Distinguishing Ecological Engineering from Environmental Engineering

Ecological Engineering 20 (2003) 389–407

Distinguishing ecological engineering from environmental engineering

T.F.H. Allen a,∗, M. Giampietro b, A.M. Little a

a Department of Botany, University of Wisconsin Madison, Madison, WI 53706-1381, USA b INRAN-Unit of Technological Assessment, Rome, Italy Accepted 4 August 2003

Abstract

This paper uses complex system thinking to identify key peculiarities of ecological engineering. In particular it focuses on the distinction between the purpose-driven design of structures in environmental engineering and the natural process of self-organization characteristic of life, which needs to be integrated into ecological engineering. Conventional engineering addresses the problem of fabrication of an organized structure, say a road, which reflects a goal at the outset, as well as considerations external to the road. At the outset there is an essence of which the organized structure is a realization. This realization belongs to a certain type (apartment building, suspension bridge). The type is in relation to: (a) the expected contexts (e.g. housing in Manhattan, a bridge in rural Africa) and (b) location-specific socio-economic constraints (low/high economic budget). Conventional engineering does not question the goals of the selected plan and can only object to the feasibility of a proposed typology in a given context. Engineers deal with the challenge of the realization of a plan at a given point in space and time. The central dogma of biology identifies organisms as informationally-closed and this makes possible their use as machines. Ecological systems, on the contrary, are informationally-open. They cannot be used as machines to create functional structures, because they are becoming in time. For engineered structures to work it is usually required that there is (1) stability of system components; (2) admissibility of a workable context; (3) validity of purpose and concept. Ecologically-engineered structures challenge these requirements because of specificity of required environments and lability of system parts over the time the engineered structure functions. Other engineering is better if it achieves flexibility, but ecological engineering must be so flexible as to take on a looping character that updates the system to meet changing requirements. Accordingly, the original goals cannot be taken for granted later in the process of ecological engineering. Ecological engineering has to be a flexible iterative process of design, in which the designer must continually update goals, essences, typologies and processes of realization. © 2003 Published by Elsevier B.V.

Keywords: Ecological engineering; Complex systems thinking; Hierarchy theory; Rosen; Multiple scales; Beavers; Self-organization

1. Introduction

Ecological engineering is one of several branches ∗ Corresponding author. of engineering that involves dealing with living ma- E-mail address: [email protected] (T.F.H. Allen). terial. Sometimes technological processes use living

