Habitat fragmentation

Jean-François Le Galliard CNRS, 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 distribution spatial 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 corridors permeability : matrix quality and spatial structure

A case example Habitat fragmentation Granville fritillary butterfly (Finland)

Hanski. Nature. 1998.

Models of habitat fragmentation The Levin’s model (occupancy model)

m×p occupied

e

empty

p’ = m p (1 – p) – e p p* = 0 p* = (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 viable above 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 that Absolute sinks would not persist in the absence of sources A large proportion of a population can exist in sink habitats In the case of density-dependent regulation Sinks are set above their carrying capacity Sources are set below their carrying capacity Asymmetric migration between habitat patches (unbalanced dispersal) Pulliam. Am. Nat. 1988.

Models of habitat fragmentation The metapopulation model discrete spatial structure two spatial scales (local and regional) local persistence for at least a few generations dominant 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 No community scale response due to a large variation in species-specific responses

3 common small mammals (from large to small)

snakes

Robinson et al. Science. 1992.

Clonal / Non-clonal plants

Habitat destruction and species decline Large-scale experimental habitat destruction experiment in Brasil (13 years, 23 patches) 12 pristine forest patches 11 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

Density-dependent dispersal

Cross correlations between weekly growth rates

Spatial correlation for population sizes

Ims and Andreassen. Proc. Roy. Soc. 2005.

Evolutionary consequences of fragmentation

Ecological consequences of short-term evolutionary responses ? Habitat fragmentation

Population dynamics Ecological responses

Environmental parameters Demographic parameters

DISPERSAL

Evolutionary responses

Adaptive dynamics Short-term evolutionary responses to habitat fragmentation ?

Evolutionary changes and fragmentation Butterfly (Plebejus argus) in UK Relative thorax mass  flight muscles  flight ability Measurements in a common environment

Thomas et al. JAE. 1998

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.