The island rule: An assessment of biases and research trends

Abstract

Aim: The island rule has been widely applied to a range of taxonomic groups, with some studies reporting supporting evidence but others questioning this hypothesis.To bring more clarity to this debate, we conducted a comparative analysis of the available literature, focussing on potential biases. Location: Worldwide.Methods: We performed a systematic review to identify studies testing the island rule and translated these studies' outcomes, so that they follow a consistent approach.The studies were assessed for differences in their analysis of the island rule. We created an authorship network showing who published studies with whom on the topic and weighted the data based on co-authorship and number of publications.Results: We identified 143 relevant studies, finding a significantly lower frequency of supporting studies according to our consistent approach (50%) than the authors' own statements (59%). Two core-author groups could be identified with a strong publication record on the island rule. The first group has predominately published studies supporting the rule, whereas the other group has mainly published studies questioning it. According to a subsequent analysis excluding studies with a high risk of HARKing (hypothesizing after the results are known), the frequency of studies supporting the rule further dropped to 42%.Main conclusions: Empirical support for the island rule is low, especially for nonmammalian taxa and when using a consistent evaluation approach. Differences among studies in supporting versus questioning this hypothesis seem to be partly due to author-related biases. Methods to address potential biases in studying ecological hypotheses are urgently needed. We offer such a method here. K E Y W O R D Sauthor biases, author groups, hierarchy of hypotheses, island biogeography, island dwarfism, island gigantism, island rule | INTRODUCTIONThe observation that species on islands are smaller or larger than their mainland counterparts has led to the formulation of the island rule which describes "the tendency for a graded series of changes in the size of island vertebrate species in relation to mainland congeners, such that small-bodied species tend to get larger, and vice versa"-definition from a standard textbook on Island Biogeography (Whittaker & Fern andez-Palacios, 2007, p. 346). The island rule was first formulated by Leigh van Valen (1973) who stated that "the regular evolution of mammalian body size on islands is an extraordinary phenomenon which seems to have fewer exceptions than any other ecotypic rule in animals" (p. 35). These findings were based on a study by Foster (1964), who had described the tendency of several mammalian orders to be either larger (Rodentia) or smaller (Carnivora, Artiodactyla, Lagomorpha) when isolated on marine islandsThe dataset underlying this analysis will be included in the expanded online edition. compared to related mainland populations. The island rule was fundamentally reshaped by Lomolino (1985) (see also Heaney, 1978), who had reviewed a simil...

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

“…The highly inconsistent results of these studies (e.g. 5,6,24 ) are perhaps unsurprising because they typically deal with single species or pool together data on different traits from numerous sources without controlling for variation in study design or accounting for measurement error. Accordingly, a recent systematic review based on a simplified scoring system 24 concluded that previous studies were undermined by author-related biases and that empirical support for the island rule is generally low, particularly for non-mammalian taxa.…”

Ecology, conservation, and phylogenetic position of the Madagascar JacanaActophilornis albinucha

“…Explanations for the island rule are complex and multifaceted, and they include foraging-preference shifts, character displacement, reduced resource competition, and reduced predation pressure (Clegg and Owens 2002;Scott et al 2003). The island rule for birds is inconsistently supported (Gaston and Blackburn 1995;Clegg and Owens 2002;Lomolino 2005;Olson et al 2009;Lokatis and Jeschke 2018), and in mammals endemic to Madagascar there is little evidence supporting this pattern (Kappeler et al 2019). Furthermore, Madagascar is considered by some as a subcontinent rather than an island due to its large land mass (de Wit 2003), such that the Madagascar Jacana may not constitute a true island endemic.…”

“…Therefore, if jacanas represent a large-sized species (as do rails, ducks and ratites, according to Lomolino [2005]), the larger size of the Madagascar Jacana compared with the African Jacana is evidence against the island rule. The inconsistencies in findings between studies testing this rule could also be attributable to methodological differences and specific ecological traits that shift the strength of the rule (Clegg and Owens 2002;Lokatis and Jeschke 2018).…”

Skull shape and Bergmann's rule in mammals: hints from Old World porcupines

“…Several ‘rules’ have been proposed to describe the patterns of size variation in animal species (Lomolino et al ., ; McNab, ). Among those, the Bergmann's rule is one of the most debated (Blackburn, Gaston & Loder, ; Raia & Meiri, ; Lokatis & Jeschke, ). The Bergmann's rule states that, within closely related endotherm taxa, body size tends to decrease from colder to warmer climates The (Bergmann, ; Salewski & Watt, ; Stanchak & Santana, ).…”