The theory of Chris van Leeuwen: Some important elements

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The theory of

Chris van

Leeuwen

Taeke de Jong - Ger de Vries

Sybrand Tjallingii - Kees Duijvestein

Dirk Sijmons

Some important elements

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The theory of

Chris van Leeuwen

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The theory of

Chris van Leeuwen

Some important elements

Taeke de Jong

Ger de Vries

Sybrand Tjallingii

Kees Duijvestein

Dirk Sijmons

(Eds.)

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Delft University of Technology Julianalaan 134

2628 BL Delft The Netherlands

Design: Cyril Strijdonk Ontwerpbureau, Gaanderen; dtp: Itziar Lasa Final editing: Dirk Dubbeling

ISBN 978-90-818111-4-9 NUR 922

No part of this book may be reproduced in any form by print, photoprint, mi-crofilm or any other means, without written permission of the copyrightholder.

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Acknowledgements

This book is a tribute to the pioneering work of Prof. C.G. van Leeuwen. The authors think his work and reflections deserve more attention of ecologists, planners and practitioners in environmental planning. This book offers an overview and we hope it will act as a source of inspiration.

Chris van Leeuwen (1920-2005) was a lecturer and later Professor of Ecology at the Faculty of Architecture of the Delft University of Technology from 1970 until his retirement in 1987. His work not only formed the basis for think-ing on sustainable built environments at the Faculty of Architecture, but also formed the basis for national policies on sustainable spatial planning. In par-ticular, the principle of ecological main structure, still in use in current poli-cies is based on his work.

After the Second World War, Chris van Leeuwen played an important role in mapping out areas of natural beauty in the Netherlands. His merit being that he had a tremendous practical knowledge of flora and fauna which he com-bined with a fundamental analysis of ecological relationships.

Van Leeuwen discovered a number of specific problem areas in relation to the environment in general. In order to explain these areas and to be able to tackle them, he made use of the general systems theory, in particular cyber-netics, and he developed a ‘regulation theory’ based on variations in time and space. Important elements within this theory are equality and difference, sta-bility and change and the boundaries, transitions or gradients in which differ-ences can more or less be distinguished as stable in the field.

The relation theory helps designers to grasp the dynamics of (natural) sys-tems and to understand these in relation to their plans. This enables them to predict the effects of interventions more effectively. A sustainable system will always have to seek a situation of stability, where development is possible, where large-scale regeneration is possible, where internal correction occurs during unexpected peaks or situations and where there is room for individual development and resistance to external influences.

Chris van Leeuwen published many papers, often in collaboration with oth-er authors. Howevoth-er, his course notes containing the basis of his theory woth-ere not published outside the university and were fragmented.

In order to keep Van Leeuwen’s work alive and accessible, we have opted for a structure in which in Chapter 1 Taeke de Jong and in the epilogue Dirk Sijmons explains what the meaning of the work of Van Leeuwen has been in their own work. This way the reader is introduced gently into the theory of Van Leeuwen. After this, we chose for a selection and adaptation of three of his more influential lectures given at Delft University of Technology. The orig-inal lecture notes have been used as far as possible.

Chapter 2 is based on Ekologie I. Beknopte syllabus [Ecology I, Abridged syl-labus] (1979-1980), which deals with ecology and the environment. Chapter 3 contains part of his lecture Ekologie [Ecology] (1973), which concerns the

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rela-tion theory as it was developed by Van Leeuwen. Chapter 4 returns to

Ekolo-gie I. Basale werking en hun effecten [Ecology I. Basic functions and their effects] (1985), which is about the tools. This text on the important elements in the theory of Van Leeuwen was produced by Ger de Vries, assisted by Taeke de Jong and Sybrand Tjallingii. In Chapter 5, Sybrand Tjallingii shows a process- oriented approach for design projects and in Chapter 6 Kees Duijvestein gives an explanation of design methods. Appendix 1 provides an abridged bibliog-raphy of Van Leeuwen’s work.

In his lecture notes Van Leeuwen often uses synonyms for similar concepts. We have attempted to harmonize this as far as possible. Where necessary, we have tried to avoid confusion by only using Van Leeuwen’s terminology.

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1 Introduction: Connecting

is easy, separating is

difficult

1 Taeke de Jong

1.1 Chris van Leeuwen

This chapter traces Dutch nature conservation thinking back to the arche-typal Dutch ecologist Van Leeuwen. As a student at the Faculty of Architec-ture in Delft my favourite lecArchitec-tures were those given by the architect Aldo van Eyck and the ecologist Chris van Leeuwen. Both emphasised the boundaries between spaces instead of the nature of the spaces themselves. “The bound-ary makes the difference; that’s where it happens”, they argued. After all, the task of urban and architectural designers is to draw boundaries to make spac-es visible and usable.

The emerging environmental awareness of the seventies ensured that the lectures of Van Leeuwen were popular. Shortly before his death he attended a conference dedicated to his work (Joustra & De Vries, 2004), organised by for-mer students in urbanism and architecture (see Figure 1.1).

His knowledge of nature was second to none, but at the same time he was an armchair scholar and writer of many widely published articles and lecture notes (Van Leeuwen, 1971) that surprised colleagues and fascinated designers (see Figure 1.2).

1.2 Open-closed theory

His ‘open-closed theory’ (Van Leeuwen, 1964) was the subject of a dispute with his friend and close colleague Victor Westhoff from the University of Nij-megen at the former national institute of nature conservation (RIN). With one of his students Westhoff developed a Dutch synecological system of life com-munities (Westhoff & Den Held, 1975), according to Braun-Blanquet (1964), that was elaborated on by his successor (Schaminee et al., 1995) and trans-lated into nature target types (Bal et al., 2001) (see Figures 1.3-1.5) that were applied in the actual policy of the Dutch ecological network (NEN). However, that operational approach now loses its foundation in the perspective of cli-mate change.

Van Leeuwen made field inventories himself for many years. Based on his experience he emphasised transitions between such supposed life commu-nities rather than determining the commucommu-nities themselves (Van Leeuwen, 1965). It was precisely there that he saw the most rare species, especially if such a transition was spun out along a broad strip (gradient) into an infinite

1 An earlier version of this chapter was published in: Jong, T.M. de, J.N.M. Dekker et al., (Eds.), 2007, Landscape

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range of unnamed particular environments on a smaller scale. There the eco-logically most interesting species settled.

