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Publication #FOR 92

Chapter 3: Biodiversity and the Restoration of the Urban Forest Ecosystem1

Eliana Kämpf Binelli2

Abstract

Biodiversity is the variety of life and all the processes that keep life functioning. Global biodiversity provides many ecosystem services, such as protection of water resources, nutrient storage and cycling, and pollution mitigation. These ecosystem services have recently been estimated to provide $33 trillion per year. Biodiversity occurs at many levels from genetic diversity to species diversity to ecosystem diversity. Biodiversity has been reduced in urban areas through ecosystem destruction, degradation, and fragmentation of remaining ecosystems. Biodiversity can be increased in urban areas by managing the landscape as a whole and improving connectivity between ecosystem fragments. Biodiversity can also be restored by (i) leaving stumps, leaves, snags and logs to improve nutrient cycling and for wildlife, (ii) planting native species that mimic composition of nearby ecosystems, (iii) controlling invasive plants and animals, (iv) enhancing the ecosystem's natural structure, and (v) creating multi-age ecosystems in several stages of succession. Ecological processes to restore include natural disturbances (e.g., fire), ecological succession, nutrient cycling, and hydrological cycling.

Introduction

While watching TV, reading newspapers, listening to the radio, or even talking to friends, we all have heard something about biodiversity. Issues such as old-growth forests and the spotted owl, tropical deforestation, hunting of whales, and many other topics related to biodiversity have made the news.

Biodiversity has emerged as one of the key environmental concerns in the debate over the worldwide depletion of natural resources. Biodiversity is now a matter not only of scientific interest but also public concern throughout the world.

But, what exactly is biodiversity? Why is it important? Are urban forests important to the conservation and maintenance of biodiversity? Why should urban foresters, citizens, policy makers and professionals be concerned about biodiversity in urban areas? Can we restore biodiversity in our cities? How? This publication will discuss these questions and how managers can incorporate biodiversity into urban forest restoration projects.

What Is Biodiversity?

Biodiversity, the short term used for biological diversity, is "the variety of life and all the processes that keep life functioning" (Keystone Center 1991). Biodiversity includes 1) the variety of different species (plants, animals - including humans, microbes and other organisms), 2) the genes they contain, and 3) the structural diversity in ecosystems.

The wealth of biodiversity supports ecological processes that are essential to maintain ecosystems on earth (Figure 1). Examples of such ecological processes are the nutrient cycle, the hydrological cycle, and natural succession.

Figure 1. 

The exact number of existing species in the world is unknown, with estimates varying from as low as 5 million to as high as 100 million species. Most are insects that play critical roles in ecosystems such as decomposition and nutrient cycling.


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One of the most fundamental attributes of biological diversity is that it is always changing. The wealth of biodiversity is the product of hundreds of millions of years of evolutionary history. The process of evolution means that the pool of living diversity is dynamic and constantly changing. Climatic, geologic, hydrologic, ecological, and evolutionary processes generate biodiversity and keep it forever changing (Noss and Cooperrider 1994). We explore this issue with more details in Chapter 4 - Succession and Disturbances.

Levels of Biodiversity

Let's explore in more detail how biodiversity occurs in ecosystems. The key to an effective analysis of biodiversity is the definition of each level of organization that is being addressed.

Biodiversity is usually considered at three different nested levels: 1) gene, 2) species and 3) ecosystem. Changes in one level of biodiversity may have impact on the next level and vice-versa. For example, imagine that an exotic disease (Dutch elm disease or Chestnut blight) is introduced to an urban forest with low species diversity (mostly elms or chestnut trees). Since the genetic pool of these urban forests is limited to species susceptible to these diseases, not only individual species will be affected but also the whole ecosystem to which these species belong.

Gene level

Biodiversity at the genetic level refers to the information contained in the genes of all individual plants, animals and microorganisms. This level of biodiversity is critical in order for species to adapt to changing conditions and to evolve.

Restoration ecologists usually recognize the genetic level of biodiversity in restoration projects. For example, after Hurricane Andrew struck in South Florida in 1992, all the Australian pines (Casuarina equisitifolia) were destroyed in Bill Baggs, a heavily used urban park in Miami. Prior to the hurricane, Australian pine, which is a highly invasive species, covered a large portion of the park. The natural removal of Australian pines by the hurricane provided a great opportunity to restore the park to conditions closer to its previous natural conditions. In this project, it was recommended that seeds be collected from local ecosystems within 50 miles radius of Bill Baggs in order to ensure a well-adapted genetic pool to the climate and soils of this specific location (Figure 2).

Figure 2.1. 
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Figure 2.2. 

The Bill Baggs Cape Florida Restoration Project, considered the genetic level of biodiversity by collecting genetic material from areas representative of the region's ecosystems. Several small ecosystems were restored including wetlands (2.1), and uplands (2.2).


Credit:

Mary Duryea


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Species level

This level is what most people have in mind when they think about biodiversity. Most simply, species diversity is the number of species present in an area. However, the specific combination of species and their relative abundance are also important considerations.

It is common in many cities across the US to find neighborhoods where streets are all planted with the same tree species. In fact, if we consider even the whole city, we would find only a few species planted over and over again. The diversity of street tree species is critically low in many U.S. cities and towns (Sun 1992). In Oakland, CA, for example, only four species make up 49 percent of the tree population (Nowak 1993), and in Chicago, IL, six species or genera constitute more than half of the population (Nowak 1994).

