictors establishing and index attribute, combined with the conditions of ecology society, and environment in Tongzhou District. The index system of ecosystem carrying capacity was constructed in terms of developing and supportive ability (Table 1), thereof, supportive indicators is a positive index, while the pressure index is a negative indicators. It was beneficial to configure them to refer to "ecological county, ecological city, building an ecological province index (Trial) ", the indicators of constructing national environmental protecting city and some advanced cities home and abroad.

Table 1 Evaluating index system of ecosystem carrying capacity in Tongzhou District Primary

targets Sub index Valuating indicators Units

Reference value of indicators Ecological supportive ability The developing force

GDP per capita˄C1) 104yuan RMB pre capita ≥2.5

Urban dwellers ' controllable income˄C2) 104yuan RMB pre capita ≥1.2

Peasants’ annual net income pre capita˄C3) Yuan pre capita ≥4500

Annual fiscal revenue pre capita˄C4) Yuan pre capita ≥3800

The ratio of urbanization˄C5) % ≥50

The ration of environment satisfactory degree˄C6) % ≥95

Number of hospital bed in thousand person˄C7) a ≥9

The satisfactory degree of home income˄C8) % ≥100

The supportive force

The converge ratio of greening in build area˄C9) % ≥50

The days that air quality is or prior to secondary level˄C10) day ≥280

The converge rate of noise reaching standards˄C11) % ≥60

The popularity ratio of residents water-saving˄C12) % ≥100

The harmless treatment rate in urban domestic refuse˄C13) % ≥100

The cultivated area per capita˄C14) ha pre capita ≥0.08

Ecological pressure

Energy intensity pre GDP˄S1) TSC/ 104yuan RMB ≤1.2

Water intensity pre GDP˄˄S2) m3/ yuan ≤150

The comparing rate between urban citizens and rural residents

in living expenditure˄S3) % ≤1.3

Unemployment rate˄S4) % ≤4.00

The annual mechanical growth rate of population˄S5) ‰ ≤20

The housing area of rural residents pre capita˄S6) m2 per capita ≤37

The density of population˄S7) Square kilometer pre capita ≤500

Engers coefficient˄S8) % ≤40

Mortality ˄S9) ‰ ≤9

The emission intensity of SO2˄S10) kg/104yuan RMB ≤5

The emission intensity of COD˄S11) kg/ 104yuan RMB ≤4.5

The average noise value of traffic arterial loads˄S12) dB ≤50

Note: Numerous date about ecological carrying capacity in Tongzhou District, the required data was obtained from the government text 2 ,3, questionnaires and other means.

The index system of ecological carrying capacity and pressure are dimensionless, by comparing the ecological carrying capacity of ecosystem carrying capacity and pressure rating. In compliance with the principle of relative capacity to establish scientific and reasonable evaluation index system of ecological carrying capacity, through the following indicators and targets factor normalization factor sequence processing, Grey relational analysis of conditions (comparability, accessibility, polarity consistency) have already been satisfied, calculated the grey relevant degree of ecological carrying capacity.

By above mentioned method, then gradually calculated the grey relevant degree of ecological carrying capacity in Tongzhou District, the concrete value as follows:

° ° ° ° ¿ °° ° ° ¾ ½ ° ° ° ° ¯ °° ° ° ® ­ ° ° ° ° ¿ °° ° ° ¾ ½ ° ° ° ° ¯ °° ° ° ® ­ 738 . 0 000 . 1 689 . 0 560 . 1 789 . 0 842 . 1 359 . 0 373 . 0 457 . 0 820 . 0 009 . 1 576 . 1 139 . 1 836 . 0 625 . 0 000 . 1 680 . 0 417 . 1 789 . 0 837 . 1 350 . 0 362 . 0 456 . 0 782 . 0 025 . 1 432 . 1 997 . 0 684 . 0 800 . 0 000 . 1 670 . 0 465 . 1 679 . 0 830 . 1 340 . 0 411 . 0 455 . 0 724 . 0 822 . 0 297 . 1 840 . 0 592 . 0 875 . 0 950 . 0 660 . 0 400 . 1 643 . 0 770 . 0 330 . 0 418 . 0 454 . 0 670 . 0 527 . 0 171 . 1 709 . 0 444 . 0 038 . 1 900 . 0 650 . 0 350 . 1 625 . 0 760 . 0 320 . 0 427 . 0 453 . 0 638 . 0 389 . 0 090 . 1 595 . 0 384 . 0 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 5 4 3 2 1 0 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 w w w w w w W   ° ° ° ° ¿ °° ° ° ¾ ½ ° ° ° ° ¯ °° ° ° ® ­ ° ° ° ° ¿ °° ° ° ¾ ½ ° ° ° ° ¯ °° ° ° ® ­ 883 . 0 196 . 1 824 . 0 866 . 1 944 . 0 007 . 1 429 . 0 446 . 0 547 . 0 981 . 0 207 . 1 885 . 1 362 . 1 000 . 1 914 . 0 462 . 1 994 . 0 072 . 2 154 . 1 224 . 1 512 . 0 529 . 0 667 . 0 143 . 1 499 . 1 094 . 2 458 . 1 000 . 1 351 . 1 689 . 1 132 . 1 475 . 2 147 . 1 402 . 1 574 . 0 694 . 0 769 . 0 223 . 1 389 . 1 191 . 2 419 . 1 000 . 1 971 . 1 140 . 2 486 . 1 153 . 3 448 . 1 743 . 1 743 . 0 941 . 0 023 . 1 509 . 1 187 . 1 637 . 2 597 . 1 000 . 1 703 . 2 344 . 2 693 . 1 516 . 3 628 . 1 979 . 1 833 . 0 112 . 1 180 . 1 611 . 1 013 . 1 839 . 2 549 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 000 . 1 @ 5 4 3 2 1 0 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 x x x x x x GRF

