Losing tidal wetlands in the midpoint of East Asian-Australasian Flyway in the pasting 100 years

doi:10.21203/rs.3.rs-2987880/v1

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Losing tidal wetlands in the midpoint of East Asian-Australasian Flyway in the pasting 100 years

https://doi.org/10.21203/rs.3.rs-2987880/v1

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Migratory shorebirds rely on tidal flats as stopover sites for refueling and wintering. Taiwan is in the middle of the East Asian–Australasian Flyway, where the sandy/muddy shores on the western coast support thousands of migratory birds with important stopover habitats. Among these habitats, tidal flats are the most crucial habitats for shorebird survival. However, in the last century, the landscape has been largely altered for agriculture, aquaculture, and industrial development. To assess the current status of shorebird habitats, we evaluated landscape changes between the 1920s and 2020s based on historical topographic map data, Landsat Archive images, and the Normalized Difference Water Index. Our analysis showed that the area of tidal flats increased before the 1950s but has decreased since the 1980s. The area dropped rapidly from 459.72 km² to 194.05 km², and almost 60% of the tidal flat coverage has been lost in the last 60 years. These results indicate that the tidal wetlands in western Taiwan have reached an endangered status, as per the International Union for Conservation of Nature Red List of Ecosystems criteria. Although the decrease in speed seems to have slowed in recent decades, energy and industrial development still threaten tidal wetlands.

coastal wetlands

intertidal flat

land cover

mudflat

Normalized Difference Water Index (NDWI)

shorebirds’ habitats

Tidal flats lie between marine and terrestrial environments. They are known for their high biodiversity, supported by both marine- and terrestrial-derived nutrients. Tidal flats are among the most productive ecosystems in the world and serve as essential habitats for millions of migratory shorebirds. Therefore, shorebirds have become indicator species for this ecosystem (Beninger et al. 2018; Mathot et al. 2018; Passarelli et al. 2018).

This productive coastal area is an important natural resource and living region for people. However, natural coastline landscapes have been reclaimed to create artificial and highly managed wetlands, given the rapid increase in the human population in the last century. Human-contributed wetland fields, including rice paddies, saltpans, aquaculture ponds, and reservoirs, have become a new type of wetland. However, both natural and anthropogenic wetlands have been lost in recent years due to residential and commercial development, energy production, transportation facilities, and industrial development (Amano et al. 2010; Davidson 2014; Gopal 2013). Upstream weirs, dams, and reservoirs cut off sediments from the land to the coastline, indirectly threatening tidal flat ecosystems (Walling 2006).

Tidal flats are disappearing globally, with 16% loss between 1984 and 2016 (Murray et al. 2019). This situation is particularly challenging in Asia, where many tidal flats that serve as habitats for migratory shorebirds have disappeared from the Chinese coastline over the last 50 years due to seawalls, reclamation projects, and changes in sediment regimes (Choi et al. 2018; Ma et al. 2014; Murray et al. 2014; Murray et al. 2012). In South Korea, the Saemangeum Seawall, built in 2006, has impounded almost 29,000 hectares of tidal flats. After the closure, at least 128,000 shorebirds were lost between 2006 and 2008 (Moores et al. 2016).

Since the tidal flats of East Asia belong to the East Asian-Australasian Flyway (EAAF), which extends from Siberia and Alaska through East Asia to Australasia, they are an annual migration route for migratory birds and support the largest number of shorebirds among the eight known flyways in the world (Hansen et al. 2022). The loss of habitat has coincided with the decline in the numbers of migratory shorebirds in their non-breeding grounds in Australasia (Studds et al. 2017). However, most studies on tidal flat changes and their effects on shorebirds within the EAAF have focused on the Yellow Sea (Choi et al. 2018; Ma et al. 2014; Murray et al. 2014; Murray et al. 2012). Recent research has shown that even simple, predictable linkages among sites caused by "to-and-fro" migration can make migratory species especially vulnerable to habitat loss, substantially affecting conservation prioritization (Runge et al. 2014). Therefore, gathering information on each vital shorebird habitat throughout the EAAF is crucial. Furthermore, the threats that lead to losses in tidal flats have rarely been analyzed in detail.

