Conservation biology: predicting birds' responses to forest fragmentation.
ABSTRACT Understanding species' ecological responses to habitat fragmentation is critical for biodiversity conservation, especially in tropical forests. A detailed recent study has shown that changes in the abundances of bird species following fragmentation may be dramatic and unpredictable.
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ABSTRACT: Despite the alarming rate of tropical deforestation, the long-term conservation value of forest fragments remains poorly understood. We report on the avifaunal turnover in an isolated 4 ha tropical forest fragment in Singapore (i.e. Singapore Botanic Gardens rainforest fragment (SBGRF)) between 1898 and 1998. Over 100 years, the SBGRF lost 18 (49%) species and gained 20 species. More forest-dependent species (3) were lost from the SBGRF than survived (1) or colonised it (no species). Conversely, significantly more introduced species (4) colonised the fragment than were previously recorded (1 species). Significantly more nectarivores survived (8 species) or colonised (9 species) than were lost (two species). In essence, while the avian species richness in the SBGRF remained relatively constant after a century, its species composition underwent significant changes. The avian species composition in the SBGRF in 1998 appeared to be more similar to that of the contemporary smaller and younger Singaporean secondary forest patches than to either the larger and older forest reserves or to the SBGRF 100 years ago. Our study suggests that small isolated tropical forest fragments may have limited long-term conservation value for native forest bird species.Animal Conservation. 01/2005; 8(02):217-222.
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ABSTRACT: Long-term (> 50 years) extinction patterns and processes in isolated tropical forest patches are poorly understood. Considering that forest fragments are rapidly becoming the common feature of most tropical landscapes, data on the long-term conservation value of such fragments are urgently needed. We report on avifaunal turnover in a tropical woodlot (Bogor Botanical Gardens; 86 ha; 54% native and 46% introduced plants; mean 83,649 visitors/month) that has been surveyed several times before and after its isolation in 1936. By 2004 the original avifaunal richness of this woodlot declined by 59% (97 to 40 species) and its forest-dependent avifauna declined by 60% (30 to 12 species). Large-bodied birds were particularly prone to extinction before 1987, but following this time none of the species traits we studied could be considered predictive of extinction proneness. All seven forest-dependent bird species that attempted to colonize this woodlot by 1987 perished thereafter. Our results show that area reduction, isolation, intense human use, and perverse management (e.g., understory removal) of this patch have probably negatively affected the long-term sustainability of its forest avifauna.Conservation Biology 06/2006; 20(3):772-9. · 4.36 Impact Factor
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ABSTRACT: In an ecological community, groups of species may or may not have a nested structure. Furthermore, any nested structure detected could have several causes, each of which would have to be identified, since they may have important theoretical and management implications. In this paper, I assessed the nested structure of bird communities using cloud forest fragments in eastern Mexico to identify bird species and groups of species sensitive to cloud forest fragmentation. Sensitive species were expected to have a nested arrangement highly correlated with forest fragmentation intensity. Analysis identified the following groups of birds as showing a nested structure highly correlated with cloud forest fragmentation: birds with a body mass between 100 and 300 g and larger than 600 g, some trophic behavioural guilds (terrestrial granivore, terrestrial granivore–frugivore, understory granivore–frugivore, arboreal granivore–frugivore, gleaning terrestrial insectivore, gleaning understory insectivore, cliff gleaning insectivore, nocturnal aerial insectivore, diurnal raptor, nocturnal raptor, terrestrial omnivore, scavenger), forest interior and generalist birds, species restricted to cloud forest, and threatened species. This study shows that the analysis of the nested species assemblage can be considered a useful tool to identify species sensitivity to ecological or landscape patterns and processes, in this case, species or groups of species affected by a fragmented landscape pattern.Oikos 02/2005; 108(3):634 - 642. · 3.33 Impact Factor
Conservation Biology: Predicting Birds’ Responses
to Forest Fragmentation
critical for biodiversity conservation, especially in tropical forests. A
detailed recent study has shown that changes in the abundances of bird
species following fragmentation may be dramatic and unpredictable.
Cagan H. Sekercioglu1
and Navjot S. Sodhi2
Because of unprecedented rates
of deforestation and forest
degradation, habitat fragmentation
has become a major issue in
conservation biology. Forest
fragments are often scattered
among human-dominated urban
and agricultural areas. Forest
fragmentation results in three
prominent changes: reduced forest
area, increased isolation among
fragments, and creation of edges
where forests abut non-forested
habitats. Because of factors such
influx of invasive competitors and
predators, fragments significantly
decline, over time, in their
conservation value for many forest
species [1,2] (Figure 1). Which
species flourish or perish as
question. Ecological traits such as
body size, diet, mobility and
specialization often correlate
strongly with fragmentation
sensitivity [3–5]. In some
ecosystems, however, it may be
impossible to predict species’
responses to fragmentation .
