0198566123.pdf

STOCHASTIC VERSUS DETERMINISTIC EXTINCTIONS 291

Although approximately 1.7 million species have
been described, typical estimates of the total diver-
sity of the planet range from 5 million to 30 million,
with figures as high as 100 million occasionally
being cited. Many of the supposed undescribed
species are thought to occur amongst invertebrate
taxa in tropical forests, and ocean floors, although
huge uncertainty remains as to how diverse these
systems really are (see, e.g. Lambshead and
Boucher 2003).
Islands, too, are part of this uncertainty, undoubt-
edly holding many species currently scientifically
unknown that are waiting to be discovered and
described. For instance, the cataloguing of the
Canarian biota has been under way since the
French priest Louis Feuillee came to the archipel-
ago in 1724. Despite almost three centuries of scien-
tific attention, species new to science continue to be
discovered, in recent decades at a rate approximat-
ing one species every 6 days (Fig. 11.1), including
two large lizards (Gallotia intermedia and G. gomer-
ana) and two trees (Myrica rivas-martineziiand
Dracaena tamaranae) (Izquierdo et al. 2004). Many
new finds have escaped detection until recently

because they persist only in small populations.

Hence, no sooner are they discovered than many

are categorized as in danger of extinction.

11.2 Stochastic versus deterministic extinctions

Two main types of extinction events operating on

islands can be differentiated (Table 11.1): largely

stochastic extinctions, inherent to the natural

dynamics of island environments, and determinis-

tic extinctions, related directly or indirectly to

human activity. In the first group we include natu-

ral disasters, such as volcanic eruptions, but also

the consequences of the taxon cycle dynamics,

which involve species moving down an evolution-

ary cul-de-sac in becoming highly restricted local

endemics, increasingly prone to natural disasters.

The second group includes habitat-loss, degrada-

tion or fragmentation, the introduction of alien

species and predation by humans, whether for the

pot or for other purposes.

When one species goes extinct, any evolutionary

interactions involving that species must also cease.

1838 Brullé

(59 species of

various insect

orders)

1839

Brullé (14

lepidoptera),

Mcquart

(16 diptera)

1852

Shuttleworth

(27 molluscs)

1862, Wollaston

(93 coleoptera)

1864, Wollaston

(224 coleoptera)

1865, Wollaston

(59 coleoptera)

1865, Mousson

(33 molluscs)

1936, Frey

(23 diptera)

1908, Walshingham

(42 lepidoptera) 1954, Lindberg(65 hemiptera)

1987, Wunderlich

(90 araneids)

1992, Wunderlich

(94 araneids)

1833 Webb &

Berthelot

(16 molluscs)

17921810182018301840 1850 1860 187018801890190019101920193019401950 1960 1970 1980 19902000

0

500

1,000

1,500

2,000

2,500

3,500

3,000

250

225

200

175

150

125

100

75

50

25

0

Years

Species

Cumulative Species Number

Figure 11.1The historical pattern of description of endemic invertebrates of the Canaries. The bars represent the number of species described
each year, and the line the cumulative total. (After Izquierdo et al. 2001.)