Taiwan is a vital waterbird biodiversity hotspot in the Central EAAF region. The sandy/muddy shores on the western coast support thousands of migratory birds and provide important stopover sites and wintering habitats (Kasahara et al. 2020; Li et al. 2019; Lin et al. 2023). However, the homes and workplaces of millions of people are located in this area. Over the last 100 years, industrial and commercial development has caused the natural coastline to shrink drastically, with only 5% of the original coast remaining in the western part of Taiwan (Construction and Planning Agency 2021). Because Taiwan provides habitats for thousands of migratory birds and is located in the middle of the EAAF, research is needed to fill crucial knowledge gaps in the literature on the conservation of migratory birds. Additionally, it will provide an excellent opportunity to highlight the main threats and how they have caused the loss of tidal flat ecosystems in a rapidly developing region in East Asia.

Long-term data are needed to explore the relationship between tidal flat area changes and the various driving forces behind tidal flat changes. Maps are valuable documents for recording the historical environment and provide valuable environmental information for studying habitat changes based on overlapping maps and satellite images. Taiwan’s land cover was reconstructed and divided into six classes, excluding tidal flats, between 1904 and 2015 (Chen et al. 2019). Previous studies have shown the probability of historical analyses providing information on habitat changes (Shin 1980; Stein et al. 2010; Chen et al. 2019). Satellite sensors have been widely used to monitor the Earth since the 1960s, including the IKONAS, Landsat Thematic Mapper (TM), Enhanced Thematic Mapper (ETM+), MODIS, and SPOT. Recently, Landsat TM and ETM + images have been used to estimate tidal flat areas (McFeeters 2013; Murray et al. 2012; 2014).

Our research focused on the changes in the area and risks to the tidal flat ecosystem in Taiwan. To estimate the current status of the tidal flat ecosystem in Taiwan, we (1) mapped the spatial distribution of tidal flats over the last 100 years and evaluated the area-changing patterns; (2) applied the International Union for Conservation of Nature (IUCN) Red List of Ecosystems criteria (Bland et al. 2016; Keith et al. 2013), a global standard for assessing and comparing the threatened status of tidal flat ecosystems on the EAAF; and (3) reviewed the development events to classify the threats to the tidal flat ecosystem.

Our study area included the Taiwan western coastline from the Dansuie River estuary (25°10'27"N, 121°24'48"E) to the Fenggang River estuary (22°11'39"N, 120°41'05"E) (Fig. 1). We obtained the spatiotemporal distribution of tidal flats from historical topographic maps and satellite data and digitized these images to estimate the area of tidal flats.

Historic topographic map data

At the beginning of the 20th century, Taiwan became a developing country. We used the historical Taiwan Fortress map published in 1921 as the baseline to estimate the area of the tidal flats. This map covers 60% of the island, including the intertidal and lowland areas of Taiwan's main island, and is the oldest map that can be used for land cover reconstruction (Academia Sinica 2003; Chen et al. 2019). The other available map was obtained from 110 series of 1:500,000 topographic maps reproduced in 1956. The collection of these maps was based on the AMS L892 (Formosa [Taiwan]) topographic maps by the US Army Map Service in 1944 and 1945 (Academia Sinica 2003).

Satellite data

First, we used the U.S. Geological Survey (USGS) Earth Explorer (U.S. Geological Survey 2020) to visually review all Landsat archived images of Taiwan from 1972 to 2021 from the USGS, which contained information on geographic location, cloud cover, and acquisition time and date. Then, we selected images with less than 30% cloud cover (1,687 images) from the Landsat 1–5 MMS (Multi-Spectral Scanner, LM1-5), Landsat 4–5 TM (Thematic Mapper, LT4-5), Landsat 7 Enhanced Thematic Mapper Plus (ETM+, LE7), and Landsat 8 Operational Land Imager (LC8). Second, we chose images in the low- and high-tide timing periods based on annual reports of marine meteorological data from the Taiwan Central Weather Bureau (https://www.cwb.gov.tw/V8/E/D/publication.html). The final images comprised Landsat MSS, TM, and ETM + satellite images at spatial resolutions of 60 m (LM1-5) or 30 m (LT4-5, LE7, and LC8). All available bands of Landsat imagery were used to determine the tidal flats (Murray et al. 2012).

