Habitat Loss and Fragmentation

Habitat loss and fragmentation have been termed the greatest worldwide threats to wildlife and the primary causes of species extinction (Simberloff, 1986). Although habitats can be fragmented or lost because of natural disturbances, such as earthquakes or geological forces, humans are the principal threat. People have modified landscapes for thousands of years for agricultural production. Today, agricultural activities are the major cause of habitat loss and fragmentation

Tropical Loggers
The charred remains of logging slash in a Brazilian rain forest. Tropical forests worldwide were severely reduced in area and fragmented during the latter half of the twentieth century. (Stephanie Maze/Corbis)

throughout the world. Human settlement, resource extraction, and industrial development generally result in small, isolated areas or patches of natural habitat surrounded by developed land (Gascon et al., 1999). Humans also alter landscapes through the construction of fences, parking lots, roads, buildings, and hydroelectric dams.

Tropical forests worldwide have been severely reduced in area and fragmented during the latter half of the twentieth century. Much of the world's natural prairie grasslands and steppes have been replaced by wheat, corn, and other crops. Wetlands have been drastically reduced in area and number in many regions of the world as they are drained or filled. Although we often think of fragmentation only in a terrestrial context, frag mentation also occurs in the marine environment. Mangroves, seagrasses, salt marshes, coral reefs, kelp forests, and rocky shorelines are fragmented by natural forces such as wave action, currents, tides, and storm surge, as well as human activities such as dredging, boating, and nutrient enrichment.

Many of the world's major riverine systems are highly fragmented, or their flow has been modified by human intervention. According to the World Register of Dams, between 1950 and 1986 the number of large dams in the world increased sevenfold. Dams fragment habitat and change natural water-flow patterns. In the Pacific Northwest, dams have seriously affected salmon populations by preventing salmon from returning to their native streams to reproduce. Humans also affect river ine systems by diverting their flow for reservoirs or irrigation.

Natural versus Human Fragmentation Several differences exist between human-caused and naturally fragmented landscapes. In human-modified landscapes, most patches are usually unsuitable for wildlife. A naturally patchy landscape has a complex structure with many different types of patches, all suitable for wildlife. A human-fragmented landscape tends to have a simplified patch structure with distinct edges and a few small patches of natural habitats in a large area of developed land. In natural landscapes there is less contrast between adjacent patches, and therefore there are potentially fewer "edge effects" (see below). Certain features of human-fragmented landscapes, such as roads, are novel in the evolutionary history of most wild species and pose unusual threats. Besides fragmenting the environment, roads, especially those that are heavily traveled, are a direct threat to wildlife. Roads also make remote areas more accessible to hunters and aid invasion by exotic species.

Effects of Habitat Fragmentation There are four major consequences of habitat fragmentation: increased habitat loss; decreased patch size; increased number of edge effects; and increased patch isolation.

Habitat loss. Habitat loss is the (permanent) conversion of land to other uses. Habitat loss drives habitat fragmentation, and the two are tightly linked. Fragmentation of habitats is typically a consequence of habitat loss. However, the level of fragmentation may vary even if the same amount of habitat is lost. For example, assume that 100 hectares of trees will be removed from a 200-hectare forest reserve. This habitat loss could occur in one location, leaving one large fragment of 100

hectares. Alternatively, the trees could be removed from several locations across the reserve leaving 100 forest fragments of one hectare each. In both cases the reserve has lost 100 hectares of forest, but in the second scenario there is a much higher level of fragmentation. The total area of remnant forest in the landscape is the same, but the degree of fragmentation and thus the consequences for plants and animals are quite different.

Decreased patch size. The size of the fragments that remain in a landscape is a critical factor in determining the number and type of species that can survive there. Some species (such as bears, tigers, elephants, and migratory birds) require large areas of continuous habitat and simply cannot survive in small patches. They are referred to as area-sensitive species. Larger patches can support larger populations of a given species and thereby buffer them against extinction, inbreeding depression, and genetic drift. For all species—large or small— that cannot cross a forest edge or leave a patch, all requirements to complete their life cycles must be met within the patch. This is especially important for species with complex life cycles. Amphibians, for example, have an aquatic larval stage and an upland adult phase, and require distinct habitats to meet those needs.

Increased edge effects. Many studies have examined the effects of edges on the physical environment and on biological communities that remain after fragmentation (Laurance and Bierregaard, 1997).

Some of the most significant edge effects are the microclimatic changes that take place along a fragment's edge. Edge areas are warmer, more exposed to light and wind, and drier than interior forest. These microclimatic gradients extend from the edge of the fragment into the interior, approximately 15 to 75 m. Changes to the microclimate along the edge can have secondary effects, such as altering vegetation structure and eventually plant and animal communities.

