Source Patch Ecology

Patch dynamics is a conceptual approach to ecosystem and habitat analysis that emphasizes dynamics of heterogeneity within a system i.e. that each area of an.

Source–sink dynamics is a theoretical model used by ecologists to describe how variation in habitat quality may affect the population growth or decline of organisms.

Since quality is likely to vary among patches of habitat, it is important to consider how a low quality patch might affect a population. In this model, organisms occupy two patches of habitat. One patch, the source, is a high quality habitat that on average allows the population to increase. The second patch, the sink, is very low quality habitat that, on its own, would not be able to support a population. However, if the excess of individuals produced in the source frequently moves to the sink, the sink population can persist indefinitely. Organisms are generally assumed to be able to distinguish between high and low quality habitat, and to prefer high quality habitat. However, ecological trap theory describes the reasons why organisms may actually prefer sink patches over source patches. Finally, the source-sink model implies that some habitat patches may be more important to the long-term survival of the population, and considering the presence of source-sink dynamics will help inform conservation decisions.

Although the seeds of a source-sink model had been planted earlier, 1 Pulliam 2 is often recognized as the first to present a fully developed source-sink model. He defined source and sink patches in terms of their demographic parameters, or BIDE rates birth, immigration, death, and emigration rates. In the source patch, birth rates were greater than death rates, causing the population to grow. The excess individuals were expected to leave the patch, so that emigration rates were greater than immigration rates. In other words, sources were a net exporter of individuals. In contrast, in a sink patch, death rates were greater than birth rates, resulting in a population decline toward extinction unless enough individuals emigrated from the source patch. Immigration rates were expected to be greater than emigration rates, so that sinks were a net importer of individuals. As a result, there would be a net flow of individuals from the source to the sink see Table 1.

Pulliam s work was followed by many others who developed and tested the source-sink model. Watkinson and Sutherland 3 presented a phenomenon in which high immigration rates could cause a patch to appear to be a sink by raising the patch s population above its carrying capacity the number of individuals it can support. However, in the absence of immigration, the patches are able to support a smaller population. Since true sinks cannot support any population, the authors called these patches pseudo-sinks. Definitively distinguishing between true sinks and pseudo-sinks requires cutting off immigration to the patch in question and determining whether the patch is still able to maintain a population. Thomas et al. 4 were able to do just that, taking advantage of an unseasonable frost that killed off the host plants for a source population of Edith s checkerspot butterfly Euphydryas editha. Without the host plants, the supply of immigrants to other nearby patches was cut off. Although these patches had appeared to be sinks, they did not become extinct without the constant supply of immigrants. They were capable of sustaining a smaller population, suggesting that they were in fact pseudo-sinks.

Watkinson and Sutherland s 3 caution about identifying pseudo-sinks was followed by Dias, 5 who argued that differentiating between sources and sinks themselves may be difficult. She asserted that a long-term study of the demographic parameters of the populations in each patch is necessary. Otherwise, temporary variations in those parameters, perhaps due to climate fluctuations or natural disasters, may result in a misclassification of the patches. For example, Johnson 6 described periodic flooding of a river in Costa Rica which completely inundated patches of the host plant for a rolled-leaf beetle Cephaloleia fenestrata. During the floods, these patches became sinks, but at other times they were no different from other patches. If researchers had not considered what happened during the floods, they would not have understood the full complexity of the system.

Dias 5 also argued that an inversion between source and sink habitat is possible so that the sinks may actually become the sources. Because reproduction in source patches is much higher than in sink patches, natural selection is generally expected to favor adaptations to the source habitat. However, if the proportion of source to sink habitat changes so that sink habitat becomes much more available, organisms may begin to adapt to it instead. Once adapted, the sink may become a source habitat. This is believed to have occurred for the blue tit Parus caeruleus 7500 years ago as forest composition on Corsica changed, but few modern examples are known. Boughton 7 described a source pseudo-sink inversion in butterfly populations of E. editha. 4 Following the frost, the butterflies had difficulty recolonizing the former source patches. Boughton found that the host plants in the former sources senesced much earlier than in the former pseudo-sink patches. As a result, immigrants regularly arrived too late to successfully reproduce. He found that the former pseudo-sinks had become sources, and the former sources had become true sinks.

One of the most recent additions to the source-sink literature is by Tittler et al., 8 who examined wood thrush Hylocichla mustelina survey data for evidence of source and sink populations on a large scale. The authors reasoned that emigrants from sources would likely be the juveniles produced in one year dispersing to reproduce in sinks in the next year, producing a one-year time lag between population changes in the source and in the sink. Using data from the Breeding Bird Survey, an annual survey of North American birds, they looked for relationships between survey sites showing such a one-year time lag. They found several pairs of sites showing significant relationships 60–80 km apart. Several appeared to be sources to more than one sink, and several sinks appeared to receive individuals from more than one source. In addition, some sites appeared to be a sink to one site and a source to another see Figure 1. The authors concluded that source-sink dynamics may occur on continental scales.

