Habitat loss and fragmentation are widely recognized as the main cause of increased species extinction, and are thus perhaps the most important conservation issues in the world today.
Fragmentation is the transformation of a relatively homogenous area into smaller homogenous areas that are interspersed with disturbed areas. Disturbances are either natural (e.g. fire, insects, wind) or human-caused (e.g. logging). Agriculture, urbanization and forestry disturb the greatest amount of natural area and are considered to be the main sources of human-caused fragmentation worldwide.
Forestry vs. agriculture and urbanization
From a simplistic perspective, urban and agricultural areas can be viewed as consisting of a few ‘islands’ of trees in a permanent ‘sea’ of treeless land. In contrast, areas of about the same size that have been logged, particularly in British Columbia, usually consist of the opposite pattern – impermanent ‘islands’ of cleared trees in a ‘sea’ of trees.
Agriculture and urbanization have larger localized and immediate effects than forestry on biodiversity in forested areas.1,2 For example, ‘edge effects’ on nest predation seem to be stronger at forest edges in agricultural landscapes than in logged landscapes.3
Habitat loss and fragmentation due to forestry occur over a larger area and at a faster rate than has resulted from agriculture and urbanization. Commercial forest harvesting is the main human cause for loss and fragmentation of late seral forest in the Pacific Northwest.4 In 1988, about 40% of British Columbia was fragmented by roads, while some BEC zones like the Interior Douglas-Fir and Coastal Douglas-Fir were more than 95% fragmented.5 Thus, at larger spatial scales and over the long term, habitat loss and fragmentation from forestry is likely to have an impact on biodiversity equivalent to that from urbanization and agriculture.
Two events occur when forests are fragmented. Habitat is lost and the spatial arrangement of habitat changes. Hypothesized impacts to biodiversity are both positive (i.e. increases in abundance of some species, particularly generalists) and negative, such as loss of gene flow due to barriers to movement, loss of species and individuals that were restricted to the lost habitat, crowding and increased competition of individuals and species in habitat remnants and the degradation of remaining habitat due to edge effects and species invasions.6 Though the overall number of species may not change or may even increase when a landscape is fragmented, the biotic community is altered.7
Habitat loss vs. fragmentation
Habitat loss and fragmentation are two separate processes.8-9 Two areas may have the same amount of habitat, but the spatial arrangement of remnant habitat and thus the amount of fragmentation within each can be drastically different. Fragmentation per se can degrade habitat for some species while improving it for others, and these effects may be independent of habitat loss. Currently, the majority of evidence suggests that overall habitat loss has a much larger effect on biodiversity than the spatial arrangement of remnant habitat.10
Forests are naturally fragmented. Geology, climate, and natural disturbances such as fire create a mosaic of habitat patches. Wetlands, sub-alpine forests, and old growth areas, particularly in forests with frequent disturbance intervals (e.g. boreal forests) – all naturally exist as ‘patches’.11 And there are likely many ‘patches’ beyond our human perceptions. For some insects, a single species of a shrub may be habitat and each individual shrub a habitat patch. The problem with forestry-created fragmentation is that the scale, rate, and pattern of fragmentation differs from that created naturally.12
The Search for Generality
As of May 2004, more than 2000 studies on habitat fragmentation effects on biodiversity had been published. Despite this plethora of research, reviews of experimental approaches and observational studies do not provide clear insights that can be used in management.13,14 This lack of clarity can be partially attributed to human limitations. Only a fraction of studies separate the effects of habitat loss from those of fragmentation; fragmentation has mostly been measured at the patch not the landscape scale, and has been conceptualised and measured differently.6,12 The majority of studies have been on vertebrates, though it is likely that non-vascular plants and insects are more sensitive to fragmentation. For some species, habitat loss and fragmentation effects may exist but may not be quantified by a particular study. Finally, in many studies, habitat fragments have been assumed to act like islands surrounded by a sea of non-habitat. In reality, the surrounding land may provide resources that do not exist in the habitat or may augment those that are found in the habitat, especially for generalist species.11,15
While these human limitations can be overcome to some degree, vast ecological variability may continue to veil generality in the effects of habitat loss and fragmentation on species.16,17 The following factors illustrate this variability: landscape effects are multi-causal; fragmentation effects are species-specific; landscape context and disturbance history vary by location; and time lags – time between cause and effect – result in the occurrence of effects after studies are completed.13,14,16 One generality however is clear. Cutting less forest will have the most positive impact on the maintenance of biodiversity.
