Happy Star Wars Day! (May the Fourth... as in, "May the Fourth be With You..." get it?). What better day to talk about aliens... but not the Yoda kind.
- Island has unique wildlife, found nowhere else on earth
- Island is visited by humans
- Humans introduce domestic animals and/or invasive pests
- Island looses unique wildlife, now found nowhere on earth
To be sure, extinctions occur on the mainland also, both because of invasive enemies as well as other causes (habitat loss, hunting, etc), but islanders seem to be especially vulnerable. Part of it is that islands are small, and so the populations that live their tend to be small. Makes sense - smaller the land mass, smaller the number of animals it can supports, easier it is for predators to kill all of said animals off. Because the landmass is smaller, the animals that inhabit them also tend to be more specialized, which works great when conditions are stable, but is very problematic when they suddenly change.
Island animals, especially those that evolved in the absence of predators, also tend to be naive in the face of danger, a tameness that some visitors mistake for stupidity. No bird exemplifies (or exemplified, emphasis on past tense) this more than the dodo, a hulking, turkey-sized pigeon-like bird from Mauritius, off the east coast of Africa. Unaccustomed to the indignities of being hunted, the dodo waddled up the first sailor with a club, and was driven to extinction shortly after its discovery.
The dodo calls to mind another problem that many island birds face when dealing with invasive predators - many are flightless. It seems silly at first - flight is the most awesome advantage birds have over mammals. Why give up that gift? The fact is, flight is expensive. It takes a lot of energy for a bird to fly, and it requires specialized muscles and feathers that are - biologically - costly to maintain. Fear is biologically expensive, too. Imagine if you ran away from every single thing you saw that could be possibly dangerous - strangers you see while out walking, cars, unfamiliar dogs, unexplained noises - you'd run yourself ragged in no time. Time spent running is time not spent eating, or mating. If an animal has no predators in its environment, it makes sense to loose this fear, and if there is plenty of food on the ground, and no need to get up high, why bother flying? Current flightless island birds include the kakapo and kiwis of New Zealand, the flightless cormorant of the Galapagos, and the steamer ducks of the Falkland Islands.
It all makes perfect sense when you look at it naturally. The problem is that we've made this an increasingly unnatural world, and it's getting harder for specialized, predator-naive animals to survive. Invasive predators have eaten their way through populations of many island birds around the globe, driving some to extinction. Others are found only in captivity now, stranded in zoos and breeding centers until a safe release site can be located. Sometimes, this means waging costly, difficult eradication campaigns to cleanse habitats of invasive pests. In other cases, it means playing Noah's Ark and trying to establish new populations of birds on predator-free islands (which, ironically, involves introducing non-native species... again).
Some islands birds are lost and gone forever - the moas of New Zealand, the elephant birds of Madagascar - and nothing (at least right now) - can bring them. For others, however - species which are still hanging in the balance - there is still time, still a chance to pull them back from the brink.
Guam and the other Mariana Islands have birds in both categories.