Randvoorwaarden voor natuurlijke kwaliteit in pleistocene zandgebieden : een onderzoek vanuit de persistentietheorie in het perspectief van planning

RESEARCH FRAMEWORK:SUSTAINABILITY AS A PLANNING PROBLEMThe quality of soil, water and air, and the number and diversity of plant and animal species in the Netherlands, show a sharp decline. This decline in the quality of the natural substratum poses a major problem for environmental policy and planning. Government policy and planning aim at sustainable land use. However, the concept of sustainability is subject to various interpretations. This study concentrates on the ecological aspects of sustainability. The problem of the current decline in the quality of the natural substratum can be solved only on the basis of knowledge of ecological limits to human management. To this end it is assumed that planning needs an ecological approach, an angle from which ecological aspects of human management can be analysed, and adequate solutions to the problem of the declining quality of the natural substratum can be found. In order to devise sustainable trajectories for the environment, indicators of ecological sustainability are needed together with their limiting values. In addition, an ecological approach should provide planning with insight into the underlying factors. These factors are called the building blocks of sustainability. Finally, knowledge of the implications of respecting ecological limits is prerequisite to planning.In this study, an ecological approach is constructed on the basis of theoretical notions as to the persistence of natural ecosystems.The study focuses on the ecological limits to agricultural management in the pleistocene areas in the Netherlands, as it is here that the decline in the quality of the natural substratum is most dramatic. A distinction is made between agro-ecosystems and nature-ecosystems. Agro-ecosystems are defined as ecosystems whose functioning is regulated with a view to agricultural production. Natureecosystems are defined as ecosystems which are controlled with a view to nature conservation and/or nature expansion.LIMITS TO SUSTAINABILITYThe theoretical concept of persistence of ecosystems with a plural hierarchical organisation is chosen as the basis of the search for ecological limits. The hierarchical organisation of ecosystems results from differences in the speed and spatial scale of processes, relevant to the system as a whole. Within this hierarchical organisation, three dimensions are distinguished. The biotic dimension refers to the hierarchically structured network of species. The functional dimension refers to the hierarchy of processes and functions within the ecosystem. The spatial dimension refers to the hierarchical spatial arrangement of ecosystems and their interrelationships. Each ecosystem is characterized by a specific coherence between the biotic, functional and spatial dimensions. The hierarchical organisation of ecosystems and the coherence between the three dimensions play an important role in the self-regulation of natural ecosystems, notably when they are faced with disturbances. These disturbances are defined as significant changes in the characteristics of an ecosystem, which result, for instance, from changes in environmental factors. In contrast to the concept of stability, the concept of persistence implies the self-regulation of ecosystems that can exist in more than one - equilibrium state or attraction domain and even in a non-equilibrium state. Persistence of hierarchically organised ecosystems is defined as the ability to maintain the characteristic coherence of species, functional aspects and spatial components. When confronted with disturbances, these ecosystems are able to persist by shifting from one attraction domain to another. The possibility of alternative equilibrium states is, however, limited to a specific range of disturbances.Ecological theory focuses mostly on population density or population size, elements of the biotic dimension, as indicators of persistence. Limits to the persistence of natural ecosystems are then defined as limits to fluctuations in these indicators, acceptable with a view to the long-term survival of the population in question. Owing to the coherence between the dimensions of hierarchically organised ecosystems, changes in population density or size are the result of underlying functional, biotic or spatial processes. These processes are therefore regarded as the building blocks of persistence. The extent to which the persistence of ecosystems is threatened is related to the extent to which the limits to changes in these building blocks are exceeded.PERSISTENCE OF AGRO-ECOSYSTEMS AND NATURE-ECOSYSTEMSThe processes in both agro-ecosystems and nature-ecosystems are the same as in natural ecosystems. However, they are regulated by a specific form of human management. Agro and nature-ecosystems differ in the extent to which their selfregulation is replaced by human regulation. It is assumed that these types of ecosystems are also characterized by a three-dimensional hierarchical organisation.In agro-ecosystems, the coherence between the dimensions is regulated with a view to animal and plant production. The yield per ha and/or per animal is regarded as the indicator of the persistence of agro-ecosystems. Important building blocks of the persistence of agro-ecosystems are the quality of the soil in the functional dimension, the health of livestock and crops in the biotic dimension, and the possibility of multiple land use in the spatial dimension.In nature-ecosystems, the coherence