Impacts of urbanisation on birds : Disentangling the effects of multiple pollutants on avian behaviour and physiology

Abstract: Anthropogenic pollution is a pervasive feature of urbanisation, reaching into ecosystems worldwide and posing novel challenges to wildlife. Not surprisingly, differences in behaviour, and physiology, have been found between urban and rural populations. Most studies on anthropogenic impacts have so far either used a dichotomous approach, comparing urban-rural sites, or investigated impacts of just one stressor. However, urban environments create a complex matrix of co-occuring pollutants, leading to potentially complex interactive effects between stressors. We currently lack a deeper knowledge of the combined and single effects and the underlying mechanisms creating urban-rural phenotypic variation. In this thesis, I investigated the single and combined effects of urban pollutants of avian behaviour and physiology. Specifically, I used the oxidative stress system, immune system, and plasma fatty acid composition, as the key physiological traits responding to human-induced environmental change. Urban pollutants of interest were artificial light at night (ALAN), anthropogenic noise, ozone and soot, and as human-influenced additional factors I looked at impacts of differential diets, and vegetation structure. I utilised full-factorial experimental exposure experiments in the wild and in the laboratory, and a correlative study in the wild, using wild and captive birds.I found that ALAN exposure alone decreases activity and noise exposure alone decreases the proportion of birds found feeding. The combined exposure to these two pollutants led to a non-additive effect on the proportion of birds resting, with ALAN as the driving stressor. ALAN-exposed nestlings mounted a less strong immune response, with a reduction of melatonin levels being the likely mechanistic link to an impaired immune functioning. Simultaneous exposure to ALAN and noise increased levels of an important antioxidant, total glutathione, more than the additive effect from single pollutant effects would have estimated (positive synergistic effect). Furthermore, I found that ozone is a potent pro-oxidant, negatively affecting antioxidant capacity, but we found no increased levels of oxidative damage due to ozone exposure. Soot exposure, on the other hand, did not affect avian oxidative stress status. Dietary ω6- and ω3-polyunsaturated fatty acids (PUFAs) modulated oxidative stress response to ozone exposure, but also act alone, with ω3-PUFAs decreasing non-enzymatic antioxidant capacity. Likewise, ω6:ω3 ratios of circulating PUFAs of wild nestlings are changed by human-influenced environmental factors, as well as their antioxidant capacity is negatively affected by air pollution and number of oak trees around their nest box. We also showed in this latter study, that using multi-stressor approach gives a more profound mechanistic understanding of phenotypic effects, then using a dichotomous comparison, which might obscure certain effects. Overall, I show that pollutants affect behaviour and key fitness related physiological traits and that the combined exposure to multiple stressors can lead to unexpected non-additive effects. This highlights the need of a more thorough mechanistic understanding of multi-stressor effects. A deeper understanding of single and combinatory effects of anthropogenic stressors will help gaining crucial insight into populations and species resilience to environmental change, thereby targeted actions can be proposed to maintain biodiversity in cities and have a future development of sustainable cities.

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