This is illustrated by a recent study
 which has shown that, in
southeast Australia, bird species’
abundances changed very
unpredictably after the
fragmentation of Eucalyptus
woodlands and even some species
in the same genus differed greatly
in their responses.
Area has been reported as the
strongest predictor of species
richness in forest fragments [8–10].
However, isolation can also affect
species richness and abundance in
fragments . The species–area
relationship (Figure 2) has been
biotic losses from human-modified
landscapes [12,13]. The
rule-of-thumb is that a 90% loss of
habitat area leads to a w25–50%
loss of species . The predictive
power of this relationship may be
weak, because it does not account
for either habitat heterogeneity or
fragmentation, but it is the only
such existing model . Although
the identities of disappearing
species are as important as their
number, how the abundances of
species change because of habitat
degradation has been conceptually
and empirically little developed.
This is a critical lacuna as the
disappearance of functionally
such as specialists, scavengers or
seed dispersers can affect the
entire community .
Fragmentation frequently results
in the ‘cutting’ of the long tail of the
rank-abundance curve, as rare
species, particularly diverse in
tropical forests, often disappear
first (see Figure 1 in ). Such
‘nested’ distributions where
‘‘species present at species-poor
sites are subsets of those present
at species-rich sites’’  mean
that species that will disappear
from fragments can be predicted.
Species that are rare, sedentary
or specialized in their habitat
requirements are expected to have
lower persistence .
Mac Nally  examined whether
the relative abundances of
woodland bird species can be
predicted following fragmentation.
He surveyed birds in 73 remnants
ranging from 15 ha to 2900 ha in
size in south-eastern Australia.
Because of the absence of pre-
fragmentation data, three large
remnants (16,000–41,000 ha) were
used as reference sites. Although
changed little, suggesting
reorganization of abundances at
ecological time frames, the
changes in the abundances of
individual species were not
predictable. Fragmentation did not
necessarily favor species common
at reference sites. The bird
communities in smaller fragments
werealso not nestedsubsets of the
reference ones, contrary to what
some studies have shown
elsewhere [4,9,17–19]. Mac Nally
 suggests that our knowledge of
how bird communities organize
themselves in fragmented
landscapes is shallow and that
more research should be
Figure 1. An example of
Southeast Asian forest spe-
Barbet (Megalaima oorti).
Photo by Malcolm Soh.
Current Biology Vol 17 No 19
conducted in this direction. We
agree with Mac Nally that we must
look beyond predicting species
richness in human-altered habitats
and focus on the factors that
change species abundances,
which he originally depicted with
the ‘abundance spectrum’.
We suggest that nestedness
analysis be more widely (albeit
judiciously ) employed in
fragmentation studies, and that
existing fragmentation data be
meta-analyzed to understand
regional and ecological differences
in nestedness. Bird communities
are often comprised of distinctive
groups of species with similar
responses to fragmentation . In
nestedness analyses, pooling all
species can obscure important
patterns  as a result of the
idiosyncratic responses of more
mobile and less specialized
species . Specialized and
sedentary species, typical of
tropical humid forests, are more
likely to show nested distributions
, and these species are also the
ones that are more extinction-
prone [9,10,20]. Other examples of
more nested groups are forest
understory species, large-bodied
species and species of
conservation concern [4,9].
Comparing the nestedness of
ecologically distinct groups will
reveal those that are more nested
and hence more vulnerable
to fragmentation [4,9].
of the nestedness matrix (a graphic
representation of species’
distributions, see Figure 1 in ) is
also valuable, as some species,
including sensitive ones, may be
affected by factors other than
fragmentation . Used in this
manner, nestedness analysis
becomes an important
Highly nested distributions imply
that a few large patches can
conserve most or all species.
However, statistically significant
nestedness does not mean perfect
nestedness , and many species
can exhibit idiosyncratic patch
occupancies . Differences in
species extinction or colonization
rates are the two main causes of
nestedness in fragments. Species
richness patterns become less
nested and more unpredictable if
species vary little in their
vulnerability to extinction or in their
ability to colonize fragments [4,18].
Birds that are less specialized
and/or have small home ranges
are less vulnerable to extinction.
