SCOPE 34 - Practitioner's Handbook on the Modelling of Dynamic Change in Ecosystems

2.1 DEFINITION OF ECOLOGICAL SYSTEMS

As early as 1935, Sir Arthur Tansley suggested the term 'ecosystem' should be used to describe' ...not only the organism complex but also the whole complex of physical factors forming what we call the environment' (Tansley, 1935). This concept of the interaction of living organisms with the physical and chemical factors of their environment has proved to be one of the most important ideas of science, and ecological systems have become the basic units for studies of the interactions between organisms or between organisms and their environment. Various ways of ordering and studying ecological systems have been suggested, dependent on the scale of investigation and the main focus of attention in the research. The role of any organism depends on its place in the ecosystem, and our ability to manipulate or conserve organisms, communities of plants and animals, or whole ecosystems, depends on our understanding of the complex interrelationships between them.  

Particular interest has always been expressed in the ways in which organisms combine in communities which are characteristic of a particular type and place, and which reflect past and present land use. Plant communities and associations over the world as a whole can be broadly classified in biomes. Biomes embrace the major vegetation types of the world, and, within broad limits, have a characteristic productivity. Udvardy (1975) has extended this classification into biogeographical realms, biogeographical provinces within the realms, and major biomes or biome complexes. Biome classifications of this kind, combined with hypotheses about the effects of climatic factors, facilitate comparisons of productivity at regional and world scales.                                   

Measurement of changes in biomes provides information on the effects of major influences such as the clearing of forest, desertization, reafforestation, etc., which are usually a combination of man's activities and climate. Within communities, changes in populations of organisms reflect the responses of communities to climate, to modification by man, and to the natural processes of succession through which one complex of organisms gives way to another , leading ultimately to a climax or truncated climax community. Dynamic changes within communities occur at different rates at different localities, so that variations occur from place to place even within the same communities. Spatial and dynamic heterogeneities, therefore, have a marked effect on the  patterns of change in ecological systems. It is essential to develop models or analogues of ecological systems which are capable of representing spatial and dynamic heterogeneity so as to avoid assumptions of homogeneity which cannot be substantiated. Changes in populations are measured by comparing numbers of organisms at particular points of time and space, sometimes distinguishing between stages in the development of the organisms, e.g. eggs, larvae, pupae, adults, etc., or between sexes and age classes. In more detailed studies, it may be possible to follow particular marked individuals at various points in time, and so determine patterns of distribution, survival, reproduction, feeding and death.

An alternative to the study of an ecosystem by investigating changes in populations and communities of organisms is the tracing of flows of energy, nutrients and pollutants through the system. Radiant energy from the sun is trapped by green plants and combined with various chemical elements to form organic compounds which enable all the essential properties of life to proceed in living organisms. Such organisms can be classified according to their trophic levels, i.e. by their mode of nutrition. Green plants which obtain their energy from the sun are the primary producers and form the first of these trophic levels. The herbivorous animals which range from minute invertebrates to large mammals feed on these living plants, and are therefore described as being primary consumers; they occupy a second trophic level. Other animals prey upon the herbivores and form a third trophic level and, in turn, these carnivores are preyed upon by top or predatory carnivores to produce a fourth trophic level. The decomposers and detritus feeders form yet another trophic level, which accepts the residues from the other levels and turns them back into nutrients to be used by the primary producers, together with the sun's energy, to store new energy for the whole ecological system.                                  

All ecosystems possess a characteristic trophic structure, and may therefore be studied by the investigation of that structure. In most systems, the only external source of energy is the sun, but, for many sub-systems, energy enters in the form of live or dead organisms, or in the form of decomposed organisms from another system. The energy is then used by organisms for synthesizing new compounds for growth and reproduction, and also to maintain the cells in their bodies, for movement, and to maintain body temperatures. While the energy for these processes can be made available through the breakdown of organic molecules in respiration, not all of the energy released in this way is utilized by the organisms, so that a proportion is lost and dissipated as heat. As a result, there is a constant flow of energy through the ecosystem from primary producers to carnivores and decomposers, and a constant loss of energy to the atmosphere as a by-product of respiration.

Change in ecological systems may therefore be assessed by an examination of the changes in the flows of energy. Such investigations formed an important part of the International Biological Programme (IBP) and many techniques were developed during that Programme for the measurement of biomass and the consequent changes of energy within different compartments of ecosystems. Without the kinds of models which are described in this Handbook, however, measurement of energy flow can never provide more than a static description of a particular ecosystem at a particular point of time. By the correct use of models, such descriptions may be developed to the point at which prediction can be made of the changes that will take place if the system is modified in some particular way. 

