In some continuous habitats, all the individuals of a population may live together and interact as a single widespread but discrete population (Forman 1995), however many species have evolved to live in several patches, with very few species naturally occurring in just one area (Spellerberg and Sawyer 1999; Lande and Barrowclough 1987). However threats to the viability of wildlife populations are numerous (Lacy 2000), and when habitats are destroyed, populations residing in these habitats can go extinct and eventually the whole species may do also (Ehrlich 1995). While deforestation can obviously result in the removal of plant species from an area, forest alteration and/or commercial collection are also important endangering processes to a range of biota (Aikan and Leigh 1995). As a result of natural forest conversion to industrial tree plantations, areas of formerly continuous forests are fragmented into smaller patches; a process that is believed to have a great impact on wildlife species and populations (Latiff and Zakri 1998). To this end, the processes of habitat loss and fragmentation are thought to be the primary causes of the extinction of populations, metapopulations (discussed below) and species worldwide (Gu et al. 2002)
The process of fragmentation typically results in a landscape-level mosaic of small and large patches of primary forest, variously degraded forest, and completely degraded and transformed ecosystems (Davies 1998), analogous with the landscape of the PFZ. Through human induced habitat loss, modification and fragmentation, biological communities are becoming smaller and smaller, often leading to the isolation of populations (Spellerberg and Sawyer 1999; Davies 1998). In extreme cases of patchiness, suitable habitat can be so widely spaced that no movement or interaction between populations occurs between the patches at all, and thus exist as several discrete populations (Forman 1995). However where these sub populations (demes) are
Page | 38 connected by the movements or gene flow, extinction and re-colonisation events, metapopulations; (populations of populations) arise (Hilty et al. 2006; Lande and Barrowclough 1987; Merriam 1991).
The effects of forest fragmentation have been widely reported in the literature for birds (Newmark 1991; Fuhlendorf et al. 2002; Kurosawa and Askins 2003; Lampila et al. 2005; van Houtan et al. 2006), mammals (Verboom and Apeldoorn 1990; Gaines et al. 1997; Zollner 2000;
Bakker and van Vuren 2004; Strevens 2007; Lees and Peres 2008; Mortelliti et al. 2009;
Mortelliti and Boitani 2008; Holland and Bennett 2009; Shadbolt and Ragai 2010; Charles and Ang 2010), trees (Cordeiro and Howe 2003; Tabarelli et al. 2004), insects (Dunley et al. 2009) and for a range of other ecological processes (Chiarello 2000; Hobbs 2001; Hoffmeister et al.
2005; Goosem 2007; Gould et al. 2008; Jaquiery et al. 2008) to name but a few. Small isolated populations, for example those likely to occur throughout the PFZ landscape, and those that require large areas of continuous habitat face additional threats to stability and persistence (Lacy 2000), and are more prone to extinction through proximate causes: 1) demographic stochasticity, 2) environmental stochasticity, 3) genetic deterioration and 4) social dysfunction (Lawton 1995) than are larger populations. Catastrophes; events that occur at random intervals, are also predicted to kill a percentage of the population outright. These events include fires, floods, extreme storm events and droughts (Hunter 1996), extraterrestrial impacts, large earthquakes, tsunamis and volcanic eruptions (Corlett 2009).
Demographic stochasticity is the uncertainty resulting from random variations in reproductive success and survival, immigration and emigration at the level of the individual (Hunter 1996). It can lead to the demographic structure of a population being radically altered (Hilty et al. 2006), and is recognised as a potential threat to small populations (Lacy 2000; Lindenmayer and
Page | 39 Franklin 2000; Strevens 2007). Environmental stochasticity; the uncertainty due to random variation in a range of parameters that measure habitat quality, also threatens small populations.
This can include such qualities as provision of protective cover, availability of water, relationships with other species including parasites, pathogens predators, prey or competitors, levels of nutrients and pollutants in the environment, and climatic variables (Hunter 1996).
Genetic stochasticity refers to the random variation of gene frequencies occurring within a population that result from genetic drift, population bottlenecks and inbreeding (Hunter 1996).
Lacy (2000) illustrates that in theory, populations with less than 50 breeding adults may suffer from inbreeding depression within a few generations, even much larger populations that have been fragmented into isolated sub-populations of fewer than 50 breeding animals each, may theoretically lose variability at a much faster rate than would be estimated from the total metapopulation. However while it is assumed that species that have passed through population bottlenecks should suffer genetic problems as a result of the deleterious effects of inbreeding, there are examples of where species have passed through such bottlenecks and have since begun to establish stable populations. In New Zealand the black robin (Petroica traverse) (Freeman 1994), kakapo (Strigops habroptilus) (Balance 2010) and takahe (Notornis mantelli) are examples, however all have, and are likely to continue to require intensive human intervention in order to secure their long term viability.
Many species are shown to exhibit a metapopulation structure whereby stochastic extinctions are common but are balanced with recolonisations from neighbouring patches, thus allowing the species to persist (Ehrlich 1995). A metapopulation structure may therefore be the only mechanism by which a species’ persistence can be assured for a defined period of time (Shaffer 1987) in a patchy landscape, and dispersal amongst patches in a metapopulation is therefore of
Page | 40 central significance, and is likely to be critical to the long term persistence of a species (Hilty et al. 2006).
Reasons for dispersal from the natal territory include the avoidance of inbreeding and competition for prospective mates where losers may find it advantageous to move away from a superior competitor (Alcock 1993). However moving about in a landscape is likely to be costly in terms of energy expended (Alcock 1993), and the mobility of vertebrate species invariably brings them into contact with a wide range of environments, objects (Pough et al. 1989) and predators (Alcock 1993) and other threats. Conditions within the matrix can therefore greatly influence the movement of individuals and thus also the connectivity between habitat patches (Lindenmayer and Franklin 2000). However likewise, the internal conditions of forest fragments may be as, if not more important, than their spatial configuration in the landscape in terms of determining their biotic abundance and distribution, and their corresponding value for conservation purposes (Hobbs 2001).
In multi-species metapopulations it is believed that superior competitors exclude inferior species from co-occupied patches, but that inferior competitors are able to maintain their populations as a result of higher rates of re-colonization and lower patch mortality rates (May et al. 1995).
However as the number of habitat patches occupied by a species diminishes, so too does the probability of vacant patches being recolonised, to a point where regional, potentially global extinctions can occur (Ehrlich 1995). Logically therefore, a species will become extinct at that point in time when the last local population becomes extinct (Lawton 1995), and the point at which a population becomes functionally extinct will be the point at which the population no longer has individuals of both sexes that are capable of reproduction (Miller and Lacy 2003).
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