habitat fragmentation (lecture 8) Flashcards
How are naturally patchy habitats and habitats that have been fragmented by human activities different?
human fragmented landscape are typically
- more fragmented
- a harsher and more homogenous matrix
What’s the fragmentation process?
- initially very small gaps form
- little effect other than simple reductions in habitat quality
- gaps get larger
- fragments of habitat are isolated from eachother
- impacts of fragmentation get worse with extent of fragmentation
How are habitat loss and fragmentation different?
- lost habitat is always replaced by new habitat
- fragmentation consists of habitat loss (& gain), smaller habitat patches, leading to isolated habitat patches surrounded by matrix
- as more habitat is lost, impact becomes due to patch size and isolation (fragmentation) rather than the effect of habitat loss alone
What ecological principles relate to habitat fragmentation?
- species-area relationships
- island biogeography theory (IBT)
- extinction debt
- small population paradigm
- edge effects
How do species-area relationships relate to habitat fragmentation?
- more species in larger patches
- bigger area = more micro habitats
- bigger area = larger, more viable populations
e.g. rainforest butterflies fragmented by oil palm in Sabah (Benedick et al 2006) - species richness correlates with patch area size
What is island biogeography theory & how does it relate to habitat fragmentation?
- MacArthur & Wilson, 1976
- developed it to explain patterns of oceanic island species richness
- balance between colonisation & extinction
- relevant to habitat fragmentation if consider islands as patches, surrounding ocean as unsuitable habitat (matrix)
What does island biogeography theory predict?
- colonisation rates are higher on islands close to mainland, decreases with number of species
- extinction rates are higher on small islands, increases with number of species
- number of species on an island at equilibrium of colonisation and extinction
- continual turnover of species, some become
extinct & others colonise - number of species eventually becomes constant.
- loss of species, i.e. relaxation, if patches
become smaller or more isolated - more species in large, less isolated patches
How does extinction debt relate to habitat fragmentation?
- extinction debt is the idea that further extinctions will occur in a patch even without further change
- 78% of related studies found evidence (Kuussaari et al., 2009)
- sub-saharan african countries committed to losing 33% of forest primates due to past habitat loss
How does the small population paradigm relate to habitat fragmentation?
- fragmentation increases number of small populations
e. g. groups of black & gold howler monkeys in fragments have reduced genetic diversity compared to those in continuous forest (Oklander et al., 2010)
How do edge effects relate to habitat fragmentation?
- greater relative impacts from external pressures in small fragments
e. g. rainforest trees have higher mortality at patch edges
How does the matrix influence species response to fragmentation?
- matrix quality influences species response
forest bird species in south africa (Neuschultz et al., 2011):
- greater decline in specialist species in natural forest fragment surrounded by eucalyptus plantation matrix than by agricultural matrix
- IBT and species-area relationships assume species can’t survive in the matrix
- matrix quality is crucial and predictions improve when taken into account
How does the species traits influence species response to fragmentation?
forest bird species in south africa (Neuschultz et al., 2011):
- generalist species barely declined
e. g. grassland butterflies differ in ability to disperse through the woodland matrix
- effects can vary hugely with subtle differences in matrix
- different butterfly species have different resistances e.g. to conifers vs willows
How can species maintain viable populations in single fragments?
likely in species with:
- small home ranges
- tolerant of edge effects
quantified by area sensitivity
How do species integrate multiple patches to survive?
- integrate multiple patches into single home range
- integrate multiple patches into a metapopulation
- metapopulation = several smaller subpopulations linked together by recruitment of individuals into breeding population (gene flow)
How do metapopulations work?
- not all fragmented populations are metapopulations: must be gene flow
- source-sink dynamics: source generates recruits, sink only receives
- most have habitat patches that aren’t constantly occupied but also crucial in ensuring overall viability
- must consider when interpreting survey data
