- Landscape ecology
Landscape ecology is the science and art of studying and improving the relationship between spatial pattern and ecological processes on a multitude of scales and organizational levels (Wu 2006, 2008; Wu and Hobbs 2007). In a broad sense, landscape ecology represents both a field of study and a scientific perspective or paradigm. As a highly interdisciplinary and transdisciplinary enterprise, landscape ecology integrates biophysical and analytical approaches with humanistic and holistic perspectives across natural and social sciences. Landscapes are spatially heterogeneous geographic areas characterized by diverse interacting patches or ecosystems, ranging from relatively natural terrestrial and aquatic systems such as forests, grasslands and lakes to human-dominated environments including agricultural and urban settings (Forman 1995, Turner et al. 2001; Wu and Hobbs 2007). The most salient characteristics of landscape ecology are its unequivocal emphasis on the relationship among pattern, process and scale and its focus on broad-scale ecological and environmental issues that necessitates the coupling between biophysical and socioeconomic processes. Key research topics in landscape ecology include ecological flows in landscape mosaics, land use and land cover change, scaling, relating landscape pattern analysis with ecological processes, and landscape conservation and sustainability (Wu and Hobbs 2002).
The term landscape ecology was coined by
Carl Troll, a German geographer in 1939 (Troll 1939). He developed this terminology and many early concepts of landscape ecology as part of his early work applying aerial photograph interpretation to studies of interactions between environment and vegetation.
Heterogeneity is the measure of how different parts of a landscape are from one another. Landscape ecology looks at how spatial structure affects organism abundance at the landscape level, as well as the behavior and functioning of the landscape as a whole. This includes the study of the pattern, or the internal order of a landscape, on process, or the continuous operation of functions of organisms (Turner 1989). Landscape ecology also includes geomorphology as applied to the design and architecture of landscapes (Allaby 1998).
Geomorphologyis the study of how geological formations are responsible for the structure of a landscape.
Evolution of theory
One central landscape ecology theory originated from MacArthur & Wilson's "
The Theory of Island Biogeography". This work considered the assembly of flora and fauna on islands as the result of colonization from a mainland stock and stochastic extinction. The concepts of island biogeographywere generalized from physical islands to abstract patches of habitat by Levins' metapopulationmodel. This generalization spurred the growth of landscape ecology by providing conservation biologists a new tool to assess how habitat fragmentationaffects population viability. Recent growth of landscape ecology owes much to the development of geographic information systems (GIS) technology and the availability of large-extent habitat data (e.g. remotely sensed satellite images or aerial photography).
Development as a discipline
Landscape ecology developed in
Europefrom historical planning on human-dominated landscapes. In North America, concepts from general ecology theory were integrated. While general ecology theory and its sub-disciplines focused on the study of more homogenous, discrete community units organized in a hierarchical structure (typically as populations, species, and communities), landscape ecology built upon heterogeneity in space and time, and frequently included human-caused landscape changes in theory and application of concepts (Sanderson and Harris 2000).
By 1980, landscape ecology was a discrete, established discipline, marked by the organization of the
International Association for Landscape Ecology(IALE) in 1982 and landmark book publications defining the scope and goals of the discipline, including Naveh and Lieberman (1984) and Forman and Godron (1986) (Ryszkowski 2002). Forman (1995) wrote that although study of “the ecology of spatial configuration at the human scale” was barely a decade old, there was strong potential for theory development and application of the conceptual framework. Today, theory and application of landscape ecology continues to develop through a need for innovative applications in a changing landscape and environment. Landscape ecology today relies more on advanced technologies such as remote sensing, GIS, and simulation models, with associated development of powerful quantitative methods to examine the interactions of patterns and processes (Turner et al. 2001). An example would be determining the amount of carbon present in the soilbased on landform over a landscape, derived from GIS maps, vegetationtypes, and rainfalldata for a region.
Relationship to ecological theory
Although landscape ecology theory may be slightly outside of the “classical and preferred domain of scientific disciplines” because of the large, heterogeneous areas of study (Sanderson and Harris 2000), general
ecologytheory is central to landscape ecology theory in many aspects. Landscape ecology consists of four main principles, which include: 1. the development and dynamics of spatial heterogeneity, 2. interactions and exchanges across heterogeneous landscapes, 3. influences of spatial heterogeneity on biotic and abioticprocesses, and 4. the management of spatial heterogeneity. The main difference from traditional ecological studies, which frequently assume that systems are spatially homogenous, is the consideration of spatial patterns (Turner and Gardner 1991).
