Further Reading

To consider 'edge effect', one must have a definition of an edge. Edges in ecology are frequently represented by a physical transition from one kind of habitat to another (Figure 1). The physical attribute of this edge may be manifested in habitat metrics such as density, size, color, texture, and salinity. Often edges are in the eye of the beholder; that is, an apparent edge between two habitats may be organized at one scale for a human, but at an altogether different scale for, say, a vole - which would arguably detect habitat gradients and edges at finer resolutions and shorter distances than the human. Therefore, as one considers the effect of edge, such consideration is inextricably bound up in scale, and, therefore, requires careful definition of the components of scale (resolution of examination and extent of the edge in question).

The reasons to carefully consider edges and their effects are manifold. These boundaries or transitions from one unit of organization (e.g., habitat, community, ecosystem, depending on your scale of choice) to another often exhibit high levels of species richness or biodiversity and perhaps, most importantly, may be sensitive sentinels of environmental change. These transitional areas, which are also commonly called ecotones, often reveal species composition, structure, and function representative of the unit of organization as compared with the adjacent area as well as having their own unique array of species and characteristics (e.g., species diversity, population dynamics, local extinction rates, sediment geochemistry, and soil processes, to name a few). Moreover, edges are often important controls over a range of ecosystem processes. These boundaries influence the structure and function of landscapes serving as a sort of valve, regulating ecological processes across a wide range of temporal and spatial scales. As in all branches of ecology, choice of

Figure 1 Examples of edges. Clockwise from top left: underwater seagrass meadow edge, Long Island, New York (C. Pickerell); power line corridor through forested deciduous forest (unknown); modification by fire of edge between scrub and shrub-grassland; spruce forest-upland meadow edge. Top left: Reproduced by permission of Chris Pickerell.

Figure 1 Examples of edges. Clockwise from top left: underwater seagrass meadow edge, Long Island, New York (C. Pickerell); power line corridor through forested deciduous forest (unknown); modification by fire of edge between scrub and shrub-grassland; spruce forest-upland meadow edge. Top left: Reproduced by permission of Chris Pickerell.

scale explicitly determines the range of possible interpretations that can emerge from a study and investigating the role of edge effects in mediating plant and animal populations is also bounded by this choice. This makes the study of 'edge effect' a rich subject for application of modeling and statistical innovation.

Before edge effect per se is considered, the distribution of the edge in question must be documented in space and time and again, depending on the scale chosen, the edge may be sharply defined or a slowly changing gradient. In any event, a line feature does not always provide a useful context for evaluation. When considered simply as a line with an adjacent zone of influence based on proximity to that edge, the edge implies the portion of a unit of measure near its perimeter, where influences of the adjacent unit can cause an environmental difference between the interior of the unit and its edge - the 'edge effect'.

The edge effect in its simplest terms may be represented as a geometric concept: perimeter-to-area (P/A) ratio of a distinct habitat or landscape unit (Figure 2). As that ratio increases, the perimeter, or edge per unit area, increases - making interior portions of the unit progressively closer to an edge. Thus, any processes that are affected by transition across the edge (or perimeter, or boundary) may in turn have progressively greater influence on the area inside or outside that edge as the P/A ratio increases. Thus, the 'edge effect' is a term used to describe the various consequences on plants and animals that occur as a result of one type of habitat conjoining with another. These edges may be anthropogenically derived, such as farmland cut out of a forest, a roadway through, well, just about anything natural such as salt-marsh grading into mudflat, an ephemeral stream entering an arid plain, and of course, burned versus unburned areas.

In any event, edge effect may evoke a distinctive species composition or abundance in the outer part of the landscape patch. For example, when a landscape is a mosaic of perceptibly different types, such as a forest adjacent to 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. Edge effect may have a

Core area = • Declining edge effect

Figure 2 Examples of how various patch geometries can influence edge effect. Dark, core areas conceptually illustrate how with decreasing perimeter and increasing area, portions of the habitat emerge that are not markedly influenced by edge effects. Redrawn from Rowley L, Edwards R, and Kelly P (1993) Edges -Their Effect on Vegetation and Wildlife. http://www.dse.vic.gov.au/ dpi/nreninf.nsf/childdocs/-9599F8E44B161F63CA256BC800079622-738D5F5AA8EE28A2CA256BC800090A63-6F1B33D2E88612BE4A256DEA002933B4-3AAE61565EC0FDF3CA256BCF00088820?open (accessed November 2007).

variety of consequences on wildlife. For example, when thin, narrow strips of roadside vegetation and larger, square forest blocks in northern Victoria, Canada, were compared, they shared only one common bird species. This can be attributed to the large amount of edge habitat typical of roadsides and its consequences for the fauna of this habitat.

In general, we may group classes of mechanisms through which edge effect can have similar impacts on dissimilar types of species interactions into four general categories.

Dispersal barriers or filters. These occur when an edge provides a signal or physical impediment to an organism or its propagule that is attempting to move. An example would be when an animal encounters an edge and is behaviorally influenced not to risk movement across the boundary as there was a perception of exposure to predators - but others of the same species do not make the same choice. Another example could be when a change (here, increase) in vegetation density at an edge lodges a plant propagule and prevents it from dispersing.

