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A study of the population dynamics of Spartina alterniflora at Jiuduansha shoals, Shanghai, China
e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 164–172
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A study of the population dynamics of Spartina alterniflora at Jiuduansha shoals, Shanghai, China Huamei Huang, Liquan Zhang ∗ State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan Road North, Shanghai 200062, China
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a b s t r a c t
Article history:
This study investigated the population expansion pattern of an exotic species of Spartina
Received 22 February 2006
alterniflora for a period of 7 years, after it had been newly introduced to the neonatal shoals
Received in revised form 7 June 2006
of Jiuduansha (GPS), in the Yangtze Estuary, Shanghai. Remote sensing, in conjunction with
Accepted 10 June 2006
geographical information systems (GIS) and global positioning systems (GPS) was used to map saltmarsh vegetation on the Jiuduansha shoals and the classifications were then checked using in situ field surveys of selected areas. The results showed that the S. alterni-
Keywords:
flora population had expanded from 55 hm2 when first introduced in 1997, to 1014 hm2 in
Jiuduansha shoals
2004. The population expansion pattern of S. alterniflora on the Jiuduansha shoals was com-
Exotic plant
patible with the common feature of invasions, i.e. the initial colonization, a lag time and
Spartina alterniflora
the onset of rapid population growth and range expansion. In the first year of plantation
Population dynamics
(1997), about 35 hm2 of S. alterniflora was successfully colonized on the Jiuduansha shoals.
Remote sensing
The period between 1998 and 2000 was characterized by a lag time, and the area of S. alterniflora increased only to 101.6 hm2 . The year 2000 marked an onset of rapid population growth and range expansion and the annual expanding rate reached 25–116%, which exceeded any of the indigenous species and indicated the strong competitive capability, rapid range expansion and wide ecological niche of S. alterniflora. The advent of remote sensing, in conjunction with geographical information systems and global positioning systems, provides a potential tool for mapping vegetation, and for monitoring population dynamics and range expansion of invasive species on a large scale. The implications for population and community dynamics, biodiversity conservation and wetland management in terms of the analysis of the sequence of events associated with the initial colonization, a lag time, rate of geographic spread and features of geographic spread of the exotic S. alterniflora and the native P. australis on the Jiuduansha shoals are discussed. © 2006 Elsevier B.V. All rights reserved.
1.
Introduction
The Yangtze Estuary is a typical medium tidal estuary with multi-order bifurcations, shoals and sand bars. Due to the distinct interactions between runoff and tidal currents, the dynamic and geomorphologic processes of the estuary
∗
display a unique kinematical rule among the estuaries of the world (Yun, 2004). The region of mouth bars in the estuary and the submerged delta nearby the estuary are the major locations for sedimentation of the huge amount of silt brought by the Yangtze River (Fig. 1). The neonatal shoals of Jiuduansha initially emerged above the water surface
Corresponding author. Tel.: +86 21 6223 2599; fax: +86 21 6254 6441. E-mail addresses: [email protected], [email protected] (L. Zhang). 0925-8574/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2006.06.005
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Fig. 1 – The location of study area showing the Yangtze Estuary, Shanghai metropolitan region and Jiuduansha shoals.
in the 1920s and have been growing rapidly since (Fig. 2). According to recent measurements in 2004, the areas above the 0 and −5 m isobaths of the shoals have amounted to 127.3 and 348 km2 , respectively, and the highest elevation was 4 m. The Jiuduansha shoals have never been colonized by humans and have been in a natural condition since they were formed; they were set as a wetland nature reserve of Shanghai Municipality in 2003 and a national wetland nature reserve in 2005. Spartina alterniflora originated in the eastern and gulf coasts of the USA. The species dominates on salt marshes and has important ecological functions in its native ecosystems (Simenstad and Thom, 1995). With its great capacity for reducing tidal wave energy, mitigating erosion and trapping sediment, S. alterniflora was introduced to many coastal and estuarine regions of the world as a species for ecological engineering (Callaway and Josselyn, 1992; Chung et al., 2004; Chen et al., 2004; Zhang et al., 2004). In recent years, however, some evidence has been reported that this exotic species may outcompete native plants, threaten the native ecosystems and coastal aquaculture, and cause declines in native species rich-
ness (Callaway and Josselyn, 1992; Daehler and Strong, 1996; Chen et al., 2004). As an ecological engineering project carried out by the State Key Laboratory of Estuarine and Coastal Research, 40 hm2 of Phragmites australis and 55 hm2 of S. alterniflora were planted on the Jiuduansha shoals in 1997, as a measure to mitigate the negative effects of land reclamation during the construction of Pudong International Airport (Chen et al., 2001). By their very nature, salt marshes and inter-tidal zones are dynamic and their management relies on up-to-date spatial information (Mitsch and Gosselink, 2000; Thomson et al., 2003; Philipp, 2005). Remote sensing (RS) is becoming the main resource for land cover inventory information gathering, and has been widely used to provide a useful tool for mapping vegetation, and monitoring vegetation dynamics and biodiversity conservation on a large scale (Ayres et al., 2004; Zhang et al., 2004; Philipp and Field, 2005; Huang et al., 2005; Hinkle and Mitsch, 2005; Gao and Zhang, 2006). The aim of this study was to map the salt marsh vegetation and analyze quantitatively the population expansion pattern of S. alterniflora for a period of 7 years after being introduced to the Jiuduansha shoals.
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Identifying these time-based data is an important first step to providing a scientific basis for wetland management and biodiversity conservation.
2.
