biome (Whitmore 1997). The results of such rapid clearing will be apparent ..... tropical agricultural landscape. M. Sc. Thesis, James Cook University, Townsville,.
Landscape connectivity and biological corridors Susan G. W. Laurance1,2 1
Smithsonian Tropical Research Institute, Apartado 2072, Balboa Ancón, Republic of Panama 2
Biological Dynamics of Forest Fragments Project, National Institute for Amazonian Research (INPA), C.P. 478, Manaus, AM 69011-970, Brazil
Introduction In the tropics, natural habitats are being converted to agricultural land faster than any other biome (Whitmore 1997). The results of such rapid clearing will be apparent in the next few decades when most of the remaining tropical forest will occur as isolated remnants (Myers 1984). The type of habitat that surrounds these remnants may play a crucial role in their conservation. Adjoining habitats (e.g. agroforestry lands) that are more similar to the remnants in terms of structure and floristic composition will be the most beneficial to the long-term preservation of biodiversity.
In addition to supporting native species of plants and animals, agroforestry systems may contribute to the conservation of biodiversity by facilitating the connectivity of populations, communities, and ecological processes in fragmented landscapes. Habitats that can maintain this connectivity across the landscape are commonly referred to as biological corridors, or simply corridors. Corridors can consist of various types of habitat, but by definition will differ from the surrounding vegetation, and will link habitat remnants that were once originally connected (Saunders and Hobbs 1991).
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If agroforestry systems are to be used to increase landscape connectivity, it is important to understand which characteristics of a corridor will make it effective for a given organism. As few studies have been carried out on the movement of wildlife through agroforestry corridors, this chapter will review the relevant literature on tropical forest corridors. I will describe how rainforest animals select and use linear habitat remnants and which features appear to be most important for corridor effectiveness. I will also consider some of the relevant research on wildlife use of agroforestry plantations in order to discuss their usefulness in connecting landscapes.
Landscape connectivity is a function of both the environmental features of a corridor and the behavior of wildlife species that may attempt to use the corridor (Merriam 1984). The general assumption underlying the value of landscape connectivity is that a fragmented landscape that is interconnected is more likely to support viable faunal and floral populations and intact ecological processes, than a landscape that is comprised of only isolated fragments (Harris 1984; Bennett 1998).
This assumption is based on two theoretical concepts: island biogeography theory (MacArthur and Wilson 1967) and metapopulation models (Levins 1970). Island biogeography theory proposed that the number of species contained on an isolated community (such as an oceanic island or forest fragment) is the result of a dynamic equilibrium between opposing forces of colonization and extinction. An application of the theory predicts that with increasing isolation between forest fragments, there will be a decreasing rate of immigration by species unable to traverse the modified habitat. Numerous studies have documented that land-bridge islands and forest fragments lose species after isolation, a phenomenon termed species relaxation (Diamond 1972; Terborgh
1974; Laurance and Vasconcelos, chap.--). 2
Rather than examining entire species assemblages, metapopulation models consider the population of a single species, which occurs in spatially-separate subpopulations that are connected by dispersal (Levins 1970; Forman 1997). While there are various types of metapopulation models, there are two general forms. The first identifies a major source population that dispersers outwards to smaller sink populations. This refers to a situation where small habitat fragments are partly separated from a larger area of intact habitat. The second is a population that is patchily distributed throughout the landscape and connected by dispersal, as would occur when only small forest fragments remain in a formerly forested landscape. Small populations in the models are prone to local extinction, and the movement of individuals among patches can both bolster dwindling populations via their genetic and demographic contributions (termed the “rescue effect”; Brown and Kodric-Brown 1977) and recolonize local patches where the species has gone extinct.
In both island biogeography theory and metapopulation models, the extinction rate of a species will depend upon the size and quality of the remnant habitat(s), whereas recolonization will depend on the level of landscape connectivity. Various landscape features may affect connectivity, such as the distance between remnant patches, the type of surrounding matrix (modified habitats), and the presence of corridors or small habitat patches that can function as stepping-stones.
