Convention on biological diversity

MECHANISMS

Agricultural development

B
Unsustainable levels of consumption and production

Human population growth and density

. An historical and present perspective of forest biological diversity

While the causes of past extinctions remain under debate, the recent phenomenal rate of extinctions associated with humans has been substantiated for a few cases, some noted in Pimm et al. (1995). Humans have caused extinctions for several thousands of years (Martin, 1973). A particular example is the Polynesian occupation of Pacific islands, which resulted in the extinction of more than 2000 bird species [reference needed]. In the north-eastern United States and southern Canada, an area with trees was logged continuously since European colonisation. During that period, four of 26 endemic bird species (16%) have gone extinct and it has been calculated that humans now cause about 1000 - 10000 extinctions per million species-years (Pimm et al., 1995). Extinction is not always a rapid process. Many species may be driven to extinction over several hundred years by now immutable processes. However, in the 15^th and 16^th centuries, the global spread of European cultures and their livestock, crops, weeds and diseases increased the loss of island flora and fauna and added to the threats to continental species. In later centuries, the growth of trans-oceanic human travel and commerce led to the spread of a tremendous variety of species to new regions of the world and human colonisation of many uninhabited islands. Native species may be out-competed by the exotic species introduced, deliberately or accidentally, by humans. Current estimated rates of extinction for most higher life-forms in tropical rain forests are 1-10% of these species in the next 25 years (or 1000 to 10,000 times background rates) (UNEP, 1995).

Causes of extinction are many: Allee effects in small populations, environmental change, demographic and genetic stochasticity, excessive killing by humans, competition from exotic species, habitat loss and fragmentation and other habitat changes (Box 5). Most extinctions faced by contemporary animal species are the result of habitat loss and competition (or predation) by exotic species (WCMC, 1992). Indeed, habitat loss is the major threat to 75% of endangered birds (Pimm et al., 1995). For trees, the number of species at risk is largely a result of logging and clearing for agriculture, which together threaten about 2300 species (Oldfield et al., 1998).

Extinctions may be caused by isolation of populations, followed by inbreeding and loss of fitness to react to environmental changes, competition and natural selection (see Chapter 1). Lessons from population genetics indicate that in forest areas, large areas of habitat must exist to maintain populations of large-bodied species, there must be connections between habitat patches to enable gene flow, populations should always be larger than that which is minimally viable, there should be multiple separate populations to avoid loss by local catastrophic events, and great care should be taken with island ecosystems. Of course, in biological systems, cascading effects can result from individual extinctions, when a species with particular functional importance to an ecosystem (keystone species) is removed (see earlier chapters).

Some countries are attempting to redevelop historical forest types, based on retrospective studies of biodiversity and forest structures. The idea is to establish a ‘benchmark’ of forest types, structures and age classes in the landscape and then to try and redevelop historical ecosystems through silvicultural techniques. For instance, in eastern Canada, which has had a history of clearcutting and high-grade logging over 300 years, several important conifer species, such as white pine (Pinus strobus), red spruce (Picea rubens) and eastern hemlock (Tsuga canadensis) have been severely depleted. Redevelopment of the historical forest ecosystem, as part of the biodiversity objectives set for the forest, is occurring within the Canadian Model Forest Network Programme (see Box 9 in Chapter IV).

C. Direct causes of loss of forest biodiversity

As mentioned earlier, human actions are the most important direct cause of forest loss. Their destructive actions include: adverse agriculture practices (conversion to agricultural land and industrial agriculture, overgrazing and unmitigated shifting cultivation); unsustainable forest management (such as poor logging practices, over-exploitation of wood for fuel and charcoal manufacture, over collection of non-timber forest resources); introduction of exotic and/or invasive plant and animal species; infrastructure development (road building, hydro-electrical development, improperly planned recreational activities); mining and oil exploitation. In addition, human cause forest fires, pollution and climate change all of which are likely to lead to forest loss.