Introduction

Several studies have reported that habitat loss and fragmentation are leading causes of imperilment for plants and animals. Very frequently, habitat loss proceeds in ways that habitat fragments are left resulting in smaller, more isolated areas of habitat. Humans also regularly erect barriers to movement in forms such as fences, roads, canals, dams, and urban areas. It is therefore almost inevitable that the capacity of many organisms to move through a landscape will be reduced. The following section reviews types of changes in habitat connectivity and their consequences. This is followed by a description of how we measure habitat connectivity, and a section on how habitat corridors are being used to attempt to reduce the effects of fragmentation.

Habitat connectivity is defined as the degree to which a landscape facilitates or impedes movement of organisms among habitat or resource patches. The term is used synonymously with habitat connectance. (This article does not review food web connectance, which refers to the density of trophic links in a food web - see Food Chains and Food Webs.) Habitat connectivity includes elements of both the physical structure of habitat and the movement ability and behavior of the organisms in question. Structural connectivity denotes the physical structure of habitat, and is evaluated using measures of habitat extent, subdivision, and contagion, or some combination of these. By contrast, functional connectivity refers to the potential for movement of organisms among habitat or resource patches, and depends on the organisms' perception of, and reaction to, their environment, as well as on their ability to move and costs of movement. Therefore functional connectivity may vary between species or between individuals of the same species. For some species, natural currents (e.g., stream flow or oceanic currents) or winds may influence functional connectivity. Both structural and functional connectivity may vary among natural environments and as a result of human activities that alter the composition and patterning of the environment and/or an organism's movement behavior. Some studies have defined connectivity on a purely structural basis, and others have used the presence of habitat corridors as an indicator of connectivity.

Habitat connectivity can also influence the flux of materials across ecosystem boundaries, which is considered under the topic ecosystem subsidy. Here, the overlap between subsidy and habitat connectivity is considered. Ecosystem subsidy is defined as the transport of a donor-controlled resource (prey, detritus, or nutrients) from one habitat to another where it is utilized by a recipient plant or consumer. Transport here involves both physical fluxes and flows, including river flows, winds, and oceanic currents. A network of channels or rivers joining lakes or ponds fits naturally into ideas of structural connectivity. However, transport via winds and oceanic currents are not usually considered in landscape studies of structural connectivity, which usually involve the quantification of static landscape patterns. Ecosystem subsidy is therefore dependent on connectivity, but the literature for patterns with fluxes and flows is largely separate from the landscape ecology literature about habitat connectivity. Studies of ecosystem subsidy typically investigate processes at ecosystem and community levels such as fluxes of nutrients and levels of biomass at different trophic levels. By contrast, landscape ecological studies of habitat connectivity are more frequently motivated by maintaining species diversity and population persistence. Hence connectivity in a broad sense influences processes at levels from individuals up to entire ecosystems, and at the ecosystem level it is termed ecosystem subsidy.

The prime importance of the movement of organisms to biodiversity is revealed by a variety of ecological and evolutionary studies. Below we briefly review the importance of island biogeography studies of species diversity, metapopulation studies ofthe population dynamics ofone or a few species, and studies of inbreeding in relation to isolation.

MacArthur and Wilson's equilibrium theory of island biogeography theorizes that the level of species diversity on habitat islands arises as a balance between the processes of colonization and extinction of species. Colonization is related to distance from a large mainland area of habitat such that more isolated habitat islands should contain fewer species than islands that are closer to a large mainland area of habitat. Extinction of species is related to island size, such that smaller islands can hold fewer species than large islands. Hence, smaller more remote islands should contain fewer species than larger and more connected islands. Furthermore, there is a turnover of species and colonization is required to maintain species diversity. There is a broad range of empirical evidence supporting this theory in a diverse array of taxa. These ideas were brought to the attention of conservation biologists by Wilson and Willis, who in 1975 published a figure laying out the idea that islands closer together and those connected by habitat corridors support more species than islands that are less connected. The ideas both from the equilibrium theory of island biogeo-graphy and from Wilson and Willis' figure have been broadly applied to habitat islands of many kinds, both aquatic and terrestrial, and have been influential in perpetuating the importance of habitat connectivity.

