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

Abstract

The niche variation hypothesis predicts insular populations exhibit increased sexual size dimorphism (SSD), to minimize intraspecific competition. Although many animal taxa conform to this prediction, insular patterns of SSD have yet to be investigated in plants. Here, we tested for differences in SSD of dioecious plants that colonised four island groups (Kermadec, Three Kings, Chatham and Auckland Islands) from New Zealand. Using herbarium collections, we quantified leaf and stem sizes of 263 individuals from 28 dioecious taxa. We developed a novel analytical technique to explore changes in the direction of SSD on islands. Lastly, we tested for evolutionary size changes of male and female plants on islands. The degree of SSD did not vary predictably between insular and mainland taxa, contrary to predictions of the niche variation hypothesis. Furthermore, the direction of SSD was not predictable on islands, while it was consistently female biased on the mainland. Our results suggest that selection favours increased size of both sexes on islands and that SSD is unpredictable for insular plants.

Dietary niche variation and its relationship to lizard population density

“…A phenomenon whereby insular populations expand their niches through expansion of intra‐population variation, usually thought to derive through release from interspecific competition, is often termed the “niche variation hypothesis” (Bolnick, Svanbäck, Araújo, & Persson, ; Soule & Stewart, ; Van Valen, ; although the term also have other, related meanings, see, e.g. Costa, Mesquita, Colli, & Vitt, ; Kavanagh & Burns, ). Insular species are hypothesized to have wider niches than mainland ones because they are, presumably, free to exploit resources that on the mainland would be used by their competitors (Dunham, Tinkle, & Gibbons, ; Soule & Stewart, ).…”

“…(), however, found similar or even higher morphological variability in mainland settings, which they ascribed to the influence of gene flow. Kavanagh and Burns () found no evidence for increased sexual size dimorphism (SSD) in island plants compared to mainland ones.…”

The island rule and its application to multiple plant traits

“…Research from Cox & Burns (2017) has shown that leaves on Chatham Island species showed stronger evidence for island gigantism than plant stature, which was also greater for the island species compared to their closest mainland relative. Kavanagh & Burns (2015) also found that leaves are significantly larger on islands. Other traits, such as seed size has also been demonstrated to increase on islands through inter-specific comparison (Kavanagh & Burns, 2014).…”

“…Corner (1949) demonstrated previously that seed size is correlated with traits such as stem size and plant height. Kavanagh & Burns (2015) and Cox & Burns (2017) also found evidence that insular seed size tended to display gigantism, and correlate with plant stature. However, the same studies found that not all traits correlate, and that some, such as leaf size, display size independently of other traits.…”

“…Studies have shown that insular dwarfism in plants occurs (Shinohara & Murakami, 2006;Burns, 2016a;Sun et al, 2016). Gigantism in insular plant species has also been observed (Carlquist, 1974;Böhle et al, 1996;Grant, 1998;Panero et al, 1999;Percy & Cronk, 2002;Kavanagh & Burns, 2014;Kavanagh & Burns, 2015). However, studies that tie the two together to better understand the graded trend of the Island Rule are significantly less (Kavanagh & Burns, 2014;Cox & Burns, 2017;Hendriks & Burns, unpubl.…”