Forest Fragmentation Flashcards
There’s rapid conversion of tropical forest…
… to agriculture, often managed in intensive plantations
But forest clearance to agriculture is rarely wholesale…
…and often fragments of forest are retained in the landscape from tiny patches to massive swathes that are now cut off from contiguous forest.
Habitat fragmentation
Three stages to fragmentation process:
- Initial habitat loss
- Isolation of blocks of habitat
- Increased isolation due to further land-use change
Deforestation is usually nonrandom
Some habitats cleared preferentially
Geography (topography, soils, climate)
Physical accessibility (e.g., roads, rivers)
Fragments are often a non-random subset of original habitats & microhabitats
Some species are absent or poorly represented in fragments from outset (“Sample effect”)
How big is the problem?
Brazilian Amazon - ~180,000 fragments 1-100 ha - 25% forest within 1 km of edge Brazilian Atlantic - ~230,000 fragments 1-100 ha - 91% forest within 1 km of edge
Ecological Foundations to Fragmentation Research:Species-Area Relationships & Island Biogeography Theory
What do islands and mountain tops have in common?
They both demonstrate relationships between area of habitat and number of species that survive in that area
This fundamental field of ecology began when biologists started to see patterns in the number of species that occurred on islands and mountain tops and the relationship that appeared to occur between species richness and habitat area
Species-area relationship
Power function: S = cA^z
The curve is characterized by an equation called the Arrhenius equation
S = species richness, A=Area, and c & z are constants (c = species richness factor and z = species accumulation factor)
The Theory of Island Biogeography (MacArthur & Wilson 1967)
Oceanic islands were viewed as relatively static (i.e., slow change) in species composition
Colonization and extinction events were considered rare
M&W postulated that the number of species on islands is a dynamic equilibrium between the opposing forces of extinction and colonization
Basic Tenets of Island Biogeography Theory
Local extinction rate is a function of island size Bigger islands larger population sizes lower extinction rates more species Colonization rate (arrival of new species) is a function of island isolation from the mainland More isolated islands lower rates of colonization fewer species
Can IBT be applied to biodiversity impacts of fragmentation in tropical forest fragments?
Area and isolation effects
Impact of area on species richness?
Species richness increases with area
There are also impacts on species composition, as smaller patches have different composition to larger forest patches
Which species are lost?
Rapid loss of species with large area requirements:
- Predators
- Large-bodied species
- Elevational migrants
Plus forest-interior specialists
Isolation effects
(1) Increasing distance from contiguous forest means that fragments are more isolated
(2) Habitat matrix impacts on connectivity
What is the habitat matrix?
The habitat that surrounds forest fragments and ‘connects’ them to other fragments or contiguous forest
Depending on the matrix it often supports populations of some species found in forest
Fragments are connected for these species
Disparity between forest and matrix
Inhospitable matrix for forest species
- Colonization
rates decline
- Useable habitat shrinks
Hospitable matrix reduce the effects of fragments size on some species
Also allows for dispersal movements and recolonisation of fragments where species have gone extinct
Extinction debt over time
Major confounding issue in identifying impacts of fragmentation is effect of time
Immediately after fragmentation, many species remain, but…
Many species committed to extinction over time
Termed “Extinction debt” (Tilman et al. Nature 1994)
Future ecological cost of current habitat destruction
Extinction debt in the Amazon
100 ha fragments lose half of their species in <15 yrs
Reducing the extinction debt
Increase fragment size to reduce debt, but…
To reduce species loss 10-fold requires 1000-fold increase in fragment area (Ferraz et al. 2003 PNAS)
Edge effects
A disturbance that penetrates into the fragment
Abiotic and Biotic edge effects?
Wind, tree mortality, disturbance-adapted species invade, altered spp composition, increased temperature, reduced moisture.
Myriad changes:
Microclimate and wind
Vegetation structure (e.g., treefall gaps)
Floristic composition
Animal composition
Can extend very far into forest (to 400 m)
Critically, processes can effectively cause a reduction in fragment area for many species
Functioning of ecosystems
Rainforest communities are underpinned by networks of interactions between species
Species play different functional roles in tropical forest systems:
Predators
Seed dispersers and pollinators
Trees store carbon & structure
An impact of fragmentation on ecological functions?
Large predator loss causes…
x10 increase in reptiles, x25-50 increase in monkeys, x100 increase in leaf cutter ants, x35 increase in rodents means reduced numbers of saplings
Reduced seed dispersal mutualism, fewer seeds dispersed and are dispersed less far.
Reduced wood density
What does fragmentation tell us about protected area management?
Maintaining connectivity
IBT stresses the importance of isolation to colonization rates
Key recommendation for protected area management is to retain connectivity
Wider and better quality corridors
A value of corridors?
Corridor is a linear habitat remnant, surrounded by the modified matrix, that facilitates wildlife movement between patches
Could retention or creation of corridors improve species survival?
E.g., Brazil requires land-owners to retain riparian (riverine) corridors of minimum width depending on size of river
Biodiversity value of Brazil’s corridors?
Summary
Forest fragmentation major disturbance
Strong species-area and isolation effects
There is an extinction debt in fragments
Edge effects further degrade value of patches (especially small ones)
Functioning in fragments is reduced
Breakdown of interactions causing cascading impacts throughout communities
Creating corridors is important method