It is not, however, solely the flow of energy to the tropic levels of an ecosystem which is of importance in the study of ecology. An almost equal interest is focused upon the flow of chemical elements, either as nutrients or as pollutants, through ecological systems. Primary producers take up mineral elements such as nitrogen and phosphorus, in the form of soluble mineral salts from surrounding soil or water. Herbivores and carnivores obtain nitrogen and phosphorus mainly as organic compounds in their food, though cattle and human beings may require supplements of raw minerals such as salt or copper . The dead organisms, together with their waste and excretory products, are broken down by decomposers (mainly bacteria and fungi) which release mineral nutrients in a form available for re-use by the primary producers. Similarly, there is a cycling of carbon released in the atmosphere as carbon dioxide as a result of respiration of plants, animals and decomposers. This carbon dioxide is taken up by green plants during photosynthesis, and the carbon passed on to animals when they eat the plants. There are similar cycles for nitrogen and oxygen and, indeed, for any chemical elements which we need to consider .             

In recent times, it has become increasingly important to study the flow of pollutant elements and compounds through ecological systems. Many of these pollutants are stored and concentrated in animal tissues, reaching their highest levels in predatory animals, after successive concentrations at lower trophic levels. The mechanisms by which these substances pass through the trophic levels are therefore of particular significance, and it has become important to measure the changes taking place in such systems by the uptake and storage of elements and compounds which are otherwise foreign to the natural system. 

Finally, the heritable characteristics of organisms are passed from parents to  offspring by complex series of genetic events. Successive generations of organisms are subjected to varying pressures by their environment and by competing organisms; differentiated selection of genotypes thus induces change into the ecological system through the genetic make-up of the component organisms. It is, therefore, possible to describe dynamic change in ecosystems by documenting the genetic composition of organisms, and to predict future changes by modelling heritability and selection processes within defined populations. Similarly, it may frequently be possible to predict the effects of selection pressures upon populations of organisms through genetic mechanisms. Such models require particular information about the extent to which various characteristics of organisms are associated with genetic components, together with a knowledge of the fundamental laws of genetics.

In considering the dynamic change of ecosystems, we should not, of course, forget the important physical and chemical factors of the environment. Changes in these factors may require to be measured because of their impact upon organisms, or because they have themselves been altered by the organisms. Some of the more important factors may be related to the climate within which the organisms exist, either for a large area or for relatively small parts of the system, as in the soil.

Some dynamic changes involve regular cycles, as in the diurnal rhythms and in the seasons, but many of the changes are relatively unpredictable, as is the climate from year to year at a particular location. Other longer-term changes are associated with such factors as the melting of the ice in polar regions, sun spots, etc. Closely associated with climate are the factors .of physiography which, particularly in Britain, have a strong influence upon the effects of climate through slope, aspect, and the drainage of soils. Similarly, the physical and chemical properties of soils are constantly changing, either through the effects of organisms themselves, or through deliberate attempts at management by man. The measurement and modelling of such changes become, therefore, an essential part of the modelling of dynamic change in ecosystems.

2.2 MEASUREMENT OF CHANGE IN ECOLOGICAL SYSTEMS

In the past, much of the measurement of change in ecological systems has been done through repeated survey. Such measurement requires an initial survey to provide a baseline against which change can be measured, followed by successive surveys to establish the direction and extent of change. There are, however, difficulties about the design of surveys to measure change, particularly when it is not clear what those changes may be. Thus, although a detailed survey may be made with the intention of providing the baseline for the monitoring or surveillance of change, and even though the precision of estimates made in the baseline survey may be high, the change itself may not be monitored with any great precision unless there is some clear hypothesis or hypotheses to direct the design of these surveys. Where, however, there are relevant hypotheses about the nature of the change, it may be possible to design repeated surveys which are capable of detecting change with reasonably high precision. Sampling with partial replacement, as developed for continuous forest inventory, is a particularly appropriate technique for use in such circumstances (Ware and Cunia, 1962; Cunia and Chevrou, 1969). A statistical checklist, highlighting some of the more important questions to be asked in the design of sample surveys, is given by Jeffers (1979).

Some authors make a distinction between monitoring and surveillance, depending on whether or not there is an attempt to correct the change taking place and bring it back to some stable state. No attempt will be made to maintain this distinction in this Handbook. Whether or not the ecological system is to be maintained in some preassigned state, it is necessary to determine the change which is taking place and to measure its extent. In essence, any investigation of dynamic change requires the definition and bounding of the problem to be framed as hypotheses which can be tested formally, even if that test can only be conducted after a chain of deductive reasoning from one or more hypotheses which are capable of direct verification (Jeffers, 1978a).

