Inconsistent patterns of body size evolution in co‐occurring island reptiles

Abstract

Aim Animal body sizes are often remarkably variable across islands, but despite much research we still have a poor understanding of both the patterns and the drivers of body size evolution. Theory predicts that interspecific competition and predation pressures are relaxed on small, remote islands, and that these conditions promote body size evolution. We studied body size variation across multiple insular populations of 16 reptile species co‐occurring in the same archipelago and tested which island characteristics primarily drive body size evolution, the nature of the common patterns, and whether co‐occurring species respond in a similar manner to insular conditions. Location Aegean Sea islands. Time period 1984–2016. Major taxa studied Reptiles. Methods We combined fieldwork, museum measurements and a comprehensive literature survey to collect data on nearly 10,000 individuals, representing eight lizard and eight snake species across 273 islands. We also quantified a large array of predictors to assess directly the effects of island area, isolation (both spatial and temporal), predation and interspecific competition on body size evolution. We used linear models and meta‐analyses to determine which predictors are informative for all reptiles, for lizards and snakes separately, and for each species. Results Body size varies with different predictors across the species we studied, and patterns differ within families and between lizards and snakes. Each predictor influenced body size in at least one species, but no general trend was recovered. As a group, lizards are hardly affected by any of the predictors we tested, whereas snake size generally increases with area and with competitor and predator richness, and decreases with isolation. Main conclusions No factor emerges as a predominant driver of Aegean reptile sizes. This contradicts theories of general body size evolutionary trajectories on islands. We conclude that overarching generalizations oversimplify patterns and processes of reptile body size evolution on islands. Instead, species’ autecology and island particularities interact to drive the course of size evolution.

Environmental correlates of reptile variation on the Houtman Abrolhos archipelago, eastern Indian Ocean

“…Our results support the conclusions of Itescu et al. (2018) in Aegean island reptiles that body size and environmental factors show few general trends, and size is most likely a consequence of autecological responses to island peculiarities, as was indicated by Novosolov and Meiri (2013) documenting species adaptive demographic responses to environmental resources. All island populations should be managed and conserved to retain variation within the composite species populations and should have a high priority in archipelagos where both diversity and threatening processes are greatest.…”

Divergent selection along elevational gradients promotes genetic and phenotypic disparities among small mammal populations

“…This finding is consistent with that of previous studies on animals along elevational gradients at intraspecific (Caro et al, 2013;Eastman, Morelli, Rowe, Conroy, & Moritz, 2012;Liao, Zhang, & Liu, 2006;Wasserman & Nash, 1979) and interspecific (Du et al, 2017;Geraghty, Dunn, & Sanders, 2007;Hu, Xie, Li, & Jiang, 2011) levels. Such conflicting patterns might reflect idiosyncratic responses due to inherent ecological, physiological, and evolutionary constraints (Adams & Church, 2008;Itescu et al, 2018). Such conflicting patterns might reflect idiosyncratic responses due to inherent ecological, physiological, and evolutionary constraints (Adams & Church, 2008;Itescu et al, 2018).…”

“…Studies on the effects of climatic variation have mainly focused on temperature-size correlations, testing the validity of Bergman's and Allen's rules along altitudinal gradients (Brehm & Fiedler, 2004;Du et al, 2017;Freeman, 2017;Gutiérrez-Pinto et al, 2014). Considering well-established island hypotheses, we would expect biotic interactions to play an important role in shaping morphological diversity in mountains as they do in islands (Heaney, 1978;Itescu et al, 2018;Michaux, Bellocq, Sarà, & Morand, 2002). Increasing altitude leads to a reduced area, enhanced spatial isolation, and relaxed predation risk, similar to the patterns observed in insular systems (Körner, 2007).…”

Global patterns of body size evolution in squamate reptiles are not driven by climate

“…We also calculated the mean value of each of our environmental predictors across the cell. We omitted island cells (all landmasses smaller than Australia) from this analysis in order to remove the potential bias to our results from effects of insularity on body size evolution (e.g., Itescu et al, ).…”