‘On being the right size’ – Do aliens follow the rules?

Abstract

Aim: To assess whether mammalian species introduced onto islands across the globe have evolved to exhibit body size patterns consistent with the 'island rule,', and to test an ecological explanation for body size evolution of insular mammals.Location: Islands worldwide. Methods:We assembled data on body mass, geographical characteristics (latitude, maximum elevation) and ecological communities (number of mammalian competitors, predators and prey) for 385 introduced populations across 285 islands, comprising 56 species of extant, non-volant mammals. We used linear regression, ANCOVA and regression tree analyses to test whether introduced populations of mammals exhibit the island rule pattern, whether the degree of body size change increased with time in isolation and whether residual variation about the general trend can be attributed to the geographical and ecological characteristics of the islands.Results: Introduced populations follow the predicted island rule trend, with body size shifts more pronounced for populations with greater residence times on the islands. Small mammals evolved to larger body sizes in lower latitudes and on islands with limited topographic relief. Consistent with our hypothesis on the ecology of evolution, body size of insular introduced populations was influenced by co-occurring species of mammalian competitors, predators and prey. Conclusion:The island rule is a pervasive pattern, exhibited across a broad span of geographical regions, taxa, time periods and, as evidenced here, for introduced as well as native mammals. Time in isolation impacts body size evolution profoundly.Body size shift of introduced mammals was much more pronounced with increasing residence times, yet far less than that exhibited by native, palaeo-insular mammals --

Space–time patterns of body size variation in island bovids: The key role of predatory release

“…The principal product of RTA is a recursively branching tree that describes the direct, interactive and contextual relationships between the response variable (here Si) and a subset of the predictor variables (geographic and ecological variables) (see e.g. Durst & Roth, 2012Lomolino et al, 2012Lomolino et al, , 2013Lyons et al, 2016;Van der Geer et al, 2018 for descriptions and application of classification and regression tree analyses to body size patterns). For each RTA, I obtained an optimal tree, selected based on K-fold cross-validation, and a maximal tree, produced by growing interactively the respective optimal tree as large as possible (SAS Institute, 2017).…”

“…The island rule pattern was later described as a graded trend from gigantism in smaller species to dwarfism in larger species (Lomolino, 1985), with a diversity of alternative explanations, either confirming (e.g. Benton et al, 2010;Faurby & Svenning, 2016;Heaney, 1978;Lomolino, 2005;Lomolino et al, 2012Lomolino et al, , 2013McClain, Durst, Boyer, & Francis, 2013;Palombo, 2009; Van der Geer, Lomolino, & Lyras, 2018) or challenging (e.g. Itescu, Karraker, Raia, Pritchard, & Meiri, 2014;Karagkouni, Sfenthourakis, & Meiri, 2017;Meiri, Cooper, & Purvis, 2008) the generality and putative causal factors for body size variation among insular mammals.…”

“…Aside from comprehensive studies (e.g. Faurby & Svenning, 2016;Lomolino, 1985Lomolino, , 2005Lomolino et al, 2012Lomolino et al, , 2013McClain et al, 2013;Meiri et al, 2008;Van der Geer et al, 2018), investigations of the island rule pattern in insular mammals have mainly focused on proboscideans and rodents, which include the most spectacular cases of dwarfism and gigantism, respectively (see e.g. Durst & Roth, 2012Palombo, 2007;Van den Hoek Ostende, Van der Geer, & Wijngaarden, 2015; Van der Geer, Van den Bergh, et al, 2016).…”

Peer Review #2 of "Effect of isolation on coat colour polymorphism of Polynesian rats in Island Southeast Asia and the Pacific (v0.2)"

“…Body mass evolution of island mammals typically follows a graded trend from gigantism in small-sized species to dwarfism in large-sized species (Foster, 1964;Van Valen, 1973;Lomolino, 1985Lomolino, , 2005, where the degree and direction of body size evolution vary in a predictable manner in accordance with characteristics of the ancestral taxon and the levels and nature of various factors like competition, predation, ecological complexity, climate, latitude, island area and isolation (Lomolino et al, 2012(Lomolino et al, , 2013. Other morphological features regularly shared by phylogenetically distant fossil island mammals are proportionally shortened limbs (Sondaar, 1977), reduced brain size (Weston & Lister, 2009;see, however, Lyras et al, 2006), increased convolution of the brain cortex (Desachaux, 1961;Falk et al, 2005), fusions of limb bones (Leinders & Sondaar, 1974;Moyá-Solá, 1979;van den Bergh et al, 2008;van der Geer, 2014), high-crowned teeth (Bover, 2004;van den Bergh, 1999;van der Geer et al, 2006;van der Made, 1999), loss of dental elements (Leinders, 1984;Lyras et al, 2006;Boekschoten & Sondaar, 1972), shortened muzzles (van der Geer, 2005;Quintana, Köhler & Moyà-Solà, 2011;Kaifu et al, 2011;van der Geer et al, 2018), increased stereoscopy (van der Geer, 2005) and increased size variation (de Vos & van der Geer, 2002;van der Geer, 2014). Isolated mammal populations may also show shared ecological differences such as increased tameness or ecological naiveté (see also below), higher and more stable population densities, reduced intraspecific aggression, reduced reproductive output (see, however, Raia, Barbera & Conte, 2003, who suggested increased reproductive prod...…”

“…Evolutionary divergence is expected to be greatest in the following populations: those on islands with reduced competitor and predator stress through relaxed selection (Whittaker & Fernandez-Palacios, 2007;Lomolino, Riddle & Whittaker, 2017), those on small islands (Lomolino et al, 2012; and those with greater residence times (Lomolino et al, 2013;van der Geer, Lomolino & Lyras, 2018). The hypothesis is that these populations of Polynesian rats will show the highest incidence of aberrant coat patterns.…”