Sodhi and Ehrlich: Conservation Biology for All. http://ukcatalogue.oup.com/product/9780199554249.do CHAPTER 5 Habitat fragmentation and landscape change Andrew F. Bennett and Denis A. Saunders Broad-scale destruction and fragmentation of fragmentation” for issues directly associated with native vegetation is a highly visible result the subdivision of vegetation and its ecological of human land-use throughout the world (Chap- consequences. ter 4). From the Atlantic Forests of South America This chapter begins by summarizing the con- to the tropical forests of Southeast Asia, and in ceptual approaches used to understand conserva- many other regions on Earth, much of the original tion in fragmented landscapes. We then examine vegetation now remains only as fragments the biophysical aspects of landscape change, and amidst expanses of land committed to feeding how such change affects species and commu- and housing human beings. Destruction and nities, posing two main questions: (i) what are fragmentation of habitats are major factors in the implications for the patterns of occurrence of the global decline of populations and species species and communities?; and (ii) how does (Chapter 10), the modification of native plant and landscape change affect processes that influence animal communities and the alteration of ecosys- the distribution and viability of species and com- tem processes (Chapter 3). Dealing with these munities? The chapter concludes by identifying changes is among the greatest challenges facing the kinds of actions that will enhance the conser- the “mission-orientated crisis discipline” of conser- vation of biota in fragmented landscapes. vation biology (Soulé 1986; see Chapter 1). Habitat fragmentation, by definition, is the “breaking apart” of continuous habitat, such as 5.1 Understanding the effects tropical forest or semi-arid shrubland, into dis- of landscape change tinct pieces. When this occurs, three interrelated 5.1.1 Conceptual approaches processes take place: a reduction in the total amount of the original vegetation (i.e. habitat The theory of island biogeography (MacArthur loss); subdivision of the remaining vegetation and Wilson 1967) had a seminal influence in sti- into fragments, remnants or patches (i.e. habitat mulating ecological and conservation interest in fragmentation); and introduction of new forms of fragmented landscapes. This simple, elegant land-use to replace vegetation that is lost. These model highlighted the relationship between the three processes are closely intertwined such that number of species on an island and the island’s it is often difficult to separate the relative effect of area and isolation. It predicted that species rich- each on the species or community of concern. ness on an island represents a dynamic balance Indeed, many studies have not distinguished be- between the rate of colonization of new species to tween these components, leading to concerns that the island and the rate of extinction of species “habitat fragmentation” is an ambiguous, or even already present. It was quickly perceived that meaningless, concept (Lindenmayer and Fischer habitat isolates, such as forest fragments, could 2006). Consequently, we use “landscape change” also be considered as “islands” in a “sea” of de- to refer to these combined processes and “habitat veloped land and that this theory provided a 88 © Oxford University Press 2010. All rights reserved. For permissions please email: [email protected] 1 HABITAT FRAGMENTATION AND LANDSCAPE CHANGE 89 quantitative approach for studying their biota. live fences, and pastures with dispersed trees This stimulated many studies in which species each support diverse assemblages of birds, bats, richness in fragments was related to the area dung beetles and butterflies (Harvey et al. 2006). and isolation of the fragment, the primary factors To these species, the landscape represents a mo- in island biogeographic theory. saic of land uses of differing quality, rather than a The development of landscape ecology contrib- contrast between “habitat” and “non-habitat”. uted new ways of thinking about habitat Recognizing landscapes as mosaics emphasizes fragments and landscape change. The concept the need to appreciate all types of elements in of patches and connecting corridors set within the landscape. This perspective is particularly rel- a matrix (i.e. the background ecosystem or evant in regions where cultural habitats, derived land-use type) became an influential paradigm from centuries of human land-use, have impor- (Forman and Godron 1986). It recognized the tant conservation values. importance of the spatial context of fragments. Different species have different ecological The environment surrounding fragments is great- attributes, such