1.3 Gradient map in national planning

The line of thought proposed in the open-closed theory became the guide-line for the Dutch Second National Policy Document on Spatial Planning (RPD, 1966), in which Van Leeuwen’s ‘Gradient map’ was published (see Figure 1.6). Citing RPD (1966):

Gradients are narrow zones with gradual intermediate stages between landscapes that have mutually very different life circumstances. Examples are contact zones between salt-water and freshsalt-water environments, between relatively dry and wet areas, between poor-ly and richpoor-ly nutritious landscapes and slopes in high areas. Within or directpoor-ly near these gradual zones a great gradation of environmental types exists within a small area and as a result a diversity of plant and animal species. Nearly all rare plant species in our coun-try can be found in such areas. Moreover, these are the regions in the Netherlands where natural forest edge thickets can develop.

What is also typical of these transitional environments is their ‘conservative’ nature. This ensures the continued existence of species found at these loca-tions, subject to the transitional environment not being disturbed fully by changes due to modern agricultural methods.

Van Leeuwen surprised colleagues by predicting to the square metre where a specific rare plant species could be found. For example, I once witnessed him when he was already at an advanced age looking around and indicating the place where the Carex pulicaris (‘flea sedge’) should grow. The warden of the area however had never found that species on his territory. Bystanders

Figure 1.1-1.2 Conference reader dedicated to Van Leeuwen and some books based on Van

Leeuwens work

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went down on their knees and sure enough found the predicted flea sedge. Van Leeuwen did it intuitively, based on his ‘phenomenal’ field knowledge.

1.4 Regulation theory

1.4.1 Relation theory

It was not possible, however, for Van Leeuwen to record his experience in writings other than by sketching a very theoretical framework known as ‘re-lation theory’. This theory has been referred to in many articles and elabo-rated on in different separate directions; always surprising by the unexpect-ed connections between ‘down to earth’ examples. It lunexpect-ed to his being made an honorary doctor at the University of Groningen in 1974. Almost 10 years lat-er critique came up about the mathematical robustness of the relation theo-ry (Sloep, 1983). The same critique, however, would apply to many other eco-logical theories. Gregory Bateson states it very clearly: “Science probes, it does not prove” (Bateson, 1979). Van Leeuwen’s theory is part of an explorative in-ductive process that generates general ideas about complex patterns and pro-cesses from specific observations. The rigour of reduction to formal logic does not do justice to the fruitfulness of van Leeuwen’s thinking in coming to grips with the complexities of man and nature interactions. Moreover, most read-ers and certainly listenread-ers got the feeling of a crystal-clear and simple frame-work that was relevant to many issues concerning design, spatial planning, urban renewal and nature conservation. Finally Van Leeuwen agreed to give his theoretical framework the more precise name ‘regulation theory’, accord-ing to his cybernetic references of steeraccord-ing and disturbaccord-ing.

Figure 1.3-1.5 Westhoﬀ’s plant life communities (Westhoﬀ & Den Held, 1975), Braun Blanquet’s

Plantsociol-ogy (J. Braun-Blanquet, 1964) and Westhoﬀ’s synecolPlantsociol-ogy translated into Dutch nature target types (Bal et

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1.4.2 Spatial and temporal variation

One of the first diagrams I remember from Van Leeuwen’s lectures shows some basic notions of that theory (see Figure 1.7). Firstly it shows the possibil-ity of a negative relation between pattern and process in ecosystems in terms of spatial and temporal variation. So, in general, diversity correlates to stabil-ity (often found near vague boundaries) and equalstabil-ity correlates to change (of-ten found near sharp boundaries). But I realised many years later that this rule cannot be applied on any level of scale if you take the scale paradox in-to account.

According to Ross Ashby (1957, 1956) ‘equality’ is not regarded as the oppo-site of ‘difference’ but as its near-zero-value. After all, any imagined

differ-Figure 1.6 ‘Gradient map’ in the Dutch Second National Policy Document on Spatial

Planning

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ence can always be made more different by adding attributes of difference (for instance difference of place, distance), but it cannot always be made less different. A difference less than the least difference we can observe or imag-ine is called ‘equality’. So, ‘difference’ and ‘change’, ‘equality’ and ‘stability’

Figure 1.7 Spatial and temporal variation in the theories of Van Leeuwen

Source: De Jong, 2007, derived from the lectures of Van Leeuwen in 1972

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in the diagram are all taken as values of ‘variation’ (the variable to be distin-guished spatially and temporally).

To see equality as a special kind of difference is contrary to the main pre-suppositions of standard mathematics, the science of equality (you can-not count different categories) and equations. However, chaos equations like yx+1=a*yx-(a*yx)2 where a>3.6 produces chaotic behaviour. The result of that formula is even different on different computers using different roundings off (see Figure 1.8). The same applies to very small differences in initial values in complex models producing very different results. The main problem is that the mathematical treatment of quantities presupposes qualitative categorisa-tion reducing differences to an ‘average’ (see Figure 1.9), tacitly supposed in set theory.

1.4.3 Disturbing and steering

Taking spatial and temporal variation further, Van Leeuwen supposed pro-cesses of a second order on both pattern (‘process on pattern’) and process (‘process on process’) called ‘differentiating’ and ‘steering’ with ‘equalis-ing’ and ‘disturb‘equalis-ing’ as zero-values (see the grey arrows in left Figure 1.10). Because these processes are changes as well, they are by definition disturb-ing and equalisdisturb-ing. Stoppdisturb-ing a process of disturbance is also a disturbance. Suddenly cleaning a ditch or decreasing the number of grazers could deteri-orate the condition of the ecosystem unexpectedly. The consequence of this view appeared to be a recommendation not to change conditions too sudden-ly: clean the ditch or decrease the number of grazers slowly according to the adaptation speed of the system.

So, according to Van Leeuwen, it is easier to break down differences

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ising) than to create them (differentiating) and at the same time it is easier to introduce changes (disturbing) than to guarantee duration (steering). This is a simple verbal expression of the second law of thermodynamics in the per-spective of cybernetics. Within that interpretation ‘life’ is represented as a phenomenon climbing up into local diversity and duration at the cost of glob-al disturbance located elsewhere.

1.4.4 Separation and discontinuity

Second order patterns and processes

Van Leeuwen’s regulation theory became more complicated as soon as he started to look for a second order of patterns as well: ‘pattern on pattern’ (‘structure’, ranging from ‘separation’ causing difference, into its zero value ‘connection’ causing equality) and ‘pattern on process’ (‘dynamics’, gradu-al (‘continuity’) or sudden (‘discontinuity’) changes and stops, causing stabil-ity or change). Later I realised distinguishing levels of spatial and temporal scales might simplify the argument and put it into perspective. Perhaps the primary supposition about a negative relation between pattern and process is limited to certain levels of scale that would explain exceptions. Perhaps con-cepts like ‘pattern on pattern’ are simply a question of scale. After all, ‘differ-ence’ is a scale-sensitive concept. Moreover, difference, equality, separation and connection are direction-sensitive.

