Executive Summary
SCOPE 55 Functional Roles of Biodiversity: A Global Perspective, H.A. Mooney, J.H. Cushman, E. Medina, O.E. Sala, and E.-D. Schulze eds., 1996, 493 pp.
This file includes:
Chapter 1. The SCOPE Ecosystem
Chapter 17. What We Have Learned about the Ecosystem Functioning of Biodiversity
1. The SCOPE Ecosystem
Functioning of the Biodiversity Program
H.A. Mooney, J. H. Cushman, E. Medina, O. E. Sala & E.-D. Schulze1.1 BACKGROUND
As natural ecosystems are increasingly impacted by human activities, resulting in disruptions of system interactions and losses of populations, and even species, there has been increasing concern about how we are modifying the ecosystem processes that originate from and maintain these systems, and that benefit humankind. The Scientific Committee on Problems of the Environment (SCOPE) launched a program in 1991 to assess the state of our knowledge of the role of biodiversity, in all its dimensions, in ecosystem and landscape processes. This effort was part of the larger program, DIVERSITAS, which focuses on the science of biodiversity and was initially cosponsored by SCOPE, the International Union of Biological Sciences (IUBS) and UNESCO (United Nations environmental, Scientific, and Cultural Organization). The SCOPE program was guided by a Scientific Advisory Committee that included David Hawksworth, Brian Huntley, Pierre Lasserre, Brian Walker, Ernesto Medina, Harold Mooney, Valeri Neronov, ErnstDetlef Schulze and Otto Solbrig.
The overarching questions that were agreed upon for this program were:
1. Does biodiversity 'count" in system processes (e.g. nutrient retention, decomposition, production, etc.), including atmospheric feedbacks, over short- and long-term time spans, and in face of global change (climate change, land-use, invasions)?
2. How is system stability and resistance affected by species diversity, and how will global change affect these relationships?
The SCOPE program was designed not only to synthesize our knowledge for the functional role of biodiversity, but also to develop the basis for an experimental program for inclusion in the International Geosphere Biosphere Program. As is discussed in Chapter 17 (Conclusions), the information base from which we build was not especially designed to answer the questions posed above. Until recently, and with exceptions in part due to stimulation from this program, there has been virtually no experimentation in this rather central area. One reason for this has been the past separation of the research areas of population ecology and ecosystem ecology, a separation which this program attempted to bridge.
In the following sections we outline the structure of the SCOPE program, followed by a description of an expansion of the program under the auspices of the Global Biodiversity Assessment conducted by the United Nations Environmental Program.
1.2 THE SCOPE PROGRAM
The SCOPE program consisted of a series of activities between 1991 and 1994, culminating in an overall synthesis meeting at Asilomar, California, in 1994. The program was launched in October 1991 with a meeting on background issues held in Bayreuth, Germany. This meeting brought together ecologists and population biologists, both directed toward evaluating the consequences of human-driven disruption of natural systems. In particular, there was an examination of the degree of redundancy within systems, the ubiquity of keystone species, the tightness of species interactions (from mutualisms to food webs), the resilience of system to perturbations, and the interaction of landscape units. The few direct studies on species numbers and ecosystem function were evaluated. The interaction of policy and science in this area was also explored. The highlights of this meeting were described in Chapin el al. (1992) and the full results in Schulze and Mooney (1993).
The second phase of the program consisted of a series of meetings focusing on specific biotic regions of the world. These meetings took place during 1992-1993. The regions selected represent particularly critical areas in terms of threats to diversity losses, or are particularly sensitive to global change effects, or are especially amenable to experimentation. The same issues were discussed for each system, as noted below, in order to get uniform treatment of the nature of the diversity of that system, how that particular system is being modified, and the potentially differential structural/functional relationships among systems. However, the reality of the information available meant that not all issues were necessarily discussed, or if they were the discussion was not even across systems. It will be seen that, as always, the material available determines the structure and hence the diversity of ways of approaching the same theme.
SCOPE ECOSYSTEM FUNCTIONING OF BIODIVERSITY PROGRAM
Each regional symposium was designed to address the following issues:
1 Natural diversity of systems
Species
Populations
Functional groups
Systems
Landscapes2. Impact of change on diversity
Climate and atmosphere
Land use
Invasions3 Assessing diversity role on ecosystem function
Additions (invasion analog)
Subtractions (harvesting, disease, etc.)
