Decrease in social cohesion in a colonial seabird under a perturbation regime

Abstract

Social interactions, through influence on behavioural processes, can play an important role in populations' resilience (i.e. ability to cope with perturbations). However little is known about the effects of perturbations on the strength of social cohesion in wild populations. Long-term associations between individuals may reflect the existence of social cohesion for seizing the evolutionary advantages of social living. We explore the existence of social cohesion and its dynamics under perturbations by analysing long-term social associations, in a colonial seabird, the Audouin's gull Larus audouinii, living in a site experiencing a shift to a perturbed regime. Our goals were namely (1) to uncover the occurrence of long-term social ties (i.e. associations) between individuals and (2) to examine whether the perturbation regime affected this form of social cohesion. We analysed a dataset of more than 3500 individuals from 25 years of monitoring by means of contingency tables and within the Social Network Analysis framework. We showed that associations between individuals are not only due to philopatry or random gregariousness but that there are social ties between individuals over the years. Furthermore, social cohesion decreased under the perturbation regime. We sustain that perturbations may lead not only to changes in individuals' behaviour and fitness but also to a change in populations' social cohesion. The consequences of decreasing social cohesion are still not well understood, but they can be critical for the population dynamics of social species.

Figure 1

Number of breeding pairs in the Punta de la Banya colony from colonization in 1981 to 2017. The observed phases in the population dynamics (initial growth, dynamic equilibrium and collapse) are separated by dashed lines. Red arrow indicates the arrival of predators (mainly foxes) to the colony. The shadow area shows the temporal window for data analysis for comparing the periods before and over the collapse (separated by a red dashed line).

Figure 2

Graphical representation of social networks by the association between individuals of Audouin’s gulls in breeding patches comparing (a) the stability phase (2002–2011) and (b) the transition phase to colony collapse (2012–2017) (see Fig. 1). Each node represents an individual and each edge links those individuals that have bred in the same patch. We used the half-weight association index (HWI) to estimate the strength of the relationship between pairs of individuals. The HWI is more suitable than other metrics when not all individuals within each group have been identified^,.

Figure 3

Map of the study area comprising the 3 main colonies and the distribution of patches within colonies during the study period at (a) Sant Antoni and (b) Punta de la Banya. Sant Carles de la Rápita colony is considered to have only one patch. Map made by the authors from the Orthophoto of Catalonia 1:5000 of the Institut Cartogràfic i Geològic de Catalunya (ICGC)( https://www.icc.cat/vissir3/), used under a CC BY 4.0 license with the use of CorelDraw X5 v.15.2.0.686.