The significance of shared leaf shape in Alseuosmia pusilla and Pseudowintera colorata

Abstract

Leaf shape, size, and colour are used by herbivores to identify sources of palatable foliage for food. It is possible, therefore, that an undefended plant might gain protection from herbivores by matching leaf characteristics of a chemically defended species. We demonstrate the use of a geometric morphometric approach to quantify spatial variation in leaf shape and size across populations of Pseudowintera colorata (Raoul) Dandy, and a putative Batesian mimic, Alseuosmia pusilla (Colenso) A. Cunningham. These are unrelated, sympatric species that, to the human eye, bear strikingly similar foliage. Using the Cartesian coordinates of leaf margins as descriptors of leaf shape, we found that in the chemically defended P. colorata, leaves were morphologically distinct from all of the neighbouring species except for the undefended A. pusilla. Alseuosmia pusilla individuals were more similar to neighbouring than to distant P. colorata, and 90% of leaf shape variation in the two species varied similarly across an altitudinal gradient. The data are consistent with Batesian mimicry, wherein the conspicuous characteristic of a defended model is replicated by an undefended mimic across its entire growing range. Our study provides the first detailed and powerful quantitative leaf shape evidence of leaf shape being matched between an undefended plant species to a chemically defended unrelated species across a shared growing range, and highlights the importance of using a spatially explicit morphometric method when investigating leaf shape, especially in relation to plant mimicry.