Fragmentation
Disturbance4. Reconstructing and maintaining diverse systems
5. Refining our knowledge through Explicit experiments Long-term observations
Thus, from the start, the focus on the program was on all elements of biodiversity-, not just species, although species were, without question, the focus of the work since this is where the greatest information is available, and further, where the most concern has been voiced. Global change effects were addressed in their full context, i.e. land-use change, atmospheric change and invasions, rather than concentrating on a single driver, e.g. climate as is often the case.
Since little experimentation is available, as noted, surrogates were utilized for these in the syntheses. For example, biotic invasions can be considered a surrogate for experiments on the addition of biotic diversity to a system, just as selective harvesting in forestry or species specific lethal diseases can be considered as experiments on biotic subtractions from ecosystems. However, in the case of surrogates there are usually no control measurements, nor are ecosystem functional responses necessarily measured. The main objective was to lay the groundwork for a better database for the future based on experimentation.
The greatest challenge facing the science and practice of ecology today is developing the tools to reconstruct, or repair, ecosystems that have been degraded through human activities. This research area is still poorly developed and needs considerable attention. The basis for this science lies in the kind of material discussed in this book - what species and in what combinations provide the greatest ecosystem services? What sort of species representation is needed to ensure stability in face of fluctuating climates?
The biotic regions that were selected, on the basis of the criteria noted above, were:
Estuaries, lagoons and mangroves
Mediterranean systems
Islands
Boreal forest
Tundra
Coral reefs
Savanna
Coastal systems
Tropical forest
Lakes and rivers
Temperate forest
Arid zonesNote that although most of the above can be considered a biome type, islands of course are representative of most of the biomes. However, they are special in view of their generally relative simplicity and because of the disproportionately high human impacts they have received.
To produce some of these assessments full-scale symposia were held that included a large number of experts. In these cases a system-specific book was produced on these systems, as happened with islands, Mediterranean systems, Arctic and alpine areas, savannas and tropical forests, as noted below. The chapters in this volume represent condensations of the fuller treatment contained in these books. The other systems were assessed by small groups of experts, as indicated in the authorships of these chapters. Representatives of all these systems met in Asilomar, California, in 1994 for a final discussion of the material and for cross-system comparisons (Baskin, 1994).
1.3 THE GLOBAL BIODIVERSITY ASSESSMENT
The SCOPE program was expanded somewhat following initiation by the United Nations Environmental Program (UNEP) of a Global Biodiversity Assessment (GBA). In mid-1993, a group met in Trondheim, Norway, to prepare an outline of such an assessment. It was decided that the SCOPE effort (as well as other DIVERSITAS components) would be incorporated into the GBA, as noted in the publications below. The GBA is intended to provide the scientific underpinnings for the Biodiversity Convention.
The constraints on space for the Global Biodiversity Assessment meant that each system could only receive a few pages of text. Thus the material had to be greatly condensed and tightly structured For each system a number of ecosystem processes or properties were considered, and the human impacts on them were described, and the ecosystem consequences of these impacts were assessed. There were then comparisons across systems for commonalities in responses. These assessments were reviewed by a large international peer group and their comments incorporated. Since the initial and amended program all represent a single effort to understand the consequences of a change in diversity on ecosystem services, we take our concluding chapter from all of them. Thus the information gathered for this SCOPE program is held at several levels of detail. First the system specific volumes noted above, this volume, which has lengthy considerations of a larger set of biomes, and then the GBA which has highly condensed considerations of an even larger set of biomes.
We gratefully acknowledge support for this program from the John D. and Catherine T. Mac Arthur Foundation, the A.W. Mellon Foundation, the European Commission, and from the United Nations Environment Program (UNEP).
1.4 PROGRAM PUBLICATIONS
Published
Baskin, y. Ecologists dare ask: How much does diversity matter? Science 264:202-203.
Chapin, F.S. III and Körner, Ch. (Eds) U9) Arctic and Alpine Biodiversity: Ecosystem Consequences in a Changing Climate. Springer, Berlin, 323 pp.
Chapin, F.S., III., Schulze, E.-D. and Mooney, H.A. (1992) Biodiversity and ecosystem processes. Trends Eco. Evol. 7: 107-108.
