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A Planner's Handbook
By André Botequilha Leitão, Joseph Miller, Jack Ahern, Kevin McGarigal
ISLAND PRESSCopyright © 2006 Island Press
All rights reserved.
A Spatial and Human-Oriented Ecology
During the eighteenth and nineteenth centuries, many scientists developed the basis for what would become the science of ecology (Forman and Godron 1986). Haeckel introduced the term ecology in 1866, originally meaning "knowledge of the house (hold)" (Capra 1996), as a sister science of economy, which is literally "the management (and counting) of the house (hold)" (Zonneveld 1995).
Ecology is concerned with the interactions between organisms and their environment and how those interactions determine the distribution of both plants and animals (O'Callaghan 1996).
Ecology focuses on the study of ecosystems, and on the vertical relationships (topology) between the different components of ecosystems, such as climate, water, soil, bedrock, flora, and fauna.
1.1. Foundations of Landscape Ecology
Landscape ecology emerged in Europe in the 1950s and shares its heritage with the related disciplines of biogeography and ecology. One of the principal distinctions between landscape ecology and other branches of ecology is the emphasis and focus on the spatial patterning of multiple ecosystems in heterogeneous landscapes.
Landscape ecology introduced several perspectives and principles that have become fundamental for planners. One such perspective is the spatial dimension of ecological processes. Vertical (topological) relationships are considered together with horizontal (chorological) relationships between the ecosystems that comprise a landscape. Landscape ecology offers theory and empirical evidence that enables scientists and planners to understand and compare different spatial configurations of land cover types (Forman 1995), and enables planners to anticipate and manage the ecological consequences of a plan.
A second fundamental perspective is the focus of landscape ecology on human ecology, and on the application in planning and management. In landscape ecology, human activities are considered part of ecosystems, not as a separate component.
A third perspective consists of adopting the landscape as the principal unit of study. Together with a systemic, holistic approach, landscape ecology provides an integrated analysis of the complex, human-made landscapes that are fast becoming dominant worldwide.
In nature, form and function constitute a unity because they are reciprocally influential in a closely integrated relationship responsible for landscape evolution. In this context, the form and function principle is particularly useful to planning since it allows one to relate physical characteristics of the landscape and the spatial configuration of a plan with landscape functions and the processes that shape and alter those same characteristics. Although these might have slightly different meanings, land use, land cover, spatial structure, and pattern are fundamental concepts for both landscape ecologists and planners (Antrop 2001). Landscape ecologists focus on detecting structure where planners work on creating new structures. The former look at spatial patterns to learn about landscape processes and functions, the latter focus on guiding these according to planning goals (Antrop 2001).
1.2. A Landscape Perspective
There is enormous global diversity in landscape types, from grasslands and deserts, to forests and tundra, with many gradations between these types and the level of human activity occurring within them—activities such as agriculture, urban and suburban development, forestry, and mining. Each of these landscape types has several dimensions: ecological, economic, social, cultural, and aesthetic. Depending on one's professional or disciplinary viewpoint, landscapes can be seen from multiple perspectives. One perspective views the visible component of the landscape, the so-called "phenosystem" (González Bernáldez 1981), primarily as an aesthetic phenomenon as in the seventeenth century landscape painting. Another perspective views landscapes as "closer to the eyes than to the mind, more related to the heart, the soul, the moods than to the intellect" (Hardt 1970 cited in Bastian 2001, 758). Others view landscape as a socio-spatial entity (Linehan and Gross 1998), or as landscape products (Taborda 2000). Others perceive landscapes as geographic surface units, focused on their natural components including: water, hills, fields, and forests (Wascher 2000). Since the beginning of the twenty-first century, the landscape concept has been evolving towards a transdisciplinary perspective (Naveh 1991; Tress and Tress 2001) (see Box 1.1).
1.3. Main Characteristics of Landscape Ecology
To use landscape pattern metrics appropriately, it is important to understand their scientific context. While the range of applications for landscape metrics may be diverse, most metrics were developed and adapted specifically for landscape ecological research applications. Therefore, the more knowledge planners possess about landscape ecology principles and concepts, the easier it becomes for them to use landscape metrics appropriately.
In this handbook we have adopted a widely accepted definition of landscape: a kilometer-wide mosaic over which particular local ecosystems and land uses recur (Forman and Godron 1986; Forman 1995). A landscape mosaic is comprised of spatial elements (e.g., patches, corridors, and matrices, described below), and landscape metrics help to measure, describe, and understand the significance of these elements or their spatial pattern. Although we focus on a spatial approach to understand landscapes, we acknowledge the need to perceive landscapes as multidimensional entities that can be understood from a transdisciplinary perspective (see Box 1.1). Additionally, we find the spatial approach to understanding landscapes to be compatible with other approaches from other disciplines including: anthropology, sociology, history, and economics.
Landscape ecology focuses on the relationship between landscape structure and function and the ways landscapes change over time. To introduce this we will first examine the conceptual fundamentals of landscape structure and function. Then we will examine fundamental conceptions of landscape change, focusing on issues that are highly relevant for planning.
