Just like some lizards and insects evolve to larger sizes on islands, a new study has found plant seeds do the same.
Among other things, this could help increase the weight of seeds to make sure they don’t disperse too far and end up in the ocean, says plant biologist Patrick Kavanagh, a PhD student at inst Victoria University of Wellington.
He is co-author of a new study on the effect of islands on plant seed size, published in today’s issue of Proceedings of the Royal Society B.
It is well known that islands do strange things to the size of animals. Over evolutionary time under the “island rule,” large creatures tend to become dwarfed because of smaller habitats and fewer resources.
On the other hand, fewer predators and competitors of small creatures mean they can become giant. The result includes dwarf elephants, at one extreme, and giant insects and lizards at the other.
Previous research has also suggested that the size of plants may also be affected by being on islands. For example, the leaves of plants on islands tend to be larger, and small herbaceous plants like daisies can become more woody and tree-like.
Kavanagh wondered whether the size of plant seeds would be affected as well by island life.
“The seed is a very important life stage for plants. Not only does it affect dispersal but it affects establishment of the seedling.”
Kavanagh analysed the seed size of 40 herbaceous plants, shrubs and trees from islands around New Zealand, including the Chatham Islands. He compared the seed sizes of each plant with its closest mainland relative.
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“Across the board the general pattern was an increase in seed size.”
Dispersal and establishment
Kavanagh says there are a couple of reasons why this might be the case.
First, the bigger the seed, the heavier it is and the less distance wind or animals can disperse it and he says this is important given that islands are surrounded by ocean.
“The plants’ available habitat space is smaller than if they’re on a mainland area,” says Kavanagh.
“If the seeds go too far they’ll end up in the ocean.”
Second, the bigger the seed is, the more resources it provides the seedling, and the greater chance the plant has of establishing.
“The larger seed gives rise to a larger seedling which is then more likely to outcompete its neighbours and establish and be successful,” says Kavanagh.
Even a small increase in seed size is likely to give a plant the edge over other plants on an island, he says.