Important terms in Landscape ecology
Landscape ecology not only embraced a new vocabulary of terms but also incorporated general ecology theory terms in new ways. Many of the terms used in landscape ecology are as interconnected and interrelated as the discipline itself. Landscape can be defined as an area containing two or more
ecosystems in close proximity (Sanderson and Harris 2000).
cale and heterogeneity (incorporating composition, structure, and function)
A main concept in landscape ecology is scale. Scale represents the real world as translated onto a map, in the relationship between distance on a map image and the corresponding distance on earth (Malczewski 1999). Scale is also the spatial or temporal measure of an object or a process (Turner and Gardner 1991), or level or degree of spatial resolution (Forman 1995). Components of scale include composition, structure, and function, which are all important ecological concepts. Applied to landscape ecology, composition refers to the number of patch types (see below) represented on a landscape, and their relative abundance. For example, the amount of
forestor wetland, the length of forest edge, or the density of roads can be aspects of landscape composition. Structure is determined by the composition, the configuration, and the proportion of different patches across the landscape, while function refers to how each element in the landscape interacts based on its life cycle events (Turner and Gardner 1991). Pattern is the term for the contents and internal order of a heterogeneous area of land (Forman and Godron 1986).
A landscape with structure and pattern implies that it has spatial heterogeneity, or the uneven, non-random distribution of objects across the landscape (Forman 1995). Heterogeneity is a key element of landscape ecology that separates this discipline from other branches of ecology.
Patch and mosaic
Patch, a term fundamental to landscape ecology, is defined as a relatively homogeneous area that differs from its surroundings (Forman 1995). Patches are the basic unit of the landscape that change and fluctuate, a process called patch dynamics. Patches have a definite shape and spatial configuration, and can be described compositionally by internal variables such as number of trees, number of tree species, height of trees, or other similar measurements (Forman 1995). Matrix is the “background ecological system” of a landscape with a high degree of connectivity. Connectivity is the measure of how connected or spatially continuous a corridor, network, or matrix is (Forman 1995). For example, a forested landscape (the matrix) with fewer gaps in forest cover (open patches) will have higher connectivity. Corridors have important functions as strips of a particular type of landscape differing from adjacent land on both sides (Forman 1995). A network is an interconnected system of corridors while mosaic describes the pattern of patches, corridors and matrix that form a landscape in its entirety (Forman 1995).
Boundary and edge
Landscape patches have a boundary between them which can be defined or fuzzy (Sanderson and Harris 2000). The zone composed of the edges of adjacent ecosystems is the boundary (Forman 1995). Edge means the portion of an ecosystem near its perimeter, where influences of the adjacent patches can cause an environmental difference between the interior of the patch and its edge. This edge effect includes a distinctive species composition or abundance in the outer part of the landscape patch (Forman 1995). For example, when a landscape is a mosaic of perceptibly different types, such as a forest adjacent to a
grassland, the edge is the location where the two types adjoin. In a continuous landscape, such as a forest giving way to open woodland, the exact edge location is fuzzy and is sometimes determined by a local gradient exceeding a threshold, such as the point where the tree cover falls below thirty-five percent (Turner and Gardner 1991).
Ecotones, ecoclines, and ecotopes
A type of boundary is the
ecotone, or the transitional zone between two communities (Allaby 1998). Ecotones can arise naturally, such as a lakeshore, or can be human-created, such as a cleared agricultural fieldfrom a forest (Allaby 1998). The ecotonal community retains characteristics of each bordering community and often contains species not found in the adjacent communities. Classic examples of ecotones include fencerows; forest to marshlands transitions; forest to grassland transitions; or land-water interfaces such as riparian zones in forests. Characteristics of ecotones include vegetational sharpness, physiognomic change, occurrence of a spatial community mosaic, many exotic species, ecotonal species, spatial mass effect, and species richness higher or lower than either side of the ecotone (Walker et al. 2003).
An is another type of landscape boundary, but it is a gradual and continuous change in environmental conditions of an ecosystem or community. Ecoclines help explain the distribution and diversity of organisms within a landscape because certain organisms survive better under certain conditions, which change along the ecocline. They contain heterogeneous communities which are considered more environmentally stable than those of ecotones (Attrill and Rundle 2002).
ecotopeis a spatial term representing the smallest ecologically-distinct unit in mapping and classification of landscapes (Forman 1995). Relatively homogeneous, they are spatially-explicit landscape units used to stratify landscapes into ecologically distinct features for measurement and mapping of landscape structure, function, and change over time, and to examine the effects of disturbance and fragmentation.
Disturbance and fragmentation
Disturbance is an event that significantly alters the pattern of variation in the structure or function of a system, while fragmentation is the breaking up of a habitat, ecosystem, or land-use type into smaller parcels (Forman 1995). Disturbance is generally considered a natural process. Fragmentation causes land transformation, an important current process in landscapes as more and more development occurs.
An important consequence of repeated, random clearing (whether by natural disturbance or human activity) is that contiguous cover can break down into isolated patches. This happens when the area cleared exceed a critical level, which means that landscapes exhibit two phases: connected and disconnected (Green et al. 2006).