Study area
The Jiuduansha Wetland Nature Reserve is located between the southern and northern watercourse of the Yangtze Estuary, 12 km east of Pudong International Airport and between 31◦ 03 –31◦ 17 N, 121◦ 46 –122◦ 15 E (Fig. 1). It covers 423.2 km2 , with an east-west distance of 25.9 km and north-south length of 46.3 km, and consists of the Jiangyanan shoal, upper shoal, middle shoal and lower shoal. After a half-century of dynamic evolution, the Jiuduansha shoals have become more or less stable (Fig. 2). The region of the Jiuduansha shoals has a northern subtropical monsoon climate, with an average annual temperature 16 ◦ C; summer temperatures average 28 ◦ C while winters are cold with an average temperature of 4 ◦ C. Average annual precipitation is approximately 1200 mm, with 60% of rainfall occurring during May–September and few typhoons during summer and autumn. The dynamic and geomorphologic processes of the shoals are profoundly influenced by the distinct interactions between runoff and tidal (semidiurnal) currents. The maximum and average tidal range is 4.62 and 2.67 m, respectively. Due to the short history of the shoals, the soil type is original and simple, consisting mainly of the coastal alluvial and solonchalk soils. Since 1987, the pioneer Scirpus mariqueter salt marsh community could be discerned from satellite images, which showed the community first colonizing on the tidal flats above the 2 m elevation. The tidal flats closest to the low water mark, elevation less than 2 m, are characterized by mud flats that
are devoid of any vascular plants. As sedimentation and succession progressed, the P. australis community replaced the S. mariqueter community above the 2.9 m elevation (Zhang and Yong, 1992; Huang et al., 2005). An additional species in this zone was S. alterniflora, which was introduced to the Jiuduansha shoals in 1997. Over the last 7 years this species has gradually invaded large areas where could formerly be covered by P. australis and has also started to invade the upper parts of the S. mariqueter zone. Except for the Jiangyanan shoal that is still below the 2 m elevation, a typical zonation of salt marsh vegetation has developed on the other three shoals over the same time period.
3.
Materials and methods
Fig. 3 gives a general data processing chart for the materials and methods used in the study, which applies to all of the images, and more details are presented in the following sections.
3.1.
Data source
For tracing S. alterniflora population developments, treatments and analyses of satellite images were conducted. A set of multi-temporal landsat thematic mapper images of 20 October 1997, 24 May 2000, 26 July 2001, 2 August 2003 and 19 July 2004, and a landsat enhanced thematic mapper image of 11 November 2002 were used, which basically covered the state of the low tide at the time the images were taken and the areas of different stages from first planting to coalescence. These satellite images were geometrically corrected by a series of nautical charts, 1:120,000 scale, using ERDAS Imagine software. Quadratic polynomials were applied to correction equations according to the distribution of control points. All images were
Fig. 2 – Historical changes in the 0 m isobaths of Jiuduansha shoals in the Yangtze Estuary.
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represented the “brightness”, the second “greenness” and the third “wetness” (Crist and Cicone, 1984; Crist and Kauth, 1986; Zhao, 2004). After the K-T transformation, considerable differences among the major salt marsh communities could be identified on the first three axes. A number of vegetation indices have been formulated for mapping vegetation (Liu et al., 1998; Lyon, 2000) and among them, NDVI, an important index reflecting the vegetation growth status and its coverage, has been used most widely (Tappan et al., 1992; Lyon et al., 1998). Based on the above-mentioned image spectral enhancement, a series of classes, i.e. water, muddy flat, dyke, S. alterniflora, P. australis and S. mariqueter communities, were identified and selected as training samples. A supervised classification, using the Maximum Likelihood Classifier in ERDAS Imagine, was then carried out. The classified images were then integrated into a GIS platform for analyzing the spatial and temporal dynamics patterns of salt marsh vegetation and S. alterniflora.
3.3. Fig. 3 – A general flow chart for data processing in this study and more details see the text.
then resampled to a resolution of 30 m × 30 m and the error was less than 0.5 pixels.
3.2.
Data processing
The multispectral TM and ETM+ remote sensing data makes it possible for spectral transformations to generate new sets of image components or bands (Schmidt and Skidmore, 2003). On the basis of a previous study on the coastal salt marsh in Shanghai (Huang et al., 2005), two spectral enhancement methods, Tasseled Cap (K-T) Transform and Normal Difference Vegetation Index (NDVI), were used to interpret satellite images more efficiently. The K-T transformation could facilitate finding the data structures inherent to a particular sensor and set of scene classes, and adjust the viewing perspective so that these structures could be more easily and completely observed (Crist and Kauth, 1986). In K-T transformation, the coefficient was determined by satellite sensor. The first axis of the output data
Field verification
The purpose of field verification was to validate the results of image classification as well as to provide reliable ground information for the training samples. An in situ field survey was carried out in July 2004 and a total of 83 sites were selected, with 13 on the upper shoal, 41 on the middle shoal and 29 on the lower shoal, using GPS to match the locations as accurately as possible. The potential error of the GPS receiver used was less than 15 m and the topical error usually less than 10 m. The historic records of the ecological engineering project and field surveys (Tang and Lu, 2003) were used for the results of image classification before the year 2004. Accuracy assessment involved the derivation of accuracy metrics based on a comparison between the class labels in the thematic map and ground data for a set of specific locations (Foody, 2002). We assessed the accuracy of initial classification and the results showed that the initial overall accuracy for the year 2004 was 71%, year 2003 73%, year 2002 78%, year 2001 67%, year 2000 65%, and year 1997 68%, respectively. After field verification, some misclassifications were corrected, especially near the boundaries of different salt marsh community types, and the overall accuracy for revised classification of the different years reached to 80–90%.
Table 1 – The spatial-temporal dynamics of saltmarsh communities on the Jiuduansha shoals Date
Spartina alterniflora Area (hm2 )
20 October 1997 24 May 2000 26 July 2001 11 November 2002 2 August 2003 19 July 2004 a
55a 101.6 283.7 377.1 469.6 1014.4
Phragmites australis (hm2 )
Scirpus mariqueter (hm2 )
Total area (hm2 )
Annual spread rate (%) – – 87.2 47.1 24.6 116.0
Including 40 hm2 P. australis and 55 hm2 S. alterniflora planted in 1997.
