198
Applied island biogeography
These macroecological approaches to island data,
generated by the stimulus of the MacArthur – Wilson
theory, have promoted the wide use of species – area
curves for conservation purposes. These include predicting species endangerment globally, regionally and
locally (McDonald & Brown, 1992 ; Tilman et al. , 1994 ;
Pimm & Askins, 1995 ; Brooks & Balmford, 1996 ) as
functions of habitat loss and fragmentation; devising
general reserve - design principles (Diamond, 1975a ;
Wilson & Willis, 1975 ); and identifying conservation
targets for specifi c habitat types (Desmet & Cowling,
2004 ). Among the most controversial uses of the
species – area relationship based implicitly on ETIB is its
application in the forecasting of future species extinctions as a function of habitat loss due to factors such
as deforestation (e.g. Brooks et al. , 1997 , 2002 ) or
future climate change (e.g. Thomas et al. , 2004 – see
Box 7.3 ).
Projected extinctions based on species – area models
involve several uncertainties (Heywood et al. , 1994 ;
Whittaker & Fern á ndez - Palacios, 2007 ) and can never
completely replace species - level assessments for the
identifi cation of extinction threat (e.g. Kotiaho et al. ,
2005 ). However, for many species of conservation
concern, the collection of appropriately detailed information is an unrealistic target. It is vital, therefore, that
conservation biogeographers develop more realistic
indirect measures and theoretical projections of extinctions, based on as pragmatic a set of assumptions as
possible (May et al. , 1995 ; Laurance, 2007 ).
The wide variations in outcomes can be seen from
efforts to estimate likely extinctions arising from tropical deforestation. Results of current and future rates of
deforestation have varied dramatically, ranging from
the alarming (e.g. Ehrlich & Wilson, 1991 ) to more
modest (but still signifi cant) losses (Wright & Muller -
Landau, 2006 ), thus strongly affecting projections of
future species losses.
Recently, Wright & Muller - Landau (2006) noted
that the estimates of net tropical deforestation rates
during the 1990s differ by 250 per cent (see their Table
2). Using a number of criteria, they considered 45
humid tropical countries that support 89.6 per cent of
all extant closed tropical forest and 89.9 per cent of all
potential tropical forest cover. They concluded that
deforestation rates will decrease as population growth
slows, and that a much larger area will continue to be
forested than previous studies suggest.
Such uncertainties, along with differences arising
from choice of assumptions about species persistence
in degraded habitats, from the high sensitivity of predictions to uncertainty or errors in species – area slopes
and from large uncertainties about both the global
species totals and the geographical distribution of
biodiversity, mean that all currently available predictions of future losses inherently posses great
uncertainty (see Table 8.2 , and Chapter 7 and see
Laurance, 2007, 2008 ; Willis & Bhagwat, 2009 , for
general discussion). Although the most recent of the
estimations presented in Table 8.2 was made in 1992,
we consider the information to be useful in pointing
out the problems in predicting global extinctions that
can arise through different assumptions on a number
of critical issues.
In short, extinction rate estimates based on
species – area projections involve many uncertainties
(Heywood et al. , 1994 ). The precise form of the
relationship describing the loss of species from an
original habitat as a function of the remaining habitat
area is still an open question. There are two main
associated issues. First, many species are not restricted
to their ‘ native ’ habitat and can persist in certain
anthropogenic habitats. Second, the slope of the
species – area relationship used for the loss of total
area of a habitat is still uncertain; there is no strong
theoretical or empirical justifi cation for the use of a
‘ global ’ slope value of z = 0.25 (or any other single
value).
Whittaker & Fern á ndez - Palacios (2007) have criticized the use of SAR as a means of forecasting species
threatened by, or committed to, extinction, noting ‘ the
way in which the species – area models are used … is
conceptually decoupled from the island theory from
which it seemingly derives ’ . They argue fi rst that, a z
of 0.25 is a subjective ‘ middle ’ value to take (see discussion above about the z - values of the different SAR
categories). Second, and more crucially, this z - value
has been derived from analyses of true isolates. It
describes approximately how many species are held
in each of a series of isolates/islands of different
size. Yet, in several recent studies the z - value is applied
not to separate fragments but to an entire region
(e.g. Brooks & Balmford, 1996 ; and see also Box 10.2,
pp. 272 – 273 in Whittaker & Fern á ndez - Palacios,
2007 ).
