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Habitat Fragmentation: the Theories Which Provide the Framework for the Study of Habitat Fragmentation

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Habitat Fragmentation: the Theories Which Provide the Framework for the Study of Habitat Fragmentation Kelly Marie Henry NRS 534 – Ecology of Fragmented Landscapes May 18, 2005 Habitat fragmentation: the theories which provide the framework for the study of habitat fragmentation The common occurrence of habitat destruction results not only in habitat loss and habitat degradation, but also in fragmentation of the remaining habitat. A focal point for the research of many landscape ecologists is on the effects of habitat fragmentation to the individuals, populations, and ecosystems experiencing this phenomenon. Habitat fragmentation can be defined as a process resulting in the transformation of a large section of relatively homogeneous habitat into smaller habitat patches of a heterogeneous composition (Fahrig 2003; Reed et al. 1995). Of particular importance is that the resulting smaller habitats are typically isolated from each other by a matrix of habitats unlike the original. The concepts many landscape ecologists use when formulating habitat fragmentation hypotheses are not new ideas to the discipline of ecology. Two of the main theories providing the backbone to habitat fragmentation studies, the island biogeography theory and the metapopulation theory, were originally developed to explain observations made in community and population ecology (Collinge 1996). Some common parameters examined when studying habitat fragmentation include the size, the degree of isolation, the context (matrix), and the degree of heterogeneity of the fragment as well as the impact of edge on the fragment. Island Biogeography Theory The island biogeography theory, originally proposed by MacArthur and Wilson in 1967, states that the size of an oceanic island and its distance from a mainland source of colonizing species influences the number of species present on that island. While it is evident that the island biogeography theory was originally developed to explain species composition on oceanic islands, landscape ecologists studying habitat fragmentation apply the theory to the terrestrial islands created by habitat fragmentation (Fahrig 2003; Whittaker 1998; Collinge 1996). Habitat fragmentation research structured around the island biogeography theory typically focuses on the size of the fragment and the degree of isolation of the fragment. 1 The theory of island biogeography formed the basis of a study conducted by Walters et al. (1999) who examined the adverse effects of habitat fragmentation on brown treecreepers (Climacteris picumnus). Upon completing their research, Walters et al. (1999) found evidence that brown treecreepers experience lower male to female pairing success due to disrupted dispersal patterns in fragmented habitats versus unfragmented habitats. Another study with framework centered around the island biogeography theory was that of Davies and Margules (1998) who studied populations of carabid beetles. Based on previous studies, these scientists hypothesized that carabid beetle species richness would decrease in fragmented habitats, and carabid beetle abundance would decrease with the occurrence of fragmentation, with decreasing size of the fragment and with proximity to the edge of the fragment. Upon completing the study, Davies and Margules found that habitat fragmentation did, in fact, impact carabid beetle species richness and it appeared to alter species composition as well. Furthermore, habitat fragmentation did appear to decrease the abundance of the two species experiencing complete isolation due to habitat fragmentation (the other six species examined were not completely isolated). Metapopulation Theory The second theory supporting habitat fragmentation research is the metapopulation theory. Collinge (1996) defines a metapopulation as “a set of spatially separated groups of conspecific individuals.” Originally developed by Levins (1969) in the late 1960’s, the metapopulation theory states that while local populations of organisms experience periods of colonization and extinction events, the metapopulation as a whole continues to thrive. In order to be considered a metapopulation, the subpopulations must remain interconnected by gene flow, extinction, and recolonization (Whittaker 1998). There are two types of metapopulation models, the classic model and the source-sink model. The classic model assumes all subpopulations are of the same size, while the source-sink model assumes a large core population that persists indefinitely during the periods of extinction and recolonization experienced by the smaller sink populations. Habitat fragmentation research incorporating the metapopulation theory typically focuses on the connectivity and the exchange of individuals between habitat fragments. The metapopulation theory was applied in a study by Dunham et al. (2003) examining the short and long-term impacts of fire on native fish populations. The study found that the persistence of local native fish populations in larger habitat patches could 2 be explained by the larger population size able to inhabit the patch or by the increased habitat heterogeneity frequently existing in these larger habitat patches. However, the persistence of populations in smaller habitat patches was likely due to the dispersal of individuals from nearby populations. It is the combination of both the island biogeography theory and the metapopulation theory, which together support habitat fragmentation studies. Although the island biogeography theory focuses on the size and degree of isolation of habitat fragments and the metapopulation theory focuses on connectivity and exchange between habitat fragments, many researchers study different combinations of these effects on individuals, populations, and ecosystems. Both theories have a strong spatial component, which causes the two theories to become tightly interwoven in studies of habitat fragmentation. A strong emphasis is placed when studying habitat islands on the degree of isolation or the connectivity of the habitat with surrounding patches. The spatial location of individuals, populations, and communities influences the metapopulation dynamic. Hanski (1998) states that the spatial structure of the metapopulation is as important in determining the metapopulation dynamic as birth and death rates since the spatial structure allows for immigration. Following an event of habitat destruction resulting in habitat fragmentation, a patch typically experiences faunal relaxation (Viveiros de Castro and Fernandez 2004; Whittaker 1998). Once fragmentation occurs, the newly created patch is supersaturated with species. After the “extinction debt” or time lag following the fragmentation is over, the number of species slowly decreases until a new equilibrium level is achieved (Hanski 1998). However, immigration and extinction both continue to occur during the relaxation process and after equilibrium has been established. In general, the response to fragmentation by a metapopulation is non-linear due to the manner in which habitat connectivity is lost (Hanski 1998). However, evidence does suggest that relaxation and the sequence of species lost are highly structured and should be predictable (Viveiros de Castro and Fernandez 2004). The idea of a metapopulation allows for satellite populations to come and go, with the core population remaining indefinitely. However, due to the stochastic nature of faunal relaxation, species can become extinct throughout entire metapopulations. Common parameters used in habitat fragmentation studies When studying the effects of habitat fragmentation on an ecosystem, landscape ecologists focus on a wide variety of habitat parameters. Some recurring themes in the 3 study of habitat fragmentation include the size, the degree of isolation, the context (matrix), and the degree of heterogeneity of the fragment, as well as the impact of edge on the fragment. The size of the fragment influences the ecological process able to occur within the fragment (Collinge 1996). As habitat fragments become more isolated, the dynamics within the fragment become increasingly important. It is therefore important to maintain a minimum dynamic area, defined by Pickett and Thompson in 1978 as the “smallest area with a natural disturbance regime, which maintains internal recolonization sources, and hence minimizes extinction” (Dunham et al. 2003). Many studies have found a positive correlation of decreasing species richness and individual abundance with decreasing fragment size due to low reproduction and survival rates in the smaller fragments (Smith and Hellmann 2002; Collinge 1996). It should be noted that not all studies examining the size of a fragment relative to its impact on a population find negative effects associated with decreasing patch size (Eggleston et al. 1999; Davies and Margules 1998). The degree of connectivity and the composition of the surrounding matrix influence the species interactions between habitat fragments. The persistence of a population in the face of increasing habitat fragmentation can be explained by the metapopulation theory only if some degree of connectivity is maintained between the fragments. As connectivity decreases, population persistence decreases due to isolation from the supporting metapopulation (Dunham et al. 2003). The context of surrounding habitat will influence the degree and type of interaction between the fragment and the surrounding area. The degree of dissimilarity strongly influences the flow of nutrients and materials, as well as the persistence of plant and animal species between the fragment and the surrounding matrix (Mesquita et al. 1999; Collinge
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