Influence on mortality. Elaborating on the animal example above, an example here would be when one of the animals that did make the choice to disperse across the edge became a predator's meal whereas the animal that did not disperse survived. In general, edges have been a focus of studies on predation and competition, particularly when edges are anthropogenically introduced, creating a quasi-experimental opportunity to observe how inter- and intraspecific dynamics and mortality (fitness) are controlled by habitat architecture.

Involvement in spatial subsidies (in which dispersers' intrapatch impacts are maintained by their activities in other habitats). Subsidies essentially describe situations where crossing edges is needed for an organism to survive as there are insufficient amounts or kinds of resources in one patch -therefore crossing edges into other habitats must be risked.

Novel interactions. Examples of this include, perhaps most famously, the interplay of newly invading species that occurs as edges are created. Because species have not, at least locally, encountered one another before, these interactions can have profound consequences for those species' survival, growth, and reproduction, particularly though competition and predation. Moreover, anthropo-genically created edges are susceptible to many secondary and new interactions, such as nutrient and herbicide/ pesticide loads when edges arise from farming, herding, and husbandry. The physical change imposed by an edge often invariably leads to changes in microclimate which in itself becomes a novel interaction of biotic and abiotic components of the ecosystem.

With the aforementioned mechanisms, there is an endless variety of consequences that can arise for species interacting with ecosystem edges - some of which facilitate species' success while others may be decidedly negative. Some examples of consequences of edge effect mechanisms are outlined in the following.

Microclimate shifts. As habitat architecture changes across an edge, many other physical factors may also change: temperature, light, relative humidity, wind, and, in the cases of aquatic settings, current speeds, salinity, wave energy, etc. In turn, these physical gradients can feed back into driving additional changes across the edges, such as soil or sediment type, hydrology, advantages for different plant and animal species (e.g., stress tolerance). Such feedback cycles can cause edge characteristics to continue changing over time, long after any perturbation that may have contributed to the formation of the edge has ceased.

Species shifts - edge versus interior species. As edges are created, species that have requirements of habitat continuity, contiguity, or critical minimum ranges may disappear. Species that are adapted to changing conditions or whose habitat needs are not (comparatively to habitat 'core' species) spatially and temporally sensitive may thrive. These vacancies created by loss of core species may therefore introduce previously rare or absent species to the local environment - including exotic or invasive species. Moreover, the arrival of some new species, particularly those that may be ecosystem engineers (organisms that create, modify, or maintain habitats (or microhabitats) by causing physical state changes in biotic and abiotic materials that, directly or indirectly, modulate the availability of resources to other species; e.g., humans, beavers) may then drive the characteristics of the edge architecture in yet new directions. However, species shifts arising from edge development need not be overtly negative; these locations may often enrich species diversity by providing a range of new and not necessarily incompatible niches (a niche being the combination of physical conditions where a species can best grow, feed, and reproduce; the confluence of these optimal conditions determining its success and abundance is termed the ecological 'niche') for new species.

Human proximity. Many habitat edges are anthropogenic in origin (but contrast this with natural, migrating gaps such as storm-induced blowouts in underwater seagrass beds or regeneration waves (gaps) in spruce forests). When edges are created (and maintained) by humans, such as road cuts, power line corridors, pipeline swaths, etc., the intimidating proximity of humans, our machines, and the concomitant movement and noise can quickly drive species away, creating vacant niches for new ones.

How do edge effects vary with shape and size? Generally speaking, the longer the edge, the larger the area disturbed (Figure 2). The more angular the patch edges, the greater the edge effect. Changes in edge direction increase disturbance whereas rounded or smooth-edged shapes minimize edge effects. The smaller the area of a patch, the greater the risk of an impact occurring, often with the core or center of the habitat being altered or eliminated. Edge effects are likely to be most influential in narrow bands of habitat, those of small extent and generally those with the highest P/A ratios. Consequently, edge effects are an important issue in the management of corridors and small habitat units. Larger areas are also vulnerable where long, narrow intrusions pierce the core of otherwise continuous habitat (e.g., small trails) and serve as a conduit for species introductions, disease introduction, and human activity.

How far do edge effects extend?: The 'depth' of the effect in habitat varies greatly with the length of the edge, the contrast in edge, the width of the habitat, its composition (vegetation, rock, biogenic detritus, etc.), the species of consideration, and the stability of the edge composition. One study showed that in terms of vegetation structure, the width of a forest edge was less than 13 m, but based upon the distribution of birds' nests, the functional width of the edge ranged from ~10 to 65 m. Certainly, in the case of disease introduction, the extent of effect can be grossly disproportionate to the physical size of the edge area, especially when epidemics erupt as a consequence of edge formation and sweep far across the landscape.

