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The effect of human disturbance on animal distribution has received considerable attention in recent years (Owens 1977; Stalmaster & Newman 1978; Bélanger & Bédard 1989; Keller 1991; Stockwell, Bateman & Berger 1991; Pfister, Harrington & Lavine 1992; Reijnen et al. 1995; Madsen 1998). Assessing the severity of the effects of disturbance has important practical consequences; if it has serious impacts, conservationists are justified in recommending that access to wildlife areas be limited (Burger 1981; Tuite, Hanson & Owen 1984; Klein, Humphrey & Percival 1995). However, if the impacts of disturbance are trivial, then such measures cannot be justified. Restricting human access to the countryside can be expensive and time-consuming but, more importantly, it goes against the increasing view that rural access should be increased. Moreover, access to areas of conservation value can be the best way to protect them, as it increases the value placed on them by society (Adams 1997). There is therefore a need to quantify the extent to which disturbance adversely affects animal populations, in the context of a wider debate of how much human access to wildlife areas should be sanctioned or discouraged.
There are two components to the problem of disturbance: whether human presence causes animals to avoid areas that they would otherwise use, and whether this in turn affects mortality, reproductive success or population size (Gill & Sutherland 2000). The majority of studies of disturbance refer to the first component and use one of two approaches. The first compares animal distribution between sites with differing levels of disturbance (Tuite, Hanson & Owen 1984; Pfister, Harrington & Lavine 1992; Sutherland & Crockford 1993; Milsom et al. 2000; Suárez, Balbontín & Ferrer 2000). Interpreting such studies can be difficult because variations in disturbance are often confounded by factors such as prey density, competitor or predator density, or the locations of territories, nests or roost sites. Nevertheless, such information is necessary in order to demonstrate whether disturbed sites support fewer animals than the resources would potentially allow, and to quantify the extent to which the use of such sites could be increased if disturbance was absent. The second approach involves recording short-term behavioural responses to disturbance (Draulans & van Vessem 1985; Bélanger & Bédard 1989). It is, however, impossible to relate such short-term responses to the pattern of use of sites over a whole season. This is because animals may be displaced from disturbed sites in the short term but may return at a later date; over the course of a season the overall use of these sites may then be unaffected by disturbance. Studies of disturbance need, therefore, to identify the major factors related to the distribution and behaviour of the species in question and then to examine the role of disturbance in altering these relationships (Gill, Sutherland & Watkinson 1996).
Much of the concern about the effects of disturbance relates to coastal areas, because they sustain high levels of human recreational use (Davidson et al. 1991) and because they are important for wildlife (Smit & Piersma 1989; Piersma & Baker 2000). Such studies have often focused on shorebirds (Charadrii) as they frequently occur on areas subject to high human pressure and because their tendency to take flight in response to human presence suggests that they may be particularly susceptible (Burger 1981; Kirby, Clee & Seager 1993; Smit & Visser 1993). Sites with high levels of human activity often have lower densities of birds than sites with low levels (Burger 1981; Klein, Humphrey & Percival 1995). However, none of these studies has addressed whether disturbed sites could have supported more birds in the absence of human presence. While these studies may suggest an effect of disturbance on habitat use, it is clearly important to establish whether avoidance of human presence results in reduced use of the habitat over the course of a season or whether it is simply a short-term change in spatial distribution that will be reversed at a later date (Gill, Sutherland & Watkinson 1996).
During winter, estuaries at northern latitudes are an extremely important source of food for many thousands of shorebirds. Studies of many species have demonstrated the overwhelming importance of the invertebrate prey population of estuaries in determining the spatial and temporal distribution of the birds that consume them (Zwarts & Blomert 1992; Goss-Custard et al. 1995; Piersma et al. 1995). Thus, if human presence alters the distribution of these birds, the most important consequence is likely to be an alteration in use of the food supplies. Several studies of shorebirds have demonstrated the importance of quantifying the fraction of the prey populations that are both accessible to the birds and profitable to consume. For example, studies of predator–prey relationships frequently show that prey which can bury deeper in sediments are less accessible to predatory shorebirds (Myers, Williams & Pitelka 1980; Zwarts & Wanink 1984; Wanink & Zwarts 1985; Piersma et al. 1993; Zwarts & Wanink 1993). In addition, the size of prey can affect its availability to predators; prey may be too large to be consumed or too small to be profitable (Zwarts & Blomert 1992; Piersma et al. 1993; Zwarts & Wanink 1993). Assessing the impact of disturbance on the use of prey populations by shorebirds will therefore require measures of prey selection.
We aimed to assess the extent to which different types of human activity might reduce the number of shorebirds that can be supported by invertebrate food supplies at a range of spatial scales. We focused on the black-tailed godwit Limosa limosa islandica L., which winters on the estuaries of north-west Europe. Our study area included estuaries with some of the highest levels of recreational use in Britain (Davidson et al. 1991). Black-tailed godwits are a species considered to be at risk from disturbance in the wintering grounds (Batten et al. 1990).
The effect of disturbance on site use can be examined in two ways. If the species in question is food-limited and responsible for virtually all of the depletion of the available prey, levels of human activity can simply be related to the abundance of prey at the end of the season (Fig. 1). This method assumes that initial resource abundance is not related positively to levels of human activity. This method has been used successfully to identify the extent to which the use of sugar beet fields by pink-footed geese Anser brachyrhynchus was constrained by human disturbance (Gill 1996). The resource need not be food supply, but must be the major cause of the occupation of the area by the species in question, for example breeding territories, nesting sites or roosting locations. The territory use and consequent productivity of ringed plovers Charadrius hiaticula has also been shown to be markedly affected by human disturbance, resulting in significant reductions in local population size (Liley 1999).
