Conservation planning in a changing world
209
accumulate so - called deleterious genes, i.e. genes that
reduce survival or fertility (see Caughley, 1994 ).
A further complication in assessing genetic effects
of fragmentation 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 within each population, but increased genetic differentiation between
populations (see Leberg, 1991 ). The viability of an
isolated population may also be infl uenced by the
occurrence of environmental change or disturbance,
and indeed it has been argued that it is critical to
take such environmental catastrophes and fl uxes into
account when estimating the MVP and designing
conservation measures based around protecting
such small, endangered populations (Mangel & Tier,
1994 ).
An example of synergetic effects of a catastrophic
event and inbreeding is provided by song sparrows
( Melospiza melodia ) living on Mandarte Island in
western Canada. The inbred birds died at a much
higher rate during a severe storm than did outbred
birds (Keller et al. , 1994 ). Although the severe weather
was what caused this mortality, it appeared that
inbreeding determined, in part, which individuals survived the storm.
Recently, Reed et al . (2003) considered the effects of
age structure, catastrophes, demographical and environmental stochasticity, and inbreeding depression, to
derive MVP estimates for 102 vertebrate species. They
defi ned an MVP as ‘ one with a 99 per cent probability
of persistence for 40 generations ’ . Across this data set,
mean and median estimates of MVP were 7,316 and
5,816 adults, respectively. The estimated values did not
differ systematically between major taxa, or with
trophic level or latitude, but were negatively correlated
with population growth rate. Reed et al . (2003) stress
that although MVPs provide a useful rule of thumb for
species conservation (which is that the size of vertebrate populations needed for successful long - term
conservation is about 7,000 adults), MVPs should not
be used as precise conservation targets. For further
discussion see also Brook et al . ’ s (2006) study on the
MVP of 1,198 species.
Closely related to the concept of MVP is the idea of
the minimum viable area (MVA). For some species, e.g.
snail populations, a fairly small area may suffi ce to
maintain the requisite number of individuals. Species
of higher trophic levels generally require more area or
space to ensure good survival prospects. It has been
environmental variation (Reed et al. , 2003 ; Brook
et al. , 2006 ), with the available empirical evidence
pointing to the upper end of this range (e.g. Reed et al. ,
2003 ). It has been estimated that the maximum tolerable rate of inbreeding is 1 per cent per generation,
which has in turn been translated to approximately 50
individuals to ensure short - term fi tness (see Shafer,
1990 ). However, typically only a proportion of the
adult population participates in breeding and it is these
animals that form the effective population size, which
is often substantially smaller than the total population
size (Shafer, 1990 ; see Crandall et al. , 1999 , for discussion on the concept).
A study of grizzly bears in the Yellowstone National
Park showed that to prevent inbreeding rates exceeding
1 per cent required an overall population size of at least
220 rather than 50 animals (Shafer, 1990 ). Further
rule of thumb estimates have been collated by
Frankham et al . ( 2002 ; their Table 14.1, p. 339) as
follows: the population numbers required to avoid
inbreeding depression and to retain fi tness in the short
term, > 50; to retain evolutionary potential, 500 –
5,000; and to avoid the accumulation of deleterious
mutations, 12 to 1,000 individuals.
Attempts to calculate the viability of single populations (i.e. whether the population is likely to persist for
a given period of time) are referred to as population
viability analyses (PVA) (see Reed et al. , 2003 ). PVA
can take into account the combined impacts of stochastic factors (demographical, environmental and
genetic stochasticity) and deterministic factors (e.g.
habitat loss, over - exploitation). According to Brook
et al . (2006) , PVA and the threat categories of IUCN
(Box 4.1 ) each offer an assessment of a species ’ probability of extinction based on its current population
size and structure and the characteristics of the threatening processes it faces. On the other hand, the main
feature of MVP analysis is that the risk of extinction is
fi xed and the critical question asked is how large a
population must be to avoid this risk.
Demographical stochasticity of initially small populations can lead to losses from a series of isolates
without a need to invoke any specifi c mechanism such
as predation or loss of fi tness. However, where small
populations persist for a reasonable length of time (e.g.
several generations), they may also lose genetic variability as they pass through bottlenecks. They may then
lose fi tness by lacking the genetic fl exibility to cope
with either the normal fl uctuations of environment
or an altered environment, and they may also
209
accumulate so - called deleterious genes, i.e. genes that
reduce survival or fertility (see Caughley, 1994 ).