A classic example of problems associated with lower species diversity is the extensive use of American elm (Ulmus americana) as a street and urban tree in U.S. cities after World War II. American elm constituted 95 percent of all street trees (200,000 elms) in Minneapolis, MN, for instance (Price 1993). When Dutch elm disease, a fungus spread by bark beetles that causes wilting and dieback of elms, was introduced in the late 1960's, nearly all American elms were killed in Minneapolis and in the rest of the country (Figure 3). Besides the obvious aesthetic problems, this lack of biodiversity necessitated major and expensive efforts to eradicate and dispose of the killed elm trees.

Figure 3.1. 

American elms (Ulmus americana) were once extensively planted in streets and parks in many cities and towns across the U.S.


Credit:

Mary Duryea


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Figure 3.2. 

The introduction of Dutch elm disease killed nearly all the elms, and reminds us that the species level of biodiversity is critical when managing urban forests.


Credit:

Edward Gilman


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The Dutch elm disease outbreak and the loss of virtually all American elms illustrate the consequences of lack of species diversity. Besides, by planting only a few tree species or genera, the age diversity of the species planted may be extremely reduced. The end result of this practice is many old, decaying trees to be removed, pruned, and managed at the same time, increasing the city's or municipality's tree maintenance costs. Biodiversity can be enhanced at the species level by simply increasing the number of different tree species planted (preferably native species present in natural ecosystems in the region). Additionally, by planting species each year instead of all in one year, the age diversity in urban forests can also be increased.

Ecosystem level

The structure of the urban forest is an important biodiversity consideration at the ecosystem level. Structure in forests is characterized by the nature and abundance of the various vegetation layers (canopy, subcanopy, shrub layer, and ground cover) and the presence of dead logs and snags. It is important that ecosystems retain their natural structure.

In most ecosystems, a greater structural diversity will support a greater diversity of wildlife and will ensure better ecosystem functioning. A forested ecosystem should have snags (dead standing trees) (Figure 4) and logs (Figure 5), which provide habitat for small mammals, amphibians and reptiles and food for many insects and fungi (which in turn are food for birds).

Figure 4.1. 

Figure 4.2. 

Snags provide important ecosystem structure. They are habitat for birds (4.1), mammals, termites, insects, frogs and several microorganisms and are also important for the nutrient cycle (4.2).


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Figure 5. 

In a natural forest, there will be snags and logs in different stages of decay. Different living organisms use these different stages.


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Structural diversity should be reintroduced in restoration projects. There are several ways in which this can be accomplished. For example, a snag can be created by cutting a hazard tree but leaving a taller stub to decay. Many urban forest restoration projects also import logs and snags by salvaging trees in areas slated for development. These trees are then used as either downed logs or "planted" back in the ground like giant posts to decay, increasing the structural diversity and enhancing nutrient cycling.

Why Is Biodiversity Important?

Recently, all natural ecosystems on earth have been estimated to provide $33 trillion annually in ecosystem services (Costanza et al. 1997). This is twice the combined gross domestic product of all nations in the world. Ecosystem biodiversity provides us with these services, which include the protection of water resources, nutrient storage and cycling, pollution bioremediation (biologically based environmental cleanup), maintenance of ecosystems, soil formation, climate regulation and other natural processes, recreation and food production.

Biodiversity occurs at several spatial scales (locally, regionally, globally). This means that biodiversity has significance at a global scale as well as in our own city backyards. Some of the values associated with biodiversity include:

  • ecosystem functioning,

  • future value, and

  • educational and recreational benefits.

Ecosystem functioning

When ecosystems are diverse, there is a range of pathways for many ecological processes and for primary production. If one of these pathways is damaged or destroyed, an alternative pathway may be used and the ecosystem can continue functioning (Kimmins 1996). For example, when a particular bacteria species is missing from the nutrient cycle, in a diverse ecosystem another organism may be present to carry out the same function (Figure 6). However, some organisms, such as top predators, also play an important role in ecosystem functioning but cannot be easily replaced. In any case, if the biological diversity is greatly diminished, the functioning of the ecosystem may be at a risk.

Figure 6. 

An example of ecosystem services is the decomposition of organic matter by microorganisms and other species, such as these fungi.


Credit:

Larry Korhnak


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The associated costs of losing the ability of ecosystems to function are extremely high. The degradation of wetlands is a dramatic example of the problems associated with loss of ecosystem biodiversity. Floods, problems in water quality and quantity for natural and human systems, and declines in fish and wildlife populations, have all been linked to wetlands destruction, degradation and fragmentation. The Everglades is an extensive ecosystem in Florida, which currently faces such problems. Costs for restoring natural ecosystem services and biodiversity to the Everglades have been estimated to be hundreds of million of dollars. Congress recently approved the expenditure of $1.5 billion to restore only some areas of the Everglades (South Florida Ecosystem Task Force 1998).

Future value

Natural ecosystems are a reservoir of continually evolving genetic material, irrespective of whether their values have been recognized. The same genetic material may have important but yet to be discovered medicinal, economic, aesthetic, recreational or intrinsic values for future generations.

An example of one of the most promising discoveries in recent years has been taxol, which was initially isolated from the Pacific yew (Taxus brevifolia Nutt.), a tree species in the Douglas-fir forests of the Pacific Northwest that was until recently considered unimportant (Figure 7). Taxol has been used in the treatment of ovarian and breast cancers. In the U.S., approximately 25% of all prescriptions contain active ingredients derived from plants (Principe 1989).

Figure 7. 

The bark of Pacific yew (Taxus brevifolia Nutt.) trees contains taxol, a new drug for treating several forms of cancer.


