220
Applied island biogeography
(2) threshold effects, (3) assembly rules, and (4) the
role of the matrix for conservation in habitat islands.
1 Life history of habitat islands: As Whittaker
et al . (2005) comment: ‘ It is disappointing that we still
know so little about the power and timescale of “ species
relaxation ” . ’ Here we suggest that the consideration
of the life history (ontogeny) of habitat islands could
be particularly insightful in revealing the patterns
and processes shaping species richness, species
assembly and disassembly. Although a number of
theoretical frameworks have been put forward to
describe the sequential process of species relaxation
after habitat loss and fragmentation (Section 8.2.2),
the temporal scale of habitat loss and fragmentation
has received the least attention. This has restricted our
knowledge on how, for example, the abiotic characteristics of a fragment (e.g. net primary productivity) and
rates of nutrient cycling change through time after its
isolation, and how this affects the fragment ’ s capacity
in maintaining biodiversity.
Habitat conversion is almost always a non - random
process (e.g. Raheem et al. , 2009 ). In forest landscapes,
for example, the most accessible and productive areas
tend to be deforested fi rst. Thus, the remaining fragments show a non - random spatial distribution with
respect to age, because the geographical distribution of
older fragments (i.e. isolated earlier) is different from
that of those isolated later. Moreover, other environmental factors, ranging from anthropogenic disturbance (e.g. hunting) to physical gradients (e.g.
topography and climate) may be correlated with fragmentation and forest loss (Laurance et al. , 2002 ). We
believe that the integration of research on the ontogeny of habitat islands will help us towards estimating
more accurately the rates at which species extinctions
are likely to occur. The time - lags and ‘ extinction debt ’
involved in such extinction processes are still poorly
explored and in need of much attention (Box 8.2 ;
Tilman et al. , 1994 ). By focusing further work on the
above questions, we will be able to approach more analytically questions related to the time - lag for relaxation
and extinction debt.
2 Thresholds: Taxon -
and
system - dependent
thresholds, beyond which species losses accelerate (see
Ewers & Didham 2006 ; Whittaker & Fern á ndez -
Palacios, 2007 ; Suding & Hobbs, 2009 ) have received
very limited attention. Analyses of critical value
ranges, where even small changes in environmental
variable(s) will lead to large changes in the system, will
help us towards understanding relaxation as a result
‘ islands ’ , but they carefully differentiate them from
true islands. MacArthur ( 1972 , p. 105) pointed out
that true islands are ‘ separated by a vacuum insofar
as land birds and insects are concerned ’ , whereas
habitat islands are ‘ separated by other habitats fi lled
with birds and insects ’ , thus the spill - over of organisms
from adjacent habitats is a primary factor for habitat
islands.
Island biogeography theory and the subsequent
theories and applications it has inspired and infl uenced
have made an important contribution to conservation
biogeography. The theory has inspired much thinking
about the importance of the size and connectivity of
protected areas in the maintenance of species diversity,
and it has stimulated an avalanche of research on fragmented ecosystems. However, generalizations derived
from this theory have given rise to models that are too
simplistic (e.g. Laurance, 2008 ).
Recent advances in island theory demonstrate that
we are moving towards a new synthesis, identifying
and incorporating aspects of the island systems that
were not considered in the past. For example:
i Within oceanic island biogeography, efforts have
recently been made to adjust the MacArthur – Wilson
(1967) model to accommodate the dramatic changes
in the carrying capacity and environmental characteristics of islands that occur through the life history
(ontogeny) of an oceanic island itself (see e.g. Whittaker
et al ., 2008, 2010 ).
ii Application of genetic analyses are producing a
more nuanced grasp of species and gene fl ow between
insular and mainland habitats.
iii Scale - dependency of isolation and fragmentation
effects are beginning to be quantifi ed.
iv Efforts have been made to incorporate matrix effects
and to consider the implications of longer term changes
within habitat islands post - isolation.
v Assumptions of initial equilibrium in pre -
fragmentation landscapes have been challenged (for
discussion and exemplifi cation of the foregoing, see
Whittaker & Fern á ndez - Palacios, 2007 ).
These considerations suggest that the dynamic
process at the heart of the island equilibrium theory
needs to be embedded in a much more dynamic model
of the physical environment (a point argued more generally in Chapter 7 ).
