Published in Ekológia (ČSFSR), Bratislava, Vol. 11, No. 4, pp. 369-377.
Some aspects of the theory of the ecosystem spatial structure. I. Theory
Some theoretical aspects of the spatial structure of (plant) communities and whole ecosystems are discussed. The spatial structure of a plant community or an ecosystem is understood as a part of the total structure of the community (ecosystem). It forms a skeleton, in the framework of which different ecosystem processes take place. The distinction between the space heterogeneity of the environment and that of the biotic component of the ecosystem is emphasized. The heterogeneity can be evaluated on different levels (using different scales). In this way a whole system of space-structure units can be formed, encompassing the geobiocoenosis of the landscape, the local complex of ecosystems or communities, the ecosystem or community, the microcoenosis and the structural group of species. The units of a higher order than an ecosystem (community) mark the complex geobiocoenotic approach, those of the lower order represent the coenotic approach. The spatial structure of ecosystems is associated with gradients of both the environment and the communities. Special effects (under term 'ecotone') can appear along the gradients. There are several definition of ecotones but their applicability is often limited. The theoretical analysis of the possibility of ecotone formation and its identification is given.
If a structure is considered as an ordered pair of elements [M,S], where M is a set of objects of the structure and S is a set of relationships among the objects of M (compare Ad mek, 1983), then the study of an ecosystem structure involves the identification of its components and of the respective relationships. The concept of an ecosystem is identical here with the classical concept (Odum, 1971 etc.). It is necessary to note that many of the studying phenomena pertaining to ecosystems also occur within the communities of some organisms, and vice versa. Where it will be possible, we shall therefore speak about the ecosystems, and the level of the community will be mentioned in parenthesis.
It is very difficult to study the structure of an ecosystem as a whole, since it involves innumerable relationships among the organisms and the environment, as well as among the organisms themselves. Of importance is therefore a further specification and delimitation of the relationships concerned. This contribution deals with the organization of ecosystems in space, e.g. their spatial structure. The spatial structure of ecosystems is the basic characteristic of ecosystems (communities), since it affects a great number of their functions (Greig-Smith, 1979). This is why considerable attention has been paid to such tasks as the study of minimal areas of communities (Moravec, 1973), the spatial organization of populations of higher plants (Bertness, Ellison, 1987; Ford, Renshaw, 1984; Goodall, 1974; Greig-Smith, 1981; Kershaw, 1973, and others), or the species associations on small plots (Mahdi, Law, 1987). Other authors were concerned with the relationships of understory associations to the overstory (Crozier, Boerner, 1984). In spite relatively common data on the spatial organization of vegetation in different ecosystems (e.g. Bonnicksen, Stone, 1981; Bouxin, 1983; Bouxin, Gautier, 1982; Erschbamer et al., 1983; Whittaker et al., 1979; Whittaker et al., 1979), the terminological problems pertaining to a level of spatial structure under a community level remain to be cleared.
The ecotone is a related but was not too often used term. It is sometimes used to describe the transition zone between different ecosystems (communities), which is characterized by several features. It is dealt with the ecotonal (or edge) effect in this connection (see Odum, 1971 or, lately, Hansen et al., 1988). The relations between the ecotones and the diversity of species are also the object of a contemporary international SCOPE projects (Holland, 1988; Rusek, 1986).
Here is it necessary to turn the attention to the importance of the scale applied to the study of a spatial structure, since the methodology applied predetermines the particular level at which the structure is studied. The relationships between the scale and the space-structure units is also clearly evident from the work of Wiens et al. (1986), where the following levels of the spatial organization (and the scale) are distinguished:
It is not obligate to think about the itemization into these categories absolutely. There are gradual transitions between these categories. It is possible effectively to describe this reality using the hierarchy theory (Allen, Starr, 1982).
- the area used by a single individual,
- the scale of a local population or patch,
- the scale of the closed system,
- the regional scale,
- the biogeographical scale.
Sources and types of spatial heterogeneity
When studying the spatial heterogeneity, it is necessary to distinguish the heterogeneity of environmental factors and the heterogeneity of biocoenoses (of the vegetation, e.g.). The first one can be separated into two groups:
A particular unit of vegetation of the certain level is produced in dependence on the distribution of environmental factors and on the type of response given to them by individual species of the coenoses, inclusive of the response modification due to the co-actions among the organisms (the hierarchical organization of such levels is dealt with further on). Secondary biotic (for example vegetational) gradient can be therefore observed along the gradient of environmental factors, sometimes (incorrectly) designated as the abiotic gradient. Their complex can be designated as the abiotic-vegetational gradient. Vegeta- tional gradients independent of the heterogeneity of the environment reflecting the internal relationships within the phytocoenosis can be also observed, if our interest is focused on the vegetation. They result from the co-actions and competition of individual components of the community.