0925-8574/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.ecoleng.2003.08.007 390 T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 organisms as machines, as in draft animals, fermen- are genuinely ecological. There are good reasons to tation agents or genetically-modified organisms. For cleave ecological engineering from civil and industrial the purposes of this paper, whether the horse power engineering, the orthodox categories of engineering to pulling a given load is generated by real horses or which environmental engineering naturally belongs. by a horse-equivalent generated by a steam engine is We believe that a solid body of knowledge already not the critical part of the story. The important point exists in the field of theoretical ecology, hierarchy the- for our narrative is that biological material introduced ory, and complex systems thinking that can clarify the into an engineering process or engineered structure situation. This clearer vision can generate in ecolog- requires a richer characterization of what should be ical engineering something of the sound predictions considered as a machine or an ecological system. This and proper precaution that are characteristic of engi- paper addresses that richer characterization, and what neering at large. However, this can only occur when it means for grouping types of engineering with their ecological engineers are conscious of the distinctive- respective distinct styles. ness of their practice relative to other types of en- Use of biological material has generated some con- gineering. The theory to which we refer introduces fusion as to the categorization of engineering in bio- a clear distinction between: (i) a process of design logical and ecological settings. Various adjectives and and fabrication of machines driven by human purpose, nouns have been freely combined in terms such as i.e. environmental engineering as described above and “ecological engineering”, “genetic engineering”, “bi- (ii) the processes of autopoiesis (self-definition) and ological monitoring” or “bio-chemical engineering”. self-organization (emergence or order) typical of life Sometimes entirely new terms have been coined, such and ecological systems (Maturana and Varela, 1980, as “bionics”. This new terminology, with its prefixes 1998) i.e. ecological engineering. of bio-, genetic-, or eco-, encourages a mistaken view This paper is divided in three sections. Section 1 that systems engineered to employ living things still deals with basic differences between conventional en- belong to the realm of ecology and ecological systems. gineering (which in our view still includes environ- Organisms employed as machines may not be as reli- mental engineering) and ecological engineering. This able as physical machine, but they can be constrained distinction is framed using concepts developed in the well enough so as to behave like ordinary machines. field of complex system thinking, especially in rela- In beer brewing or in cheese vats the context of the tion to theoretical ecology (i.e. the characterization organisms is tightly and skillfully constrained so that of “life-itself” as developed by Robert Rosen (1991, the cultures perform with reliability. The required con- 2000)). stancy of those created environments means we are Section 2 focuses on the peculiar challenges faced dealing there with engineering not ecology. Such sys- by ecological engineers. While organisms may be tems do not suffer from the constant change that is used as machines, we deny that ecological systems can ubiquitous in ecology at all levels. By contrast, what be used as machines to create ecologically-engineered the authors here call ecological engineering is still part functional structures. Unlike organisms, ecological of ecology and so must deal with the process of change systems are informationally-open, and cannot be used that can be expected at all levels. Environmental, not reliably in the medium term, let alone for extended ecological, engineering generally works only with the long-term periods. Rather than work as agents of structure. That is to say, environmental engineering creation, ecological processes act as constraints and lists its components and evaluates the effects of the perturbations on both the environment of the realiza- ecosystem on the components. Therefore environmen- tion process and the functional engineered structure. tal engineering remains part of engineering, although Koestler (1967) refers to the associative context as it has an awareness of ecology. Environmental engi- the context in which a process may occur or a struc- neering uses organisms as machine, but that does not ture may exist, part of which may be the context that make it ecological engineering. Confusing organismal makes the function of an engineered structure useful. machines with ecological situations is dangerous, be- Ecological engineering deals with structures whose cause it encourages over-confidence in the calculations realization process and structured functionality is dis- and predictions as to the engineering of situations that rupted by failure of the associative context such that T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 391 the original goals must be revisited. This is due to the give living material the constancy of behavior that systemic mismatch in time scale between: (a) the pace normally characterizes non-living materials. An ex- at which human decision making and engineering op- ample might be the engineered environment that keeps erates and (b) the pace at which ecological processes bacteria behaving in the desired manner of a sewage update their typologies and mechanisms of control. works. By contrast, the ecological engineer attempts Section 3 discusses the implications of a case study to live with the dynamical behavior of life and its con- provided by ecological engineering performed by stant change of purpose. There is a biological learning beavers: beavers show flexible/multiple scale strate- process here that is not often part of conventional gies. Beavers engineer at different levels of analysis engineering. In contrast to all other engineering, the simultaneously. This enables them to deal better with ecological engineer co-opts a creative process that is the complexity of ecological processes on different intrinsic to the emergent biological structure. Engi- scales. The cost to the beavers is that this forces them neers facing ecological dynamics would do well to to adapt their identity in the cause of flexibility. Adap- pay attention to the inescapable emergence that is em- tation of identity translates for humans into changing bodied in living material, for it demands a distinctive the set of driving goals, a solution that often would style and invokes a new approach. Ecological engi- be unacceptable to the client of a conventional human neering amounts to surfing the vortex of some emer- engineer. gent property, and so is often perceived as uncertain. Some ecological issues span a long period of time, when considering the pace of human affairs. All eco- 2. The basic difference between conventional and logical issues, even the ones that span a short time, ecological engineering last a long time relative to the stability properties of individual ecological players and their immediate There is a continuum between environmental en- ecological context. Many of the differences between gineering and ecological engineering. Nevertheless, engineered and ecological material may be seen as ecological engineering becomes distinctive in its fun- simply an issue of relative scale. The goals for con- damental approach. Environmental engineering is an ventionally engineered material are set at a scale such extension of the engineering process that considers that the substance of the engineered product is rela- the environment in as many aspects as are thought to tively inert over the time it takes to fulfill the goal. be to be relevant. Engineering requires models to be In the end all bridges do come down, but usually by constructed of the action to be undertaken, consider- design of demolition after the many decades, or even ing safety factors only at the anthropomorphic level, a century or two of service. Over the expected life of not at the level of the safety of the ecosystem. Soci- an