Abstract
The degree and direction of morphological change in invasive species with a long history of introduction is insufficiently known for a larger scale than the archipelago or island group. Here, I analyse data for 105 island populations of Polynesian rats, Rattus exulans, covering the entirety of Oceania and Wallacea to test whether body size differs in insular populations and if so what biotic and abiotic features are correlated with it. All insular populations of this rat, except one, exhibit body sizes up to twice the size of their mainland conspecifics. Body size of insular populations is positively correlated with latitude, consistent with thermoregulatory predictions based on Bergmann's rule. Body size is negatively correlated with number of co-occurring mammalian species, confirming an ecological hypothesis of the island rule. The largest rats are found in the temperate zone of New Zealand as well as on mammalian species-poor islands of Polynesia and the Solomon Islands. Carnivory in the form of predation on nesting seabird colonies seems to promote 1.4- to 1.9-fold body size increases.
Highlights
Humans have been transporting vertebrates to islands worldwide since prehistoric times
The 95 islands vary in size (0.1–785 753 km2; mean 15 592.1 km2), absolute latitude (0.25–47.21°; mean 16.5°), maximum elevation (1–5030 m; mean 872 m), numbers of ecologically relevant native predators (n = 0–13; mean n = 0.2) and native competitors (n = 0–73; mean n = 2.3) and total number of mammalian species (n = 1–164; mean n = 8.4) (Supplementary Table S1)
Island populations with a mean body mass twice that of their mainland conspecifics or heavier are found in remote Eastern Polynesia (Henderson, Si = 1.92; Pitcairn, Si = 2.04; Nuku Hiva, Si = 2.07) and New Zealand (Stanley, Si = 1.94; Red Mercury, Si = 1.95)
Summary
Humans have been transporting vertebrates to islands worldwide since prehistoric times. I present data from a ubiquitous invasive species, the Polynesian rat (R. exulans), which provides an excellent proxy to measure and model evolutionary changes for several reasons. It vastly extended its geographic range through human-mediated transport (Roberts 1991) (Fig. 1) and is currently a commensal in islands of the Lesser Sunda Archipelago, the Moluccas, Sulawesi, the Philippines, New Guinea, New Zealand and practically all inhabited Pacific islands (Corbet & Hill 1992; Musser & Carleton 2005), and has recently been recorded in Taiwan and the southern Ryukyu Islands (Motokawa et al 2001). Rats in general have the potential to show evolutionary changes within a relatively short time span due to their high reproduction rate
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