Action: Ensure connectivity between habitat patches
Key messagesRead our guidance on Key messages before continuing
- A replicated, controlled study in Canada found significantly higher abundances of some birds, but not forest specialists, in forest patches connected to a continuous area of forest, than in isolated patches.
- Another study of the same system found evidence that corridors were used by some bird species more than clearcuts between patches, although corridors near cut forest were not used more than those near uncut stands.
Habitat fragmentation, as well as destruction, may be an important driver of population declines. Small areas hold fewer species than large ones and if individuals are unable to cross areas of converted habitat then populations in separate habitat patches will become isolated. This potentially makes them more vulnerable to extinction, from natural variations in birth and death rates or sex ratios; from inbreeding depression and from outside pressures, both natural (such as storms or wildfires) and man-made (such as hunting or continued habitat loss), although the precise effects of habitat fragmentation, as opposed to loss, are debated (e.g. Fahrig 1997).
Theoretically, the number of species surviving in a habitat fragment is determined by its size and its effective distance to other habitat patches (MacArthur & Wilson 1967). Connecting remaining areas of habitat is therefore often seen as a way to increase the viability of populations, but there is considerable debate as to the effectiveness of such ‘wildlife corridors’ (e.g. Beier & Noss 1998).
Studies describing the effects of creating habitat corridors, rather than retaining them are described in ‘Habitat restoration and creation’. Beier, P. & Noss, R.F. (1998) Do habitat corridors provide connectivity? Conservation Biology, 12, 1241–1252.
Fahrig, L. (1997) Relative Effects of Habitat Loss and Fragmentation on Population Extinction. The Journal of Wildlife Management, 61, 603–610.
MacArthur, R.H. (1967) The Theory of Island Biogeography. Princeton University Press, Princeton, N.J.
Supporting evidence from individual studies
A replicated, controlled study in boreal forests in 1993-5 in Alberta, Canada (Machtans et al. 1996), found that significantly higher abundances of the ten most common understorey birds were found in three riparian corridors between forest patches than in three clearcuts between patches. Only two of the ten were found nesting or foraging in clearcuts. In addition, significantly more juveniles used corridors following logging, than before, but only in one site. No more birds used the buffer strips near logged forest than similar strips near un-logged forest, when controlling for local abundances. Corridors consisted of 1-5-m of riparian vegetation and 90-110 m of forest. Visual surveys were used in clearcuts and mist nets in corridors.
A replicated, controlled study as part of the same study as Mactans et al. (1996) in mixed boreal forests in northern Alberta, Canada (Hannon et al. 2002), found significantly higher abundances of resident songbirds and woodpeckers, but not of forest specialists, in forest plots connected to a continuous block when compared to isolated fragments. Resident species were found at similar abundances in connected fragments and unfragmented forests, whilst habitat generalists were found at similar abundances across all forest types. None of the individual species analysed appeared to benefit from connectivity. Forest fragments were 10 or 40 ha, either in continuous forest, isolated by a 200 m strip of clearcut on all sides or isolated on three of four sides for connected fragments. Three replicates of each treatment were established in the winter of 1993-4 and monitored until 1998.
- Machtans C.S., Villard M.A. & Hannon S.J. (1996) Use of riparian buffer strips as movement corridors by forest birds. Conservation Biology, 10, 1366-1379
- Hannon S.J. & Schmiegelow F.K.A. (2002) Corridors may not improve the conservation value of small reserves for most boreal birds. Ecological Applications, 12, 1457-1468