ecologically-engineered structure, its context may ety is blessed by the caution and integrity that exists cause it to change in form and function. Meanwhile the across engineering disciplines. Environmental engi- material of the ecologically-engineered structure may neering is a branch of civil and sometimes industrial also completely change its components—replaced, engineering. As such it remains within the purview as the woodsman’s axe that is five handles and two of standard engineering protocol as it imposes an ex- heads old. ternal design on material that is the passive recipient If ecologists built bridges, they would commonly of engineered limits. Not so for ecological engineers, fall down. Petroski (1993) notes that there is a cycle whose engineered material offers no such constancy. of bridge failure over about 25 years where almost all This one fact puts ecological engineering in a different bridges of a type serve well and safely, but in the end class from all other engineering, including environ- one particularly ambitious extension fails, for instance, mental engineering. Plants, animals and bacteria are the Tacoma Narrows bridge disaster. Ecologists are not as predictable as steel and concrete, because life no less intelligent, able, or diligent than engineers. It indeed has a life of its own. Environmental engineers is rather that the basic assumptions and time differ- constrain that creative force of life, so that it can be ential typical of ecological processes force a different used successfully similar to the successful use of steel style. Ecologists expect failure of their predictions and in civil engineering projects. Environmental engineers engineered solutions, because of the nature of their 392 T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 material. Simulations in complex ecological systems As to change in biologically engineered material, are not designed as predictors, but rather as offerings over only a decade or so insects became resistant to of reasonable scenarios, so as to generate better spec- DDT. Although the dimensions of the problem may be ulation not prediction. The apparent rigor of popula- large in conventional engineering, they can be counte- tion biology is illusory, because it is rarely applied to nanced. By contrast, in biology the number of dimen- material outcomes, and consists of arcane issues with sions to the problem can be so large as to be incalcu- little application (Grime, 2002). Of course, ecological lable. In the biology of DDT, two apparently innocent engineers could design structures to function within facts became crucial before anyone realized that they the short periods over which ecological entities do not mattered: organisms differentially scavenge for fat so turn over, but society generally demands longer term as to hold on to it for themselves, and DDT happens service from ecologically-engineered devices. The be fat-soluble. The model was that DDT would go ev- style of ecologists is not particularly useful in design- erywhere, and so be diluted below toxic levels. DDT ing or checking the stability of a bridge in the here behaved unexpectedly because it did go everywhere, and now. but then bioaccumulated in exactly the wrong places. What happens when the goals for ecological design The ecological bridge fell down long before abandon- and management are stretched over a time window ment of DDT had been designed. much wider than the one at which the stability of the Ecologists are no engineers, but they are used to engineered structures can be assumed as a given? Such wallowing in the mire that might spatter the engineer an extension of time for desired function happens with at margins of ecological engineering. Ecologists spend attempts at sustainable development. Sustainability ex- much more time describing what nature offers than do tends the time, but the development means there are engineers. The combination of the richness of itinerary too many changes for the ecological engineer to main- and richness of outcome makes ecology very differ- tain a solution for the problem. A dramatic enlarge- ent from engineering. There is much more description ment of the relative time horizon requires discussing in ecology than in engineering, because there are so not only the validity of the assumptions about the sta- many more types of component and self-organizing bility intrinsic to the given structure, but also the sta- pattern to describe. Moreover, there are more overt bility of the original associated context in which the levels of control in living systems. Whereas steel has engineering took place. In turn, this translates into the predictable properties because of what it is, living ma- need to discuss anew the validity of the original set of terial is created by explicit codes that exist indepen- goals that started the process of engineering in the first dent of anyone who would manipulate life. place. That is why ecological engineers should avail themselves of the patterns of thinking of ecologists, 2.1. The process of design of machines and the so that they can see a problem as one of ecological process of self-organization of life engineering when it presents itself. At its limits, ecological engineering gives way com- We now turn to a few general concepts emerging in pletely to ecology, with ecology’s low batting average the field of complexity which are related to hierarchy in relation to the satisfaction of human goals. Ecolo- theory and scaling. This is to show that some of our gists are used to having their predictions go awry. By more radical statements later do indeed have an intel- making the style of ecological thinking explicit, this lectual history; we cannot be accused of pulling our paper is written to make biological thinking accessible ideas from nowhere. to those engineers who need it. Almost paradoxically, In order to make sense of their perceptions of an ecological thinking can make the ecological engineer external reality, humans have to organize and share more predictive. Agriculture is a matter of ecologi- their perceptions. They do this by assigning pieces of cal engineering. It expects varieties of crops to yield experience into types, technically called epistemic cat- to agents of disease, and the agricultural researcher egories. An epistemic category is a device for recog- is always breeding new varieties to accommodate the nizing things that are different while being equivalent change. Ecologists are used to their projects being on- in some way. In this light, epistemic categories earn going, and they engineer accordingly. the alternative appellation of an equivalence class to T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 393 emphasize difference touched with similarity (Rosen, For example, when using the word “dog” we re- 2000). The process through which humans organize fer to any individual organism belonging to the their perceptions so as to make sense of them, and so species “Canis familiaris”. The characterization as to facilitate development of a common language, of the holon “dog” refers to the set of relational has been studied by several authors. The list includes functions (the niche of that species) expressed by Koestler (1967, 1978), Polanyi (1977), Simon (1962), members of an equivalence class (the organisms Allen and Starr (1982), Allen and Hoekstra (1992), belonging to that species). This means that when O’Neill (1989), Ahl and Allen (1996). We introduce using the word “dog” we loosely refer both to the here only a few concepts that we need later for our characteristics of the niche occupied by the species discussion. in the ecosystem and to the characteristics of any Holons and holarchies—Each component of a dis- individual organism belonging to it (including the sipative nested hierarchical system may be called a dog of our neighbor). Every “dog”, in fact, be- ‘holon’, a term introduced by Koestler (1967, 1978). longs to an equivalence class (the species “canis He stresses that a holon has a double nature as a whole familiaris”) even though, each particular individual, made of smaller parts (e.g. a human being made of or- has some “special” characteristics (e.g. generated gans, tissues, cells, atoms) while it is at the same time by stochastic events of its personal history) which itself a part