A further complication in assessing genetic effects
of fragmentation 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 within each population, but increased genetic differentiation between
populations (see Leberg, 1991 ). The viability of an
isolated population may also be infl uenced by the
occurrence of environmental change or disturbance,
and indeed it has been argued that it is critical to
take such environmental catastrophes and fl uxes into
account when estimating the MVP and designing
conservation measures based around protecting
such small, endangered populations (Mangel & Tier,
1994 ).
An example of synergetic effects of a catastrophic
event and inbreeding is provided by song sparrows
( Melospiza melodia ) living on Mandarte Island in
western Canada. The inbred birds died at a much
higher rate during a severe storm than did outbred
birds (Keller et al. , 1994 ). Although the severe weather
was what caused this mortality, it appeared that
inbreeding determined, in part, which individuals survived the storm.
Recently, Reed et al . (2003) considered the effects of
age structure, catastrophes, demographical and environmental stochasticity, and inbreeding depression, to
derive MVP estimates for 102 vertebrate species. They
defi ned an MVP as ‘ one with a 99 per cent probability
of persistence for 40 generations ’ . Across this data set,
mean and median estimates of MVP were 7,316 and
5,816 adults, respectively. The estimated values did not
differ systematically between major taxa, or with
trophic level or latitude, but were negatively correlated
with population growth rate. Reed et al . (2003) stress
that although MVPs provide a useful rule of thumb for
species conservation (which is that the size of vertebrate populations needed for successful long - term
conservation is about 7,000 adults), MVPs should not
be used as precise conservation targets. For further
discussion see also Brook et al . ’ s (2006) study on the
MVP of 1,198 species.
Closely related to the concept of MVP is the idea of
the minimum viable area (MVA). For some species, e.g.
snail populations, a fairly small area may suffi ce to
maintain the requisite number of individuals. Species
of higher trophic levels generally require more area or
space to ensure good survival prospects. It has been
environmental variation (Reed et al. , 2003 ; Brook
et al. , 2006 ), with the available empirical evidence
pointing to the upper end of this range (e.g. Reed et al. ,
2003 ). It has been estimated that the maximum tolerable rate of inbreeding is 1 per cent per generation,
which has in turn been translated to approximately 50
individuals to ensure short - term fi tness (see Shafer,
1990 ). However, typically only a proportion of the
adult population participates in breeding and it is these
animals that form the effective population size, which
is often substantially smaller than the total population
size (Shafer, 1990 ; see Crandall et al. , 1999 , for discussion on the concept).
A study of grizzly bears in the Yellowstone National
Park showed that to prevent inbreeding rates exceeding
1 per cent required an overall population size of at least
220 rather than 50 animals (Shafer, 1990 ). Further
rule of thumb estimates have been collated by
Frankham et al . ( 2002 ; their Table 14.1, p. 339) as
follows: the population numbers required to avoid
inbreeding depression and to retain fi tness in the short
term, > 50; to retain evolutionary potential, 500 –
5,000; and to avoid the accumulation of deleterious
mutations, 12 to 1,000 individuals.
Attempts to calculate the viability of single populations (i.e. whether the population is likely to persist for
a given period of time) are referred to as population
viability analyses (PVA) (see Reed et al. , 2003 ). PVA
can take into account the combined impacts of stochastic factors (demographical, environmental and
genetic stochasticity) and deterministic factors (e.g.
habitat loss, over - exploitation). According to Brook
et al . (2006) , PVA and the threat categories of IUCN
(Box 4.1 ) each offer an assessment of a species ’ probability of extinction based on its current population
size and structure and the characteristics of the threatening processes it faces. On the other hand, the main
feature of MVP analysis is that the risk of extinction is
fi xed and the critical question asked is how large a
population must be to avoid this risk.
Demographical stochasticity of initially small populations can lead to losses from a series of isolates
without a need to invoke any specifi c mechanism such
as predation or loss of fi tness. However, where small
populations persist for a reasonable length of time (e.g.
several generations), they may also lose genetic variability as they pass through bottlenecks. They may then
lose fi tness by lacking the genetic fl exibility to cope
with either the normal fl uctuations of environment
or an altered environment, and they may also