Davis, G.W. and Richardson, D.M. (Eds) U995) Mediterrean- Type Ecosystems: The Function of Biodiversity. Springer, Berlin, 366 pp.
Hobbs, R.J. (1992) Biodiversity in Mediterranean Ecosystems of Australia. Surrey Beatty, Chipping Norton, Australia, 246 pp.
Mooney, H.A., Lubchenco, J., Dirzo, R. and Sala, 0. (Eds) (1995) Biodiversity and Ecosystem Function: Basic Principles. In UNEP Global Biodiversity Assessment. Section 5. Cambridge University Press, Cambridge.
Mooney, H.A., Lubchenco, J., Dirzo, R. and Sala, 0. (Eds) (1995) Biodiversity and Ecosystem Function: Ecosystem Analyses. In UNEP Global Biodiversity Assessment. Section 6. Cambridge University Press, Cambridge.
Orians, G., Dirzo, R. and Hall, J. (Eds) (1996) Ecosystem Function of Biodiversity in Tropical Forests. Cushman, for Springer, Berlin, 229 pp.
Paine, R.T. (1995) A conversation on refining the concept of keystone species. Conserv. Biol. 9: 962-964.
Richardson, D.M. and Cowling, R.M. U993) Biodiversity and ecosystem processes:Opportunities in Mediterranean-type ecosystems. Trends Ecol. Evol. 8: 79-80.
Schulze, E.-D. and Mooney, H.A. (Eds) (1993) Ecosystem Function of Biodiversity. Springer, Berlin, 525 pp.
Solbrig, O.T., Medina, E. and Silva, J.F. (Eds) (1996) Biodiversity and Savanna Ecosystem Process: A Global Perspective. Springer, Berlin, 233 pp.
Vitousek, P.M., Loope, L.L. and Adsersen H. (Eds) (1995) Islands. Biological Diversity and Ecosystem Function. Springer, Berlin, 238 pp.
SCOPE 55 Functional Roles of Biodiversity: A Global Perspective, H.A. Mooney, J.H. Cushman, E. Medina, O.E. Sala, and E.-D. Schulze eds., 1996, 493 pp.
In Press
Smith, T., Shugart, H. and Woodward I. Plant Functional Types. Cambridge University Press. (SCOPE and IGBP-GCTE were joint sponsors of this activity).
Chapter 17. What We Have Learned about the Ecosystem Functioning of Biodiversity
H.A. Mooney, J. H. Cushman, E. Medina, O.O E. Sala & E.-D. Schulze17.1 BACKGROUND
Here we summarize the contents of this book as well as the results of the overall SCOPE Ecosystem Functioning of Biodiversity Program and the Global Biodiversity Assessment (GBA) (UNEP 1995). This book gives in-depth syntheses of the ecosystem functioning of biodiversity for a number of the worlds' major biomes. A number of biomes (tropical, Orians et al. 1996; savannas, Solbrig et al. 1996; Mediterranean, Davis and Richardson 1995; arctic and alpine, Chapin and Körner 1995; those found on islands, Vitousek et al. 1995) are covered in even greater detail in the specific volumes cited above. The GBA, in contrast, treats these same ecosystems, plus a few others, in a highly condensed form that facilitates cross-biome comparisons.
We start this chapter with the summary statements of the overall program that are taken directly from the GBA Sections 5 and 6. Documentation for these conclusions are contained in the GBA volume. These summaries noted that understanding the role of elements of biodiversity in the functioning of ecosystems is a relatively new research endeavor that addresses the structural and functional properties of ecosystems, and the degree of sensitivity of these properties to changes in the underlying diversity. Understanding the functional role of biodiversity has crucial implications for the management of the Earth System. Valuable scientific principles and guidelines for making ecosystem management decisions are beginning to emerge in spite of the field's youth and the relatively small number of experimental studies from which it draws. These emerging principles are embodied in a series of statements that deal with the importance of diversity at different levels of integration.
17.2 GENERAL PROGRAM CONCLUSIONS
1. The loss of genetic variability within a population of a species of a given area can reduce its flexibility to adjust to environmental change and narrow the options for adjustments to climate change, for example, as well as for rehabilitating specific habitats.