of a larger whole (an individual human make it unique. That is, any particular organized being is a part of a household, a community, a coun- structure (the dog of the neighbor) can be identi- try, the global economy). For a discussion of this con- fied as different from other members of the same cept within hierarchy theory see also Allen and Starr class, but at the same time, it must be a legitimate (1982, pp. 8–16). member of the class (Giampietro, 2003). Holons have their own composite “organized structure” at the focal level (Simon, 1962). They Simon (1962) in his seminal work proposes the represent an emergent property generated by the combined use of two concepts: “organized structure” organization of their lower level components, the and “relational function” as a general way to describe emergence of the holon’s wholeness. However, there elements of complex systems. Kenneth Bailey (1990) is also a partness to the holon. This necessitates a proposes the same approach, but using the different given context that is associated with the persistence more transparent terms of “role” and “incumbent” of the organization. Thus the associative context of a when dealing with human societies. Using his ap- human holon would be at least air to breathe, water proach, G.W. Bush would be the actual incumbent to drink, and food to eat. When interacting with the in the role of US President. Salthe (1985) suggests a rest of the hierarchy, holons perform relational func- similar combination of mapping based on yet another tions that contribute to a set of emergent properties selection of terms: “individuals” (as the equivalent that are not their own, but are expressed at a higher of organized structures or incumbents) and types (as level of analysis by the next holon up the hierarchy. equivalent of relational functions or roles). Finally, An example here would be that in a beaker full of Rosen (2000) proposes, within a more general the- water molecules, one cannot to find a wet one. When ory of modeling relation, a more provocative pair dealing with holons we face a standard epistemologi- of terms. He suggests making a distinction between cal problem. A space–time domain must be adopted individual realizations and essences. Individual re- in order to characterize a given holon’s relational alizations are always special, and being individual functions, and consider higher-level perception and they cannot be fully described by any scientific description of events. This does not coincide with representation—science needs the replicates that oc- the space–time domain which must be adopted when cur in an equivalence class. There is more to an characterizing the holon’s own organization as it is essence than an equivalence class, because the essence derived from the relational functions of its own parts occurs in observation above and beyond observer de- (lower-level holons). The implications of this mis- cisions. The part of epistemology that is observable match of scale are discussed in detail in Giampietro but beyond observer decisions is called the other. The (2003). equivalence class is entirely a creation of the observer, 394 T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 and so does not exist in the other (McCormick et al., ing from a client at some level. Engineers commonly 2004). Essences exist in the realm of the other, and require those goals to be explicit and settled before often pertain to system behavior. We see the same dis- the actual engineering starts. There are large social tinction between essence and type, so Salthe’s types consequences that can be entailed by the engineered are more local than Rosen’s essences. Science seeks structure (e.g. population displacement in the building to address the essence that makes the equivalence of a large dam). But these issues (e.g. who checked class members equivalent. The logical similarity be- the validity of assumptions and goals behind the se- tween the various couplets of terms is quite evident. lection of the plan?) are settled in some way before These patterns of organization allow expression of the engineer enters the discourse. Sometimes these are some emergent property of the system as a whole. settled by ignoring them, and then, through no fault Koestler (1967) coined “associative context” in his of the engineer, the project comes home to haunt the “The ghost in the machine” when he uses the term client. In fact, no matter how the goals given to the in a description of cognition. Emergent properties engineer were selected, we have to expect that new themselves exist in a context, and the pattern of structures tend to drive the emergence of new func- organization require particular contexts that are asso- tions (a modification of the given associative context), ciative contexts in Koestler’s sense. Epistemic cate- which means the unexpected arrival of side effects. gories, which we use to organize our perceptions and Consider bridge building in the San Francisco Bay to communicate meaning, arise from collections of Area. The Oakland Bay Bridge has been responsible realizations. We presume that these human intellec- for relegating Oakland to a secondary consideration. tual constructs reflect essences and larger roles, and Oakland was a center of wealth and swinging cul- so expect the equivalence in epistemic categories to ture in the 1930s. Not so now, for Oakland sits in indicate a particular associative context. Change the the shadow of San Francisco. It is the place you go expected associative context of an expression and you through to reach the City by the Bay. With this knowl- get a joke. This concept can be extended, on the side edge, a planner of the extensions of the rapid transport of the other, to include the way that the existence subway, the BART, will need to consider the urban and survival of an organism invokes an appropriate planning implications of extending the transport sys- environment as a required association (an “admissible tem here or there or even nowhere. The communities environment” is the term used by Rosen (1985) for on the west bay, for various reasons, have resisted ex- dissipative systems). There is no realized organized tensions of the BART directly south of San Francisco. structure that can perform a given function without The engineer dealing with the fabrication of bridges its required associative context. and tunnels usually wants no part of those decisions. Allen and Hoekstra (1992) made much of the fact The planners need to decide on a plan. Only then do that a type, in itself, does not carry a scale-tag. A given the municipalities come to the engineer for solutions ratio between the size of the head, the body and the for the design of how to put the BART where the legs of a given shape of organism can be realized at planners have decided to send it. Before that, the en- different scales, and this is modeled in the allometric gineer has probably identified the costs of a tunnel as equations. It is only when a particular organized struc- opposed to a bridge, and the planners will use those ture is realized, that the issue of scale enters into play. figures. However, the real engineering does not start At that point scale matters in relation to: (1) identity until the planners have made their final decisions. of lower-level elements (level n − 1) responsible for With the goals set, the next step is to address how the structural stability of the system (at the focal level the essence of the project of engineering can be tai- n); (2) identity of the context (level n + 1) in which lored to the associative context using one of the avail- the system has to be able to express its function. able types of organized structure. The term proposed Now we can get back to the discussion of the differ- by Rosen, essence, is not anything mystical, although ence between conventional and ecological engineer- it does raise, on purpose, uncomfortable feelings for ing. An abstract description of engineering starts by reductionistic practitioners. Through Rosen’s device, recognizing the existence of a given set of goals at the the message we want to convey is that at this initial outset. Often the goal can be a general statement com- stage of the process of design we have a set of goals T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 395

