about 400 individuals. Genetic variation within the
population is still remarkably high (Tudge 1991).
Nonetheless, the Drosophilastudies reviewed in
Chapter 7 showed that sustained or repeated bot-
tlenecks do lead to loss of heterozygosity. Hence,
although you may be able to rescue some, perhaps
many, species from a bottleneck of short duration,
you cannot assume that this will apply to a sus-
tained or repeated bottleneck. Just because a suite
of threatened species are hanging on in their
reserves now, it doesn’t mean that the same level of
biodiversity can be sustained for a long time in a
fragmented landscape.
A further complication in assessing genetic
effects is that where a species is split into numerous
separate populations in fragmented habitats, there
may be multiple bottlenecks involved. This may
result in reduced variation withineach population
but increased genetic differentiation betweenpopu-
lations (see Leberg 1991). This may be of signifi-
cance to lengthening persistence in metapopulation
scenarios (below).
Pimmet al. (1988) have undertaken an analysis of
island turnover that illustrates how some species
traits may be important to determining minimum
population sizes. They analysed 355 populations
belonging to 100 species of British land birds on 16
islands. They found that the risk of extinction
decreases sharply with increasing average popula-
tion size. They examined extinction risk for large-
bodied species, which tend to have long lifetimes
but low rates of increase, and for small-bodied
species. They found that at population sizes of
seven pairs or below, smaller-bodied species are
more liable to extinction than larger-bodied species,
but that at larger population sizes, the reverse is
true. This can be explained as follows. Imagine that
both a large-bodied and a small-bodied species are
represented on an island by a single individual.
Both will die, but the larger-bodied species is liable
to live longer and so, on average, such species will
have lower extinction rates per unit time. On the
other hand, if the starting population is large, but it
is then subject to heavy losses, the small-bodied
species may climb more rapidly back to higher
numbers, whereas the larger-bodied species might
remain longer at low population size, and thus be
vulnerable to a follow-up event. They also found
that migrant species are at slightly greater risk of
extinction than resident species. Numerous factors
might be involved in migrant losses, such as events
taking place during their migration or in their alter-
native seasons’ range (e.g. Russell et al. 1994).
Further studies will be necessary to establish the
generality of this particular finding (see discussions
between Pimm et al. 1988; Tracy and George 1992;
Diamond and Pimm 1993; Haila and Hanski 1993;
and Rosenzweig and Clark 1994).
Isolates subject to significant environmental
change or disturbance may need to have much
larger populations than is otherwise the case to
ensure survival. A number of studies have recog-
nized the potentialof environmental catastrophes in
this context but, typically, they have not attempted
to evaluate the general significance of such catas-
trophes (see Pimm et al. 1988; Williamson 1989b;
Menges 1992; Korn 1994). Of course, it is inherently
difficult to incorporate extreme events into such
analyses. As Bibby (1995) remarks ‘how do you esti-
mate the probability of a cat being landed on a par-
ticular small island in the next five years?’ Yet, such
an event may have a crucial impact on an endan-
gered prey species.
Mangel and Tier (1994) argue that environmen-
tal catastrophes may often be more important in
determining persistence times of small popula-
tions than any other factor usually considered, and
therefore should be incorporated explicitly when
formulating conservation measures. They go on to
provide computational methods for modelling per-
sistence times which do take account of catastro-
phes and which allow for quite complicated
population dynamics. Unsurprisingly, they find
that the resulting MVPs are larger than those
derived from variants of the MacArthur and
Wilson model, or indeed other analyses of popula-
tion viability which ignore catastrophes (Fig. 10.1)
(see also Ludwig 1996).
An empirical demonstration is provided by
Hurricane Hugo, which in 1989 caused extensive
damage to the forests of the Luquillo mountains,
the home of the last remaining wild population
of the Puerto Rican parrot (Amazona vittata). In 1975
the parrot population had reached a low of 13 wildMINIMUM VIABLE POPULATIONS AND MINIMUM VIABLE AREAS 255