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The loss and degradation of many important bird habitats in lowland Britain has resulted in population declines and range contractions in a number of species (Gibbons et al. 1993; Fuller et al. 1995; Chamberlain et al. 2000). For example, four breeding wader species have undergone severe declines on lowland wet grassland habitats during the last few decades (lapwing, snipe Gallinago gallinago, curlew Numenius arquata and redshank Tringa totanus, Wilson et al. 2005). In lowland wet grassland habitats, these species are now primarily restricted to nature reserves and sites managed for birds within agri-environment schemes (Ausden & Hirons 2002). Although the timing of these declines coincides with a period of increase in a number of their avian predators, such as carrion crows Corax corone and magpies Pica pica (Gregory & Marchant 1996), declines in wader populations are considered to be driven primarily by changes in agricultural grassland management in the UK, particularly improved drainage and increased use of fertilizers (Baines 1988, 1989; Chamberlain et al. 2000; Wilson et al. 2001). These diminished populations might be more vulnerable to predation than previously, and predation may be restricting breeding densities possibly leading to further declines and local extinction. For example, some studies have shown that the nests of lapwing breeding as isolated pairs or at low densities are more likely to suffer predation compared with those breeding at high densities or in large colonies (Elliot 1985; Berg et al. 1992; Seymour et al. 2003). However, this is not a universal finding (see Galbraith 1988) and is likely to depend on the predator species involved. Conservation management of wet grassland outside nature reserves, through agri-environment schemes such as the Environmentally Sensitive Areas and Countryside Stewardship, has so far failed to reverse population declines in the UK (Ausden & Hirons 2002; Wilson et al. 2005). In this context, predator control may be necessary as a short-term measure, to enable populations of prey species to increase.
To date, most experimental studies of the role of predators in determining bird densities and productivity have been restricted to gamebirds and waterfowl (Newton 1993; Côté & Sutherland 1997). In these cases, the objective of predator control is usually to produce a ‘harvestable surplus’ of birds in the autumn and winter for sport shooting. Whilst predator control for game management can be effective in increasing the size of the postbreeding population of target species such as grey partridge Perdix perdix and may also lead to an increase in the size of the breeding population (Tapper et al. 1996; Reynolds & Tapper 1996), the role of predators in avian population limitation generally, is less clear (Newton 1993, 1998; Côté & Sutherland 1997; Evans 2004). Comparatively few studies have examined the effect of predation on the population dynamics of wader species (Hill 1988; Parr 1993; Byrd et al. 1994), although there is evidence to suggest that in some areas nest predation rates of species such as curlew are sufficient to account for the observed population decline and would certainly prevent recovery (Grant et al. 1999). A review by Newton (1993) found that predator removal resulted in improved nest survival in 23 of 27 studies, increased postbreeding population size in 12 of 17 studies, and increased breeding numbers in 10 of 17 studies. A meta-analysis of 20 predator control studies published up to 1995 (Coté & Sutherland 1997), including many included in the earlier review by Newton (1993), concluded that whilst predator control generally resulted in significant increases in nest survival and postbreeding population size of the prey species, the increase in the size of the subsequent breeding population did not attain statistical significance. Whilst predator control may consistently increase annual breeding performance, the effect on populations is less uniform.
The aim of this study was to assess the impact of fox and crow control on the productivity and population size of breeding lapwing Vanellus vanellus. Of the common breeding waders of wet grassland, the lapwing was selected for this work because its breeding ecology has been well studied and lapwing nests are placed in sparse vegetation which makes them both vulnerable to predators and relatively simple to locate and monitor for research purposes. In common with other wet grassland wader species, the lapwing has suffered a considerable decline in recent decades in response to changes in agricultural practice (Wilson et al. 2001). Between 1987 and 1998, the declines have been most severe in Wales (77%) and south-west England (64%, Wilson et al. 2001), and the species is currently on the Amber list of Birds of Conservation Concern (Gregory et al. 2002). In many areas in the west and south-west of Britain, lapwing breeding populations are highly fragmented and vulnerable to local extinction (Gates & Donald 2000). In England and Wales, the highest breeding concentrations generally occur within nature reserves that are managed specifically for breeding waders (Ausden & Hirons 2002), and this study was carried out in the context of nature conservation management of lowland wet grassland reserves in the UK. However, even in such situations, losses of clutches and young to predators may be considerable, and populations may be prevented from increasing due to high losses to predators. In consequence, control of predator species has been proposed as a reserve management tool to enable populations of waders such as lapwing to increase and/or provide a source of fledged young to recruit into surrounding areas.
Predator control focused on foxes and carrion crows since these species are known to be important predators of wader nests and young (Green et al. 1987; Cotgreave 1995). Neither stoat Mustela erminea nor weasel Mustela nivalis were included in the experiment due to the difficulties involved in trapping these species on grassland and the risk of killing water voles Arvicola terrestris in traps located along ditches. This study does not therefore address the general issue of lapwing population limitation by all its predators, but examines the practical conservation benefit, at a site-scale, of levels of predator control that would be achievable on nature reserves in the UK, using legal and humane methods.