167.5a 353.8 368.9 401.9 463.4 563.5
966.6 1017.1 1382.9 1608.2 1850.2 1789.0
1094.1 1472.5 2035.5 2387.2 2783.3 3366.9
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Fig. 4 – The spatial-temporal dynamics of salt marsh vegetation and Spartina alterniflora on the Jiuduansha shoals. (A) The map of salt marsh vegetation of 1997 and the plantation locations were imposed on the map; (B) the map of 2000 marked an onset of rapid population growth and range expansion after the initial colonization; (C–F) the maps for the period of 2001–2004 showing the rapid population growth and range expansion.
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The results of salt marsh vegetation classification based on the multi-temporal satellite images for the Jiuduansha shoals are presented in Fig. 4. The distribution areas of each type of saltmarsh vegetation for the period of 1997–2004 are summarized in Table 1. From this information, three distinct stages could be recognized according to the expansion pattern of S. alterniflora.
S. alterniflora and P. australis on the middle shoal then began to expand towards the surroundings, which were mostly the habitats of the S. mariqueter community. The width of the expanding range would reach 2–4 m and many small clumps appeared within the range of 50 m outside the plantation belts. At the same time, many small patches of S. alterniflora clumps emerged among the original S. mariqueter community on the lower shoal, which indicated that S. alterniflora had succeeded in colonization and had begun to expand along with the accretion of elevation.
4.1.
4.3.
4.
Results
Initial colonization—year 1997
In April and May, 40 hm2 of P. australis and 50 hm2 of S. alterniflora were planted on the middle shoal as an “Ecological Engineering Project for Pudong International Airport”. Culms of these two species were planted along belts, each 100 m in width, in a regular arrangement with 1 m spacing. In addition, ca. 5 hm2 of S. alterniflora were planted where the elevation reached above 2.5 m on the lower shoal. Due to the small size of the clumps, the newly planted S. alterniflora and P. australis could not be discerned on the satellite image (with the resolution of 30 m × 30 m) of this year. The plantation locations recorded by GPS (Chen et al., 2001) were thus imposed on the map of salt marsh vegetation (Fig. 4A). The total salt marsh vegetation on the Jiuduansha shoals was 1094.1 hm2 , which included 167.5 hm2 of P. australis mainly on the upper shoal and 966.6 hm2 of S. mariqueter (Table 1). The documents kept at our laboratory reported that at the end of the year 1997, 60–70% of S. alterniflora and 80% of P. australis planted on the middle shoal colonized successfully and reached a height of 0.7–1.3 m. The mean number of tillers produced by these neophytes was 27 and 12 per clump for S. alterniflora and P. australis, respectively. Due to the low elevation and a severe typhoon in August, only 41 culms among the 5 hm2 of S. alterniflora planted on the lower shoal had been observed as exhibiting any growth sign.
4.2.
Lag time—year 1998–2000
The S. alterniflora community had not been discerned on the satellite image until the year 2000, and this image clearly showed the plantation belt on the middle shoal and many small patches on the lower shoal (Fig. 4B); as well, the S. alterniflora clumps began to invade towards their surroundings. One common feature of invasions is a lag time between initial colonization and the onset of rapid population growth and range expansion (Sakai et al., 2001). This lag time could also be observed for the population of S. alterniflora introduced into the Jiuduansha shoals for the period of 1998–2000. Although the total saltmarsh vegetation on the Jiuduansha shoals increased from 1094.1 hm2 in 1997 to 1472.5 hm2 in 2000, the area of S. alterniflora on the middle and lower shoals was only 101.6 hm2 (Table 1). The field observation in the growing seasons of 1998–1999 showed that both S. alterniflora and P. australis planted in 1997 had covered the plantation belt, forming a lush monocommunity. The mean height of the P. australis community reached 1.7 m and that of S. alterniflora 1.5–2 m (maximum height 2.3 m in some lush patches). The mono-community of
Rapid range expansion—year 2000–2004
The period during 2000 and 2001 marked an onset of rapid population growth and range expansion for S. alterniflora (Fig. 4B and C). The area of S. alterniflora community on the Jiuduansha shoals increased to 283.7 hm2 in 2001, at an annual expansion rate of 87% (Table 1). It was notable that the S. alterniflora community expanded much faster than the P. australis community planted on the middle shoal. On the lower shoal, several major patches of S. alterniflora larger than 5 hm2 had formed, and some scattered small patches of P. australis could be discerned as the accretion of elevation and natural succession progressed. The S. alterniflora population kept on rapidly spreading, increasing to 377.06 hm2 and the annual expansion rate reached 47% in 2002 (Fig. 4D and Table 1). The P. australis community kept also on spreading, but at a much lower rate than that of S. alterniflora. The S. alterniflora population was still in the process of rapid expansion in the years 2003 and 2004, and the areas amounted to 469.6 and 1014.4 hm2 with an annual expansion rate of 25% and 116%, respectively (Fig. 4E and D and Table 1). As most of the suitable niche on the middle shoal had been occupied by this time, however, the expansion rates of both the S. alterniflora and P. australis communities began slowing down. On the lower shoal, it was noticeable that an enormous range expansion of S. alterniflora was taking place, which could be attributed mainly to the rapid accretion of area and elevation of the lower shoal.
5.
Discussion
5.1. The nature of saltmarsh vegetation on the Jiuduansha shoals The Jiuduansha shoals are a neonatal wetland and develop very fast owing to sedimentation of the huge amount of silt brought by the Yangtze River. The climate, substrate and geomorphologic processes are suitable for development of salt marsh vegetation and the growth of S. alterniflora. Zonation of vegetation is widely recognized in salt marsh and three distinct zones of salt marsh vegetation, related to elevation, can be identified within the tidal flats in the region of the Yangtze Estuary (Zhang and Yong, 1992; Huang et al., 2005). The salt marsh vegetation on the Jiuduansha shoals also displays a typical vegetation zonation for this region. The dynamic, neonatal and natural characteristics of the Jiuduansha shoals, both in terms of silt deposition, vegetation succession and biological invasion, linked to its importance as a wetland for biodiver-
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sity conservation and resource management, makes it an ideal study area for this research.
