What are the contributions in "Extinction debt on oceanic islands" ?

Triantis et al. this paper proposed a taxonomic model for tree-canopy insects and found that tree canopy insects are more likely to be related to the species of the genus `` Canopy Insects ''.

What future works have the authors mentioned in the paper "Extinction debt on oceanic islands" ?

Hence, the authors would also anticipate a correspondingly large number of archipelagic-scale species extinctions for Azorean endemic arthropods in the future as the extinction debt is settled. In the absence of focused and well-resourced interventions, the legacy of past and current deforestation on oceanic islands will be an inexorable process of biodiversity loss stretching well into the future. Amongst the three studied taxa, their analyses suggest that Araneae and Coleoptera are at greater risk of extinction per island, compared to Hemiptera. The scarce available information suggests that delayed extinctions are more likely to occur in species with longer generation times, e. g. mammals as opposed to insects, ( see review in Kuussaari et al. 2009 ), but recent studies on invertebrates ( Raheem et al. 2009, Sodhi et al. 2009 ) have shown a resilience of some invertebrate species to the effects of forest loss ; with many species requiring only very small areas to persist for extended periods ( see also discussion in Samways 2006 ).

What is the impact of deforestation on the oceanic islands?

In the absence of focused and well-resourced interventions, the legacy of past and current deforestation on oceanic islands will be an inexorable process of biodiversity loss stretching well into the future.

How many sites were sampled in Terceira?

In addition, in Terceira (see also Borges and Brown 1999, Cardoso et al. 2009), Pico (Borges and Brown 1999), Graciosa (Borges et al. 2006a) and Santa Maria (Borges unpubl.), an additional 64 sites were sampled (2970 pitfall traps), covering all the available habitat types present, i.e. natural grasslands, exotic forests, semi-natural pastures and intensively managed pastures.

How many species of beetle have been recorded since 1965?

In point of fact, at least five SIE beetle species (Bradycellus chavesi, Calathus extensicollis, Calathus vicenteorum, Nesotes azorica, Ocydromus derelictus), recorded early in the 20th century, have not been recorded since 1965 and might therefore be considered extinct (Borges et al. 2000).

What is the way to prove the extinction of a small arthropod species?

Conclusively proving the extinction of a small arthropod species will be practically impossible within such a large area as the Azorean archipelago (2328 km2), but the authors concur with others (Hanski et al. 2009, Ladle 2009), that given the great importance of understanding the processes and rates of species extinctions, analyses based on indirect evidence can be informative.

What is the model for the endemic species?

The species area model for the archipelagic endemic species was the best model (i.e. lower value of AIC) only for Araneae (see Alternative mechanism in Supplementary material and Table S4), indicating that at least for Coleoptera and Hemiptera, the hypothesis that larger islands have more species, independent of the current area of the native forests, can be ruled out.

(Open Access) Extinction debt on oceanic islands (2010) | Kostas A. Triantis

Q: What is the way to explain the lack of relationship between the current extent of native forest and?

An alternative mechanism for explaining the lack of relationship between the current extent of native forest with the number of forest dependent species, is that larger islands have more species, independent of the current area of their native forests, due to their larger size.

Q: How many species were considered as endemic to the Azorean islands?

Only the forest-dependent species endemic to the archipelago (59 species in total) were considered for further analyses; these species represent 56% of all the endemic species of the taxa considered.

Extinction debt on oceanic islands

Kostas A. Triantis, Paulo A. V. Borges, Richard J. Ladle, Joaquı

n Hortal, Pedro Cardoso,

Clara Gaspar, Francisco Dinis, Ene´sima Mendonc

a, Lu

cia M. A. Silveira, Rosalina Gabriel,

Catarina Melo, Ana M. C. Santos, Isabel R. Amorim, Se´rvio P. Ribeiro, Artur R. M. Serrano,

Jose´ A. Quartau and Robert J. Whittaker

K. A. Triantis (konstantinos.triantis@ouce.ox.ac.uk), Biodiversity Research Group, Oxford Univ. Centre for the Environment, South Parks