Landscape ecology theory
Elements of landscape ecology theory
Landscape ecology, as a theory, stresses the role of human impacts on landscape structures and functions and proposes ways for restoring degraded landscapes (Nave and Lieberman 1984). Landscape ecology explicitly includes humans as entities that cause functional changes on the landscape (Sanderson and Harris 2000). Landscape ecology theory includes the landscape stability principle, which emphasizes the importance of landscape structural heterogeneity in developing resistance to disturbances, recovery from disturbances, and promoting total system stability (Forman and Gordon 1986). This principle is a major contribution to general ecological theories which highlight the importance of relationships among the various components of the landscape. Integrity of landscape components helps maintain resistance to external threats, including development and land transformation by human activity (Turner et al. 2001). Analysis of land use changes has included a strongly geographical approach within landscape ecology. This has led to acceptance of the idea of multifunctional properties of landscapes (Ryszkowski 2002). There are still calls for a more unified theory of landscape ecology due to differences in professional opinion among landscape ecologists, and the interdisciplinary approach to the discipline (Bastian 2001).
An important related theory is hierarchy theory, which refers to how systems of discrete functional elements operate when linked at two or more scales. For example, a forested landscape might be hierarchically composed of
drainage basins, which in turn are composed of local ecosystems or stands, which are in turn composed of individual trees and tree gaps (Forman 1995). Recent theoretical developments in landscape ecology have emphasized the relationship between pattern and process, as well as the effect that changes in spatial scale has on the potential to extrapolate information across scales (Turner and Gardner 1991). Several studies suggest that the landscape has critical thresholds at which ecological processes will show dramatic changes, such as the complete transformation of a landscape by an invasive specieswith a small change in average temperatures per year which favors the invasive habitat requirements (Turner and Gardner 1991).
Landscape ecology application
Developments in landscape ecology illustrate the important relationships between spatial patterns and ecological processes, and incorporate quantitative methods that link spatial patterns and ecological processes at broad spatial and temporal scales. This linkage of time, space, and environmental change can assist land managers in applying land management plans to solve environmental problems (Turner et al. 2001). The increased attention in recent years on spatial dynamics has highlighted the need for new quantitative methods that can analyze patterns, determine the importance of spatially explicit processes on the landscape, and develop reliable landscape models (Turner and Gardner 1991).
Multivariateanalysis techniques, a type of statistics incorporating many variables, are frequently used to examine landscape level vegetation patterns. A number of studies in ripariansystems and wetlands use a variety of statistical techniques, such as cluster analysis, canonical correspondence analysis(CCA), or detrended correspondence analysis(DCA), for classifying vegetation. Gradient analysisis another way to determine the vegetation structure across a landscape, or to help delineate critical wetland habitat for conservation or mitigation purposes (Lyon and Sagers 1998, Choesin and Boerner 2002). Climate changeis another major component in structuring current research in landscape ecology. Ecotones, as a basic unit in landscape studies, may have significance for management under climate change scenarios, since change effects are likely to be seen at ecotones first because of the unstable nature of a fringe habitat (Walker et al. 2003). Research in northern regions has examined landscape ecological processes, such as the accumulation of snow during winter, snow melting, freeze-thaw action, percolation, soil moisture variation, and temperature regimes through long-term measurements in Norway (Loffler and Finch 2005). The study analyzes gradients across space and time between ecosystems of the central high mountains to determine relationships between distribution patterns of animals in their environment. Looking at where animals live, and how vegetation shifts over time, may provide insight into changes in snow and ice over long periods of time across the landscape as a whole.
Other landscape-scale studies maintain that human impact is likely the main determinant of landscape pattern over much of the globe (Wilson and King 1995). Landscapes may become substitutes for
biodiversitymeasures because plant and animal composition differs between samples taken from sites within different landscape categories. Taxa, or different species, can “leak” from one habitat into another, which has implications for landscape ecology. As human land use practices expand, and continue to increase the proportion of edges in landscapes, the effects of this leakage across edges on assemblage integrity may become more significant and important in conservation because taxa may be conserved across landscape levels, if not at local levels (Dangerfield et al. 2003).
Relationship to other disciplines
Landscape ecology has important links to application-oriented disciplines such as
agricultureand forestry. In agriculture, landscape ecology has introduced new options for the control and management of environmental threats brought about by the intensification of agricultural practices. Agriculture has always been a strong human impact on ecosystems (Ryszkowski 2002). In forestry, changes in consumer needs have changed conservation and use of forested landscapes from structuring stands for fuelwood and timber to ordering stands across landscapes to enhance aesthetics, habitats and biological diversity. Landscape forestry provides methods, concepts, and analytic procedures for shifting management from traditional to landscape forestry (Boyce 1995). Landscape ecology has been cited as a major contributor to the development of fisheries biology as a distinct biological science discipline (Magnuson 1991), and is frequently incorporated in study design for wetland delineation in hydrology(Attrill and Rundle 2002).
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