5.3. Implication for biodiversity conservation and resource management
5.2.
Studies of invasive species may provide opportunities to better understand aspects of community dynamics, and are crucial for applications of community theory in restoration biology (Sakai et al., 2001; Teal and Peterson, 2005; Philipp and Field, 2005). The spatial pattern of population and community is a product of the interactions among processes like colonization, dynamics, renewal, competition, succession and evolution. The Jiuduansha shoals provide an ideal setting for analysis of the sequence of events associated with the initial colonization, lag time, rate of geographic spread and features of geographic spread of the exotic S. alterniflora and the native P. australis. In addition, with little anthropogenic disturbance imposed upon the wetland ecosystem, examination of the ecological theory of population and community dynamics can be carried out. It is noticeable that P. australis planted in 1997 showed a similar pattern and process of range expansion, although at a slower rate, as did S. alterniflora, which indicated that the exotic S. alterniflora shares traits (or niches) with the native P. australis. Once either of these two species occupied the new habitat, they could not out-compete and replace each other as reported by some studies (e.g. Chen et al., 2005), which is clearly compatible with the classical space-preemption model for plant community dynamics (Watt, 1947). Range expansion models have been applied directly to resource management questions, such as whether barrier zones can effectively be used to slow the spread of the invaders (Sakai et al., 2001). Such models can also lead to valuable insights into the population and community ecology of invasive species, and can be used to estimate the potential effectiveness of control strategies (Liu et al., 2005). Based on the results of this study, a spatially explicit cellular automata will be used to model population and community dynamics, and to study the interactions between spatial pattern and ecosystem processes for the salt marsh vegetation on the Jiuduansha shoals, which is very important for wetland biodiversity conservation and resource management.
Expansion pattern of S. alterniflora
One common feature of invasions is a lag time between initial colonization and the onset of rapid population growth and range expansion (Sakai et al., 2001). The expansion pattern of S. alterniflora on the Jiuduansha shoals is clearly compatible with this common feature, and the lag time in this case can be interpreted mainly as the evolution of adaptations to the new habitat. Once initial colonization and establishment have occurred, a rapid expansion of S. alterniflora has been reported by a number of researchers (Tang and Zhang, 2003; Li et al., 2006). For example, S. alterniflora at the San Francisco Bay on the Pacific coast of California had been in the phase of exponential increase with an annual expansion rate of 18–20% (Ayres et al., 2004). On the coast of Jiangsu Province, China, the range expansion of S. alterniflora had been delineated into three stages: the first stage was the period 1993–1995, with an annual mean expansion rate of 30%; the second stage was the period 1995–1999, with an annual rate of 43%; and the third stage was the period 1999–2001, with an annual rate of 10% (Zhang et al., 2004). After initial colonization and a lag time, the year 2000 marked an onset of rapid range expansion of S. alterniflora on the Jiuduansha shoals. The rates were higher than those mentioned above and could be attributed to the dynamic and fast growing nature of the neonatal shoals. It could be also anticipated that the rapid range expansion of S. alterniflora would last for a considerable period in future on the Jiuduansha shoals. Once initial colonization and establishment have occurred, invasive species may spread from continuing long distance dispersal (saltation dispersal) as well as from short-distance dispersal (diffusion dispersal) with lateral expansion of the established population (Davis and Thompson, 2000; Sakai et al., 2001). Our results indicate that diffusion dispersal with lateral expansion of the established population is the predominant spreading pattern for S. alterniflora, while long distance dispersal by their propagules may play almost no role for the range expansion. Factors influencing the number of dispersal modes, and vital rates (births, deaths) are critical factors regulating the spread of invasive species (Sakai et al., 2001). Compared with native species, S. alterniflora has a stronger reproductive capacity and wider ecological niche. A study carried out on Chongming Island, very close to our study site, indicated that one individual of S. alterniflora could produce 86–222 tillers and the diffusion dispersal with lateral expansion of the established population could reach 107–263 cm during a growing period of 9 months (Zhang et al., 2006). Another study showed that seed production of S. alterniflora was 369 ± 52 per individual and the seed germination rate was about 72.3 ± 2.3% (Chen et al., 2005). Given their importance in the continued range expansion of S. alterniflora in our study site, quantifying both the number and distribution of propagules (both seed and tiller) involved in establishment and spread needs further study.
6.
Conclusion
This study investigated the population expansion pattern of an exotic species of S. alterniflora for a period of 7 years after being newly introduced to the neonatal Jiuduansha shoals in the Yangtze Estuary, Shanghai. The population expansion pattern of S. alterniflora on the Jiuduansha shoals was compatible with the common feature of invasions, i.e. the initial colonization, a lag time and the onset of rapid population growth and range expansion. The year 2000 marked an onset of rapid population growth and range expansion of S. alterniflora, which exceeded any of the indigenous species and indicated its strong competitive capability, rapid range expansion and wide ecological niche. An analysis of the sequence of events associated with the initial colonization, lag time, rate of geographic spread and features of geographic spread of the exotic S. alterniflora and the native P. australis on the Jiuduansha shoals indicated
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a pattern which was clearly compatible with the classical space-preemption model for plant community dynamics. Range expansion models could be applied directly to resource management questions and the results of this study indicated the potential of this approach for providing timely data for biodiversity conservation of intertidal zones, resource management and sustainable development on coastal regions.
Acknowledgements The authors would like to thank members of the Ecological Section of the State Key Laboratory of Estuarine and Coastal Research, East China Normal University, for their assistance with the collection of the field data. We also thank Professor Bruce Anderson, Queen’s University, Canada for valuable comments and linguistic checking. The research has been funded by National Key Fundamental Research and Development Program (2004CB720505), Key Project of the Shanghai Scientific & Technological Committee (04DZ19304) and the State’s 10th Five-Year “211 Project” – supported key academic discipline program of ECNU.