Legitimate questions

Anyway, many legitimate questions remain. I will summarise some of them, but not answer them here. The very first question is: “Is this science?” How could you make categories as general as difference and change or separation and connection operational for tests by empirical research? Should you not distinguish different kinds of difference (for example abiotic, biotic differenc-es, differences observed on different levels of scale) to find mutual relations? What causes what? Are the second order variations dominant? Does separa-tion cause difference or the reverse? How could you imagine separasepara-tion with-out difference?

Elaborating on these questions we come across fundamental epistemologic questions similar to those I am familiar with from the debate about academ-ic design (De Jong and Van der Voordt, 2002). They go beyond critacadem-ics such as Sloep (1983) because equality itself is disputed. Consequently the use of cate-gorisation and classification within categories presupposed in any variable is attacked. The very core of that debate in practice is the question how to gen-eralise solutions of context-sensitive problems bound to specific unique loca-tions and contexts. That question applies to ecology as well, confronted with a confusing diversity of species multiplied by a diversity of specimens and con-texts. Management theory also struggles with the inapplicability of reduction

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into the ‘average’ (see Figure 1.9) from empirical science (Riemsdijk, 1999). From a designer’s point of view, many design decisions in specific contexts cannot be supported by empirical research aimed at generalisation. “That conclusion does not apply to this specific location!”, designers complain. Van Leeuwen’s approach offered a terminology directly fitting to design acts par excellence: separating and connecting. It functioned as a great heuristic tool, but many applications fell prey to confusion of scale through a lack of scale articulation. Let us now go back to ecological practice.

1.5 Meadowland as an extended fringe

Shortly before his death Van Leeuwen provided me with an illustrative exam-ple of an extended fringe. In between meadowland and forestland, in natural circumstances, a fringe emerges as a result of herbivore grazing (see Figure 1.10 and Figure 1.11).

Herbivores use their long necks to ‘mow’ across to the boundary of their reach without treading on or manuring the ground (floating head). By doing so, they create prototypes of meadowland. In meadowland (an extended fringe) that has not been manured and mowed without being treaded on you find species like Serratula tinctoria (saw-wort) which are not found elsewhere. Spe-cies-rich steppe grasslands like in the Ukraine and Russia are comparable to meadowlands. Why are there species-rich (hundreds per m2_{) grasslands and}

species-poor (one per ha) grasslands? Instability of a specific temporal scale between dry and wet, cold and warm, freshwater and saltwater seems to be the most important explaining factor. Such instability reinforces itself: a dense, solid soil emerges with Plantago major (the tread plant ‘common plan-tain’). Water is retained, but it also flows away easily. That is why even more powerful alternations between wet and dry, cold and warm arise, which can-not be endured by many plant species. In Moscow dryness is locally sup-pressed by the fire brigade, again reinforcing disturbance and condensation of the soil. A slope, however, acts as a stabilizer.

In the Netherlands Plantago major never grows on a slope, because the con-trast between wet and dry is too small. There, other plant species can sur-vive stabilising the environment even further. The Russian species-rich steppe, unlike a desert, has a stable water balance horizontally and vertically. A desert becomes brackish through evaporation and the resulting rising water (ascending moist flow). Salinization by irrigation is a well known phenome-non. So, a link between wet-dry, cold-warm, saltwater-freshwater alternation arises there, which does not happen in species-rich steppes. Against tempo-ral changes there are stable spatial transitions based on selective separation.

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1.6 Selectors and regulators in the landscape

1.6.1 Connection supposes separation

What I would like to emphasize in this case is the importance of inaccessibil-ity or isolation for large mammals. The awarded ‘Plan Ooievaar’ (Figure 1.12) concerning the future of an area around the Rhine and Meuse, from the Ger-man border downstream to the North Sea coast, is primarily based on separa-tion (De Bruine et al., 1987, see Figure 1.13). But since the concept of ecological networks (ecological infrastructure) gained momentum in the Netherlands, now connections (how organisms can reach a habitat) are primarily empha-sised, as opposed to separation of different habitats (why a habitat is a suita-ble environment for a species).

But to counterbalance this emphasis on connections, I would now like to concentrate on the importance of separations to arrive at the middle

(mi-Source: Vera, 1997

Figure 1.10-1.11 Metaphors for the wilderness, including an illustration from the book

(pruners, alternating grubbers, grazers)

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lieu). The concept of ‘structure’ (literally ‘brickwork’) comprises both sepa-ration and connection. It is this combination that produces particular envi-ronments where ‘specialist’ species are at ease. Researching such an environ-ment could be named ‘structure ecology’. In terms of regulation theory both isolation and connection are a value of separation. Connection is solely a zero value of separation. Connection supposes separation, not the reverse. There are no windows without walls. But there is ‘difference in separation’, always a combination of separation and connection while separation directs connec-tion into one direcconnec-tion.

1.6.2 Selectors and regulators

The first notable combination follows on from the ‘basic paradox of spatial arrangement’ as Van Leeuwen referred to it: the phenomenon of separation perpendicular to connection. A road is laid out to connect, but perpendicular to that connection it separates. A situation painfully obvious at crossings. The solution to connecting perpendicularly to the other connection is separating vertically (viaduct) or separation in time (traffic lights, see Figure 1.14 and 1.15). However, there are more combinations of separating and connecting. Features such as a deck, dam, gutter, pipe or bowl are examples of ‘selectors’ in one, two, three, four and five directions, selectively connecting into the other directions. That direction-sensitive connection quality cannot be envisaged without sep-aration into the other directions. Selectors ensure that not everything is go-ing anywhere. Taps, lids, valves, wedges and wheels are regulators ensurgo-ing that not everything is always going somewhere. Living organisms are com-plex combinations of selectors and regulators known in technology as mecha-nisms on different levels of scale (see Figures 1.16 and 1.17).

Figure 1.12-1.13 The ‘Plan Ooievaar’, with separation of nature and agriculture: zoning, selection, regulation

in one direction, ‘ecological networks’ in another direction

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1.6.3 Operational and conditional steering

The complex world of selective separation and connection occurs right down to the smallest scale in biology: the cell and its membranes. On these inter-faces, substances are selected and allowed to make connections with each other. The conditions for specific connections are created primarily by sep-arating substances that should not be connected (preselection). This already begins with the external membrane separating the inner environment from the entropic outside world, making less probable processes possible inside. This range of conditions and the endoplasmatic apparatus necessary to cre-ate the right conditions for the right connection is often forgotten when un-derstanding the isolated process of connection operationally (monocausally).

