Transcript BDC321_L05 - Fragmentation & connectivity

BCB 322:
Landscape Ecology
Lecture 5:
Emerging processes II
Fragmentation & connectivity
Introduction
• A core issue for modern
conservation planning &
landscape ecology (Saunders et
al, 1991)
• Has been identified as one of
the biggest challenges to
biodiversity
• Proceeds extremely rapidly – in
large parts of the world almost
all fragmentation occurred in
the last century (Australia,
Amazonia)
• Negative impact on many
species due to geographical
isolation and sundering of
metapopulation communication
• Interior species affected more
than others
Deforestation &
development, Tanzania
Introduction
• Large predators die out first, causing population
overruns of other species
• These species may then cause further degradation
• (eg): removal of the top predator in Zimbabwean
national parks (man) led to overpopulation and habitat
trashing by elephants
• Although island biogeography is used to describe them,
fragments are not true islands.
• Connectivity & ecotones must also be considered when
studying landscape fragmentation
• In fact, since all landscapes are inherently patchy,
fragmentation may be considered as filling a continuum
from untouched wilderness to fully- fragmented
• Fragmentation is scale specific for different organisms
Scale dependence
• If a large area is
fragmented into smaller
intact areas, it is termed
“geographical
fragmentation” (coarsegrained)
Wiens, 1994
• Fragmentation on the scale of plots is extremely finegrained (eg): native vegetation in a matrix of invaders
• Effects tend to be species-specific due to scale &
resource considerations
• Specialists are generally worse affected by fine grain
fragmentation than generalists
• Fragments tend to be more vulnerable to external
disturbance (wind, drought, disease)
Diversity in fragments
• Species assemblages in fragments are usually subsets
of those of larger plots
• Species assemblages in smaller woodlots tend to be
lower than in large ones
• Fragmentation method, habitat type and surrounding
matrix effect all play a role in the effect of fragmentation
on species
– Temperate forest birds show high resilience to fragmentation
into woodlots
– Tropical deforestation immediately reduces biodiversity, and
separations of as little as 80m can act as barriers for insects,
small mammals & understorey birds
– Burning in Chilean forests promotes the growth of Vismia
around remnants, whilst logging does not. Remnants
surrounded by this species are considered more isolated by
birds (Stouffer & Bieregaard, 1995)
Fragmentation: species response
• Some species suffer more from fragmentation due to
habitat size sensitivity (large predators & interior
species)
• Eg: amphipods in Australian eucalyptus forest showed
marked reductions in populations after fragmentation
(Margules et al, 1994)
• By contrast, scorpions showed no significant change in
the same plots, possibly because it is capable of
fossorial behaviour.
• Small beetles often consider
fairly small open spaces to be
impassable due to increased
risk of dessication
• Conservation responses
should therefore look at
individual species responses
and not just α-diversity
Fragmentation: species responses
• Forest tent caterpillar (Malacostoma
disstria) outbreak durations are
related to the extent of forest
edge/km: parasites & predators are
less efficient at the edges (Roland,
1993)
• In a study by Kattan et al (1994),
showed that between 1959 & 1990,
31% of avifauna in the upland forest
of Colombia were eliminated
Roland, 1993
• Species that fed in the understorey, and those that required
large canopies for fruit provision were hit worse than other
species.
• Clearly, the effect of fragmentation depends on the
biogeography of the species in question
Fragmentation & predation
• Species in fragments are often
more vulnerable to predation
(Wilcove, 1985)
• Many other factors affect
predation rates at the local level,
including vegetation type & cover
of the surrounding matrix,
• In a fragmentation gradient in
Southwestern California, avian
nest predators were found to
increase with fragmentation,
whilst snakes decreased.
• Hence, primarily snake-predated
species were less impacted by
predators in fragments than other
species (Patten & Bolger, 2003)
Wilcove, 1985
http://www.rfadventures.com/images/Animals/Re
ptiles/Snakes/Non%20Ven/
Fragmentation measurement
• There are many measures useable for fragmentation, and
often severable variables must be combined by regression
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Area (& ratios of area: long axis length; area: perimeter)
Structure
Isolation
Surrounding land use
Edges (external & internal) per km
• Species turnover in a fragment is calculated as:
(E  C)
100%
( S1  S 2)
where
E= extinctions
C = colonisations
S1 & S2 = # breeding species in years 1 & 2
(Diamond, 1969)
• Turnover is inversely related to area, with plots over a certain
size having a constant turnover rate.
