Lecture 13 - forest fragmentation Flashcards
when does forest fragmentation occur?
- rapid conversion of tropical forest 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
definition of habitat fragmentation
a large expanse of habitat is transformed into a number of smaller patches of smaller total area, isolated from each other by a matrix of habitats unlike the original
what are the 3 stages of the fragmentation process
1) initial habitat loss
2) isolation of blocks of habitat
3) increased isolation due to further land use change
is deforestation a random or non random process?
usually non random - some habitats cleared preferentially e.g. geography (soils etc) or physical accessibility (roads
- fragments are often a non random subset of original habitats and microhabitats
what is the ‘sample effect’?
some species are absent or poorly represented in fragments from outset
how vast is the problem of fragments?
huge - Amazon = 180,000 fragments
Atlantic=230,000 fragments
what are the ecological foundations to fragmentation research
1) species area relationships
2) island biogeography theory
describe species area relationships
relationships between area of habitat and number of species that survive in that area e.g. islands and mountain tops
- pattern is such a strong relationship it can graphed across many taxa
SARS equation
S=cA^z - curve is characterised by the Arrhenius equation
s = species richness
A= Area
c and z are constants (c = species richness factor and z= species accumulation factor)
- species area curve appears with almost law-like consistency in empirical data sets
who came up with the theory of Island Biogeography?
MacArthur and Wilson - 1967
describe the theory of IBT
- oceanic islands were viewed as relatively static (i.e. slow change) in species composition - colonisation 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 colonisation
what are the basic principles of IBT
- local extinction rate is a function of island size
i. e. bigger islands = larger populations = lower extinction rates = more species - colonisation rate is a function of island isolation from the mainland
i. e. more isolated = lower colonisation rates= fewer species
what does IBT predict
the number of species on an island is determined by its size and isolation
- predicts that every island has characteristic colonisation and extinction curves determined by its area of isolation
- where the extinction and colonisation curves cross is hte number of species
what can species area effects be applied to?
applied to forest fragmentation and also has an impact on composition
- rapid loss of species with large area requirements (e.g. predators, large bodies species) , elevational migrants and forest-interior specialists
what are isolation effects in forest fragmentation?
increasing distance from contiguous forest means increased isolation of fragments
- local extinction is much lower for connected patches than isolated ones
- habitat matrix impacts on connectivity
what is a 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)
what happens when there is large disparity between forest and matrix?
inhospitable matrix for forest species = colonisation rates decline = useable habitat shrinks
(small disparity = opposite)
what is the effect of a hospitable matrix?
reduces the effect of fragment size on some species - allows for dispersal movements and recolonisation of fragments where species have gone extinct
what is extinction debt over time and why is it a problem?
- immediately after fragmentation many species remain but are committed to extinction over time
- big issue in identifying impacts of fragmentation
- future ecological cost of current habitat destruction
what is the though effect of fragments in the Amazon?
100ha fragments thought to lose half of their species in less than 15 years
what happens if you increase fragment size?
it decreases debt but to decrease species loss 10 fold requires 1000 fold increase in fragment area
what are edge effects?
a disturbance that penetrates into the fragment (abiotic and biotic) e.g. microclimate, vegetation structure, flora and fauna composition
impact of edge effects
- can extend very fair into forest (to 400m)
- processes can effectively cause a reduction in fragment area for many species
- plots close to the edge have a higher rate of species turnover per year than plots far from edge
what effect can large predator loss have on the functioning of an ecosystem?
large predator loss = increase herbivore and seed predators = decreased number of saplings and decreased seed dispersal = decreased wood density in fragments
