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Understanding the life history response of animal populations to environmental selection pressure is a central research theme in evolutionary ecology and conservation biology. Our current knowledge of life history traits in animal populations is, however, mostly based on studies conducted on temperate systems, contrasting with the fact that a large number of species live in the tropics. The roseate tern (Sterna dougallii) is a oceanic seabird showing mainly a tropical distribution with a relatively fragile conservation status, making it an interesting case study. In this thesis, ten years of data (1998-2007) were used to determine the main demographic traits of the tropical roseate tern population breeding on Aride Island, Seychelles (western Indian Ocean), and to explore their relation with environmental factors. We focused on the estimation of reproductive success, age at first reproduction (sexual maturation) and age-specific survival probabilities in relation to both oceanography of the study area (food availability) and quality of the breeding habitat. By using chlorophyll concentrations as a proxy measure of marine fish stocks, we showed that the overall low reproductive success in this population (range 0.0 0.57 chick/pair) is mainly dictated by the strong inter-annual fluctuations in local food supply conditions around Aride Island. Reproductive success was also found to be related to the spatial variation in vegetation characteristics of the nesting (woodland) habitat used by the terns. Birds nesting in densely-vegetated areas with a closed canopy cover had higher chick mortalities, presumably through a high level of parasitism by ticks, when compared to those breeding in more open areas such as forest clearings. By relying on capture-mark-recapture methods, age of first reproduction was estimated at 3-4 years and age-specific survival probabilities at 0.62, 0.77 and 0.81 in juveniles (1-year), immature individuals (2-years), and breeding adults (3-years and older), respectively. Most of these demographic parameters were also found to be influenced by oceanographic conditions (e.g. Indian ocean Dipole) and the levels of tick infestation during the chick-growing period. We further show how these vital rates can be incorporated into a simple population viability analysis in order to model population dynamics (i.e. population growth rate) and, ultimately, to provide local managers with conservation measures. Finally, the contribution of this work to the global knowledge of tropical seabird life histories is discussed through a comparison with demographic parameters of temperate roseate tern populations. |