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A study of the population dynamics of Spartina alterniflora at Jiuduansha shoals, Shanghai, China Huamei Huang, Liquan Zhang ∗ State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan Road North, Shanghai 200062, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
This study investigated the population expansion pattern of an exotic species of Spartina
Received 22 February 2006
alterniflora for a period of 7 years, after it had been newly introduced to the neonatal shoals
Received in revised form 7 June 2006
of Jiuduansha (GPS), in the Yangtze Estuary, Shanghai. Remote sensing, in conjunction with
Accepted 10 June 2006
geographical information systems (GIS) and global positioning systems (GPS) was used to map saltmarsh vegetation on the Jiuduansha shoals and the classifications were then checked using in situ field surveys of selected areas. The results showed that the S. alterni-
Keywords:
flora population had expanded from 55 hm2 when first introduced in 1997, to 1014 hm2 in
Jiuduansha shoals
2004. The population expansion pattern of S. alterniflora on the Jiuduansha shoals was com-
Exotic plant
patible with the common feature of invasions, i.e. the initial colonization, a lag time and
Spartina alterniflora
the onset of rapid population growth and range expansion. In the first year of plantation
Population dynamics
(1997), about 35 hm2 of S. alterniflora was successfully colonized on the Jiuduansha shoals.
Remote sensing
The period between 1998 and 2000 was characterized by a lag time, and the area of S. alterniflora increased only to 101.6 hm2 . The year 2000 marked an onset of rapid population growth and range expansion and the annual expanding rate reached 25–116%, which exceeded any of the indigenous species and indicated the strong competitive capability, rapid range expansion and wide ecological niche of S. alterniflora. The advent of remote sensing, in conjunction with geographical information systems and global positioning systems, provides a potential tool for mapping vegetation, and for monitoring population dynamics and range expansion of invasive species on a large scale. The implications for population and community dynamics, biodiversity conservation and wetland management in terms of the analysis of the sequence of events associated with the initial colonization, a lag time, rate of geographic spread and features of geographic spread of the exotic S. alterniflora and the native P. australis on the Jiuduansha shoals are discussed. © 2006 Elsevier B.V. All rights reserved.
1.
Introduction
The Yangtze Estuary is a typical medium tidal estuary with multi-order bifurcations, shoals and sand bars. Due to the distinct interactions between runoff and tidal currents, the dynamic and geomorphologic processes of the estuary
∗
display a unique kinematical rule among the estuaries of the world (Yun, 2004). The region of mouth bars in the estuary and the submerged delta nearby the estuary are the major locations for sedimentation of the huge amount of silt brought by the Yangtze River (Fig. 1). The neonatal shoals of Jiuduansha initially emerged above the water surface
Corresponding author. Tel.: +86 21 6223 2599; fax: +86 21 6254 6441. E-mail addresses: [email protected], [email protected] (L. Zhang). 0925-8574/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2006.06.005
e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 164–172
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Fig. 1 – The location of study area showing the Yangtze Estuary, Shanghai metropolitan region and Jiuduansha shoals.
in the 1920s and have been growing rapidly since (Fig. 2). According to recent measurements in 2004, the areas above the 0 and −5 m isobaths of the shoals have amounted to 127.3 and 348 km2 , respectively, and the highest elevation was 4 m. The Jiuduansha shoals have never been colonized by humans and have been in a natural condition since they were formed; they were set as a wetland nature reserve of Shanghai Municipality in 2003 and a national wetland nature reserve in 2005. Spartina alterniflora originated in the eastern and gulf coasts of the USA. The species dominates on salt marshes and has important ecological functions in its native ecosystems (Simenstad and Thom, 1995). With its great capacity for reducing tidal wave energy, mitigating erosion and trapping sediment, S. alterniflora was introduced to many coastal and estuarine regions of the world as a species for ecological engineering (Callaway and Josselyn, 1992; Chung et al., 2004; Chen et al., 2004; Zhang et al., 2004). In recent years, however, some evidence has been reported that this exotic species may outcompete native plants, threaten the native ecosystems and coastal aquaculture, and cause declines in native species rich-
ness (Callaway and Josselyn, 1992; Daehler and Strong, 1996; Chen et al., 2004). As an ecological engineering project carried out by the State Key Laboratory of Estuarine and Coastal Research, 40 hm2 of Phragmites australis and 55 hm2 of S. alterniflora were planted on the Jiuduansha shoals in 1997, as a measure to mitigate the negative effects of land reclamation during the construction of Pudong International Airport (Chen et al., 2001). By their very nature, salt marshes and inter-tidal zones are dynamic and their management relies on up-to-date spatial information (Mitsch and Gosselink, 2000; Thomson et al., 2003; Philipp, 2005). Remote sensing (RS) is becoming the main resource for land cover inventory information gathering, and has been widely used to provide a useful tool for mapping vegetation, and monitoring vegetation dynamics and biodiversity conservation on a large scale (Ayres et al., 2004; Zhang et al., 2004; Philipp and Field, 2005; Huang et al., 2005; Hinkle and Mitsch, 2005; Gao and Zhang, 2006). The aim of this study was to map the salt marsh vegetation and analyze quantitatively the population expansion pattern of S. alterniflora for a period of 7 years after being introduced to the Jiuduansha shoals.
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Identifying these time-based data is an important first step to providing a scientific basis for wetland management and biodiversity conservation.
2.