- space-fluctuant changes, where the level of a factor at any point in space is relatively independent on the coordinates (the fluctuation pertains here to the localization in space, not to the time),
- gradient changes which can be observed as a gradual changes of the studied factor in space.
The continuity of gradients has also to be mentioned with respect to the problems discussed. It is possible to say in general that strictly discontinuous changes of either environmental or biotic factors are extremely rare. Most of such discontinuities turn out to be gradual transitions as soon as the scale applied is changed.
Questions falling under the term of vegetation-environment relationships are not dealt with here, although belonging to the most important ones, since they are adequately discussed in a number of contemporary publications (see e.g. TerBraak, Looman, 1986; TerBraak, 1986, 1987; TerBraak, Gremmen, 1987; Roberts, 1987; Ellenberg, 1974). Co-activity and competition have been paid considerable attention even at the ecophysiological level at least in some fields (for plant communities see, e.g., Latůvka, 1986).
The system of space-structural units
The problems of spatial organization were studied by Solon (1983) at the level higher than ecosystem (coenotic) level. He discriminated structural levels formed by:
New units of a lower hierarchical level as pertaining to the vegetation are specified further on:
- landscapes (or vegetation of the respective landscapes),
- local complexes of ecosystems (phytocoenoses),
- ecosystems (coenoses).
The term community has been already specified , though not always quite exactly (compare Odum, 1971; Braun-Blanquet, 1964; Zlatník, 1976). It remains questionable what is the relationship of the basic classification unit of the vegetation represented by a community (the type of which is often called, e.g., an association by the Central European geobotanical school - Braun-Blanquet, 1964; Barkman et al., 1986), and the community representing one of the space-structural units. This difference can be designated as the paradox of the double way of looking on the communities. This is why we have to ask the question: Does a syntaxonomically defined community situated on a gradient in space correspond to a space-structural unit? Not probably so in a number of cases. It is impossible to identify with an adequate accuracy the limits of a syntaxonomically characterized community, due to the continuity of the vegetation gradient.
- microcoenoses (e.g. plant microcoenoses or micro-phytocoenoses), and
- structural group of species.
A (plant) microcoenosis (further MC or PMC) can be defined according to the distinction of the space-fluctuant and the gradient changes in the community. It is such a part of the stand where its fluctuant changes due to the gradient ones are negligible. In this case, an increased scale does not give a transition to the lower structural level (i.e. the structural group of species). The MC (or PMC) can be therefore considered to represent the basic building units of communities. They from the elementary stand mosaic, which makes it possible to study the relationships between adjacent communities and their transient forms. The contact zones between communities appear from this point of view to have developed into the MCs (PMCs). The elements of the stand mosaic (the PMCs) can be classified according to the species composition. In this way, the system of types of MCs (types of PMCs) can be formed.
A structural group of species (SGS) is defined as a set of species occurring more or less regularly together (i.e., of those with a significant positive linkage) on areas not exceeding the average active range (e.g. maximal distance between two organisms by which the immediate relationships on physiological bases is possible) of any member of the group. The SGSs are determined in the first place by the biology and ecophysiology of all species, and by the mutual relationships between individuals from the species of SGS. The factors effective here involve: kind of response to individual ecological factors, growth and life forms of individual species, growth strategies (Grime 1979), competition, allelopathy, mycorhizas, relations among parasites, haemiparasites and their hosts, etc. The importance of individual factors for the formation of the spatial structure of vegetation has been assessed by Whittaker (1975), for example.
Ecotones as special effects of the spatial structure
The term 'ecotone' can be given different meanings. It can be defined as a transitional zone of any kind between different ecosystems or as a zone with definite characteristic features. At the beginning, there is in any case a compromise between the discontinuous and continuous approach to the conception of communities. In the first place, it is necessary to be aware between what is the ecotone to be looked for. Two different approaches are available according to the outcome of the answer:
One of the contemporary ecotone definitions was presented in connection with the 'Ecotones' project of SCOPE by Hansen et al. (1988):
'Ecotone is the zone of transition between adjacent ecological systems, having a set of characteristics uniquely defined by space and time scales and by the strength of the interactions between adjacent ecological systems.'
- The ecotone between different ecosystems. In this case, the ecosystem has to be regarded as a whole, comparing all abiotic factors and populations of all organisms present. We have thus to begin with the classification of complete ecosystems on the given area or region under study. To work out such a classification is in reality extremely difficult and this is why the most projects of complex ecosystemic ecotone studies get restricted by choosing a definite biotic component as the main one. It is quite regularly the vegetation of vascular plants. The whole approach is changed, however, by this step, so that the following has to be said.
- The ecotone between different communities of a particular group of organisms under specific ecological conditions. Such conditions may involve the abiotic factors as well as the presence and functions of other groups of organisms. The classification of communities (and the identification of those between which we are looking for the ecotone) can be mostly achieved without particular difficulties. This approach requires that the ecotone is situated between two communities of specific organisms. Therefore it is possible to identify the ecotone at a particular locality for one group of organisms, but not necessarily so for another one.