2. The addition or deletion of a species can have profound effects on the capacity of an ecosystem to provide services. We are beginning to develop the potential to predict which species these will be. They are those with unique traits within an ecosystem for fixing nitrogen, capturing water, emitting trace gases, causing disturbance and so forth. We can predict the consequences of their removal or addition a priori. Although the success of an alien species in a new habitat may be difficult to forecast, its impact on ecosystem functioning upon establishment can be predicted based on whether the new species utilizes or produces a unique resource. Certain species, without readily recognized traits, when deleted can have profound effects on ecosystem functioning. These are so-called "keystone" species and at present, due to our lack of a general theory, their potential effects on removal can only be assessed by direct experimentation.
3. Recent studies are confirming the proposition that the capacity of ecosystems to resist changing environmental conditions, as well as to rebound from unusual climatic or biotic events, is related positively to species numbers.
4. The simplification of ecosystems in order to produce greater yield of individual products comes at the cost of the loss of ecosystem stability and of such free services as controlled nutrient delivery and pest control, which thus needs to be subsidized by the use of fertilizers and pesticides.
5. Certain ecosystems, such as those found in arid regions and on islands, appear particularly vulnerable to human disruptions and hence alteration of their functioning. These sensitive systems all have low representation of key functional types (organisms that share a common role).
6. Fragmentation and disturbance of ecosystems and landscapes have profound effects on the services provided, since they result in shifting the balance of the kinds of species present-from large, long-lived species to small, short-lived ones. These shifts result in the reduction of the capacity of these systems to store nutrients, sequester carbon and provide pest protection, among other things. Ecosystems, and the services they provide, must be considered in a total landscape context, and in some cases even on an intercontinental basis.
7. We have been more successful in simplifying than in reconstructing ecosystems. Our lack of success in ecosystem restoration suggests the need for great caution in reducing biodiversity through management practices because of the potential 1055 of goods and services over the long run. As society exerts ever greater control and management of the ecosystems of the world, great care must be taken to ensure their sustain-ability, which is in large part due to the buffering capacity provided by biotic complexity.
17.3 LESSONS FROM SPECIFIC BIOMES
Information from specific biomes, as well as comparisons among them, give us insights into diversity/functional relationships. Below we note observations from particular ecosystems, taken from this volume, that particularly illustrate specific biodiversity issues. The order of the systems discussed here differs from that of the text since here they are organized by specific lessons learned.
17.3.1 Mediterranean ecosystems
On disturbance/diversity/functioning It has been well documented that disturbance, at a moderate level, can promote diversity. In the Mediterranean basin, human-induced landscape variation - fields, pastures, scrublands and forests - leads to high diversity at all levels as well as to multiple services to humankind. Even complex manipulation of ecosystems, such as in the "dehesa" agro-ecosystem in southern Spain, has provided a diversity of organisms and services and has been sustainable, at least until recently. Contrast this with the recent dramatic massive conversion of the native vegetation of western Australia to wheat fields, resulting in large alterations of ecosystem functioning, particularly related to water balance. This conversion has resulted in salinization of soils and losses of many services, including nutrient supply, and pest and erosion control. There are now efforts to repair the damage by reintroducing perennial systems. In the Mediterranean case, in a sense, there has been adaptive management practices through the centuries - with any management experiment being small-scale because of the lack of the mechanical means to do otherwise, as well as the complex land tenure system which also led to small-scale, and patchy, alterations. In western Australia the recent development of this region was rapid and extensive, aided by fully mechanized conversion of vast areas. By the time the problems were recognized, extensive damage had already been done.
The lesson is that even in a world that is increasingly impacted by human activities, we can manage landscapes to produce sustainable ecosystems that provide ample services, but there is a considerable challenge in doing so. The examples are there: we need to learn from them.
17.3.2 The open ocean
Managing in the dark We lack details on the ecological structure and function of the open-ocean ecosystem, and for good reasons. This system is vast and difficult to study. We are only now learning of the rich diversity of the benthic system. In general, we have very little understanding of the interactions of the components of the open oceans. This lack of knowledge has put us all at peril, as evidenced by the sudden and dramatic decline of the oceans fisheries. The drivers of this demise are no doubt complex, but most certainly includes the overfishing made possible by "industrial" harvesting. There is little understanding of the ecosystem consequences of the demise of specific fisheries, since research has generally been more commodity-based (a particular fish) rather than system-based. It could be that total ocean productivity has not declined, but that there has only been a shift in the abundances of various species. We do not know. It is stated in this volume that there may not be as much functional equivalency in the oceans as there is on the land, and thus the possibilities for functional replacement are low, for functions other than production. This is an important proposition that needs further study. We are already seeing a great interest in a new approach to fisheries management. It is quite clear that this new approach will, for the first time, be imbedded in an ecosystem paradigm, where functions and services are considered, and where humans are considered an increasingly dominant element in this ecosystem.
