Effects of environmental variability and perturbations on seabirds' population dynamics = Efectos de la variabilidad ambiental y las perturbaciones en la dinámica de poblaciones de las aves marinas

Abstract

The overarching aim of my thesis was to assess the role of environmental variability and specially of perturbations in the population dynamics of social vertebrates. Our results emphasize the need to collect individual, population and metapopulation data at long term and large spatial scales. Such information is crucial to understand wild animal populations respond to environmental variability and perturbations. We demonstrate that environmental variability and perturbations induce complex responses and lead to covariance processes within demographic traits at different spatio-temporal scales. As expected for long-lived species, perturbations encouraged species to halt breeding investment in order to ensure future survival. However, under strong perturbation regimes an unknown population resilience threshold is surpassed and foregoing reproduction is insufficient to buffer population changes. Cumulative effects of perturbations magnified population decreases across large spatio-temporal scales. Consecutive perturbations are potentially useful tools to effectively manage overabundant species. However, caution must be taken because their long-term consequences are still poorly understood. Severe environmental conditions caused both lethal and non-lethal responses on populations. Non-lethal responses include changes in foraging behaviour, development cultural innovations such non-random dispersal movements or colonization of novel environments. Individuals responded differentially to environmental variability and consecutive perturbations depending on their individual quality, age, experience and cohort of birth. Such differential responses resulted in severe selection filters and represent a structuring force — potentially destabilizing population dynamics. Processes underlying the strong early-life selection pressures on early life survival are not limited to the rearing period, first winter is also critical. Consecutive cohort effects driven by density dependence and climate influence early-life survival of long-lived species, but that influence faded at older ages. Experienced individuals act as an information repository. They are more likely to engage large distance dispersal and colonize new environments. Populations that include older individuals are more likely to be able to effectively and rapidly adapt to environmental change, at least in long lived, social species that exhibit social learning. Colonisations are temporally and spatially unpredictable and respond to an accumulation of perturbations exceeding an unknown threshold for colonising new patches. The temporal mismatch between perturbations and colonisations events suggests that colonisers need a longer prospecting period to compensate for the lack of public information. Non-lethal, density independent perturbations, such as predators’ presence, can drive changes in populations age structure and, therefore in the population reproductive value, through selection for young and inexperienced individuals with lower breeding success. We provide strong evidences showing that consecutive perturbations, non-lethal effects and non-random individuals’ responses across different life stages are more important than previously thought. These results are especially relevant for understanding how populations will respond to the predicted increases in magnitude and frequency of perturbations as a consequence of global change.

La tesis evalúa los efectos de la variabilidad ambiental y las perturbaciones en poblaciones naturales a diferentes escalas espacio-temporales. Todos los resultados mostrados proporcionan información útil para el modelado poblacional y metapoblacional. En particular, proporcionamos fuertes evidencias que demuestran que las perturbaciones consecutivas, los efectos no letales y las respuestas diferenciales de los individuos a lo largo de las distintas etapas de la vida son más importantes de lo que se pensaba anteriormente. Estos resultados son especialmente relevantes para entender cómo las poblaciones naturales responderán a los incrementos previstos en la magnitud y la frecuencia de las perturbaciones como consecuencia del cambio global.