e.g. Making it possible for cars and trucks to go e.g. Tunnel versus Bridge over an obstruction

Available Location specific Environment Know-how Constraints Engineering Human-given Typology of essence of Organized structures Goals realization a function performing functions

FABRICATION Planning admissible stable Cultural Economic boundary context Context Constraints conditions

Fig. 1. The various steps entailed by the process of fabrication of an engineered structure. Note the planning phase is separate from the engineering phase. The fabrication and function of the engineered structure resides in an environmental context. which bound the essence, but the essence is still an process. The goals are received as an input in the form open information space. In the case of a bridge, the of a specification of a typology reflecting the semantic essence is an informationally-open set of everything given by an essence (Fig. 1). that could fill the need that arises from the goal. The At this point, the engineer uses mechanical pro- essence is the full set of possibilities that would allow cesses to create a structure that realizes the chosen passage of people, vehicles or whatever the goal pre- type of structure. This process of fabrication entails scribes, to move across some obstruction. The essence the definitive closure of the information space. The may be bounded, but it is informationally-open in that closure of the information space arises through the im- there is a functionally infinite number of possible ways position of two sets of constraints: (a) those reflecting to achieve the goal. the decisions made by the planners and the character- The openness of the essence means that before get- istics of the associative context (this is what drives the ting into the mechanical process of fabrication we have selection of a type); (b) those imposed by technical to move to choose a given type of organized structure and economic aspects of the processes of realization that can be used to generate the blue-print for the actual (this is what presses the argument for a particular pro- realization of the original essence. The design accord- cess of fabrication reflecting the selected type). Put ing to a given blue print represents a necessary move in another way, the planning constraint entails a finite to an informationally-closed system. The fabrication set of options at the outset (e.g. tunnel versus bridge) occurs following a particular model of the selected in relation to a specified goal (e.g. moving cars and type, a formalization that, like all formalizations used trucks but no trains—from a point A to a point B). for guiding action, is closed and finite. The essence The technical and economic limits are additional ex- captures the meaning of the issue. It does so in terms ternal constraints (e.g. technical feasibility, available of the purposes of any and all the possible realizations know-how, economic budget). that could arise in a specified context associated with Out of all this comes an organized structure that per- the first plan. The specified associative context is set forms functions. There are constraints and perturba- in the planners’ plan and not by the engineer’s plan, tions that can interfere with the mechanical processes for the engineer’s plan must take the planners’ plan as that effect the realization (fabrication). As the build- given. Organized structures that come out of the en- ing phase proceeds, unexpected externalities may in- gineering exercise are just mechanical systems. The terfere with construction. In Milwaukee, a crane being goals are very firmly set before the engineer enters the used to construct the new sports arena roof fell over, 396 T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 killing construction workers, and setting back the con- civil engineering is green, and insistence on that ethic struction, while increasing costs significantly. In gen- makes environmental engineering distinct. All envi- eral, the perturbations that occur in the realization of a ronmental engineers who ignore the ecological setting conventional engineering process have a limited time do so at some risk. window in which to have their effect, since the re- In environmental engineering, biotic material may alization of bridge building is a finite process that is be present, often as organisms that are being used finished before the bridge or sports stadium becomes as machines to achieve the realization of the essence functional. As will become clear below, there is of- given to the engineers by the client. Industrial fermen- ten no such finite window for perturbations to affect tation uses yeast in this way, as do sewage works use ecologically-engineered structures, because the phase bacteria. Biological engineering, while clearly sepa- of realizing the engineered structure is open with re- rate from environmental engineering, uses organisms gard to the time when perturbations can interfere with most particularly as machines. Molecular biologists the realization process. insert genes to fine tune the biological machine. There is of course much more to life than it being a machine, but in an important sense the carriage horse has been 3. The distinctive character of ecological over used exactly as a machine, as the forerunner of the ob- environmental engineering viously mechanical railway engine pulling a carriage. Genetically-modified organisms are used as machines In environmental engineering, the various essences much as biochemical horses. Using organisms as ma- leading to the selection of typologies of organized chines introduces a strong constraint on the stability of structures generally have one thing in common, they the solution to the fabrication issue. More than steel, include concern for environment as an explicit ethic organisms are particularly sensitive to environmental (Fig. 2). The meaning of environmental engineering change. It is easy for some shift in context to deny the involves more than lip service to the proper function- associative context the organism needs to function as a ing of the environment. To that extent, this version of machine. Using organisms in realization of typologies

Technical processes

Engineering

Human-given Realizations Goals essences of essences Concern for Biological stable ecological Organized structures Processes processes performing functions expected context Boundary conditions

species used as machines ecosystems used to OK, BUT ONLY within fulfill human goals! AY their natural associative context W NO expect problems when these processes are scaled up, since it becomes impossible to keep the same identity for the context

Fig. 2. The various steps leading to the realization of organized structures within environmental engineering. T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 397 linked to essences meets head on not only the prob- as machines inserts them in the wrong place in the lem of getting the right process of fabrication, but also process of realizing an essence. the problem of guaranteeing the particular associative Realizing either an ecologically-engineered func- context required for the proper operation of the real- tion or an ecosystem is less of a finite process than ized structure. realizing environmental or other conventionally engi- The issue of associative environment, the need to neered structures. The realization in both ecological keep the engineered structure in a workable environ- engineering and ecosystems is distinctly iterative. Eco- ment, is elaborate with narrow tolerances when biological engineering problems demand solutions that logical material is used in engineering. However, we are very labile, by virtue of the importance of the should not underestimate those very same needs for changes in the environment to the components of the an admissible environment for steam engines and the realization. Untamed ecosystems share something of like. Often the surprise of finding an inadmissible the same problem. An ecosystem is able to stabilize its environment comes with a move upscale. For steam own identity by making a network of organized struc- trains the admissible environment must have supplies tures performing the function of validating the associ- of coal and water at regular intervals along the track. ation between types and expected associative contexts. The problem becomes acute when the many working Thus ecosystem identity is a matter of becoming in engineered copies of the prototype take the realized time, so it is necessary for the process of realization to structure into new environments, the move up scale. keep going. Realization is never complete. Similarly, Steam locomotives in the Western United States of ecological engineering is also a problem of becom- course required an environment that provided water ing. More than a problem with a solution that just is, and coal, but such an environment had to be engi- a problem of ecological engineering is continuously neered. Those supplies would have been there already active. A bridge, while it functions, is a solution that in the frequent towns and villages of more populous just is, not one that does much becoming. In a trivial Europe, the environment of the first steam engines. case of ecological engineering, one cannot brush one’s In the United States, the West was populated because teeth well enough so one does not have to do it again. of the creation of an admissible environment for the Engineering oral hygiene never stops, as is character- steam engine. Such ecological requirements create pit- istic of ecological engineering at large. At the scale falls even more readily when organisms are used as of engineering genetically-modified organisms, they machines. Expect failure if the organism is denied its need to be made continuously for them to play their natural associative context, and that tends to happen role in society. Ecological engineering usually has an very easily in a move up scale when the organism open-ended process of realization. that worked in the prototype is moved into production Ecosystems are not available to be part of the me- models. chanical processes that effect the realization of an Some may imagine that ecosystems can be inserted engineered structure. The big distinction here be- as machines instead of organisms, to use ecosystem tween ecosystems and organisms is that organisms function as a means of fabricating ecologically- are merely realizations of essences generated at the engineered structures (Fig. 2). In fact for some, mak- ecological level. An ecosystem is a different sort of ing ecosystems into machines might be the benchmark creature. An ecosystem is not a realized structure, in distinction between merely environmental as opposed the same way as is an organism. A mature organism to ecological engineering. The argument would be is a relatively fixed realization of a type (associated that we understand ecosystems in much the same way with a given context), translated through DNA into we understand organisms. Therefore we should be a concrete structure. In an organism there is a fixed able use ecosystems for multidimensional manipula- being, but there is no such fixation to give a body that tions of our environment at a scale much larger than is an ecosystem. As we said above, an ecosystem is a that which is achievable using organisms. The authors becoming, not a being. The critical difference is that here disagree. It is not possible to use ecosystems as the set of essences associated to the self-organization machines in the same way that we can, with caution, of an ecosystem is constantly being updated in a way use organisms as machines. The use of ecosystems that is sufficient to deny any ecological configuration 398 T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407