The endless range of conditional functions in the environment seems to require a different, perhaps typically ecological way of thinking, other than the single function with one clear product. Such processes are imitated in systems of retorts and pipes being the armamentarium of chemistry. Madame Curie needed four years to isolate 1/10 gram of radium from tons of pitch-blende. To dissolve sugar in our coffee is a daily activity taking seconds, but separating it afterwards requires much more effort. A heap of manure is easi-ly dispersed, but it takes years to extract it from the ecosystem.

Similarly, it is easier to destroy the subtle system of selectors and regula-tors of a living organism than to rearrange and synthesise it. A violent murder means demolishing separations, starting with those of the skin. Suppose now, an ecologically rare location is surrounded by a range of conditional func-tions we still do not understand completely. Is it wise then to make connec-tions for a few ordinary populaconnec-tions with botanically doubtful funcconnec-tions, but well known as cuddly by holyday-making visitors? Their equalising function in small areas could be that of an elephant in a china shop. Other (migrating) animals besides grazers do not fit in our small nature reserves. They belong in vast eutrophic areas elsewhere in the world. There they are needed as miner-al transporters comparable with pipelines connecting one-sided highly pro-ductive communities. A much larger number of smaller and more rare species of animals needing a smaller area could be better supported by diversification of the botanical foundation. You can wait which superstructure then develops instead of taking the summit of a food web as a nature target beforehand. You don’t start building a house with the roof.

Figure 1.14 Basic paradox of directions

Figure 1.15 Selectors

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1.6.4 Ecological networks

In the doctoral thesis of Van Bohemen (2004, see Figure 1.18-1.20), it is strik-ing that the ecological effect of the hundreds of million Euros spent on eco-logical connections is hardly ever assessed. The argument is this: you have to build a wildlife viaduct before you can measure its effect. That phase is now upon us. However, it is recognised that, just like in epidemiological research, cause and effect are difficult to separate. We still focus solely on the effect on populations of some popular species. Which effect the constructed

connec-Figure 1.17 Sluice closed and open

Source: Arends, 1994

Figure 1.16 Mechanical forms of selection and regulation by separating and connecting

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tions show on other species is even more difficult to determine. The deterio-rating effect of positive discrimination is well known from hanging up nest-ing-boxes: other bird species were ousted, insects died and the plant species for which the insects were postillons d’amour disappeared.

The impact of connections is sometimes demonstrably negative. Examples include the import of alders from Eastern Europe in the seventies or the con-nection of the Main-Danube canal. The concon-nection of all parts of the world to each other (globalisation) may be the greatest danger. Connecting genetically different races could cause loss of biodiversity. This leads to the subject that fascinates me most: levels of scale. At what level of scale is connecting the best strategy, and at what level of scale is separating the best strategy? The best argument for separating areas is the emergence of subspecies, though this can take a long time. A crucial question is: are we in the Netherlands in need of other large mammals besides grazers if they have better and more sustainable conditions elsewhere? In our wet country could we not create much more interesting ‘ecological conditions’ through separation (Tjallingii, 1996, see Figure 1.18-1.20 and Figure 1.21), conditions that are lacking every-where else?

A more moderate conclusion is that ecology cannot produce general state-ments, though politicians would like to seduce you into thinking so. That is what I learned from the PhD thesis of Mechtild de Jong (2002, see Figure 1.18-1.20). The methodological problem of scientific generalisation in the context-sensitive relations between one and a half million species of which we know so little, is something that ecology shares with context-sensitive design (De Jong and Van der Voordt, 2002) and management sciences.

The problem of the classical empirical ideal to produce generalising ments (out of bits and pieces and subsequently to deduce from these state-ments conclusions for specific cases) increases when realising that any spe-cies is made up of individuals that react differently to each other. This prob-lem increases even more so, when taken into account that any individual

Figure 1.18-1.20 Doctoral theses of Van Bohemen (2004) with ecological connection on the cover, Tjallingii

(1996) and De Jong (2002)

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grows up in a different environment and context.

An ecologist is not invited to copy solutions, but to find a common solution for a local field of problems using a unique concept. This is not solely an eco-logical network, but a more complete ecoeco-logical infrastructure.

1.7 Conclusions

Generalisation is a dangerous thing, especially if small differences can pro-duce great effects. This is the case in ecology. Biodiversity between species and between specimens within any species is multiplied by the number of contexts they live in. The physical and social context of any location is differ-ent from any other location because each location is unique, if only because of its situation between other locations on the Earth’s surface.

This diversity is insurance for life. But there are all sorts of differenc-es. Some of them we call ‘equality’: the basis of expectations. The ecological expectations for our common future are gloomy. But our imagination covers more than expectations; it opens up possible futures as well as probable ones. The modality of possibility requires a different way of reasoning than proba-bility.

Figure 1.21 Separating dry and wet flows

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In the advanced technology of pattern recognition the emphasis on simi-larity shifts to a focus on dissimisimi-larity (Pekalska, 2005). Following that track broadens the view into unexpected and improbable possibilities, opened up by difference. Differences are observable at boundaries. So, it is worth the effort to study boundaries rather than homogeneous areas. The boundaries determine the areas, not the reverse. Perhaps it will produce cross-border insight.

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bouw-meester van het natuurbeheer [Chris van Leeuwen, master builder of nature preservation], De levende natuur 86 (3), pp. 66-73.

Sloep, P.B., 1983, Patronen in het denken over vegetaties. Een kritische be-schouwing over de relatietheorie [Patterns in thoughs about vegetations. A critical view of the relation theory], Groningen (Stichting Drukkerij C.

Regen-boog).

Tjallingii, S.P., 1996, Ecological conditions: strategies and structures in en-vironmental planning (PhD thesis), Delft (Technische Universiteit Delft,

Fa-culteit Bouwkunde).

Vera, F., 1997, Metaforen voor de wildernis [Metaphores for the wildernes],

Den Haag (Ministerie van Landbouw, Natuurbeheer en Visserij).

Vogel, G. & H. Angermann, 1970, Sesam atlas bij de biologie (twee delen) [Biol-ogy atlas (two parts], Baarn (Bosch en Keuning).

Westhoff, V. & A.J.D. Held, 1975, Plantengemeenschappen van Nederland [Plants communities in the Netherlands], Zutphen (Thieme en Cie).

