IV.3 Ecological Dynamics in Fragmented Landscapes
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IV.3 Ecological Dynamics in Fragmented Landscapes Jianguo Wu OUTLINE 1. Spatial heterogeneity and landscape fragmentation 2. Population and species dynamics in fragmented landscapes 3. Ecosystem dynamics in fragmented landscapes 4. Hierarchical patch dynamics Landscapes will likely become increasingly fragmented for biological organisms and ecological processes as the human population and its demands for resources continue to escalate. Landscape fragmentation results in habitat loss and alterations in the composition and spatial arrangement of landscape elements, consequently affecting population and ecosystem processes. Thus, to protect biodiversity and ecosystem functioning and to understand how nature works in the changing world, we must understand how organisms, populations, communities, and ecosystems interact with spatially heterogeneous landscapes in which they reside—that is, ecological dynamics in fragmented landscapes. This chapter discusses the effects of landscape fragmentation on population and ecosystem processes as well as major approaches to studying these effects. GLOSSARY landscape connectivity. The ability of a landscape to facilitate the flows of organisms, energy, or material across the patch mosaic. Landscape connectivity is a function of both the structural connectedness of the landscape and the movement characteristics of the species or process under consideration. landscape ecology. The science and art of studying and influencing the relationship between spatial pattern and ecological processes on multiple scales. Land use and land cover change and its ecological consequences are key research topics in landscape ecology. landscape fragmentation. The breaking up of vegetation or other land cover types into smaller patches by anthropogenic activities, or the human introduction of barriers that impede flows of organisms, energy, and material across a landscape. Habitat fragmentation is a similar term to landscape fragmentation but has a more explicit focus on changes in habitat of organisms. landscape pattern. The composition (diversity and relative abundance) and configuration (shape and spatial arrangement) of landscape elements, consisting of both patchiness and gradients. metapopulation. The total population system that is composed of multiple local populations geographically separated but functionally connected through dispersal. patch dynamics. A perspective that ecological systems are mosaics of patches exhibiting nonequilibrium transient dynamics and together determining the system-level structure and function. 1. SPATIAL HETEROGENEITY AND LANDSCAPE FRAGMENTATION To study ecological dynamics in fragmented landscapes , it is necessary to characterize the spatial pattern of landscapes and understand the causes and mechanisms of the pattern. As described in chapter IV.2, landscapes are spatially heterogeneous geographic areas in which patches and gradients of different kinds, sizes, and shapes are interwoven. This spatial heterogeneity is ubiquitous in both terrestrial and aquatic systems on all scales in space and time. Several types of factors are responsible for the creation of landscape heterogeneity. First, the physical template of landscapes is usually heterogeneous in terms of geomorphological features and distribution of energy and abiotic resources . Second,disturbances,bethey natural(e.g., fires, droughts, floods, and windstorms) or anthropogenic (e.g., human-induced fires, urbanization, deforestation , and highway construction), are frequently the primary cause for landscape heterogeneity. Third, biological processes (e.g., herbivory, species competition, deceases, and allelopathy) and fine-scale variability in topography and soil resources can also contribute to landscape heterogeneity. In general, abiotic conditions (e.g., climate, topography, and geomorphology) provide the context in which biological and anthropogenic processes often interact to generate landscape pattern. For example, the spatial pattern of temperature and precipitation determine the broad-scale distribution of biomes, within which the characteristics of ecosystem types are influenced by topographical features and mesoscale climatic variations. The structure and function of local ecosystems, however, are often affected significantly by biological processes. Spatial heterogeneity gives rise to landscape pattern, of which patches—relatively homogeneous areas that differ from their surroundings—are the fundamental units. Patches can be characterized by their size, shape, content, duration, structural complexity, and boundary characteristics. Landscape pattern is usually considered to have two components: composition (the diversity and relative abundance of different kinds of patches) and configuration (the shape and spatial arrangement of patches) (see chapter IV.2 for more detail on this). Spatial heterogeneity is an important source for the biological diversity, ecosystem services, and scenic wonders of the natural world. In other words, the world is naturally and wonderfully patchy. However , landscape fragmentation—the process of breaking up contiguous landscapes or their elements by human activities—has profoundly transformed the spatial pattern of most if not all natural landscapes around the world and has become one of the greatest threats to biodiversity and ecosystem functioning. As landscapes are fragmented, extant vegetation is removed, and...