birds, as a result of a combination of habitat
destruction, capture for pets, and slaughter (for
food and to protect crops). Given protection, the
population built up from this low point, to pre-
hurricane levels of 45–47, from which it was
reduced by the hurricane to about 25 birds (Wilson
et al. 1994). Intriguingly, post-hurricane conditions
may have suited the parrots, as 6 pairs bred in 1991,
the highest total since the 1950s, and 11 young
fledged the following year. The post-breeding
population thus swiftly climbed to between 34 and
37 parrots. However, the number of birds is clearly
far too small for comfort, and the species could
easily be pushed to extinction by repeated cata-
strophic hurricanes.
As endemic species tend not to occur on the
smallest of islands, extinctions in the global sense
caused primarily by natural disturbances are likely
to be rare events on ecological timescales, unless
the species concerned has already been reduced in
range (Hilton et al. 2003). One possible example of a
species finished off in this way, by hurricanes in
1899, is the St Kitts subspecies of bullfinch Loxigilla
portoricensis grandis, although the last record of this
bird was actually several years earlier, in 1880
(Williamson 1989b). A more recent example of a
species reduced to the status ‘critically threatened’
is the Montserrat oriole (Icterus oberi), which
although confined to only 30 km^2 of forest within
the island of Montserrat, was not considered glob-
ally threatened before the eruption of the Soufriere
Hills volcano in 1995. The eruptive activity (as often
the case) spanned several years (indeed it had not
ceased as of the end of 2005), devastating much of
the forest habitat of the oriole. The dramatic popu-
lation decline extended even to the small areas of
intact forest, suggesting knock-on consequences
of the eruption such as reduced food supplies or
increasing nest predation. It proved difficult to esti-
mate the oriole population but it is thought that it
had declined from several thousand individuals
before the eruptions to perhaps 100–400 pairs in the
year 2000 (Hilton et al. 2003). The populations are
currently being closely monitored (www.rspb.
org.uk/science/diaries/montserrat/index.asp).
In order to make MVAs more realistic, efforts
have now been made to include data on age struc-
ture, catastrophes, demographic and environmental
stochasticity, and inbreeding depression. Reed et al.
(2003) use this approach to derive MVP estimates
for 102 vertebrate species, using a working defini-
tion of MVP as ‘one with a 99% probability of per-
sistence for 40 generations’. Their chosen species
included two amphibians, 28 birds, one fish, 53256 ISLAND THEORY AND CONSERVATION
Time (t)Population size at timet, N(t)0 100
Time (t)0 10050 40(a) (b)Figure 10.1Simulation runs of population trajectories. (a) In the absence of catastrophes all five trajectories persist for the 100 time units
shown. (b) In the presence of catastrophes two populations become extinct and two are at low population sizes after 100 time units. (From
Mangel and Tier 1994, Fig. 5; original figure supplied by M. Mangel.)