Processes that affect species in fragmented landscapes

                The size of any population is determined by the balance between four parameters: births, deaths, immigration, and emigration. Population size is increased by births and immigration of individuals, while deaths and emigration of individuals reduce population size. In fragmented landscapes, these population parameters are influenced by several categories of processes.    

Deterministic processes

                Many factors that affect populations in fragmented landscapes are relatively predictable in their effect. These factors are not necessarily a direct consequence of habitat fragmentation, but arise from land uses typically associated with subdivision. Populations may decline due to deaths of individuals from the use of pesticides, insecticides or other chemicals; hunting by humans; harvesting and removal of plants; and construction of roads with ensuing road kills of animals. For example, in Amazonian forests, subsistence hunting by people compounds the effects of forest fragmentation for large vertebrates such as the lowland tapir (Tapir terrestris) and white-lipped peccary (Tayassu pecari), and contributes to their local extinction (Peres 2001). Commonly, populations are also affected by factors such as logging, grazing by domestic stock, or altered disturbance regimes that modify the quality of habitats and affect population growth. For example, in Kibale National Park, an isolated forest in Uganda, logging has resulted in long-term reduction in the density of groups of the blue monkey (Cercopithecus mitza) in heavily logged areas: in contrast, populations of black and white colobus (Colobus guereza) are higher in regrowth forests than in unlogged forest (Chapman et al. 2000). Deterministic processes are particularly important influences on the status of plant species in fragments (Hobbs and Yates 2003).

Isolation

                 Isolation of populations is a fundamental consequence of habitat fragmentation: it affects local populations by restricting immigration and emigration. Isolation is influenced not only by the distance between habitats but also by the effects of human land-use on the ability of organisms to move (or for seeds and spores to be dispersed) through the landscape. Highways, railway lines, and water channels impose barriers to movement, while extensive croplands or urban development create hostile environments for many organisms to move through. Species differ in sensitivity to isolation depending on their type of movement, scale of movement, whether they are nocturnal or diurnal, and their response to landscape change. Populations of one species may be highly isolated, while in the same landscape individuals of another species can move freely.Isolation affects several types of movements, including: (i) regular movements of individuals between parts of the landscape to obtain different requirements (food, shelter, breeding sites); (ii) seasonal or migratory movements of species at regional, continental or inter-continental scales; and (iii) dispersal movements (immigration, emigration) between fragments, which may supplement population numbers, increase the exchange of genes, or assist recolonization if a local population has disappeared. In Western Australia, dispersal movements of the blue breasted fairy-wren (Malurus pulcherrimus) are affected by the isolation of fragments (Brooker and Brooker 2002). There is greater mortality of individuals during dispersal in poorly connected areas than in well-connected areas, with this difference in survival during dispersal being a key factor determining the persistence of the species in local areas. For many organisms, detrimental effects of isolation are reduced, at least in part, by habitat components that enhance connectivity in the landscape (Saunders and Hobbs 1991; Bennett 1999). These include continuous “corridors” or “stepping stones” of habitat that assist movements (Haddad et al. 2003), or human land-uses (such as coffee-plantations, scattered trees in pasture) that may be relatively benign environments for many species (Daily et al. 2003). In tropical regions, one of the strongest influences on the persistence of species in forest fragments is their ability to live in, or move through, modified “countryside” habitats (Gascon et al. 1999; Sekercioglu et al. 2002).

Stochastic processes

                When populations become small and isolated, they become vulnerable to a number of stochastic (or chance) processes that may pose little threat to larger populations. Stochastic processes include
the following.

• Stochastic variation in demographic parameters such as birth rate, death rate and the sex ratio of offspring. 

• Loss of genetic variation, which may occur due to inbreeding, genetic drift, or a founder effect from a small initial population size. A decline in genetic diversity may make a population more vulnerable to recessive lethal alleles or to changing environmental conditions. 

• Fluctuations in the environment, such as variation in rainfall and food sources, which affect birth and death rates in populations. 

•  Small isolated populations are particularly vulnerable to catastrophic events such as flood, fire, drought or hurricanes. A wildfire, for example, may eliminate a small local population where as in extensive habitats some individuals survive and provide a source for recolonization.

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Land-use intensification and abandonment

              Humans have transformed a large fraction of the Earth’s land surface. Over the past three centuries, the global extent of cropland has risen sharply, from around 2.7 to 15 million km2, mostly at the expense of forest habitats (Turner et al. 1990). Permanent pasture lands are even more extensive, reaching around 34 million km2 by the mid-1990s (Wood et al. 2000). The rate of land conversion has accelerated over time: for instance, more land was converted to cropland from 1950 to 1980 than from 1700 to 1850 (MEA 2005). 

               Globally, the rate of conversion of natural habitats has finally begun to slow, because land readily convertible to new arable use is now in increasingly short supply and because, in temperate and boreal regions, ecosystems are recovering somewhat. Forest cover is now increasing in eastern and western North America, Alaska, western and northern Europe, eastern China, and Japan (Matthews et al. 2000; MEA 2005). During the 1990s, for instance, forest cover rose by around 29 000 km2 annually in the temperate and boreal zones, although roughly 40% of this increase comprised forest plantations of mostly non-native tree species (MEA 2005). Despite partial recovery of forest cover in some regions (Wright and Muller-Landau 2006), conversion rates for many ecosystems, such as tropical and subtropical forests and South American cerrado savanna-woodlands, remain very high. Because arable land is becoming scarce while agricultural demands for food and bio fuel feed stocks are still rising markedly (Koh and Ghazoul 2008), agriculture is becoming increasingly intensified in much of the world. Within agricultural regions, a greater fraction of the available land is actually being cultivated, the intensity of cultivation is increasing, and fallow periods are decreasing (MEA 2005). 

                Cultivated systems (where over 30% of the landscape is in croplands, shifting cultivation, confined-livestock production, or freshwater aquaculture) covered 24% of the global land surface by the year 2000. Thus, vast expanses of the earth have been altered by human activities. Old-growth forests have diminished greatly in extent in many regions, especially in the temperate zones; for instance, at least 94% of temperate broadleaf forests have been disturbed by farming and logging (Primack 2006). Other ecosystems, such as coniferous forests, are being rapidly converted from old-growth to semi-natural production forests with a simplified stand structure and species composition. Forest cover is increasing in parts of the temperate and boreal zones, but the new forests are secondary and differ from old-growth forests in species composition, structure, and carbon storage. Yet other ecosystems, particularly in the tropics, are being rapidly destroyed and degraded. For example,marine ecosystems have been heavily impacted by human activities. The large-scale transformations of land cover described here consider only habitat loss per se. Of the surviving habitat, much is beingdegraded in various ways such as by habitat fragmentation, increased edge effects, selective logging, pollution, overhunting, altered fire regimes, and climate change.

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