Credit:

Dr. A.C. Mitchell


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Biodiversity is also essential in biological control and for the breeding of disease resistant species. Use of genetically resistant plant species for food production, clothing, commercial and urban forestry is derived from a wide array of diverse native species.

Educational and recreational benefits

One of the most important reasons to manage and protect biodiversity in urban centers is their educational and recreational values. Recreational benefits are perhaps the most important value of biodiversity in urban areas. People value natural areas for a variety of reasons: psychological renovation through contact with nature, jogging and hiking, birdwatching, photographing, and many other activities. The aesthetic value of ecosystems also contributes to the emotional and spiritual well-being of a highly urbanized population (Figure 8).

Figure 8.1. 
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Figure 8.2. 

Recreational benefits of biodiversity are closely related to aesthetic, psychological (8.1 and 8.2) and educational values.


Credit:

Mario Binelli


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In 1991, 30 million Americans participated in wildlife watching, and another 14 million adults went fishing (U.S. Fish and Wildlife Service 1992). Nationwide, wildlife viewers spent $18 billion (Norris 1992). Watchable wildlife recreational activities provide local economies with important income generated by sales, employment, and tax revenues. For example, Florida's watchable wildlife generated $3.5 billion in 1996 (Florida Game and Fresh Water Fish Commission 1998).

Some ecosystems, especially those close to metropolitan centers are becoming extremely rare. For example, Florida's scrub ecosystems are now surrounded by the greater Orlando urban area and are threatened by human encroachment and development. Ultimately, it will be up to these urban citizens to protect such ecosystems and their benefits. In this case, there is some evidence that the Florida scrub jay, an endangered bird in the scrub ecosystem, may persist in residential areas, provided adequate patches of the scrub ecosystem remain preserved nearby. (Florida Game and Fresh Water Fish Commission 1997). These urban remnant ecosystems could be powerful tools for educating urban citizens about the importance and value of such diverse ecosystems (Figure 9).

Figure 9.1. 

Figure 9.2. 

Managing for biodiversity in urban areas is an excellent opportunity for integrating ecological, educational (9.1) and recreational values (9.2).


Credit:

Larry Korhnak


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Increasing urbanization accelerates human pressures on remaining natural ecosystems. At the same time, however, recreational spaces have to be managed for this increasing population. In 1996, 2.7 million Floridians participated in wildlife recreational activities within a mile of their homes and 543,000 visited natural areas around their homes (Florida Game and Fresh Water Fish Commission 1998). Urban forests may play an important role in integrating recreational demands and conservation of natural resources.

Now that we have discussed some values of biodiversity, why should urban managers consider biodiversity in the restoration of urban forests as ecosystems? Urban and community forests have been estimated to provide nationwide $3 billion a year in social, ecological and economic benefits (McPherson and Rowntree 1991). These benefits include conservation of energy, pollution control, and improvement of aesthetic quality of cities. By managing and restoring urban forests for biodiversity, such benefits could be greatly enhanced. For example, by restoring ecosystems and their associated natural processes, such as nutrient and hydrological cycling, local communities could save money, energy and resources. Restoring an urban wetland to provide habitat for wildlife would also contribute to recreational and economic opportunities. Removal of invasive species from a city's park, for instance, may bring back the natural diversity and functioning of the ecosystem, which in turn might improve its recreational and aesthetic value for the local community.

Managing for biodiversity in urban areas will require a more holistic approach than usually seen. Urban forests are more than a collection of street trees. Remnants of natural areas, waterways, parks, backyards, right-of-ways, and industrial parks both in public and private properties are all part of the urban forest ecosystem.

Can Biodiversity Be Protected in Urban Forests?

Most human-made habitats, such as a landscaped park, have lower biodiversity than natural forests. However, urban environments usually include a great diversity of habitats (such as water retention ponds, industrial parks, railway rights-of-way, greenways, and others), which may support some wildlife and plant species. In some cases, urban habitats may even play a significant role in the conservation of 'rare' or 'threatened' species.

For example:

  1. Rare prairie plant species in the Midwestern US are found alongside railroads and highways. In such areas these species are protected from the agricultural activities that destroyed much of the original prairie habitat (Ahern and Boughton 1994).

  2. Of the 144 threatened and endangered wildlife species of Illinois, 14% (20 species) have been recorded in recent times in Cook County, the most urbanized county of the Chicago Metropolitan area (Friederici 1997).

How Is Biodiversity Reduced In Urban Areas?

The ultimate threat to global biodiversity is an increasing human population and the consequent increased use and development of the world's remaining natural ecosystems. The largest threat to biodiversity in urban areas is the reduction and alteration of the total area of natural ecosystems available to native animal and plant species (Figure 10). Ecosystem destruction, degradation and fragmentation may significantly reduce biodiversity.

Figure 10.1. 

Figure 10.2. 

Figure 10.3. 

Figure 10.4. 

Biodiversity is lost by ecosystem destruction (10.1), fragmentation (10.2) and degradation. Figure 10.3 illustrates a degraded longleaf pine ecosystem that has been invaded by exotic species whereas figure 10.4 illustrates a healthy longleaf pine ecosystem. The diversity of the longleaf pine ecosystem is associated with its herbaceous layer and a relatively open canopy.


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Ecosystem destruction

Frequently, urban natural areas are completely eliminated during residential and/or commercial development. Usually, after construction exotic trees, shrubs and lawns are established. Additional amounts of fertilizers and irrigation, frequent mowing and mulch are required for such intensively managed areas.