Estimating the area of tidal flats

We digitized the coastal areas of the two historical maps as spatially distributed polygons and estimated the tidal flat areas in 1921 and 1956. To map the tidal flats from the satellite images, we calculated the Normalized Difference Water Index (NDWI) of all selected Landsat images for each pixel to separate the water area and terrestrial environments. The equation for the NDWI was as follows:

\({R}_{TM/ETM}=\frac{{\rho }_{green}-{\rho }_{NIR}}{{\rho }_{green}+{\rho }_{NIR}}\) Formula 1

where \({\rho }_{green}\) and \({\rho }_{NIR}\) are the radiances of the green and near-infrared wavelengths of the Landsat images (Murray et al. 2012; Xu, 2006).

Estimating trends in distributions and assessing the risk to the ecosystem

We subtracted the terrestrial area of high tide from that of low tide to obtain the tidal flat area. To detect changes and assess the threatened status, we compared tidal flat areas among seven time periods (the 1920s, 1950s, 1980s, 1990s, 2000s, 2010s, and 2020s) over the last 100 years.

We also applied criterion A (declining ecosystem distribution) of the IUCN Red List of Ecosystems criteria (Bland et al. 2016; Keith et al. 2013) to assess the status of the western Taiwan tidal flat ecosystem. We used the getDeclineStats() function in the R package “redlistr” to calculate the Absolute Rate of Decline (ARD) and the Proportional Rate of Decline (PRD) of the ecosystem distribution change (Lee et al. 2019) and estimated the tidal flats loss rate in each time period.

There are three timescales for ecosystem assessment with criterion A: (1) past 50-year period, (2) present and future 50-year period, and (3) historical change. We compared all available data from the historical maps, satellite images, historical data, and extrapolated data to estimate the loss of geographic distribution during these three time periods. The quantitative estimate of a given area was considered to be vulnerable when the decline in its distribution was between 30% and 50%; endangered when the decline was between 50% and 80%; and critically endangered when the decline was > 80% in the past 50-year period, the present, and future 50-year periods. Regarding historical change, an area was considered vulnerable when the decline in its distribution was between 50% and 70%, endangered when the decline was between 70% and 90%, and critically endangered when the decline was > 90%. Finally, the overall status of an ecosystem type was the highest risk category obtained using any criterion (Bland et al. 2016; Keith et al. 2013).

Furthermore, we wanted to understand the threats of changes in coastline areas. We digitized the development area and surveyed the years of coastal development and river construction over the last century. All events were classified and modified into categories of IUCN direct threats based on land use and land cover types (IUCN, 2022), which can be divided into four main types: (1) residential and commercial development, (2) agriculture and aquaculture, (3) energy production and mining, and (4) natural system modifications. The first type includes residential and commercial areas, such as harbors, thermal power plants, industrial factories, and terrestrial areas. The second category encompasses agriculture and aquaculture, which are further categorized into agriculture and aquaculture areas. The third type comprises mining and quarrying, including deserted salt ponds and renewable energy sources such as solar plate farms. The fourth type involves natural system modifications, including dams and water management/use such as dams, reservoirs, and river weirs.

Our data covered more than 400 km of Taiwan’s coastline and revealed a rapid and widespread decline in tidal flats across the entire region from 1921 to 2021 (Figure 1). Historical topographic maps indicate that tidal flats occupied 368.29 km² in the 1920s and 459.72 km² in the 1950s. During the first period (1920s–1950s), the area of tidal flats in western Taiwan increased by 24.83%. However, we did not have sufficient images for tidal flat area assessment from the 1960s to the 1970s because of the limited number of satellite images and cloud coverage. As per satellite image estimations, the area of tidal flats decreased rapidly to 339.78 km² from 1980 to 1989. During the second period (1950s–1980s), the proportion of tidal flat areas decreased (26.09%). From 1990 to 1999, the area decreased continually to 270.51 km². After 2000, the decline rate slowed down (11.53%); the tidal flats area was 239.31 km² in the 2000s and 194.05 km² in the 2010s. The area decreased rapidly from 459.72 km² to 194.05 km². However, in the recent decade, the area of tidal flats increased slightly to 236.08 km² in the 2020s (see Table 1, Figure 2, Figure 3 in appendix).