between the dimensions is regulated with a view to a specific species composition. The focus is mainly on the conservation of rare species and communities, characteristic for Dutch environmental conditions. This species composition is regarded as the indicator of the persistence of nature-ecosystems. Important building blocks are the quality of soil and groundwater and the level of the groundwater table in the functional dimension, the minimum viable population size of characteristic species, defined as the population size that will ensure (at some statistical level) its persistence for a specified time, in the biotic dimension, and the surface and spatial heterogeneity together with the possibility of connection with similar nature-ecosystems in the spatial dimension.In order to implement an ecological approach for planning, a quantitative indicator of the acceptable limits to the fluctuations in both agricultural yield and species composition is needed. When such an indicator is lacking, this study focuses on limits to changes in the mentioned building blocks, as for example limits to the accumulation of heavy metals in agro-ecosystems and limits to the isolation of nature- ecosystems.AGRO-ECOSYSTEMS AND NATURE-ECOSYSTEMSIN PLEISTOCENE AREASIn order to define the current problems relating to persistence, the relationship between agriculture and nature in pleistocene areas is re-examined through the use of the theoretical framework. The extent to which disturbances in agro and nature- ecosystems threaten their respective persistence depends both on the intensity of agricultural land use, and the susceptibility to related disturbances of the natural substratum, the totality of soil, water, air, plants and animals. Initially, the pleistocene areas were characterized by many gradual transitions from oligotrophic to mesotrophic and eutrophic conditions. The development of agriculture in the pleistocene areas was structurally influenced by the pattern of plains, sand ridges and stream valleys.In the 1960s dairy farming in pleistocene areas expanded enormously. The 1970s showed a large increase in intensive husbandry, especially in the eastern and southern pleistocene areas. Later, a decrease in acreage was compensated by developments in the respective dimensions of agro-ecosystems, resulting in an annually increasing production per hectare and per animal. Factual information concerning possible threats to the persistence of agro-ecosystems is scarce, except for arable farmland in the northern pleistocene area, where agricultural yields show a decline as a result of the intensive tillage practices.Using 'yield' as an indicator of persistence proves to be difficult because of the continuous adjustment and extension of the management of agro-ecosystems. Thus, it is not yet possible to quantify the variation in yields which would be acceptable in terms of persistence. Changes in several building blocks, however, signal possible future threats to the persistence of agro-ecosystems. These changes include the accumulation of potassium and heavy metals in the soil, the speed with which weeds, soil organisms and insects become resistant to pesticides, and the possibility of limited use of land which was heavily manured in the past.Moreover, changes in agricultural land use have affected and still affect other aspects of the quality of the natural substratum through the release of nitrogenous and acidifying compounds, the lowering of the groundwater table and the decrease in spatial heterogeneity.Around 1900, the Dutch nature movement was born out of an increasing concern about the general changes in agricultural land use. In the period 1900-1960 the acreage of nature-ecosystems in the Netherlands sharply decreased. Since 1960, the quality of the remaining nature-ecosystems has been declining. The situation with respect to the number and diversity of species in these nature-ecosystems has deteriorated. Consequently, it is assumed that the persistence of nature-ecosystems in pleistocene areas has been strongly affected. To a large extent, this deterioration is caused by the above mentioned effects of agricultural land use. These disturbing effects have led to a decreasing viability of plants and animals characteristic of nature- ecosystems. As a result of the cumulative effect of disturbances, it can be assumed that nature-ecosystems in. pleistocene areas have only one attraction domain left. This limits their capacity to absorb further disturbances. In order to sustain this assumption, it is necessary to quantify the acceptable variation in species composition for specific types of nature-ecosystems,such as e.g. heaths and moorland pools. So, nature conservation schemes should state explicitly which type of ecosystem is intended. Only then, limits to changes in both the species composition and the relevant building blocks can be determined.From a planning perspective, maintaining the persistence of nature-ecosystems of in the pleistocene areas is a more pressing problem than maintaining the persistence of agro-ecosystems.POLICY PLANS FOR THE QUALITY OF THE NATURAL SUBSTRATUMThe decline in the quality of the natural substratum in the Netherlands prompted the government to revise its policy. In the last few years a number of government reports on this problem have been published: the National Report on Environmental Policy, the Report on Nature Policy, the Third Report on Watermanagement, the Fourth Report on Spatial Organisation (Extra) and the Report on Agricultural Structures. All these reports focus on sustainable development, particularly on the sustainable preservation and development of nature-ecosystems. Besides the traditional preservation of the so-called 'half-natural' ecosystems like heaths and moorlands, there is a strong interest in developing original, self-regulating ecosystems. Restoring the quality of the natural substratum is a prerequisite in both approaches. 