Changes in social cohesion in a long-lived species under a perturbation regime

O_LIUnderstanding the behaviour of a population under perturbations is crucial and can help to mitigate the effects of global change. Sociality can influence the dynamics of behavioural processes and plays an important role on populations resilience. However little is known about the effects of perturbations on the social cohesion of group-living animals.\nC_LIO_LITo explore the strength of social cohesion and its dynamics under perturbations, we studied an ecological system involving a colonial, long-lived species living in a site experiencing a shift to a perturbed regime. This regime, caused by the invasion of predators, led this colony to hold from 70% to only 3% of the total world population in only one decade. Because birds breed aggregated in discrete and annually changing patches within large colonies, we could disentangle whether annual aggregation was random or resulted from social bonding among individuals. Our goals were 1) to uncover if there was any long-term social bonding between individuals and 2) to examine whether the perturbation regime affected social cohesion.\nC_LIO_LIWe explored social cohesion by means of contingency tables and, within the Social Network Analysis framework, by modeling interdependencies among observations using additive and multiplicative effects (AME) and accounted for missing data. We analysed 25 years of monitoring with an individual capture-recapture database of more than 3,500 individuals.\nC_LIO_LIWe showed that social bonding occurs over years in this species. We additionally show that social bonding strongly decreased after the perturbation regime. We propose that sociality and individual behavioural heterogeneity have been playing a major role driving dispersal and thus population dynamics over the study period.\nC_LIO_LIPerturbations may lead not only to changes in individuals behaviours and fitness but also to a change in populations social cohesion. The demographic consequences of the breaking down of social bonds are still not well understood, but they can be critical for population dynamics of social species. Further studies considering individual heterogeneity, sociality and different types of perturbations should be carried out to improve our understanding on the resilience of social species.\nC_LI