One of the more confusing issues involves identifying sources and sinks in the field. 9 Runge et al. 9 point out that in general researchers need to estimate per capita reproduction, probability of survival, and probability of emigration to differentiate source and sink habitats. If emigration is ignored, then individuals that emigrate may be treated as mortalities, thus causing sources to be classified as sinks. This issue is important if the source-sink concept is viewed in terms of habitat quality as it is in Table 1 because classifying high-quality habitat as low-quality may lead to mistakes in ecological management. Runge et al. 9 showed how to integrate the theory of source-sink dynamics with population projection matrices 10 and ecological statistics 11 in order to differentiate sources and sinks.

Table 1. Summary characteristics of variations on the source-sink dynamics model.

Habitat patches are represented in terms of their 1 inherent abilities to maintain a population in the absence of immigration, 2 their attractiveness to organisms that are actively dispersing and choosing habitat patches, and 3 whether they are net exporters or importers of dispersing individuals. Note that in all of these systems, source patches are capable of supporting stable or growing populations and are net exporters of individuals. The major difference between them is that in the ecological trap model, the source patch is avoided or at least not preferred to the low quality trap patch. All of the low quality patches whether sinks, pseudo-sinks, or traps are net importers of dispersing individuals, and in the absence of dispersal, would show a population decline. However, pseudo-sinks would not decline to extinction as they are capable of supporting a smaller population. The other major difference between these low quality patch types is in their attractiveness; sink populations are avoided while trap patches are preferred or at least not avoided.

Why would individuals ever leave high quality source habitat for a low quality sink habitat. This question is central to source-sink theory. Ultimately, it depends on the organisms and the way they move and distribute themselves between habitat patches. For example, plants disperse passively, relying on other agents such as wind or water currents to move seeds to another patch. Passive dispersal can result in source-sink dynamics whenever the seeds land in a patch that cannot support the plant s growth or reproduction. Winds may continually deposit seeds there, maintaining a population even though the plants themselves do not successfully reproduce. 12 Another good example for this case are soil protists. Soil protists also disperse passively, relying mainly on wind to colonize other sites. 13 As a result, source-sink dynamics can arise simply because external agents dispersed protist propagules e.g., cysts, spores, forcing individuals to grow in a poor habitat. 14

In contrast, many organisms that disperse actively should have no reason to remain in a sink patch, 15 provided the organisms are able to recognize it as a poor quality patch see discussion of ecological traps . The reasoning behind this argument is that organisms are often expected to behave according to the ideal free distribution, which describes a population in which individuals distribute themselves evenly among habitat patches according to how many individuals the patch can support. 16 When there are patches of varying quality available, the ideal free distribution predicts a pattern of balanced dispersal. 15 In this model, when the preferred habitat patch becomes crowded enough that the average fitness survival rate or reproductive success of the individuals in the patch drops below the average fitness in a second, lower quality patch, individuals are expected to move to the second patch. However, as soon as the second patch becomes sufficiently crowded, individuals are expected to move back to the first patch. Eventually, the patches should become balanced so that the average fitness of the individuals in each patch and the rates of dispersal between the two patches are even. In this balanced dispersal model, the probability of leaving a patch is inversely proportional to the carrying capacity of the patch. 15 In this case, individuals should not remain in sink habitat for very long, where the carrying capacity is zero and the probability of leaving is therefore very high.

An alternative to the ideal free distribution and balanced dispersal models is when fitness can vary among potential breeding sites within habitat patches and individuals must select the best available site. This alternative has been called the ideal preemptive distribution, because a breeding site can be preempted if it has already been occupied. 17 For example, the dominant, older individuals in a population may occupy all of the best territories in the source so that the next best territory available may be in the sink. As the subordinate, younger individuals age, they may be able to take over territories in the source, but new subordinate juveniles from the source will have to move to the sink. Pulliam 2 argued that such a pattern of dispersal can maintain a large sink population indefinitely. Furthermore, if good breeding sites in the source are rare and poor breeding sites in the sink are common, it is even possible that the majority of the population resides in the sink.

The source-sink model of population dynamics has made contributions to many areas in ecology. For example, a species niche was originally described as the environmental factors required by a species to carry out its life history, and a species was expected to be found only in areas that met these niche requirements. 18 This concept of a niche was later termed the fundamental niche, and described as all of the places a species could successfully occupy. In contrast, the realized niche, was described as all of the places a species actually did occupy, and was expected to be less than the extent of the fundamental niche as a result of competition with other species. 19 However, the source-sink model demonstrated that the majority of a population could occupy a sink which, by definition, did not meet the niche requirements of the species, 2 and was therefore outside the fundamental niche see Figure 2. In this case, the realized niche was actually larger than the fundamental niche, and ideas about how to define a species niche had to change.

