Savard, JPL; Clergeau, P; Mennechez, G (1999): Biodiversity concepts and urban ecosystems
Savard, JPL; Clergeau, P; Mennechez, G (1999): Biodiversity concepts and urban ecosystems
Abstract
The association of biodiversity and urban ecosystems has usually concerned the impact of urbanization on biodiversity. However, biodiversity concepts can easily be applied to the urban ecosystem itself. As more and more people live in cities, restoration, preservation and enhancement of biodiversity in urban areas become important. Concepts related to biodiversity management such as scale, hierarchy, species identity, species values, fragmentation, global approaches can be used to manage urban biodiversity. Application of these concepts in such arti®cial ecosystems may yield important insights for the management of natural ecosystems. Birds are highly visible and quite sensitive to changes in habitat structure and composition. Bird species richness in urban ecosystems is in¯uenced both by local and landscape characteristics and a multiscale approach is essential to its proper management. People±wildlife con¯icts are an integral component of wildlife management in urban ecosystems and must be addressed. Enhancement of biodiversity in urban ecosystems can have a positive impact on the quality of life and education of urban dwellers and thus facilitate the preservation of biodiversity in natural ecosystems.
131
(...) biodiversity (...) has generally been used in a very comprehensive manner meaning the variability of life (composition, structure and function). Biodiversity can be represented as an interlocked hierarchy of elements on several levels of biological organization (Noss, 1992).
Urban ecosystems have usually been examined in terms of their impact on biodiversity (...) However (...) much can be learned by applying biodiversity concepts to the urban ecosystem itself.
132
(...) from an ecological perspective, urban ecosystems are highly dynamic (Gilbert, 1989; Adams, 1994) and can provide useful insights into the management of biodiversity in other ecosystems. They are characterized by a high level of heterogeneity often organized along gradients extending from the surrounding landscape to the town centre (McDonnell and Pickett, 1990; McDonnell et al., 1993).
Biodiversity concerns related to urban ecosystems can be divided into three major groups: (1) those
related to the impact of the city itself on adjacent ecosystems; (2) those dealing with how to maximize biodiversity within the urban ecosystem and (3) those related to the management of undesirable species within the ecosystem. While species diversity and abundance are often related to the quality of urban life (Adams, 1994; Middleton, 1994) the overabundance of some species can be at times undesirable (Cooper, 1987; Clergeau et al., 1996).
This paper deals with concerns related to the urban ecosystem itself using birds as a target group. Birds are quite sensitive to changes in habitat structure and composition and are therefore excellent indicators of changes and stresses in the urban ecosystem (Savard and Falls, 1982; Clergeau et al., 1998).
2. Important concepts related to biodiversity
2.1. Hierarchy of scales
Life is structured in a hierarchical fashion starting with cells forming individuals, which regroup into populations, which in turn form species, who end up as communities. Biodiversity concerns can occur at any level of organization. Levels of biological organization often correspond to speci®c spatial and temporal scales and must be addressed at their appropriate scale.
(...) In this case, we confine ourselves to birds but the same argument could be applied to
plants, insects or mammals. In fact, biodiversity includes all forms of life.
As biodiversity is expressed in a multitude of spatial scales, it is essential to specify the scale(s) of interest when using the term biodiversity (Savard, 1994). Failure to do so will quickly generate confusion and misunderstanding, since it is frequent to find different and even contradictory results on different scales (Wiens, 1989). A multi-scale approach is thus necessary to properly address biodiversity questions. Scales are not independent from one another but linked in a hierarchical way (Allen and Starr, 1982) so the effects of an action at a given scale must be considered on higher and lower scales (Savard, 1994). Within urban ecosystems, actions taken to preserve or enhance
biodiversity should be so at scales ranging from individual plants to the entire city itself and even its
surrounding areas (Table 1). Temporal scales are also important to consider.
(...) When discussing and studying bird diversity it is also important to specify the type of diversity that interests us. For example, a park may have only two breeding species but be used for feeding by 10 species during the summer and five during the winter.
The hierarchical approach emphasizes logical and functional linkages among scales. Because biodiversity transcends several spatial and temporal scales as well as several organizational levels (from genes to communities), its management should be incorporated into a hierarchical decision system (Rice, 1992) as in some species management (Clergeau, 1995). In such a system, higher levels constrain to various degrees the actions taken at lower levels.
