Kevin Burns examines the concept of ‘island syndrome’ in plants and animals as he attempts to solve some of the riddles of island evolution
The world’s weirdest animals live on islands. The dodo bird, perhaps the most renowned island inhabitant, was far from normal. It was big and bulky, fully flightless and totally ambivalent to predators. But the dodo actually wasn’t too unusual by island standards. Many other island species share its weird ways. Kiwi are similarly large, flightless and defenceless, as are kakapo and takahe, two similar species that also waddle around New Zealand forests.
Biologists have long been fascinated by the suite of traits that typically evolve in island birds. Collectively known as the ‘island syndrome’, these traits present several serious challenges.
First, they generate a huge headache for wildlife managers and conservationists.
Island animals often evolve in the absence of mammalian predators, so when humans arrive and bring these predators along with us, they can have a devastating effect on island species. As a result, many animals conforming to the island syndrome are now gone – eaten to extinction by continental invaders. Those that survive cling to an existence subsidised by intensive management.
Second, the island syndrome is an unsolved evolutionary riddle. Despite their morphological similarities, the dodo, Kiwi, kakapo and takahe reside on very different branches of the avian family tree. The dodo was a pigeon, Kiwi are ratites, kakapo are parrots and takahe are rails. This seems to indicate that island environments shape species in similar ways, but the reason this happens is still poorly understood.
It turns out that plants could hold the key to solving the riddles posed by island syndrome. Island plants show many of the same characteristics as island animals, and so the lessons learned from plants could help explain some of the mysteries posed by animals.
Charles Darwin argued that flightlessness in island birds results from selection favouring smaller wings, which reduces the likelihood of getting blown out to sea. However, reduced dispersal isn’t always observed in island plants. When it is, large seeds are the cause, not smaller dispersal aids (like smaller wings in birds). Therefore, evolution favours large seeds on islands, perhaps to make seedlings more competitive, rather than to reduce dispersal abilities. This suggests that island birds are flightless not because large wings are maladaptive, but because large body size is somehow more adaptive.
My research also shows that island plants lose their defences just like island animals, but the story isn’t so straightforward. Plant defences against mammalian herbivores (like thorns) are often lost on islands, but plants seem to have other forms of defence. For example, strange leaf colours and odd branch patterns are common in island plants and may have helped protect them from giant browsing birds, which were important predators prehistorically. Island birds are similar – cryptic colouration and a habit of freezing when threatened helped protect them from predatory birds in the past. However, these behaviours are next to useless when faced with introduced mammalian predators, which tend to hunt using a sense of smell.
It’s beginning to look like plants and animals walk the same evolutionary pathway on islands, leading to a unified island syndrome in all types of island life. While this helps solve some of the riddles of island evolution, it also implies that plants and animals are faced with the same dire conservation circumstances, suggesting both might need similarly close conservation attention moving into the future.
Professor Kevin Burns is author of Evolution in Isolation: The Search for an Island Syndrome in Plants (Cambridge University Press).