Rapid evolution of great kiskadees on Bermuda: an assessment of the ability of the island rule to predict the direction of contemporary evolution in exotic vertebrates

Abstract

Aim To determine whether an exotic bird species, the great kiskadee (Pitangus sulphuratus), has diverged in morphology from its native source population, and, if so, has done so in a manner predicted by the island rule. The island rule predicts that insular vertebrates will tend towards dwarfism or gigantism when isolated on islands, depending on their body size. For birds, the island rule predicts that species with body sizes below 70-120 g should increase in size. The great kiskadee has a mean mass of c. 60 g in its native range, therefore we predicted that it would increase in size within the exotic, and more insular, Bermudan range.Location The islands of Bermuda (exotic population) and Trinidad (native source population).Methods We took eight morphological measurements on 84 individuals captured in the exotic (Bermudan) population and 62 individuals captured in the native source (Trinidadian) population. We compared morphological metrics between populations using univariate and principal components analyses. We assessed whether the effects of genetic drift could explain observed differences in morphology. We calculated divergence rates in haldanes and darwins for comparison with published examples of contemporary evolution. Finally, we used mark-recapture analysis to determine the effects of the measured morphological characters on survivorship within the exotic Bermudan population.Results Individuals in the exotic Bermudan population have larger morphological dimensions than individuals in the native source population on Trinidad. The degree of divergence in body mass (g) and bill width (mm) is probably not due to genetic drift. This rate of divergence is nearly equal to that observed amongst well-documented examples of contemporary bird evolution, and is within the mid-range of rates reported across taxa. There is no clear effect of body size on survivorship as only one character (bill width) was found to have an influence on individual survivorship.Main conclusions Exotic species provide useful systems for examining evolutionary predictions over contemporary time-scales. We found that divergence between the exotic and native populations of this bird species occurred over c. 17 generations, and was in the direction predicted by the island rule, a principle based on the study of native species.

Body size changes in passerine birds introduced to New Zealand from the UK

“…Siemann and Rogers 2008), crustaceans (Torchin et al 2001), reptiles (Losos et al 1997), birds (Mathys and Lockwood 2009) and mammals (Simberloff et al 2000). However, with the exception of experimental manipulations (e.g.…”

“…We compare an aseasonal measure of body size -tarsus length -between specimens from present day New Zealand and present day UK, and between individuals from both these populations and individuals collected from the UK in the middle of the nineteenth century. Our expectation is that body sizes should be larger in New Zealand populations than in both historical and contemporary UK populations, based on the generally greater mean body size of island birds (Olson et al 2009), evidence of size increases in natural avian colonists in New Zealand (Cassey and Blackburn 2004), and previous analyses of bird introductions (Mathys and Lockwood 2009) and invasions to islands . As far as we are aware, this is the first time that the body sizes of individuals in exotic bird populations have been compared with individuals in historical source populations, rather than just with individuals in contemporary source populations that may also in theory have undergone morphological changes over the period since the exotic population was introduced.…”

“…For example, show that the natural sequential colonization by silvereyes (Zosterops lateralis) from Tasmania to South Island, New Zealand (in 1830), and the Chatham Islands (in 1856) has been accompanied by successive increases in size (wing length). Mathys and Lockwood (2009) showed that exotic great kiskadees (Pitangus sulphuratus) on Bermuda had larger morphological dimensions than individuals from the source population on Trinidad 50 years after introduction. Mathys and Lockwood (2011) also showed that five out of six exotic passerine bird species they examined on the Hawaiian islands showed morphological divergence across islands 80 to 140 years after introduction, although they do not present comparisons with the source population.…”

Island Life Shapes the Physiology and Life History of Eastern Bluebirds (Sialia sialis)

“…Efforts to better understand this divergence have resulted in generalities concerning the direction of change. The "island rule" predicts that small species should increase in size when isolated on islands (e.g., Mathys and Lockwood 2009), while the "island syndrome" relates an array of changes including increased body size, slower maturation, and reduced reproductive output (Adler and Levins 1994). These changes are partially the product of ecological differences between islands and continents.…”

“…Island organisms tend to diverge from their closest continental relatives in terms of their morphology, demography, physiology, and behavior (Foster 1964;Case 1978;Adler and Levins 1994;Grant 1998;Clegg and Owens 2002;Lomolino 2005;Mathys and Lockwood 2009). Efforts to better understand this divergence have resulted in generalities concerning the direction of change.…”