134
Constraints may include mandatory coordination across scales and levels. For example, a grassland
park in a city may have only five breeding bird species, three of which are speci®c to that park. Local bird diversity of the park could be doubled or tripled by converting this grassland park into a wooded one. However, this would result in the loss of three species for the city as a whole. So, in this case, an increase in local diversity would result in a decrease in diversity at the scale of the city. Obviously, a choice has to be made here, as to which scale we want to give priority to. It doesn't necessarily have to be the greater scale as long as efforts are coordinated across the scales. Since urbanized areas already function within a hierarchical decision system, it should be relatively easy to extend to biodiversity management. In the example described here, there could be a general policy on the scale of the city to preserve and enhance the unique features of some parks. Once those features are identified and protected, a general goal of increased species richness can then be applied to all parks. Food webs and trophic relationships illustrate well the hierarchical organization of life and stress an other important point to consider when managing biodiversity: all species are not equal.
2.2. Species
One important aspect in dealing with biodiversity is that not all species are equal. Species vary in size, shape, abundance, distribution, trophic position, ecological function, feeding habits and desirability. Therefore, diversity indices which assume all species as being equal are of little use for properly managing biodiversity (Savard, 1994). Some species may play important roles in the community, so their absence would significantly affect several other species. Identification of these species is difficult since their role may be seasonal or even habitat specific (Bond, 1993). It is important to identify desirable species.
(...) Other species-related concepts used in biodiversity conservation are the concept of umbrella species and flagship species (Hunter, 1990). Umbrella species are at the top of food chains or with large home ranges so that by protecting these species we protect all species on which they depend or the species with similar requirements but smaller home ranges.
(...)
Birds can also be excellent indicators of various aspects of urban ecosystems (biodiversity; vegetation
structure; contaminants).
(...)
People+wildlife interactions are crucial in urban ecosystems (Brown et al., 1979) and any enhancement of biodiversity must take them into consideration (Michelson, 1970; Gilbert, 1989; Clergeau et al., 1997).
135
2.3. Habitat fragmentation and habitat quality
Habitat fragmentation can be extreme within urban ecosystems. Its effect occurs at all spatial scales and affects all organisms. Fragments of natural vegetation may be too small or even too isolated to support some species.
(...)
Vegetation corridors linking urban green areas between themselves and/or with rural habitats are
important to maintain and enhance urban biodiversity (Flink and Searns, 1993). They facilitate movements and ensure colonization of isolated natural areas (Clergeau, 1998). Streams form natural corridors in urban areas and should be managed as such.
(...)
Statements like `our goal is to enhance urban biodiversity' are not workable because of the encompassing nature of the word biodiversity. Something like `our goal is to increase bird diversity in urban parks' is more appropriate as it specifies the group of organisms, as well as the scale of action. It is essential to elaborate achievable goals. It is also important not to confound biodiversity and species diversity as they are quite different concepts. Species diversity is only a small portion of biodiversity and deals mostly with species richness, sometimes incorporating abundance as well. Biodiversity includes all forms of life, as well as structural and functional aspects.
3. Importance of local and landscape attributes for bird abundance and biodiversity
New residential developments with little or no original vegetation have a low diversity of birds, but depending on the architectural structure of their buildings, can sometimes support high densities of
birds. Bird diversity typically increases as vegetation develops (Hohtola, 1978; Luniak, 1994). Urban woodlands are an important component of the urban landscape in terms of bird species diversity. The larger the woodland, the more species it supports (Sasvari, 1984; Tilghman, 1987a). For similar size woodlots, spatial heterogeneity, complex vertical structure and diverse species composition of vegetation are all associated with higher bird species richness (Cavareski, 1976; Tilghman, 1987b). Composition of the native component of urban bird communities varies according to the geographical location of the city (Table 2), its landscape context and the amount of natural vegetation contained in the city.
136
Urban ecosystems are quite similar worldwide in terms of structure, functions and constraints. They
differ in terms of their geographical location, their size and the type of landscape they modify. The
landscape surrounding a city greatly in¯uences plant and wildlife species that will be found within the new artificial ecosystem. It is essential to consider landscape factors in the management of urban biodiversity.
137
The impact of urbanization on a given landscape is partially a function of the original composition of the landscape. Urbanization will increase bird diversity of simple landscapes such as desertic areas or grassland areas by creating new habitats and supporting exotic species of birds (Emlen, 1974). In diversified landscapes that support an initial high diversity of habitats, a decrease in bird diversity is likely to occur as urbanization becomes more prevalent, eliminating unique natural habitats (Batten, 1972). Landscape factors such as the composition and arrangement of habitat types around a local area have significant impacts on bird populations of that local area and must be taken into account when managing the local area (i.e. a park, a residential area, etc.). Landscape context is especially important in complex landscapes such as found in urban ecosystems (Dunning et al., 1992).
