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Landscape ecology principles are listed and illustrated below in four sections: Patches; Edges; Corridors; and Mosaics. Each section begins with an introduction to important terms and concepts, and ends with a list of key references. For additional references please refer to the bibliography.
In a densely populated world plant and animal habitat increasingly appears in scattered patches. Ecologists first considered habitat patches analogous with islands, but soon largely abandoned the analogy due to the major differences between the sea and the matrix of countryside and suburban developments surrounding a "terrestrial" patch. Patches, however, do exhibit a degree of isolation, the effect and severity being dependent on the species present.
Four origins or causes of vegetation patches are usefully recognized: remnants (e.g., areas remaining from an earlier more extensive type, such as woodlots in agricultural areas); introduced (e.g., a new suburban development in an agricultural area, or a small pasture within a forest); disturbance (e.g., a burned area in a forest, or a spot devastated by a severe windstorm); and environmental resources (e.g., wetlands in a city, or oases in a desert).
Patches are analyzed below and differentiated in terms of (1) size, (2) number, and (3) location. Patches may be as large as a national forest, or as small as a single tree. Patches may be numerous in a landscape, such as avalanches or rock slides on a mountainside, or be scarce such as oases in a desert. The location of patches may be beneficial or deleterious to the optimal functioning of a landscape. For example, small, remnant forest patches between large reserves in an agricultural matrix can be beneficial. In contrast, a landfill located adjacent to a sensitive wetland may have a negative impact on the ecological health of the landscape.
PATCH SIZE: LARGE OR SMALL?
P1. Edge habitat and species
Dividing a large patch into two smaller ones creates additional edge habitat, leading to higher population sizes and a slightly greater number of edge species, which are often common or widespread in the landscape.
P2. Interior habitat and species
Dividing a large patch into two smaller ones removes interior habitat, leading to reduced population sizes and number of interior species, which are often of conservation importance.
P3. Local extinction probability
A larger patch normally has a larger population size for a given species than a smaller patch, making it less likely that the species (which fluctuates in population size) will go locally extinct in the larger patch.
The probability of a species becoming locally extinct is greater if a patch is small, or of low habitat quality.
P5. Habitat diversity
A large patch is likely to have more habitats present, and therefore contain a greater number of species than a small patch.
P6. Barrier to disturbance
Dividing a large patch into two smaller ones creates a barrier to the spread of some disturbances.
P7. Large patch benefits
Large patches of natural vegetation are the only structures in a landscape that protect aquifers and interconnected stream networks, sustain viable populations of most interior species, provide core habitat and escape cover for most large-home-range vertebrates, and permit near-natural disturbance regimes.
P8. Small patch benefits
Small patches that interrupt extensive stretches of matrix act as stepping stones for species movement. They also contain some uncommon species where large patches are absent or, in unusual cases, are unsuitable for a species. Therefore small patches provide different and supplemental ecological benefits than large patches.
PATCH NUMBER: HOW MANY?
P9. Habitat loss
Removal of a patch causes habitat loss, which often reduces the population size of a species dependent upon that habitat type, and may also reduce habitat diversity, leading to fewer species.
P10. Metapopulation dynamics
Removal of a patch reduces the size of a metapopulation (i.e., an interacting population subdivided among different patches), thereby increasing the probability of local within-patch extinctions, slowing down the recolonization process, and reducing stability of the metapopulation.
P11. Number of large patches
Where one large patch contains almost all the species for that patch type in the landscape, two large patches may be considered the minimum for maintaining species richness. However, where one patch contains a limited portion of the species pool, up to four or five large patches are probably required.
P12. Grouped patches as habitat
Some relatively generalist species can, in the absence of a large patch, survive in a number of nearby smaller patches, which although individually inadequate, are together suitable.
PATCH LOCATION: WHERE?
The probability of a species going locally extinct is greater in an isolated patch. Isolation is a function not only of distance, but also of the characteristics (i.e., resistance) of the intervening matrix habitat.
A patch located in close proximity to other patches or the "mainland" will have a higher chance of being (re-)colonized within a time interval, than a more isolated patch.
P15. Patch selection for conservation
The selection of patches for conservation should be based on their: 1) contribution to the overall system, i.e., how well the location of a patch relates or links to other patches within the landscape or region; and 2) unusual or distinctive characteristics, e.g., whether a patch has any rare, threatened, or endemic species present.