Road, Oxford, OX1 3QY, UK, and Dept de Cie

ncias Agra

rias, Univ. dos Ac

ores, CITAA (Azorean Biodiversity Group), Terra-Cha

, PT-9700-

851, Angra do Heroı

smo, Terceira, Ac

ores, Portugal.  P. A. V. Borges, P. Cardoso, C. Gaspar, F. Dinis, E. Mendonc

a, L. M. A. Silveira,

R. Gabriel, C. Melo and I. R. Amorim, Univ. dos Ac

ores, Dept de Cie

ncias Agra

rias, CITAA (Azorean Biodiversity Group), Terra-Cha

, PT-

9700-851, Angra do Heroı

smo, Terceira, Ac

ores, Portugal.  R. J. Ladle, Biodiversity Research Group, Oxford Univ., Centre for the

Environment, South Parks Road, Oxford, 0X1 3QY, UK.  J. Hortal, NERC Centre for Population Biology, Imperial College at Silwood Park,

Ascot, SL5 7PY, UK.  A. M. C. Santos, Div. of Biology, Imperial College at Silwood Park, Ascot, SL5 7PY, UK and Dept de Cie

ncias Agra

rias,

Univ. dos Ac

ores, CITAA (Azorean Biodiversity Group), Terra-Cha

, PT-9700-851, Angra do Heroı

smo, Terceira, Ac

ores, Portugal.  S.

Ribeiro, Univ. Federal de Ouro Preto, DEBIO/Inst. de Cie

ncias Exatas e Biologicas, Lab. Evolutionary Ecology of Canopy Insects, 35400-000,

Ouro Preto, MG, Brazil.  A. R. M. Serrano and J. A. Quartau, Centro de Biologia Ambiental/Dept de Biologia Animal, Faculdade de

Ciencias da Univ. de Lisboa, R. Ernesto de Vasconcelos, C2, PT-1749-016 Lisboa, Portugal.  R. J. Whittaker, Biodiversity Research Group,

Oxford Univ. Centre for the Environment, South Parks Road, Oxford, OX1 3QY, UK, and Centre for Macroecology, Evolution and Climate,

Dept of Biology, Univ. of Copenhagen, DK-2100 Copenhagen, Denmark.

Habitat destruction is the leading cause of 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-specific. We overcome this challenge by applying a method that uses the historical sequence of deforestation in the

Azorean Islands, to calculate realistic and ecologically-adjusted speciesarea relationships. The results reveal dramatic and

hitherto unrecognized levels of extinction debt, as a result of the extensive destruction of the native forest:95%, in

B600 yr. Our estimations suggest that more than half of the extant forest arthropod species, which have evolved in and

are dependent on the native forest, might eventually be driven to extinction. Data on species abundances from Graciosa

Island, where only a very small patch of secondary native vegetation still exists, as well as the number of species that have

not been found in the last 45 yr, despite the extensive sampling effort, offer support to the predictions made. We argue

that immediate action to restore and expand native forest habitat is required to avert the loss of numerous endemic species

in the near future.

In their natural state, oceanic islands typically support

a substantial proportion of endemic species, many of

which have been lost as a direct consequence of recent

human habitation (Steadman 2006, Whittaker and

Ferna

ndez-Palacios 2007). The biodiversity ‘‘crisis’’ is thus

nowhere more apparent and in need of urgent action than

on remote islands (Paulay 1994). The majority of the

documented extinctions since ca AD 1600 are of species

endemic to oceanic islands. Although the specific causes of

these extinctions are often difficult to attribute (Whittaker

and Ferna

ndez-Palacios 2007), the primary drivers are the

habitat destruction and fragmentation universally associated

with human colonization, in combination with other factors

such as the introduction of non-native species (Paulay 1994,

May et al. 1995, Blackburn et al. 2004, Steadman 2006,

Hanski et al. 2007, Whittaker and Ferna

ndez-Palacios

2007).

Habitat destruction is rarely absolute and typically results

in many species being reduced to a few small, isolated

populations, each susceptible to a variety of stochastic

factors such as random fluctuations in demography, chan-

ges of the local environment and the erosion of genetic

variability (Lande 1993). Hence, it can take several

generations for the full impact of habitat destruction and

fragmentation to be visible in the number of extinctions

(Tilman et al. 1994, Helm et al. 2006, Vellend et al. 2006).

Ecography 33: 285294, 2010

doi: 10.1111/j.1600-0587.2010.06203.x

# 2010 The Authors. Journal compilation # 2010 Ecography

Subject Editor: Helmut Hillebrand. Accepted 12 March 2010

285

IBS SPECIAL ISSUE

This time-lag represents an ‘‘extinction debt’’ (Tilman et al.