Table 1 The net loss of tidal flats in Taiwan from the 1920s to 2030s

Decades	Area (km²)	Declines (%)	Period		% Loss
1920s	368.29	-		(1) past 50-year
1950s	459.72	-24.83		1960s-2010s	53.29
1960s	415.44*	-		1970s-2020s	36.39
1970s	371.16*	-		(2) present and future 50-year
1980s	339.78	26.09		1980s-2030s	18.15
1990s	270.51	20.39		(3) historic change
2000s	239.31	11.53		1920s-2010s	47.31
2010s	194.05	18.91		1920s-2020s	35.90
2020s	236.08	-21.66
2030s	278.11**	-

* Values were extrapolated using the absolute rate of decline from the 1950s to the 2010s.

** Value was extrapolated using the absolute rate of decline from the 2010s to the 2020s.

During the last 100 years, the lowest value of tidal flats was 194.05 km² in 2010, and the maximum loss in the geographic distribution of western Taiwan’s tidal flats during the last 60 years (the 1950s–2010s) was 57.79%. According to the extrapolated data, 53.29% of the tidal flat coverage has been lost in the last 50-year period (the 1960s–2010s) and 36.39%, in the 1970s–2020s, indicating that the ecosystem has reached the VU-EN status based on the ecosystem distribution decline rate. Based on the extrapolated data in the 2030s, the tidal flats area will be 278.11 km², and the future decline status will be 18.15% (least concern [LC]) from the 1980s to 2030s, which includes a present and future 50-year period. The total decline of the tidal flat area since 1920 indicated that the losses were 35.90–47.31%, and the ecosystem had an LC status due to historic changes (Table 1, Figure 2). Taiwan’s tidal flat ecosystem is estimated to be endangered.

We discovered 48 reservoirs and river dams in 17 major western river systems, nine commercial and industrial harbors, six thermal power plants and industrial factories, and seven agriculture and aquaculture developments in the western coastal region, which were built in the 1900s. The lost tidal flat areas were converted into agriculture and aquaculture areas (53%; 9% agriculture, and 44% aquaculture), residential and commercial development (46%), and energy production and mining (2%) (Figure 4 in appendix).

The development of tidal flat ecosystems can be classified into four stages. In the first stage (1920s–1950s), the area of tidal flats increased because of the continual supply of sediments from the upstream rivers. From the 1950s to the 1970s, tidal flat abstraction and reclamation for aquaculture and agriculture caused huge shrinkage in the southwest part of Taiwan. The third stage (the 1980s–2010s) was the industrial development period, and the tidal flat area continuously decreased. Many harbors, thermal power plants, and industrial factories have been constructed on the coastline since the 1980s, and the last huge industrial factory area was built in 1998. In the fourth stage, in the most recent decade (2010s–2020s), industrial construction slowed down, and the tidal flat area slightly increased.

Half of the world's wetlands have been lost since 1900. Degradation and reclamation of coastal wetlands are global phenomena. The rate of natural coastal wetland loss has remained high since the 1980s and is many times faster than that of inland wetlands (Davidson 2014). Coastal areas have rapidly developed and are now occupied by large populations, resulting in the global loss of tidal flats. Since 1984, 16.02% of this area has been lost (Hood 2004; Kirwan and Megonigal 2013; Murray et al. 2019). Tidal flats are major wetlands with high productivity and biodiversity in both marine and terrestrial areas. Asia has 44% of the global tidal flats, but they have been lost due to urban and industrial land reclamation and weir construction. Between the 1950s and 2000s, 65% of the tidal flats were lost around the Yellow Sea coastline, the most important shorebird habitat in the EAAF (Moores et al. 2016; Murray et al. 2014; 2019). The degradation of tidal flat ecosystems in Taiwan is similar to that observed in Asia and other parts of the world. In Taiwan, there were 45% artificial coastlines, and in the western part, less than 10% of the natural coastlines in 2017. From the 1950s to the 2010s, 57.79% of the tidal flat area was lost.

