Evolutionary size changes in plants of the south‐west Pacific

Abstract

Aim To investigate evolutionary changes in the size of leaves, stems and seeds of plants inhabiting isolated islands surrounding New Zealand. Location Antipodes, Auckland, Campbell, Chatham, Kermadec, Three Kings and Poor Knights Islands. Methods First, we compared the size of leaves and stems produced by 14 pairs of plant taxa between offshore islands and the New Zealand mainland, which were grown in a common garden to control for environmental effects. Similar comparisons of seed sizes were made between eight additional pairs of taxa. Second, we used herbarium specimens from 13 species pairs to investigate scaling relationships between leaf and stem sizes in an attempt to pinpoint which trait might be under selection. Third, we used herbarium specimens from 20 species to test whether changes in leaf size vary among islands located at different latitudes. Lastly, we compiled published records of plant heights to test whether insular species in the genus Hebe differed in size from their respective subgenera on the mainland. Results Although some evidence of dwarfism was observed, most insular taxa were larger than their mainland relatives. Leaf sizes scaled positively with stem diameters, with island taxa consistently producing larger leaves for any given stem size than mainland species. Leaf sizes also increased similarly among islands located at different latitudes. Size changes in insular Hebe species were unrelated to the average size of the respective subgenera on the mainland. Main conclusions Consistent evidence of gigantism was observed, suggesting that plants do not obey the island rule. Because our analyses were restricted to woody plants, results are also inconsistent with the weeds-to-trees hypothesis. Disproportionate increases in leaf size relative to other plant traits suggest that selection may favour the evolution of larger leaves on islands, perhaps due to release from predation or increased intra-specific competition

Sexual size dimorphism in island plants: the niche variation hypothesis and insular size changes

“…Environmental differences between islands may be important, as many plant traits vary predictably along environmental gradients (Diaz et al ). Environmental variables scale strongly with latitude (Clarke and Gaston , Kreft and Jetz ), however a recent test found consistent size increases of plants on islands separated by more than 20° latitude (Burns et al ). Similarly, our results displayed consistent increases in the size of both sexes, across all islands.…”

The repeated evolution of large seeds on islands

“…Seed size is strongly correlated with traits evident later in ontogeny, such as plant height and stem size [2,7,43]. Many herbaceous lineages develop woodiness on islands, and recent research suggests that an increase in leaf size is common [8,9]. As a result, it could be that selection first acting on seeds may facilitate evolutionary changes at later-life-history stages.…”

“…This allowed the inclusion of species with different fruiting phenologies. Seed sizes were measured on Chatham Island taxa in January 2008 [8] [17]). This was not always achievable; therefore, a variable number of seeds were used to characterize seed size (table 1).…”

“…Many of these other traits evolve predictably on islands (e.g. growth form and leaf area; [8,9]); however, detailed quantitative investigations of how seed size is affected by insularity are lacking.…”

The island rule: An assessment of biases and research trends

“…Palmer, ), molluscs (McClain, Boyer, & Rosenberg, ; Welch, ), reptiles (e.g. Boback & Montgomery, ; Meiri, , ) and even plants (Burns, Herold, & Wallace, ). While it has been supported for enigmatic taxa such as spring tails (Ulrich & Fiera, ) or cave‐dwelling plant hoppers (Hoch, Bourgoin, Stelbrink, & Wessel, ), several studies have questioned the rule's validity, e.g.…”