Source–sink dynamics has also been incorporated into studies of metapopulations, a group of populations residing in patches of habitat. Though some patches may go extinct, the regional persistence of the metapopulation depends on the ability of patches to be re-colonized. As long as there are source patches present for successful reproduction, sink patches may allow the total number of individuals in the metapopulation to grow beyond what the source could support, providing a reserve of individuals available for re-colonization. 20 Source–sink dynamics also has implications for studies of the coexistence of species within habitat patches. Because a patch that is a source for one species may be a sink for another, coexistence may actually depend on immigration from a second patch rather than the interactions between the two species. 2 Similarly, source-sink dynamics may influence the regional coexistence and demographics of species within a metacommunity, a group of communities connected by the dispersal of potentially interacting species. 21 Finally, the source-sink model has greatly influenced ecological trap theory, a model in which organisms prefer sink habitat over source habitat. 22

Land managers and conservationists have become increasingly interested in preserving and restoring high quality habitat, particularly where rare, threatened, or endangered species are concerned. As a result, it is important to understand how to identify or create high quality habitat, and how populations respond to habitat loss or change. Because a large proportion of a species population could exist in sink habitat, conservation efforts may misinterpret the species habitat requirements. Similarly, without considering the presence of a trap, conservationists might mistakenly preserve trap habitat under the assumption that an organism s preferred habitat was also good quality habitat. Simultaneously, source habitat may be ignored or even destroyed if only a small proportion of the population resides there. Degradation or destruction of the source habitat will, in turn, impact the sink or trap populations, potentially over large distances. 8 Finally, efforts to restore degraded habitat may unintentionally create an ecological trap by giving a site the appearance of quality habitat, but which has not yet developed all of the functional elements necessary for an organism s survival and reproduction. For an already threatened species, such mistakes might result in a rapid population decline toward extinction.

In considering where to place reserves, protecting source habitat is often assumed to be the goal, although if the cause of a sink is human activity, simply designating an area as a reserve has the potential to convert current sink patches to source patches e.g. no-take zones. 23 Either way, determining which areas are sources or sinks for any one species may be very difficult, and an area that is a source for one species may be unimportant to others. Finally, areas that are sources or sinks currently may not be in the future as habitats are continually altered by human activity or climate change. Few areas can be expected to be universal sources, or universal sinks. 23 While the presence of source, sink, or trap patches must be considered for short-term population survival, especially for very small populations, long-term survival may depend on the creation of networks of reserves that incorporate a variety of habitats and allow populations to interact. 23

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a b c d Pulliam, H. R. 1988. Sources, sinks, and population regulation. American Naturalist 12-661.

a b Watkinson, A. R., and W. J. Sutherland. 1995. Sources, sinks and pseudo-sinks. Journal of Animal Ecology 6-130.

a b Thomas, C. D., M. C. Singer, and D. A. Boughton. 1996. Catastrophic extinction of population sources in a butterfly metapopulation. American Naturalist 17-975.

a b Dias, P. C. 1996. Sources and sinks in population biology. Trends in Ecology and Evolution 6-330.

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a b Tittler, R., L. Fahrig, and M. A. Villard. 2006. Evidence of large-scale source-sink dynamics and long-distance dispersal among wood thrush populations. Ecology 29-3036.

a b c Runge, J. P., M. C. Runge and J. D. Nichols. 2006. The role of local populations within a landscape context:defining and classifying sources and sinks. American Naturalist 15-938.

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Fernández, L. D. 2015. Source-sink dynamics shapes the spatial distribution of soil protists in an arid shrubland of northern Chile. Journal of Arid Environments 11-125.

a b c Diffendorfer, J. E. 1998. Testing models of source-sink dynamics and balanced dispersal. Oikos 7-433.

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Grinnell, J. 1917. The Niche-Relationships of the California Thrasher. The Auk 7-433.

Hutchinson, G. E. 1957. Concluding remarks. Cold Spring Harbor Symposium Quantitative Biology 5-427.

Howe, R. W., G. J. Davis, and V. Mosca. 1991. The Demographic Significance of Sink Populations. Biological Conservation 9-255.

Leibold, M. A., M. Holyoak, J. M. Chase, M. F. Hoopes, R. D. Holt, J. B. Shurin, R. Law, D. Tilman, M. Loreau, and A. Gonzalez. 2004. The metacommunity concept: a framework for multi-scale community ecology. Ecology Letters 1-613.

Robertson, B. A., and R. L. Hutto. 2006. A framework for understanding ecological traps and an evaluation of existing evidence. Ecology 75-1085.

a b c Roberts, C. M. 1998. Sources, sinks, and the design of marine reserve networks. Fisheries -19.

Battin, J. 2004. When good animals love bad habitats: Ecological traps and the conservation of animal populations. Conservation Biology 82-1491.

Delibes, M., P. Gaona, and P. Ferreras. 2001. Effects of an attractive sink leading into maladaptive habitat selection. American Naturalist 17-285.

Dwernychuk, L. W., and D. A. Boag. 1972. Ducks nesting in association with gulls-an ecological trap. Canadian Journal of Zoology 9-563.

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Retrieved from https://en.wikipedia.org/w/index.php.title Source–sink_dynamics oldid 691197144

Categories: Landscape ecologyEcological theoriesPopulationConservationBehavioral ecology.

Table 1. Summary characteristics of variations on the source-sink dynamics model. Source-sink Source-pseudosink Ecological trap; Source patch high quality habitat.