1994)  a future ecological cost of habitat destruction that

may not be initially apparent in studies made shortly after

habitat loss has occurred. For this reason it is probable that

the true ecological costs of the historically recent spate of

habitat destruction, disturbance and fragmentation on many

oceanic islands are yet to be realised (Diamond 1989), i.e.

there exist many extant but seriously imperilled species.

Developing methods to quantify the magnitude and

taxonomic distribution of the extinction debt is clearly

important for effective conservation planning and prior-

itization. However, accurate assessment of extinction rates

and their extrapolation into the future requires robust long-

term data on species occurrences  data which are rarely

available, especially for less conspicuous taxa such as

invertebrates. The lack of appropriate knowledge has led

to an inevitable reliance on indirect measures and theore-

tical projections of extinctions (McDonald and Brown

1992, Heywood et al. 1994, May et al. 1995, Pimm et al.

1995, Brooks et al. 1997, Rosenzweig 2001, Brook et al.

2003, Whittaker et al. 2005, Kuussaari et al. 2009, Ladle

2009).

One of the most commonly used methods for estimating

future extinctions is to extrapolate from the characteristic

form of the classic island speciesarea relationship [ScA

where S is the number of species, A is (island) area, and c

and z are constants] derived from island biogeography

theory (Preston 1962, MacArthur and Wilson 1967). The

consequences of habitat loss under this framework can be

predicted following the ‘‘rule of thumb’’ calculation that a

10-fold decrease in area results in a twofold decrease in

species (Darlington 1957), or alternatively, when an area of

habitat is reduced by 90%, the number of species eventually

drops to one half. This approach has been applied at

varying  sometimes very coarse  scales to forecast species

losses as a function of habitat loss due to factors such as

deforestation (Brooks et al. 2002) or future climate change

(Thomas et al. 2004). Even though the accuracy of this

approach critically rests upon accurate estimation of the

slope (z) of the relationship (Rosenzweig 2001, Whittaker

et al. 2005, Lewis 2006, Whittaker and Ferna

ndez-Palacios

2007), it has been commonplace to assume z0.25 across

a range of different taxonomic groups, scales and ecogeo-

graphical systems (May et al. 1995, Brooks et al. 2002,

Thomas et al. 2004).

Although arthropods represent the bulk of all known

living species, the level of threat imposed by global

environmental changes to arthropod diversity remains

poorly documented (Brooks et al. 2006, Fonseca 2010).

Dunn (2005) has estimated that roughly 44 000 insect

extinctions have occurred in the last 600 yr, but the number

of extinctions documented during this period is 61 species

(IUCN 2009; the respective number for arachnids is zero).

Here, we apply a method that uses the historical informa-

tion on deforestation on the Azores (a remote Atlantic

Ocean archipelago) to generate more accurate estimates of

local extinctions or extirpations (hereafter extinctions) for

the endemic forest-dependent species of three well-studied

groups of arthropods from the Azores, namely the spiders

(Araneae), the true bugs (Hemiptera) and the beetles

(Coleoptera). This approach has been used in a few

mainland systems (Pimm and Askins 1995, Helm et al.

2006, see also Kuussaari et al. 2009 for a recent review) but

we are not aware of any similar study on islands, despite the

widely accepted notion that islands and especially oceanic

islands have suffered and will probably suffer increased

extinctions following habitat loss.

The Azores constitute an ideal model system for

assessing extinction debt because: 1) they have lost95%

of their original native forest during the six centuries of

human occupation; 2) being one of the most isolated

archipelagos on Earth they support a significant number of

single island endemic species (SIE; i.e. endemic species

restricted to one island) (Borges et al. 2005b, Borges and

Hortal 2009, Cardoso et al. 2010); 3) the history of human

settlement and deforestation is well known (Frutuoso 1963,

Silveira 2007), and; 4) extensive distributional data exist for

a range of taxa (Borges et al. 2005b).