As will be discussed in the section on nestedness
below, depending on the degree of shared species
between different habitat islands, it is possible for relatively low or very high proportions of the original
species found in a region to be represented in a
Applied island biogeography
These macroecological approaches to island data,
generated by the stimulus of the MacArthur – Wilson
theory, have promoted the wide use of species – area
curves for conservation purposes. These include predicting species endangerment globally, regionally and
locally (McDonald & Brown, 1992 ; Tilman et al. , 1994 ;
Pimm & Askins, 1995 ; Brooks & Balmford, 1996 ) as
functions of habitat loss and fragmentation; devising
general reserve - design principles (Diamond, 1975a ;
Wilson & Willis, 1975 ); and identifying conservation
targets for specifi c habitat types (Desmet & Cowling,
2004 ). Among the most controversial uses of the
species – area relationship based implicitly on ETIB is its
application in the forecasting of future species extinctions as a function of habitat loss due to factors such
as deforestation (e.g. Brooks et al. , 1997 , 2002 ) or
future climate change (e.g. Thomas et al. , 2004 – see
Box 7.3 ).
Projected extinctions based on species – area models
involve several uncertainties (Heywood et al. , 1994 ;
Whittaker & Fern á ndez - Palacios, 2007 ) and can never
completely replace species - level assessments for the
identifi cation of extinction threat (e.g. Kotiaho et al. ,
2005 ). However, for many species of conservation
concern, the collection of appropriately detailed information is an unrealistic target. It is vital, therefore, that
conservation biogeographers develop more realistic
indirect measures and theoretical projections of extinctions, based on as pragmatic a set of assumptions as
possible (May et al. , 1995 ; Laurance, 2007 ).
The wide variations in outcomes can be seen from
efforts to estimate likely extinctions arising from tropical deforestation. Results of current and future rates of
deforestation have varied dramatically, ranging from
the alarming (e.g. Ehrlich & Wilson, 1991 ) to more
modest (but still signifi cant) losses (Wright & Muller -
Landau, 2006 ), thus strongly affecting projections of
future species losses.
Recently, Wright & Muller - Landau (2006) noted
that the estimates of net tropical deforestation rates
during the 1990s differ by 250 per cent (see their Table
2). Using a number of criteria, they considered 45
humid tropical countries that support 89.6 per cent of
all extant closed tropical forest and 89.9 per cent of all
potential tropical forest cover. They concluded that
deforestation rates will decrease as population growth
slows, and that a much larger area will continue to be
forested than previous studies suggest.
Such uncertainties, along with differences arising
from choice of assumptions about species persistence
in degraded habitats, from the high sensitivity of predictions to uncertainty or errors in species – area slopes
and from large uncertainties about both the global
species totals and the geographical distribution of
biodiversity, mean that all currently available predictions of future losses inherently posses great
uncertainty (see Table 8.2 , and Chapter 7 and see
Laurance, 2007, 2008 ; Willis & Bhagwat, 2009 , for
general discussion). Although the most recent of the
estimations presented in Table 8.2 was made in 1992,
we consider the information to be useful in pointing
out the problems in predicting global extinctions that
can arise through different assumptions on a number
of critical issues.
In short, extinction rate estimates based on
species – area projections involve many uncertainties
(Heywood et al. , 1994 ). The precise form of the
relationship describing the loss of species from an
original habitat as a function of the remaining habitat
area is still an open question. There are two main
associated issues. First, many species are not restricted
to their ‘ native ’ habitat and can persist in certain
anthropogenic habitats. Second, the slope of the
species – area relationship used for the loss of total
area of a habitat is still uncertain; there is no strong
theoretical or empirical justifi cation for the use of a
‘ global ’ slope value of z = 0.25 (or any other single
value).
Whittaker & Fern á ndez - Palacios (2007) have criticized the use of SAR as a means of forecasting species
threatened by, or committed to, extinction, noting ‘ the
way in which the species – area models are used … is
conceptually decoupled from the island theory from
which it seemingly derives ’ . They argue fi rst that, a z
of 0.25 is a subjective ‘ middle ’ value to take (see discussion above about the z - values of the different SAR
categories). Second, and more crucially, this z - value
has been derived from analyses of true isolates. It
describes approximately how many species are held
in each of a series of isolates/islands of different
size. Yet, in several recent studies the z - value is applied
not to separate fragments but to an entire region
(e.g. Brooks & Balmford, 1996 ; and see also Box 10.2,
pp. 272 – 273 in Whittaker & Fern á ndez - Palacios,
2007 ).
As will be discussed in the section on nestedness
below, depending on the degree of shared species
between different habitat islands, it is possible for relatively low or very high proportions of the original
species found in a region to be represented in a