Abstract
AbstractAimPopulation size changes can lead to changes in local abundance and/or site occupancy, depending on the processes influencing site use by individuals. Here, we quantify such changes for wintering waterbirds and assess their implications for widely used conservation strategies in which sites that support in excess of a given proportion of a population are prioritized for protection.LocationBritain.MethodsWe use long‐term survey data to quantify changes in population size and distribution for 19 waterbird species across Britain.ResultsPopulation changes in these species have varied greatly (from declines of ~25% to increases of >1,600%) over 26 years, and we show that change in local abundance was the predominant consequence of these changes, while colonization of new sites mainly occurred in response to large population increases. For most species, changes in abundance and occupancy were spatially dependent over relatively short distances, consistent with (but not conclusive of) density‐dependent spillover. Levels of occupancy among species were negatively associated with proportions of sites, and populations within sites, exceeding the 1% of total population threshold for site protection. For species increasing in overall population size, the occurrence of small incipient populations at new sites resulted in declines in the number of sites supporting >1% of the total population and the proportion of the population supported by these sites.Main conclusionsFluctuations in waterbird population size are more likely to result in changes in local abundance than distribution. Consequences of population change for site protection when abundance thresholds are used for site designation depend on shifts in the evenness of distribution of abundances across sites, and whether occupancy is increasing or decreasing. Range‐expanding species have an increased likelihood of losing some sites, and populations within sites, exceeding the 1% of total population threshold for site protection.
Highlights
Understanding the dynamic ecological processes through which species’ distribution, site occupancy and abundances alter with changes in population size is a key issue in ecology and conservation (Borregaard & Rahbek, 2010; Gaston et al, 2000)
We evaluate the consequences of variation in abundance and occupancy changes across species for the 1% population threshold and levels of population and site protection
To understand how changes in occupancy and population size lead to changes in proportions of populations and sites qualifying for protection, we examined the relationship between numbers of sites occupied by species in each time period and the proportions of sites, and total proportions of populations within those sites, exceeding the 1% population threshold for site protection
Summary
Understanding the dynamic ecological processes through which species’ distribution, site occupancy and abundances alter with changes in population size is a key issue in ecology and conservation (Borregaard & Rahbek, 2010; Gaston et al, 2000). | 25 temporal intraspecific abundance–occupancy relationships, making them highly variable and less consistently positive than interspecific relationships (Gaston et al, 1999) Migratory species such as wading birds may be viewed as a particular case where external drivers potentially act on both abundance and occupancy (e.g., at wintering sites) via impacts on overall population numbers occurring at a remote distance (e.g., at breeding sites, Newton, 1997). The extent to which changes in abundance and/or occupancy are spatially dependent, for example as a result of density-dependent population spillover effects (Gaston, Blackburn, & Lawton, 1997; Gregory, 1998; Newton, 1997), has a bearing on which sites are likely to exceed threshold population numbers through time, as overall population size fluctuates (Gill et al, 2001). We evaluate the consequences of variation in abundance and occupancy changes across species for the 1% population threshold and levels of population and site protection
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