EDGES AND BOUNDARIES
An edge is described as the outer portion of a patch where the environment differs significantly from the interior of the patch. Often, edge and interior environments simply look and feel differently. For example, vertical and horizontal structure, width, and species composition and abundance, in the edge of a patch, differ from interior conditions, and together comprise the edge effect. Whether a boundary is curvilinear or straight influences the flow of nutrients, water, energy, or species along or across it.
Boundaries may also be "political" or "administrative," that is artificial divisions between inside and out, which may or may not correspond to natural "ecological" boundaries or edges. Relating these artificial edges with natural ones is important. As human development continues its expansion into natural environments, the edges created will increasingly form the critical point for interactions between human-made and natural habitats.
The shapes of patches, as defined by their boundaries, can be manipulated by landscape architects and land-use planners to accomplish an ecological function or objective. Due to the diverse significance of edges, rich opportunities exist to use this key ecological transition zone between two types of habitat in designs and plans.
E1. Edge structural diversity
Vegetative edges with a high structural diversity, vertically or horizontally, are richer in edge animal species.
E2. Edge width
Edge width differs around a patch, with wider edges on sides facing the predominant wind direction and solar exposure.
E3. Administrative and natural ecological boundary
Where the administrative or political boundary of a protected area does not coincide with a natural ecological boundary, the area between the boundaries often becomes distinctive, and may act as a buffer zone, reducing the influence of the surroundings on the interior of the protected area.
E4. Edge as filter
Patch edges normally function as filters, which dampen influences of the surroundings on the patch interior.
E5. Edge abruptness
Increased edge abruptness tends to increase movement along an edge, whereas less edge abruptness favors movement across an edge.
BOUNDARIES: STRAIGHT OR CONVOLUTED?
E6. Natural and human edges
Most natural edges are curvilinear, complex, and soft, whereas humans tend to make straight, simple, and hard edges.
E7. Straight and curvilinear boundaries
A straight boundary tends to have more species movement along it, whereas a convoluted boundary is more likely to have movement across it.
E8. Hard and soft boundaries
Compared with a straight boundary between two areas, a curvilinear "tiny-patch" boundary may provide a number of ecological benefits, including less soil erosion and greater wildlife usage.
E9. Edge curvilinearity and width
Curvilinearity and width of an edge combine to determine the total amount of edge habitat within a landscape.
E10. Coves and lobes
The presence of coves and lobes along an edge provides greater habitat diversity than along a straight edge, thereby encouraging higher species diversity.
SHAPES OF PATCHES: ROUND OR CONVOLUTED?
E11. Edge and interior species
A more convoluted patch will have a higher proportion of edge habitat, thereby slightly increasing the number of edge species, but sharply decreasing the number of interior species, including those of conservation importance.
E12. Interaction with surroundings
The more convoluted the shape of a patch, the more interaction, whether positive or negative, there is between the patch and the surrounding matrix.
E13. Ecologically "optimum" patch shape
An ecologically optimum patch provides several ecological benefits, and is generally "spaceship shaped," with a rounded core for protection of resources, plus some curvilinear boundaries and a few fingers for species dispersal.
E14. Shape and orientation
A patch oriented with its long axis parallel to the route of dispersing individuals will have a lower probability of being (re-)colonized, than a patch perpendicular to the route of dispersers.
CORRIDORS AND CONNECTIVITY
The loss and isolation of habitat is a seemingly unstoppable process occurring throughout the modern world. Landscape planners and ecologists must contend with this continuing process if further reductions in biodiversity are to be slowed or halted.
Several dynamic processes cause this isolation and loss over time. The key spatial processes include: fragmentation (i.e., breaking up a larger/intact habitat into smaller dispersed patches); dissection (i.e., splitting an intact habitat into two patches separated by a corridor); perforation (i.e., creating "holes" within an essentially intact habitat); shrinkage (i.e., the decrease in size of one or more habitats); and attrition (i.e., the disappearance of one or more habitat patches).
In the face of continued habitat loss and isolation, many landscape ecologists stress the need for providing landscape connectivity, particularly in the forms of wildlife movement corridors and stepping stones. Despite residual discussion over the effectiveness of corridors in enhancing biodiversity, a growing empirical body of research underlines the positive net benefits accruing from incorporating higher quality linkages between habitat patches.