Study area
The Jiuduansha Wetland Nature Reserve is located between the southern and northern watercourse of the Yangtze Estuary, 12 km east of Pudong International Airport and between 31◦ 03 –31◦ 17 N, 121◦ 46 –122◦ 15 E (Fig. 1). It covers 423.2 km2 , with an east-west distance of 25.9 km and north-south length of 46.3 km, and consists of the Jiangyanan shoal, upper shoal, middle shoal and lower shoal. After a half-century of dynamic evolution, the Jiuduansha shoals have become more or less stable (Fig. 2). The region of the Jiuduansha shoals has a northern subtropical monsoon climate, with an average annual temperature 16 ◦ C; summer temperatures average 28 ◦ C while winters are cold with an average temperature of 4 ◦ C. Average annual precipitation is approximately 1200 mm, with 60% of rainfall occurring during May–September and few typhoons during summer and autumn. The dynamic and geomorphologic processes of the shoals are profoundly influenced by the distinct interactions between runoff and tidal (semidiurnal) currents. The maximum and average tidal range is 4.62 and 2.67 m, respectively. Due to the short history of the shoals, the soil type is original and simple, consisting mainly of the coastal alluvial and solonchalk soils. Since 1987, the pioneer Scirpus mariqueter salt marsh community could be discerned from satellite images, which showed the community first colonizing on the tidal flats above the 2 m elevation. The tidal flats closest to the low water mark, elevation less than 2 m, are characterized by mud flats that
are devoid of any vascular plants. As sedimentation and succession progressed, the P. australis community replaced the S. mariqueter community above the 2.9 m elevation (Zhang and Yong, 1992; Huang et al., 2005). An additional species in this zone was S. alterniflora, which was introduced to the Jiuduansha shoals in 1997. Over the last 7 years this species has gradually invaded large areas where could formerly be covered by P. australis and has also started to invade the upper parts of the S. mariqueter zone. Except for the Jiangyanan shoal that is still below the 2 m elevation, a typical zonation of salt marsh vegetation has developed on the other three shoals over the same time period.
3.
Materials and methods
Fig. 3 gives a general data processing chart for the materials and methods used in the study, which applies to all of the images, and more details are presented in the following sections.
3.1.
Data source
For tracing S. alterniflora population developments, treatments and analyses of satellite images were conducted. A set of multi-temporal landsat thematic mapper images of 20 October 1997, 24 May 2000, 26 July 2001, 2 August 2003 and 19 July 2004, and a landsat enhanced thematic mapper image of 11 November 2002 were used, which basically covered the state of the low tide at the time the images were taken and the areas of different stages from first planting to coalescence. These satellite images were geometrically corrected by a series of nautical charts, 1:120,000 scale, using ERDAS Imagine software. Quadratic polynomials were applied to correction equations according to the distribution of control points. All images were
Fig. 2 – Historical changes in the 0 m isobaths of Jiuduansha shoals in the Yangtze Estuary.
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represented the “brightness”, the second “greenness” and the third “wetness” (Crist and Cicone, 1984; Crist and Kauth, 1986; Zhao, 2004). After the K-T transformation, considerable differences among the major salt marsh communities could be identified on the first three axes. A number of vegetation indices have been formulated for mapping vegetation (Liu et al., 1998; Lyon, 2000) and among them, NDVI, an important index reflecting the vegetation growth status and its coverage, has been used most widely (Tappan et al., 1992; Lyon et al., 1998). Based on the above-mentioned image spectral enhancement, a series of classes, i.e. water, muddy flat, dyke, S. alterniflora, P. australis and S. mariqueter communities, were identified and selected as training samples. A supervised classification, using the Maximum Likelihood Classifier in ERDAS Imagine, was then carried out. The classified images were then integrated into a GIS platform for analyzing the spatial and temporal dynamics patterns of salt marsh vegetation and S. alterniflora.
3.3. Fig. 3 – A general flow chart for data processing in this study and more details see the text.
then resampled to a resolution of 30 m × 30 m and the error was less than 0.5 pixels.
3.2.
Data processing
The multispectral TM and ETM+ remote sensing data makes it possible for spectral transformations to generate new sets of image components or bands (Schmidt and Skidmore, 2003). On the basis of a previous study on the coastal salt marsh in Shanghai (Huang et al., 2005), two spectral enhancement methods, Tasseled Cap (K-T) Transform and Normal Difference Vegetation Index (NDVI), were used to interpret satellite images more efficiently. The K-T transformation could facilitate finding the data structures inherent to a particular sensor and set of scene classes, and adjust the viewing perspective so that these structures could be more easily and completely observed (Crist and Kauth, 1986). In K-T transformation, the coefficient was determined by satellite sensor. The first axis of the output data
Field verification
The purpose of field verification was to validate the results of image classification as well as to provide reliable ground information for the training samples. An in situ field survey was carried out in July 2004 and a total of 83 sites were selected, with 13 on the upper shoal, 41 on the middle shoal and 29 on the lower shoal, using GPS to match the locations as accurately as possible. The potential error of the GPS receiver used was less than 15 m and the topical error usually less than 10 m. The historic records of the ecological engineering project and field surveys (Tang and Lu, 2003) were used for the results of image classification before the year 2004. Accuracy assessment involved the derivation of accuracy metrics based on a comparison between the class labels in the thematic map and ground data for a set of specific locations (Foody, 2002). We assessed the accuracy of initial classification and the results showed that the initial overall accuracy for the year 2004 was 71%, year 2003 73%, year 2002 78%, year 2001 67%, year 2000 65%, and year 1997 68%, respectively. After field verification, some misclassifications were corrected, especially near the boundaries of different salt marsh community types, and the overall accuracy for revised classification of the different years reached to 80–90%.
Table 1 – The spatial-temporal dynamics of saltmarsh communities on the Jiuduansha shoals Date
Spartina alterniflora Area (hm2 )
20 October 1997 24 May 2000 26 July 2001 11 November 2002 2 August 2003 19 July 2004 a
55a 101.6 283.7 377.1 469.6 1014.4
Phragmites australis (hm2 )
Scirpus mariqueter (hm2 )
Total area (hm2 )
Annual spread rate (%) – – 87.2 47.1 24.6 116.0
Including 40 hm2 P. australis and 55 hm2 S. alterniflora planted in 1997.
