mammals, and 18 reptiles. Across this data set,
mean and median estimates of MVP were 7316 and
5816 adults, respectively. The values estimated did
not differ systematically between major taxa, or
with trophic level or latitude, but unsurprisingly
were negatively correlated with population growth
rate. Interestingly, they also found that MVPs based
on shorter studies systematically underestimated
extinction risk, which appeared to be due to the
greater temporal variation in population size
captured in studies of longer duration. Although
Reedet al. (2003) stress that the values they
obtained do not provide a ‘magic number’, they do
serve to indicate the rough size of vertebrate popu-
lations that may be needed for successful conserva-
tion, i.e. about 7000 adults, which is higher than
most previous estimates. In many cases, the area
required for a large vertebrate population of this
size to be contained in a single reserve will be unob-
tainable, requiring conservation strategies that
either allow population exchange to occur between
major reserves, or that are not dependent on
reserve systems.
How big an area?
How does a figure for a MVP translate into a
minimum viable area(MVA)? With some species
and communities, e.g. butterfly populations, a
fairly small area may suffice to maintain the requi-
site number of individuals. In general, the higher
up the trophic chain, the larger the area needed.
Simply considering the home range of a few verte-
brates is illustrative. It has been calculated that a
single pair of ivory-billed woodpeckers (Campephilus
principalis) may require 6.5–7.6 km^2 of appropriate
forest habitat; that the European goshawk (Accipiter
gentilis) has a home range of about 30–50 km^2 ; and
that male mountain lions (Felis concolor) in the west-
ern United States may have home ranges in excess
of 400 km^2 (Wilcove et al. 1986). Even some plant
and insect species may need surprisingly large
areas, as illustrated by Mawdsley et al.’s (1998)
analyses of strangler fig species, which indicate that
the MVA for some species may be as large as
200 000 ha because of their low population densi-
ties. Thus, for many species, reserves must be really
rather large if their purpose is to maintain a MVP
entirely within their bounds.
There is of course, one very large proviso
attached to the calculation of a minimum critical
area. The approach assumes that the area con-
cerned acts rather like a large enclosed paddock in
a zoo, with freedom of association within it, but no
exchange with any other paddocks or zoos. Unless
the reserve does indeed contain the only popula-
tion of the species, or they are completely immobile
creatures, there is always the possibility of immi-
gration of other individuals from outside the
reserve. Where a significant degree of exchange
takes place, this may buffer populations from
genetic erosion and extinction, such that the con-
cept of a fixed critical minimum area loses its mean-
ing. In practice, this must be assessed on a taxon by
taxon, and probably species by species, basis.Applications of incidence functions
Area requirements of particular species can be
examined by means of incidence functions estimat-
ing the probability of a species occurring for habitat
islands of given combinations of area and isolation
(Box 5.2; Wilcove et al. 1986; Watson et al. 2005). The
information returned from such incidence func-
tions is not equivalent to estimating MVAs. This is
because the incidence functions constitute the
product of population exchange within the net-
work of suitable habitats within a region. They
therefore tell you the properties of ‘islands’ on
which a target species currently occurs, not those
on which it may persist in the long term, or in an
altered ecological context.
Simberloff and Levin (1985) examined the inci-
dence of indigenous forest-dwelling birds of a
series of New Zealand islands, and of passerines of
the Cyclades archipelago (Aegean Sea). In both
systems they found that most species occur remark-
ably predictably, with each species occupying all
those and only those islands larger than some
species-specific minimum area. A minority of
species in each avifauna did not conform to this
pattern, possibly because of habitat differences
among islands and because of anthropogenic
extinctions. Although stochastic turnover might beMINIMUM VIABLE POPULATIONS AND MINIMUM VIABLE AREAS 257