Thomas G. Barnes, Extension Wildlife Specialist

This publication is intended to be a companion to FOR-75, An Ecosystems Approach to Natural Resources Management.

This publication introduces the concepts and principles of landscape ecology for managing wildlife and other natural resources. It is intended to raise public awareness and give an overview of a new philosophy and method for managing natural resources at the landscape level.

A landscape is a heterogenous area composed of a cluster of interacting ecosystems that are repeated in various sizes, shapes, and spatial relationships throughout the landscape. Landscapes have different land forms, vegetation types, and land uses. Another way of looking at a landscape is as a mosaic of habitat patches across which organisms move, settle, reproduce, and eventually die and return to the soil. The best way to envision a landscape is to look at the land from an aerial perspective or to examine aerial photographs to see how a particular piece of land fits into the larger picture.

Landscape ecology is the study of structure, function, and change in a heterogenous land area composed of interacting ecosystems. It is an interdisciplinary science dealing with the interrelationship between human society and our living space. Landscape ecology is a relatively new science, although Europeans have been using its principles much longer than Americans. We can learn a great deal from examining how the Europeans have taken an almost completely human-dominated landscape and attempted to restore ecological functions to its systems.

Principles of Landscape Ecology

To understand landscape ecology, we have to focus on some of its important principles: landscape composition, structure, function, and change.

Composition involves the genetic makeup of populations, identity and abundance of species in the ecosystem, and the different types of communities present.

Structure involves the variety of habitat patches or ecosystems and their patterns the size and arrangement of patches, stands, or ecosystems including the sequence of pools in a stream, snags and downed logs in a forest, and vertical layering of vegetation.

Function involves climatic, geological, hydrological, ecological, and evolutionary processes such as seed dispersion or gene flow.

Change involves the continual state of flux present in ecosystems.

A landscape consists of three main components: a matrix, patches, and corridors Figure 1. If we understand these components and their interrelationships, we can make better management decisions at the landscape level.

The matrix, the dominant component in the landscape, is the most extensive and connected landscape type, and it plays the dominant role in landscape functioning. If we try to manage a habitat without considering the matrix, we will likely fail to provide what wildlife need in that area.

For instance, if your goal is to enhance the number of different species in a 40-acre forest patch surrounded by soybean fields, you will not create wildlife openings in the forest. That is, you will not want to create more edge the outer zone of a patch that differs from its interior because in an agricultural matrix, any type of opening will create more and smaller forested patches in that area, further reducing the amount of interior habitat available to the wildlife that need it.

The characteristics of matrix structure are the density of the patches porosity, boundary shape, networks, and heterogeneity. If an area has been broken up but the patches are fairly close together, the patches are still dense enough to be useful for animal movement. However, if you open up a large forested area by creating small openings, the patches may not be dense enough to sustain certain kinds of animals, and you could have a problem with predation on other wildlife by raccoons, opossums, black rat snakes, or blue jays. A reduction in density might also increase nest parasitism by brown-headed cowbirds on neotropical migrant songbirds. We can illustrate how lack of density can create problems with brown-headed cowbirds. Some parts of eastern Kentucky do not have a large problem with brown-headed cowbirds because the matrix there is forested land. However, these birds pose a potential problem in other areas of eastern Kentucky where the matrix has been highly fragmented by coal mining, agriculture, and urban development Figure 2.

Boundary shape also has implications for neotropical migrant birds and edge species of wildlife. The more uneven the boundary, the more edge. Within matrix areas, networks connect habitats of different size and shape, creating what is called heterogeneity within the landscape. These different habitat patches usually are replicated throughout the matrix.

For example, the forests of eastern Kentucky vary by slope, landscape position, and soil type. Ridgetop forests are dominated by pine and oaks, cove sites are mixed hardwood stands, south- and west-facing slopes are oak-hickory forest, etc. If a chance event like a tornado were to occur, it might tear up one or two areas, but it would not wipe out all habitat for a species because the same habitat type is replicated several times in an area. The overall damage to wildlife would not be as great because that type of habitat would still be close by.

When natural resources are managed at the landscape level, context where the biological element is placed in the landscape is just as important as content. In other words, you must consider the surrounding matrix when attempting to conserve an area for its unique ecological attributes. Thus, if the land is being set aside to preserve rare plants such as wildflowers in a glade or animals such as bats in a cave, the content or community we are interested in is affected by the context of the environment if the surrounding landscape is altered.

It is well documented that adjacent habitats affect each other through changes in microclimates and the transfer of nutrients, materials, or seeds, etc. between communities. These changes ultimately affect ecological processes such as gene flow and species composition in each community. For example, breaking up the forest or creating openings in the forest matrix creates smaller forest patches, with the matrix becoming open land Figure 3. Thus, fragmentation of forest patches results in drier microclimates, which:

alter species composition and favor exotic, invasive species

increase the susceptibility of windthrow of existing trees

exacerbate a loss of forest interior wildlife species like neotropical migrant songbirds

reduce the genetic diversity of the remaining populations, and

allow for the invasion of exotic, weedy species.