167.5a 353.8 368.9 401.9 463.4 563.5
966.6 1017.1 1382.9 1608.2 1850.2 1789.0
1094.1 1472.5 2035.5 2387.2 2783.3 3366.9
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Fig. 4 – The spatial-temporal dynamics of salt marsh vegetation and Spartina alterniflora on the Jiuduansha shoals. (A) The map of salt marsh vegetation of 1997 and the plantation locations were imposed on the map; (B) the map of 2000 marked an onset of rapid population growth and range expansion after the initial colonization; (C–F) the maps for the period of 2001–2004 showing the rapid population growth and range expansion.
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The results of salt marsh vegetation classification based on the multi-temporal satellite images for the Jiuduansha shoals are presented in Fig. 4. The distribution areas of each type of saltmarsh vegetation for the period of 1997–2004 are summarized in Table 1. From this information, three distinct stages could be recognized according to the expansion pattern of S. alterniflora.
S. alterniflora and P. australis on the middle shoal then began to expand towards the surroundings, which were mostly the habitats of the S. mariqueter community. The width of the expanding range would reach 2–4 m and many small clumps appeared within the range of 50 m outside the plantation belts. At the same time, many small patches of S. alterniflora clumps emerged among the original S. mariqueter community on the lower shoal, which indicated that S. alterniflora had succeeded in colonization and had begun to expand along with the accretion of elevation.
4.1.
4.3.
4.
Results
Initial colonization—year 1997
In April and May, 40 hm2 of P. australis and 50 hm2 of S. alterniflora were planted on the middle shoal as an “Ecological Engineering Project for Pudong International Airport”. Culms of these two species were planted along belts, each 100 m in width, in a regular arrangement with 1 m spacing. In addition, ca. 5 hm2 of S. alterniflora were planted where the elevation reached above 2.5 m on the lower shoal. Due to the small size of the clumps, the newly planted S. alterniflora and P. australis could not be discerned on the satellite image (with the resolution of 30 m × 30 m) of this year. The plantation locations recorded by GPS (Chen et al., 2001) were thus imposed on the map of salt marsh vegetation (Fig. 4A). The total salt marsh vegetation on the Jiuduansha shoals was 1094.1 hm2 , which included 167.5 hm2 of P. australis mainly on the upper shoal and 966.6 hm2 of S. mariqueter (Table 1). The documents kept at our laboratory reported that at the end of the year 1997, 60–70% of S. alterniflora and 80% of P. australis planted on the middle shoal colonized successfully and reached a height of 0.7–1.3 m. The mean number of tillers produced by these neophytes was 27 and 12 per clump for S. alterniflora and P. australis, respectively. Due to the low elevation and a severe typhoon in August, only 41 culms among the 5 hm2 of S. alterniflora planted on the lower shoal had been observed as exhibiting any growth sign.
4.2.
Lag time—year 1998–2000
The S. alterniflora community had not been discerned on the satellite image until the year 2000, and this image clearly showed the plantation belt on the middle shoal and many small patches on the lower shoal (Fig. 4B); as well, the S. alterniflora clumps began to invade towards their surroundings. One common feature of invasions is a lag time between initial colonization and the onset of rapid population growth and range expansion (Sakai et al., 2001). This lag time could also be observed for the population of S. alterniflora introduced into the Jiuduansha shoals for the period of 1998–2000. Although the total saltmarsh vegetation on the Jiuduansha shoals increased from 1094.1 hm2 in 1997 to 1472.5 hm2 in 2000, the area of S. alterniflora on the middle and lower shoals was only 101.6 hm2 (Table 1). The field observation in the growing seasons of 1998–1999 showed that both S. alterniflora and P. australis planted in 1997 had covered the plantation belt, forming a lush monocommunity. The mean height of the P. australis community reached 1.7 m and that of S. alterniflora 1.5–2 m (maximum height 2.3 m in some lush patches). The mono-community of
Rapid range expansion—year 2000–2004
The period during 2000 and 2001 marked an onset of rapid population growth and range expansion for S. alterniflora (Fig. 4B and C). The area of S. alterniflora community on the Jiuduansha shoals increased to 283.7 hm2 in 2001, at an annual expansion rate of 87% (Table 1). It was notable that the S. alterniflora community expanded much faster than the P. australis community planted on the middle shoal. On the lower shoal, several major patches of S. alterniflora larger than 5 hm2 had formed, and some scattered small patches of P. australis could be discerned as the accretion of elevation and natural succession progressed. The S. alterniflora population kept on rapidly spreading, increasing to 377.06 hm2 and the annual expansion rate reached 47% in 2002 (Fig. 4D and Table 1). The P. australis community kept also on spreading, but at a much lower rate than that of S. alterniflora. The S. alterniflora population was still in the process of rapid expansion in the years 2003 and 2004, and the areas amounted to 469.6 and 1014.4 hm2 with an annual expansion rate of 25% and 116%, respectively (Fig. 4E and D and Table 1). As most of the suitable niche on the middle shoal had been occupied by this time, however, the expansion rates of both the S. alterniflora and P. australis communities began slowing down. On the lower shoal, it was noticeable that an enormous range expansion of S. alterniflora was taking place, which could be attributed mainly to the rapid accretion of area and elevation of the lower shoal.
5.
Discussion
5.1. The nature of saltmarsh vegetation on the Jiuduansha shoals The Jiuduansha shoals are a neonatal wetland and develop very fast owing to sedimentation of the huge amount of silt brought by the Yangtze River. The climate, substrate and geomorphologic processes are suitable for development of salt marsh vegetation and the growth of S. alterniflora. Zonation of vegetation is widely recognized in salt marsh and three distinct zones of salt marsh vegetation, related to elevation, can be identified within the tidal flats in the region of the Yangtze Estuary (Zhang and Yong, 1992; Huang et al., 2005). The salt marsh vegetation on the Jiuduansha shoals also displays a typical vegetation zonation for this region. The dynamic, neonatal and natural characteristics of the Jiuduansha shoals, both in terms of silt deposition, vegetation succession and biological invasion, linked to its importance as a wetland for biodiver-
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sity conservation and resource management, makes it an ideal study area for this research.
