Abstract
The spatial structure of breeding seabirds is increasingly recognised as a metapopulation—a network comprising connected subpopulations, each facing distinct risks, demographic dynamics, and environmental pressures. Yet, risk assessments conducted via population viability analysis (PVA) have typically treated these subpopulations in isolation. In addition, subpopulations have been assumed to be density independent (i.e., insensitive to the effects of sparsity or crowding). These assumptions are made for simplicity and as a precaution. However, both connectivity and density dependence are common in nature, and ignoring these mechanisms may lead to biased assessments.
This issue is particularly important for the black-legged kittiwake (Rissa tridactyla, hereafter kittiwake), a UK red-listed species in decline. Conservation efforts have focused on individual Special Protection Areas (SPAs), but the broader network of kittiwake colonies—including non-SPA sites— remains poorly understood. Key questions include how colonies are connected, whether non-SPA sites support or deplete SPA colonies, and how the system as a whole responds to disturbance.
We developed the first seabird metapopulation model that integrates connectivity, three forms of density dependence, and floater birds that could buffer population shocks. Our goals were to: 1) fit this advanced model to all available UK and Irish kittiwake data, 2) produce a next-generation metapopulation PVA tool for seabirds, 3) assess the roles of connectivity and density dependence, and 4) explore both local and system-wide mitigation or compensation options.
We compiled a comprehensive dataset on kittiwake populations in Britain and Ireland, including both SPA and non-SPA colonies. The data span the period 1985–2023 and include population counts, breeding success, survival of adults and immatures, and dispersal based on bird ringing data.
We tested three versions of our broad modelling framework: Model 1 allowed each colony independent demographic time series; Model 2 grouped demographic rates into six UK regions; and Model 3 added time trends to Model 2. All models showed a consistent decline in the metapopulation, with only some persistence in central and eastern Britain.
Using the most parsimonious Model 2, we forecast trends for the next 50 years, in the absence of any environmental drivers. All colonies are predicted to decline towards extinction, with no consistent geographic pattern in the rate of decline. In this situation, connectivity had limited impact because the entire metapopulation functions as a collection of sink populations.
We explored what improvements in demographic rates could reverse these trends, testing combinations of increases and decreases in breeding success, adult survival, pre-breeder survival, and floater survival. Adult survival emerged as the most critical factor. We also examined the potential of artificial nesting structures (ANS) as a compensation tool, offering a practical framework for evaluating their placement and impact.
Ultimately, for a declining metapopulation composed entirely of sink colonies, widespread and drastic improvements are needed to reverse the downward trends predicted by this modelling. Nonetheless, understanding connectivity and density dependence—particularly Allee effects in small colonies—is essential for evaluating and designing effective conservation strategies, when these are undertaken.
Despite the unprecedented realism and computational feasibility of this framework considerable validation and development work lies ahead. We outline priority areas for future work that will improve the realism of the model and extend its reliability and predictive ability. Chief among them is the inclusion of environmental covariates that will connect our predictions to processes such as climate and ocean change, and the connection of particular mitigation and compensatory measures to the quantitative population improvements they may confer.