17.3.3 Tropical forests
The time dimension Tropical forests illustrate the importance of the time dimension in considering the roles of species in ecosystem functioning. Through selective harvesting of plants and animals we have performed many "experiments" to test the role of various species and functional groups in the functioning of total ecosystems. However, interpreting the results of these experiments may take a long time, since many organisms live for centuries and many ecosystem processes have very long time constants, e.g. soil formation. Tropical forests provide a good example of this. As pointed out in the tropical forest chapter, only 50 tree generations have elapsed since the last glacial retreat, when temperatures were considerably cooler. In much more recent time there has been a massive, and selective, harvesting of the large mammals of the tropical forests by humans. Because of the long lifespan of the dominant trees, we may not readily see the dramatic and long-term impact of the shifting balance of herbivores and carnivores in this system on plant reproductive biology, and hence the structure and function of the forest.
17.3.4 Mangroves
The link between the land and the sea Mangrove systems illustrate many dimensions of the diversity-function relationship. At the landscape level these systems represent a crucial link between the land and the sea. On the one hand, they protect the land from erosion induced by storms, and on the other they provide the foundation, in terms of nurseries, for many fisheries that lie off the coast. The amount of destruction of mangrove forests is staggering in many parts of the world, and represents severe losses of the multiple services that these systems provide. The mangrove systems provide a particularly good test system for refining our knowledge of the relationship of ecosystem functioning and biodiversity. Along the east coasts of Australia, for example, mangrove forests exist with over 30 dominant species. Going eastward, along the islands of the South Pacific, mangrove plant diversity declines progressively until only one species is found in Samoa. This striking gradient is apparently driven by dispersal distance from the mainland. The climate does not vary much along this tropical longitudinal gradient. We do not yet know how system functioning responds to this loss of diversity, or if the diversity of other components of the ecosystem scale in the same manner as the dominant plants. These systems provide abundant material for examining the role that particular keystone organisms play in regulating decomposition. In many regions, crabs apparently play a central role in the initial shredding of litter, whereas in other regions gastropods play this role.
17.3.5 Agroecosystems
On simplification and substitution Agroecosystems provide a particularly good example of how we have substituted the services provided by natural ecosystems for those provided by organisms of particular interest to humankind. It would seem, on the face of it, that comparisons of the diversity-function relationship would be easy between natural and managed ecosystems. However, in agroecosystems, no matter how simple or intensive, the services lost, such as nutrient and water regulation, are compensated for by human-provided substitutions, often at a considerable energy cost.
The data available suggest that, for a given function such as productivity or organic matter accumulation, it does not take many species to provide full services. However, the few analyses available are generally unidimensional in nature and do not consider all functions and their interactions in a system context at a given time, much less through time. Clearly, there is ample opportunity to explore more fully the role of genetic, species and landscape diversity in ecosystem functioning in agroecosystems versus natural systems. One message that emerges clearly is that in agroecosystems, landscape diversity is an essential component of sustainability.
17.3.6 Island ecosystems
On simplicity due to dispersal Islands, of course, do not represent a special ecosystem type since virtually all of the world's major ecosystems can be found on islands in one place or another. What is special about them is that they generally represent a special case of any ecosystem, in that they are simpler than their continental counterparts. This generally means that they have fewer representatives of a given functional group, or may even have whole groups missing. It is thus not surprising that islands are of particular value in studying the role of species in ecosystem functioning. Most of our knowledge on these issues come not from observing deletions, or species extinctions, but from documenting the effects of additions brought about by successful invasions. Most of the spectacular cases of deletions, such as flightless birds, have preceded the era of scientific inquiry. On the other hand, there are a number of well-studied examples of the ecosystem consequences of relatively recent species additions which have shown the dramatic effects that can result, particularly if the addition represents a new functional type. It is clear that islands will most certainly be utilized as the testing ground for emerging hypotheses on the role of diversity and function.
17.3.7 Cold and dry ecosystems
On simplicity due to limiting water Arid ecosystems share with islands the characteristic that they have few representatives of any functional group. In this case, however, climatic severity rather than dispersability is the filter on diversity. The results are apparently the same, however. Some of the most dramatic examples of the consequences of species removals and additions come from these arid systems, where resulting major shifts in functioning have been documented with the alteration of species composition. Also, the fact that the structural dominance of desert ecosystems is dependent on only a few species makes any loss result in cascading effects on the whole system. The removal of a single arboreal species, or a shift from grasses to a single tree being dominant, totally alters the structure and function of the entire ecosystem.
On simplicity due to cold temperature As in deserts, arctic and alpine systems have both structural and taxonomic simplicity. Because of evolutionary constraints, entire functional groups are missing from the extremes of cold-dominated ecosystems. Humans have also been responsible for deletion of many of the large grazers. The arctic, because of the ease of performing certain types of ecosystem manipulation, has been an important testing ground for the diversity-function issue.
17.3.8 Lakes and rivers
Responses to massive impacts Fresh water bodies, and the organisms that inhabit them, have been impacted more by humans than virtually any other system on Earth. In a direct sense humans compete, and win, against organisms for limiting fresh water. Rivers are massively dammed or diverted, and lakes are extensively utilized for recreation. The biotic composition of water bodies has been greatly impacted by these activities, as well as others which include deliberate biotic introductions and the effects of pollutants, including acid deposition.
Lakes in particular provide excellent examples for examining the consequences of changes in biotic composition on ecosystem functioning. They are relatively clearly circumscribed systems, and limnologists, by training, generally have a more holistic view of their systems than terrestrial ecologists. Many important insights about ecosystem functioning and population dynamics have come from lake studies.
Chapter 12 in this volume illustrates these issues and system advantages. The enormous and complex impact of a single species addition, such as the cases of the opossum shrimp and the Nile perch, has been well documented. These effects have been mainly through food web alterations, or trophic cascades. At the same time, lake systems have been shown to undergo dramatic shifts in species structure under stress conditions, and yet certain ecosystem processes, such as primary productivity, have shown little change at first because of species compensations.
Because of the ease of experimentally manipulating lake systems, they offer particularly powerful models for deriving general rules of where species additions or deletions will, or will not, have a major impact on ecosystem processes. Unfortunately, we already have thousands of uncontrolled experiments in progress on biotic additions and deletions to lake and river systems, the consequences of which are poorly monitored. We should certainly make the effort to remedy this as soon as possible.
17.3.9 Coastal ecosystems
Keystones and compensations It was in an intertidal system that the presence of a keystone species was first experimentally demonstrated. Since then, many other examples of keystones have been illustrated, and extensions of the concept have been made. One important finding is that the role of a species may vary along with its distribution; it may play a strong keystone role in one place but not in another, since the complex of associate species changes in widely distributed species. Also, in intertidal systems it has been shown how humans themselves play a keystone role. Where keystones are lacking, species compensations are evident, with function being maintained after species removals by replacement of the activity by the remaining species.
17.3.10 Coral reefs
Complexity Coral reefs represent a remarkable collection of organisms, many of which have co-evolved commensal relationships. Thus it is no surprise that dramatic instances of major ecosystem rearrangements have been noted by either the deletion or increase of one species or another. These systems also provide strong support for the notion of the cascading influence of the loss of a single guild, such as algal grazers, on the health of entire coral systems, and in turn on the loss of such ecosystem services as coastal protection and attributes of interest to tourists. Since these systems are bathed in water and colonized by larvae, the distances between reef systems and the currents between them are crucial. The importance of virtually all dimensions of biodiversity, from genes to seascapes, is readily demonstrated in coral ecosystems.
17.3.11 Boreal forests
Low diversity and low redundancy Boreal systems illustrate many aspects of diversity-function relationships. Low species richness translates into low representation in any functional type. Thus, the impact of the removal of any single species can be great. No doubt the characteristic boom and bust cycles of many animal grazers in these systems is related to system simplicity, and thus intrinsic instability. There are many examples of large influences by single species, not only on local systems but on whole landscapes, as in the case of beaver. Also remarkable because of system simplicity is the dramatic ecosystem impact of a single trait within the small functional group of tree species, depending on whether they are evergreen or deciduous. It is the boreal forests, along with the deserts and tundra, which provide the best evidence for the importance of functional group diversity in maintaining ecosystem stability.