If instead natural areas are preserved and incorporated during development, biodiversity could be maintained. Natural areas have much lower maintenance requirements when compared to traditional landscaping. Additionally, aesthetically pleasing environments, such as natural urban remnants, increase the economic value of residential and commercial areas.

Ecosystem degradation

Ecosystem degradation may not be easily noticed in the short-term and is difficult to detect and harder to quantify. Degradation is of greater long-term concern, since its effects are cumulative and may build up only very slowly. Degradation deteriorates and disrupts ecosystem processes. Some examples of causal degrading agents are pesticides, chronic air pollution and invasive species. Erosion, or removal of the litter from a forested site would also cause ecosystem degradation by interrupting nutrient cycling.

Microorganisms in the soil (such as invertebrates, fungi and bacteria) carry out critical ecosystem functions (such as decomposition and nitrogen fixation). Yet these organisms are so small that they usually go unnoticed until the consequences of their disruptions are too obvious to neglect.

In metropolitan centers, for instance, air pollutants slowly accumulate in urban forest soils over time. The gradual accumulation of hydrocarbons in a New York urban forest, for example, formed a hydrophobic soil layer, which in turn has decreased the population and activity of soil microbes and invertebrates. This hydrophobic layer, coupled with trampling and high concentrations of heavy metals in urban soils, has also reduced the rates of microbial processes, affecting the nitrogen cycle in these forests (White and McDonnell 1988).

Ecosystem fragmentation

Landscapes become fragmented when natural ecosystems are broken up into remnants of vegetation that are isolated from each other (Figure 11). Therefore, fragmentation results in a landscape that consists of remnant areas of native vegetation surrounded by other land uses. At a larger scale the landscape is composed of cities, farms, rivers, rural areas and natural areas (Figure 12). In the urban area the landscape might include strips of street trees, backyards, schoolyards, shopping centers, creeks, rivers, parks, landfills, industrial parks and fragments of natural areas (Figure 13).

Figure 11. 

In urban areas, ecosystems that used to be continuous are now fragmented in the landscape.


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Figure 12.1. 
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Figure 12.2. 

At a larger scale, the landscape is composed of cities (12.1), farms, rivers, rural areas, natural areas and fragments of natural areas (12.2).


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Figure 13.1. 

Figure 13.2. 

Figure 13.3. 

In urban areas, the landscape is composed of street trees (13.1), backyards, shopping centers (13.2), parks, industrial parks and fragments of natural areas (13.3).


Credit:

Paul West, Seattle Department of Parks and Recreation


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Ecosystems Are Connected and Inter-related

The landscape is a mosaic of several different ecosystems. It is important to recognize that natural ecosystems are connected and inter-related. Fragmentation of natural ecosystems will affect ecosystem processes, plants, and wildlife. Turtles, for example, live in water but need upland ecosystems to lay their eggs. If we fragment upland ecosystems, by either constructing a road between the ecosystems or putting a fence around the upland, turtles will be prevented from reproducing (Figure 14).

Figure 14. 

This yellow bellied turtle (Trachemys scripta) was stranded by a road while trying to move to an upland ecosystem to lay eggs. This usually happens when the interconnectedness of ecosystems is not taken into account.


Credit:

Joseph Schafer


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This example shows that we need integrated management and restoration efforts, where ecosystems are allowed to interact with each other. Roads, fences, or other human-made boundaries may limit the flow of nutrients and water and the movement of plants and animals between ecosystems.

What happens to ecosystem fragments?

Let's take a closer look at an ecosystem that has been fragmented and isolated. Usually, conditions in the surrounding landscape are different from conditions in the ecosystem fragment. As a result, an edge is formed between the landscape and the ecosystem fragment. Every ecosystem has an edge, but the amount of edge in urban ecosystem fragments increases tremendously as a result of external factors in the landscape. As the edge increases, the size of the interior core is reduced.

The core area of an ecosystem fragment is the undisturbed interior area of that ecosystem. In this core area we usually have:

  • functional ecological processes,

  • a greater diversity of native species,

  • a diversified structure with multilayered vegetation (trees, shrubs, herbaceous and ground cover plants), logs, and snags,

  • a greater diversity of wildlife with area-sensitive birds, mammals, and other animals, and

  • an undisturbed microclimate.

Several external factors from the landscape can affect ecosystem fragments (Figure 15). Along the edge of the ecosystem fragment there is increased solar radiation. Since there is more light available, species that grow better in full sun will become established closer to the forest edge, while shade tolerant species will be restricted to the interior core (Saunders et al. 1991). Invasive species will also be favored in edges and more disturbed areas.

Figure 15. 

External factors from the landscape affect ecosystem fragments. The greater these external influences, the greater the edge and smaller the core area.


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Trees at the edge will also be more susceptible to wind, air pollution, and increased temperatures, resulting in a drier microclimate (Saunders et al. 1991). In turn, nutrient cycling may be affected because the heating of the soil may affect microorganisms, litter decomposition, and soil moisture retention.

Therefore, fragmentation alters the structure, composition and function of ecosystems. A principle to remember is that the more you alter the structure, composition and function of ecosystems, the greater the energy needed to restore the ecosystem back to its original condition.

