Insular Woodiness

Insular gigantism and insular dwarfism has occured (and is occuring) on isolated islands. If there are no predators and enough food, mammals can grow to enormous sizes. If there are predators and not enough food, mammals tend to shrink in size.

But what happens to plants on isolated islands, you could wonder.

Insular woodiness, the evolution of woodiness in plant species that inhabit islands, is the most conspicuous aspect of island floras, comparable to flightlessness in birds and dwarfism and gigantism in mammals.

A recent scientific report identified more than 1,000 insular woody species on 375 islands resulting from at least 175 independent evolutionary transitions on 31 different archipelagos around the world[1]. In combination with global data on island species composition, climate, and environment conditions, the researchers tested multiple longstanding hypotheses regarding the origins of insular woodiness and find that the absence of herbivores and traits related to drought stress are best correlated with the occurrence of insular woody species across all islands.

Structural equation models reveal that the insular woody species richness on oceanic islands correlates with a favorable aseasonal climate, followed by increased drought and island isolation (approximating competition). When continental islands are also included, reduced herbivory pressure by large native mammals, increased drought, and island isolation are most relevant.

The results illustrate different trajectories leading to rampant convergent evolution toward Insular woodiness and further emphasize archipelagos as natural laboratories of evolution, where similar abiotic or biotic conditions replicated evolution of similar traits.

An example is thyme (Thymus vulgaris). It is a bushy, woody-based evergreen subshrub that is perfectly adapted to the sometimes arid environment of the Greek islands.

Moreover, the extensive research also offers an interesting look into the future. "In 2022 we had another dry summer in Europe. The fact that drought most likely has a positive influence on wood formation offers promising opportunities for agricultural research to secure our food production", co-author Lens explains. "Suppose you could make any non-woody crop woody through breeding. Then you would get a larger crop with a higher yield per plant. But just as important: you could make such more lignified crops more drought-resistant. we really can't ignore it in a world dealing with climate change and a growing global population."

[1] Zizka et al: The evolution of insular woodiness in Proceedings of the National Academy of Sciences of the United States of America - 2022