Instead of exploring the effects of colonization and extinction on species diversity, metapopulation theory relates these processes to the regional population dynamics of one or a few species. Metapopulation studies show how reduced connectivity increases extinction rates of local populations and reduces rates of recolonization of vacant habitat patches. The net effects of such dynamics depend on whether species have their own independent dynamics, as represented in single species models, or whether species can be driven locally extinct by predators or competitors. As shown in Figure 1, the isolation of increasingly small habitat patches can only reduce the likelihood of regional persistence for single or noninter-acting species, whereas interacting species can actually benefit from moderate reductions in connectivity. For example, in predator and prey metapopulation models, a voracious predator might drive its prey species extinct

Noninteracting (single) species, or species with stable within-patch interactions

Noninteracting (single) species, or species with stable within-patch interactions

Large, well connected

Figure 1 The influence of habitat subdivision on single species and interacting species where one species can drive another extinct within a habitat patch (e.g., a predator and prey, or a dominant and subordinate competitor species). In highly connected habitat patches, the predator is capable of driving the prey species regionally extinct whereas a single species can persist. At extremely low levels of connectivity in small habitat fragments, all species go extinct regardless of their interactions. Overall, the figure illustrates that the degree to which species interact may alter how they are influenced by changes in connectivity and habitat patch size.

Large, well connected

Small isolated fragments

Figure 1 The influence of habitat subdivision on single species and interacting species where one species can drive another extinct within a habitat patch (e.g., a predator and prey, or a dominant and subordinate competitor species). In highly connected habitat patches, the predator is capable of driving the prey species regionally extinct whereas a single species can persist. At extremely low levels of connectivity in small habitat fragments, all species go extinct regardless of their interactions. Overall, the figure illustrates that the degree to which species interact may alter how they are influenced by changes in connectivity and habitat patch size.

from large well-connected areas of habitat. Reduced connectivity might reduce the ability of predators to reach areas containing prey, thereby weakening the net effect of predators on prey by providing prey with a refuge from predation. If connectivity is reduced too much, prey may be hindered from reaching habitat areas from which they have been driven extinct, so that local extinction rate exceeds the recolonization rate and eventually prey are driven extinct and predators would starve to death. In predator and prey metapopulation models, the dynamics of prey at extremely low levels of connectivity are similar to those of single species. There is a moderate amount of evidence supporting such dynamics for predators and prey, but mostly from microcosms and other highly manipulable systems. Metacommunity models of many competing species show similar effects of reduced connectivity, with the added prediction that species that are the worst at dispersing should be least able to withstand reductions in connectivity even if they are strong competitors in local communities.

The isolation of a habitat patch refers to the rate of immigration into that habitat patch. Hence, isolation and connectivity are inversely related. Mortality during dispersal may add to isolation. In the absence of immigration, populations are expected to suffer from several effects. Small populations are likely to have reduced genetic variability as a result of the small number of individuals present. This may be compounded by the accumulation of deleterious mutations through inbreeding, leading to mortality and reduced reproductive capacity in a population. Small populations are also more likely to go extinct, such that a metapopulation with lots of small populations in a region may experience a regional reduction in genetic diversity, which may exasperate the effects of small population size through reducing the potential for gene flow. Populations with low heterozygosity may have a reduced potential for future evolution. A meta-analysis (quantitative review) by Derek Spielman, Barry Brook, and Richard Frankham of over 170 threatened taxa from the IUCN (World Conservation Union) Red List showed that on an average, threatened taxa had a 35% reduction in heterozygosity compared to nonthreatened relatives. Several empirical studies indicate that the loss of heterozygosity is frequently associated with a reduction in reproductive rates through inbreeding. Furthermore, computer simulation models that involve reasonable assumptions suggest that this 35% level of loss of heterozygosity could cause 24-78% reductions in the median time to population extinction because of reduced reproduction (estimates vary depending on assumptions about the extent of juvenile mortality caused by inbreeding). Habitat loss and fragmentation are the prime causes for imperilment of taxa (at least in the US), and therefore reductions in connectivity are likely to be involved in these effects. It is important to realize that in severe cases reduced connectivity likely produces genetic changes that can impact populations before purely demographic processes cause these populations (or species) to go extinct. Hence, both demographic and genetic changes are important to understanding the effects of habitat fragmentation and reduced habitat connectivity.

Implicit in the idea of measuring connectedness is that there is a sharp demarcation between habitat and nonhabitat. By contrast, many organisms are not strict habitat specialists and may use different landscape elements to different extents. A step toward recognizing that species may not sharply demarcate habitat from nonhabitat while still simplifying the landscape is to recognize the matrix that lies between habitat patches. Differentiating between species that use the matrix and those that do not may improve the predictability of analyses that look at species diversity in relation to connectivity because connectivity is only relevant to species that do not use the matrix as habitat. The same is true of metapopulation studies that investigate patch occupancy in relation to connectivity.

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