Three basic classes of hypotheses may be distinguished:

1. Hypotheses of relevance identifying and defining the variables, species and sub-systems which are relevant to the problem.

2. Hypotheses of processes, linking the sub-systems within the problem, and defining the impacts imposed on the system.

3. Hypotheses of relationships, and of the formal representations of those relationships by linear, non-linear and interactive mathematical expressions.

These three classes of hypotheses may well be linked within a formal chain of deductive argument, leading to processes which can be summarized by a decision table enumerating all the hypotheses, and combinations of hypotheses, that must be specified in order to solve a particular problem. The decision table also specifies, for each combination of hypotheses, the decisions or actions that should be taken to ensure that the problem is correctly solved. Because decision tables provide a clear concise format for specifying a complex set of hypotheses and the various consequent courses of action, they are ideal for describing the conditions for interaction between component parts of a model. The extension of these techniques to the enumeration of the necessary combinations of hypotheses for particular courses of action where uncontrolled events may intervene, so that we are unable to control or predict with certainty, has been the main thrust of recent research into decision analysis (Raiffa, 1968).

The clearer definition of hypotheses about the nature and rate of change in ecological systems enables survey to be replaced by short-term and long-term experiments. The increased control over experimental areas provided by well-considered experimental design increases the precision with which change can be measured, and also offers the possibility of testing the interacting effects of various factors, including methods of management, conservation measures, and various forms of protective legislation. Indeed, by effective planning and design, it may be possible to provide a carefully assessed programme of environmental management linked to the detection of desirable or undesirable change in ecosystems. At the same time, unexpected change may be detected and included in subsequent measurements in the experimental areas. Regrettably, however, rather little emphasis has been given so far to the use of experiments as opposed to surveys taken as cross-sections in time, possibly because research workers are reluctant to commit their successors to maintaining the measurement and assessment of their experiments, or, alternatively, because research workers are reluctant to be committed to a programme of research by their predecessors.

It may well be that the academic and institutional organization of science, in developed and developing countries alike, precludes the rational design of investigations to determine and measure change in ecological systems over anything more than half a decade. A statistical checklist of the questions to be asked when designing experiments to detect change in ecological systems is given by Jeffers (1978b).

2.3 CONCEPTUAL MODELS OF DYNAMIC CHANGE

2.3.1 Vegetation

The species composition of all biological communities varies in time and space, the rate and character of the change being a function of the scale at which a community is examined, and of the external and internal factors which influence different populations in different ways. The interaction between organisms and environment that characterizes the point in time and space then becomes part of a larger system when arrays of sub-systems are combined into a biological community. It follows that communities and landscapes constitute a range of possibilities. These possibilities arise from combinations of the extent of differences between small sites and their patterns of diversity, the degree of biological influence on sites and on population regulation, the behaviour and longevity of dominant populations, the relative stability of populations, the roles of disturbance and succession, the kinds of succession and the extent to which species are replaced during successions.

Classical ecology has been mainly concerned with changes are reflected in vegetation, particularly in vegetation redevelopment following perturbation of some community. This emphasis arises because plants provide the energy base for all other biological activity, and because the vegetation provides the mechanical structure of the biological environment in which other organisms exist. The classical concepts of ecological succession involve two essential assumptions: first, that species replacement during succession occurs because populations tend to modify the environment, making conditions less favourable for their own persistence and leading to progressive substitution; and second, that a final and stable system, or climax, ultimately appears which is self-perpetuating, and is in balance with the physical and biological environment.

This view of ecological succession, developed over the centuries, and based largely on observed relationships between vegetation and environment, and on patterns of revegetation on agricultural and forest land, has been generally accepted by the scientific community, even though obvious exceptions to the form of succession exist. Egler (1954) was probably the first person to suggest formally that the classical model of succession may not apply in all situations, and he referred to this classical model as 'relay floristics' and suggested that, in many cases, the initial floristic composition following a perturbation may dominate the entire pattern of subsequent succession. He suggested that, unless species persisted throughout the perturbation, or were able to enter the perturbed site shortly afterwards, they would not subsequently be represented in the community that developed.

Evidence has gradually accumulated to suggest that the concept of the initial floristic composition may have wider applicability than was originally envisaged by Egler (Colinvaux, 1973; Drury and Nisbet, 1973; Horn, 1974). More recently, Connell and Slatyer (1977) have proposed a broader overall system of successional processes which incorporates the possibility of the pathways of relay floristics and initial floristic composition operating independently or in combination, and which, in addition, includes a pathway in which succession is truncated at a point short of the expected climax, a phenomenon which has been frequently observed (but seldom explicitly recognized) in successional theory.

