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TRENDS IN THE CAUSES OF DECLINE 303

taxa, and for example are regarded as the principal

threat to over 90% of 282 endangered plant species

on Hawaii (Cox and Enquist 2000).

In terms of habitat change, the loss of forest and

woodland is the biggest concern (Fig. 11.3). Johnson

and Stattersfield (1990) estimate that about 402

threatened species of birds are restricted to islands (a

further 67 threatened species have both island and

continental distributions). Of these 402 species, 77%

are forest dwellers, divided between 200 rain forest

(lowland and montane) species and 113 seasonal/

temperate-forest species. A recent assessment of

threatened birds in insular South-East Asia found a

close relationship between the numbers of bird

species considered threatened, and the numbers of

extinctions predicted from deforestation using a

species–area approach (Brooks et al. 1997). Although

certain caveats must be attached to such crudely

derived estimates (cf. Chapter 10; Bodmer et al. 1997),

there is no doubt that habitat loss is a key driving

force no matter which means of assessment is used.

Table 11.4Principal causes of rarity, decline, and extinction of
island endemic bird species (from data in Johnson and Stattersfield
1990)

Threat Number of species

Extinct since Rare or declining

c.AD 1600

Habitat destruction 19 206
Limited range – 165
Introduced species 34 76
Hunting 25 35
Trade – 16
Human disturbance 1 10
Natural causes 1 or 2 11
Fisheries – 2
Unknown 41 64

For many species in both ‘extinct’ and ‘rare or declining’ categories,
more than one cause has been assigned and the values given above
reflect this; the overall totals in the data set were 97 ‘extinct’ and
402 ‘rare or declining’.

Box 11.3 The brown tree snake in Guam

The most notorious recent example of the impact

that the accidental introduction of a predator can

have on an oceanic island is provided by the

brown tree snake (Boiga irregularis). A native of

the Australasian region, it was introduced

accidentally to Guam, possibly as a stowaway in

military cargo, soon after the end of the Second

World War. Guam is the largest (541 km^2 ) and

most populated (100 000 inhabitants) island of

the Marianas archipelago, in Micronesia. The

snake was first sighted in Guam in the 1950s, and

by 1968 it had spread all over the island. Its

nocturnal activity, its ability to live in close

proximity to humans and, especially, the wide

range of prey it consumes (lizards, rats, fruit bats,

birds- including native, introduced and domestic

species), enabled a demographic explosion, with

densities of 12 000 to 15 000 snakes per square

mile being reported (Patrick 2001).

Before the arrival of the brown tree snake, the

only snake on the island was a small, blind snake,

which lives in the soil and feeds on ants and

termites. The vertebrate fauna of Guam, having

evolved for millennia without such native

predators, lacked the usual defensive behaviour

mechanisms of continental forms, and this is

thought to have been an important contributory

factor to their demise (Rodda et al. 1999). Brown

tree snakes are considered responsible for the

extirpation from Guam of all its breeding

population of seabirds, as well as 10 of the 13

species of native forest birds: Guam rail (Rallus

owstoni), white-browed rail (Poliolimnas cinereus),

white-throated ground-dove (Gallicolumba

xanthonura), Mariana fruit-dove (Ptilinopus

roseicapilla), Micronesian kingfisher (Halcyon

cinnamomina), nightingale reed-warbler

(Acrocephalus luscinia), Guam flycatcher (Myiagra

freycineti), rufous fantail (Riphidura rufifrons),

cardinal honeyeater (Myzomela cardinalis), and

bridled white-eye (Zosterops conspicillatus) (Rodda

et al. 1999). Two of the extinct endemic species,

the Guam rail and the Micronesian kingfisher, are

being bred in zoos in the hope that they can