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Human disturbance and its potential impacts upon bird populations are currently topical and contentious issues for conservationists. Although many studies have revealed a behavioural impact, or even direct effect on breeding success or survival, these cannot usually be extended to predict the impact on population size. Here we present a population model that allows predictions of the effect that changes in human numbers, visiting a 9-km-long section of the coastline, may have upon the size of a Ringed Plover Charadrius hiaticula population. Human disturbance affects Ringed Plovers in our study area through birds avoiding areas of high disturbance and, in addition, through the accidental trampling of a small number of nests by people walking on the beach. Using the level of human disturbance and habitat variables (which define territory quality) it is possible to predict which areas of beach are occupied and therefore the sites available to the population. Breeding success, for a given area of beach, can be predicted from habitat data. Incorporating known, density-independent, adult mortality allows the equilibrium population size to be predicted. This provides a model that predicts population size. This model is then used to predict the population that the site would support with different, hypothetical, levels of disturbance. If nest loss from human activity was prevented, for example by fencing nests, we predict the Plover population size would increase by 8%. A complete absence of human disturbance would cause a population increase of 85%. If the numbers of people were to double, we predict the population would decrease by 23%.
There is a large volume of published information on the effects of disturbance to birds (see reviews by Hockin et al. 1992, Carney & Sydeman 1999, Nisbet 2000, Woodfield & Langston 2004). Various studies have shown a wide range of effects such as taking flight, an increase in heart rate or accidental trampling of nests. Despite this volume of literature, there is little information as to when disturbance is a serious issue and what levels of disturbance might cause a problem.
Such information is particularly important where the species is of conservation concern, yet to our knowledge, apart from Mallord et al. (2007), no published study of a breeding bird species quantifies the population consequences of disturbance. This is despite the fact that disturbance has been implied as a factor causing population decline for a wide range of species (BirdLife International 2000).
Population regulation occurs through density-dependent, negative feedback mechanisms and the strength of the density-dependence will determine the extent of the regulation. Territorial behaviour can provide an obvious mechanism for density-dependence (see Sutherland & Norris 2002), and through an understanding of territorial behaviour it is then possible to predict how density-dependence may operate for a population. There are essentially three mechanisms by which density-dependence can arise through territorial behaviour: when population size increases territories may compress, poorer quality sites may be used and individuals may refrain from breeding (Sutherland 1996, 2006).
Territory size is elastic (Krebs 1971) and smaller territories may occur at high densities. In Davies's (1992) study of the Dunnock Prunella modularis, complex mating and territorial behaviour is shown to relate to the spatial distribution of the population. Mating behaviour is determined by territory size and the degree of territory overlap.
A wide range of studies shows that breeding sites vary in quality, for example in their habitat structure (Korpimäki 1988, Catchpole & Phillips 1992, Ens et al. 1992, Olson & Rohwer 1998, Murison et al. 2007), predation risk (Møller 1988, Penloup et al. 1997), nest-site availability (Plissner & Gowaty 1995), prey abundance (Komdeur 1992) or ease of defence (Eason 1992). Where such variation in site quality occurs, it can act as a mechanism for population regulation if at higher population sizes there is greater use of poorer sites with lower productivity (Rodenhouse et al. 1997). For this mechanism to work, individuals must be able to distinguish between sites that differ in quality. There is strong evidence that this is the case (Rodenhouse et al. 1997). A number of studies have shown that individuals will switch to better quality sites as they become available (Krebs 1971, Petersen & Best 1987, Switzer 1993, Greenberg & Gradwohl 1997). Territories ranked by the permanency of occupation (Korpimäki 1988, Møller 1982, Baeyens 1981, Bunzel & Druke 1989, Newton 1989), order of settlement (Brooke 1979, Lanyon & Thompson 1986, Bensch & Hasselquist 1991) or age (Lanyon & Thompson 1986) usually show a correlation with such rank and breeding success. The mechanisms by which these correlations occur are usually assumed to involve individuals sampling many sites. However, if dispersal is reduced or survival is higher on good quality sites, the same relationship between occupancy and quality will result (Rodenhouse et al. 1997).
A further aspect of territoriality is a phenomenon known as floating, whereby individuals present on the breeding grounds do not hold their own territories, even when such territories are available (Smith 1978, Smith & Arcese 1989). Following Ens et al. (1992), Komdeur (1992), Sutherland (1996) and Kokko and Sutherland (1998), we think of individuals as deciding not to breed rather than being prevented from doing so. Such individuals ‘queue’ for good quality territories rather than adopting a poor quality territory. In this way, the decision to adopt a territory or not is dependent on perceived territory quality, survival and the length of the queue.
