Biology Reference
In-Depth Information
Fortunately, most mainland mammals—be they mongooses or monkeys—and many rep-
tiles and amphibians and even ants rarely occupied distant islands before human navigation.
The exceptions are continental islands that were once connected to a large landmass, such as
those islands that were part of the Sunda Shelf, once connecting Southeast Asia that sits west
of Wallace's Line, and another once linking Australia to New Guinea by a land bridge (see
chapter 2). There are good reasons why this is so: most terrestrial creatures simply can't swim
far enough or survive the passage from the mainland by clinging to a raft of floating veget-
ation, although snakes and lizards are much better at it than mammals. Terrestrial mammals
(with the exception of bats) and amphibians are especially poor dispersers over marine barri-
ers for another vital reason: like humans, if they drink seawater they lose more water in trying
to excrete the excess salt ingested. Here, two lines from Samuel Taylor Coleridge's
Rime of
theAncientMariner
are as apt as basic physiology to explain the conundrum: “Water, water,
everywhere, / Nor any drop to drink.” In the absence of most terrestrial mammals, different
ecological worlds evolve. For example, there are places without large cats as top predators,
such as on the islands of Komodo and New Caledonia. Komodo has a dinosaur ecosystem
where a large monitor lizard has become the apex predator, even hunting mammals. In New
Caledonia, in the absence of mammalian predators, the geckos of the island have undergone a
divergence of species, the largest of which has become the “tiger” of the island, eating smal-
ler vertebrates.
Even the most remote archipelagoes that, like Hawaii, have never been connected to a
mainland, however, experience waves of potential invasions by various species. Some come
as stowaways on human-powered vessels, like Polynesian rats, or, like the mongoose, are
deliberately introduced. But dispersal is a natural process that happens without human assist-
ance. Some colonists do survive on floating mats of vegetation, carried by the ocean's cur-
rents to their new destinations. The vast majority of natural colonizations fail, however. Most
species drown en route or die of dehydration, as already noted, or starve to death soon after
arrival, or fail to find a mate or to leave enough offspring to carry on. This chapter focuses on
the devastating effects of successful invasive species and the ecological disruption they cause
to island life, especially the rarities endemic to the islands.
Invasive species can disrupt local ecologies anywhere, but for several evolutionary reasons
they often hit remote oceanic islands especially hard. Being a great disperser allows a species
to colonize a remote island such as Hawaii. But once there, in the absence of significant
predators, natural selection strongly favors against dispersing offspring because they end up
drowning if they head out to find new places to live and breed. One outcome is that on ocean-
ic islands flightless birds and insects have repeatedly evolved. A paradox of nature: once ar-
rived on such an island, it is better to become sedentary, but as a result, those species that do
are more vulnerable to predators that are later introduced.
