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Comment on N.A. Richards' diversity and stability in a street tree population
Urban Ecology, 7 (1982/1983) 173. 176 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Comments
173
and Critiques
COMMENT ON N.A. RICHARDS’ STREET TREE POPULATION
GLENN GUNTENSPERGEN
DIVERSITY
and FOREST
Department of Bo tany, University WI 53201 (U.S.A.)
AND STABILITY
IN A
STEARNS
of Wisconsin-Milwaukee,
P. 0. Box 413, Milwaukee,
The linkage of two persistent themes in ecology, diversity and stability was a logical development with its intellectual roots based on observations of catastrophic events in natural ecosystems and those modified by human activity (Elton, 1958). Those who have watched the elimination of American elm from city after city, as Dutch elm disease moved northward and westward, and who have seen the resulting devastation when those dead elms were removed, can be excused their acceptance of a relationship between diversity and stability. Similarly, observers of urban economic systems have ample reason to relate diversity in the economic base to a measure of stability. We should like to emphasize several points. First, present ideas of diversity and stability are muddled at best and at worst indicate that no systematic relationship exists. Second, we suggest that, although species richness does not relate directly to population stability, all approaches to population stability, both biological and economic should be considered. Finally, we would argue that diversity is important in urban forestry and may be incorporated into an effective management program. Based on his experience with street tree populations and their practical problems, Richards is reacting to the uncritical acceptance of the diversitystability relationship by land managers and the general public. Richard’s paper invites comment because, in taking the view that diversity is of negligable value in urban forestry, he has used economic rather than ecological arguments in dealing with an ecological issue. Our comments are directed toward clarification of the diversity and stability concepts from a biological perspective - with the intent to express a different and more sympathetic view of the value of diversity to the street tree community. Solutions to practical problems in urban forest management should have a sound scientific basis as well as economic feasibility. To be successful, the perceptions of the taxpayer must be considered equally with the criticism of the specialist. The relationship between diversity and stability is complex and the popular concept as Richards states it, i.e., that diversity results in stability could in many instances more accurately state that stability is the cause of diversity (May, 1976). Ecological concepts, such as diversity and stability that describe the inter-
174 actions within populations and communities and between those entities and the environment, originated from intuitive arguments and observation of how communities function. Additional information has been derived from simple trophic experiments in the laboratory, and the concept is further supported by simplified mathematical models. However, in the application of the argument to urban systems, one must remember that natural selection is often replaced by economic considerations. As Richards noted, management tends to negate natural process. Populations of street trees do not reproduce themselves nor does the street tree community change through the normal successional process. Additional energy in the form of maintenance is essential if street tree populations are to serve their function and remain intact systems. The price we pay for creating our own systems is the cost of continual management. As a biological concept, stability has several meanings. A population is stable if it peAsilts at a c&am level, i.e., resistance or static stability, or if population numbers fluctuate over a greater or lesser period of time but return to an original equilibrium state, it is resilient (Holling, 1974). Unfortunately, the definition Richards has chosen for stability includes both biological and economic constructs and, for this reason the rest of his argument cannot be strictly ecological or biological. Stability, as Richards defines it, describes a population having a low probability that it will experience serious disruption of values and management in the forseeable future. This concept of static stability also implies the energy inputs required to maintain it. His managed population is one that is expected to remain stable over a considerable period of time and to which energy is provided to aid in maintaining that stability. Certainly stable systems may last longer than unstable systems. However, there is more concern here with destabilizing elements that have economic rather than biological implications. We agree that individuals are important in street tree populations and that early mortality must be avoided as must high losses of older individuals within any short time period. Length of life, though, is primarily an economic argument. Short-lived species can produce stable communities if stability is considered in the appropriate time frame with species life span. Likewise, a stressful environment does not necessarily produce instability - another myth introduced into the argument that has nothing in particular to do with diversity. Young trees of new or different species need not necessarily be destabilizing, provided that the species added have a reasonable chance of adapting to the urban environment. We agree, however, that ideas of stability or instability should be viewed in the context of response of individual species populations. The concept of diversity requires discussion. Ecological practice divides diversity into several components, species richness refers only to the number of species present; while evenness describes the relative distribution of individuals among species. This ecological definition of diversity fits more appropriately with Richards welcome suggestion that a variety of age classes is desirable. Richards presents the diversity stability argument in a much too simplified