We have selected four key areas that we consider
important for a more successful application of island
theory to conservation biogeography. These include
examination of (1) the life history of habitat islands,
Applied island biogeography
(2) threshold effects, (3) assembly rules, and (4) the
role of the matrix for conservation in habitat islands.
1 Life history of habitat islands: As Whittaker
et al . (2005) comment: ‘ It is disappointing that we still
know so little about the power and timescale of “ species
relaxation ” . ’ Here we suggest that the consideration
of the life history (ontogeny) of habitat islands could
be particularly insightful in revealing the patterns
and processes shaping species richness, species
assembly and disassembly. Although a number of
theoretical frameworks have been put forward to
describe the sequential process of species relaxation
after habitat loss and fragmentation (Section 8.2.2),
the temporal scale of habitat loss and fragmentation
has received the least attention. This has restricted our
knowledge on how, for example, the abiotic characteristics of a fragment (e.g. net primary productivity) and
rates of nutrient cycling change through time after its
isolation, and how this affects the fragment ’ s capacity
in maintaining biodiversity.
Habitat conversion is almost always a non - random
process (e.g. Raheem et al. , 2009 ). In forest landscapes,
for example, the most accessible and productive areas
tend to be deforested fi rst. Thus, the remaining fragments show a non - random spatial distribution with
respect to age, because the geographical distribution of
older fragments (i.e. isolated earlier) is different from
that of those isolated later. Moreover, other environmental factors, ranging from anthropogenic disturbance (e.g. hunting) to physical gradients (e.g.
topography and climate) may be correlated with fragmentation and forest loss (Laurance et al. , 2002 ). We
believe that the integration of research on the ontogeny of habitat islands will help us towards estimating
more accurately the rates at which species extinctions
are likely to occur. The time - lags and ‘ extinction debt ’
involved in such extinction processes are still poorly
explored and in need of much attention (Box 8.2 ;
Tilman et al. , 1994 ). By focusing further work on the
above questions, we will be able to approach more analytically questions related to the time - lag for relaxation
and extinction debt.
2 Thresholds: Taxon -
and
system - dependent
thresholds, beyond which species losses accelerate (see
Ewers & Didham 2006 ; Whittaker & Fern á ndez -
Palacios, 2007 ; Suding & Hobbs, 2009 ) have received
very limited attention. Analyses of critical value
ranges, where even small changes in environmental
variable(s) will lead to large changes in the system, will
help us towards understanding relaxation as a result
‘ islands ’ , but they carefully differentiate them from
true islands. MacArthur ( 1972 , p. 105) pointed out
that true islands are ‘ separated by a vacuum insofar
as land birds and insects are concerned ’ , whereas
habitat islands are ‘ separated by other habitats fi lled
with birds and insects ’ , thus the spill - over of organisms
from adjacent habitats is a primary factor for habitat
islands.
Island biogeography theory and the subsequent
theories and applications it has inspired and infl uenced
have made an important contribution to conservation
biogeography. The theory has inspired much thinking
about the importance of the size and connectivity of
protected areas in the maintenance of species diversity,
and it has stimulated an avalanche of research on fragmented ecosystems. However, generalizations derived
from this theory have given rise to models that are too
simplistic (e.g. Laurance, 2008 ).
Recent advances in island theory demonstrate that
we are moving towards a new synthesis, identifying
and incorporating aspects of the island systems that
were not considered in the past. For example:
i Within oceanic island biogeography, efforts have
recently been made to adjust the MacArthur – Wilson
(1967) model to accommodate the dramatic changes
in the carrying capacity and environmental characteristics of islands that occur through the life history
(ontogeny) of an oceanic island itself (see e.g. Whittaker
et al ., 2008, 2010 ).
ii Application of genetic analyses are producing a
more nuanced grasp of species and gene fl ow between
insular and mainland habitats.
iii Scale - dependency of isolation and fragmentation
effects are beginning to be quantifi ed.
iv Efforts have been made to incorporate matrix effects
and to consider the implications of longer term changes
within habitat islands post - isolation.
v Assumptions of initial equilibrium in pre -
fragmentation landscapes have been challenged (for
discussion and exemplifi cation of the foregoing, see
Whittaker & Fern á ndez - Palacios, 2007 ).
These considerations suggest that the dynamic
process at the heart of the island equilibrium theory
needs to be embedded in a much more dynamic model
of the physical environment (a point argued more generally in Chapter 7 ).
We have selected four key areas that we consider
important for a more successful application of island
theory to conservation biogeography. These include
examination of (1) the life history of habitat islands,