1Schmiegelow, F.K.A. and M. Mönkkönen. 2002. Habitat loss and fragmentation in dynamic landscapes: avian perspectives from the boreal forest. Ecological Applications 12: 375-389.
2McComb, W.C. 1999. Forest fragmentation: wildlife and management implications synthesis of the conference. In J.A. Rochelle, L.A. Lehmann, J. Wisniewski (eds.). Forest fragmentation: wildlife and management implications (pp. 295-301). Brill, Leiden, Netherlands.
3Chalfoun, A.D., F.R. Thompson and M.J. Ratnaswamy. 2002. Nest predators and fragmentation: a review and meta-analysis. Conservation Biology 16: 306-318
4Lehmkuhl, J.F. and L.F. Ruggiero. 1991. Forest fragmentation in the Pacific Northwest and its potential effects on wildlife. In L.F. Ruggiero, K.B. Aubry, A.B. Carey, and M.H. Huff (technical coordinators) . Wildlife and vegetation of unmanaged Douglas-fir forests (pp. 35-46). US Forest Service Technical Report PNW-285.
5Harding, L.E. and E. McCullum. 1994. Overview of ecosystem Diversity. In L.E. Harding and E. McCullum (eds.). Biodiversity in British Columbia: our changing environment (pp.227-244). Environment Canada, Ministry of Supply and Services, Ottawa.
6Fahrig, L. 2003. Effects of habitat fragmentation on biodiversity. Annual Review in Ecology, Evolution, and Systematics 34: 487-515.
7Harrison, S.E. Bruna. 1999. Habitat fragmentation and large-scale conservation: what do we know for sure? Ecography 22: 225-232.
8Andrén, H. 1994. Effects of habitat fragmentation on birds and mammals in landscapes with different proportions of suitable habitat: a review. Oikos 71: 355-364.
9With, K.A. and A.W. King. 1999. Extinction thresholds for species in fractal landscapes. Conservation Biology 13: 314-326.
10Fahrig, L. 2001. How much habitat is enough? Biological Conservation 100:65-74.
11Bunnell, F.L. 1999. What habitat is an island? In J.A. Rochelle, L.A. Lehmann, J. Wisniewski (eds.). Forest fragmentation: wildlife and management implications (pp. 1-31). Brill, Leiden, Netherlands.
12McGarigal, K. and W.C. McComb. 1999. Forest fragmentation effects on breeding bird communities in the Oregon Coast range. In J.A. Rochelle, L.A. Lehmann, J. Wisniewski (eds.). Forest fragmentation: wildlife and management implications (pp. 223-246). Brill, Leiden, Netherlands.
13Debinski, D.M. and R.D. Holt. 2000. A survey and overview of habitat fragmentation experiments. Conservation Biology 14: 342-355.
14McGarigal, K. and S.A. Cushman. 2002. Comparative evaluation of experimental approaches to the study of habitat fragmentation studies. Ecological Applications 12: 335-345.
15Brotons, L., M. Mönkkönen and J.L. Martin. 2003. Are fragments islands? Landscape context and density-area relationships in boreal forest birds. The American Naturalist 162: 343-357.
16Bissonette, J.A. and I. Storch. 2002. Fragmentation: is the message clear? Conservation Ecology. www.consecol.org/vol6/iss2/art14/main.html.
17Villard, M-A. 2002. Habitat fragmentation: major conservation issue or intellectual attractor? Ecological Applications 12: 319-320.