Colonization likelihood is higher
if birds are more mobile (e.g.
long-distance migrants, nomadic
species)  or if fragments are
Because mobile and generalist
bird species reduce nestedness,
temperate, more seasonal, and
more open ecosystems, which
harbor higher proportions of
migrant species and habitat
generalists (Figure 3), should be
less nested than tropical humid
forests [4,9,19]. But global
comparisons are lacking.
Mac Nally’s  study took place
in a temperate, dry habitat
(300–700 mm of rain per year) and
open Eucalyptus woodlands. As
expected, the bird community is
relatively mobile and generalized,
helping explain the reduced
nestedness and high idiosyncrasy
observed. The study species, on
average, resemble other
temperate woodland birds in
their mobility and habitat
specialization, and are more
mobile and more flexible in their
habitat use than most tropical
forest birds (Figure 3).
Increased habitat similarity
between fragments and the
surrounding matrix also reduces
isolation and nestedness. A more
‘permeable’ matrix can increase
the presence of some common,
generalist species in small
fragments, but reduced isolation
can also hamper the persistence of
some rare, specialized species in
larger fragments if these birds
leave and do not come back .
contributes to the idiosyncrasy of
species distributions in the Mac
Nally study . Compared to
closed, humid tropical forests,
open, dry, temperate woodlands
are more similar to surrounding
farmlands. Interestingly, the study
fragments do not exhibit a
species–area relationship —
S = 75.6*A20.0027, r2= 0.0033,
based on species lists provided
by Mac Nally  — further
suggesting that the fragments
in this landscape are less
isolated than the surrounding
matrix than is typical of
tropical forest remnants.
The presence of an unusual
‘despotic’ species, the colonial and
aggressive Noisy Miner (Manorina
melanocephala), is another factor
that contributes to the difficulty of
abundances in this system. This
species dominates forest
fragments and drives out most
native forest species, regardless of
their ecological differences .
Such a native species is hardly
equaled in other forest
fragmentation studies, especially
in the tropics. As open, dry
woodland habitats that are
frequented by relatively
generalized and mobile bird
species and where fragments
are dominated by the unique
Noisy Miners, Australian
log10 Forest patch area (ha)
S2 (adj. R2 = 97.1%)
S3 (adj. R2 = 83.4%)
S4 (adj. R2 = 0.0%)
S1 (adj. R2 = 92.2%)
Number of bird speciesNumber of bird species
log10 Forest patch area (ha)
Figure 2. Species–area relationships for four avian functional groups that vary in their
Reproduced with permission from .
ironbark woodlands are not
representative of most forests,
underlining the need for global
syntheses of avian responses to
The difference between
a species’ abundance rank in the
reference plots and its rank in
fragments is a key measure of its
response to fragmentation.
Ecological correlates of rank
differences can illuminate the
causes of fragmentation
sensitivity. Based on the
fragments’ species lists we
obtained from Mac Nally , we
compared the basic ecological
characteristics of species that
declined versus increased in the
smallest (A2) study fragments in
relation to the reference areas.
Forest specialists, insectivores,
nectarivores, and species with
lower clutch sizes were
open habitat species,
granivores, and raptors
increased in small fragments.
Interestingly, so did the large-
bodied species (median body
mass 142 g versus 21 g),
contrary to most tropical forest
Even when species responses
to fragmentation are highly
idiosyncratic, analyses of rank
differences can reveal interesting
ecological patterns and
highlight potentially vulnerable
species. There is a major need for
global meta-analyses of
measures [17,18] with existing
data. These analyses will help
formulate the drivers of
fragmentation sensitivity and
nestedness, explain regional
differences, and contribute to
the development of ecological
theory . For example, the
same characteristics that
make fragments less nested
(more permeable matrix, high
proportion of generalist, mobile,
species) can also make them
more vulnerable to invasive
species, but this has been little-
studied. We hope that Mac
Nally  will inspire similar
studies worldwide, particularly
in the tropics where an improved
understanding of species’
responses to fragmentation is
critical to the conservation of
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1Center for Conservation Biology,
Department of Biological Sciences,
Stanford University, 371 Serra Mall,
Stanford, CA 94305, USA.2Department
of Biological Sciences, National
University of Singapore, 14 Science
Drive 4, Singapore 117543, Republic of
Percent of bird species
Mac Nally (2007)
Mac Nally (2007)
(5086 species) and temper-
ate woodland birds (472
species), with the 123 study
Sedentary species do not
movements (for example,
migration, altitudinal migra-
tion, nomadic movements).
confined to only one main
habitat (for example, forest
The list of study species
was provided by MacNally.
For a description of the
bird ecology database that
was used for this analysis,
Current Biology Vol 17 No 19