With knowledge of the behavioural decisions that individuals make, density-dependent reproductive success can be determined, as at higher densities poorer quality sites may be adopted or, if all suitable sites are occupied, individuals may refrain from breeding. Equilibrium population size will occur when the mean lifetime reproductive success per individual is equal to 1. Thus, if mortality rates are known, it is possible to determine the density at which the population will reach equilibrium.
In this paper we illustrate the application of this approach and determine the consequences of human disturbance to the Ringed Plover Charadrius hiaticula population at a study site in Norfolk, England. Ringed Plovers provide an ideal species with which to explore the approach as they are associated with coastal areas which often receive high visitor pressure, especially during the summer, and there is evidence for a decline in the coastal breeding population of Ringed Plovers in the UK, with human disturbance having been implicated as a cause (e.g. Briggs 1983, Pienkowski 1984). The species is amber-listed as a species of conservation concern within the UK, owing to a moderate decline in the wintering population (Gregory et al. 2002). A number of other members of the genus are also threatened, and human disturbance is widely implicated as a cause, especially for species such as the globally threatened Piping Plover C. melodus (BirdLife International 2000).
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Field data, theory and a modelling approach have been dovetailed in order to consider behavioural responses at a population scale. The model has been shown to represent the actual population, and allows floating to be considered in relation to density-dependence. The model is then extended to allow changes in human disturbance levels to be considered in a population context.
Disturbance clearly has a major impact on Ringed Plover population size. The provision and management of access to sites require careful planning and the results presented allow such planning to be made in relation to maintaining or enhancing Ringed Plover population size. Sites that are highly disturbed are not used by breeding birds, and therefore any increase in disturbance levels on these sites will not alter population size. By contrast, large increases in disturbance levels to previously undisturbed sites would adversely affect population size, as these sites would no longer be used by breeding pairs. Areas of wide beach support higher densities of breeding birds, per metre of sea-wall, than narrow beaches. Thus, managing disturbance levels so that the widest sections are away from access points is likely to result in the largest population size possible per length of coastline.
Using the logistic regression equation it is possible to suggest the disturbance level above which the site would not be occupied. If the mean site quality value for all sections is used, in order to gain a probability of greater than 0.50 that a section will be occupied, the total people value is 30.90. This value is a total from all 47 transects (February–August 1996–97) and equates to an encounter rate of approximately 0.37 people per minute at average walking pace or an average of 0.005 people per metre of sea-wall per transect.
Assumptions used in the framework
The importance of perception
There are a number of alternative indicators of perceived quality. These include age, breeding chronology (such as arrival date or laying date), permanency of occupancy and various behaviours (see Table 5). These gave reasonably consistent indications of the importance of beach width and lack of disturbance.
The density-dependence is produced by the model from the process of individuals selecting territories that have higher reproductive output. Of course, the presence of ecological traps, in which there are areas that are selected but are actually of poor quality (see Orians & Wittenberger 1991; Székely 1992) will greatly affect the shape of the density-dependent curve (Kokko & Sutherland 2001). The evidence from this study was that preference was related to breeding output.
In order to predict equilibrium population size in the future it is necessary to make some assumptions about survival. For Ringed Plovers at Snettisham we assume winter survival to be independent of breeding density. This is because ringing recoveries of Ringed Plovers ringed at Snettisham and recorded during the winter (see Liley et al. 1999) clearly show that the Snettisham population winters across a very broad spread of sites, including northeast Britain, Ireland, southwest England and Brittany. Hence, we are focusing on a site supporting a local population which mixes with birds from other sites over the winter. Any factor affecting a single site will be unlikely to impact the winter population as a whole, and therefore density-independent winter survival is likely. This assumption would not apply for an analysis that affected the whole population, such as climate change.
To conclude, this paper demonstrates that it is possible to determine the impact of human disturbance through an understanding of density-dependence. In terms of conservation it is at this level, the population context, that issues such as disturbance should be addressed. Human disturbance can be seen to have a major impact on Ringed Plover population size, and the estimated 85% increase in population size that would occur, were human access to be restricted, represents the impact of disturbance on the species. The framework utilized in this paper is flexible and potentially applicable to almost any aspect of environmental change or any species. The predictions presented here were obtained, in the main, over three breeding seasons. Whilst further years’ data will provide them with greater strength, this paper illustrates that very long datasets are not necessarily essential to predict population-scale change.