Seabirds, climate and prey. A population study of two seabird species.

Abstract

Many seabird populations have declined dramatically over recent decades, and studying the demography and population dynamic in seabird populations is important to gaining a better understanding of causes and mechanisms lying behind such changes and to identify targets for conservation and management. Climate and prey availability are known to affect demography and population dynamics of seabirds. Climate is now changing and a warming of the ocean may lead to changed availability of prey species and will affect seabirds through their demographic traits and ultimately their abundance and distribution. The main aim of this thesis was to study the impact of climate and prey abundance on seabird demography and population dynamics, using two seabird populations; the black-legged kittiwake Rissa tridactyla and the Common guillemot Uria aalge breeding on Hornøya in NE Norway whose populations are declining in Norway. The first question addressed in the thesis was how environmental conditions affect the demography and population variability of kittiwakes and guillemots. The non-breeding distribution of the kittiwakes has recently been documented using year-round light-based geolocators, and we investigated the relationship between environmental conditions in these non-breeding areas and the adult survival of kittiwakes breeding on Hornøya. We found for the kittiwakes that Thecosomata, a group of pteropods (also called sea butterflies), in the Grand Banks/Labrador Sea area in winter and the capelin Mallotus villosus stock in Barents Sea in the pre-breeding season together explained as much as 52 % of the yearly variation in adult survival rate. Further we found that the availability of 0-group cod Gadus morhua was a much more important factor affecting demography and determining the population dynamics of the common guillemot in the Barents Sea than previously considered. The second question was which demographic trait drives the population dynamics of the steeply declining kittiwake population on Hornøya. We showed, by using demographic matrix modeling and LTRE analyses, that both the variability in breeding success (and hence the recruitment to the population) and adult survival rate contributed to the steep decline in the kittiwake population on Hornøya. The strong reduction in breeding success (and hence a decline in recruitment to the population) had, however, the highest impact. The final question was whether populations can adapt to environmental changes through micro-evolutionary processes. This study found that adult survival of the two different genetic colour morphs (bridled and non-bridled morph) of the common guillemot responded differently to yearly variability in sea surface temperatures. Colour dimorphism may, in this case serve as a genetic marker in the guillemot population, and can potentially visualize the direction of selection and micro-evolutionary processes driven by climate. The studies in this thesis contribute to the understanding of the population dynamic of two declining seabird species which are both on the Norwegian Red List, and to the directions of future studies and conservation targets of both species.