Change in the demographic dynamics of a spatially structured population of a colonial waterbird after a perturbation affecting a large colony.

Localized perturbations may affect spatially structured populations (SSPs) as a whole by altering dispersal between patches. Yet, such large-scale consequences remain poorly understood and may depend on the proportion of the SSP affected by the perturbation. Here, we used correlative analyses and integrated population models to investigate the impact of pond draining-localized perturbations of breeding site availability-on the structure and demographics of an SSP of black-headed gulls (Chroicocephalus ridibundus). Pond draining regularly affected patches occupied by colonies, reducing the occupancy probability of those patches in subsequent years, but causing no visible changes in population growth or structure as breeders relocated to unperturbed patches. However, the draining of the pond occupied by the largest colony in the system (64% of total breeders) triggered the immediate emigration of over 60% of breeders outside the SSP, with no subsequent recovery. In the following years, apparent survival showed increased interannual variability, suggesting greater sensitivity of the SSP to the perturbation of smaller colonies. Dispersal between colonies also became more uniform, reflecting the homogenization of colony sizes after the emigration event in a context of informed dispersal. Considering perturbation magnitude and dispersal mechanisms appears essential for understanding the response and resilience of SSPs to local perturbations.