The island rule (1, 2) posits that large mainland species evolve to be small on islands, while small mainland species evolve to be large. Biddick et al. (3) assess the signal of an island rule in plant traits on islands off the coast of New Zealand, concluding that some plant traits show evidence of an island rule while others do not. We applaud Biddick et al. for their application of the island rule to plants. However, we observe significant heterogeneity in their data, and while analysis on a trait-by-trait basis may reveal an island rule, it may obscure evidence of island lineages exploring more of trait space than their mainland relatives.
A single plant species may demonstrate considerable variation in growth form, as their data suggest. For example, they use Veronica elliptica for multiple comparisons; leaf area shows a 5-fold difference between 2 different sources (rows 38–40 vs. 114–116 in their dataset S1). As both sources were literature derived for this within-species data, it is difficult to see how such large variation is accounted for through their modeling of “collection method” and “species identity” as random effects; this may create a gradient that reflects within-species variation rather than an evolutionary trend. Additionally, this variation may disguise a different trend than size convergence. For example, for observations that had both leaf and stature measurements, 36 showed different patterns between leaf and stature, suggesting they are not uniformly becoming gigantic or dwarfed. Closely related species can also display significant heterogeneity. For example, the comparison of Coprosma repens to the closely related Coprosma chathamica reveals a case of island gigantism in stature but dwarfism in leaves. However, C. repens is more closely related to Coprosma petiolata (4), which appears to represent a case of island dwarfism in both stature and leaves, indicating that this process can be extremely variable.
We consider that this variation may represent a process of island plants exploring more of trait space than mainland relatives (5, 6), even in situations of so-called anagenesis (5, 7). Analyses of separate traits can reflect an island rule but hide divergent patterns of among-trait evolution. In general, we expect that populations colonizing islands will be more subject to drift (8), which could drive nonadaptive morphological change (9) and cause island lineages to explore more of trait space for both traits and trait covariance. Although in a separate study these authors did not detect allometric change for island vines (10), it remains to be tested for more plant groups.
This dataset represents a valuable resource to understand the evolution of plant morphology on islands. A more holistic approach, focusing on multivariate analysis of plant traits, making full use of the available phylogenetic data for these taxa, is capable of revealing the dynamics of evolution on islands. Such dynamics may be obscured when searching for size convergence on a trait-by-trait basis.
Acknowledgments
J.I.B. and N.W.-H. were supported by the Woolf Fisher Trust. We thank Caroline Parins-Fukuchi and Stephen Smith for their helpful discussion.
Footnotes
The authors declare no competing interest.
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