Corridors were first proposed for conservation planning in 1975, based on fragmentation and island studies (Diamond 1975; Wilson and Willis 1975). Since that time there have been many studies that have demonstrated the major benefits of corridors, which include: 1) facilitating wildlife movement; 2)
providing habitat; and 3) benefiting 3
ecosystem processes (Bennett 1998). Alternatively, some studies have warned of the potential costs of corridors, such as 1) the risk of spread of biotic and abiotic disturbances to remnant populations and habitats, 2) the potential for increased wildlife mortality in corridors, and 3) insufficient information on whether the financial costs of corridors could be better invested in other conservation initiatives (e.g. purchasing land). The benefits and costs of corridors can be summarised into four major landscape functions: a pathway for movement, a habitat for residency and source of new individuals, a population sink, and a complete or partial barrier for movement (Figure 1).
****Insert Figure 1 here****
Benefits of corridors Facilitating different types of movement An array of studies has demonstrated that habitat corridors can facilitate the movement of wildlife. Three types of movement have been described: local, migratory, and dispersal (Bennett 1990). Although few studies have been conducted in tropical rainforest, there is evidence that some species of mammals, birds, butterflies and beetles will undertake local movements through corridors within their home range and/or between foraging and nesting sites (Lovejoy et al. 1986; Isaacs 1995; and see Bennett 1998 for review).
Migratory movements of wildlife through corridors have been observed in mammals, birds, and amphibians (Newmark 1993; Powell and Bjork 1995; Forman and Deblinger 2000). Wildlife species in these studies have been observed preferentially moving through forested corridors rather than the surrounding agricultural matrix. One study in Costa Rica, for example, showed that large tropical
frugivores (e.g. Resplendent Quetzal, 4
Pharomachrus mocinno) required forested corridors from montane to lower elevational forests, so that birds could follow seasonal changes in their food supply (Powell and Bjork 1995). Whereas, for some Nearctic migrants, shade coffee and cacão plantations in Central America provide useful and even crucial habitats for these bird species (Robbins et al. 1992; Prefecto et al. 1996; Greenberg et al. 1997).
Dispersal movements are normally described as the one-way movements of young animals seeking unoccupied territories in order to breed. Corridor studies have demonstrated that the dispersal movements of mammals, birds, butterflies and plants can be assisted by habitat linkages (Bennett 1998). Dispersal movements are important for population dynamics because they allow individuals to immigrate to new populations or to recolonize locally extinct populations. One of the key challenges concerning the effectiveness of corridors is to demonstrate that dispersing individuals not only move through corridors but also become established in fragment populations. Only in this way can immigrants reduce the negative consequence of insularization on fragment populations. Genetic evidence is beginning to accumulate that confirms that gene flow is occurring between fragmented faunal populations linked by corridors, thereby demonstrating the successful establishment of immigrants in the population (Mech and Hallett 2001).
Providing habitat for resident species Depending upon the shape, habitat structure, and floristic composition of corridors, a range of wildlife species may reside in them. Edge and generalist species are probably the most common occupants of corridors, predominating in narrow habitat strips that occur along roadsides, riparian areas, and windbreaks (Crome et al. 1994; Hill 1995; Laurance and Laurance 1999; de Lima and Gascon
1999; Forman 1997). These types of 5
corridors are frequently just slender strips of edge habitat with little or no interior. Rare and endangered species usually avoid such areas and are more likely to reside in wider corridors with a higher-quality (or interior) habitat (Laurance and Laurance 1999).
Residency in corridors is the most effective way of maintaining population connectivity, particularly for less mobile species or those that will move long distances (Bennett 1990). For such species, corridors will need to provide adequate resources such as food and shelter. If habitat is suitable for residency then population continuity (and gene flow) will be maintained by both the movements of individuals in and out of the corridor, and by their movements and reproduction within the corridor (Figure 1) (Bennett 1990). In addition to dispersing animals, reproduction within corridors --as detected in the rainforest corridors of northern Australia and central Amazon (Laurance 1996; de Lima 1998), will also provide an additional source of individuals for the corridor population.