Methods

Study area

The first human settlements were established in the Azores

(Supplementary material Fig. S1) around AD 1440. More

than 550 yr of human presence has taken its toll on the

local fauna and flora, 420 species of which (out of the 4467

total terrestrial taxa known from the Azores) are endemic to

the archipelago (Borges et al. 2005b). Today, ca 70% of the

vascular plant species and 58% of the arthropod species

found in the Azores are exotic, many of them invasive

(Borges et al. 2005b, 2006). The native ‘‘laurisilva’’,

a humid evergreen broadleaf laurel forest, was the pre-

dominant vegetation form in the Azores before human

colonization in the 15th century (ca AD 1440). Here, we

consider as ‘‘native forest’’ both the humid evergreen

broadleaf laurel forest and other native forest types such

as the Juniperus brevifolia- and Erica azorica-dominated

forests. The Azorean laurisilva differs from that found on

Madeira and on the Canary Islands as it includes just a

single species of Lauraceae (Laurus azorica), although also

featuring several species of sclerophyllous and microphyl-

lous trees and shrubs (e.g. J. brevifolia and E. azorica), and

luxuriant bryophyte communities, covering all available

substrata (Gabriel and Bates 2005).

The destruction of the native forest in the Azores has

followed a clear temporal sequence. At the time of human

colonization the archipelago was almost entirely covered by

forest (ca AD 1440) (Martins 1993, Silveira 2007). By

300 yr ago (ca AD 1700) human activities had restricted the

native forest in most islands to areas above 300 m a.s.l. and

by ca AD 1850, areas with native forest were mainly present

above 500 m a.s.l. (Silveira 2007). 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, with the clearing of large fragments at

mid- and high-altitude for pasture, further decreasing the

native forest to its current extent of 2.5% of the total area

of the archipelago (B58 km

in total). Thus, inB600 yr

95% of the original native forest has been destroyed

(Gaspar 2007, Gaspar et al. 2008, Table 1).

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Data

As a result of the exhaustiveness of taxonomic work, the

relative poorness of the Azorean fauna, and the intensive

sampling during the last ten years (see Supplementary

material for an analytical description of the sampling

method), the Borges et al. (2005b) checklist (updated also

with recent unpublished data) includes virtually all arthro-

pod species native to the Azores, reported and described

from 1859 (Droue

t 1859) up to today, as well as an

accurate account of their presence or absence in all the

islands of the archipelago. The data for the Araneae,

Hemiptera and Coleoptera are particularly comprehensive

(Borges et al. 2005b, Borges and Wunderlich 2008,

Cardoso et al. 2010). In this context, even if more

species remain to be discovered from the islands in the

future (e.g. Borges and Wunderlich 2008), we can reason-

ably regard each island as being currently proportionally

equally well-sampled.

In 1998, 60 native species (excluding Crustacea, Acari,

Collembola, Hymenoptera and Diptera) were known to be

SIE. During 1999 and 2000, 64 transects were set up,

covering all remnants of native forest in the Azorean islands

(BALA project) (Borges et al. 2005a, Ribeiro et al. 2005,

Table 1). Eight species out of the original 60 SIE were

found in other islands, but also 13 new species were

described, nine of them being SIE (Borges and Wunderlich

2008). During 2003 and 2004, 38 new transects were set up

in the same forest remnants (Gaspar 2007, Gaspar et al.

2008). After this intensive additional round of surveys, only

one further species previously thought to be a SIE was

found in another island, demonstrating the high reliability

of the current checklist at the island level.

Based on previous work (Borges and Brown 1999,

Borges et al. 2005a, 2006, 2008, Ribeiro et al. 2005,

Gaspar 2007, Borges and Wunderlich 2008, Gaspar et al.

2008) the endemic arthropods were classified as native

forest dependent and non-forest dependent species (e.g.

cave-adapted species, native grassland specialists, species

also surviving in exotic forests or other man-made habitats).

A species was considered forest-dependent (i.e. forest

specialist) when 85% or more of its individuals have been

collected in native vegetation (see Forest dependent

endemic species in Supplementary material Table S1).

Only the forest-dependent species endemic to the archi-

pelago (59 species in total) were considered for further

analyses; these species represent 56% of all the endemic

species of the taxa considered. Despite the intensive survey

effort recently carried out in anthropogenic habitats on

some of the islands (Terceira, Pico, Graciosa and Santa

Maria; Borges and Brown 1999, Borges et al. 2005a, 2006,

2008, Borges and Wunderlich 2008; see also Supplemen-

tary material), none of the species considered as a native

forest endemic here has been found to have large popula-

tions in any other type of land use (B15% of their total

numbers of individuals, after standardising for sampling

effort; see details in Supplementary material Table S1). The

completeness and comparability of these surveys was

verified using a number of sampling effort algorithms (see

Sampling effort analysis in the Supplementary material).

