208
Applied island biogeography
structure changes the extinction threshold – the
amount of habitat destruction a population can tolerate – by making the threshold sensitive to the rates of
destruction.
8.3 SPECIES INCIDENCE
8.3.1 Minimum v iable p opulations, m inimum
a reas and i ncidence f unctions
In his seminal paper, Caughley (1994) identifi ed two
prevailing paradigms in conservation biology: the
‘ declining population paradigm ’ and the ‘ small population paradigm ’ . The declining population paradigm is
the identifi cation and management of the processes
that depress the demographical rate of a species and
cause its populations to decline deterministically,
whereas the small population paradigm is the study of
the dynamics of small populations that have declined
owing to some (deterministic) perturbation, and which
are more susceptible to extinction via chance (stochastic) events. These concepts underpin the formulation
of extinction - risk criteria.
Theoretical and empirical work has repeatedly
shown that, once reduced in size and geographical
range, populations face a considerably elevated risk of
extinction (MacArthur & Wilson, 1967 ). Or, as Darwin
( 1872 , p. 133) put it: ‘ Rarity … is the precursor to
extinction ’ .
There are actually several different forms of rarity
(Box 4.1 ), the most extreme form of which is when a
species is reduced to a small population entirely isolated from supplementary immigration, or indeed to
the very last such population of the species. In the late
1970s, researchers identifi ed the need to characterize
quantitatively the long - term viability of such small and
entirely isolated populations (Soul é & Wilcox, 1980 ).
This led to the concept of the minimum viable population (MVP), the smallest number of individuals
required to provide a specifi ed probability of persistence over a given period of time (Shaffer, 1981 ). For
instance, the MVP could be operationalized as ‘ the
population size required to ensure a 99 per cent probability of the species ’ population persisting for 40
generations or for 1,000 years ’ (see, e.g. Reed et al. ,
2003 ).
Theoretical estimates of MVPs typically vary from as
few as 50 to as many as 10,000 individuals, based on
the postulated effects of demographical, genetic and
consequences for producers (Oksanen & Oksanen,
2000 ).
Terborgh et al . (2001) studied a set of large predator -
free islands created by a hydroelectric impoundment in
Venezuela. The small area of the islands restricted the
predator community to species predating invertebrates
(e.g. birds, lizards, anurans and spiders) and seed predators (rodents), alongside herbivores (howler monkeys,
iguanas, and leaf - cutter ants). Predators of vertebrates
were absent, and densities of rodents, howler monkeys,
iguanas and leaf - cutter ants were found to be 10 to
100 times greater than on the nearby mainland, suggesting that larger predators normally limit their
populations.
Moreover, the densities of seedlings and saplings of
canopy trees are severely reduced on herbivore - affected
islands. Terborgh et al . (2001) found support for the
idea that hyper - abundant folivores could reduce
species - rich forests to an odd collection of herbivore -
resistant plants. The endpoint of such a process is likely
to be a biologically impoverished system.
All of the above examples suggest the existence of
taxon - and system - dependent thresholds, beyond
which species losses accelerate (Ewers & Didham,
2006 ; Whittaker & Fern á ndez - Palacios, 2007 ). Such
thresholds are highly pertinent to understanding
relaxation as a result of habitat loss and fragmentation. The most dramatic changes seem to be those
following the loss of a trophic tier, typically the loss of
top predators. However, similarly dramatic changes
can follow the addition of a tropic tier, as seen when
terrestrial vertebrate predators are introduced to
remote islands previously lacking them (Terborgh,
2010 ).
Given the central importance of the topics of habitat
fragmentation and species relaxation in predicting
current and future extinction rates, it is surprising that
more attention has not been given to experimental
analyses of threshold effects and to studies of the
timescales over which the ‘ extinction debt ’ persists
(see Simberloff & Martin, 1991 ; Brooks et al. , 1999 ;
Laurance, 2002 ). Although restricted to metapopulation model simulations (of which, more follows below),
Keymer et al . (2000) have shown that persistence in
dynamic landscapes depends on the interaction
between three factors: the amount of habitat in the
landscape; the rate of change of the amount of habitat;
and the life history of the species living in the landscape. More generally, they suggest that including
temporal considerations into models of landscape
Applied island biogeography
structure changes the extinction threshold – the
amount of habitat destruction a population can tolerate – by making the threshold sensitive to the rates of
destruction.
