The Ecology Book

148

they counted returning species

to observe their recolonization.

The Florida Keys experiment

showed that distance did indeed

play an important role: the farther

an island was from the mainland,

the fewer invertebrates returned to

recolonize the area.

New waves of immigration can,

however, save even faraway island

species from extinction. This is

more likely to happen with certain

bird species—which can travel long

distances quickly—than with, for

example, small mammals. There is

also the so-called target effect,

where some islands are more

favored destinations because of the

habitat they provide. Given the

choice of a treeless island and one

with woodland, a tree-nesting bird

will naturally opt for trees.

Human impact

The key factors influencing the

species mix on an oceanic island

are its degree of isolation, how long

it has been isolated, its size, the

suitability of its habitat, its location

relative to ocean currents, and

chance arrivals (for example,

organisms washed up on mats of

ISLAND BIOGEOGRAPHY

floating vegetation). Most of these

factors apply to any similar, isolated

habitats, not just actual islands.

The impact of humans—who

probably began visiting isolated

islands in the Pacific at least 3,000

years ago—has sometimes been

dramatic. In recent centuries,

people took dogs, cats, goats,

and pigs with them when they

colonized islands in the Pacific

and elsewhere; inadvertently, they

also carried rats on their boats.

On many islands, rats ate the eggs

of breeding seabirds and the

seeds of endemic plants, some of

which grew nowhere else. On the

Galapagos Islands, dogs ate

tortoise eggs, native iguanas, and

even penguins. Goats competed

with Galapagos tortoises for food

and wiped out up to five species of

plant on the Santiago Islands.

The arrival of humans, however,

has not always reduced species

richness on islands. Researchers

discovered the important role of

ship-assisted colonization of

islands in the Caribbean. Despite

its relatively small size, Trinidad,

for instance, has more species of

anole lizards than the much larger

island of Cuba, because economic

sanctions since the 1960s have

meant that fewer boats (and their

lizard stowaways) dock in Cuba.

“Island” habitats

In the early 1970s, American

biologist James H. Brown applied

the Wilson–MacArthur model to

“islands” of coniferous forest on 19

mountain ridges in the Great Basin

of California and Utah. The ridges

are separated from each other by

a vast sagebrush desert. Brown

discovered that the diversity and

distribution of small mammals

(excluding bats) in the isolated

forests could not be explained in

terms of an equilibrium between

colonization and extinction. Some

species had become extinct, but

no new species had arrived for

millions of years, so Brown dubbed

the mammals “relicts.” A few years

later, his analysis of resident bird

populations on the ridges revealed

that new bird species had arrived

from larger, similar forests in the

Rocky Mountains to the east and

in the Sierra Nevada to the west.

Brown concluded that certain

species groups—especially those

that fly—are more likely to be

successful immigrants than others.

Destroying rainforest

for economic gain

is like burning a

Renaissance painting

to cook a meal.

Edward O. Wilson

Islands are

surrounded by water.

The water acts as

a physical barrier,

preventing many species

from entering or leaving.

As species arrive

and depart, island

populations evolve.

Islands are

ecological systems.

US_144-149_Island_biogeography.indd 148 12/11/18 6:25 PM