During the first period (1920s–1950s), the area of the tidal flat ecosystem increased. However, the ecosystem area declined rapidly between the 1980s and 2010s. In the recent decade (2010s–2020s), the degradation pattern stopped, and the area began to increase again. Our research provides information on the speed and progress of mudflat accumulation before systematic human development. The results also showed that more than half of the tidal flats in Taiwan have disappeared in the last 60 years, a pattern similar to that observed in the Yellow Sea (65%) in North Asia, which is the most important migratory bird habitat in the EAAF (Murray et al. 2015). The changes in socioeconomic patterns in East Asia were similar to those in Taiwan, but Taiwan experienced these changes a decade earlier. We also found that the disappearance of the mudflat area slowed down with the reduction in human disturbance after the 2010s.

Previous research focused only on inland areas without discussing coastline development (Chen et al. 2019). Our results reconstructed the spatial and temporal changing patterns of the coastal area over the last 100 years, and we conducted local and regional coastline long-term assessments. Furthermore, in the process of our research, we enhanced the spatial resolution of regional mapping, avoiding situations of image misinterpretation such as misidentifying fishponds as intertidal mudflats, as seen in other global-scale research (Murray et al. 2022).

Satellite images have been available since the 1960s, and historical topographic maps must be used to obtain past land cover information. Despite the potential impact of cartographic generalization, maps are valuable documents that provide information about historical environments. Overlapping historical maps can provide evidence of environmental change. Using this information, we estimated the changes in the distribution and area of tidal flats over the last century. However, comparisons with historical mapping should be interpreted with caution (Murray et al. 2014). The original maps from 1921 to 1956 were topographic maps that employed precise survey techniques (Academia Sinica 2003; Chen et al. 2019).

Shin (1980) compared maps published in 1904, 1924, 1954, and 1975, and found that the area of tidal flats in western Taiwan has continually increased. According to maps and field studies, 538.7 km² of tidal flats existed during the 1970s. In our research, the quality and coverage of the Landsat images were insufficient to estimate the entire area of tidal flats in Taiwan in the 1960s and 1970s. However, Shin's (1980) research showed that the area of tidal flats in western Taiwan continually increased before 1975.

According to the IUCN Red List of Ecosystems criteria, the tidal flat ecosystem in Taiwan has reached EN status. This assessment is useful for evaluating the impact of human activities on ecosystems and the major direct threats to the tidal flat ecosystem in Taiwan: (1) residential and commercial development, (2) agriculture and aquaculture, (3) natural system modifications, and the potential threat of (4) energy production and mining.

The systematic reclamation of tidal flats began in 1947, and the government published a development plan that included the construction and allocation of tidal flats; 10% of the area of tidal flats was transferred for agricultural, fishery, and salt production purposes over 30 years (Shin 1980). Since the 1960s, broad coastal plains have been reclaimed as fishponds and saltpans (Ho 2016), and the landscape has already changed due to agriculture and aquaculture. Heavy, petrochemical, and shipbuilding industries and large public construction projects, such as transportation and electric power, have developed in coastline areas since the 1970s. Nine commercial and industrial harbors, six thermal power plants and industrial factories, and seven major agricultural and aquaculture developments have been built in the western coastal region since the 1900s.

Tidal flats are typically located near estuarial delta areas supplemented with sediments from rivers. The erosion of coastlines and disappearance of tidal flats have been attributed to a decrease in sand supply (Chang and Chen 2001). In Taiwan, mountainous buildings have transported sufficient sediment to the western coastal area, resulting in broad tidal flats (Hsu et al. 2007). While the area of tidal flats increased during the first three-quarters of the 20th century, it has continually decreased since the 1970s. This reduction in the sediment supply may be due to sediment deposition, dams, or upstream reservoirs. Tidal flats in Taiwan depend heavily on ongoing sediment supply. However, at least 48 reservoirs and river dams in 17 major western river systems have caused a substantial decline in sediment output from rivers. The number of existing tidal flats has slightly increased in recent years. After the Jiji Weir was built in 2002, the construction broke down the sediment transport from upstream, thus threatening the last and only surviving tidal flat, located on the north side of the Choshui River estuary. As the Choshui River is the largest river in Taiwan, the Jiji Weir also threatens the southwestern part of the tidal flats and leads to coastal erosion (Huang 2010; Hsu et al. 2007).