8.3 SPECIES INCIDENCE
8.3.1 Minimum v iable p opulations, m inimum
a reas and i ncidence f unctions
In his seminal paper, Caughley (1994) identifi ed two
prevailing paradigms in conservation biology: the
‘ declining population paradigm ’ and the ‘ small population paradigm ’ . The declining population paradigm is
the identifi cation and management of the processes
that depress the demographical rate of a species and
cause its populations to decline deterministically,
whereas the small population paradigm is the study of
the dynamics of small populations that have declined
owing to some (deterministic) perturbation, and which
are more susceptible to extinction via chance (stochastic) events. These concepts underpin the formulation
of extinction - risk criteria.
Theoretical and empirical work has repeatedly
shown that, once reduced in size and geographical
range, populations face a considerably elevated risk of
extinction (MacArthur & Wilson, 1967 ). Or, as Darwin
( 1872 , p. 133) put it: ‘ Rarity … is the precursor to
extinction ’ .
There are actually several different forms of rarity
(Box 4.1 ), the most extreme form of which is when a
species is reduced to a small population entirely isolated from supplementary immigration, or indeed to
the very last such population of the species. In the late
1970s, researchers identifi ed the need to characterize
quantitatively the long - term viability of such small and
entirely isolated populations (Soul é & Wilcox, 1980 ).
This led to the concept of the minimum viable population (MVP), the smallest number of individuals
required to provide a specifi ed probability of persistence over a given period of time (Shaffer, 1981 ). For
instance, the MVP could be operationalized as ‘ the
population size required to ensure a 99 per cent probability of the species ’ population persisting for 40
generations or for 1,000 years ’ (see, e.g. Reed et al. ,
2003 ).
Theoretical estimates of MVPs typically vary from as
few as 50 to as many as 10,000 individuals, based on
the postulated effects of demographical, genetic and
consequences for producers (Oksanen & Oksanen,
2000 ).
Terborgh et al . (2001) studied a set of large predator -
free islands created by a hydroelectric impoundment in
Venezuela. The small area of the islands restricted the
predator community to species predating invertebrates
(e.g. birds, lizards, anurans and spiders) and seed predators (rodents), alongside herbivores (howler monkeys,
iguanas, and leaf - cutter ants). Predators of vertebrates
were absent, and densities of rodents, howler monkeys,
iguanas and leaf - cutter ants were found to be 10 to
100 times greater than on the nearby mainland, suggesting that larger predators normally limit their
populations.
Moreover, the densities of seedlings and saplings of
canopy trees are severely reduced on herbivore - affected
islands. Terborgh et al . (2001) found support for the
idea that hyper - abundant folivores could reduce
species - rich forests to an odd collection of herbivore -
resistant plants. The endpoint of such a process is likely
to be a biologically impoverished system.
All of the above examples suggest the existence of
taxon - and system - dependent thresholds, beyond
which species losses accelerate (Ewers & Didham,
2006 ; Whittaker & Fern á ndez - Palacios, 2007 ). Such
thresholds are highly pertinent to understanding
relaxation as a result of habitat loss and fragmentation. The most dramatic changes seem to be those
following the loss of a trophic tier, typically the loss of
top predators. However, similarly dramatic changes
can follow the addition of a tropic tier, as seen when
terrestrial vertebrate predators are introduced to
remote islands previously lacking them (Terborgh,
2010 ).
Given the central importance of the topics of habitat
fragmentation and species relaxation in predicting
current and future extinction rates, it is surprising that
more attention has not been given to experimental
analyses of threshold effects and to studies of the
timescales over which the ‘ extinction debt ’ persists
(see Simberloff & Martin, 1991 ; Brooks et al. , 1999 ;
Laurance, 2002 ). Although restricted to metapopulation model simulations (of which, more follows below),
Keymer et al . (2000) have shown that persistence in
dynamic landscapes depends on the interaction
between three factors: the amount of habitat in the
landscape; the rate of change of the amount of habitat;
and the life history of the species living in the landscape. More generally, they suggest that including
temporal considerations into models of landscape