Increased wind along the edge of the fragments can physically damage trees, causing stunted growth or tree falls. This is especially obvious when a fragment first forms, since interior plant species are often not adapted to handle high wind stress. Furthermore, wind tends to dry out the soil, decrease air humidity, and increase water loss (evapotranspiration rates) from leaf surfaces, creating a drier microclimate. This drier environment may increase the risk and frequency of fires.

Along the edge of a fragment, biotic changes, such as changes in plant communities and nutrient cycling, invasions by generalist animal species, and transmission of disease from domesticated animals to wildlife, often extend much farther than the physical changes. In one study, invasion by a disturbance-adapted butterfly species extended nearly 250 m into the forest (Laurance et al., 2000).

Edges are more susceptible to invasion by generalist or "weedy" plant species (such as lianas, vines, creepers, and exotic weeds) that are better adapted to handle disturbance and the new microclimate. Simultaneously, long-lived interior canopy species, epiphytes, and other mature forest taxa decline in abundance. Wind can also increase the transfer of seeds from outlying areas, thereby aiding invasion of foreign, generalist, or weedy species. The increased light along the edges affects both the rate and type of plant growth, favoring light-loving species at the expense of shade-loving ones.

Since many tree species have long life spans, it may take hundreds of years to truly understand the dynamics and effects of fragmentation. The longest running and perhaps the most detailed study of fragmentation effects ever conducted is the Biological Dynamics of Forest Fragments project, which began in 1979. This pioneering project, located in the

Amazon region north of Manaus, Brazil, has generated many of the findings described here and has informed much of our understanding in general of the effects of forest fragmentation. Forest fragments from this area experienced a dramatic loss of plant biomass. Although secondary vegetation (especially vines and lianas) proliferated, the new biomass did not compensate for the loss of "interior" tree species. Loss of biomass in the tropics could also be a source of increased greenhouse emissions from decomposition.

Edge effects alter insect communities and as a result have a profound effect on leaf litter decomposition and hence nutrient cycling (Didham, 1998). Beetles (Carabidae, Staphy-linidae, Scarabaeidae) common to continuous interior forest disappear from forest fragments, which is surprising, given their small size and generalist habitat requirements. Possibly this is a result of the drier microclimate or loss of species they depend on (that is, less mammal dung and fallen fruit on which to reproduce). Another reason for the change is that these insects actually travel tremendous distances in search of decaying material for their reproduction, and they may not be able to cross the area between patches. Whatever the cause, the implications for ecosystem function are significant. Unless there is another organism to fill their role as decomposers, more decaying matter is left on the ground for a longer time, and nutrient cycling may be slowed. Also, the incidence of disease may be elevated as dung is left on the ground longer, allowing flies to breed there.

Increased patch isolation. The degree of isolation of a patch helps determine what biological communities can be sustained in it. In a very isolated patch, species that cannot disperse may become separated from other populations and thus prone to genetic inbreeding and possibly local extinction. The degree of connectivity between patches is similarly important in maintaining sustainable populations of some species. Although patches may appear isolated, their actual biological connectivity depends on whether the habitat that separates them (called the matrix) is hostile to plant and animal dispersal. Note, however, that if the matrix contains similar elements of the patches (for example, forest patches surrounded by grassland or savanna), many species will travel out of the patches for extended periods, thereby greatly expanding the effective area of patches. This movement will generally not occur where dissimilar habitats meet (for example, forest and crop land).

In a given landscape, the effects of connectivity and isolation vary greatly from species to species. For example, species that fly (birds, bats, flying insects) are less affected by patch isolation than less mobile species (such as frogs and beetles).

Fragmentation and Species Diversity Fragmentation causes the loss of animal and plant populations by a process termed faunal relaxation. During relaxation, species loss is nonrandom with respect to their place on the food chain or trophic role, with species at higher trophic levels, such as large-bodied vertebrates, being most vulnerable and typically among the first species to disappear. Thus predators are often lost before their prey, and those species that do manage to persist in small fragments (often herbivores) tend to become far more abundant than populations of the same species in larger, species-rich fragments. Increased abundance is partly a result of decreased competition: when competing species are removed, the resources they utilized become available to the remaining species. Another reason for increased abundance is that prey populations are no longer limited by predators. The overabundance of herbivores on small fragments weeds out palatable plant species and converts the landscape into a forest of "herbivore-proof plants. Fragmentation thus triggers distortions in ecological interactions that result in changes in community composition and structure. Such distortions drive species loss, ending in a greatly simplified ecological system lacking much of the initial diversity.

Species Vulnerable to Fragmentation Behavior, resource needs, reproductive biology, and natural history can be used to identify species that are most vulnerable to fragmentation (Laurance and Bierregaard, 1997). Examples of species that are expected to be most affected by fragmentation include rare species with narrow distributions or small populations; species with large home ranges, such as top carnivores or large animals; species that need heterogeneous landscapes; species that avoid matrix habitats or that have very specialized habitat requirements; species with limited dispersal abilities or low fecundity; and coevolved species (that is, plants with specific pollinators).

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