4. Approaches to enhance urban biodiversity
There is no single best starting point when dealing with biodiversity. Local actions as well as regional
actions are equally important on their respective scale. In urban areas home-owners can take various actions on the scale of their lot. It is important to realize that it is the concerted efforts at various scales that produce the best results. It is essential that home-owners realise that their own local action can contribute to a larger collective effort that would culminate in the creation of a real biological corridor that facilitates the movements of several species throughout the city.
(...)
A first approach for urban planners would be to inventory resources within the city (natural area, lake, river. . .) and their organization (unique structures, corridors) and to assess the abundance, distribution and possible concerns about these resources. Once resources have been identi®ed on the scale of the city itself, strategies can be established for preserving and using them whenever necessary (...) Another important aspect to consider is the landscape context. This is important, as the maintenance of some resources within the city may depend on what is happening outside the city.
Some activities to enhance urban bird diversity are worth mentioning here:
4.1. Plantation of trees and shrubs
Bird diversity in urban areas tends to be proportional to the existing volume of vegetation (Emlen,
1974; Lancaster and Rees, 1979). During migration, numbers and diversity of migrating birds within the city are also proportional to vegetation abundance (Savard, 1978).
Table 4
A multi-scale approach: to enhance bird abundance and diversity in the city we may want to
1. Examine the country side surrounding the city and secure or restore important bird habitats that may act as a source of birds for the city;
establish a greenbelt around the city.
2. Identify and consolidate vegetation corridors linking these areas to the city and link parks whenever possible; make use of natural streams and right of ways.
3. Increase the volume and diversity of vegetation in the city (along streets, right of ways, industrial, commercial and residential sectors).
4. Exploit and enhance the features of some parks that may attract particular species of birds.
5. Increase the structural diversity of vegetation in natural and recreational parks of the city.
6. Plant conifers and fruit trees to provide cover and food for birds; promote the night blackout of tall buildings during bird migration and encourage architecture that minimizes bird collisions.
7. Erect special nesting structure for cavity or cliff nesting species (nest boxes for owls, chimneys for swifts, nesting platforms for falcons or storks).
8. Encourage homeowners to manage their property for birds, to restrain their pets and to minimize the use of pesticides and herbicides.
9. Distribute guidelines as to the proper planting of trees and shrubs to attract birds.
10. Maintain bird feeders, nest boxes, bird baths in backyards.
Abstract
The association of biodiversity and urban ecosystems has usually concerned the impact of urbanization on biodiversity. However, biodiversity concepts can easily be applied to the urban ecosystem itself. As more and more people live in cities, restoration, preservation and enhancement of biodiversity in urban areas become important. Concepts related to biodiversity management such as scale, hierarchy, species identity, species values, fragmentation, global approaches can be used to manage urban biodiversity. Application of these concepts in such arti®cial ecosystems may yield important insights for the management of natural ecosystems. Birds are highly visible and quite sensitive to changes in habitat structure and composition. Bird species richness in urban ecosystems is in¯uenced both by local and landscape characteristics and a multiscale approach is essential to its proper management. People±wildlife con¯icts are an integral component of wildlife management in urban ecosystems and must be addressed. Enhancement of biodiversity in urban ecosystems can have a positive impact on the quality of life and education of urban dwellers and thus facilitate the preservation of biodiversity in natural ecosystems.
131
(...) biodiversity (...) has generally been used in a very comprehensive manner meaning the variability of life (composition, structure and function). Biodiversity can be represented as an interlocked hierarchy of elements on several levels of biological organization (Noss, 1992).
Urban ecosystems have usually been examined in terms of their impact on biodiversity (...) However (...) much can be learned by applying biodiversity concepts to the urban ecosystem itself.
132
(...) from an ecological perspective, urban ecosystems are highly dynamic (Gilbert, 1989; Adams, 1994) and can provide useful insights into the management of biodiversity in other ecosystems. They are characterized by a high level of heterogeneity often organized along gradients extending from the surrounding landscape to the town centre (McDonnell and Pickett, 1990; McDonnell et al., 1993).
Biodiversity concerns related to urban ecosystems can be divided into three major groups: (1) those
related to the impact of the city itself on adjacent ecosystems; (2) those dealing with how to maximize biodiversity within the urban ecosystem and (3) those related to the management of undesirable species within the ecosystem. While species diversity and abundance are often related to the quality of urban life (Adams, 1994; Middleton, 1994) the overabundance of some species can be at times undesirable (Cooper, 1987; Clergeau et al., 1996).