IDENTITY OF HUMAN SOCIETY

FABRICATION Typology of Essences of Organized structures Goals realizations functions performing functions

REPRESENTATION right Location specific associative Constraints Larger scale contexts Constraints

Fig. 3. The processes that define the identity of human society as the result of multiple scale constraints. Human systems are able to define, change and update their identity at a pace that is much faster than that of ecological systems (cf. biological vs. cultural evolution for a similar mismatch). That is why the essences that humans generate through their technology are not compatible over the full range of scales that apply to those generated by ecological processes. Consequently in ecologically-engineered situations humans must be prepared to continually change their goals. the status of a fixed structure. If ecosystems are not As Rosen (2000, 1991) might put it, biological sys- fixed structures, then they are not available to be part tems are made of non-equivalent observers (species of the mechanical processes. and organisms) able to self-entail their identities in Ecological processes produce the essences of or- loops of either physiological or evolutionary adapta- ganisms, but ecological processes can do that only tion. This establishes reciprocal constraints on behav- through a continuous self-adjustment. That adjustment iors and realizations operating across hierarchical lev- must able to guarantee the matching between: (i) pro- els, each level adjusting to the demands of its neigh- cesses of realization of organized structures encoded boring levels. by genetic information and (ii) the expected asso- It may be challenging to use organisms as prac- ciative context. From this perspective it easy to see tical realizations by environmental engineers, but at that the essence of an organism is not written in its least they are tangibly bounded entities. With draft DNA in a way that molecular biologists might imag- animals or genetically-modified organisms one knows ine. Rather, the essence of an organism is preserved what one is using in fairly concrete terms. Ecosystems by the integrated combination of expected behaviors are neither tangibly bounded nor tightly coded. Be- and expressed behaviors in the ecological community cause ecosystems keep changing their set of essences, context. The expressed behaviors refer to the species the model one uses of them is at risk of becoming interacting within an ecosystem (Fig. 3). Allen and soon outdated. The process of generation of the set Hoekstra (1992) identify the community as a wave in- of essences that are making up ecosystem identity is terference pattern between the particular periodicity of exactly characterized by the fact that it cannot be for- organisms of different species. In this way, the iden- malized using simple models (Rosen, 1991, 2000). We tity of the community becomes part of the essence of cannot model ecosystems to the degree we can model all its members. Organisms belonging to interacting organisms. This is not just because it is too hard and species must be capable not only of producing copies complicated, rather the failure to model ecosystems is of themselves successfully, but they must also main- endemic to ecosystems in principle. It is wishful think- tain the stability of the mosaic of the reciprocal asso- ing to imagine that ecosystems can be used as devices ciative context. That reciprocal context is captured in for realizing human-given goals to create functioning the accommodations that species make to each other. structures to serve human goals. T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 399

Knowledge managing human goals in relation to ecological constraints 2 of Ecological Processes Engineering Technical checking finding processes 3 local 1 relevant Validity of Realizations feasibility Goals factors essences of typology affecting context Concern Biological for the Processes Organized structures stable integrity of performing functions expected ecological species used as context processes Boundary conditions machines BUT ONLY within their natural associative context

Ecological Processes = Constraints & Emergence (formal models will unavoidably be affected by uncertainty)

Fig. 4. The key role of knowledge of ecological processes in developing sound engineering strategies. The three key areas of required knowledge for ecological engineering are: (1) identifying the context in which the engineered structure can function meaningfully; (2) managing goals so that they ﬁt the ecological possibilities; if the goals are infeasible, look for substitute goals that are satisfying while also realistic; and (3) checking feasibility in the local construction domain.

That being said, the crucial need for knowledge Ecological processes can make obsolete the very and analysis of ecosystem function is still what dis- definition of the original plan used in the engineer- tinguishes ecological engineering from environmental ing program. Changes in context imposed by ecolog- engineering (Fig. 4). Ecosystems may not be inserted ical processes can move the focus from the technical into the process of the creation of ecologically- soundness of the process of realization to the sound- engineered structures, but they are nevertheless critical ness of the process of planning for translating goals to the process of ecological engineering. Ecological into action. Proper consideration of systemic features systems and their functioning enter the process as of ecological systems tend to change the engineering key factors determining the character of the various activity. The change is from a one-way process aimed associative contexts. They do this in two ways. First at the realization of a structure associated with the they act as the context of any biological material achievement of a given goal to an iterative process. that is being used as a machine in the realization Each iteration achieves an update of the definition of process. Therefore, ecological systems can work to goals and essences used to decide how to continue to preserve the context that the engineer hopes will be make the organized structures. Thus the diagrammatic the stable context for the functioning of the orga- summary of ecological engineering is captured in the nized structure at the end of the engineering process. iterative process shown in Fig. 5. Second, by changing context, ecological systems can Ecologically-engineered systems accommodate by impose a change on the validity of the set of goals, co-opting ecological settings and co-evolving with essences and typologies of organized structures that them. Co-evolved structures are always being tested, was used to start the engineering process in the first hence the continuous process of realization in ecolog- place. ical engineering. Ecological systems achieve a forced 400 T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407

