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.