The distribution of diversity: challenges and applications
59
The typical reliance of such estimates on the dominant island biogeographical paradigm of the last half
century, MacArthur & Wilson ’ s (1967) equilibrium
theory, is discussed further in Chapter 8 , but for present
purposes it is worth noting that estimates following
this rationale involve a variety of ecological methods
and assumptions (e.g. Wilson, 1992 ; Thomas et al .,
2004 ), yet tend to make use of the same generalization
regarding the relationship between the loss of habitat
(area) and inferred loss of species, that the slope (z) of
the species – area relationship expressed by the power
model (S = cA
z , where S = species number, A = Area,
and c and z are constants) can be approximated as
0.25. As Gershwin put it in the song from Porgy and
Bess, ‘ It ain ’ t necessarily so … ’ (Chapter 8 ; Thuiller
et al ., 2004 ; Whittaker & Fern á ndez - Palacios, 2007 ).
2 Wallacean extinctions is a term given by Ladle & Jepson
(2008) to the apparent extinction of a species, whereby
the species can no longer be detected in areas in which
it previously occurred, but where there is a chance of it
persisting in areas within the range that have either
knowledge of tropical tree diversity to arrive at a global
species number fi gure in the region of 30 million.
Indeed, a fi gure of this order of magnitude is implicit
in many estimates used by scientists in popular discourse about extinction rates. By starting with a global
estimate of this magnitude, and then eating away at it
by using estimates of habitat loss (e.g. rates of tropical
deforestation) according to a given species – area relationship, it is possible to derive startling fi gures. Such
estimates are characteristically provided in terms of
numbers of species that are being lost per annum,
often expressed in relation to the size of a country or,
if using a shorter time frame, the size of a football
pitch. When projected into the future, round number
dates such as 2020 or 2050 are used, which allow
fi gures in the order of a million or several million to be
deployed within a time frame of relevance to the
human life span; such estimates grab the attention of
the media (Ladle et al ., 2004 ), signifying the risk of
serious losses and the need for action, but are also easy
to criticize scientifi cally.
Table 4.2 Typology and defi nitions of extinction (modifi ed from Ladle & Jepson, 2008 ) .
Type
Defi nition
Linnean extinction
Extinctions of undiscovered species inferred from the species – area relationship and
estimates of species diversity for a given ecosystem or region. The assumed losses of
these inferred species are termed Centinelan extinctions by Wilson (1992) .
Wallacean
extinction
Species that have not been documented for many years but in which extinction is
uncertain because populations might survive in areas that have not been surveyed
within the potential distributional range.
Phoenix extinction
Extinct in wild but genetic material available in the form of stored material or closely
related conspecifi c or congeneric variety/breed/hybrid, allowing the possibility of a
future reintroduction of the same or a functionally equivalent form.
Ecological
extinction
Extinct in the wild but with extant captive bred population, or present in the wild but at
such low densities that it no longer interacts to a meaningful degree with other species
in the community (i.e. it is functionally extinct).
Local extinction
Extinct in the wild within a clearly defi ned geographic area but with extant free - living
populations outside that area.
True extinction 1:
Contemporary
extinction
Extinction since the birth of the international conservation movement (mid - 19th century).
Last known population has been monitored and surveyed and is now considered
globally extinct in the wild. No captive - bred population or genetic material available.
True extinction 2:
Historical
extinction
Extinction prior to the birth of the international conservation movement. No
authenticated record of an extant population. No captive - bred population or viable
genetic material available.
59
The typical reliance of such estimates on the dominant island biogeographical paradigm of the last half
century, MacArthur & Wilson ’ s (1967) equilibrium
theory, is discussed further in Chapter 8 , but for present
purposes it is worth noting that estimates following
this rationale involve a variety of ecological methods
and assumptions (e.g. Wilson, 1992 ; Thomas et al .,
2004 ), yet tend to make use of the same generalization
regarding the relationship between the loss of habitat
(area) and inferred loss of species, that the slope (z) of
the species – area relationship expressed by the power
model (S = cA
z , where S = species number, A = Area,
and c and z are constants) can be approximated as
0.25. As Gershwin put it in the song from Porgy and
Bess, ‘ It ain ’ t necessarily so … ’ (Chapter 8 ; Thuiller
et al ., 2004 ; Whittaker & Fern á ndez - Palacios, 2007 ).
2 Wallacean extinctions is a term given by Ladle & Jepson
(2008) to the apparent extinction of a species, whereby
the species can no longer be detected in areas in which
it previously occurred, but where there is a chance of it
persisting in areas within the range that have either
knowledge of tropical tree diversity to arrive at a global
species number fi gure in the region of 30 million.
Indeed, a fi gure of this order of magnitude is implicit
in many estimates used by scientists in popular discourse about extinction rates. By starting with a global
estimate of this magnitude, and then eating away at it
by using estimates of habitat loss (e.g. rates of tropical
deforestation) according to a given species – area relationship, it is possible to derive startling fi gures. Such
estimates are characteristically provided in terms of
numbers of species that are being lost per annum,
often expressed in relation to the size of a country or,
if using a shorter time frame, the size of a football
pitch. When projected into the future, round number
dates such as 2020 or 2050 are used, which allow
fi gures in the order of a million or several million to be
deployed within a time frame of relevance to the
human life span; such estimates grab the attention of
the media (Ladle et al ., 2004 ), signifying the risk of
serious losses and the need for action, but are also easy
to criticize scientifi cally.
Table 4.2 Typology and defi nitions of extinction (modifi ed from Ladle & Jepson, 2008 ) .
Type
Defi nition
Linnean extinction
Extinctions of undiscovered species inferred from the species – area relationship and
estimates of species diversity for a given ecosystem or region. The assumed losses of
these inferred species are termed Centinelan extinctions by Wilson (1992) .
Wallacean
extinction
Species that have not been documented for many years but in which extinction is
uncertain because populations might survive in areas that have not been surveyed
within the potential distributional range.
Phoenix extinction
Extinct in wild but genetic material available in the form of stored material or closely
related conspecifi c or congeneric variety/breed/hybrid, allowing the possibility of a
future reintroduction of the same or a functionally equivalent form.
Ecological
extinction
Extinct in the wild but with extant captive bred population, or present in the wild but at
such low densities that it no longer interacts to a meaningful degree with other species
in the community (i.e. it is functionally extinct).
Local extinction
Extinct in the wild within a clearly defi ned geographic area but with extant free - living
populations outside that area.
True extinction 1:
Contemporary
extinction
Extinction since the birth of the international conservation movement (mid - 19th century).
Last known population has been monitored and surveyed and is now considered
globally extinct in the wild. No captive - bred population or genetic material available.
True extinction 2:
Historical
extinction
Extinction prior to the birth of the international conservation movement. No
authenticated record of an extant population. No captive - bred population or viable
genetic material available.