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2 The theory of Van

Leeu-wen: Ecology and

envi-ronment

1 Ger de Vries

2.1 Functional relationships

An organism cannot simply survive somewhere as an individual or as a spe-cies. An environment has to make that possible, it has to be able to offer that organism the opportunity to survive. If an environment has such opportuni-ties then it has the capacity to protect that organism or it has an ecological functional value for that organism. This ecological functional value is con-nected to the spatial structure of that environment and to the way in which it operates or functions.

Organisms vary from species to species with regard to their specific requirements for survival. Through differences in these requirements, a cer-tain environment will be of use ecologically to one species but not to another. The ecological functional value of a certain environment can remain con-stant throughout time or it can change. If the functional value remains the same then with the passage of time the same type of organisms will be found that existed there before.

Changes in the environment can entail for a species: the environment now has a functional value for it, or the level of the functional value has increased. Such a change signals an improvement or progress for that species. Howev-er, to a different species the same change can mean: that the environment has lost its functional value for it or the level of the functional value has decreased. Such a change signals deterioration or decline for that species.

2.1.1 The environment of an organism

So, the specific environment of an organism is connected to the question whether a certain environment has an ecological value or not in relation to the requirements for existence that apply to that organism. If a certain envi-ronment does display a functional value for that organism, then the specif-ic environment for that organism is present. Each single species has its own specific environment that differs from that of all other species. The specific environment of an organism can be seen as an optimum curve.

This specific environment runs as a line on the given scale between two limits, the minimum and the maximum. The limit in the direction ‘too little’ is the tolerance limit of the minimum requirement, the direction ‘too much’ is the tolerance limit of the maximum requirement permitted. Only when the

1 Chapter 2 is based on Ekologie I, Beknopte syllabus [Ecology I, Abridged syllabus] (1979-1980) which deals with ecology and the environment. The original lecture notes have been used as far as possible.

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specific conditions exist for that species in a certain environment, does that environment have a functional ecological value for that organism. It then has a protective effect for that organism. If the specific environment has not been achieved we find ourselves below the minimum limit (too little of something) or above the maximum limit (too much of something) and the environment will be useless to that organism and the organism will be harmed. De Jong (2006) describes this clearly as follows (see also Figure 2.1):

The curve of ecological tolerance relates the chance of survival of a species or ecosys-tem ‘y’ to any environmental variable ‘x’, the presence of water for example. In that spe-cial case survival runs between drying out and drowning. Imagine the bottom picture as a slope from high and dry to low and wet. Species A will survive best in its optimum. There-fore we see flourishing specimens on the optimum line of moisture (A); above or below it there are marginally growing specimens (a). But the marginal specimens are important for the survival of the species as a whole.

Suppose for instance that there are long-lasting showers. The lower, marginal specimens that are situated in an area that is too wet will die. The flourishing specimens become marginal but the specimens that are located high and dry will start to flourish. Long-lasting dry weather has a reverse effect. Levelling the surface and water supply for agri-cultural purposes in favour of one useful species means loss of another species and an increased risk for the remaining (De Jong, 2006).

Figure 2.1 The curve of ecological tolerance

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Duijvestein (2008) gives the temperature environment ‘life on earth’ as an ex-ample. Earth is located in a cold, dark space 150 million kilometres from the sun, its source of light and heat. If earth was closer to the sun, then there would be no liquid water present on earth and no life as a result (for it would be too warm). But if earth was further away from the sun than it is now, then all the water on earth would be ice and again there would be no chance of survival (for it would be too cold)(see Figure 2.2).

2.1.2 Disturbance or stabilization

If the specific environment for a certain organism is present, it is possible that:

1. the specific environment disappears as a result of a change in the sur-roundings;

2. the specific environment continues.

In the first case, the specific environment for that organism is disturbed. The protective capacity of the surroundings has been damaged for that organism. In the second case the ecological functional value of the surrounding for that organism is maintained. The protective capacity of that surrounding becomes protective itself. This latter protection is achieved by steering or stabilization. We can refer to this steering or stabilization as balance. Two types of balance can be distinguished:

1. a static balance: the stabilizing effect is called elasticity;

2. a dynamic balance (steady state): the stabilizing effect is called a counter reaction or feedback.

The question is what elasticity and/or feedback can offer. A system that is able to stabilize or protect in order to maintain the ‘form’ (the spatial struc-ture) and the ‘behaviour’ (the effect throughout time) of a different system. Such a system is called a selector-regulator, it can select and regulate. A se-lector-regulator is therefore a system that has the capacity to protect. In

oth-Figure 2.2 Earth is located at the correct medium between too cold and too hot Milieu

Sun Mercury

Milieu

mi-lieu

Venus Earth Mars

Too hot

The right centre Too cold

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er words, it is a functional system; a mechanism also referred to as an appli-cation or a device.2

2.2 Devices and their functions

In Section 2.1.1 it was stated that the specific conditions for an organism lie in a given scale between two limits (minimum and maximum) where the op-timum lies somewhere in between (a zone on the scale of ‘not too much’ and ‘not too little’; a zone in which the protective capacity of the environment is maintained).

Effects coming from the surroundings that steer towards ‘too much’ or ‘too little’ diminish the chance of survival. In the case of steering or stabilization, the question is therefore which types of disturbance or damage can be distin-guished that devices have to work against if they are to function by prevent-ing a ‘surplus’ or ‘shortage’.

In addition, the point of departure is a system or device in an environment with a source and a drain. The source is the part of the environment from where a flux such as energy, matter information flows. This flow (or at least a part of it) enters the system or device, then exits (or partly exits) the system or device and ends up in the drain as part of the environment where the flux or flow eventually ends up (see Figure 2.3).

Disturbance or degradation occurs when the tolerance limit for minimum conditions (towards too little) or the tolerance limit for maximum conditions (towards too much) is exceeded.

2.2.1 Four protective functions

In principal, there are four conceivable types of disturbance or degradation: too little in, too little out, too much in and too much out. The first type is re-ferred to as undernourishment, the second type as obstruction, the third type as overfeeding and the fourth type as leakage, plundering or loss.

A system or device can only retain its specific characteristics or function-2 To select = ensuring the correct quality and quantity of anything that contributes to the maintenance of the balance in question.

To regulate = ensuring that something constantly remains the same (= does not change) = stabilization = protec-tion.

