Bergmann's rule in alien birds

David W. Redding,Ellie E. Dyer,Tim M. Blackburn

doi:10.1111/ecog.03750

David W. Redding, Ellie E. Dyer + Show 1 more

Open Access

https://doi.org/10.1111/ecog.03750

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Journal: Ecography	Publication Date: Aug 16, 2018
Citations: 13	License type: CC BY 3.0

Affiliation: University College London, Zoological Society of London

Abstract

Native bird species show latitudinal gradients in body size across species (Bergmann's rule), but whether or not such gradients are recapitulated in the alien distributions of bird species are unknown. Here, we test for the existence of Bergmann's rule in alien bird species worldwide, and investigate the causes of the observed patterns. Published databases were used to obtain the worldwide distributions of established alien bird populations, the locations of alien bird introductions, and bird body masses. Randomisation tests and linear models were used to assess latitudinal patterns in the body masses of introduced and established alien bird populations. Established alien bird species exhibit Bergmann's rule, but this is largely explained by where alien bird species have been introduced: latitudinal variation in the body masses of established alien bird species simply reflects latitudinal variation in the body masses of introduced species. There is some evidence that body mass is implicated in whether or not established species’ alien ranges spread towards or contract away from the Equator following establishment. However, most alien bird ranges are encompassed by the latitudinal band(s) to which the species was introduced. Bergmann's rule in alien birds is therefore a consequence of where humans have introduced different species, rather than of natural processes operating after population introduction.

Highlights

Bergmann (1847) was the first to propose that species of homeothermic animals living in cold climates should be larger-bodied than phylogenetic close relatives living in warm climates, and that there should be latitudinal gradients in body size in such taxa (Blackburn et al 1999)
The relationship is generally well supported, with regional and global analyses of the distribution of avian body masses finding a general trend of larger body masses at high latitudes (Blackburn and Gaston 1996a, Olson et al 2009)
Low phylogenetic correlation in the extent of Pole-ward spread or contraction (using the ‘caper’ package in R (Orme et al 2015) applied to a Hackett all species phylogeny (Jetz et al 2012), lambda [95% confidence intervals] = 0, [0, 0.18]) means that phylogenetic generalised least square (PGLS) models give identical answers; we report only the generalised linear models (GLM) results

Summary

Introduction

Bergmann (1847) was the first to propose that species of homeothermic animals living in cold climates should be larger-bodied than phylogenetic close relatives living in warm climates, and that there should be latitudinal gradients in body size in such taxa (Blackburn et al 1999). The relationship that this posits has come to be generally known (and will be known here) as Bergmann’s rule, debate rumbles on as to whether spatial variation in body size should be analysed across or within species (cf Watt et al 2010, Meiri 2011). Evidence suggests that this pattern is a consequence in part of turnover among lineages, and in part of adaptation to temperature gradients and resource availability within lineages (Olson et al 2009)

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