Aiding ecosystem processes Habitat corridors can also provide additional landscape services by aiding ecosystem processes such as protecting watersheds and stream quality (Karr and Schlosser 1978), and providing windbreaks. Riparian vegetation along streams can reduce soil erosion and maintain water quality and stream flows by shading streams and thereby reducing the excessive growth of exotic aquatic plants (Parendes and Jones 2000, Tucker et al., chap. --). Furthermore, adequate streamside vegetation can reduce the in-flow of chemicals and nutrients, thereby helping to maintain water quality and inhibit the unrestricted growth of aquatic algae. Windbreaks and fencerows can reduce windspeeds and consequently help to protect pastures, crops, and livestock as well as natural habitats from exposure (Harvey et al., chap --). 6
Costs of Corridors Wildlife corridors have their critics. A number of potential detrimental effects have been suggested (Simberloff and Cox 1987; Simberloff et al. 1992; Hess 1994) that should be considered when recommending corridors as a component of a regional conservation strategy.
Spread of biotic and abiotic disturbances First, as a result of increased immigration, wildlife corridors could potentially facilitate the spread of disease, exotic species, weeds, and undesirable species (Simberloff and Cox 1987, Hess 1994). Increased immigration might also disrupt local adaptations of species and even decrease the level of genetic variation by causing outbreeding depression, which occurs with the mating of highly dissimilar individuals (Simberloff and Cox 1987). Such events, however, are unlikely to occur when corridors are being used to reconnect habitats that have been isolated by human land uses rather than connecting naturally unconnected habitats (Noss 1987).
Second, corridors might facilitate the spread of abiotic disturbances such as fire (Simberloff and Cox 1987). In tropical landscapes, ignition sources of fire most commonly occur in managed pastures. Agroforestry areas will normally not be susceptible to fire because the closed or semi-closed tree canopy creates a dark humid environment with low fuel loads and because of man-made firebreaks. Therefore, agroforestry areas could act as potential fire buffers to remnant habitats and are more likely to inhibit rather than promote the spread of fire.
Population sinks 7
Corridors could act as population sinks by attracting individuals to areas where they experience reduced survival or reproduction. By aiming to promote wildlife movement across the landscape, corridors may funnel wildlife through private lands increasing their exposure to hunters, poachers, predators and domestic animals (see Treves and Salafsky, chap.--). Moreover, corridors manoeuvre organisms into an environment with limited resources and potentially superior competitors such as generalist and edge species, which could reduce their reproductive success (e.g. nest predation, Angelstam 1986) and compromise their survival (Simberloff and Cox 1987). To date most research into increased mortality in corridors has been extrapolated from edge effect studies in fragmented habitats (Gates and Gysel 1978), and little evidence is available on mortality rates in corridors compared to other habitats.
Financial costs vs. benefits of corridors Finally, there has been some suggestion that the financial cost of corridors may outweigh their benefits, and that scarce conservation dollars could be better spent on other initiatives (Simberloff et al. 1992). For example, in areas such as eastern Madagascar where 250 m width, were found to support the most sensitive of the arboreal mammals in tropical Australia (Laurance and Laurance 1999). Similarly, many understory rainforest birds in central Amazonia had reduced abundances within at least 70 m of forest edges, and some probably exhibited even stronger edge avoidance (Laurance 2001). For this bird community, a 250 m-wide corridor might provide only ca. 100 m of forest-interior and could still fail to support very sensitive species.
Corridor length Corridor length can also be a key factor. For slow-moving species with a short lifespan, a long corridor may be a population sink. Successful dispersal through a corridor will be probably be negatively related to corridor length. A long corridor will need to contain all the habitat requirements of a species. If this is not possible, then the presence of larger habitat patches serving as stepping-stones along the corridor route may help provide critical habitat requirements for dispersing individuals (Newmark 1993).
Canopy and corridor continuity Closed-canopy ecosystems like tropical forests may be more strongly affected by habitat discontinuities than other habitats. Canopy connectivity can be an important factor for the movement of canopy and understory species. A discontinuous canopy may act as a barrier for species that require cover from predators or for arboreal species that mainly move through upper forest strata.
Some corridors may be bisected by road or rivers, which could be a potential barrier to animal movements. In tropical forests, even relatively narrow road and powerline clearings (