1.3.1. Landscape Structure
Landscape structure is a description of the spatial relationships among ecosystems, or more specifically the distribution of energy, materials, and species in relation to the size, number, types, and configurations of ecosystems.
There are several principal ways to describe the structure of landscapes, each using different kinds of data. With point data, the property of interest is usually the geographic location of each point, although measured attributes at each location may also be of interest. Linear networks within a landscape may be useful in the study of hydrologic systems (such as rivers and streams), wildlife corridors, or transportation and energy networks. Surficial, or continuous surface data is useful to address landscape variability as gradients (McGarigal and Cushman 2005). Categorical data assumes a patchy landscape structure, as commonly seen in soil or land cover maps. In this handbook the categorical data model is used, which has been widely adopted by planners.
Forman and Godron (1986) use three fundamental landscape elements to define landscape structure: patches, corridors, and the matrix. With these three elements any landscape (e.g., urban, agricultural, forested) can be described. According to Forman (1995, 7), the model that coalesces these landscape elements, the mosaic model, has analogies in other disciplines such as art, architecture, urban planning, and medicine. In the circulatory system of the human body, an organ (heart) and tubes (veins, arteries) together form a structure that allows blood to move (flow) and transport oxygen (function) within an overall context of other systems in the body (matrix). Over time body shape and size changes, thus altering body functions. Another example of this analogy is provided by Kevin Lynch's typology of urban form including: districts, edges, nodes (patches), paths (corridors), and landmarks (Lynch 1960).
In addition to landscape elements per se, it is important to account for the spatial relationships among the elements that make up a landscape. Are they clustered and adjacent to one another, or dispersed and far apart? In a landscape ecological approach, landscape elements can only be fully understood by understanding their context. The ecological significance of spatial characteristics (size, shape, or spatial distribution) of landscape elements is given not by these characteristics per se, but by considering the effect of those characteristics on each other and on other elements of the landscape. All landscape elements, regardless of their specific land cover type, influence landscape functions through their spatial characteristics. This is a fundamental interrelationship applicable to any landscape type—urban, rural, or natural (see Box 1.2).
LANDSCAPE STRUCTURAL ELEMENTS: PATCH, CORRIDOR, AND MATRIX
A patch is defined as a relatively homogeneous nonlinear area that differs from its surroundings (Forman 1995). Patches provide multiple functions including wildlife habitat, aquifer recharge areas, or sources and sinks for species or nutrients. A parcel of native forested land surrounded by farm fields is a patch, as is a large asphalt parking lot surrounded by golf courses. Thus, there are many kinds of patches: agricultural fields, wood lots, or villages (Figure 1.4). What constitutes a patch ultimately depends on the application and what is deemed meaningful as a way of representing the landscape mosaic in the context of that application.
A corridor is defined as a linear area of a particular land cover type that is different in content and physical structure from its context (Forman 1995). Corridors serve many functions within the landscape including habitat for wildlife, pathways or conduits for the movement of plants, animals, nutrients, and wind, or as a barrier to such movement. There are many types of corridors, ranging from riparian or river corridors, to interstate highway systems, to canals within an agricultural landscape (Figure 1.5).
The matrix is the dominant land cover type (LCT) in terms of area, degree of connectivity and continuity, and control that is exerted over the dynamics of the landscape (Forman 1995) (Figure 1.6). Examples of matrices include: a city with patches of parkland, forest with patches created by timber harvesting, agricultural fields with occasional small woodlots, or an agricultural landscape with a dense network of hedgerows and riparian vegetation (Figure 1.4). In the last example, hedgerows may exert significant control over the functioning of the landscape by controlling the movement of nutrients, wind, water, and wildlife across the landscape, as well as controlling the movement of people in the landscape. This control is accomplished without being the proportionately dominant land cover type—spatial arrangement of the hedgerow is key.
The spatial arrangement of patches, corridors, and matrices, and their profound interactions are, in a sense, the hallmarks of landscape ecology.
1.3.2. Landscape Function
Landscape function can refer to the broad categories of "services" that landscapes provide: production, protection, and regulation. Production services support the human needs for food, wood, recreation, and transport. Landscape protection provides for natural functions, such as rainfall infiltration, oxygen production and absorption of carbon dioxide, water cleansing by soils and wetlands, nutrient buffering by riparian corridors, and maintenance of biological diversity. Landscape regulation provides negative feedback loops that assure the overall stability of a landscape (Naveh 1994; 1998, 9; 1999).
Landscape function can also refer specifically to the flows of energy, materials, nutrients, species, people, and finally to ecological processes such as the production of biomass or the infiltration and percolation of rainfall. Materials like water or nutrients like carbon, phosphorus, and nitrogen either cycle within or flow through ecosystems, either between air and organisms (carbon), soil and organisms (phosphorus), or between air, soil, and organisms (nitrogen) (Forman 1995).