The respective species lists of endemic forest specialists

for the above three taxa were extracted for the areas of native

forest corresponding to four points in time (below). This

step was undertaken using SQL-based queries on the

ATLANTIS-Azores database by means of the Atlantis

Tierra 2.0 software (Zurita and Arechavaleta 2003, Borges

et al. 2005b, Table 2). The ATLANTIS-Azores database

includes an exhaustive checklist created by many taxono-

mists, who have recently performed a detailed revision of

the taxonomic status of many species, identified many

synonyms and improved the list of Azorean arthropods

(Borges et al. 2005b). This database includes the spatial

distribution of all recorded species specimens in a 500

500 m grid, based on both literature and unpublished field

data, hence allowing us to obtain the list of species for any

region within any of the islands. Here we extracted four

different species lists for each taxon, each one of them

chosen to correspond to the extent of native forest at four

known points in time before and since human coloniza-

tion (Table 1; Fig. 2 with the island of Terceira as an

illustration). They were as follows: a) for the total area of

each island, i.e. all known forest specialist species reported

from the island. This reflects the near 100% forest cover of

the islands before the arrival of humans; AD 1440, herein

. b) For areas above 300 m, including only those species

reported above this elevational limit and corresponding

to the extent of the native forest ca AD 1700, T

. c) For

areas above 500 m, the extent of the native forest at ca

AD 1850, T

. d) for the present area occupied by native

forest, including only those species currently reported from

native forest remnants within each island, AD 2000, T

The slight differences in the number of species denoted

for (a), (b) and (c) are due to the fact that some species have

been recorded only from the lowland areas which have been

sequentially lost over time. As Raheem et al. (2009) have

recently shown, the influence of pre-fragmentation patterns

of species turnover can persist despite habitat loss and

fragmentation, with the spatial pattern in species distribu-

tion before disturbance persisting to the present. Thus, we

avoided considering each island as a priori biogeographically

homogeneous before habitat destruction, in terms of species

distribution in the different elevational zones considered.

The differences between the species number for the total

island area (a) and for the current extent of the native forest

(d) (Table 2) are due to the inclusion in (a) of historical

records of species presences in low and mid altitudes where

the native forest is now absent. This means that if a species

has been reported in the past from a lowland area where the

native forest is now absent and this species is not found in

any of the areas currently covered by native forest, the

species was included in list (a) but not in list (d). Thus, for

this latter category we are not following the simple

elevational criterion used for (b) and (c) but we are instead

using the actual distribution of the native forest patches.

The current area of native forests for all the islands

(Table 1) was estimated based on digital aerial photo-

graphy of the islands and field work (Gaspar 2007, Gaspar

et al. 2008).

Calculation of extinction debt

To explore the impact of native forest destruction on

current levels of endemic arthropod species richness, we

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IBS SPECIAL ISSUE

Table 2. The number of forest-dependent endemic arthropod species in the four different habitat areas, corresponding to the extent of native forest at four known points in time, before and following

human colonization (Supplementary material Table S2 and Methods for details).

Island Coleoptera Araneae Hemiptera

Total area,

Area 300 m,

Area 500 m,

Present area,

Total area,

Area300 m,

Area500 m,

Present area,

Total area,

Area 300 m,

Area 500 m,

Present area,

Graciosa 2 2 32 31 

Corvo 1 1 1  00 0  22 2

Flores 8 7 6 6 11 11 11 10 5 5 4 3

Faial 4 3 3 3 8 8 7 7 5 5 5 3

Pico 14 13 13 13 10 10 10 10 4 4 4 4

o Jorge 4 4 4 4 11 11 11 11 6 6 6 4

Terceira 11 10 9 9 11 11 11 10 8 7 7 5

o Miguel 17 17 11 11 11 10 9 9 6 5 5 5

Santa Maria 14 13 12 12 7 7 6 6 3 3 3 3

Table 1. Basic characteristics of the islands of the Azores (main source: Borges and Hortal 2009; see also Methods). Latitude and longitude refer to the centre of the island, and are given in decimal

degrees. Total area of the island approximates the forest cover before the arrival of humans; AD 1440, T

; area above 300 m corresponds to the extent of the native forest ca AD 1700, T

; area above

500 m, the extent of the native forest ca AD 1850, T

; and the present area of forest remnants is for AD 2000, T

. : absence of native forest; *currently there is no primary native forest on Graciosa and

Corvo Islands. On Graciosa only a very small patch of secondary native vegetation occurs; this patch is dominated by small-sized

Erica azorica

, an early successional endemic shrub.

