Conceptual approaches ~ Conservation Biology

The theory of island bio geography (MacArthur and Wilson 1967) had a seminal influence in stimulating ecological and conservation interest in fragmented landscapes. This simple, elegant model highlighted the relationship between the number of species on an island and the island’s area and isolation. It predicted that species richness on an island represents a dynamic balance between the rate of colonization of new species to the island and the rate of extinction of species already present. It was quickly perceived that habitat isolates, such as forest fragments, could also be considered as “islands” in a “sea” of developed land and that this theory provided a quantitative approach for studying their biota. This stimulated many studies in which species richness in fragments was related to the area and isolation of the fragment, the primary factors in island bio geographic theory. The development of landscape ecology contributed new ways of thinking about habitat fragments and landscape change. The concept of patches and connecting corridors set within a matrix (i.e. the background ecosystem or land-use type) became an influential paradigm (Forman and Godron 1986). It recognized the importance of the spatial context of fragments.

The environment surrounding fragments is greatly modified during landscape changes associated with fragmentation. Thus, in contrast to islands, fragments and their biota are strongly influenced by physical and biological processes in the wider landscape, and the isolation of fragments depends not only on their distance from a similar habitat but also on their position in the landscape, the types of surrounding land-uses and how they influence the movements of organisms (Saunders et al. 1991; Ricketts 2001). The influence of physical processes and disturbance regimes on fragments means that following habitat destruction and fragmentation, habitat modification also occurs. Mcintyre and Hobbs (1999) incorporated this complexity into a conceptual model by outlining four stages along a trajectory of landscape change.

These were:

(i) Intact landscapes, in which most original vegetation remains with little or no modification.

(ii) Variegated landscapes, dominated by the original vegetation, but with marked gradients of habitat modification.
(iii) Fragmented landscapes, in which fragments are a minor component in a landscape dominated by other land uses.
(iv) Relict landscapes with little (<10%) cover of original vegetation, set within highly modified surroundings.

This framework emphasizes the dynamics of landscape change. Different stages along the trajectory pose different kinds of challenges for conservation management. Many species are not confined solely to fragments, but also occur in other land uses in modified landscapes. In Nicaragua, for example, riparian forests, secondary forests, forest fallows, live fences, and pastures with dispersed trees each support diverse assemblages of birds, bats, dung beetles and butterflies (Harvey et al. 2006). To these species, the landscape represents a mosaic of land uses of differing quality, rather than a contrast between “habitat” and “non-habitat”. Recognizing landscapes as mosaics emphasizes the need to appreciate all types of elements in
the landscape.

This perspective is particularly relevant in regions where cultural habitats, derived from centuries of human land-use, have important conservation values. Different species have different ecological attributes, such as their scale of movement, life history stages, longevity, and what constitutes habitat. These each influence how a species “perceives” a landscape, as well as its ability to survive in a modified landscape. Consequently, the same landscape may be perceived by different taxa as having a different structure and different suitability, and quite differently from the way that humans describe the landscape. A “species-centered” view of a landscape can be obtained by mapping contours of habitat suitability for any given species (Fischer et al. 2004).