Bergmann's rule in alien birds

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.

Colonization pressure: a second null model for invasion biology

“…Where species are introduced also influences spatial variation in the traits of established alien bird species. Thus, Blackburn et al (2019b) showed that the body masses of alien bird species established at higher latitudes are larger, on average, than the masses of species established at lower latitudes. This recapitulates the pattern, known as Bergmann's Rule, shown by native bird species (Olson et al 2009).…”

“…Once again, however, the pattern for established alien birds is largely a consequence of where alien species were introduced: we now find larger-bodied, on average, bird species at higher latitudes because the species introduced there were larger-bodied, on average. Differential failure of introduced populations of different body masses modifies these patterns to a degree, but does not erase them (Blackburn et al 2019b).…”

“…The absolute magnitudes of X and Y made little difference to these effects. For studies of alien species, this means that alien species richness will be strongly determined by colonization pressure when a relatively high proportion of introductions succeed (Y C X), but will tend to be more weakly correlated with colonization pressure if most introductions fail (Y ( X) (Blackburn et al 2019a). Data for alien island birds suggest that the numbers of successful and failed introductions are about equal overall (Blackburn et al 2008), and so we would expect colonization pressure to be a strong determinant of alien species richness in this example.…”

Evidence for Rapoport’s rule and latitudinal patterns in the global distribution and diversity of alien bird species

“…To address these issues, some authors have turned to alien species introductions as a form of natural experiment to test the mechanisms driving latitudinal patterns (e.g. Blackburn, Redding, & Dyer, ; Blanchet et al, ; Guo, Sax, Qian, & Early, ; Sax, ). An alien species is one which has been introduced by a human vector beyond the limits of its native range, either accidentally or on purpose (Blackburn, Lockwood, & Cassey, ).…”

“…Yet, these analyses only examine those species with established distributions: they do not fully consider the effects of where species have been introduced, and where introduced species have succeeded or failed. Their transitory nature means that data on failed introductions are harder to obtain than data on successfully established species (Blackburn et al, ; Dyer, Cassey, et al, ; Pyšek et al, ), and therefore patterns in alien species latitudinal distributions have generally been described without fully taking them into account (although see Blackburn et al, ). Sax () rejected an association between the distribution of species introductions and alien species richness, arguing that the latitudinal pattern of introduction events for alien birds in North America did not match the latitudinal pattern of established species richness.…”