Ultimately, small preserves that are set aside for their content may fail unless people intervene with intensive management, which is expensive and time consuming. To refer again to the example of the glade, opening the surrounding forest habitat increases the kinds and numbers of exotic plant species that overcome the rare plants unless intensive, site-specific management like herbicide treatment, hand pulling, etc. is implemented.

Similarly, fragmenting the forest surrounding a cave could alter the cave s microclimate; certainly, nonpoint source pollution would alter its climate and make the cave unsuitable for bats or other unique organisms like blind cavefish.

Patches are nonlinear surface areas that differ in vegetation and landscape from their surroundings. They are units of land or habitat that are heterogeneous when compared to the whole. They include four different types: disturbance, remnant, environmental resource, and introduced.

Disturbance patches are either natural or artificial. They result from various activities, including agriculture, forestry, urbanization, and weather i.e., tornados, hurricanes, ice storms, etc.. If left alone, a disturbance patch will eventually change until it combines with the matrix.

Remnant patches result when humans alter the landscape in an area and then leave parcels of the old habitat behind. Remnant patches are generally more ecologically stable and persist longer than disturbance patches.

Environmental resource patches occur because of an environmental condition such as a wetland or cliff line.

Introduced patches are ones in which people have brought in nonnative plants or animals or rearranged native species. Animals moving from one area to another can also bring in these nonnative elements.

Several aspects of patches are important from an ecological perspective and affect landscape-level management decisions. The approach used most often in analyzing patch habitats is to think of them as islands. Much of the current thinking about landscape patch management has its roots in the theory of island biogeography. This theory was developed in 1967 by MacArthur and Wilson to explain the patterns of species diversity on oceanic islands. It has also proven useful and applicable to a variety of ecological situations because an island is simply defined as a patch or parcel of favorable habitat surrounded by unfavorable habitat. Just as wildlife disperse to oceanic islands, terrestrial wildlife and plants move between habitat islands. MacArthur and Wilson s theory suggests that various dispersal events could therefore be predicted.

A key concept in MacArthur and Wilson s theory is that an equilibrium point exists in a population between the rate that new species come in and the rate that previously existing species become extinct. Once this point is reached, the island s populations of species are then maintained at this equilibrium diversity. Island populations, then, have a tendency to seek out this equilibrium.

Island size and relative isolation distance to the mainland affect both these rates and their equilibrium point. Relatively isolated and small fragments offer the lowest equilibrium species diversity, while nearby large islands offer the highest. Thus, from a habitat standpoint, the first important concept is patch size, which determines how much energy can be stored in that patch as well as the number of species that can reside there. A larger patch can normally support a larger number of species and a greater variety of habitat types Figure 4.

A concept getting closer consideration these days is the relationship between habitat patch size and the edge effect. In 1933, in Game Management, Aldo Leopold wrote that creating edge and maximizing the amount of interspersion, or the juxtaposition, of habitats was beneficial for wildlife. Held as dogma by wildlife biologists until recently, this philosophy is unfortunately the most overused concept Leopold discussed. He stated that increasing the edge increases the number of wildlife species in an area. However, if we look at things from a landscape perspective, edge is the one habitat not in short supply. Although edge is good for certain species, particularly generalist or game species, it favors those species over interior species, or species that require specific habitat types.

Unfortunately, fragmented habitats with a large percentage of edge can become an ecological trap. These islands of habitat may look good for some species of birds to build their nests in, but they also attract a wide host of nest predators, including raccoons, skunks, opossums, blue jays, and rat snakes. These animals decrease the nesting success of any birds in that area. For instance, in a recent study, scientists compared nesting success of loggerhead shrikes in fencerow habitat versus those in more contiguous forested habitat. In fencerow habitats, the bird s nesting success dropped almost to the point that they could not replace themselves due to nest predation.

Additionally, patch size has implications for neotropical migrant songbirds if the surrounding matrix is good habitat for brown-headed cowbirds. If present in the matrix and they will be present in an agricultural matrix, cowbirds will lay their eggs in the nests of neotropical migrant birds. The neotropicals cannot recognize the cowbirds eggs, and they end up raising cowbirds rather than their own species.

It is important to keep patches in the landscape as large as possible because the habitat in shortest supply in the landscape is contiguous forest or grassland. An important consideration from a landscape perspective is how to maximize patch size and minimize the edge effect because nest parasitism begins to drop off significantly at 50 yards from a forest edge. Therefore, anything more than 50 yards into a patch could be considered interior habitat.

So, in a patch 100 yards across, how much would be interior habitat. None would be interior habitat because if you go 50 yards in on each side, there is nothing left. In general, as patches get larger, there is more interior habitat. And if a patch is large enough, there is significantly more interior habitat. But how big should habitat patches be to minimize the influence of exotic or edge species. This requirement varies by species; nest parasitism by cowbirds, for example, may extend up to 900 yards into the forest interior. There are no definitive guidelines except the bigger, the better.

Furthermore, patch shape and configuration also influence how large the patches need to be. Scientists estimate that if we are to maintain minimum viable populations for many neotropical migrants, the minimum patch size should be 10,000 acres.