1.3.3. Relationships Between Landscape Structure and Function
Structure and function relationships are illustrated by the form and function principle, which states that the interaction between two objects is proportional to their common boundary surface (or edge) (Forman and Godron 1986, 177). The size and shape of patches determines to a large degree their ecological and functional characteristics. Large agricultural fields, for example, have greater evapotranspiration rates per unit of area than small fields. This is due in part to the greater expanse of vegetation (crops) that the wind may travel over, and the large proportion of topsoil exposed to wind and sunlight. Large forests, on the other hand, may serve to protect water and soil resources by providing vegetative cover for an entire aquifer, and by limiting the amount of soil that is exposed to weathering forces of wind and sunlight.
Large patches of native ecosystems are more likely to possess a greater variety of habitats than small patches, and therefore are more likely to support greater biodiversity (Dramstad et al. 1996). More food, fiber, and biochemical products are likely to be found in large patches (Forman 1995). Ecosystem services such as moderating fluctuations in surface water levels, the recycling of minerals and nutrients, and the removal of toxins from circulation in the environment are also more likely to be achieved as the size of the patches is increased. In addition, the inspirational and aesthetic experiences of the public may be greater when experienced in larger patches, such as wide-open spaces, spacious urban parks or gardens, or even urban neighborhoods of high aesthetic quality.
Variation in the size and shape of patches and corridors, and the area of the matrix, has a strong influence on the resulting landscape pattern. Size and shape determine the amount of boundary shared with other patches, corridors, and the matrix. Linear patches and corridors have greater amounts of boundary than compact, rounded patches. And complex, convoluted patch shapes have greater amounts of boundary than simple patch shapes. The boundaries between landscape elements are termed "edge" by ecologists, and they have significant implications for how a patch, corridor, or matrix ecosystem will function. For example, Hardt and Forman (1989) found that natural succession of a reclaimed strip mine site or other open area could be managed by manipulating the boundary (edge) shape at the scale of tens of meters. Planting trees and shrubs to form concavities along a straight forest boundary proved to effectively enhance the colonization of a mine. In fact, reclaimed mine areas where forest patch perimeters were characterized by concavities experienced more colonization than those by straight boundaries or convexities.
Spatial distribution, the relative location of patches and corridors within the matrix, matters as well. Each type of land cover has distinct physical characteristics. For example, a parking lot or a pasture exposed in the sun is hotter than the adjacent woods or a pond. Flows are created by measurable differences, such as those in pressure and temperature, across the landscape (Forman and Godron 1986). Wind is caused by a differential of air pressure, flowing from high pressure to low pressure areas. Landscape flows behave and move differently throughout the landscape depending on what land cover types (LCTs) are adjacent or near to one another.
Connectivity is a landscape property that nicely illustrates the relationship between landscape structure and function. In general, connectivity refers to the degree to which the landscape facilitates or impedes the flow of energy, materials, nutrients, species, and people across the landscape. Connectivity is an emergent property of the landscape that results from the interaction between landscape structure (i.e., the composition and configuration of the landscape mosaic) and landscape function (e.g., water flow, nutrient cycling, maintenance of biological diversity). Because connectivity is essential to proper ecosystem functioning, it is of great relevance in conservation planning and management (Naveh 1994; Forman 1995; Bennett 1999). For example, the greenway concept recognizes connectivity as key to providing multifunctional corridors for hydrological management, species movement, recreation, and cultural landscape preservation (Ahern 2004).
The concept of connectivity is perhaps easiest to understand in the context of plant and animal movement. In this context, connectivity refers to the degree to which the landscape facilitates or impedes movement of individuals among habitat patches. Connectivity affects the rate of movement among local populations in a spatially-structured population (or metapopulation) and is therefore critical to the persistence of populations in fragmented landscapes (Forman and Godron 1986; McDonnell and Pickett 1988; Opdam 1991; Opdam et al. 1993; Naveh 1994; Forman 1995; Bennett 1999). By affecting movement rates and patterns, connectivity also affects gene flow, which is essential for the long-term survival of populations (Selman and Doar 1992). An abrupt change in the connectivity of the landscape may interfere with dispersal success such that formerly widespread populations may suddenly become fragmented into small, isolated populations. This may in turn lead to an abrupt decline in patch occupancy and ultimately to the extinction of the population in the landscape. Thus, connectivity is often a critical issue regarding the conservation of populations.
Excerpted from Measuring Landscapes by André Botequilha Leitão, Joseph Miller, Jack Ahern, Kevin McGarigal. Copyright © 2006 Island Press. Excerpted by permission of ISLAND PRESS.
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Table of Contents
ContentsAbout Island Press,
Preface - The Need for Sustainable Planning,
1 - Landscape Ecology,
2 - Planning the Landscape,
3 - The Selected Set of Landscape Metrics,
4 - Applications of Landscape Metrics in Planning and Management,
5 - Recommendations on the Use of Landscape Metrics to Support the Planning of Sustainable Landscapes,
Island Press Board of Directors,