One example is Forest Park, a 5,000-acre urban park in Portland, Oregon. This park is an ecosystem fragment that has been greatly impacted by the surrounding land uses. The neighboring communities landscape their yards with English ivy (Hedera helix), an invasive and aggressive species. By bird dispersal and vegetative growth, English ivy has spread and invaded this forest (Figure 16). English ivy alters the structure of the forest (by impeding the growth and development of native plants), its composition (now there is only English ivy underneath the canopy) and, consequently, this ecosystem's functioning (alteration of nutrient cycling, since decomposition of organic matter may be affected). The amount of energy required to restore this ecosystem is tremendous. It is an ongoing effort, but, as a result, native species are regenerating and biodiversity is slowly coming back to Forest Park.

Figure 16. 

These high school students are removing English ivy, an invasive species that completely took over Forest Park, an urban park in Portland, Oregon.


Credit:

Mary Duryea


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How Can Buffer Zones Help?

Buffer zones are semi-natural areas located around areas of higher natural values, such as core areas. A buffer zone around an ecosystem fragment will minimize external influences and help maintain the ecological integrity of the ecosystem's core area. Establishment of buffer zones around natural and semi-natural areas permits integration of human land uses while still managing for biodiversity (Figure 17).

Figure 17. 

Buffer zones in urban settings can minimize external influences of the surrounding landscape and maintain the ecological integrity of urban ecosystem fragments.


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How Does Fragmentation Affect Biodiversity?

Fragmented ecosystems are isolated, and in urban areas the distance between fragments may be large. This, coupled with the increase in edge area and reduction of the core area, will decrease flow of genes and seed dispersal. Animals and plants that used to be in the whole area are now restricted to smaller patches.

Connected ecosystems or unfragmented landscapes will have a greater diversity of native species (Figure 18) because of their larger core area, a lower edge:core area ratio and less isolation (compared to smaller fragments).

Figure 18. 

The greater the area, the greater the number of species in the ecosystem (adapted from MacArthur and Wilson 1967).


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Let's examine the consequences of fragmentation on bird populations. Area sensitive birds, such as flycatchers, vireos, and warblers, will be reduced with fragmentation and reduction in core area. Area sensitive birds are those that need a large undisturbed area and, hence, would only live in the interior core area of a large fragment (Adams and Dove 1989) (Figure 19.1). Habitat generalist birds can be quite common in more urbanized areas and may thrive in many different conditions. Cardinals, jays, house wrens, and catbirds are examples of habitat generalist birds (Figure 19.2).

Figure 19.1. 
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Figure 19.2. 

Area sensitive birds, such as certain types of owls (19.1) may have their diversity reduced with fragmentation and a reduction in core area. However, habitat generalist birds, such as common sparrows (19.2) may be favored in a patchy environment.


Credit:

Thomas G. Barnes


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If we want to enhance the diversity and the presence of area sensitive birds in urban areas, we need to restore and connect core areas of ecosystems (for more information on wildlife, see Chapter 8-Wildlife).

How Can We Connect Fragmented Ecosystems in the Urban Landscape?

The search for solutions to the problems of ecosystem loss, degradation and fragmentation has led to a growing number of new projects and solutions. Most projects are based on ecologically sound principles. Basically, we attempt to connect fragmented ecosystems in the urban landscape and manage the landscape as a whole. By doing so, the distance between ecosystems fragments will be shortened, improving connectivity of isolated fragments.

Connectivity is essentially the opposite of fragmentation. Instead of breaking landscapes into pieces we are seeking ways to restore broken connections between fragmented ecosystems (Figure 20).

Figure 20.1. 

Figure 20.2. 

In Figure 20.1, patches A and B used to be part of the same contiguous ecosystem. A corridor may provide linkage between these ecosystem fragments. Riparian coridors (20.2) are landscape linkages that may connect several ecosystem fragments in the urban-rural interface.


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Effective connectivity is measured by the potential for movement and flow of genes, that is, movement and migration of animals (especially birds) and dispersal of plants. Many factors determine the effectiveness of connectivity, and it varies depending on the ecosystem of interest. Usually, effective connectivity will depend on:

  • presence of barriers (e.g., fences which would limit migration),

  • distance between ecosystem fragments,

  • amount of edge in the landscape linkage,

  • nature of the surrounding landscape, and

  • species that will benefit from promoted connectivity (e.g., whether a bird, a mole, a plant).

Connectivity can be promoted by using corridors, greenways, and stepping stones.

Corridors

Corridors are strips of natural vegetation linking ecosystem fragments. They can be defined as "any area of habitat through which an animal or plant propagule has a high probability of moving" (Noss 1991). Preserves or fragmented ecosystems with high biodiversity level or rare species may be linked by corridors (Figure 21).

Figure 21. 

This corridor may be serving as linkage for birds between fragmented ecosystems.


Credit:

Henry Gholz


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Whether corridors will provide all or none of the benefits listed in Table 1 will depend on several factors. For instance, a corridor that has a high proportion of edges compared to the interior forest may facilitate spread of pests, diseases, and catastrophic fires or increase exposure of wildlife to predators and domestic animals.

Groups of corridors can be combined to form corridor networks. By adding several corridors and integrating them with buffer zones and natural preserves, connectivity may be increased (Figure 22).

Figure 22. 

The proposed network of natural areas, buffer zones and corridors forms a bigger regional network of ecosystems for the state of Florida. This corridor network connects two important waterways, Ockefenokee (North Florida) and Everglades (South Florida), which have been disconnected for decades.


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Many restoration projects in cities begin with river connections. Why are rivers and creeks considered good linkage corridors? First, because riparian ecosystems are considered to be one of the richest habitat types, with alluvial soils, abundant insects and plant species. They constitute one of the most biologically productive and diversified habitat types with complex and multilayered vegetation (see Chapter 6 - The Hydrologic Cycle). Second, rivers and creeks are natural corridors that pass through many ecosystems, so the linkages between these ecosystems already exist.

