Habitat fragmentation. Jean-François Le Galliard CNRS, University of Paris 6, France

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1 Habitat fragmentation Jean-François Le Galliard CNRS, University of Paris 6, France

2 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?

3 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

4 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

5 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

6 A case example Habitat fragmentation Granville fritillary butterfly (Finland) Hanski. Nature

7 Models of habitat fragmentation The Levin s model (occupancy model) m p occupied empty e p = m p (1 p) e p Very fast local dynamics p* = 0 p* = (m-e)/m 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

8 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

9 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/a x,1] extinction rate decreases with patch area C = β exp(-α d ij ) p j A j colonisation rate decreases with distance and increases with patch crowding and patch areas Hanski. Metapopulation ecology

10 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

11 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 Clonal / Non-clonal plants

12 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

13 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

14 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

15 Evolutionary consequences of fragmentation Ecological consequences of short-term evolutionary responses? Habitat fragmentation Ecological responses Population dynamics Environmental parameters Demographic parameters DISPERSAL Evolutionary responses Adaptive dynamics Short-term evolutionary responses to habitat fragmentation?

16 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

17 Evolutionary changes and fragmentation Butterfly (Plebejus argus) in UK Thomas et al. JAE. 1998

18 Key references Fahrig, L Effects of habitat fragmentation on biodiversity. Annual Review of Ecology and Systematics. 34: Ferraz, G. et al A large-scale deforestation experiment: effects of patch area and isolation on Amazon birds. Science 315: Hanski I Metapopulation dynamics. Nature 396: Hanski I Metapopulation ecology. Oxford University Press.