Figure 1. The potential influence of human disturbance on prey abundance in spring. If human presence influences animal distribution such that disturbed sites are used less, then levels of prey depletion will be reduced in disturbed sites, resulting in higher spring prey densities than in undisturbed sites.
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Alternatively, if the predator in question is not responsible for virtually all prey depletion, the effect of human activity can be included along with all other relevant variables in an analysis of the factors determining distribution.
In this study we adopted both techniques, although we have shown previously (Gill, Sutherland & Norris 2001b) that black-tailed godwits are the major cause of over-winter depletion of available prey in our study areas. Both techniques require accurate assessments of prey availability.
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Shorebirds often show strong avoidance of humans (Burger 1981; Kirby, Clee & Seager 1993; Smit & Visser 1993) and some studies have shown their numbers to be lower in disturbed than undisturbed sites (Klein, Humphrey & Percival 1995). However, we found no evidence that human presence reduced the number of black-tailed godwits that were supported on coastal sites at a range of spatial scales. The study took place on estuaries that varied widely in both the level and type of human activity, but neither had any influence on godwit distribution or abundance. There was also no effect of the presence of marinas or footpaths on the number of godwits supported on the adjacent mudflats.
The prey types involved in this study, estuarine bivalves, are relatively sedentary once they have settled in the sediment. It may seem intuitively obvious that using depletion as a measure of the effect of disturbance will not work if the prey are mobile. However, avoidance of disturbed sites only matters if it reduces the amount of prey that is available to be consumed. Consider the example of a predator that avoids disturbed sites and only feeds in undisturbed areas. In the absence of prey movement, only the prey in the undisturbed sites will be available to the predator and the increased depletion in these sites may impact on the fitness of the predator. However, in the case of prey that can move between sites, all prey will be available to the predator during the periods when they occur in the undisturbed sites. Levels of prey depletion will therefore be equal across all sites and the intake of the predator will be unaffected by disturbance, even though disturbance has restricted predator distribution. In such cases, the traditional method of comparing predator densities in disturbed and undisturbed sites will show an effect of disturbance on distribution but only measuring depletion will identify whether or not there are associated costs.
The advantage of the approaches used in this study over examining either behavioural responses to disturbance or relating numbers of animals across sites to disturbance levels, is that they identify whether sites could support more animals in the absence of disturbance. If disturbance constrains numbers of animals using sites, it also allows calculation of the increased numbers of animals that could use the site in the absence of disturbance (Gill 1996).
In the case of wintering black-tailed godwits, current levels of human activity did not influence distribution or habitat use in our study. The level of sampling of both predators and prey was sufficient to produce clear relationships between godwit and bivalve abundance; if human presence was important in determining current godwit distribution, it is likely that these relationships would be much less clear. However, this does not mean that there are not other circumstances under which human presence could be a significant conservation problem, either for this or similar species, or on these or similar sites. For example, during periods of severe weather when wading birds are often under extreme stress (Dugan et al. 1981; Davidson & Evans 1982), any additional effects of human disturbance may be extremely important. Shorebirds are also present on coastal areas during the late summer when food may not be limiting but they are experiencing the stress of moult. At these times, high levels of recreational activity may be important.
A major factor likely to influence whether or not species respond to humans by avoiding specific areas is the risk of mortality associated with human presence. Thus, species that are hunted by humans might be expected to avoid humans more than species that are not hunted (Gill & Sutherland 2000; Gill, Norris & Sutherland 2001a). Although these estuaries are amongst the most heavily used for human recreation in England (Davidson et al. 1991), black-tailed godwits are not hunted in Britain, nor are they hunted on their Icelandic breeding grounds. This may therefore be one reason for the lack of a detectable response to human presence by this species.
Implications for conservation
It may appear that a study showing that human presence has no impact on the species in question is relatively unimportant in conservation terms. However, a common problem in conservation science and policy is the failure to distinguish critically important conservation issues from trivial ones (Caughley 1994; Sutherland 2000). The consequences of this may be a dissipation of effort and a failure to use resources in the most cost-effective manner. Thus, of the 113 red data book bird species in the UK, 76 have disturbance included under ‘Threats to survival’ alongside factors such as habitat loss, poisoning and persecution (Batten et al. 1990), although the impact of disturbance has not been examined in any detail for any of these species. The protection of sites and conservation of the species inhabiting them is likely to be easier to achieve if other interested parties have access to the sites without diminishing their conservation importance (Adams 1997). It is thus clearly important to be able to distinguish cases where human presence results in significant changes in habitat use (as in the pink-footed goose and ringed plover examples above) from cases where it does not.
Quantifying the effect of human presence on habitat use is, however, only the first step, as altering the distribution and habitat use of individuals need not have any consequences for the population as a whole (Gill, Norris & Sutherland 2001a). Understanding the population consequences will require information on levels of density-dependent mortality and fecundity in a population and how these are affected by changes in distribution in response to human presence (Sutherland 1998; Stillman et al. 2000). Thus, while the role of human presence in constraining numbers of animals on particular sites can be assessed using the methods described here, quantifying the population consequences of these constraints will be far more complex.