'Me different policy plans seek to counteract the current decline in quality by realising both an Ecological Main Structure and a general level of quality of the environment.The above mentioned policy plans are generally lacking in an ecological approach as defined in this study. An explicit definition of sustainability, together with relevant indicators and limiting values, is commonly not given. The objectives as to the quality of the natural substratum have not been made explicit and applicable on the regional level. The reports pay too little attention to the coherence between functional, biotic and spatial conditions for the persistence of ecosystems. Furthermore, the measures proposed to attain the general level of environmental quality are insufficient.BOUNDARY CONDITIONS FOR THE QUALITY OF THE NATURAL SUBSTRATUM IN THE CATCHMENT AREAS OF BEERZE AND REUSELThe case-study evaluates regional plans concerning nature-ecosystems in the catchment areas of two lowland streams in the southern pleistocene area. The pivotal question is whether or not these schemes are adequate in terms of persistence. The case-study comprised three parts: the conservation of forests and heaths as core areas in the Ecological Main Structure, the realisation of corridor zones for forest birds, and the creation of nature expansion areas in stream valleys.The evaluation encountered a number of difficulties. Firstly, the schemes were not very specific about the desired species composition. Secondly, data on the current species composition in potential core areas and nature expansion areas were incomplete. Thirdly, quantitative knowledge as to the limits to fluctuations in the species composition of the nature- ecosystems concerned, is rather limited. Consequently, it proved to be impossible to demarcate entirely the different attraction domains in which these ecosystems can persist.Since an evaluation using the indicator 'species composition' could not be carried out satisfactorily, the study was orientated towards limits to fluctuations in the building blocks of persistence. Again, the knowledge of these limits is patchy. In the functional dimension, the values of the critical loads of acid deposition in nature-ecosystems are in part arbitrary. Knowledge concerning limits to desiccation and eutrophication is limited to estimated original values. In the biotic dimension, no general rule for the minimum viable population size can be given. In the spatial dimension, knowledge concerning the size, location and quality of corridor zones between core areas, required for the dispersal of species, was mostly qualitative.As far as possible, the level of quality of the natural substratum that can be realised on the basis of the proposed policy measures was compared with the level of quality necessary to ensure the persistence of nature-ecosystems. The latter corresponds to the limits on fluctuations in the different building blocks, and is seen as the complex of boundary conditions for persistence. Not only do the results of this comparison show a large gap between measures and conditions, they also suggest that some ecological conditions have not been taken into account at all.Respecting the boundary conditions for the quality of the natural substratum in nature- ecosystems will have far-reaching consequences for the management of the agro-ecosystems in the catchment areas of Beerze and Reusel. These consequences include, in the functional dimension, a reduction of the release of nitrogenous compounds and phosphate, and a reduction of the use of groundwater; in the spatial dimension, a reduction in acreage and in some zones a restriction on land use, due to the requirements of nature-ecosystems; in the biotic dimension, a reduction in the number of animals may be necessary. However, standards for the translation of the conditions for persistence of natureecosystems into acceptable agricultural management have yet to be developed.AN ECOLOGICAL APPROACH TO PLANNINGThe case-study in the Beerze-Reusel area leads to the conclusion that, without an ecological approach, the balance between political objectives, ecological conditions and adequate policies is seriously impeded. The regional plans do not lead to sustainable conservation of existing nature-ecosystems, and the realization of both adequate corridor zones and nature expansion areas in stream valleys seems doubtful. On the other hand, meeting the conditions for the quality of the natural substratum of nature-ecosystems would seriously affect the outlook for agriculture in the Beerze-Reusel area.In theory, combining of the concept of persistence with the hierarchical concept of ecosystems could provide planning with a promising basis for an ecological approach. From this study it is apparent that the coherence between the different ecological aspects of sustainable development necessitates coherence and integration of the different domains of physical planning. The implementation of the theoretical approach elaborated in this study will depend on whether or not the lacunae in ecological knowledge which were encountered can be remedied. In particular there is need for more extensive knowledge of the boundaries of the different attraction domains in which ecosystems can persist.