Connectedness
• Isolation obviously causes
problems for species
movement.
• Three measures of patch
isolation can be considered:
– connectedness: the degree of
physical connection between
patches (structural attribute).
Obviously, the matrix is the
most connected element, but
generally connectedness is a
measure of the patch structure
– connectivity: extent to which
subpopulations are connected
into a functional demographic
unit (functional attribute).
– corridors: functional or
structural connection between
different subpopulation.
Connectivity
Farina,1998
Corridors
• Functional structures in a landscape which are
fundamental for mitigation of the effects of fragmentation
(also for invasive species penetration)
• Important concept for conservation planning, to allow
movement of organisms through the reserve and
surrounds
• Consequently, they may be structurally recognisable
(such as hedgerows in a field matrix), but are not
necessarily so
• In fact, there’s little evidence that animals use
hedgerows/fencelines as corridors
• Many plant species soil conditions for growth & seed
conditions that are not guaranteed by a narrow strip of
vegetation
• Hence, “corridor” is an unclear concept, and is used in
different contexts in different places in the literature.
Corridors
• Can be created by topography (mountain ridges),
hydrology (riverbeds) or human forest clearance and
other disturbances
• Rivers are the best studied corridor structures – often
associated with alien invasion due to the patchiness of
the riparian structure
• Patchiness due to flooding, temporary ponds, seasonal
dryness.
• Invasibility differs according to hyrdological &
geomorphological zones in the river
• Furthermore, alien plant invasion can be mitigated due
to high seasonal variation
www.in.gov/dnr/public/novdec02/corridor.jpg
Corridors
• Vital for large home range
mammals in humanimpacted landscapes (eg:
cougar - Felix concolor –
can travel up to 5
miles/night (Beier, 1993)
• Satellite telemetry is useful
for measuring ranges of
such species
http://www.dsbn.edu.on.ca/Schools/MarthaC/cougar%2082.jpg
• Telemetry measurements of leatherback turtles showed
they followed the same route every year to beaches on
Costa Rica (Morreale et al, 1996)
• In fact, it appears many species follow “marine
corridors” & without testing, these can easily be
disrupted by human activity such as fishing
• Corridor width plays a role in the viability of a corridor –
too narrow and dispersal capacity is limited
Summary
• Fragmentation is a global process that reduces
biodiversity & accelerates local & global extinctions
• Fragmentation increases habitat edges and the potential
for predation
• Fragmentation is a species specific measure, and
although one species may see the environment as
fragmented, it may be homogeneous for another
• Animal dispersion/movements increase with
fragmentation
• Connectivity is a functional measure of fragmentation
• Connectedness is the structural corollary to connectivity
• Corridors are essential for the maintenance of
biodiversity in a fragmented landscape, although
definitions of corridors vary
References
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Diamond, J.M. (1969) Avifaunal equilibria and species turnover rates on the
Channel Islands of California. Proceedings of the National Academy of
Sciences, USA 69:3199-3203
Farina, A. (1998) Principles and Methods in Landscape Ecology. Chapman
and Hall, London, UK
Kattan, G.H., Alvarez-Lopez & H., Giraldo, M. (1994) Forest fragment and
bird extinctions: San Antonio eighty years later. Conservation Biology 8:
138-146
Margules, C.R., Gaston, A.J. & Hitier, S. (1994) Contrasting effects of
habitat fragmentation on the scorpion Cercophonius squama and an
amphipod.Ecology 75: 2033-2042
Patten, M. A. and Bolger, D. T. 2003. Variation in top-down control of avian
reproductive success across a fragmentation gradient. – Oikos 101: 479–
488.
Roland, J. (1993) Large-scale forest fragmentation increases the duration
of tent caterpillar outbreak. Oecologia 93: 25-30
Saunders, D.A., Hobbs, R.J. & Margules, C.R. (1991) Biological
consequences of ecosystem fragmentation: a review. Conservation Biology
5: 18-32
Stouffer, P.C. & Bierregaard, R.O. (1995) Use of amazonian forest
fragments by understory insectivorous birds. Ecology 76: 2429-2445
Wiens, J.A. (1994) Habitat fragmentation: island v landscape perspectives
on bird conservation. Ibis 137: S97-S104
Wilcove, D.S. (1985) Nest predation in forest tracts and the decline of
migratory songbirds. Ecology 66:1211-1214