Habitat fragmentation
Jean-François Le GalliardCNRS, University of Paris 6, France
Habitat fragmentation : facts
Habitat fragmentation describes a state (or a process) of discontinuities (fragments) within the preferred living area (habitat) of a species.
The classical paradigm of population ecology is that of a single, large and homogeneous population, but it is widely recognised that most populations are fragmented and heterogeneous
-> implications for ecological and evolutionary processes ?
Habitat destruction vs. habitat fragmentation
Habitat destruction is associated with massive habitat loss, fragmentation and habitat degradation
~ 83 % land surface affected by human activities
Forest fragmentation (green area)in Finland from 1752 to 1990
Habitat destruction includes several processes:• Reduction in the total area of the habitat• Increase in number of habitat patches• Decrease in habitat patches area• Increase in isolation of habitat fragments• Possibly, a decrease in habitat quality
Fahrig. Ann. Rev. Ecol. Syst. 2003.
Effects of habitat destruction on biodiversity
Habitat destruction is considered as one of the main cause of species loss on earth with overexploitation and species invasion according to the 2006 IUCN statistics
• 16,119 species are threatened with extinction in the Red List. • 99% of threatened species are at risk from human activities. Humans are the main cause of extinction and the principle threat to species at risk of extinction. • Habitat loss and degradation are the leading threats. They affect 86% of all threatened birds, 86% of the threatened mammals assessed and 88% of the threatened amphibians.
Examples of species threatened by habitat loss in Europe (21 listed endangered)
Erismature à tête blanche Grenouille des Pyrénées Silene diclinis
Ecology of fragmented habitats
Spatial structure : existence of discrete, localised patches of preferred habitat separated by a matrix of non-preferred habitat
patchy distributionspatial organisation : number and spatial distribution of patches
Local demography : small patches are more likely to go extinct and more variable than large populations
Connectivity : patches are separated by a matrix of non-preferred habitat putting limits on dispersal abilities
connectivity : number, size and spatial distribution of corridorspermeability : matrix quality and spatial structure
A case example
Habitat fragmentationGranville fritillary butterfly (Finland)
Hanski. Nature. 1998.
Models of habitat fragmentation
The Levin’s model (occupancy model)
occupied
empty
m × p
e
p’ = m p (1 – p) – e p
p* = 0p* = (m-e)/m
Very fast local dynamics
The population is in a balance between migration and extinction
There is a threshold migration rate for population viability (m = e)below the threshold, the population is viableabove the threshold, the population goes extinct
Levins. Bull. Ent. Soc. Entom. USA. 1969.
Models of habitat fragmentation
The source-sink model (Pulliam)
Productive habitats
Non-productive habitats
Source : net exporter of migrants (high productivity)Sink : net importer of migrants (low productivity)
The simple source sink-models predict thatAbsolute sinks would not persist in the absence of sourcesA large proportion of a population can exist in sink habitats
In the case of density-dependent regulationSinks are set above their carrying capacitySources are set below their carrying capacity
Asymmetric migration between habitat patches (unbalanced dispersal)
Pulliam. Am. Nat. 1988.
Models of habitat fragmentation
The metapopulation modeldiscrete spatial structuretwo spatial scales (local and regional)local persistence for at least a few generationsdominant effects of extinction-colonisation dynamics
Hanski’s metapopulation model : incidence functions
« occupancy » models designed for butterflies populations
extinction rate depends on patch area
colonisation rate depends on size of and distance to neighbouring patches
State variable : occupancy of a given patch i
Model parameters and incidence functions
E = min[e/Ax,1] � extinction rate decreases with patch area
C = β ∑ exp(-α dij) pj Aj � colonisation rate decreases with distance and increases with patch crowding and patch areas
Hanski. Metapopulation ecology. 1999.
Rescue effect and alternative equilibria
Very low metapopulation occupancy = negative metapopulation growth rate due to low colonisation rate
Higher occupancy = higher colonisation rate (rescue effect) favors increased growth rate
Very high occupancy = crowding and population regulation at the regional level
Predicted (theory) Observed (66 networks) Predicted (empirical model)
Hanski. Nature. 1998.
Contrasted effects of habitat destruction
snakes
3 common small
mammals(from large to small)
Clonal / Non-clonal plants
No community scale response due to a large variation in species-specific responses
Robinson et al. Science. 1992.
Habitat destruction and species decline
Large-scale experimental habitat destruction experiment in Brasil(13 years, 23 patches)
12 pristine forest patches11 isolated patches from 10 to 600 ha
Monitoring of the bird community and analysis with a statistical model of patch turnover in species presence/absence
Extinction rate according to the « best »statistical model
Positive effect of fragmentation on extinction rates, but results are highly variable and many species are insensitive to habitat fragmentation
Negative effect of patch size on extinction rate
Ferraz et al.. Science. 2007.
Diverse effects of habitat fragmentation: why ?
Details that can matter
Landscape structure : corridors and matrices, spatial scale
Behavioural flexibility : context-dependent dispersal
Community processes : species interactions (eg competition-colonisation trade-off, complementarities …)
Example: density-dependent dispersal
Constant dispersal = can cause rescue at low population density and synchronises local population dynamics
Negative density-dependent = precipitates population extinction and limits spatial synchronisation
Dispersal and synchronisation
Example in root voles (Microtus oeconomus) from Norway
Ims and Andreassen. Proc. Roy. Soc. 2005.
Density-dependent dispersal
Cross correlations between weekly growth rates
Spatial correlation for population sizes
Evolutionary consequences of fragmentation
Population dynamics
Ecological responses
Adaptive dynamics
Evolutionary responses
Short-term evolutionary responses to habitat fragmentation ?
Ecological consequences of short-term evolutionary responses ?
Environmental parametersDemographic parameters
DISPERSAL
Habitatfragmentation
Evolutionary changes and fragmentation
Butterfly (Plebejus argus) in UK
Thomas et al. JAE. 1998
Relative thorax mass � flight muscles � flight abilityMeasurements in a common environment
Evolutionary changes and fragmentation
Butterfly (Plebejus argus) in UK
Thomas et al. JAE. 1998
Key references
Fahrig, L. 2003. Effects of habitat fragmentation on biodiversity. Annual Review of Ecology and Systematics. 34:487-515.
Ferraz, G. et al. 2007. A large-scale deforestation experiment: effects of patch area and isolation on Amazon birds. Science 315:238-241.
Hanski I. 1998. Metapopulation dynamics. Nature 396:41-49.
Hanski I. 1999. Metapopulation ecology. Oxford University Press.