Temporal Variation in Population Size of European Bird Species: Effects of Latitude and Marginality of Distribution

Abstract

In the Northern Hemisphere, global warming has been shown to affect animal populations in different ways, with southern populations in general suffering more from increased temperatures than northern populations of the same species. However, southern populations are also often marginal populations relative to the entire breeding range, and marginality may also have negative effects on populations. To disentangle the effects of latitude (possibly due to global warming) and marginality on temporal variation in population size, we investigated European breeding bird species across a latitudinal gradient. Population size estimates were regressed on years, and from these regressions we obtained the slope (a proxy for population trend) and the standard error of the estimate (SEE) (a proxy for population fluctuations). The possible relationships between marginality or latitude on one hand and slopes or SEE on the other were tested among populations within species. Potentially confounding factors such as census method, sampling effort, density-dependence, habitat fragmentation and number of sampling years were controlled statistically. Population latitude was positively related to regression slopes independent of marginality, with more positive slopes (i.e., trends) in northern than in southern populations. The degree of marginality was positively related to SEE independent of latitude, with marginal populations showing larger SEE (i.e., fluctuations) than central ones. Regression slopes were also significantly related to our estimate of density-dependence and SEE was significantly affected by the census method. These results are consistent with a scenario in which southern and northern populations of European bird species are negatively affected by marginality, with southern populations benefitting less from global warming than northern populations, thus potentially making southern populations more vulnerable to extinction.

Interactive effects of fearfulness and geographical location on bird population trends

“…In addition, population trends vary across the distribution range of species. Cuervo and Møller (2013) found stronger increases in northern populations and greater fluctuations in marginal populations, somewhat expected from influences of global warming on climatic niches (Hampe and Petit 2005), and Donald et al (2001) and Reif et al (2011) showed longitudinally varying trends. Reif et al (2011) also showed an interesting difference in the effect of relative brain size on trends at both sides of the iron curtain, consistent with the differences in land-use intensity across Europe.…”

“…FID was measured by a number of trained observers, and therefore, data were pooled for analysis. We used the FID estimates for rural populations in each location, which consisted of paired rural and urban sites (Díaz et al 2013), because the population size estimates used to assess trends for each country are mostly based on data coming from nonurban populations (Cuervo and Møller 2013). Data for the 2 Spanish sites were averaged to obtain a single country-level estimate.…”

“…Population trends for breeding birds in all European countries for which we had information on FID (Finland, Norway, Denmark, Poland, Czech Republic, Hungary, France, and Spain) were obtained from Cuervo and Møller (2013). Available population size estimates for each bird species and country were regressed on years, and the slope of this regression was used as a proxy for population trend.…”

Urbanization and species occupancy frequency distribution patterns in core zone areas of European towns

“…The breeding range (=northern distribution limit degree-southern distribution limit degree), northern distribution limit and migration distance of each bird species was measured using data provided in the maps of the book 'Birds of the Western Palearctic' (Cramp & Perrins 1977-1994 and following the method used by Møller 2008 andCuervo andMøller 2013. The year of urbanization of species was taken mainly from Møller et al 2012 by averaging the individual countries' values.…”

“…However, depending on the species pool, range centres of different groups of species may differ, for example, the forest dwelling bird species have, on an average, more northern range centres than steppic or shrubland species in Europe (Covas & Blondel, 1998). Therefore, it is important to also study the role of species population size, distribution range, marginality and northern distribution limit of species on species occupancy (e.g., Møller 2008, Cuervo & Møller 2013.…”