In Figure 1, the various pathways are indicated, and the likely effects of perturbation, at different stages in each pathway, are also shown. The basic dichotomy between the classical 'relay floristics' concept (pathway 1) and the other models (pathways 2 and 3) is reflected in the immediate divergence of the pathways. In model 1, referred to by Connell and Slatyer as the 'facilitation' model, the classical replacement pattern occurs, each successive suite of species which occupies the site tending to make the environment less favourable for their own persistence and more favourable for their successors to invade and grow to maturity. In model 2, the 'tolerance' model, environmental modifications induced by earlier colonists may either increase or decrease the rates of recruitment and growth to maturity of later species. The latter appear later because they either arrived later, or, in present directly after the perturbation, had their germination inhibited and their growth suppressed.

In contrast to the 'facilitation' model, in model 3 ^__ termed the 'inhibition' model by Connell and ^__ Slatyer the early occupants, rather than facilitating the progressive occupancy by other species, inhibit the invasion of other species by pre-empting available space through physical occupancy, through physical competition, and the use of allelopathic substances, or through other effective means of inhibition. This inhibition has the effect of truncating the succession at a stage that would normally be regarded as being composed of  non-climax species. Later succession species may only be able to enter the site when the inhibiting species are damaged or killed. If there is a subsequent perturbation, new succession may well follow a different pathway, avoiding a repetition of successional truncation.   

Figure1. Classical and truncated ecological succession

In essence, the Connell and Slatyer concepts are based on the simple premise that the presence of a particular species in a community is dependent on the product of two probabilities. The first probability is that of a propagule being available at a site, itself a function of the ability to survive a pertubation or to reach the site by appropriate dispersal mechanisms. The second probability is that of the propagule being able to become established at the site and reach reproductive maturity, itself a function of the environmental requirements of the species, its adaptive ability and its reproductive strategy in relation to the prevailing environment.

2.3.2 Populations

Dynamic change in ecosystems may be measured directly by assessment of the numbers of plants and animals to be found in some convenient unit of space and time. The counting of numbers of plants per unit area at a particular instant of time is usually a straightforward, if tedious, task. Where the number of plants is large, sampling methods may be used to limit the numbers that have to be counted, and counts may also be limited to the larger and more conspicuous plants. Problems of identification, especially in the younger stages of most plants, will also frequently limit the extent to which a complete count of all species of plants occurring in a defined area is attempted.

The measurement of plants to determine the biomass of vegetation and the rates of growth of individual species and communities of plants is a more difficult task, and comprehensive procedures have been designed to ensure that effective standards are established for comparative measurements. The estimation of primary production of forests for the International Biological Programme is described by Newbould (1967) and the measurement of primary production of grassland is described by Milner and Hughes (1968). This Handbook will not attempt to repeat the information contained in texts on population assessment and measurement, but instead concentrates on the underlying mathematical models for the counts and measurements obtained.

Counts of animal populations and measurements of the growth rates of animals are even more difficult, if only because of the difficulties of catching the animals. It may also be necessary to identify, count, and measure animals at different stages of their development, particularly when the purpose of the investigation is to model the population dynamics of one or more species. Dempster (1975) emphasizes that a real understanding of the factors determining the abundance of animals can be obtained only by the intensive study of animal populations in the field, and describes the methods available for such studies.

The practical focus for models of population change will depend entirely on the hypotheses which are formulated for any particular investigation. For some studies, interest may be concentrated on only one species, but most studies are likely to be concerned with two or more species, as in the investigation of predator-prey relationships, or as in the elucidation of the complex interrelationships of competition between species for the critical factors of a habitat.

2.3.3 Communities

The study of ecosystems at the level of biomes has already been referred to in the discussion of the definition of ecological systems (Section 2.1). Change of one biome to another takes place only rarely, and probably only when there is a major climatic shift, or as a result of wholesale intervention by man. Within biomes, however, there are frequently smaller changes which are detectable because of the variations in the communities of plants and animals which are to be found at particular sites. Indeed, spatial variation or heterogeneity may provide broad indications of changes which will take place in time. Greatest interest is usually centred on those changes in communities of plants and animals which are related to measurable changes in the physical and chemical factors of the environment. In this way, a biogeographical interpretation of the dynamic and spatial variations may be achieved, leading to carefully structured hypotheses about the development of ecotones, climax states and seres. However, it is important to ensure that such hypotheses are capable of being tested, if the conceptual models of dynamic change are to be regarded as scientific as opposed to philosophical.