Patch shape is also important. A circular patch minimizes the amount of edge compared to a thin, rectangular strip patch, which has only a narrow band of interior habitat. If the strip is narrow enough, there is no interior habitat for interior species, and ultimately the diversity in the strip would be low.

There are also functional ramifications related to edges and patch size. In general, the higher the interior-to-edge ratio, the less patch border you have, which decreases the amount of interaction with the surrounding matrix. A higher interior-to-edge ratio also:

decreases the probability of barriers that could limit the movement of organisms

decreases the probability of habitat diversity within the patch, which would not necessarily be harmful because it would be natural, not artificial, diversity

decreases the importance of corridors for species movement, as they are able to move more freely throughout the matrix

increases species diversity and the total number of animals within the patch.

A low interior-to-edge ratio would do exactly the opposite.

One of the issues related to patch size is habitat fragmentation. Fragmentation is a process that occurs along a continuum Figure 5 in which a particular area is initially all one habitat type a forest, for example and is eventually decreased until only isolated patches remain. It results in habitat loss and discontinuity and eventually leads to habitat isolation. Fragmentation ranges from creation of small disturbance patches to widespread habitat loss and insularization.

There are two components to fragmentation: 1 a decrease in the amount of interior habitat and 2 a decrease in the connectivity between those habitat patches. As an example, suppose we started out with all forestland; then, three farmers move in and farm their small areas. As time passes, development creeps in, and the farms expand their agricultural base, resulting in larger gaps between habitat patches. At this point, the landscape is moving from a forest matrix to an agricultural matrix. In the beginning, there is still connectivity between forest patches even though it is narrow. At the endpoint of the continuum, there is a totally different type of habitat. From the wildlife standpoint, many of the original species would have two options: move to another area, or perish.

At the present time, forested wildlife habitat in the landscape often occurs in patches within an agricultural landscape matrix. Managing wildlife at the landscape level is an attempt to unite habitat patches through the use of corridors, specifically riparian forests or fencerow habitats to allow native biodiversity to flourish across the complete range of environmental gradients or between ecosystems. Viewed in another context, we do not necessarily have to connect habitat fragments, but rather direct our management to allow for the natural dispersal of wildlife.

Species can move across land more easily than across water. However, fragmentation in terrestrial systems creates something similar enough to the island effect that predictions can be made based on island biogeographic principles. How easily organisms can move across the landscape is determined by the density of the landscape. Thus, a high density of patches would tend to be similar to a large number of stepping-stones that organisms could use as cover as they go from one patch to another.

As noted earlier, the number of species that reside in a patch increases as the size of a patch increases. However, patches are not a random sample or a subset of the landscape. As a general rule, when you fragment the habitat, you section off one particular type of habitat Figure 6. Thus, if you are going to fragment a habitat patch, you need to look at the habitat types within both the patch and the matrix and attempt to maintain each patch type.

Habitat fragmentation can be viewed as either a positive or negative feature in the landscape. It can have positive effects by increasing habitat diversity, creating beneficial juxtaposition of habitats, and, as Leopold said, increasing edge, which favors generalist wildlife species like white-tailed deer, raccoon, opossum, northern bobwhite quail, etc.

Fragmentation can be viewed as negative when:

smaller habitat patches are created that lead to local extinctions or isolation,

habitats are no longer connected, particularly if the fragmentation is caused by a nonforestry activity such as urbanization, and

the amount of edge is increased because fragmented habitat is harmful to interior species like bobolinks, wood thrushes, etc.

Fragmentation and Nonnative Species

As habitats are fragmented into smaller pieces, one final negative impact occurs: the invasion of nonnative or exotic organisms. Current estimates indicate that more than 25 percent of the flora in the United States is exotic. The history and folly of premeditated and accidental introduction of exotic plants and animals are well documented.

These alien species, particularly those introduced by humans into environments they would not have reached through normal dispersal methods, have transformed, and are continuing to transform, entire ecosystems. For example, the American chestnut once accounted for one-quarter of the standing timber volume in the eastern deciduous forest. Today this species is reduced to sprouts and a few adults that were not destroyed by the introduced chestnut blight. This invasion by an alien species has dramatically altered the composition, structure, and functioning of this ecosystem.

It has been estimated that exotic animals and plants harm or threaten resources in at least 109 national parks. An introduced tree, the Australian tree, has infested more than eight million acres of native sawgrass prairies in Everglades National Park. In addition to causing loss of habitat for wading birds and increased drying of the marsh, this pest is particularly intrusive. It does not respond to burning or herbicide treatment and may increase as a consequence of these eradication methods. The Loxahatchee National Wildlife Refuge is spending at least 75,000 a year in an attempt to control the invading tree.

In addition to completely altering ecosystem structure and composition, exotics cause other problems. In native ecosystems, a species is kept in check by ecosystem control like competition for food, space, or water; pathogens that cause disease; or predators. When a new species is introduced into an ecosystem, those same control mechanisms usually are not present. What usually happens is that its population blossoms without any natural controls. Exotics have also been known to hybridize with related native species, resulting in a lack of genetic fitness for the native population. Introduced predators may cause a decline in native prey species because the naive native prey have not become adapted throughout time to deal with the new predator.

