Manipulating water habitats to optimise wader and wildfowl populations.
The artificial creation of wetlands for waders and wildfowl has been a key feature of RSPB management over the past few decades. Initial successes, measured in terms of increases in species diversity and abundance, particularly in the case of rare species, has led to the increased use of this management technique. However, little is known of the sustainability of such systems, or of the underlying principles which drive them. In particular, it is not known whether such systems are optimal nor whether they can sustain high levels of invertebrates indefinitely.
The aim of this type of conservation management is to provide a resource which is limiting at a particular time of year, such as food supplies or nest sites in the spring. The response of individuals to this resource will determine the success of the management technique. Density dependent regulating factors can allow a species to respond favourably to an increase in abundance of such a resource, whereas density independent factors are unlikely to be significantly influenced by management. Consequently, an understanding of the dynamics of populations is important since a management technique aimed at increasing numbers at one stage in the life cycle of the species concerned may, in the long term, be detrimental if this stage is followed by one which acts in a strongly density dependent way. For example, we might increase wader breeding populations by creating favourable nesting habitat, but higher predation on clutches at high density would counteract some of the gains obtained by the management technique.
Conservation management usually attempts to push a species to a new, higher stable equilibrium density, although little has been documented on stability properties of simple wetlands as examined in this chapter. These systems often support large populations of opportunistic invertebrates which are eaten by wildfowl and waders. This chapter therefore aims to document changes in species abundance and diversity following hydrological management on five RSPB reserves: Havergate Island and Minsmere in Suffolk, Titchwell in Norfolk, Blacktoft Sands in Humberside and Elmley in Kent, as a first attempt towards understanding how such systems operate. The management techniques most commonly used to create feeding and nesting habitats for waders and wildfowl can be conveniently categorised as (I) lagoon creation by topsoil removal, (2) flooding of low-lying pasture and (3) reduction of salinity. In order to identify differences due to the three management categories the analyses presented are divided accordingly. General principles regarding responses of birds to these management categories are discussed.