Figure 2.3 An example of a black box for the built environment

building or neighbourhood or city

IN

SOURCE DRAIN

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al value when a second (or even possibly third, fourth, etc.) system is involved that is ‘good enough’ to act as a stabilizer or protector. Such a (second) device has to act against the risk of undernourishment, obstruction, overfeeding and leakage in the original device and thus perform four different functions, or parts of functions. These four functions are:

1. the anti-undernourishment or nourishment function 2. the anti-obstruction or discharge function

3. the anti-overfeeding or resistance function and 4. the anti-leakage or retention function.

2.2.2 Connections between the mutual functions

Within one device, the four functions will display connections. In the mod-el device or ecodevice designed by Van Wirdum (1982) the four functions are classified in a structured manner (see Figure 2.4). Nourishment and discharge are recovery functions. They are offensive; they serve to satisfy the require-ments of the system. Resistance and retention are shielding functions, some-times also called protection functions (see Figure 2.5).

Within this given device, one can imagine a second but also third, fourth etc., that can all profit from the protective function of the given device. With-in this second (third, fourth etc.) device, one can imagWith-ine devices that profit from that second device and so on (nesting) (see Figure 2.6).

2.2.3 Improvement/deterioration

Besides recovery and shielding, there is also the possibility of improvement in the situation of a certain species. Improvement means:

■

an increase in the functional value of a device

■

an increase in the capacity to protect, stabilize, preserve, or

■

an increase in regulation.

One important aspect of improvement is a growing specialization. An in-crease in the functional value for a more limited range than before gives a shift on the line between multifunctional to more monofunctional.

An example of a shift from multifunctional to monofunctional is a clay dike (= suitable for water resistance = also suitable for plants and animal life), compared to an asphalt dike (= resistance to water is enforced (improvement) = unsuited to plants and animal life (deterioration)).

What is an improvement for one species, can be a deterioration or a decline for another species. The environment loses its functional value for that spe-cies or the level of its functional value declines. And so improvement and

Figure 2.4 Ecodevice or model appliance

Source: Van Wirdum, 1982 IN

NOT IN

OUT NOT OUT

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deterioration often co-exist. An improvement for one is often a deterioration for another. An illustrative example of the saying ‘there’s no such thing as a free lunch’.

There are two types of improvement:

■

Type 1: improvement of useless to more or less useful or from more or less useful to even more useful. An example of this is land that is made suitable for certain types of organisms that could not survive there before. This is an improvement for these species. But this also implies that the land simulta-neously becomes unsuitable for other species that could survive there be-fore but can no longer survive there. For these species it is a deterioration.

■

Type 2: improvement to a higher level. This type of improvement entails an increase in specialization and refinement of the selection and regula-tion mechanism within the given device. In addiregula-tion, the significance of the shielding functions increases and the significance of the recovery functions decreases. Likewise there is an increase in efficiency, precision work, frugal-ity (from material waste to recycling), a reduction in energy consumption and an increase in information consumption.

2.2.4 Conservation and progression

Conservation and progression sound as though they are opposites, but they are both concerned with protection. The opposite of protection is damage; a decrease in the functional value of a device. Damage is synonymous to terms like ‘deterioration’ and ‘retrogression’. Particularly when related to a shift from ‘complex’ to ‘simple’ (= from fragile to robust) we refer to ‘regression’ or ‘disintegrative succession’.

Protection is partly a question of maintenance (shielding) and partly improvement (recovery). Maintenance comes down to the retention or conser-vation of a functional value. In addition, shielding (maintenance) provides a defensive conservation and recovery (improvement) an offensive conservation. Improvement is an offensive increase in the capacity to protect or preserve. Instead of ‘improvement’ we can also refer to the ‘progress’ or ‘progression’ of

Figure 2.5 Four protective functions

Resistance

Nourishment Discharge

Retention Recovery functions

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a device, therefore a ‘better conservation’.

Progression is more to do with the development from ‘simple’ to ‘complex’, from robust to fragile, with for example an increase in the significance of the protective functions and a decrease in the share of the recovery functions. A constructive succession is therefore called ‘progressive succession’. It is based on an increasing conservation ability of the environment.

Improvement and deterioration/conservation and progression leads to a number of difficult questions. According to Van Leeuwen ‘better’ means bet-ter for one species. But which species do you choose, asks Sybrand Tjallingi (1996).

And which species is ‘more equal than others’ and how do human values fit in?

Also according to Van Leeuwen, from simple to complex is progression, is better, more robust. But is this actually true? Some simple systems are neces-sary for the protection of fragile systems. On a scale of the landscape, there-fore, a combination of simple and complex is ‘better’.

2.3 Fundamental selectors and regulators

Devices of a mechanical material nature appear in principle to be constructed from seven basic elements or fundamental selectors and regulators. They all work on the basis of spatial separation and spatial connections. These funda-mental basic elements or selectors and regulators are (see Figure 1.15):

■

the wedge: in principle a wedge can provide all four functions (nourish-ment, discharge, resistance and retention);

■

the dam (vertical) or the deck (horizontal): dams and decks provide resist-ance and retention;

■

the bowl or the roof: the primary function of bowls and roofs is resistance and/or retention, but bowls can also be used for the nourishment and dis-charge function;

■

the pipe: a pipe can be used for all four functions;

■

the net: a net is suitable for all four functions;

These selectors and regularors have a static nature. There are also dynamic ones:

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■

the tap: suitable for all four functions;

■

the wheel: an ideal regulator to obtain and sustain a dynamic balance (steady state). An example is our planet that turns on its axis and to which the rhythm of day and night and ebb and flow are connected.

2.4 Fragile as opposed to robust devices

From a functional point of view devices are always involved with other devic-es, that can again be considered to be a device. Devices serve each other or use each other. One device has a practical value for the other.

The practical value of a device B for another device A is based on the pro-tective capacity of B to sustain or improve the practical value of A. Devices serve or protect each other. You can also say that they use (are protected by) each other (see Figure 2.7).

The further we become separated from A in the sequence, the more the device concerned will have to cushion the blows for all that it serves. B’’’ serves B’’+ B’ + B + A. In the above sequence, A is dependent on everything else, everything else is essential for it.

Devices in position A are called fragile and in position B’’’ are called robust. Without the protection of B + B’ + B’’+ B’’’, A is lost. But, B’’’ can survive with-out any necessary involvement of A, B, B’ of B’’(conditionality).

It appears that the most highly developed, spatially complex living commu-nities are in position A. By contrast, the lowest developed, spatially simple liv-ing communities are in position B’’’.

Highly developed, spatially complex living communities are distinguished, among other things, by a large variety of types of organisms, by sensitive internal regulation mechanisms, reliance in particular on protection func-tions, by recycling of material and by a relatively low energy consumption, but a wide use of information. Lowly developed, spatially simple living com-munities rely, among other things, on repair mechanisms.