Introduced species may also bring new diseases into an ecosystem that native species are not adapted to combating. Finally, an introduced species may adversely affect the workings of human communities by disrupting businesses, hastening the decline of an important food or recreation resource, or affecting water quality.

The final landscape component is the corridor, the strip of land that differs from the matrix on either side. Corridors are areas that link patches together, serving as highways or conduits for organisms to transfer or move from patch to patch. Corridors are a unique mixture of environmental and biotic attributes from the surrounding matrix and patches. They have origins and types similar to those of patches: there are disturbance, remnant, environmental resource, and planted corridors. There are also stream corridors such as the path followed by a river or stream and the strips of streamside vegetation so important to migrating wildlife.

Different types of corridors foster different species. Corridors function in several ways to provide habitat for various species, especially the smaller ones like chipmunks. Line or narrow strip corridors are mainly dominated by edge species, whereas wider strip corridors, which may have mostly interior species, function for better movement of animals.

Corridors can serve as a conduit for movement or act as a barrier or filter which may serve as a barrier to gene flow. For example, roads can serve as an almost complete barrier to amphibian movement, ultimately isolating individual populations.

Corridor Structure and Function

Corridor structure and function depend on a variety of different factors, including degree of curvilinearity, breaks, narrows, nodes, and connectivity.

Curvilinearity, or the twisting and winding of the corridor, has functional ramifications related to edge. A higher degree of curvilinearity increases edge.

Breaks occur where the matrix divides up a continuous corridor. They may not affect movement for some species, but for others particularly plant species they may stop the flow of species, genes, and energy through that system.

Narrows, caused when some of the corridors narrow down, keep some species from moving through the restricted area.

Nodes are corridor intersections, where, according to studies from England, numerous interior species are sometimes found.

Connectivity of corridors should be maintained that is, they should be kept continuous and unbroken.

Corridors also have some drawbacks. They encourage predators to alter their search patterns, resulting in increased predation on native wildlife species. Small animals that use a path as a corridor for travel will be more susceptible to reduction by predation. Consider how the eastern diamond-backed rattlesnake likes to lie next to a hiking path to get sun. Additionally, think of the humans using that footpath. The humans would probably destroy every rattlesnake they encountered, ultimately leading to fewer rattlesnakes.

Another drawback is that some corridors, like roads or railroad rights-of-way, can be a conduit for the invasion of exotic organisms or diseases and pathogens.

Here is a landscape-level example: If you have corridors connecting two patches that have nest predators such as raccoons and opossums, the predators will move along these corridors in order to have cover. By their movement patterns, they are going to increase their predation on birds in that new area. The area can become very hazardous for many types of wildlife, particularly turkeys.

Current research findings in the southeastern states on turkey predation occurring along riparian habitats indicate that the narrow corridors of suitable habitat allow predators like the raccoon searching for turkey hens and nests to find and eat their prey more easily. The proximate cause of death is predation, but the ultimate problem is a lack of suitable corridor width habitat that would make it more difficult for predators to search the appropriate habitat.

The message is simple: the wider we make these corridors, the better, because doing so will decrease the chance of predators finding their prey Figure 7. These wide strips may need to be several miles across for some bigger species like black bear, but only a few yards wide for smaller species. Any time a corridor is 100 yards wide, the habitat s interior characteristics change, and nest predation drops off significantly thereafter.

Landscapes change, even though we tend to manage land with the idea that it will always be in some static community. But we all recognize that ecological communities are dynamic, ever-changing entities. For instance, when a tree falls, it creates a gap in the landscape matrix. This gap will eventually change and once again become part of the matrix.

When you create a change, you have to consider the impact it will cause on the landscape, not just for the present moment but also for 50 to 100 years in the future. For example, if you clearcut 10 of 100 acres at five-year intervals, over the next 20 to 30 years all those clearcuts and the resulting forest will end up at the pole stage small trees that are the size of poles at one time. From a biological diversity standpoint, the pole stage is the least conducive for promoting biological diversity. Therefore, it is vitally important to project the effects of management prescriptions well into the future.

Another force that alters ecosystems is the movement of plants by seeds. For example, anemochores are seeds that are blown with the wind, like maple seeds. If we cut a forest down and create a gap, this change allows the wind to move through, spreading these seeds over greater distances and affecting forest community structure. It has been hypothesized that many of the unique forest types in the Cumberland Plateau are being replaced by red maple. Could this be a result of previous harvesting strategies. Similarly, if you impact the hydrology of wetland or riparian systems, you would affect the hydrochores seeds moved by water, and any management technique that affects wildlife would affect plants that are spread by zoochores berries. The result of these activities could be good or bad, depending on whether it is a species that you want to spread.

Finally, one of the aspects of landscape ecology that ecologists have only recently explored is metapopulation. A metapopulation is a network of semi-isolated populations with some level of regular or intermittent migration and gene flow among them. In simpler terms, it is a population of populations.

