The island rule explains consistent patterns of body size evolution in terrestrial vertebrates

Abstract

Island faunas can be characterized by gigantism in small animals and dwarfism in large animals, but the extent to which this so-called ‘island rule’ provides a general explanation for evolutionary trajectories on islands remains contentious. Here we develop phylogenetic models to assess patterns and drivers of body size evolution across a global sample of paired island-mainland populations of terrestrial vertebrates. We show that ‘island rule’ effects are widespread in mammals, birds and reptiles, but less evident in amphibians, which mostly tend towards gigantism. We also found that the magnitude of insular dwarfism and gigantism is mediated by climate as well as island size and isolation, with more pronounced effects in smaller, more remote islands for mammal and reptiles. We conclude that the island rule is pervasive across vertebrates, but that the implications for body size evolution are nuanced and depend on an array of context-dependent ecological pressures and environmental conditions.

Ecological specialisation, rather than the island effect, explains morphological diversification in an ancient radiation of geckos

“…This would explain that diurnal Socotran Pristurus have not colonised ground habitats and therefore their morphology has not changed substantially, while some nocturnal geckos that have colonised the Socotra Archipelago, like Hemidactylus or Haemodracon , show a marked phenotypic divergence [29,45]. , This is consistent with results on global insular vertebrate communities suggesting that the prevalence of the island rule is subjected to system-specific ecological and environmental dynamics [44]. Furthermore, a recent study on the anole radiation in the Greater Antilles did not find evidence for an island effect, pointing instead to ecological opportunity and key innovations as the drivers of the adaptive radiation [46].…”

“…The tendency of island taxa to diverge in morphology compared to their continental relatives is a general pattern in terrestrial vertebrates, especially concerning body size [43,44]. In fact, recent studies on Afro-Arabian geckos colonising the Socotra Archipelago found support for this island effect, particularly in the genus Hemidactylus [28,29].…”

Rapid morphological change in a small mammal species after habitat fragmentation over the past half‐century

“…As rearticulated by Heaney and Lomolino in the 1980s, the island rule describes a “graded trend from gigantism in small species to dwarfism in large species” (Lomolino, 1985). Besides, rapid evolution of rodents is greater on smaller and more remote islands (Benítez‐López et al., 2021; Pergams & Ashley, 2001). Reduced predation (Steen et al., 1990), reduced interspecific competition (Millien, 2004) and increased intraspecific competition (Adler & Levins, 1994) have been proposed to explain the island rule.…”

“…Reduced predation (Steen et al., 1990), reduced interspecific competition (Millien, 2004) and increased intraspecific competition (Adler & Levins, 1994) have been proposed to explain the island rule. Despite over a century of studies conducted by ecologists, evolutionary biologists and biogeographers (Lomolino, 1985, 2005; Lomolino et al., 2012, 2013; Terborgh et al., 2001, 2006), we are far from reaching a consensus on both the generality of patterns (Benítez‐López et al., 2021; Köhler et al., 2008; Lomolino, 2005; McClain et al., 2006; Meiri et al., 2008) and the causal mechanisms of evolution in island biotas. In particular, studies that consider the role of biological factors such as predation and competition in body size changes in vertebrates are still needed (Gleditsch & Sperry, 2019; Lomolino et al., 2012).…”

Giants Among Dwarfs: The Arrival Of A New Clade Of Southern Dinosaurs In The Latest Cretaceous Of Europe

“…The pre-turnover titanosaur assemblage included small-to moderate-sized species (4-10 m body length) such as Atsinganosaurus velauciensis, Ampelosaurus atacis, Garrigatitan meridionalis, Lirainosaurus astibiae, and Lohuecotitan pandafilandi [4][5][6][7][8] , which were interpreted to have lowered metabolic rates [8][9][10][11] . Reduction of body size and shifts in growth rate relative to their contemporaneous sister taxa from continental landmasses may have resulted from selective pressures under insular, limited-resourced environments, an evolutionary process known as insular dwarfism 12,13 . In contrast, the post-turnover forms are apparently larger in size but still taxonomically undescribed 14,15 , and their first appearance in the island co-occurs with a specific group of oospecies of Gondwanan affinity.…”