Ecosystems force change in goals via context

Realized Functional Engineering process Goals Engineered Structure

Associative Context Ecosystems constantly jostling realizations

Fig. 5. A schematized diagram to summarize Fig. 4, so as to emphasize how the environment of the engineered structure suffers perturbations from ecological systems, requiring a cycle that changes goals so as to incorporate environmental influences. The cyclical nature of shifting goals and models is the principal character that sets ecological engineering apart from environmental and other engineering. dynamical cycle in ecological engineering through entirely different from all other engineering, includ- a continuous testing of the validity of their own set ing environmental engineering. Ecological engineer- of operating essences. In ecological engineering, ing mimics the way ecological systems co-opt their co-evolution through continuous validation is done environments. at each new iteration. Such an accommodation also Failure to update the essence of an ecologically- occurs in medicine, where the technical treatment engineered structure brings down the structure well changes as the biological challenge changes through before its planned demise. Organized structures per- the course of a disease. For instance, sometimes a forming a given function are linked to the usefulness single large tumor is left in situ, because it is well of their function, and responding to that functionality known that surgical removal can release many metas- gives life to useful environmentally engineered struc- tases. With the main tumor in place, metastases can tures. This usefulness is embodied in the type of en- remain so small as to be undetectable and not a med- gineered structure, but only if the structure remains in ical problem. The mechanism for this phenomenon is its associative context, or can accommodate to a new now understood to be the suppression of blood sup- context. For example, an agricultural field is main- ply by a hormone released by the main tumor (Jian tained as quite an improbable ecological community and Carmeliet, 2001). The tumor becomes co-opted functioning in a particular context. Such an organized as part of the treatment, thus delaying the onset of structure persists because the growth of crops gener- uncontrollable growth of metastases. Responding to ates enough money to allow the farmer to put in a new cancer this way is a change in the essence of the treat- crop next year. If the market changes, the farmer shifts ment. The change is from hoping to cure the cancer the realization by turning to a different crop. Thus through its removal to controlling the ultimate con- the field persists as a production unit. The makings sequences of the cancer by co-opting the main tumor. of the pattern reinforce themselves in a loop of struc- The physician has the patient live with the cancer ture feeding process, a loop that amounts to the whole rather than die as a consequence of trying to elimi- process of ecological engineering. nate it. This is an essential change in strategy. Such Ecological engineering is a process of continuous a strategy of co-option makes ecological engineering realization of a continuously changing set of essences T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 401 and typologies of organized structures. Environmen- organization. The essence that they wish to real- tal engineering implies dealing with realized structure, ize is water access to food, because water provides whose structural properties allow prediction enough to protection from predators like wolves. Initial dam meet the exacting standards of engineers. In ecologi- construction gives beavers a huge amount of safe ac- cal engineering, we must constantly change the mod- cess to food resources (woody browse) (Johnston and els because the contexts are changing all the time in Naiman, 1987). After awhile, though, they deplete the ways that deny engineering integrity. Often one must browse in the immediate vicinity of the pond. Beavers keep changing the goals because the essences that are then must change realization strategies. Rather than responsible for the realizations are changing too fast simply maintaining a primary dam, they begin to cre- for an operational solution (Fig. 4). ate accessory dams and dig canals into the uplands to obtain more safe access to food (Naiman et al., 1988)(Fig. 6). In this way, beavers adapt to changes 4. Learning from a case study: beavers as in context that they themselves created by depleting engineers food (Fig. 7). Let us cast this in terms of realizations, types and changing contexts. The consequence of Beavers make an interesting comparison to human having achieved the original goal through a successful engineers. If humans have so much difficulty with sta- piece of engineering entails a change in the strategy bility in ecologically-engineered situations, why do if the original goal is to continue being achieved. The beavers appear to be able to engineer in their world primary dam causes a change in the context that the so effectively? Recent study (Allen et al., 2001) that ecosystem offers, because the trees and herbaceous looks at the thermodynamics of beaver ecology iden- forage are depleted. A different definition of type, tifies that beavers work at a set of hierarchical lev- use smaller dams and canals, is used to realize new els, changing their strategies for resource capture as structures. These new structures, canals, have a new the environmental context changes. They are widely function, which is water access to trees further upland, recognized as “ecosystem engineers” (Pollock et al., where the waters behind the primary dam do not reach. 1995), and represent a useful example of how ecolog- Beyond this level of changing strategy, beavers ical engineers might deal with their projects. also have ways of dealing with change at a higher By recognizing different hierarchical levels of or- level of analysis. Beavers respond to changes in the ganization, we can tease apart the methods by which quality of available territories. When beavers are first beavers engineer their environments. Beavers are reintroduced to a region after a period of extirpa- effective because they have a set of identities that tion, they colonize the best sites first, resulting in translate their goals into functions. Beavers also have a highly dispersed patch structure on the landscape a set of types that constrain how they realize these (Howard and Larson, 1985). However, these very functions in different ways as responses to changes acts of colonization change the amount and quality in the associative context. This makes it possible of available territory for future beavers. These future for them to adapt to change continuously (Fig. 3). beavers change their standards of site quality in order Beavers can absorb some small change of context by to survive. The new beavers move to another corner simply repeating the same realization strategy in an within the space of admissible essences for expressing iterative process. However, there comes a threshold beaver behavior. They look in the available repertoire in magnitude of change in the context when the same of typologies of possible functions, searching for the realization strategy will not work as it did before. It most suitable one for realizing changes appropriate is then that scientists must change level of analysis to to the new setting. The change is from search for understand the new level of organization at which the a high quality site to a search for any site that will animals are operating. do. Beavers in the later stages of colonization have Allen et al. (2001) point to the fact that the change been found colonizing such marginal sites as drainage in realization strategy often corresponds to a change ditches (Müller-Schwarze and Schulte, 1999). This in available resource quality. For example, beavers amounts to redefining goals and realization strate- change resource utilization strategies of intra-territory gies in the more global planning stage just before 402 T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407