According to the function '_0i(k) x₀(k)x_i(k) , '_0iis as followed:

° ° ° ¿ °° ° ¾ ½ ° ° ° ¯ °° ° ® ­ ° ° ° ¿ °° ° ¾ ½ ° ° ° ¯ °° ° ® ­ ' ' ' ' ' ' 117 . 0 196 . 0 176 . 0 866 . 0 056 . 0 007 . 0 571 . 0 554 . 0 453 . 0 019 . 0 207 . 0 885 . 0 362 . 0 000 . 0 086 . 0 462 . 0 006 . 0 072 . 1 154 . 0 244 . 0 488 . 0 471 . 0 333 . 0 143 . 0 499 . 0 094 . 1 458 . 0 000 . 0 351 . 0 689 . 0 132 . 0 475 . 1 147 . 0 402 . 0 426 . 0 306 . 0 231 . 0 223 . 0 389 . 0 191 . 0 419 . 0 000 . 0 971 . 0 140 . 1 486 . 0 513 . 2 448 . 0 734 . 0 257 . 0 059 . 0 023 . 0 509 . 0 187 . 0 637 . 1 597 . 0 000 . 0 703 . 1 344 . 1 693 . 0 516 . 2 628 . 0 979 . 0 167 . 0 112 . 0 180 . 0 661 . 0 013 . 0 839 . 1 549 . 0 000 . 0 05 04 03 02 01 0 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 i 516 . 2 (max) 0 ' _i ˈ'_0i(min) 0 ) 0 516 . 2 5 . 0 ( 䋬 䋬 䋬 GR ' '

Then we can get:

° ° ° ¿ °° ° ¾ ½ ° ° ° ¯ °° ° ® ­ ° ° ° ¯ °° ° ® ­ ° ° ° ¿ °° ° ¾ ½ 867 . 0 867 . 0 879 . 0 597 . 0 958 . 0 994 . 0 692 . 0 698 . 0 739 . 0 985 . 0 861 . 0 591 . 0 779 . 0 000 . 1 735 . 0 735 . 0 995 . 0 544 . 0 893 . 0 851 . 0 724 . 0 731 . 0 794 . 0 899 . 0 720 . 0 539 . 0 737 . 0 000 . 1 650 . 0 650 . 0 907 . 0 465 . 0 897 . 0 761 . 0 751 . 0 807 . 0 847 . 0 852 . 0 767 . 0 518 . 0 754 . 0 000 . 1 569 . 0 529 . 0 725 . 0 373 . 0 741 . 0 636 . 0 833 . 0 956 . 0 983 . 0 716 . 0 873 . 0 439 . 0 682 . 0 000 . 1 429 . 0 488 . 0 649 . 0 337 . 0 671 . 0 567 . 0 885 . 0 920 . 0 877 . 0 659 . 0 990 . 0 411 . 0 700 . 0 000 . 1 5 4 3 2 1 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 䋬 H H H H H H

The grey relevant degrees of urban ecosystem carrying capacity of each year are listed as followed:

685 . 0

2000

J ˈJ₂₀₀₁ 0.718ˈJ₂₀₀₂ 0.769ˈJ₂₀₀₃ 0.793ˈJ₂₀₀₄ 0.826

Similarly, the grey relevant degree of ecological pressure can express as follow:

886 . 0

2000

J ˈJ2001 0.896ˈJ2002 0.914ˈJ2003 0.926ˈJ2004 0.912

On the basis of assessment by utilizing the valuating model, the calculation of ecological carrying capacity was consisted of the ecological capacity and pressure.