This paper deals with concerns related to the urban ecosystem itself using birds as a target group. Birds are quite sensitive to changes in habitat structure and composition and are therefore excellent indicators of changes and stresses in the urban ecosystem (Savard and Falls, 1982; Clergeau et al., 1998).
2. Important concepts related to biodiversity
2.1. Hierarchy of scales
Life is structured in a hierarchical fashion starting with cells forming individuals, which regroup into populations, which in turn form species, who end up as communities. Biodiversity concerns can occur at any level of organization. Levels of biological organization often correspond to speci®c spatial and temporal scales and must be addressed at their appropriate scale.
(...) In this case, we confine ourselves to birds but the same argument could be applied to
plants, insects or mammals. In fact, biodiversity includes all forms of life.
As biodiversity is expressed in a multitude of spatial scales, it is essential to specify the scale(s) of interest when using the term biodiversity (Savard, 1994). Failure to do so will quickly generate confusion and misunderstanding, since it is frequent to find different and even contradictory results on different scales (Wiens, 1989). A multi-scale approach is thus necessary to properly address biodiversity questions. Scales are not independent from one another but linked in a hierarchical way (Allen and Starr, 1982) so the effects of an action at a given scale must be considered on higher and lower scales (Savard, 1994). Within urban ecosystems, actions taken to preserve or enhance
biodiversity should be so at scales ranging from individual plants to the entire city itself and even its
surrounding areas (Table 1). Temporal scales are also important to consider.
(...) When discussing and studying bird diversity it is also important to specify the type of diversity that interests us. For example, a park may have only two breeding species but be used for feeding by 10 species during the summer and five during the winter.
The hierarchical approach emphasizes logical and functional linkages among scales. Because biodiversity transcends several spatial and temporal scales as well as several organizational levels (from genes to communities), its management should be incorporated into a hierarchical decision system (Rice, 1992) as in some species management (Clergeau, 1995). In such a system, higher levels constrain to various degrees the actions taken at lower levels.
134
Constraints may include mandatory coordination across scales and levels. For example, a grassland
park in a city may have only five breeding bird species, three of which are speci®c to that park. Local bird diversity of the park could be doubled or tripled by converting this grassland park into a wooded one. However, this would result in the loss of three species for the city as a whole. So, in this case, an increase in local diversity would result in a decrease in diversity at the scale of the city. Obviously, a choice has to be made here, as to which scale we want to give priority to. It doesn't necessarily have to be the greater scale as long as efforts are coordinated across the scales. Since urbanized areas already function within a hierarchical decision system, it should be relatively easy to extend to biodiversity management. In the example described here, there could be a general policy on the scale of the city to preserve and enhance the unique features of some parks. Once those features are identified and protected, a general goal of increased species richness can then be applied to all parks. Food webs and trophic relationships illustrate well the hierarchical organization of life and stress an other important point to consider when managing biodiversity: all species are not equal.
2.2. Species
One important aspect in dealing with biodiversity is that not all species are equal. Species vary in size, shape, abundance, distribution, trophic position, ecological function, feeding habits and desirability. Therefore, diversity indices which assume all species as being equal are of little use for properly managing biodiversity (Savard, 1994). Some species may play important roles in the community, so their absence would significantly affect several other species. Identification of these species is difficult since their role may be seasonal or even habitat specific (Bond, 1993). It is important to identify desirable species.
(...) Other species-related concepts used in biodiversity conservation are the concept of umbrella species and flagship species (Hunter, 1990). Umbrella species are at the top of food chains or with large home ranges so that by protecting these species we protect all species on which they depend or the species with similar requirements but smaller home ranges.
(...)
Birds can also be excellent indicators of various aspects of urban ecosystems (biodiversity; vegetation
structure; contaminants).
(...)
People+wildlife interactions are crucial in urban ecosystems (Brown et al., 1979) and any enhancement of biodiversity must take them into consideration (Michelson, 1970; Gilbert, 1989; Clergeau et al., 1997).
135
2.3. Habitat fragmentation and habitat quality
Habitat fragmentation can be extreme within urban ecosystems. Its effect occurs at all spatial scales and affects all organisms. Fragments of natural vegetation may be too small or even too isolated to support some species.
(...)
Vegetation corridors linking urban green areas between themselves and/or with rural habitats are
important to maintain and enhance urban biodiversity (Flink and Searns, 1993). They facilitate movements and ensure colonization of isolated natural areas (Clergeau, 1998). Streams form natural corridors in urban areas and should be managed as such.
(...)
