BCB 322:Landscape Ecology

Lecture 5: Emerging processes II

Fragmentation & connectivity

Introduction• A core issue for modern

conservation planning & landscape ecology (Saunders et al, 1991)

• Has been identified as one of the biggest challenges to biodiversity

• Proceeds extremely rapidly – in large parts of the world almost all fragmentation occurred in the last century (Australia, Amazonia)

• Negative impact on many species due to geographical isolation and sundering of metapopulation communication

• Interior species affected more than others

Deforestation & development, Tanzania

Introduction• Large predators die out first, causing population

overruns of other species• These species may then cause further degradation• (eg): removal of the top predator in Zimbabwean

national parks (man) led to overpopulation and habitat trashing by elephants

• Although island biogeography is used to describe them, fragments are not true islands.

• Connectivity & ecotones must also be considered when studying landscape fragmentation

• In fact, since all landscapes are inherently patchy, fragmentation may be considered as filling a continuum from untouched wilderness to fully- fragmented

• Fragmentation is scale specific for different organisms

Scale dependence

• If a large area is fragmented into smaller intact areas, it is termed “geographical fragmentation” (coarse-grained)

Wiens, 1994

• Fragmentation on the scale of plots is extremely fine-grained (eg): native vegetation in a matrix of invaders

• Effects tend to be species-specific due to scale & resource considerations

• Specialists are generally worse affected by fine grain fragmentation than generalists

• Fragments tend to be more vulnerable to external disturbance (wind, drought, disease)

Diversity in fragments• Species assemblages in fragments are usually subsets

of those of larger plots• Species assemblages in smaller woodlots tend to be

lower than in large ones• Fragmentation method, habitat type and surrounding

matrix effect all play a role in the effect of fragmentation on species– Temperate forest birds show high resilience to fragmentation

into woodlots – Tropical deforestation immediately reduces biodiversity, and

separations of as little as 80m can act as barriers for insects, small mammals & understorey birds

– Burning in Chilean forests promotes the growth of Vismia around remnants, whilst logging does not. Remnants surrounded by this species are considered more isolated by birds (Stouffer & Bieregaard, 1995)

Fragmentation: species response• Some species suffer more from fragmentation due to

habitat size sensitivity (large predators & interior species)

• Eg: amphipods in Australian eucalyptus forest showed marked reductions in populations after fragmentation (Margules et al, 1994)

• By contrast, scorpions showed no significant change in the same plots, possibly because it is capable of fossorial behaviour.

• Small beetles often consider fairly small open spaces to be impassable due to increased risk of dessication

• Conservation responses should therefore look at individual species responses and not just α-diversity

Fragmentation: species responses• Forest tent caterpillar (Malacostoma

disstria) outbreak durations are related to the extent of forest edge/km: parasites & predators are less efficient at the edges (Roland, 1993)

• In a study by Kattan et al (1994), showed that between 1959 & 1990, 31% of avifauna in the upland forest of Colombia were eliminated

• Species that fed in the understorey, and those that required large canopies for fruit provision were hit worse than other species.

• Clearly, the effect of fragmentation depends on the biogeography of the species in question

Roland, 1993

Fragmentation & predation• Species in fragments are often

more vulnerable to predation (Wilcove, 1985)

• Many other factors affect predation rates at the local level, including vegetation type & cover of the surrounding matrix,

• In a fragmentation gradient in Southwestern California, avian nest predators were found to increase with fragmentation, whilst snakes decreased.

• Hence, primarily snake-predated species were less impacted by predators in fragments than other species (Patten & Bolger, 2003)

Wilcove, 1985

http://www.rfadventures.com/images/Animals/Reptiles/Snakes/Non%20Ven/

Fragmentation measurement• There are many measures useable for fragmentation, and often

severable variables must be combined by regression– Area (& ratios of area: long axis length; area: perimeter)– Structure– Isolation– Surrounding land use– Edges (external & internal) per km

• Species turnover in a fragment is calculated as:

where E= extinctionsC = colonisationsS1 & S2 = # breeding species in years 1 & 2

(Diamond, 1969)

• Turnover is inversely related to area, with plots over a certain size having a constant turnover rate.

%100)21()(

SSCE

Connectedness• Isolation obviously causes

problems for species movement.