In metapopulation dynamics, individual populations may go extinct, but then they can be recolonized from other populations Figure 8. If we drive these individual populations down to low enough numbers and do not get movement between populations, serious genetic problems may develop for maintaining the species. Even a small amount of movement between populations will keep the genetic situation somewhat stable.

If there is no movement, those populations will probably go extinct, depending on whether they exhibit a source or sink patch of a metapopulation. Source patches will always stay in a particular locale and contribute individuals to all the other patches in the landscape. Sink patches allow populations or individuals to become extinct because they do not contain conducive habitat for the species to exist. For example, many times with territorial wildlife species a source patch of habitat or population will be full or at carrying capacity. Thus, some animals move to the sink patch. Even though it may not be the best habitat for some species, birds, for instance, may be able to nest and be successful there.

Another advantage of the sink population is that if some catastrophic event occurs in one sink patch resulting in the extinction of its members, another patch could help to repopulate it. Thus, the sink patches help to stabilize the population over time. What happens in one patch is asynchronous to what happens in the rest of the patches.

Another important attribute about metapopulations is that in some cases they may actually buffer the species from extinction because of the relative isolation and protection of the source and sink patches. For example, if a disease in one patch wipes out all the individuals there, despite the connectivity between the patches, there is no way the disease could spread to all the populations. Therefore, the concept of metapopulations plays a very important role in the management of wildlife today.

An example will serve to illustrate the importance of understanding metapopulations when managing wildlife at the landscape level. Northern bobwhite quail exist in a series of metapopulations. Today, they live in fragments of habitats with some movement by animals between the fragments. Sometimes that movement is very poor. In the winters of 1976 and 1977 in Kentucky, very cold weather occurred along with abundant snow and ice. Unfortunately, some of the patches of quail totally lost their populations. Over time, even with poor movement, those patches have become repopulated, although it has taken 20 years for some local populations to recover. In other cases, significant habitat alteration occurred that prevented quail from repopulating some areas, and these populations became extinct. In these cases, metapopulations allowed the species to survive and ultimately recover small populations throughout its range.

An important point concerning metapopulations has to do with the time scale used in making management decisions. Many times biologists make decisions based on what the population level is at the present time. With metapopulation dynamics, however, you have to consider extinction and indications of the area s total potential. As in the case of the quail, it has taken 20 years to restore some local populations, but many have still not recovered and may never recover due to continued habitat degradation.

Understanding metapopulation dynamics can lead to appropriate management at the landscape level. As we transform large expanses of relatively uniform habitat patches in the landscape matrix, physical changes occur that create the island effect in the resulting fragments. Such changes include:

a decrease in size of the patches

an increase in the proportion of edge, and

changes in patch microclimate, including increased sunlight, greater temperature fluctuations, and greater exposure to wind.

local extinctions of organisms

reduced dispersal and recolonization of habitat patches

invasion of exotic or nonnative species

increased nest parasitism or predation on birds, and

a reduction in the diversity of forest interior wildlife species.

Although metapopulation dynamics and the concepts of landscape ecology are complex and difficult for the general public to understand, we must make the effort to understand these concepts because of their importance in determining decisions about landscape-level and wildlife management.

Figure 1. Landscapes consist of the matrix the dominant feature, patches, and corridors that connect the patches.

Figure 2. Eastern Kentucky forest matrix has been fragmented by mining, agriculture, and human habitation.

Figure 3. Notice how the forested matrix has begun to be converted to an open lands matrix.

Figure 4. The size, shape, configuration, and number of patches all affect the amount of interior habitat in the patch. Small, single, rectangular patches provide the last amount of interior habitat, and large circular patches provide the most interior habitat.

Figure 5. Fragmentation begins with small gap formation in the matrix. Over time, the gaps may get larger, resulting in a shift in the matrix.

Figure 6. How fragmentation occurs is important. If you fragment along Line 1, over time Population or Community A will become extinct. If you fragment along Line 2, Population A does not become extinct.

Figure 7. Corridors need to be wide enough to provide more positive benefits for wildlife. This narrow riparian corridor probably produces more negative impact on wildlife.

Figure 8. A metapopulation is a population of source and sink populations.

Landscape Ecology and Ecosystems Management. many times with territorial wildlife species a source patch of habitat or population will be full or at carrying.

Dec 23, 2005  The science of ecology studies interactions between individual organisms and their environments, including interactions with both conspecifics and.

Metapopulations and Patch Dynamics: Animal dispersal in heterogeneous landscapes. Tanya Rohrbach rohrbach rci.rutgers.edu Metapopulations in the Context of Patch.

Population ecology or autoecology is a sub-field of ecology that deals with the dynamics of species populations and how these populations interact with the.

Source–sink dynamics

Landscape Ecology and Ecosystems Management

ECOLOGY AND MANAGEMENT OF SOURCE-SINK and anthropogenic impacts on habitat quality will affect whether a patch functions as a source or a sink for any.

Bowman, J., N. Cappuccino, and L. Fahrig. 2002. Patch size and population density: the effect of immigration behavior. Conservation Ecology 6 1 : 9.