Sybrand Tjallingi (1996) gives examples as city parks, tundras and tropical forests. A city park can only be peaceful if there are robust systems surrounding it,

for example roads, sport fields, water storage ponds, etc. But sometimes sport fields and water storage ponds are part of the park. In that case, the robust parts protect the fragile parts of the park. The question of scale is therefore essential.

Tundras and tropical rain forests illustrate something else:

■

in a highly dynamic environment, only robust systems survive and these are not so vulnerable to storms and frost (tundra);

■

in a less dynamic environment a very species-diverse fragile system can develop, though it is vulnerable to felling and fire (rain forest).

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2.5 Gradients

Spatial boundaries meet where different habitats converge. At this contact point there are two possible situations for the type of boundary:

1. the distinctly marked, sharp boundary (limes convergens) or 2. the blurred, vague spatial transition or gradient (limes divergens).

In the case of boundary type 1, the change between the two converg-ing habitats (for example black and white) is very abrupt. Sharp boundaries occur when the order of ranking relationships between the two habitats that meet each other are such, that the dominant, i.e. the most dynamic party, can dominate without restrictions over the underlying, less dynamic condition of both.

In the case of boundary type 1 there are concentrated processes, while the spatial pattern is characterized by coarse graininess. We refer to this as ‘limes convergens’. From a biological perspective, a coarse-grained spatial pattern is expressed in a lack of species and large amounts of each species. Coarse graininess and a lack of species is characteristic for a robust environmental device. Examples are salt pastures and reed-land.

In the case of boundary type 2 the spatial transition from white to black is represented as a slope where the two extremes exist in all possible interme-diate states in all shades of grey. Blurred boundaries exist if there is a reverse balance of power, i.e. when the less dynamic conditions can dominate above the more dynamic. This latter situation can occur through spatial connections between two opposites that converge. To this end, the underlying party from a dynamic point of view must either occupy a larger surface area (respectively volume) or must profit from gravity by being situated higher than the dynam-ically dominate condition.

Boundary type 2 depends on dispersion processes, where the spatial pat-tern is characterised by a fine texture. This is referred to as ‘limes divergens’. In vague boundaries you find a variety of species, where the numbers of each species is small. The fine grain is characteristic for a fragile environment device. Examples are dune grasslands and a verge covered in flowers.

Between both these extremes intermediate types exist. A classic of this being a waterfront (see Figure 2.8).

Sybrand Tjallingii (1996) noticed: The limes divergens is the most

interest-ing: because of the large variety of species. The shape is vague. This is interesting for designers if they want to show vague shapes. Most designers don’t want this; they look for clear, sharp shapes. The limes convergens fits here. As a result there are often conflicts between designers and ecologists.

Van Leeuwen does not tackle the design issue. The relevant question to ask

Figure 2.7 Mutual involvement of devices

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in this discussion is what combinations of robust and fragile systems, clear shapes and biodiversity are favourable.

References

Duijvestein, C.A.J., 2008, Presentation Ecological Building, Delft.

Jong, T.M. de (Ed.), 2006, Sun wind water earth life living: legends for design,

Lecture paper (Zoetermeer).

Leeuwen, C.G. van, 1971, Cursus natuurbehoud en natuurbeheer [Course on nature conservation], TH Delft, Afdeling Bouwkunde.

Leeuwen, Chr. G. van, 1979-1980, Ecologie I, Beknopte syllabus [Ecology I, abridged syllabus], Technische Hogeschool Delft, Afdeling Bouwkunde,

Vak-groep landschapskunde en Ekologie, Delft.

Tjallingii, S.P., 1996, Ecological conditions: strategies and structures in en-vironmental planning (PhD thesis), Delft (Technische Universiteit Delft,

Fa-culteit Bouwkunde).

Wirdum, G. van, 1982, The ecohydrological approach to nature protection, in: Annual Report 1981, Research Institute for Nature Management, Arnhem/

Leersum/Texel (Research Institute for Nature Management), pp. 60-74.

Front Wave front Gradient

Source: De Jong, 2006, p. 393

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3 The theory of Van

Leeu-wen: Relation theory

Ger de Vries & Taeke de Jong

Chapter 3 is based on Van Leeuwens’ lecture synopsis Ekologie [Ecology] (1973), following the original text as far as possible, supplemented with short notes by Taeke de Jong (in italics).

Van Leeuwen often used synonyms in his lecture synopsis for identical con-cepts, for instance contact for link and link for continuity. On top of that it is confusing that he uses the same terms for the basic relations and the values of the fundamental relation type Variation (Equality, Difference, No change and Change). We tried to harmonise this in this chapter. Where this could lead to vagueness we maintained Van Leeuwen’s terminology.

3.1 Introduction

Rrelations occur between the properties of systems. These properties in turn can result from relations or networks of relations. The results or consequenc-es that such a relation has for the property involved and thus the related sys-tems is called a mechanism. And the result that is related to such an mecha-nism, is an effect.

The general systems theory, including cybernetics, is related to com-plex systems or networks of relations. Hierarchic relationships exist within and among such systems and the properties of such a system are not sim-ply derived from the sum of the properties of its components, for example the games theory and the information theory.

An important principle is feedback. In the reciprocal relations or exchang-es between two elements A and B of a system, A can have an effect on B and B can produce feedback on the effect that A had on B. This occurs in the form of a loop or circle (see Figure 3.1). If the feedback of B on A is such that the effect of A on B is limited, then it is called a counter-effect (or negative feed-back). An example is the regulatory effect of a thermostat. If the feedback of B on A is such that the influence of A on B is reinforced, then it is called posi-tive feedback (or a snowball effect). An example of this is the phenomenon of unrestrained growth.

Using experiences in landscape ecol-ogy, Van Leeuwen designed an addi-tional theory that was closely connect-ed to the general systems theory. He searched for very fundamental rela-tion types and for very fundamental relations between these types. This is why he called this theory ‘relation the-ory’ or the ‘theory of basic relations’.

In relation theory, space and time play an important role in the

funda-Figure 3.1 Feedback

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mental relation types with variation, discontinuity and uncertainty as vari-ables. Van Leeuwen gave these variables absolute and relative values (0 val-ue and ∞). Therefore he sees that variation in space results in difference, for example in differences in vegetation, a relative value as opposed to the 0 value of equality. More difference is conceivable. It is impossible to be more equal than equal. Equality is therefore the absolute value of the variable ‘spa-tial variation’.