• Three measures of patch isolation can be considered:– connectedness: the degree of

physical connection between patches (structural attribute). Obviously, the matrix is the most connected element, but generally connectedness is a measure of the patch structure

– connectivity: extent to which subpopulations are connected into a functional demographic unit (functional attribute).

– corridors: functional or structural connection between different subpopulation.

               

               

               

               

               

               

               

               

Connectivity

Farina,1998

Corridors• Functional structures in a landscape which are

fundamental for mitigation of the effects of fragmentation (also for invasive species penetration)

• Important concept for conservation planning, to allow movement of organisms through the reserve and surrounds

• Consequently, they may be structurally recognisable (such as hedgerows in a field matrix), but are not necessarily so

• In fact, there’s little evidence that animals use hedgerows/fencelines as corridors

• Many plant species soil conditions for growth & seed conditions that are not guaranteed by a narrow strip of vegetation

• Hence, “corridor” is an unclear concept, and is used in different contexts in different places in the literature.

Corridors• Can be created by topography (mountain ridges),

hydrology (riverbeds) or human forest clearance and other disturbances

• Rivers are the best studied corridor structures – often associated with alien invasion due to the patchiness of the riparian structure

• Patchiness due to flooding, temporary ponds, seasonal dryness.

• Invasibility differs according to hyrdological & geomorphological zones in the river

• Furthermore, alien plant invasion can be mitigated due to high seasonal variation

www.in.gov/dnr/public/novdec02/corridor.jpg

Corridors• Vital for large home range

mammals in human-impacted landscapes (eg: cougar - Felix concolor – can travel up to 5 miles/night (Beier, 1993)

• Satellite telemetry is useful for measuring ranges of such species

http://www.dsbn.edu.on.ca/Schools/MarthaC/cougar%2082.jpg

• Telemetry measurements of leatherback turtles showed they followed the same route every year to beaches on Costa Rica (Morreale et al, 1996)

• In fact, it appears many species follow “marine corridors” & without testing, these can easily be disrupted by human activity such as fishing

• Corridor width plays a role in the viability of a corridor – too narrow and dispersal capacity is limited

Summary• Fragmentation is a global process that reduces

biodiversity & accelerates local & global extinctions• Fragmentation increases habitat edges and the potential

for predation• Fragmentation is a species specific measure, and

although one species may see the environment as fragmented, it may be homogeneous for another

• Animal dispersion/movements increase with fragmentation

• Connectivity is a functional measure of fragmentation• Connectedness is the structural corollary to connectivity• Corridors are essential for the maintenance of

biodiversity in a fragmented landscape, although definitions of corridors vary

References• Diamond, J.M. (1969) Avifaunal equilibria and species turnover rates on the

Channel Islands of California. Proceedings of the National Academy of Sciences, USA 69:3199-3203

• Farina, A. (1998) Principles and Methods in Landscape Ecology. Chapman and Hall, London, UK

• Kattan, G.H., Alvarez-Lopez & H., Giraldo, M. (1994) Forest fragment and bird extinctions: San Antonio eighty years later. Conservation Biology 8: 138-146

• Margules, C.R., Gaston, A.J. & Hitier, S. (1994) Contrasting effects of habitat fragmentation on the scorpion Cercophonius squama and an amphipod.Ecology 75: 2033-2042

• Patten, M. A. and Bolger, D. T. 2003. Variation in top-down control of avian reproductive success across a fragmentation gradient. – Oikos 101: 479–488.

• Roland, J. (1993) Large-scale forest fragmentation increases the duration of tent caterpillar outbreak. Oecologia 93: 25-30

• Saunders, D.A., Hobbs, R.J. & Margules, C.R. (1991) Biological consequences of ecosystem fragmentation: a review. Conservation Biology 5: 18-32

• Stouffer, P.C. & Bierregaard, R.O. (1995) Use of amazonian forest fragments by understory insectivorous birds. Ecology 76: 2429-2445

• Wiens, J.A. (1994) Habitat fragmentation: island v landscape perspectives on bird conservation. Ibis 137: S97-S104

• Wilcove, D.S. (1985) Nest predation in forest tracts and the decline of migratory songbirds. Ecology 66:1211-1214