175
form. It is true that many species in an ecosystem do not necessarily lead to population stability. The original intrinsic appeal that such a causal relationship exists between diversity and stability is understandable given the anecdotal information that appeared to support that view. However, while some ecologists readily accept this idea, many are more likely to dispute it. MacArthur (1955) originally proposed that stability might be a function of the interconnectedness of species in a trophic web. He reasoned that the number of links in the food web would be a means of maintaining stability in a community and suggested that a measure of diversity would be an appropriate means of measuring the number of links in the food web. Subsequently, a large body of evidence accumulated that suggested that diversity led to stability and that less diverse systems are unstable. The desirability of using ecological theory to support intutitive ideas of the functioning of natural systems led to the entrenchment of the idea that complex systems would be more stable than simple systems. May (1976) has suggested that systems evolving in a stable environment increase in diversity whereas systems evolving in unstable environments are less diverse. This says little about the relationship between diversity and stability. Recently, ecologists have criticized the hypothesis that increased diversity implies increased stability (May, 1973; Goodman, 1975; Abrams and Allison, 1982). They point out that many predictions from theory are not confirmed by empirical evidence now available. While this current view runs counter to the more traditional linkage, empirical research is needed to substantiate these ideas (McNaughton, 1977). Diversity should play a role in management planning. Although the hypothesis linking diversity and stability might be false, the policies it promotes are not necessarily unworthy. If one accepts a monoculture as the most efficient system, the management plan must include options for responding to a catastrophic event that may be species specific. Although diversity does not necessarily increase stability it may provide a variety of safety valves for simplified ecosystems. Such systems have not had the opportunity or adequate time for significant interaction between the component species and their pests or pathogens. Biologically simplified systems of one species can become economically unstable because of species specific perturbations (Pimentel, 1961). Richards’ paper contains some contradictory arguments. For example, in his discussion of genetic diversity, he states that the use of cultivars, while desirable for certain landscaping attributes, e.g. uniformity, may limit adaptation to the variety of urban environments encountered. He is probably right that the use of cultivars further reduces the potential for new plantings to contribute to population stability. However, arguing in that direction is arguing that the use of single species is also dangerous because the individuals are too similar. The point is made, but perhaps not sufficiently emphasized, that seedling populations are more desirable than vegetatively reproduced cultivars. Likewise, the abrupt loss of elms as a result of Dutch Elm Disease appears to argue against the generalization that species with proven wide
176
adaptation are more likely to withstand a new stress. Urban elms were chiefly of seedling origin, and hence had considerable intraspecific variability. In this case, the spread of the disease was exacerbated by management techniques, i.e., close planting, etc. As long as one provides adequate variety in age in the plantings, the urban tree planter could utilize species, which although perhaps not adapted to all urban conditions, would provide diversity that might help to cushion a catastrophic event. Much of the rationale for the choice of street trees in the past appears to have been economic, rather than ecological. One may question whether this situation has changed or will change. The idea that diversity can be monitored as a management tool (Sanders, 1981) may well be appropriate provided that diversity is defined as the variety present in numbers of individuals, age-classes and species, rather than as species richness alone. Managers should not strive for a maximum number of species but for a good distribution of adapted species (Levenson and Stearns, 1980). We agree that emphasis on species richness alone is misdirected, however, planting only “proven” species will also leave the manager open to catastrophic loss from unforeseen causes.
REFERENCES Abrams, P.A. and Allison, T.D., 1982. Complexity&ability, and functional response. Am. Nat., 119: 240-249. Elton, C.S., 1958. The Ecology of Invasions by Animals and Plants. Methuen, London, 181 pp. Goodman, D., 1975. The theory of diversity-- stability relationships in ecology. Q. Rev. Biol., 50: 237-266. Holling, C.S, 1974. Reslience and stability of ecological systems. Annu. Rev. Ecol. Syst., 4: l-24. Levenson, J. and Stearns, F., 1980. Application of diversity to regional ecological assessment. A review with recommendations. ANL/AA-21, Argonne National Laboratory, Argonne, IL. MacArthur, R.H., 1955. Fluctuations of animal populations, and a measure of community stability. Ecology, 36: 533-536. McNaughton, S.J., 1977. Diversity and stability of ecological communities: a comment on the role of empiricism in ecology. Am. Nat., 108: 490-498. May, R.M., 1973. Stability and Complexity in Model Ecosystems. Princeton University Press, Princeton, NJ, 265 pp. May, R.M., 1976. Patterns in multi-species communities. In: R.M. May (Editor), Theoretical Ecology: Principles and Applications. Saunders, Philadelphia, PA, pp. 142-152. Pimentel, D., 1961. Species diversity and insect population outbreaks. Ann. Etomol. Sot. Am., 54: 76-86. Sanders, R., 1981. 33. 43.
