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.
Highlights
There is currently general agreement that global warming is affecting animal and plant populations in multiple ways [1,2,3,4]
Marginality of populations, i.e., whether populations were close to the latitudinal limits of distribution of the species or close to the centre of that distribution, was positively and significantly related to coefficient of variation (CV), and this association was due to the relationship with standard error of the estimate (SEE), while it was not significantly related to the slope (Table 2)
Our study is consistent with the conclusion reached in previous studies using other approaches or investigating other taxa [3,4,45,60], namely that climate change is having a nonnegligible effect on population trends, and that this effect is more beneficial for northern than for southern populations
Summary
There is currently general agreement that global warming is affecting animal and plant populations in multiple ways [1,2,3,4]. Populations close to the southern limit of the distribution of a species would be suffering to a greater extend (or benefiting less) from global warming than northern populations of the same species. Both southernmost and northernmost populations are often marginal populations, and marginality (versus centrality) of a population across the distribution range may have a strong effect on fitness-related traits (e.g. developmental stability [9], predation rate [10], and reproductive success [11,12]) and potentially on population size variation. Marginal populations suffer greater fluctuations in abundance than central populations [18,19], and, a positive relationship between degree of fluctuation and marginality within species would be expected
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