Greenways

Greenways are a type of corridor designed to connect open spaces for ecological, cultural and recreational purposes. There are a wide variety of greenway projects around the country. We can find greenways projects that are managed as corridors between natural areas (with an ecological objective) and others that are for purely recreational purposes. Greenways range from narrow urban trails to winding river corridors to very wide, landscape level linkages.

It is important to define the goals of greenways. In some instances, an urban greenway restricted to a very narrow width, creating a beautiful space for recreation, may be the primary goal (Figure 23). However, relatively few greenways have been designed with detailed consideration of ecological functions (Smith and Hellmund 1993). Nonetheless, a greenway's ecological function should be considered and promoted whenever possible. An example is the Rio Grande Valley State Park in New Mexico. This park is a heavily used urban recreation area located only 2 to 3 miles from downtown Albuquerque, NM. The park contains extensive riparian forests of native cottonwood (Populus deltoides) and black willow (Salix nigra). These forests contrast with the typical arid Southwest areas surrounding them and for this reason host a high diversity of wildlife and migratory birds.

Figure 23.1. 
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Figure 23.2. 

Some greenways, such as the Rio Grande Valley State Park in New Mexico (23.1), provide better ecological function than this bicycle trail (23.2) in Florida. Rio Grande Valley is a heavily used urban park that also provides connectivity for wildlife and ecosystems.


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Although activities such as hiking, horseback riding, picnicking, and nature walks are encouraged, the Rio Grande Valley State Park gives high priority to recreational trail design in order to protect sensitive and unique habitats. Degraded areas have been restored with native trees and shrubs, following removal of saltcedar (Tamarix spp.), an invasive species. Connectivity between high quality areas for wildlife movement also have high priority. This greenway effort seeks to restore natural species and ecosystems processes, but also recognizes the need to make resources available and enjoyable for people.

Stepping Stones

As mentioned before, viewing the landscape holistically, instead of focusing on each separate area in isolation, should be the objective of urban managers. Even where it is not possible to connect ecosystems through corridors, stepping stones can be provided. Stepping stones (Franklin 1993) are smaller habitats that permit some plants and animals to move across the landscape from one ecosystem fragment to the other (Figure 24). Some interior species, such as many native birds, may not find them useful, but for some other species, such as small mammals and reptiles, the connectivity enhances habitat.

Figure 24. 

Stepping stones or small patches of ecosystems may help some species move from one larger ecosystem fragment (A) to another (B).


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The minimum ideal size for ecosystems to remain fully functional is often unknown. However, some scientists theorize that an optimum landscape has large patches of natural vegetation supplemented with small patches scattered as stepping stones throughout the landscape (Franklin 1993, Noss 1991, Adams 1994). In summary, stepping stones provide habitat for species that will live in small areas and help the flow of genes so birds and some plants will be able to move across the landscape.

How Can We Restore Biodiversity in Urban Areas?

There are numerous ways to enhance biodiversity in parks, neighborhoods, abandoned areas, backyards, industrial zones, and other urban forest restoration projects, including:

  • leaving stumps, leaves, snags, and logs on-site to enhance the ecosystem's natural structure, maintain the nutrient cycle, and provide habitat for wildlife and other organisms,

  • planting native species in combinations that mimic nearby ecosystems,

  • controlling invasive plants and animals that may eliminate native species,

  • enhancing the ecosystem's structural diversity, and

  • creating multi-age ecosystems (forests) in several stages of ecological succession typical of that ecosystem (see Chapter 4 - Plant Succession and Disturbances).

In these urban forest restoration projects, it is essential to maintain and/or restore the ecosystem's ecological processes, such as:

  • natural disturbances: such as fires and natural hydroperiods (for instance, re-instating flooding in drained wetlands),

  • ecological succession: understand ecological succession in nearby similar ecosystems and consider establishing these successional stages (for more information see Chapter 4 - Plant Succession and Disturbances),

  • nutrient cycle: promote and educate about the need for retaining leaves, twigs, branches and logs on site to store and cycle nutrients (see Chapter 2 - Basic Ecological Principles), and

  • hydrological cycle: find ways to aid the hydrological cycle. Examples include leaving natural mulched areas for better water infiltration and maintaining vegetative cover to prevent water erosion (see Chapter 6 - Hydrologic Cycle).

Examples of Restoration Projects

There are many projects in cities and urban areas that restore urban forests as whole ecosystem(s). Biodiversity is often an important part of these restoration projects, either at a small or large scale.

Reintroducing Fire in Gainesville, FL

Natural fire regimes are important ecological processes that should be reintroduced in fire-adapted ecosystems, including urban forest ecosystems.

For example, the longleaf pine ecosystem, a natural forest type of the Southern US, is adapted to periodic and light fires. Fires keep adjacent hardwood species from invading longleaf pine forests (Figure 25.1). In the process, these fires maintain an extremely diverse flora in the ground layer (Figure 25.2). There are more than 100 herbaceous species in sites no larger than an acre and at least 190 rare and endemic species associated with this ecosystem (Hardin and White 1998). Fires are essential to maintain this ecosystem's natural structure, that is, an open canopy of longleaf pines and the diverse ground layer. If fires are suppressed, this unique flora is largely lost.

Figure 25.1. 

Figure 25.2. 

Stepping stones or small patches of ecosystems may help some species move from one larger ecosystem fragment (A) to another (B).