2.3.4 Management

By far the greatest influence on conceptual models of dynamic change in ecosystems is that of deliberate management by man to achieve particular aims. From earliest times, man has sought to maintain and improve his standard of living by hunting animals for food, clothing, and working materials such as bone, sinew, and horn. His pursuit and capture of animals for these purposes modified ecological systems already changing for other reasons, sometimes leading to the extinction of his prey. With increasing knowledge, man was able to domesticate some animals and to adjust his rates of cropping domestic and wild animals so as to sustain human populations in the modified ecosystems in which these animals lived. Nevertheless, unexpected changes still occurred, sometimes because of diseases and pests which changed the carefully contrived balance between the animals and the rest of the ecosystem, sometimes because of economic and social pressures which induced changes in demand and supply faster than the ecological system could itself respond.

Similarly, the harvesting and cropping of wild plants quickly led to methods of cultivation which have transformed whole ecosystems throughout the world. Ecologists sometimes pretend that these cultivated ecosystems are less interesting by comparison with natural or semi-natural ecosystems, but this view is an artefact of the perceptions induced by our urban-orientated education systems, leading to a higher valuation of the 'wild' and 'natural' than of the modified and cultivated. From the point of view of the complex interrelationships between plants and animals, and of both with their environment, changes in cultivated ecosystems are as ecologically interesting as those in natural or semi-natural ecosystems. Indeed, the study of such changes, in response to economic, social and technical initiatives, is at the heart of the applied ecology of agriculture and forestry.

While much of the management of cropped ecosystems is concerned with judicious disturbance, disease and pest control, and with regeneration, an especially important influence on dynamic changes in such ecosystems is that which is due to deliberate manipulation of the genetic composition of the crop plants or animals. Such manipulation may also be extended to weed species, and to pests and diseases. In natural and semi-natural ecosystems, genetic changes are usually relatively slow. In managed ecosystems, it is usually necessary to speed up these changes by programmes of plant or animal breeding. The new strains which result from these programmes then make new demands on the ecosystems for which they were created, inducing further modification and change.

2.4 ECOLOGICAL THEORY AND APPLICATIONS

2.4.1 Theoretical concepts

In general, therefore, our search for dynamic change in ecological systems will be directed by hypotheses which we will wish to formulate about the mechanisms which govern and induce such changes. Only rarely will we be able to devise methods of detecting changes which are totally unspecified; still less can we expect to develop models of dynamic changes which are capable of predicting changes that are completely unspecified. Nevertheless, our models may frequently predict changes which are counter-intuitive, or which are unexpected. It is these unexpected changes which provide a critical test for the hypotheses which underlie the models.

Much of the ecological theory which we will wish to use in the development of models of dynamic change will be related to trophic levels in ecosystems. In particular, the changes related to the flow of energy through the various trophic levels will provide a basic concept around which many of the models can be clustered and interrelated. Closely associated with the flows of energy will be the flows of nutrient and pollutant elements and compounds which may act as stimulants or inhibitants of growth, reproduction and distribution of organisms. The ultimate fate of many of these substances will help to determine the nature and extent of dynamic changes.

At a higher level, and therefore more visibly, dynamic changes in populations and communities of organisms, through responses to environmental changes (some of which have been induced by the organisms themselves), will often provide the basis for our models. Models of population dynamics may then be used to study the processes of ecological succession, although some problems of succession will be addressed directly by appropriate models. Finally, the changes related to evolution and genetic composition will necessarily be based on genetic models which consider the patterns and mechanisms of inheritance.

2.4.2 Application

The models of dynamic change described in this Handbook have a wide range of application. Obviously, they are especially relevant to many kinds of fundamental research in ecology. Indeed, many of us believe that little progress can be made in ecology unless mathematical models are used to describe the complex interrelationships between organisms, and between organisms and the physical and chemical factors of their environment.

There are, however, many fields of practical application for which these models are relevant. Agriculture and forestry, as two broad fields of applied ecology, have already been mentioned. Wildlife conservation, including both plants and animals, is another field of applied ecology where the modelling and prediction of dynamic change is essential if wise decisions are to be made about the conservation of specific organisms, habitats, or the management of nature reserves. The more direct effects of urban man in his pursuit of recreation and visual amenity also require to be examined by the kinds of models described in this Handbook.

However, perhaps the most obvious, and urgent, requirement for models of dynamic change in ecosystems is in the assessment of environmental impact. Many books have been written on this important topic in the last ten years (e.g. Munn, 1975), but the development of methods to assess the environmental impact of proposed human actions, such as the construction of large engineering works, land reform, and legislative policy, is totally dependent on our ability to model the dynamic change of the ecological systems upon which the actions will have an impact.