Diversity
in the street trees of Syracuse,
New York.
Urban Ecol.,
5:
Comments
173
and Critiques
COMMENT ON N.A. RICHARDS’ STREET TREE POPULATION
GLENN GUNTENSPERGEN
DIVERSITY
and FOREST
Department of Bo tany, University WI 53201 (U.S.A.)
AND STABILITY
IN A
STEARNS
of Wisconsin-Milwaukee,
P. 0. Box 413, Milwaukee,
The linkage of two persistent themes in ecology, diversity and stability was a logical development with its intellectual roots based on observations of catastrophic events in natural ecosystems and those modified by human activity (Elton, 1958). Those who have watched the elimination of American elm from city after city, as Dutch elm disease moved northward and westward, and who have seen the resulting devastation when those dead elms were removed, can be excused their acceptance of a relationship between diversity and stability. Similarly, observers of urban economic systems have ample reason to relate diversity in the economic base to a measure of stability. We should like to emphasize several points. First, present ideas of diversity and stability are muddled at best and at worst indicate that no systematic relationship exists. Second, we suggest that, although species richness does not relate directly to population stability, all approaches to population stability, both biological and economic should be considered. Finally, we would argue that diversity is important in urban forestry and may be incorporated into an effective management program. Based on his experience with street tree populations and their practical problems, Richards is reacting to the uncritical acceptance of the diversitystability relationship by land managers and the general public. Richard’s paper invites comment because, in taking the view that diversity is of negligable value in urban forestry, he has used economic rather than ecological arguments in dealing with an ecological issue. Our comments are directed toward clarification of the diversity and stability concepts from a biological perspective - with the intent to express a different and more sympathetic view of the value of diversity to the street tree community. Solutions to practical problems in urban forest management should have a sound scientific basis as well as economic feasibility. To be successful, the perceptions of the taxpayer must be considered equally with the criticism of the specialist. The relationship between diversity and stability is complex and the popular concept as Richards states it, i.e., that diversity results in stability could in many instances more accurately state that stability is the cause of diversity (May, 1976). Ecological concepts, such as diversity and stability that describe the inter-
174 actions within populations and communities and between those entities and the environment, originated from intuitive arguments and observation of how communities function. Additional information has been derived from simple trophic experiments in the laboratory, and the concept is further supported by simplified mathematical models. However, in the application of the argument to urban systems, one must remember that natural selection is often replaced by economic considerations. As Richards noted, management tends to negate natural process. Populations of street trees do not reproduce themselves nor does the street tree community change through the normal successional process. Additional energy in the form of maintenance is essential if street tree populations are to serve their function and remain intact systems. The price we pay for creating our own systems is the cost of continual management. As a biological concept, stability has several meanings. A population is stable if it peAsilts at a c&am level, i.e., resistance or static stability, or if population numbers fluctuate over a greater or lesser period of time but return to an original equilibrium state, it is resilient (Holling, 1974). Unfortunately, the definition Richards has chosen for stability includes both biological and economic constructs and, for this reason the rest of his argument cannot be strictly ecological or biological. Stability, as Richards defines it, describes a population having a low probability that it will experience serious disruption of values and management in the forseeable future. This concept of static stability also implies the energy inputs required to maintain it. His managed population is one that is expected to remain stable over a considerable period of time and to which energy is provided to aid in maintaining that stability. Certainly stable systems may last longer than unstable systems. However, there is more concern here with destabilizing elements that have economic rather than biological implications. We agree that individuals are important in street tree populations and that early mortality must be avoided as must high losses of older individuals within any short time period. Length of life, though, is primarily an economic argument. Short-lived species can produce stable communities if stability is considered in the appropriate time frame with species life span. Likewise, a stressful environment does not necessarily produce instability - another myth introduced into the argument that has nothing in particular to do with diversity. Young trees of new or different species need not necessarily be destabilizing, provided that the species added have a reasonable chance of adapting to the urban environment. We agree, however, that ideas of stability or instability should be viewed in the context of response of individual species populations. The concept of diversity requires discussion. Ecological practice divides diversity into several components, species richness refers only to the number of species present; while evenness describes the relative distribution of individuals among species. This ecological definition of diversity fits more appropriately with Richards welcome suggestion that a variety of age classes is desirable. Richards presents the diversity stability argument in a much too simplified