as their scale of movement, life- ly modified during landscape changes associated history stages, longevity, and what constitutes with fragmentation. Thus, in contrast to islands, habitat. These each influence how a species “per- fragments and their biota are strongly influenced ceives” a landscape, as well as its ability to by physical and biological processes in the wider survive in a modified landscape. Consequently, landscape, and the isolation of fragments de- the same landscape may be perceived by pends not only on their distance from a similar different taxa as having a different structure and habitat but also on their position in the landscape, different suitability, and quite differently from the types of surrounding land-uses and how they the way that humans describe the landscape. influence the movements of organisms (Saunders A “species-centered” view of a landscape can be et al. 1991; Ricketts 2001). obtained by mapping contours of habitat suitabil- The influence of physical processes and distur- ity for any given species (Fischer et al. 2004). bance regimes on fragments means that following habitat destruction and fragmentation, habitat 5.1.2 Fragment vs landscape perspective modification also occurs. Mcintyre and Hobbs (1999) incorporated this complexity into a con- Habitat fragmentation is a landscape-level pro- ceptual model by outlining four stages along a cess. Fragmented landscapes differ in the size trajectory of landscape change. These were: and shape of fragments and in their spatial con- (i) intact landscapes, in which most original veg- figuration. Most “habitat fragmentation” studies etation remains with little or no modification; have been undertaken at the fragment level, with (ii) variegated landscapes, dominated by the orig- individual fragments as the unit of study. How- inal vegetation, but with marked gradients of ever, to draw inferences about the consequences habitat modification; (iii) fragmented landscapes, of landscape change and habitat fragmentation, it in which fragments are a minor component in is necessary to compare “whole” landscapes a landscape dominated by other land uses; and that differ in their patterns of fragmentation (iv) relict landscapes with little (<10%) cover of (McGarigal and Cushman 2002). Comparisons original vegetation, set within highly modified of landscapes are also important because: (i) land- surroundings. This framework emphasizes the scapes have properties that differ from those dynamics of landscape change. Different stages of fragments (Figure 5.1); (ii) many species along the trajectory pose different kinds of chal- move between and use multiple patches in the lenges for conservation management. landscape; and (iii) conservation managers must Many species are not confined solely to frag- manage entire landscapes (not just individual ments, but also occur in other land uses in mod- fragments) and therefore require an understand- ified landscapes. In Nicaragua, for example, ing of the desirable properties of whole land- riparian forests, secondary forests, forest fallows, scapes. Consequently, it is valuable to consider © Oxford University Press 2010. All rights reserved. For permissions please email: [email protected] Sodhi and Ehrlich: Conservation Biology for All. http://ukcatalogue.oup.com/product/9780199554249.do 90 CONSERVATION BIOLOGY FOR ALL a) b) many of these are intercorrelated, especially with the total amount of habitat remaining in the land- scape (Fahrig 2003). Several aspects of the spatial configuration of fragments that usefully distinguish between different landscapes include: (i) the degree of subdivision (i.e. number of fragments), (ii) the aggregation of habitat, and (iii) the complexity of fragment shapes (Figure 5.3). Some kinds of changes are not necessarily evi- dent from a time-series sequence. Landscape change is not random: rather, disproportionate Individual fragments Whole landscapes size compositional gradients change occurs in certain areas. Clearing of vegeta- shape diversity of land-uses tion is more common in flatter areas at lower eleva- core area number of fragments tions and on the more-productive soils. Such areas vegetation type aggregation are likely to retain fewer, smaller fragments of orig- isolation structural connectivity inal vegetation, whereas larger fragments are more Figure 5.1 Comparison of the types of attributes of a) individual likely to persist in areas less suitable for agricultural fragments and b) whole landscapes. orurbandevelopment,suchasonsteepslopes, poorer soils, or regularly inundated floodplains. the consequences of landscape change at both the This has important implications for conservation fragment and landscape levels. because sites associated with different soil types