Studying a specific case (space structure of an ecosystem or a community), it is probably this definition will not content a necessary criterion of resolving (the ecotone is or is not here). This is the reason why the following definition is proposed:
'Ecotone is the transition zone between adjacent ecological systems (communities of particular organisms or other space-structural units above the level of the SGS) establishing a more considerable gradient of the community change in comparison with the gradient of environment or with the topographical gradient.'
If we apply the second approach (mentioned above), under term 'environment' is embranced the abiotic factors as well as the biotic surrounding of studying community (the other taxocoenoses).
Another definition was attempted, e.g., by Z˘lyomi (1987):
'Ecotone is the transition between two, sometimes more, contrasting communities or habitats, for example, the forest margin between a stepe and a forest community. This zone is composed of a mixture of species from both communities and those occurring almost exclusively in this transitional part. The number of species and the density of some populations are often higher in the ecotone than in the neighbouring communities themselves.'
As it has been shown, there is whole system of definition of ecotones. The goals of a study and the means available are decisive for the choice of the suitable definition, but it has to be stated explicitly in advance. However, many important questions remain unsolved, such as, e.g., what is the possibility of an ecotone occurrence along environmental gradients with existing discontinuities. Or, how far is it possible to encompass the so-called 'Zono-ecotone' sensu Walter (1985), similarly also Oechel, Lawrence (1985) under the individual above given definitions. Climatically conditioned alternations of the vegetation and development of ecotones have been also observed in North America (Nielson, 1987). Further ecotonal effects, conditioned by other factors, are known from the margins of eucalyptus forests and tropical rain forest (Unwin et al., 1985). The description of a simple seashore ecotone and the detailed analysis of the environmental factors was presented by Mahal, Park (1976).
The terms of the (plant) microcoenosis, type of MC (or PMC) and the structural group of species, have been specified. However, what is their relationship to other terms used in analyses of the spatial structure of communities, such as the biotope defined by Burchfield (1972) or Goodall (1986)? The biotope represents always a unit based on functions and developed in time and space, while the two terms given above pertain primarily to (time)-space units. It is of course obvious, that their origin reflects the functional relationships of the community (ecosystem).
The concept of merocoenoses has been applied in zoology (Balogh 1958, Schaefer, Tischler, 1983). For all that it is a spatial unit of coenosis, the position of this unit in hierarchical system (given in this paper) is not delimitated clearly. Into the bargain it is not a common term but one has been used only in zoology (most of all in entomology).
The ecological terminology involves some homonyms, which may have caused that a number of opinions appear as contradictory (compare the above mentioned paradox of the double meaning given to the term community). Such a danger is also hidden in the term the structural group of species. As defined here, the term corresponds to the smallest space-structure unit participating on the composition of coenoses. The similar terms are, however, also sometimes used for reporting the results of statistical testing the joint occurrence of species on plots whatever size (e.g. Leps, not published). Such correlations can be affected (or even caused) by the heterogeneity of environment noticeable at the particular plot size. This case does not pertain of course to the biological (ecophysiological) association group representing the lowest level (the SGS).
The classification and the hierarchical ordering of the space-structure elements and units, as presented in this contribution, integrates intrinsically both the discontinuous and continuous approach. A study of gradients and spatial structure based on fully continualistic hypotheses has been performed by Gauch, Whittaker (1972, 1976) and later by Minchin (1987), for example. However, to apply this approach today and to neglect at the same time the co-evolution of species and whole communities, would be in its consequences equally unreasonable as an attempt to keep at all costs to the theory of the opposite extreme.
The ecotone can be defined in different ways (Odum, 1971; Hansen et al., 1988; Zólyomi, 1987, and others). In any case, it is necessary to make it clear between what kind of stand units do we look for the ecotone, and to select according to this the most suitable definition of ecotone.
There are at least five different levels of the spatial structure of ecosystems, their complexes or, eventually, of the respective communities and their complexes. Two units (the microcoenosis and the structural group of species) have been newly defined. The term microcoenosis is abstracted from the analysis of gradient and fluctuation changes of communities. Individual microcoenoses can be typified, so that a further term 'the type of microcoenosis' can be introduced. Next term, 'the ecotone', bears upon the analysis of gradients. It is very difficult to determine the exact and complete meaning of this term, since a number of such attempts exist. As the most suitable basis for the identification of ecotone between two communities of particular organisms appears to be a study of changes of the population structure along the gradients. Such a change is most often expressed by means of indexes of the total diversity of species and their components.
The second part of this paper will be treated of the analysis of spatial structure of three different plant communities (oak forest, wet meadow, old field). An example of application of the definitions of ecotones and of microcoenosis will be given there.
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