Human activities such as the construction of harbors or fishing ports and land reclamation for agricultural, aquacultural, and industrial areas have directly reduced the area of tidal flats. Indirect impacts include sand mining from riverbeds for construction materials and upstream construction of reservoirs or dams (Hsu et al. 2007) that decrease sediment fluxes downstream. Sediment transport by upstream rivers is the most important source of tidal flats (Walling and Fang 2003; Walling 2006).

The extent of tidal flat loss is alarming and irreparable. The loss of these flats has resulted in the disappearance of important stopover sites for migratory shorebirds over the last 100 years. Our findings are consistent with those of other sources of information regarding the loss of coastal wetlands in East Asia. Half of the lost tidal flat areas have been converted into aquaculture farms, these parts of aquaculture farms and abandoned salt ponds have become alternative habitats for migratory shorebirds in the coastal region. There are 20 (37%) Important Bird Areas (IBA) in the tidal flat region that protect migratory bird habitats (Taiwan Wild Bird Federation 2001).

The last remaining patch of complete tidal flats in Taiwan is located in the central region (Fig. 1A). This area covers the tidal flats on the Chang-Hua coastline, which is also part of the IBA (Dadu River Estuary Wetland). Based on our findings, this area is the largest and most complete tidal flat in Taiwan. As Taiwan is a crucial stopover site for migrating birds in the EAAF, the tidal flats in Taiwan serve as essential refueling sites for these migrations. A huge industrial project called the Kuokuang Petrochemical Project was conducted in this area in the 2000s, which stopped in 2011. This project included refineries and petrochemical midstream/downstream factories, and the expected development area was approximately 1,900 ha. However, there is another threat to the horizon in the form of solar panels, a new type of energy-producing building, and coverings on tidal flats that are also the cause of wetlands being endangered in recent years.

The number of migratory shorebirds that use the EAAF has declined dramatically over the last few decades. Evidence suggests that this decline is due to habitat loss at important stopover sites in the Yellow Sea (Murray et al. 2014). The rapid decline in the tidal flat area may have led to habitat and niche loss. The degradation of Taiwan’s tidal flat ecosystem is not only a local threat but also a threat to migratory shorebirds in the EAAF (Lin et al. 2023).

Acknowledgments

We would like to express our gratitude to Mao-Ning Tuanmu for his invaluable assistance in interpreting the remote sensing data, Hsiung-Ming Liao for his support in the data source of historical maps, and Ya-Jung Lu for her contributions in the initial stages of this research. We also extend our thanks to the Central Weather Bureau for providing us with the historical tidal observation data. We are grateful to David A. Keith and Dr. Nicholas J. Murray for their insightful suggestions on the early stage of this research. This project was financially supported by the Council of Agriculture, Executive Yuan, R.O.C. (Taiwan), under grant numbers 104AS-13.9.6-EI-W3, 105AS-12.9.6-EI-W3, 106AS-11.9.6-EI-W3, and 107AS-10.9.6-EI-W3.

Funding

This project was financially supported by the Council of Agriculture, Executive Yuan, R.O.C. (Taiwan), under grant numbers 104AS-13.9.6-EI-W3, 105AS-12.9.6-EI-W3, 106AS-11.9.6-EI-W3, and 107AS-10.9.6-EI-W3.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

Wan-Jyun Chen, An-Yu Chang and Ruey-Shing Lin contributed to the study conception and design. Material preparation, data collection and analysis were performed by Wan-Jyun Chen, An-Yu Chang and Chia-Chi Lin. The first draft of the manuscript was written by Wan-Jyun Chen. Ruey-Shing Lin and Pei-Fen Lee supervised this research and commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Data Availability

Data will be made available from the corresponding and first authors on request.

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Losing tidal wetlands in the midpoint of East Asian-Australasian Flyway in the pasting 100 years

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Abstract

Figures

Introduction

Methods

Historic topographic map data

Satellite data

Estimating the area of tidal flats

Estimating trends in distributions and assessing the risk to the ecosystem

Results

Discussion

Statements & Declarations

References

Supplementary Files

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