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Fires have been reintroduced in remnants of longleaf pine ecosystems in urban areas. An example is a subdividion in Gainesville, FL, that contains patches of a longleaf pine ecosystem interwoven with houses, golf courses, and streets. Periodic prescribed fire is applied to these patches of longleaf pine, maintaining its open canopy and rich herbaceous species. Education plays a key role in such innovative pratices in urban centers (Figure 26).

Figure 26.1. 

Figure 26.2. 

Figure 26.3. 

This subdivision in Gainesville, FL has patches of a longleaf pine ecosystem (26.1) interwoven with houses, golf courses and streets (26.2). Periodic prescribed fire is applied to these patches. Education plays a key role in such innovative practices in urban areas (26.3).


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Northeast Anne Greenbelt Forest Restoration in Seattle, WA

Downtown Seattle has a 35-acre restoration project developed by the Seattle Department of Parks and Recreation (SDPR), University of Washington, and the local community. This project is part of a greater effort to apply integrated landscape management practices in parks and other areas in the Seattle region (Figure 27).

Figure 27. 

The Northeast Anne Greenbelt Forest Restoration is a neighborhood restoration project in Seattle, WA (map at left). Other similar small scale projects are funded and coordinated by the Seattle Department of Parks and Recreation.


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The site was heavily invaded by exotic invasive species (English Ivy, bindweed, Himalayan blackberry, and Scotch broom), ornamental plants, and weeds, and was also a dumping ground for trash. Additional problems were soil erosion and lack of wildlife.

The partners worked together and developed a plan to:

  • remove the exotic vegetation,

  • plant varying native species to provide food and cover for wildlife and to enhance structural diversity,

  • create logs and snags to provide habitat for invertebrates, woodpeckers, and decomposers, and

  • plant trees with deep roots and understory vegetation to help stabilize the soil and reduce erosion.

Today, the area has been cleared of exotics, erosion has been stabilized and an environmental center has been established, where the local community promotes educational and recreational activities.

Chicago Wilderness in Chicago, IL

The Chicago Wilderness is a combined effort of 60 partnering organizations, including landowners, local, regional and federal agencies, universities and conservation agencies. The Chicago Wilderness' primary goal is to restore ecological processes that maintain biodiversity. Their work is to improve the region's biodiversity at all levels: genetic, species, and ecosystem diversity throughout the landscape.

To meet this goal they have several objectives:

  • to document the region's ecosystems,

  • to help restore natural communities on public and private lands,

  • to prevent further loss of critical ecosystems and, at the same time, promote carefully planned development,

  • to promote education, outreach and volunteer opportunities, and

  • to define restoration strategies (including removal of aggressive invasive species, thinning of native trees to promote growth of savannas and woodlands species, use of prescribed fire and planting of native species).

To date, there are over 109 Chicago Wilderness collaborative projects ranging from biodiversity initatives to prairie and savanna restoration projects with prescribed burning to backyard biodiversity initiatives to restoration of threatened and endangered species (Figure 28).

Figure 28. 

Outreach materials utilized by Chicago Wilderness educate citizens about the region's biodiversity and strategies for restoration.


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Monitoring Success

Monitoring is a crucial part of every ecosystem restoration project. Monitoring provides the opportunity to gather information about how ecosystems in urban areas work and how ecosystems and people interact over time. It is also a critical activity for reevaluating the success or failure of projects so that we can apply this accumulated knowledge and experience to future projects.

Ecosystems are complex and inter-related and even the best studied and planned projects might have unexpected results. One example of a learning experience is a salt marsh, 8 km south of downtown San Diego, CA. The restored ecosystem was supposed to provide habitat for an endangered bird, the light-footed clapper rail (Rallus longirostris Levipes) (Figure 29). Cordgrass species (Spartina spp.) were transplanted from nearby wetlands to provide nesting sites for the bird. However, the plant did not grow to 90 cm, the bird's preferred height. Researchers working on the project thought the problem was due to the marsh's sandy, nutrient-poor soil, so they added nitrogen fertilizers. But the fertilizer favored another plant, pickleweed, which outgrew the desired grass (Malakoff 1998). Researchers are still trying to determine the best methods for restoring this ecosystem.

Figure 29. 

Since ecosystems are complex and inter-related, careful planning and monitoring are essential elements of restoration projects. The example of this salt marsh and the light-footed clapper rails reminds us that there are no easy recipes.


Credit:

David Sarkozi


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Conclusions

Urban forest ecosystems present many opportunities for restoring biodiversity, whether in a backyard, neighborhood, park or natural area. It is essential to know and understand the natural ecosystems in these areas in terms of vegetation, structural diversity, wildlife, natural disturbance regimes and the nature of their ecological processes. When managing ecosystems for biodiversity, we should pay attention to ecosystem structure and its functioning. Ecological processes, such as nutrient cycling, hydrological cycling, and ecological succession should be reinstated in the urban forest ecosystem as a comprehensive strategy for biodiversity conservation.

Corridors, buffer zones, greenways, and stepping stones are all ways in which urban forests can be managed as ecosystems. While large scale projects may help reestablish connectivity and maintain important ecological processes, small scale projects, such as removing invasive species or restoring native species in a small city park, also contribute.

However, management of the landscape as a whole can only be accomplished if we take an interdisciplinary and integrated approach toward urban forests. This requires a combined and joint effort of local, state, and federal governments, as well as private, public, and grass-root initiatives. Education plays a critical role in generating informed citizens who are essential partners in the establishment of restoration projects in cities.

