Metapopulations and the conservation of subdivided populations

                              Small populations are vulnerable to local extinction, but a species has a greater likelihood of persistence where there are a number of local populations interconnected by occasional movements of individuals among them. Such a set of subdivided populations is often termed a “meta population” (Hanski 1999). Two main kinds of meta population have been described. A mainland-island model is where a large mainland population (such as a conservation reserve) provides a source of emigrants that disperse to nearby small populations. 

               The mainland population has a low likelihood of extinction, where as the small populations become extinct relatively frequently. Emigration from the mainland supplements the small populations, introduces new genetic material and allows recolonization should local extinction occur. A second kind of meta population is where the set of interacting populations are relatively similar in size and all have a likelihood of experiencing extinction. Although colonization and extinction may occur regularly, the overall population persists through time. The silver-spotted skipper (Hesperia comma), a rare butterfly in the UK, appears to function as a meta population (Hill et al. 1996). In 1982, butterflies occupied 48 of 69 patches of suitable grassland on the North Downs, Surrey. Over the next 9 years, 12 patches were colonized and seven populations went extinct. Those more susceptible to extinction were small isolated populations, whereas the patches more likely to be colonized were relatively large and close to other large occupied patches. 

              The conservation management of patchily distributed species is likely to be more effective by taking a metapopulation approach than by focusing on individual populations. However, “real world” populations differ from theoretical models. Factors such as the quality of habitat patches and the nature of the land mosaic through which movements occur are seldom considered in theoretical models, which emphasize spatial attributes (patch area, isolation). For example, in a meta population of the Bay checker spot butterfly (Euphydryas editha bayensis) in California, USA, populations in topographically heterogeneous fragments were less likely to go extinct than those that were in topographically uniform ones. The heterogeneity provided some areas of suitable topo climate each year over a wide range of local climates (Ehrlich and Hanski 2004). There also is much variation in the structure of subdivided populations depending on the frequency of movements between them. At one end of a gradient is a dysfunctional metapopulation where little or no movement occurs; while at the other extreme, movements are so frequent that it is essentially a single patchy population.

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Tropical and subtropical forests

               A second way to assess habitat loss is by contrastingmajor  biomes or ecosystem types. Today, tropical rainforests (also termed tropical moist and humid forests) are receiving the greatest attention, because they are being destroyed so rapidly and because they are the most biologically diverse of all terrestrial biomes. Of the roughly 16 million km2 of tropical rainforest that originally existed worldwide, less than 9 million km2 remains today (Whitmore 1997; MEA 2005). The current rate of rainforest loss is debated, with different estimates ranging from around 60 000 km2 (Achard et al. 2002) to 130 000 km2 per year (FAO 2000). Regardless of which estimate one adheres to, rates of rainforest loss are alarmingly high. Rates of rainforest destruction vary considerably among geographic regions. Of the world’s three major tropical regions, Southeast Asian forests are disappearing most rapidly in relative terms , while the African and New World tropics have somewhat lower rates of percent- annual forest loss (Sodhi et al. 2004). Such averages, however, disguise important smaller scale variation. 

               In the New World tropics, for example, the Caribbean, Meso American, and Andean regions are all suffering severe rainforest loss, but the relative deforestation rate for the region as a whole is buffered by the vastness of the Amazon. Likewise, in tropical Africa, forest loss is severe in West Africa, montane areas of East Africa, and Madagascar, but substantial forest still survives in the Congo Basin (Laurance 1999). Other tropical and subtropical biomes have suffered even more heavily than rainforests. Tropical dry forests (also known as monsoonal or deciduous forests) have been severely reduced, in part because they are easier to clear and burn than rainforests. For instance, along Central America’s Pacific coast, much less than 1% of the original dry forest survives. Losses of dry forest have been nearly as severe in Madagascar and parts of Southeast Asia (Laurance 1999; Mayaux et al. 2005).
 

                Mangrove forests, salt-tolerant ecosystems that grow in tropical and subtropical intertidal zones, have also been seriously reduced. Based on countries for which data exist, more than a third of all mangroves were lost in the last few decades of the 20th century (MEA 2005). From 1990 to 2000, over 1% of all mangrove forests were lost annually, with rates of loss especially high in Southeast Asia (Mayaux et al. 2005). Such losses are alarming given the high primary productivity of mangroves, their key role as spawning and rearing areas for economically important fish and shrimp species, and their importance for sheltering coastal areas from destructive storms and tsunamis (Danielsen et al. 2005).

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