Statements like `our goal is to enhance urban biodiversity' are not workable because of the encompassing nature of the word biodiversity. Something like `our goal is to increase bird diversity in urban parks' is more appropriate as it specifies the group of organisms, as well as the scale of action. It is essential to elaborate achievable goals. It is also important not to confound biodiversity and species diversity as they are quite different concepts. Species diversity is only a small portion of biodiversity and deals mostly with species richness, sometimes incorporating abundance as well. Biodiversity includes all forms of life, as well as structural and functional aspects.
3. Importance of local and landscape attributes for bird abundance and biodiversity
New residential developments with little or no original vegetation have a low diversity of birds, but depending on the architectural structure of their buildings, can sometimes support high densities of
birds. Bird diversity typically increases as vegetation develops (Hohtola, 1978; Luniak, 1994). Urban woodlands are an important component of the urban landscape in terms of bird species diversity. The larger the woodland, the more species it supports (Sasvari, 1984; Tilghman, 1987a). For similar size woodlots, spatial heterogeneity, complex vertical structure and diverse species composition of vegetation are all associated with higher bird species richness (Cavareski, 1976; Tilghman, 1987b). Composition of the native component of urban bird communities varies according to the geographical location of the city (Table 2), its landscape context and the amount of natural vegetation contained in the city.
136
Urban ecosystems are quite similar worldwide in terms of structure, functions and constraints. They
differ in terms of their geographical location, their size and the type of landscape they modify. The
landscape surrounding a city greatly in¯uences plant and wildlife species that will be found within the new artificial ecosystem. It is essential to consider landscape factors in the management of urban biodiversity.
137
The impact of urbanization on a given landscape is partially a function of the original composition of the landscape. Urbanization will increase bird diversity of simple landscapes such as desertic areas or grassland areas by creating new habitats and supporting exotic species of birds (Emlen, 1974). In diversified landscapes that support an initial high diversity of habitats, a decrease in bird diversity is likely to occur as urbanization becomes more prevalent, eliminating unique natural habitats (Batten, 1972). Landscape factors such as the composition and arrangement of habitat types around a local area have significant impacts on bird populations of that local area and must be taken into account when managing the local area (i.e. a park, a residential area, etc.). Landscape context is especially important in complex landscapes such as found in urban ecosystems (Dunning et al., 1992).
4. Approaches to enhance urban biodiversity
There is no single best starting point when dealing with biodiversity. Local actions as well as regional
actions are equally important on their respective scale. In urban areas home-owners can take various actions on the scale of their lot. It is important to realize that it is the concerted efforts at various scales that produce the best results. It is essential that home-owners realise that their own local action can contribute to a larger collective effort that would culminate in the creation of a real biological corridor that facilitates the movements of several species throughout the city.
(...)
A first approach for urban planners would be to inventory resources within the city (natural area, lake, river. . .) and their organization (unique structures, corridors) and to assess the abundance, distribution and possible concerns about these resources. Once resources have been identi®ed on the scale of the city itself, strategies can be established for preserving and using them whenever necessary (...) Another important aspect to consider is the landscape context. This is important, as the maintenance of some resources within the city may depend on what is happening outside the city.
Some activities to enhance urban bird diversity are worth mentioning here:
4.1. Plantation of trees and shrubs
Bird diversity in urban areas tends to be proportional to the existing volume of vegetation (Emlen,
1974; Lancaster and Rees, 1979). During migration, numbers and diversity of migrating birds within the city are also proportional to vegetation abundance (Savard, 1978).
Table 4
A multi-scale approach: to enhance bird abundance and diversity in the city we may want to
1. Examine the country side surrounding the city and secure or restore important bird habitats that may act as a source of birds for the city;
establish a greenbelt around the city.
2. Identify and consolidate vegetation corridors linking these areas to the city and link parks whenever possible; make use of natural streams and right of ways.
3. Increase the volume and diversity of vegetation in the city (along streets, right of ways, industrial, commercial and residential sectors).
4. Exploit and enhance the features of some parks that may attract particular species of birds.
5. Increase the structural diversity of vegetation in natural and recreational parks of the city.
6. Plant conifers and fruit trees to provide cover and food for birds; promote the night blackout of tall buildings during bird migration and encourage architecture that minimizes bird collisions.
7. Erect special nesting structure for cavity or cliff nesting species (nest boxes for owls, chimneys for swifts, nesting platforms for falcons or storks).
8. Encourage homeowners to manage their property for birds, to restrain their pets and to minimize the use of pesticides and herbicides.
9. Distribute guidelines as to the proper planting of trees and shrubs to attract birds.
10. Maintain bird feeders, nest boxes, bird baths in backyards.
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