Patterns of species occurrence in fragmented landscapes ~ Conservation Biology

Many studies have described the occurrence of species in fragments of different sizes, shapes, composition, land-use and context in the landscape. For species that primarily depend on fragmented habitat, particularly animals, fragment size is a key influence on the likelihood of occurrence. As fragment size decreases, the frequency of occurrence declines and the species may be absent from many small fragments. Such absences may be because the fragment is smaller than the minimum area required for a single individual or breeding unit, or for a self-sustaining population. Some species persist in fragmented landscapes by incorporating multiple fragments in their territory or daily foraging movements. In England, the tawny owl (Strix aluco) occupies territories of about 26 ha (hectares) in large deciduous woods, but individuals also persist in highly fragmented areas by including several small woods in their territory (Redpath 1995).

There is a cost, however: individuals using multiple woods have lower breeding success and there is a higher turnover of territories between years. Species that require different kinds of habitats to meet regular needs (e.g. for foraging and breeding) can be greatly disadvantaged if these habitats become isolated. Individuals may then experience difficulty in moving between different parts of the landscape to obtain their required resources. Amphibians that move between a breeding pond and other habitat, such as overwintering sites in forest, are an example. Other attributes (in addition to fragment size) that influence the occurrence of species include the type and quality of habitat, fragment shape, land use adjacent to the fragment, and the extent to which the wider landscape isolates populations. In the Iberian region of Spain, for example, the relative abundance of the Eurasian badger (Meles meles) in large forest fragments is significantly influenced by habitat quality and forest cover in the wider landscape (Virgos 2001).

In areas with less than 20% forest cover, badger abundance in forests was most influenced by isolation (i.e. distance to a potential source area >10 000 ha), whereas in areas with 20–50% cover, badgers were most influenced by the quality of habitat in the forest fragments. A key issue for conservation is the relative importance of habitat loss versus habitat fragmentation (Fahrig 2003). That is, what is the relative importance of how much habitat remains in the landscape versus how fragmented it is? Studies of forest birds in landscapes in Canada and Australia suggest that habitat loss and habitat fragmentation are both significant influences, although habitat loss generally is a stronger influence for a greater proportion of species (Trczinski et al. 1999; Radford and Bennett 2007). Importantly, species respond to landscape pattern in different ways. In southern Australia, the main influence for the eastern yellow robin (Eopsaltria australis) was the total amount of wooded cover in the landscape; for the grey shrike-thrush (Colluricincla harmonica) it was wooded cover together with its configuration (favoring aggregated habitat); while for the musk lorikeet (Glossopsitta concinna) the influential factor was not wooded cover, but the configuration of habitat and diversity of vegetation types (Radford and Bennett
2007).