It was obvious for formula in application of model to infer 0≤r_i≤1, sor₀=0 regarded as the criteria in evaluation. On account that there existed the difference between supportive ability and pressure in polarity (supportive ability

can be views the maximum polarity, contrary, the pressure can be regards the minimum polarity), the conclusion was made that the trend that supportive ability close to the referring standard was opposite to that of pressure (to map the middle of the dash as standard axis) (Figure 1). The closer to reference value in supportive ability indicated that the ecological carrying capacity increasingly enhanced year by year, but the closer to reference value in pressure demonstrated that the ecological pressure decreased.

Figure1 the carrying capacity varied trend in Tongzhou district

The chart indicated that urban ecosystem of supportive ability increasingly raised from 2000 to 2004 year, on the other hand, the ecological pressure had declined from 2000 to 2003, but began to rebound in 2003 in Tongzhou District.

4. Conclusion

It is reasonable to apply grey relevant degree method as a quantitative model to evaluate the ecological carrying capacity in Tongzhou District Beijing. In process of valuation, ecological supportive indicators and pressure indicators were selected to characterize the status of the city. The results demonstrated that from 2000 to 2004, followed with increased urbanization and increase of economic strength and more attention be paid to environmental protection in Tongzhou District, residents are satisfied with regional lifestyle and the society have been an efficient and harmonious, therefore the ecological carrying capacity had undergone increasingly enhanced. Finally, based on the valuation of ecological carrying capacity, the evaluation results, in order to improve ecological carrying capacity, it is imperative to propose measures that enhancement of resources utility, adjust the industrial structure and increasing employment opportunities and improvement of the living environment. Moreover, in this study, comparison of the relative capacity of the ecological footprint(Wei and Xu, 2009) and the different parts of the trend, the relative carrying capacity and ecological footprint pressure of the changing demands similar trends are directed from 2000 to 2003 declining rebound since 2003; the relatively supportive capacity of the supply curve and the trend of ecological footprint is not similar to the relative bearing supportive from 2000 to 2004, marked increase in the supply of eco-footprint is essentially the same. The authors think that the Ecological Footprint Supply mainly reflects the local land-use change, while the urban ecosystem carrying capacity into account the economic development of quantitative models level of scientific and technological progress, environmental control and life quality of the potential ability to improve capacity of a combination of factors such as so supportive marked increase in urban ecosystems.

This paper selected Tongzhou District as a case point, applied the new model to evaluate the relative carrying capacity. This approach views urban capacity as capacity or potential through assessing the characteristics of sustainable urban development based on establishment of ecological pressures and carrying capacity index. Although the evaluation result is relatively abstract, however comprehensively weigh the relationship between the various subsystems or its internal mechanism in urban ecosystem, in process, it is essential or necessary to grasp the information. Compared with the ecological footprint model, the relative carrying capacity considers the improvement of the potential carrying capacity from comprehensive elements such as the level of economic development, science and technology, environmental protection.

According to the assessment based on aforementioned models, there exist some drawbacks in research of urban ecosystem carrying capacity through the analysis of computation and theory consideration: urban ecological system

is similar to an open system, which always exchanges with with material flow, information flow and other functions from external sphere, so the value of urban ecosystem carrying capacity is not an absolutely fixed, varied in urban ecosystem different stages of development in city. The urban ecological carrying capacity is naturally link to dynamic development, whose theory is deeply rooted in dynamic trend of theoretical threshold; moreover there exists the spatial variation in urban ecosystem carrying capacity, as results that the condition of environmental resources in various regions conditions are different, and people's lifestyles exists some disparities. Space technology can be combined with current methods to make assessment results more practical, which can provide more detailed visual measurement in the city government for policy-making. Through comparative analysis before: urban carrying capacity should pay more attention to urban dynamics and spatial heterogeneity. In addition, urban acts as a complex system, it is heuristic for collecting multi-disciplinary knowledge and systems analysis methods to establish multi-hierarchy and dynamic model in urban ecological carrying capacity.

Acknowledgment

We would like to thank the financial support of the National Natural Science Foundation of China (Grant no.40871262) and the National Ministry of Science and Technology (Grant no. 2007BAC28B03).

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