The consequences of edge effect are also inextricably linked with the pervasive problem of habitat fragmentation especially arising from the separation of a landscape into various land uses (e.g., development, agriculture, etc.), creating numerous small, disjunct habitat patches left for use by fauna (although edge considerations are not limited to dispersal of fauna and includes all kingdoms as well). Fragmentation creates a dramatic increase in the P/A ratio of habitats (perhaps the simplest geometric assessment) and may seriously erode habitat value for species requiring large unbroken tracts of habitat. It has been broadly demonstrated that small habitat patches resulting from fragmentation often do not provide the food and cover resources for many species that do attempt to use them and often is associated with increased predation as refuge is limited (increased visibility with increasing edge) and foraging across suboptimal habitat may reveal prey to predators. Thus, with increasing complex edges, the edge effect may extend across entire habitat fragments as appears to happen with seagrass flats crisscrossed with propeller scars. A high predator population, coupled with small habitat patches that are easily penetrated, makes death by predation quite probable for many species. This is one of the factors thought to be contributing to the decline of migrant songbirds in the Eastern United States temperate forests.

One of the more recent concerns for the influence of edges in terrestrial systems is the concept of 'ecological traps'. Ecological traps are generally considered as portions of an edge where animals actively (but unwisely) select poor habitat for effective reproduction over superior habitat (for reproduction); some of the best-known examples are generally associated with birds at forest edges. Reasons for these unwise choices vary, but appear to arise from selections for perceived advantages such as preferred microclimate, vegetation types (that may be different at edges vs. interiors), food availability, and social interactions but are accompanied by novel interactions that dramatically and seemingly disproportionately reduce fitness. Traps develop when their appearance on the scene outstrips the ability of the trapped species to adapt. Such traps especially form when habitats are artificially fragmented, introducing negative edge effects, such as increased interaction with nest predators.

In another consideration, the twentieth-century debate over the SLOSS concept (single large or several small) in developing protected areas was inextricably linked to edge effect. To summarize this issue, the notion that a large reserve was preferable to several small ones emerged from a broad base of ecological investigations. The emerging consensus or at least the working hypothesis has been that large reserves support more species with greater ecological breadth of niches being occupied. Perhaps more important, it was expected that larger reserves would have lower (local) extinction rates than smaller reserves (which have more edge and thus more potential for interactions that could reduce diversity and abundance). Therefore, it was held that large reserves had the added advantage of minimizing degradation of reserve usefulness through a concurrently minimized edge effect. This implies that species loss would arise from habitat fragmentation of large, continuous habitats. Many scientists have suspected that these effects were species dependent, but that the general rule would be that larger reserves would accommodate a greater number of species and niches. The SLOSS concept led into investigations of species-area relationships, particularly with regard to human-induced habitat fragmentation, the concern again being that with greater fragmentation, species diversity and niche breadth would plummet. It is important to note, however, that fragmentation is simply a state of habitat organization but does not describe the mechanism that causes changes in species diversity, increased local extinctions, changes in niche availability, etc. Our topic of edge effect is also about the mechanisms that give rise to these observed changes (or not, in some cases) and the underlying assumptions and initial conditions of species distributions may strongly influence how or even whether edge effect is operating. The outcome and application of the SLOSS debate is somewhat tangential to this discussion, but the classic linking of edge effect, fragmentation, species diversity, island biogeography, and local extinctions should not be overlooked.

Interestingly, much of the theory regarding edge effects in aquatic ecosystems is derived from terrestrial examples and rests on our understanding of ecological and evolutionary processes and how they have produced today's ecosystems. Just as important is discerning what aspects of ecosystems derive from natural processes apart from human effects. The potential incompatibility of the aquatic and terrestrial concepts of edge effect arises from the comparatively 'open' nature (higher dispersal and connectivity) of aquatic systems and particularly marine systems that are lacking in dispersal constraints regularly found in terrestrial settings. In other words, edge effect may be extremely fuzzy in aquatic settings, with the effects varying dramatically among vagile versus sessile and nekton versus benthic strategies. Our understanding of species' movement, gene flow, trophic pathways, etc., in aquatic systems is potentially far removed from terrestrial models, and thus edge effects in aquatic systems largely await additional study and theory development.

Nonetheless, consideration of edge effect has become central to the delineation of (marine) protected areas. Edge effect has been implicated in marine protected areas (MPA) function as the basic goals of MPA designation include conservation of biodiversity and/or the preservation of intact ecosystems. But edge effect may limit species interactions and movements, especially with the presence of intense fishing pressure at boundaries. Other factors, such as increasing abundance of top predators within an MPA, may deplete prey populations and lead to trophic cascades. With intrinsically fuzzy boundaries, prediction of edge effect is likely to be especially complicated in marine ecosystems.

Finally, it should be noted that edge effect has a particular place in modeling applications. Researchers often use circular boundary conditions so as not to confound the effects of population interactions with artificial edge effects due to finite habitat length, as might be represented in a spatial model. Many models avoid this by having effects that would pass off one side of a matrix and become reintroduced on the opposite side. In general, most edge effect issues remain poorly understood in aquatic systems.

For further information, the reader is encouraged to conduct Internet searches using such key phrases as 'habitat edge', 'habitat fragmentation', and, of course, 'edge effect'.

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