175
form. It is true that many species in an ecosystem do not necessarily lead to population stability. The original intrinsic appeal that such a causal relationship exists between diversity and stability is understandable given the anecdotal information that appeared to support that view. However, while some ecologists readily accept this idea, many are more likely to dispute it. MacArthur (1955) originally proposed that stability might be a function of the interconnectedness of species in a trophic web. He reasoned that the number of links in the food web would be a means of maintaining stability in a community and suggested that a measure of diversity would be an appropriate means of measuring the number of links in the food web. Subsequently, a large body of evidence accumulated that suggested that diversity led to stability and that less diverse systems are unstable. The desirability of using ecological theory to support intutitive ideas of the functioning of natural systems led to the entrenchment of the idea that complex systems would be more stable than simple systems. May (1976) has suggested that systems evolving in a stable environment increase in diversity whereas systems evolving in unstable environments are less diverse. This says little about the relationship between diversity and stability. Recently, ecologists have criticized the hypothesis that increased diversity implies increased stability (May, 1973; Goodman, 1975; Abrams and Allison, 1982). They point out that many predictions from theory are not confirmed by empirical evidence now available. While this current view runs counter to the more traditional linkage, empirical research is needed to substantiate these ideas (McNaughton, 1977). Diversity should play a role in management planning. Although the hypothesis linking diversity and stability might be false, the policies it promotes are not necessarily unworthy. If one accepts a monoculture as the most efficient system, the management plan must include options for responding to a catastrophic event that may be species specific. Although diversity does not necessarily increase stability it may provide a variety of safety valves for simplified ecosystems. Such systems have not had the opportunity or adequate time for significant interaction between the component species and their pests or pathogens. Biologically simplified systems of one species can become economically unstable because of species specific perturbations (Pimentel, 1961). Richards’ paper contains some contradictory arguments. For example, in his discussion of genetic diversity, he states that the use of cultivars, while desirable for certain landscaping attributes, e.g. uniformity, may limit adaptation to the variety of urban environments encountered. He is probably right that the use of cultivars further reduces the potential for new plantings to contribute to population stability. However, arguing in that direction is arguing that the use of single species is also dangerous because the individuals are too similar. The point is made, but perhaps not sufficiently emphasized, that seedling populations are more desirable than vegetatively reproduced cultivars. Likewise, the abrupt loss of elms as a result of Dutch Elm Disease appears to argue against the generalization that species with proven wide
176
adaptation are more likely to withstand a new stress. Urban elms were chiefly of seedling origin, and hence had considerable intraspecific variability. In this case, the spread of the disease was exacerbated by management techniques, i.e., close planting, etc. As long as one provides adequate variety in age in the plantings, the urban tree planter could utilize species, which although perhaps not adapted to all urban conditions, would provide diversity that might help to cushion a catastrophic event. Much of the rationale for the choice of street trees in the past appears to have been economic, rather than ecological. One may question whether this situation has changed or will change. The idea that diversity can be monitored as a management tool (Sanders, 1981) may well be appropriate provided that diversity is defined as the variety present in numbers of individuals, age-classes and species, rather than as species richness alone. Managers should not strive for a maximum number of species but for a good distribution of adapted species (Levenson and Stearns, 1980). We agree that emphasis on species richness alone is misdirected, however, planting only “proven” species will also leave the manager open to catastrophic loss from unforeseen causes.
REFERENCES Abrams, P.A. and Allison, T.D., 1982. Complexity&ability, and functional response. Am. Nat., 119: 240-249. Elton, C.S., 1958. The Ecology of Invasions by Animals and Plants. Methuen, London, 181 pp. Goodman, D., 1975. The theory of diversity-- stability relationships in ecology. Q. Rev. Biol., 50: 237-266. Holling, C.S, 1974. Reslience and stability of ecological systems. Annu. Rev. Ecol. Syst., 4: l-24. Levenson, J. and Stearns, F., 1980. Application of diversity to regional ecological assessment. A review with recommendations. ANL/AA-21, Argonne National Laboratory, Argonne, IL. MacArthur, R.H., 1955. Fluctuations of animal populations, and a measure of community stability. Ecology, 36: 533-536. McNaughton, S.J., 1977. Diversity and stability of ecological communities: a comment on the role of empiricism in ecology. Am. Nat., 108: 490-498. May, R.M., 1973. Stability and Complexity in Model Ecosystems. Princeton University Press, Princeton, NJ, 265 pp. May, R.M., 1976. Patterns in multi-species communities. In: R.M. May (Editor), Theoretical Ecology: Principles and Applications. Saunders, Philadelphia, PA, pp. 142-152. Pimentel, D., 1961. Species diversity and insect population outbreaks. Ann. Etomol. Sot. Am., 54: 76-86. Sanders, R., 1981. 33. 43.
Diversity
in the street trees of Syracuse,
New York.
Urban Ecol.,
5: