This Handbook owes its origins to a Workshop jointly sponsored by the United Nations Educational, Scientific and Cultural Organization (UNESCO) Man and the Biosphere (MAB) Programme, and by the Scientific Committee on Problems of the Environment (SCOPE) of the International Council of Scientific Unions (ICSU). The Workshop was concerned with dynamic changes in ecological systems, with special reference to modern concepts relating to such changes, the techniques available for their study, and the degree to which such concepts and techniques could contribute to basic ecological research, and to the management of natural and semi-natural systems.
Five recommendations relating to research on dynamic changes in terrestrial ecosystems, and to the application of the results of such research to problems of resource use and land management, resulted from this Workshop, which was held in Santa Barbara, California, in January 1976 (Slatyer, 1977). These recommendations may be summarized as follows.
Further work on ecological succession should be encouraged, with a view to providing a thorough understanding of successional dynamics, and the interrelationship between successional processes and other community attributes associated with stability, diversity, dominance and resilience. In particular, more information is needed about the biological processes involved in truncated succession, and its associated characteristics.
Specific studies of succession in a range of ecosystems, and following
a variety of different types of perturbation, should be conducted, with a view
to providing the data necessary for the construction of comprehensive ecological
and management models. Ideally, these studies should be conducted in areas where
most of the key ingredients for model construction are already available, and
where a team of ecologists and managers could be assembled.
A practitioner's manual should be prepared to outline the techniques
available for the modelling of successional processes, the data requirements
and limitations of each of the major techniques, and methods of applying them
to ecological processes and to the management of semi-natural communities.
In association with (ii) and (iii) above, the opportunity should be taken to increase the biological realism and practical value of the various modelling approaches. It was, therefore, envisaged that research would be required both for the application of existing models and for the development of more effective modelling procedures.
Wherever possible, ecologists and managers should be located, geographically and institutionally, where they can interact to develop ecologically sound management strategies for natural, semi-natural and cultivated ecosystems. In turn, greater opportunities need to be provided for the ecologists to interact with specialists in experimental design, data analysis and interpretation, and in model building. Similarly, greater opportunities need to be provided that influence his selection of management goals and constrain his management strategies.
Rapid advances have been made in recent years, in both theory and experiment, on the general level of understanding of ecological succession. In particular, a much better appreciation now appears to exist of the interplay between different successional patterns, the effect of perturbations on the observed patterns, and on the phenomenon of successional truncation in which long-term site dominance is achieved by species which might normally be assumed to be early-successional, sub-dominant species.
The pattern of succession, in terms of the species present on a site, and their relative abundances, at various periods after a disturbance, is basically dependent on the product of two probabilities. The first of these probabilities is that of a propagule being available at the site under consideration, and the second is the probability of it being able to develop to reproductive age and complete its life cycle. Each probability can be affected, to a different degree, by the frequency, intensity, and scale or area of present perturbations. Knowledge of successional pattern can provide a useful basis for developing models of direct relevance to the management of semi-natural communities such as forests, range-lands, and lands set aside as national parks, biosphere reserves and other types of protected ecosystems. Conversely, patterns of natural succession are of very limited relevance to the problems of managing ecosystems in which human intervention has markedly altered the physical or chemical environment and the composition of plant and animal species.
In contrast, managers of semi-natural and cultivated ecosystems need an understanding of the dynamics of the systems they manage in order to predict the effects of various treatments that they apply to improve the yield of harvestable plants or animals. In particular, these managers need to be able to predict the long-term effects of intervention or perturbations on the fertility of the soil, on the dynamics of the interactions between the plants and animals of the system, and between plants, animals of the system, and between plants, animals and the physical environment. It is relatively easy to allow apparent short-term gains to mask long-term harmful effects due to the depletion of essential mineral elements, the accumulation of toxic substances in soils or in organisms, or disturbance of predator-prey relationships that lead to the extinction of either prey or predator. The study of dynamic change is, therefore, of equal importance to the managers of cropped semi-natural and cultivated systems.
In recent years, the application of powerful modelling procedures to ecological processes has stimulated research into ecological succession and into the possibilities of providing a sound ecological basis for the development of management strategies for natural, semi-natural and cropped ecosystems. These techniques have potential application to a wide spectrum of ecological problems and to the management of forests, range-lands and related systems. However, the effective application of ecological knowledge to problems of ecosystem management requires the utilization of a variety of information __ ranging from historical records, the research data, to observations obtained by direct management __ and also requires a high degree of interaction between the ecologist on the one hand and the manager on the other.
This Handbook has been designed to provide a ready source of reference to both ecologist and manager for the variety of techniques available for the modelling of dynamic change in ecological systems. Because of the different objectives and the variation in availability of data associated with different studies, and because of the characteristics of the techniques, no single approach can generally be recommended. Moreover, because some of the advantages and disadvantages of the major classes of models are complementary, there is generally something to be gained by using a combination of several approaches. As will be stressed later in the Handbook, the modelling of dynamic change needs to be carefully embedded in an overall systems analysis of the problem, and the objectives of the study must be defined with care. Techniques suited to the simulation of ecological processes for the purpose of understanding the processes may be quite inappropriate when applied to problems of direct management, and vice versa.
Alternatively, a technique may also need to be modified because of limitations in the data available. In order to improve both the general level of understanding of dynamic change in ecological systems, and the effectiveness of the methods used to explore them, well-designed surveys and long-term experiments are frequently needed to provide data sets for model generation and testing. Such research involves a recognition that it is the change in ecosystem parameters that needs to be measured rather than the state at which an ecosystem finds itself at any particular time. In the past, ecological research has tended to concentrate on static rather than dynamic models. This Handbook, therefore, ventures into relatively new ground.
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