Island Latitude

Longitude

Altitude

(m)

Total area of island

(km

), T

Area above 300 m

(km

), T

Area above 500 m

(km

), T

Present area of forest

remnants (km

), T

Maximum

age (Ma)

Graciosa 39.0 27.6 398 62 3.48 *2.50

Corvo 39.4 31.0 718 17 9.33 5.44 *0.71

Santa Maria 36.9 25.1 587 97 13.19 0.21 0.09 8.12

Faial 38.6 28.5 1043 172 80.45 36.59 2.26 0.73

o Jorge 38.7 27.9 1053 246 170.56 90.35 2.93 0.55

o Miguel 37.7 25.5 1103 757 352.39 186.02 3.31 4.01

Pico 38.5 28.2 2351 433 261.66 188.30 9.52 0.25

Flores 39.4 30.9 915 142 95.18 52.58 15.71 2.90

Terceira 38.7 27.2 1023 402 177.60 70.09 23.45 3.52

Total 2328 1163.84 629.58 57.27

288

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assumed a multiple linear relationship between species

number (S), area (A) and the geological age of each island

(G), ) i.e. Log Sb

b

Log Ab

G, for the endemic

forest-dependent species of Araneae, Hemiptera and

Coleoptera. For number of species and area we used the

conventional logarithmic transformations (log

) to esti-

mate the equation parameters (Borges and Brown 1999,

Borges and Hortal 2009, cf. Rosenzweig 2001). For the

particular case of the single island, where the number of

Araneae species was zero we used the conventional practice

of raising the values for all islands by 0.5.

Inclusion of island age (Supplementary material) follows

previous theoretical and empirical work showing that age

can influence the evolutionary dynamics of oceanic islands,

as reflected in levels of endemism (Whittaker et al. 2008,

Borges and Hortal 2009). Including island age means

that we do not assume that the islands were in a pure

‘‘ecological’’ immigrationextinction equilibrium prior to

human colonization. Instead, the number of endemic

forest species prior to human colonization is assumed to

be a longer-term outcome of immigration, speciation and

extinction dynamics.

We calculated our speciesareaage relationships using

four different ‘‘habitat areas’’ corresponding to the extent

of native forest at four known points in time: AD 1440

(total area), AD 1700 (area above 300 m), AD 1850 (above

500 m) and AD 2000 (current extent) (see above). If

‘‘relaxation’’ of species numbers has not yet taken place or is

incomplete (i.e. an extinction debt remains) then the best

fitting speciesareaage model will correspond to the

remaining area of forest at some past time. However, which

‘‘past time’’ may not be the same for each taxon due to

differences in their ecology and life history. Additionally,

we tested the effectiveness of the applied model against

a number of different models, e.g. including measures

of island elevation, log-transformed age values, and con-

sidering quadratic models of geological age, i.e. GG

(Whittaker et al. 2008).

An alternative explanation for the lack of relationship

between the current extent of native forest and the number

of forest dependent species is that larger islands originally

had more species as a consequence of their larger area. Thus,

due to their larger species pool, more species would be

expected to be found in fragments within larger islands. To

test this mechanism we evaluated the relationship between

the number of the archipelagic endemic species of the three

taxa considered here and the total area of each island and

compared its explanatory power with the respective species

areaage relationship. If larger islands have more species,

then the speciesarea model will be the best for the species

richness of the endemic taxa. We also tested the predictive

accuracy of the two speciesareaage models (for the total

area and the area above 300 m) by testing the correlation

between the observed and the predicted number of species.

Finally, in order to evaluate our predictions, we compare

the average species abundance per transect (i.e. average

number of individuals of archipelagic endemic forest-

dependent species per transect) of Graciosa Island with

the rest of the islands of the archipelago. Currently there is

no primary native forest on Graciosa; only a very small

patch of secondary native vegetation occurs, dominated by

small-sized Erica azorica, an early successional endemic

shrub. Hence we predict that the surviving forest-dependent

species that are present in several islands will show smaller

abundances within transects on Graciosa, indicative of a

progressive reduction of their populations towards extinc-

tion. All analyses were carried out using STATISTICA 6.1

(StatSoft 2003).