5.3. Implication for biodiversity conservation and resource management
5.2.
Studies of invasive species may provide opportunities to better understand aspects of community dynamics, and are crucial for applications of community theory in restoration biology (Sakai et al., 2001; Teal and Peterson, 2005; Philipp and Field, 2005). The spatial pattern of population and community is a product of the interactions among processes like colonization, dynamics, renewal, competition, succession and evolution. The Jiuduansha shoals provide an ideal setting for analysis of the sequence of events associated with the initial colonization, lag time, rate of geographic spread and features of geographic spread of the exotic S. alterniflora and the native P. australis. In addition, with little anthropogenic disturbance imposed upon the wetland ecosystem, examination of the ecological theory of population and community dynamics can be carried out. It is noticeable that P. australis planted in 1997 showed a similar pattern and process of range expansion, although at a slower rate, as did S. alterniflora, which indicated that the exotic S. alterniflora shares traits (or niches) with the native P. australis. Once either of these two species occupied the new habitat, they could not out-compete and replace each other as reported by some studies (e.g. Chen et al., 2005), which is clearly compatible with the classical space-preemption model for plant community dynamics (Watt, 1947). Range expansion models have been applied directly to resource management questions, such as whether barrier zones can effectively be used to slow the spread of the invaders (Sakai et al., 2001). Such models can also lead to valuable insights into the population and community ecology of invasive species, and can be used to estimate the potential effectiveness of control strategies (Liu et al., 2005). Based on the results of this study, a spatially explicit cellular automata will be used to model population and community dynamics, and to study the interactions between spatial pattern and ecosystem processes for the salt marsh vegetation on the Jiuduansha shoals, which is very important for wetland biodiversity conservation and resource management.
Expansion pattern of S. alterniflora
One common feature of invasions is a lag time between initial colonization and the onset of rapid population growth and range expansion (Sakai et al., 2001). The expansion pattern of S. alterniflora on the Jiuduansha shoals is clearly compatible with this common feature, and the lag time in this case can be interpreted mainly as the evolution of adaptations to the new habitat. Once initial colonization and establishment have occurred, a rapid expansion of S. alterniflora has been reported by a number of researchers (Tang and Zhang, 2003; Li et al., 2006). For example, S. alterniflora at the San Francisco Bay on the Pacific coast of California had been in the phase of exponential increase with an annual expansion rate of 18–20% (Ayres et al., 2004). On the coast of Jiangsu Province, China, the range expansion of S. alterniflora had been delineated into three stages: the first stage was the period 1993–1995, with an annual mean expansion rate of 30%; the second stage was the period 1995–1999, with an annual rate of 43%; and the third stage was the period 1999–2001, with an annual rate of 10% (Zhang et al., 2004). After initial colonization and a lag time, the year 2000 marked an onset of rapid range expansion of S. alterniflora on the Jiuduansha shoals. The rates were higher than those mentioned above and could be attributed to the dynamic and fast growing nature of the neonatal shoals. It could be also anticipated that the rapid range expansion of S. alterniflora would last for a considerable period in future on the Jiuduansha shoals. Once initial colonization and establishment have occurred, invasive species may spread from continuing long distance dispersal (saltation dispersal) as well as from short-distance dispersal (diffusion dispersal) with lateral expansion of the established population (Davis and Thompson, 2000; Sakai et al., 2001). Our results indicate that diffusion dispersal with lateral expansion of the established population is the predominant spreading pattern for S. alterniflora, while long distance dispersal by their propagules may play almost no role for the range expansion. Factors influencing the number of dispersal modes, and vital rates (births, deaths) are critical factors regulating the spread of invasive species (Sakai et al., 2001). Compared with native species, S. alterniflora has a stronger reproductive capacity and wider ecological niche. A study carried out on Chongming Island, very close to our study site, indicated that one individual of S. alterniflora could produce 86–222 tillers and the diffusion dispersal with lateral expansion of the established population could reach 107–263 cm during a growing period of 9 months (Zhang et al., 2006). Another study showed that seed production of S. alterniflora was 369 ± 52 per individual and the seed germination rate was about 72.3 ± 2.3% (Chen et al., 2005). Given their importance in the continued range expansion of S. alterniflora in our study site, quantifying both the number and distribution of propagules (both seed and tiller) involved in establishment and spread needs further study.
6.
Conclusion
This study investigated the population expansion pattern of an exotic species of S. alterniflora for a period of 7 years after being newly introduced to the neonatal Jiuduansha shoals in the Yangtze Estuary, Shanghai. The population expansion pattern of S. alterniflora on the Jiuduansha shoals was compatible with the common feature of invasions, i.e. the initial colonization, a lag time and the onset of rapid population growth and range expansion. The year 2000 marked an onset of rapid population growth and range expansion of S. alterniflora, which exceeded any of the indigenous species and indicated its strong competitive capability, rapid range expansion and wide ecological niche. An analysis of the sequence of events associated with the initial colonization, lag time, rate of geographic spread and features of geographic spread of the exotic S. alterniflora and the native P. australis on the Jiuduansha shoals indicated
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a pattern which was clearly compatible with the classical space-preemption model for plant community dynamics. Range expansion models could be applied directly to resource management questions and the results of this study indicated the potential of this approach for providing timely data for biodiversity conservation of intertidal zones, resource management and sustainable development on coastal regions.
Acknowledgements The authors would like to thank members of the Ecological Section of the State Key Laboratory of Estuarine and Coastal Research, East China Normal University, for their assistance with the collection of the field data. We also thank Professor Bruce Anderson, Queen’s University, Canada for valuable comments and linguistic checking. The research has been funded by National Key Fundamental Research and Development Program (2004CB720505), Key Project of the Shanghai Scientific & Technological Committee (04DZ19304) and the State’s 10th Five-Year “211 Project” – supported key academic discipline program of ECNU.
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