17.3.12 Temperate and tropical grasslands
Where experiments are most tractable Temperate grasslands have provided most of the available experimental evidence on the relationship between species diversity and ecosystem functioning. The relatively short life-span of grasses or their small size may partially explain the concentration of manipulative experiments in this biome. Results of these experiments, in conjunction with new conceptual models, suggest ways of predicting the effects of different species on ecosystem functioning. A common feature of most ecosystems is that a few species account for a large fraction of a given ecosystem process (e.g. primary production), but account for a small fraction of system diversity. Removal of grass species has a different impact on ecosystem functioning depending on the abundance of the removed species in the original ecosystem. Removal of subdominant species is generally compensated by the remaining species, but removal of the dominant species does not result in full compensation, at least in the short time-span of most experiments.
Grasslands have also provided experimental evidence for the relationship between species diversity and ecosystem stability. Long-term monitoring of a large set of grassland plots with differing diversities, in conjunction with the impact of a severe drought, provided the evidence to show the importance of species diversity on ecosystem resistance and resilience. The most diverse plots showed the least reduction in productivity during the drought, and were the plots which recovered their full capacity the fastest.
It was in the savannas of Africa that the first good evidence was gathered showing the importance of species richness to ecosystem resilience. The large numbers of species within a single functional type, grasses, provides enormous buffering against environmental perturbation. It is also in savannas that a clear differentiation in the ecosystem role can be seen among certain functional types, such as in the deep-rooted but sparse trees and the continuous cover of shallow-rooted herbs.
17.3.13 Temperate forests
Diversity and function over evolutionary time The temperate forests of the world are remarkable in that each continent has not only different species dominating them, as would be expected, but also a large difference in the numbers of dominants they have, which is related to the glacial history of these continents. The temperate forests of China have the greatest number of dominants, followed by the northeastern United States and then western Europe. The slim evidence we have now would indicate comparable flux rates of water and nutrients, as well as other functional similarities. Thus, in evolutionary time, comparable growth forms will utilize all of the available resources. The SCOPE project, however, focused on the impacts of humans on diversity - and the results of these perturbations - a very different issue from evolutionary niche partitioning. Results from other biomes predict that the responses to, and recovery from, species losses would potentially be greatest in the less-rich forests of Europe. We are now seeing in Europe a very substantive shift in species composition due to the effects of nitrogen and acidic deposition. These shifts are resulting in a breakdown of ecosystem functioning, with consequent resources being lost.
17.3.14 Concluding remarks
This assessment has utilized the available information, most of which is primarily observational. There is no doubt that in the years ahead greater insight and more substantive information will be brought to bear on the central question of the role of diversity in ecosystem functioning and stability. The International Biosphere Geosphere Programme has taken on the task of bringing experimentation and more organized observations to this research field. SCOPE will continue to work on the assessment, concentrating now on the poorly understood role of soil and sediment biotic diversity on ecosystem functioning. The end of this book, then is in a sense the beginning of a new research field. We add one note of caution for future workers in this field. There is no substitution for the power of experimentation. At the same time, certain issues and phenomena, mostly relating to large time and spatial scales, are just not amenable to experimentation. It is these areas where we will have to continue to utilize historical reconstructions and develop a capacity for modelling.
REFERENCES
Chapin, F.S. and Körner, C. (Eds) (1995) Arctic and Alpine Biodiversity. Vol. 113 Springer, Berlin.
Davis, G.W. and Richardson, D.M. (Eds) (1995) Mediterranean-Type Ecosystems:
The Function of Biodiversity. Springer, Heidelberg.Orians, G.H., Dirzo, R. and Cushman, J.H. (Eds) (1996) Biodiversity and Ecosystem Processes in Tropical Forests. Springer, Berlin.
Solbrig, O.T., Medina, E. and Silva, J.F. (Eds) (1996) Biodiversity and Savanna Ecosystem Process: A Global Perspective. Springer, Berlin.
United Nations Environment Programme (1995) Global Biodiversity Assessment. Cambridge University Press. Cambridge.
Vitousek, P.M., Loope, L.L. and Adsersen, H. (Eds) (1995) Islands. Biological Diversity and Ecosystem Function. Vol. 115. Springer, Berlin.
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