Island gigantism is a phenomenon sometimes seen in animal evolution in such species as the Komodo dragon, Madagascar’s extinct elephant bird and New Zealand’s extinct moa. Researchers from Victoria University of Wellington wanted to see whether the same type of gigantism could also be seen in plants and were able to show that plants that have evolved in island settings grow larger seeds than their mainland relatives.
PhD student Patrick Kavanagh and Associate Professor Kevin Burns from Victoria’s School of Biological Sciences measured the seed sizes of 40 plant species endemic to four island groups in the South Pacific surrounding New Zealand, including the Three Kings Islands to the north, the Kermadec Islands to the north-east, the Chatham Islands to the east, and two sub-Antarctic islands to the south – Auckland and Campbell Islands. The seed sizes of the plants were then compared to seed sizes from equivalent plant species measured in various locations around New Zealand. To supplement the field data, seed measurements from Seeds of New Zealand Gymnosperms and Dicotyledons (aka the New Zealand seed atlas) were also used to obtain median sizes.
The researchers showed that island plants consistently produced larger seeds than their mainland counterparts irrespective of seed dispersal modes (wind, water, fleshy fruit) and growth forms. In addition, the differences became more pronounced the larger the fruit, indicating that island gigantism occurs in plants as well as animals.
For example, island Corokia macrocarpa, a shrub with fleshy fruit, had seeds measuring in at 40.44 mm2 – more than twice the size of its mainland counterpart Corokia cotoneaster, with seeds measuring in at 18.01 mm2. Another fleshy fruit tree, Rhopalostylis aff. sapida, the Chatham Island nīkau, had seeds 175.83 mm2 – more than twice the size of the mainland nīkau Rhopalostylis sapida seeds at 80.79 mm2.
The researchers suggest that a possible reason the plants evolve larger seeds is to prevent them travelling too far and getting lost in the unfavourable conditions of the ocean. Another evolutionary advantage the authors suggest is that larger seeds tend to generate larger seedlings, “which are more likely to establish and outcompete neighbours”.
The authors write that several other plant traits are known to evolve on islands. “For example, herbaceous species often evolve to become woody, and species frequently evolve larger leaves, regardless of growth form.”
The research was published in the 20 May 2014 edition of the Proceedings of the Royal Society B: Biological Sciences.
Kavanagh, P.H. & Burns, K.C. (2014). The repeated evolution of large seeds on islands. Proc. R. Soc. B., 0675. http://dx.doi.org/10.1098/rspb.2014.0675