Taeke De Jong explains: Equality and difference do not constitute opposites. They

are values of ‘variation’ and in this way equality is considered to be a particular kind of difference: (approaching) its nil value ‘no difference’. De Jong produced a scheme showing how the mutual relations are connected to each other (see Table 3.1). The table also includes for each variable the absolute and relative value (0 and ∞). In the case of the spatial and temporal variables, these values are linked to an intermediate concept (pattern, process, structure, dynamics and feedback). Section 3.6.3 goes into more detail about these intermediate con-cepts.

Tjallingii (1996) uses partly the same variables and values for the more fun-damental relation types space and time. Whereas Van Leeuwen uses varia-tion, discontinuity and uncertainty for the relation types, Tjallingii uses vari-ance, regulation and information. Also in Tjallingii’s scheme the absolute and relative values are used per variable (see Table 3.2).

3.2 Variation

In relation theory, variation is the primary variable of the fundamental rela-tion types ’space’ and ’time’. In the case of spatial variarela-tion, Van Leeuwen us-es ‘difference’ as a relative value and ‘change’ for temporal variation. There

Table 3.1 Space and time with fundamental relation types according to Van Leeuwen

Space 0 value Equality Connection Unobservable Time ∞ Difference Separation Observable 0 value Remain unchanged Endurance Predictable ∞ Change Disruption Coincidental Variation Discontinuity Uncertainty

Table 3.2 Space and time with fundamental relation types according to Tjallingii

Space 0 value Equality Connection Familiar Time ∞ Difference Separation Unfamiliar 0 value Remain unchanged Endurance Predictable ∞ Change Disruption Coincidental Variation Regulation Information

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are, however, different layers of variation (see Figure 3.2). These layers of spa-tial end temporal variation are explained in more detail below.

3.2.1 Spatial variation: ‘equality’ vs. ‘difference’

Van Leeuwen uses the concept of difference for comparisons in space, for ex-ample ‘here’ and ‘there’ or ‘this’ and ‘that’. Difference is not categorised fur-ther (difference of place or nature) and fur-therefore remains very general within the distinction made between spatial and temporal variation. Differences ex-ist for example in altitude, soil condition, light intensity, the strength of water current and so on. But there are also differences in type, gender and behav-iour. Differences can be large and small and with a mutually different nature (a difference in difference).

When little or no difference can be observed (the level of spatial variation is approaching nil) we refer to this as equality. Equality and difference are not equal opposites, because equality is a kind of minimal or unobservable differ-ence. This asymmetric ‘relation’ is a relation between relations or a relation of the second order.

It is impossible to be more equal than equal. Equality is therefore absolute. More difference is always conceivable by the addition of other differences. Therefore there can be different differences. Difference is relative (see Figure 3.2a).

Differences in place and sort

Take for example differences in place and sort. A difference in place is a difference that is presupposed for every comparison, otherwise there can be no comparison. An object that fills the same space as a presupposed other object must be the same object and is therefore not suitable for comparison. It can be concluded from this that objects (plural) that are equal in all respects must still differ from each other in place. Equal-ity of sort with a minimum difference in place is therefore the least imaginable

differ-0 ……….……… ∞ Equality is absolute. Diﬀerence is relative. 0 ……….……… ∞ Remain unchanged is absolute. Change is relative. 0 ……….……… ∞ Connection is absolute. Separation is relative. 0 ……….……… ∞ Endurance is absolute. Disruption is relative. 0 ……….……… ∞ Observable is absolute. Unobservable is relative. 0 ……….……… ∞ Predictable is absolute. Coincidental is relative. a) b) c) d) e) f)

The theory of Chris van Leeuwen: Some important elements

The theory of

Chris van

Leeuwen

Taeke de Jong - Ger de Vries

Sybrand Tjallingii - Kees Duijvestein

Dirk Sijmons

Some important elements

The theory of

Chris van Leeuwen

The theory of

Chris van Leeuwen

Some important elements

Taeke de Jong

Ger de Vries

Sybrand Tjallingii

Kees Duijvestein

Dirk Sijmons

(Eds.)

Contents

Acknowledgements

1

Introduction: Connecting

is easy, separating is

difficult

1

Taeke de Jong

1.1 Chris van Leeuwen

1.2 Open-closed theory

1.3 Gradient map in national planning

Figure 1.1-1.2 Conference reader dedicated to Van Leeuwen and some books based on Van

Leeuwens work

1.4 Regulation theory

1.4.1 Relation theory

Figure 1.3-1.5 Westhoﬀ’s plant life communities (Westhoﬀ & Den Held, 1975), Braun Blanquet’s

Plantsociol-ogy (J. Braun-Blanquet, 1964) and Westhoﬀ’s synecolPlantsociol-ogy translated into Dutch nature target types (Bal et

1.4.2 Spatial and temporal variation

differ-Figure 1.6 ‘Gradient map’ in the Dutch Second National Policy Document on Spatial

Planning

Figure 1.7 Spatial and temporal variation in the theories of Van Leeuwen

1.4.3 Disturbing and steering

1.4.4 Separation and discontinuity

1.5 Meadowland as an extended fringe

1.6 Selectors and regulators in the landscape

1.6.1 Connection supposes separation

Figure 1.10-1.11 Metaphors for the wilderness, including an illustration from the book

(pruners, alternating grubbers, grazers)

1.6.2 Selectors and regulators

Figure 1.12-1.13 The ‘Plan Ooievaar’, with separation of nature and agriculture: zoning, selection, regulation

in one direction, ‘ecological networks’ in another direction

1.6.3 Operational and conditional steering

Figure 1.14 Basic paradox of directions

Figure 1.15 Selectors

1.6.4 Ecological networks

connec-Figure 1.17 Sluice closed and open

Figure 1.16 Mechanical forms of selection and regulation by separating and connecting

Figure 1.18-1.20 Doctoral theses of Van Bohemen (2004) with ecological connection on the cover, Tjallingii

(1996) and De Jong (2002)

1.7 Conclusions

Figure 1.21 Separating dry and wet flows

2

The theory of Van

Leeu-wen: Ecology and

envi-ronment

1

Ger de Vries

2.1 Functional relationships

2.1.1 The environment of an organism

Figure 2.1 The curve of ecological tolerance

2.1.2 Disturbance or stabilization

oth-Figure 2.2 Earth is located at the correct medium between too cold and too hot Milieu

Milieu

mi-lieu

2.2 Devices and their functions

2.2.1 Four protective functions

Figure 2.3 An example of a black box for the built environment

2.2.2 Connections between the mutual functions

2.2.3 Improvement/deterioration

■

■