Suggested Readings

Dunster, J. A. 1998. The role of arborists in providing wildlife habitat and landscape linkages throughout the urban forest. Journal of Arboriculture, 24(3): 160-167.

Argent, R. M. 1992. Ecological succession as a criterion for the selection of urban trees. Dissertation, Texas A&M University. 80p.

Sun, W. Q. 1992. Quantifying species diversity of streetside trees in our cities. Journal of Arboriculture, 18(2): 91-93.

Cited Literature

Adams, L. and L. E. Dove. 1989. Wildlife reserves and corridors in the urban environment: A guide to ecological landscape planning and resource conservation. National Institute for Urban Wildlife, Columbia, MD. 87p.

Adams, L. W. 1994. Urban wildlife habitats: A landscape perspective. University of Minnesota Press, Minneapolis, MN. 186p.

Ahern, J. and J. Boughton. 1994. Wildflower meadows as suitable landscapes. In: Platt, R.H., Rowntree, R. A. and Muick, P. C. (eds), The ecological city: Preserving and restoring urban biodiversity. pp 172-187, University of Massachusetts Press, Amherst.

Costanza, R., R. d'Arge, R. de Groot, Farber, M. Grasso, B. Hannon, K. Limburg, S. Naeem, R. V. O'Neill, J. Paruelo, R. G. Raskin, P. Sutton and M. van den Belt. 1997. The value of the world's ecosystem services and natural capital. Nature, 387(6630): 253-258.

Florida Game and Fresh Water Fish Commission. 1997. The Florida scrub jay. Tallahassee, FL.

Florida Game and Fresh Water Fish Commission. 1998. The 1996 Economic benefits of watchable wildlife recreation in Florida. Tallahassee, FL.

Franklin, J. F. 1993. Preserving biodiversity: species, ecosystems or landscapes? Ecological Applications, 3(2): 202-205.

Friederici, P. 1997. Where the wild ones are. Chicago Wilderness Magazine, Fall 1997: 6-9.

Hardin, E. D. and D. L. White. 1989. Rare vascular plant taxa associated with wiregrass (Aristida stricta) in the Southeastern United States. Natural Areas Journal, 9:234-245.

Keystone Center. 1991. Biological diversity on federal lands: Report of a keystone policy dialogue. The Keystone Center, Keystone Co., 96p.

Kimmins, J. P. 1996. Forest ecology: A foundation for sustainable management 2ed, Prentice Hall, Upper Saddle River, New Jersey, 596 p.

MacArthur, R.H. and Wilson, E.O. 1967. The theory of island biography. Princeton University Press, N.Y., 203 p.

Malakoff, D. 1998. Restored wetlands flunk real-world test. Science, 280 (5362): 371-372.

McPherson, E. G. and R. A. Rowntree. 1991. The environmental benefits of urban forests. In A National Research Agenda for Urban Forestry in the 1990s. International Society of Arboriculture, Urbana, IL. 60p.

Norris, R. 1992. Can ecotourism save natural areas? National Parks, 66:30-35.

Noss, R. F. 1991. Landscape connectivity: Different functions at different scales, p.27-39 In: Hudson, W. E. (ed), Landscape linkages and biodiversity, Island Press, Washington, D. C., 196p.

Noss, R. F. and A. Y. Cooperrider. 1994. Saving nature's legacy: Protecting and restoring biodiversity. Island Press, Washington, D. C. 416p.

Nowak, D. J. 1993. Historical change in Oakland and its implications for urban forest management. Journal of Arboriculture, 19(5): 313-319.

Nowak, D. J. 1994. Urban forest structure: the state of Chicago's urban forest. In Chicago's urban forest ecosystem: results of the Chicago Urban Forest Climate Project, E. G. McPherson, D. J. Nowak and R. A. Rowntree, eds.: 3-18.

Price, S. 1993. The battle for the elms. Urban Forests, 13(2): 11-15.

Principe, P. P. 1989. The economic significance of plants and their constituents as drugs. Economic and Medicinal Plant Research, vol.3 Academy Press, London: 1-17.

Saunders, D. A., R. J. Hobbs and C. R. Margules 1991. Biological consequences of ecosystem fragmentation: A review. Biological Conservation, 5:18-32.

Smith, D. S. and P. C. Hellmund. 1993. Ecology of greenways: Design and function of linear conservation areas. University of Minnesota, Minneapolis, MN. 222p.

South Florida Ecosystem Task Force. 1998. The Everglades on its wayback: A restoration progress report. Miami, FL.

Sun, W. Q. 1992. Quantifying species diversity of streetside trees in our cities. Journal of Arboriculture, 18(2): 91-93.

U. S. Fish and Wildlife Service. 1992. National survey of fishing, hunting and wildlife-associated recreation. U.S. Department of the Interior, Washington, D.C.

White, C. W. and M. J. Donnell. 1988. Nitrogen cycling processes and soil characteristics in an urban versus rural forest. Biogeochemistry, (5): 243-262.

Tables

Table 1. 

Benefits and disadvantages of ecological corridors.

Benefits

Disadvantages

enhance biotic movement (because they permit flow of genes)

spread of diseases

provide extra foraging areas for species that require more resources than those available in a single patch

spread of diseases

provide wildlife plant habitat

 

Footnotes

1.

This document is FOR 92, one of a series of the Entomology and Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date August 2001. Revised February 2008. Reviewed November 2012. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Eliana Kämpf Binelli, former Extension forester, School of Forest Resources and Conservation, Cooperative Extension Service, Institute of Agricultural Sciences, University of Florida, PO Box 110410, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.