Corridors in the landscape may also act as barriers or filters to species movement. Some may be population "sinks" (i.e., locations where individuals of a species tend to decrease in number). For example, roadways, railroads, powerlines, canals, and trails, may be thought of as "troughs" or barriers.
Finally, stream or river systems are corridors of exceptional significance in a landscape. Maintaining their ecological integrity in the face of intense human use is both a challenge and an opportunity to landscape designers and land-use planners.
CORRIDORS FOR SPECIES MOVEMENT
C1. Controls on corridor functions
Width and connectivity are the primary controls on the five major functions of corridors, i.e., habitat, conduit, filter, source, and sink.
C2. Corridor gap effectiveness
The effect of a gap in a corridor on movement of a species depends on length of the gap relative to the scale of species movement, and contrast between the corridor and the gap.
C3. Structural versus floristic similarity
Similarity in vegetation structure and floristics (plant species) between corridors and large patches is preferable, though similarity in structure alone is probably adequate in most cases for interior species movement between large patches.
C4. Stepping stone connectivity
A row of stepping stones (small patches) is intermediate in connectivity between a corridor and no corridor, and hence intermediate in providing for movement of interior species between patches.
C5. Distance between stepping stones
For highly visually-oriented species, the effective distance for movement between stepping stones is determined by the ability to see each successive stepping stone.
C6. Loss of a stepping stone
Loss of one small patch, which functions as a stepping stone for movement between other patches, normally inhibits movement and thereby increases patch isolation.
C7. Cluster of stepping stones
The optimal spatial arrangement of a cluster of stepping stones between large patches provides alternate or redundant routes, while maintaining an overall linearly-oriented array between the large patches.
ROAD AND WINDBREAK BARRIERS
C8. Roads and other "trough" corridors
Road, railroad, powerline, and trail corridors tend to be completely connected, relatively straight, and subject to regular human disturbance. Therefore, they commonly serve as barriers that subdivide populations of species into metapopulations; conduits mainly for disturbance-tolerant species; and sources of erosion, sedimentation, exotic species, and human effects on the matrix.
C9. Wind erosion and its control
Modest winds reduce soil fertility by selectively removing and blowing fine particles long distances, whereas heavier winds often move mid-sized particles only tens of meters. Wind erosion control reduces field size in the preponderant wind direction, and maintains vegetation, furrows, or soil clods, especially in spots susceptible to vortices, turbulence, or accelerated streamline airflow.
STREAM AND RIVER CORRIDORS
C10. Stream corridor and dissolved substances
Dissolved substances, such as nitrogen, phosphorus, and toxins, entering a vegetated stream corridor are primarily controlled from entering the channel and reducing water quality by friction, root absorption, clay, and soil organic matter; these in turn are most effectively provided by a wide corridor of dense natural vegetation.
(1) Contact with plant stems and litter slows water movement
(2) Plant roots absorb dissolved substances prior to reaching the stream
(3) Clay particles hold dissolved substances
(4) Soil organic matter absorbs dissolved substances
C11. Corridor width for main stream
To maintain natural processes, a 2nd- to ca. 4th-order stream corridor: maintains an interior upland habitat on both sides, which is wide enough to control dissolved-substance inputs from the matrix; provides a conduit for upland interior species; and offers suitable habitat for floodplain species displaced by beaver flooding or lateral channel migration.
C12. Corridor width for a river
To maintain natural processes, a ca. 5th-to 10th-order river corridor maintains an upland interior on both sides, as a conduit for upland interior species and species displaced by lateral channel migration. In addition, maintaining at least a "ladder-pattern" of large patches crossing the floodplain provides a hydrologic sponge, traps sediment during floods, and provides soil organic matter for the aquatic food chain, logs for fish habitat, and habitats for rare floodplain species.
C13. Connectivity of a stream corridor
Width and length of a vegetated stream corridor interact or combine to determine stream processes. However, a continuous stream corridor, without major gaps, is essential to maintain aquatic conditions such as cool water temperature and high oxygen content. Without these, plus other physiological conditions, viable populations of certain fish species, such as trout, will not be maintained.
Excerpted from Landscape Ecology Principles in Landscape Architecture and Land-Use Planning by Wenche Dramstad, James D. Olson, Richard T.T. Forman. Copyright © 1996 President and Fellows of Harvard College. Excerpted by permission of ISLAND PRESS.
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