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The predator control measures employed in this study resulted in a mean 40% reduction in adult fox densities and a mean 56% reduction on territorial crow densities during the course of the lapwing breeding season, across all sites. Reductions in fox numbers were apparent from the beginning of March onwards, whereas for territorial crows numbers were reduced from the beginning of April. The removal of territorial crows resulted in a compensatory influx of non-breeding birds, such that the total densities of crows remained similar between years with and without crow removal. There was evidence for a cumulative effect of predator removal on fox densities over the course of 4 years, indicating that the measures employed in one year had an impact on fox densities the following year. This was an unexpected finding, since we anticipated that the predator control measures would simply reduce fox densities during the breeding season in a local and temporary fashion. We also found that the densities of foxes were extremely variable among sites in years without predator control. On some sites in Wales, low densities of foxes during the lapwing breeding season resulted in the lack of any fox removal in years when predator control measures were operative. As a result, when adopting a simple treatment-based approach to analysis of these data, we found no consistent evidence of reduced lapwing nest or chick mortality during years of predator control. However, when we examined the effect of predator control in relation to background predator densities for each site (i.e. densities in years without predator removal), we found a significant interaction, and controlling for this effect resulted in a significant overall effect of predator control on nest failure rates. Among the six sites where detailed monitoring of chick survival was not carried out (where predator densities were generally higher), there was a significant first-order effect of predator control treatment on the proportion of lapwing with young. The variation in background fox and crow densities on the study sites is likely to be dependent on the surrounding habitat (Webbon et al. 2004) and the level of predator control practised by neighbouring land managers.
There was not strong evidence of an overall effect of fox and crow control on population trends across all sites. The magnitude of the mean annual population changes indicated that immigration and emigration occurred among sites and consequently population trends were unrelated to nest survival rates averaged across the 4-year treatment periods. During the settlement period when territory and nest-site selection occurs, lapwing are highly responsive to sward and surface water conditions (Milsom et al. 2000, 2002) and the numbers breeding on a particular site in any year will be influenced by the relative suitability of nesting habitat in neighbouring areas.
Data from nest temperature loggers suggested that non-avian predators were responsible for the great majority of nest predations. This is an intriguing finding since recorded fox densities were generally much lower for the Welsh sites than for the Ouse Washes (Fig. 2) and, taken together with the observation that fox control did not improve nest survival for any of the Welsh sites (indeed, predator control resulted in an increase in nest predation at Aberleri), suggests that mammalian predators other than foxes were principally responsible for nest failures at the Welsh sites.
Our findings support the conclusions of the meta-analysis of Côté & Sutherland (1997) and review of Newton (1993), which found fairly uniform effects of predator removal on components of avian annual productivity, but less consistent effects on breeding population size. Of the 20 studies considered by Côté & Sutherland (1997), 16 were conducted on gamebird or waterfowl species, which are characterized by very large clutch sizes. This reproductive strategy is likely to have evolved in response to intense predator pressure and the breeding success of such species is likely to respond readily to reductions in numbers of their predators. The effects of predator removal on breeding success of waders may therefore be less pronounced than for gamebirds and waterfowl. The single wader species included in the meta-analysis (golden plover Pluvialis apricaria) showed no increase in breeding success in response to predator control (Parr 1993).
The fox densities resulting from predator removal during control years were very similar to those reported, using similar survey methods, from a predator control experiment conducted on Salisbury Plain, UK, between 1985 and 1990 for the benefit of grey partridge Perdix perdix (Tapper et al. 1996; site-means of 0·38 and 0·30 sightings per hour, respectively). This suggests that efficacy of fox control was comparable in the two experiments (assuming similar detectability of foxes in the two studies). Differences in survey methods and analysis preclude direct comparisons of predator control on crow densities. Whilst the current study found significant effects of predator control on components of annual productivity, but not on population growth, Tapper et al. (1996) were able to demonstrate population effects of predator control on grey partridge. There are a number of possible reasons for these differences, including the range of potential predator species targeting by control measures, and the degree of philopatry and site-fidelity of the two prey species.
For a locally mobile species such as lapwing, the population-level benefits of improved productivity and recruitment may not be readily identifiable at the specific sites where predator control measures are implemented due to interannual shifts in breeding locality. An important step towards quantifying the consequences at a meta-population level of improvements in nest and chick survival is the development of a demographic model. This will permit an evaluation of the meta-population response to improvement in certain life-history parameters, such as nest and chick survival, that may result from local management measures such as predator control.
The analytical approach developed here should serve as a model for evaluation of predator control issues in other areas and systems. First, there is a need to parameterize the relationships between densities of the main predators and key life-history stages. For lapwing, the principal knowledge gap relates to the impact of different predator species at both the egg- and chick-stage. Remote monitoring of nests using digital recording equipment is currently underway to address this issue.
decision rules for assessment of predator control on lowland wet grassland reserves
Since predator control is a time-consuming, costly and controversial activity, especially in a nature conservation context, it cannot be viewed simply as a cost-neutral insurance measure that may yield a benefit. Rather, any potential benefit of predator control needs to be demonstrated and weighed against the benefits of other potential management actions competing for conservation resources. Unless the impact of predators and the need for their control can be demonstrated, resources may be better spent elsewhere. In the light of this, we suggest the use of a decision tree (Fig. 9) to assess the value of fox and/or crow control on a site-by-site basis, rather than a blanket approach. Predator control should only be considered for sites supporting important wader populations, with good habitat conditions and high nest loss to predators. For the nature reserves included in this study, the outcomes of the implementation of this decision tree will be monitored for 5 years from 2005 and the procedure will then be reviewed.
Figure 9. Decision tree for evaluation of fox and crow control for benefit of breeding lapwing on lowland wet grassland reserves. Parameter thresholds (nest survival, fox and crow densities) derived empirically. Nest survival is taken as surrogate for annual productivity in the absence of a viable method for economically assessing annual productivity for a large number of sites simultaneously.
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