204
Applied island biogeography
Box 8.2 Extinction d ebt in the Azores
Box prepared by K.A. Triantis and R.J. Whittaker – excerpted and modifi ed slightly from Triantis
et al . (2010) . See the source paper for a full analytical presentation of the materials and methods
used .
Habitat destruction is considered to be the leading cause of terrestrial species extinctions. However, there is
typically a time lag between the reduction in habitat area and the eventual disappearance of the remnant populations. These ‘ surviving but ultimately doomed ’ species represent an extinction debt. Calculating the magnitude
of such future extinction events has been hampered by potentially inaccurate assumptions about the slope of
species – area relationships, which are habitat - and taxon - specifi c [see text]. We have overcome this challenge
by introducing a novel method that uses the historical sequence of deforestation in the Azorean Islands to calculate realistic and ecologically - adjusted species – area relationships.
The Azores constitute an ideal model system for assessing extinction debt because:
1 they have lost more than 90 per cent of their original native forest during the fi ve centuries of human
occupation;
2 being one of the most isolated archipelagos on Earth they support a signifi cant number of single island
endemics (SIE);
3 the history of human settlement and deforestation is well known;
4 extensive biogeographical data exist for a range of taxa.
The Azorean Islands were discovered in AD 1432 by Portuguese explorers, and more than 500 years of human
settlement have taken their toll on the local fauna and fl ora, 420 species of which are endemic to the archipelago.
Today, approximately 70 per cent of the vascular plants and 58 per cent of the arthropods found in the Azores
are exotic, many of them invasive.
The destruction of the native ‘ laurisilva ’ , a humid evergreen broadleaf laurel forest, in the Azores has followed
a clear temporal sequence. At the time of human colonization (c. AD 1440), the archipelago was almost entirely
covered by forest. By 300 years ago (c. AD 1700) human activities had restricted the native forest in most
islands to areas above 300 m a.s.l. and, by c. AD 1850, areas with native forest were present only above
500 m a.s.l. The development of an economy dependent on milk production during the last decades of the 20th
century drove a further reduction of native forest area, to 2.5 per cent of the total area of the archipelago (Figure
B8.2a ).
The Azorean arthropod fauna has been intensively sampled during the last ten years. The Borges et al . (2005)
checklist includes virtually all arthropod species native to the Azores, as well as an accurate description of their
presence or absence in all the islands of the archipelago. The endemic arthropods belonging to three groups
– Araneae, Hemiptera and Coleoptera – were classifi ed as native forest dependent and non - forest dependent
species, and only the forest dependent species endemic to the archipelago were considered for further
analyses.
We used four different ‘ habitat areas ’ to calculate our species – area relationships: these were chosen to correspond to the extent of native forest at four known points in time before and since human colonization ( ≈ AD
1440, AD 1700, AD 1850, AD 2000; Figure B8.2a ). Although the historical estimates of forest cover are crude
approximations, we consider that they are accurate enough to provide a baseline for estimating the present
extinction debt.
Our analyses follow the rationale that if species ‘ relaxation ’ has not yet taken place or is incomplete (i.e. the
extinction debt has not yet been paid), then the best fi tting species – area model will correspond not to present
forest area but to a past baseline – a hypothetical dynamic equilibrium from which the system has since departed.
However, there is a complication in dealing with a system of endemic species on oceanic islands of varying
age, namely that the dynamics of colonization, speciation and extinction may be at different points, depending
on the age of the island. Accordingly, we fi tted and compared both species – area and species – area – time
models.
Applied island biogeography
Box 8.2 Extinction d ebt in the Azores
Box prepared by K.A. Triantis and R.J. Whittaker – excerpted and modifi ed slightly from Triantis
et al . (2010) . See the source paper for a full analytical presentation of the materials and methods
used .
Habitat destruction is considered to be the leading cause of terrestrial species extinctions. However, there is
typically a time lag between the reduction in habitat area and the eventual disappearance of the remnant populations. These ‘ surviving but ultimately doomed ’ species represent an extinction debt. Calculating the magnitude
of such future extinction events has been hampered by potentially inaccurate assumptions about the slope of
species – area relationships, which are habitat - and taxon - specifi c [see text]. We have overcome this challenge
by introducing a novel method that uses the historical sequence of deforestation in the Azorean Islands to calculate realistic and ecologically - adjusted species – area relationships.
The Azores constitute an ideal model system for assessing extinction debt because:
1 they have lost more than 90 per cent of their original native forest during the fi ve centuries of human
occupation;
2 being one of the most isolated archipelagos on Earth they support a signifi cant number of single island
endemics (SIE);
3 the history of human settlement and deforestation is well known;
4 extensive biogeographical data exist for a range of taxa.
The Azorean Islands were discovered in AD 1432 by Portuguese explorers, and more than 500 years of human
settlement have taken their toll on the local fauna and fl ora, 420 species of which are endemic to the archipelago.
Today, approximately 70 per cent of the vascular plants and 58 per cent of the arthropods found in the Azores
are exotic, many of them invasive.
The destruction of the native ‘ laurisilva ’ , a humid evergreen broadleaf laurel forest, in the Azores has followed
a clear temporal sequence. At the time of human colonization (c. AD 1440), the archipelago was almost entirely
covered by forest. By 300 years ago (c. AD 1700) human activities had restricted the native forest in most
islands to areas above 300 m a.s.l. and, by c. AD 1850, areas with native forest were present only above
500 m a.s.l. The development of an economy dependent on milk production during the last decades of the 20th
century drove a further reduction of native forest area, to 2.5 per cent of the total area of the archipelago (Figure
B8.2a ).
The Azorean arthropod fauna has been intensively sampled during the last ten years. The Borges et al . (2005)
checklist includes virtually all arthropod species native to the Azores, as well as an accurate description of their
presence or absence in all the islands of the archipelago. The endemic arthropods belonging to three groups
– Araneae, Hemiptera and Coleoptera – were classifi ed as native forest dependent and non - forest dependent
species, and only the forest dependent species endemic to the archipelago were considered for further
analyses.
We used four different ‘ habitat areas ’ to calculate our species – area relationships: these were chosen to correspond to the extent of native forest at four known points in time before and since human colonization ( ≈ AD
1440, AD 1700, AD 1850, AD 2000; Figure B8.2a ). Although the historical estimates of forest cover are crude
approximations, we consider that they are accurate enough to provide a baseline for estimating the present
extinction debt.
Our analyses follow the rationale that if species ‘ relaxation ’ has not yet taken place or is incomplete (i.e. the
extinction debt has not yet been paid), then the best fi tting species – area model will correspond not to present
forest area but to a past baseline – a hypothetical dynamic equilibrium from which the system has since departed.
However, there is a complication in dealing with a system of endemic species on oceanic islands of varying
age, namely that the dynamics of colonization, speciation and extinction may be at different points, depending
on the age of the island. Accordingly, we fi tted and compared both species – area and species – area – time
models.