Assessing the development of newly created habitat for wintering estuarine birds

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Abstract

The term coastal squeeze has been used to describe the process whereby hard flood defences in areas subjected to relative sea level rise constrain the ability of an estuarine system to respond naturally, thus leading to a loss of intertidal habitat, with an associated potential reduction in the carrying capacity of the estuarine system for fish and bird populations. The process of coastal squeeze, coupled with the long-term effects of climate change, has generated the need to create or restore intertidal habitats in estuaries. In Humber Estuary (UK) it is estimated that over 700 hectares of intertidal habitat will be lost due to coastal squeeze and rising sea levels over the next 50 years. In order to address this net loss, a process known as ‘managed realignment’ has been employed as an estuarine habitat management tool, whereby sections of flood defences are moved inland to create or restore intertidal habitat and offset potential losses in carrying capacity. On the Humber, the Paull Holme Strays site, to the east of Hull, has been the first managed realignment site on the estuary to become operational. The site at Paull Holme Strays was breached in September 2003 creating approximately 80 hectares of intertidal habitat. In order to assess the development of the waterbird assemblage at the Paull Holme Strays site, and ultimately the success of the habitat creation technique, monthly ornithological surveys were undertaken in the newly created habitat and on the mudflat fronting the managed realignment site. The analysis of avifaunal usage data at Paull Holme Strays has demonstrated that within 3 years of creation, the new intertidal habitat is capable of supporting at low water, a functional waterbird assemblage of similar composition to that of adjacent existing intertidal areas. At Paull Holme Strays, this has been achieved with little ongoing management of new habitats.

Introduction

Loss of intertidal habitat potentially poses a threat to the integrity of shorebird populations, in particular waders and wildfowl (collectively known as waterbirds) which are dependent upon the mosaic of habitats within estuaries. Whilst many wildfowl and waders forage on estuarine mudflats, sandflats and saltmarshes, others feed on the surrounding marshes and farmland, returning to roost on the estuarine flats (Davidson et al., 1991). British estuaries are of major national and international importance for waterbird assemblages and for many individual species (Collier et al., 2005). They are important not just as wintering sites but also as moulting sites and as migration staging areas (Prater, 1981, Davidson et al., 1991).

Land claim is one of the major causes of habitat loss in estuaries. It has been occurring for many centuries (Davidson et al., 1995) and for a variety of purposes (Davidson et al., 1991, Moser et al., 1996, Boyes and Elliott, 2006). In addition to this direct anthropogenic loss, there is also the potential for climate change, sea level rise and isostatic readjustment to have an effect in the longer term. The collective effect of these factors is often referred to as “relative sea level rise”. In areas where such a phenomenon is experienced, the restriction of the landward movement of the estuarine system by flood protection embankments may entail the progressive loss of the biologically rich intertidal areas, this process being known as “coastal squeeze” (Boorman et al., 1989, Edwards and Winn, 2006). The effects of climate change on sea level have given rise to a range of predictions for habitat loss or change. In combination with the effects of isostatic rebound, which is still being experienced in eastern and south England, relative sea level rise has been predicted to be at a rate of 1.13 cm per year in the Humber (Burd, 1994), with associated habitat loss due to coastal squeeze expected in some parts of the estuary. Whilst sea level rise in the Humber system may not necessarily lead to the direct loss of intertidal habitat in some instances, but rather the modification to its structure and function, it has been estimated that 710 hectares of new habitat will need to be created to compensate for the effects of coastal squeeze over the next 50 years on the Humber Estuary (Environment Agency, 2000a, Environment Agency, 2000b).

The Humber's intertidal area has also been greatly reduced by direct anthropogenic activity since the 17th century when large-scale land claim commenced. Land claim is still occurring but at a much reduced rate and largely limited to port development, with compensatory habitat now being required for such projects.

In order to conserve the integrity of shorebird populations, many countries now have a policy of compensating loss of intertidal areas by creating new habitat. However, the success of the newly created habitat at providing suitable foraging and roosting habitat for waterbirds can be highly variable, due, at least in part, to the ecological and topographic complexity of the intertidal habitat to be replaced (Atkinson, 2003). In north-west Europe the experience of creating new intertidal habitat, especially mudflats, is fairly limited and few studies have monitored the success of such schemes on waterbird assemblages (Evans et al., 1998, Evans et al., 2001, Atkinson et al., 2001, Atkinson et al., 2004). As a result, there is a need both to determine the rate at which a waterbird assemblage develops over time in a newly created habitat, and to compare the waterbird assemblage of the newly created habitat to that of adjacent, natural, mudflats. The capacity to create or restore high quality intertidal habitats for waterbirds has important implications for future habitat management in low-lying estuarine systems subjected to coastal squeeze, and to individual mitigation measures applied to new developments in the coastal margin.

The present paper provides an overview of such a study, on the Humber Estuary, assessing the avifaunal development of the newly created mudflat at Paull Holme Strays.

Section snippets

A newly created intertidal habitat on the Humber Estuary

The Humber Estuary is ranked as one of the top five sites in the UK for waterbirds (Collier et al., 2005). Its strategic role for waterbirds stems from its position along the East Atlantic Flyway, a broad zone stretching between the breeding grounds of waders in the Arctic and sub-Arctic and their over-wintering sites in southern Europe and Africa with wader and waterbird flocks utilising the extensive and highly productive intertidal mudflats of the estuary as a source of food (Prater, 1981,

Paull Holme Strays realignment site

Paull Holme Strays is located within the middle section of the Humber about 10 km east of the city of Kingston-upon-Hull, the largest population centre adjacent to the estuary (Fig. 1). King George Dock, 6 km upstream from the Paull Holme Strays site, has a mean high water spring tidal height of 3.7 m and neap tidal height of 2.1 m (heights given above UK Ordnance Datum (OD)) (UK Hydrographic Office, 2007).

Historically, the area within the realignment site was tidal marsh and mud, but during the

Changes in waterbird abundance

Thirty-four species of waterbird were recorded within the realignment site during the monitoring period. Waders were the dominant group with 19 species, followed by wildfowl (11 species) and with four other species present. Golden Plover, Lapwing and Black-tailed Godwit were the most abundant waders recorded on the realignment site with monthly peak maxima of 14,000, 4500 and 1300, respectively, recorded over the three-year monitoring period. Shelduck, Teal (Anas crecca), Mallard (Anas

Discussion

The present study has shown that initial colonisation of the managed realignment site by wildfowl was rapid, with Teal and Mallard recorded foraging during the first winter (2003/2004) at low water, three months after the sea defences were breached in September 2003. The rate of establishment of the wildfowl assemblage can be related to their dietary requirements, as both Teal and Mallard are largely omnivorous in their diet (Cramp, 1998). Teal are predominantly seed eaters in the autumn/winter

Conclusions

Monitoring and analysis of avifaunal usage data from the first three years of operation of the Paull Holme Strays managed realignment site have demonstrated that the habitat created from the technique can rapidly support a functional avifaunal assemblage similar in composition to that of adjacent existing intertidal areas. At Paull Holme Strays, this has been achieved with little ongoing management of the new habitats. However, a substantial body of research was undertaken at the project

Acknowledgements

Much of the thinking behind the paper was carried out for a project funded by Environment Agency. We thank the Environment Agency for permission to publish the study. We also wish to thank Ray Eades for providing valuable field help, and Professor Mike Elliott for constructive criticism of an early version of the manuscript. This manuscript has also benefited from helpful comments from Dr Phil Atkinson (British Trust for Ornithology) and Cecilia Soldatini (University of Venice).

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