Consequences of population change for local abundance and site occupancy of wintering waterbirds

Aim: Population 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 prioritised for protection. 

Location: Britain.

Methods: We use long-term survey data to quantify changes in population size and distribution for 19 waterbird species across Britain.

Results: Population changes in these species have varied greatly (from declines of ~25% to increases of >1600%) over 26 years, and we show that change in local abundance was the predominant consequence of these changes, while colonisation 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 spill-over. 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 conclusions: Fluctuations 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.