The curious case of Skokholm: equilibrium, non-equilibrium and a phase shift in an island landbird assemblage

Previous work has indicated that the landbirds of Skokholm island (Wales) are not in equilibrium as defined in MacArthur–Wilson's classic theory of island biogeography. This study takes a new dataset with over six decades of data and investigates equilibrium on Skokholm using cluster analysis to identify periods of turnover stability. The attributes of the identified periods were investigated in relation to the MacArthur–Wilson model using analyses of change in numbers of species, S, from one year to the next and measures of variability in S quantified for each of the periods identified together with a consideration of the dynamics in the numbers of species by habitat groupings. Cluster analysis identified four main periods of which two middle periods appeared to be in equilibrium but with a phase shift in-between. The first and last periods showed non-equilibrium dynamics but plots of species by habitat groupings suggested that this was due to habitat changes going on at those times. This decadal long dataset indicates that the landbirds of Skokholm exhibit periods of both equilibrium and non-equilibrium with the latter attributable to habitat change. The apparent phase shift in the equilibrium number of species was unexpected within the framework of island biogeographic theory and not easily explained using the current MacArthur–Wilson framework. There is a need to integrate the theory of island biogeography with more recent work on alternative stable states, tipping points, and phase (or regime) shifts, together with equilibrium and non-equilibrium dynamics, into a single framework.