Results

For the total island area and the area above 300 m, the

speciesareaage model applied was significant (pB0.05)

for each of the arthropod taxa considered (Table 3), with

most of the explained variance attributable to area.

However, for the area above 500 m and the present area

covered by native forest, neither the speciesareaage

relationships nor the respective speciesarea relationships

were statistically significant for any of the three taxa

considered (Supplementary material Table S2). We thus

used the first two benchmark relationships, for total area

(AD 1440, T

) and area above 300 m (AD 1700, T

)

(Fig. 1 and 2B), to represent the baseline conditions

for estimation of current extinction debt. Hence, we used

the parameters estimated for the total area of the islands

(Pred. 1; Table 4), and that of the area above 300 m (Pred.

2; Table 4) to estimate the number of endemic forest

arthropods that ‘‘should’’ be present and, by direct

comparison with the number of extant species, derive the

number of species to go extinct (i.e. the extinction debt) for

each taxon (Table 4 and Supplementary material S3).

For all three arthropod taxa considered, our results

clearly indicate that the majority of the endemic forest-

dependent species are expected to go extinct in time,

especially on those islands on which the native forest has

been restricted to small areas, namely Santa Maria,

o Miguel, Sa

o Jorge and Faial, or on which it has been

totally removed, namely Graciosa and Corvo (Table 1 and

4). Terceira, the island with the largest remnants of native

Table 3. The speciesareaage equations used for predicting extinctions. S: number of forest-dependent archipelagic endemic species;

A: area; G: geological age; b: standard error for non-standardized regression coefﬁcients (see Methods for details). The degrees of freedom

(DF), F and p-values are also presented. For all the models tested see Supplementary material Table S2.

Taxon/island area Equation SE intercept SE b

SE b

DF R

F-value p-value

Coleoptera (total area) LogS0.9150.678 LogA0.076 G 0.288 0.126 0.025 2.6 0.87 20.14 B0.01

Coleoptera (300 m) LogS0.3830.471 LogA0.116 G 0.198 0.092 0.026 2.6 0.86 18.78 B0.01

Araneae (total area) LogS0.9790.780 LogA0.026 G 0.189 0.170 0.03 2.6 0.79 11.06 0.01

Araneae (300 m) LogS0.3180.531 LogA0.067 G 0.238 0.153 0.04 2.6 0.68 6.33 0.03

Hemiptera (total area) LogS0.0600.321 LogA0.007 G 0.184 0.080 0.016 2.6 0.73 7.96 0.02

Hemiptera (300 m) LogS0.0880.347 LogA0.016 G 0.146 0.067 0.019 2.6 0.82 13.27 B0.01

289

IBS SPECIAL ISSUE

Extinction debt on oceanic islands

Figures

Citations

The Theory of Island Biogeography

The seven impediments in invertebrate conservation and how to overcome them

A reconstruction of Palaeo-Macaronesia, with particular reference to the long-term biogeography of the Atlantic island laurel forests

The island species–area relationship: biology and statistics

SYNTHESIS The island species-area relationship: biology and statistics

References

IUCN Red List of Threatened Species

The Theory of Island Biogeography

The Theory of Island Biogeography

IUCN Red List of threatened species

Extinction risk from climate change

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Frequently Asked Questions (13)

Q1. What are the contributions in "Extinction debt on oceanic islands" ?

Q2. What future works have the authors mentioned in the paper "Extinction debt on oceanic islands" ?

Q3. What is the impact of deforestation on the oceanic islands?

Q4. How many species of arthropods are endemic to the Azores?

Q5. What is the way to explain the lack of relationship between the current extent of native forest and?

Q6. What is the way to estimate future extinction rates?

Q7. What are the primary drivers of the extinctions?

Q8. How many sites were sampled in Terceira?

Q9. How many species were considered as endemic to the Azorean islands?

Q10. How did Raheem et al. (2009) show that species turnover can persist?

Q11. How many species of beetle have been recorded since 1965?

Q12. What is the way to prove the extinction of a small arthropod species?

Q13. What is the model for the endemic species?