Linking weather conditions and human induced habitat changes to population dynamics of a farmland passerine bird.
Piha, Markus*,1, Andreas, Lindén2, Pakkala, Timo1, Tiainen, Juha3, 1 Finnish Museum of Natural History, Helsinki, Finland2 Department of Biological and Environmental Sciences, Helsinki, Finland3 Finnish Game and Fisheries Research Institute, Helsinki, Finland
ABSTRACT- Birds have been studied intensively in regard to factors influencing population fluctuations. In many studies, population density, habitat quality, food availability and weather have been reported to be important predictors of population growth. However, the number of studies linking bird population monitoring data to several ecologically essential factors from different stages of species' annual cycles is scarce, especially for migrant species. We studied the effects of habitat changes and weather conditions on population dynamics of a migrant farmland bird, the skylark Alauda arvensis. The Skylark is an abundant species of open farmland landscapes all over Europe, and, it has decreased strongly in Central and Western Europe since the 1970s. The data consisted of farmland bird censuses performed annually in an agricultural landscape of 20 km2 in southern Finland in 1984-2003. The time-series model jointly used the autoregressive component, farmland habitat quality and the weather conditions in breeding and wintering areas. One of the main methodological approaches was to handle the problem with autocorrelated time series by using Monte Carlo testing of simulated autocorrelated time series. Our results show that the general trends in skylark population development can be understood by changes in agricultural land use. Weather conditions in breeding and wintering areas partly explained the between-year changes in population size. There was also evidence for first order negative density dependence. Our results highlight that both intrinsic and extrinsic factors can be simultaneously important in determining population dynamics and thus support the synthetic view of population regulation. Excluding one factor might be crucial for the visibility of the other. As migrant birds face variable ecological conditions, it is important to study population fluctuations in the light of species' life histories. The time series methodology developed for this study can serve as a useful tool in many kinds of studies on population monitoring data.
Key words: population dynamics, density dependence, visibility, alauda arvensis
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