Goal: Essence of survival and function: reproduction D Context for obtain water Harvest within access to plant browse near realization A associative food in site pond margin context C A Food consumption Typology and depletion Essence of of realization Dig canals and function: obtain C B create accessory Context for site with water dams access to plant realization B food Context for essence of function D Ultimate move upscale forces change E in Essence

Fig. 6. For beaver to maintain identity, they must survive and reproduce within their associative context. At this scale, the ecosystem specifies the essence of function for the beaver as obtaining water access to food. The beavers can pursue different typologies of realization strategies, dependent upon high (A) or low (B) quality resources. The realization of this function feeds back to cause a corresponding change in context (C), which in turns feeds back to influence both future realizations and the larger scale site context (D). However, as the site moves up scale through time, a threshold is reached in which the site context can no longer support the essence of function, causing a move up scale in essence definition (E). Comparing this scheme with that of Fig. 3, we can note that beavers here are considered parts of ecological processes. Their identity is the result of a definition of essences of their functions and realizations in parallel at different scales. Beaver identity reflects the goals of: (i) fabrication locally of members of this equivalence class (species) and (ii) expressing functions that stabilize the associative context of other realizations (organisms of other species). Achieving these goals in turn will stabilize the beaver’s own associative context in a self-entailing process. the planning stage of the actual engineering (Fig. 1). exergy carriers in the form of plant biomass—getting Those changes in plan occur as the problem changes. food). The changes are forced by changes in associa- Yet another switch occurs on the lowest, annual tive context. By changing typologies and realization scale of beaver activity. During the summer, beavers strategies, beavers are able to deal with behavioral in- consume high quality herbaceous foods in an oppor- stability arising from limits encountered at the lower tunistic fashion (Svendsen, 1980; Jenkins, 1981), but level when some types of food become unavailable. during the winter, they are forced to consume low Beavers have to go through cycles which imply dif- quality woody materials in a selective fashion in or- ferent ways of guaranteeing the essential functions for der to survive. The goal of the beavers in this case is upper level dynamic stability. The upper level function to obtain food throughout the year (Fig. 8). The selec- is stabilizing the supply of exergy harvested from the tion of the type of food to be adopted in the realiza- context in the form of plant biomass (Figs. 7 and 8). tion stage changes with season. Beavers move from We can see from the examples of beaver engineering obtaining herbaceous forage for beaver growth to for- that the very instability of lower-level structures and aging woody materials for beaver maintenance. This processes may provide the basis for the long-term dy- can be seen as the use of two different typologies of namic stability. Low level instabilities force a response realization, like some sort of tunnel versus some sort to changing scales and contexts. of bridge in examples given in Fig. 1. Meanwhile the Traditional engineers seek to avoid those lower-level reference is to the same essence of function (capturing instabilities, but from beavers we learn that low level T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 403

Fig. 7. This figure emphasizes the dynamical aspects of Figs. 6 and 8. Patterns of alternating high and low quality resource exploitation have been applied to many systems including human political systems. The benefits of complexity decline when the engineering effort of the beaver changes the admissible context. Allen et al. (2001) discuss at length the systematic differences between phases of high and low quality resource extraction, decline in resources over time drive a change in strategy over time. The beaver show a first flush of high quality resource extraction, seen here as an emergence in a sharp short burst of marginal return. This shows on the graph as an increase in benefits to complexity, as the beaver first exploit their main dam, or in their summer exploitation of herbs. Later benefits of the first burst decline, eventually requiring the beaver to put in effort actively organizing the environment to deliver lower quality resources. In fall and winter beaver organize a wood supply instead of eating herbs opportunistically. At the higher level of landscaping, the secondary phase means organizing canals over a decade or so, which is much more work than letting a dam fill for a year or two. Both secondary actions increase benefits of complexity, but this time in a more actively organized context. instabilities can give long-term stability across the landscape history. This is in contrast to structures realm of the ecological engineer. Traditional engineer- engineered in the traditional fashion of mainstream ing fails when a move upscale creates a qualitatively engineering, where there is limited capability of different context (Petroski, 1993), but beavers faced organic response to environmental change. Conven- with the same variation do not fail. The goal of the tionally engineered structures impose rigid control of traditional engineer is often long-term stability of the the environment by those structures when they are fabricated structures, and often enough engineers are in use. This overbearing character holds over when successful. In the case of beavers, the structures in the structure is no longer useful. In the absence of a use are organic and living, in that the users of the purpose, concrete structures left alone will remain as structure maintain it, and the long term stability oc- scars on the landscape. Often defunct constructions curs because of the essential organic, self-repairing are actively removed in a demolition phase that over- quality of organic solutions. comes the intrinsic and sometimes literally concrete When ecologically-engineered structures fail, nature of modern buildings. In Rome you can still their reinforcing cycles of adaptation disappear, as see the Coliseum standing pretty solid, but nowadays other organic processes remove the evidence. How- Christians have enough power to prevent it meeting ever, this disappearance does not happen so readily its original purpose. Human society has given the for conventionally engineered structures. When the ruins a new purpose, wherein Christians come as ecologically-engineered cycles of “crop generating tourists to see the remains. Other defunct classically money for more cropping” disappear, the field is engineered structures merely remain as piles of rub- overcome by forest. Evidence of the cropping system ble, after the surviving populace have used them as disappears for everyone except those trained to read quarries. 404 T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407

Goal: Essence of survival and function: reproduction D safely obtain Context for within Consume energy from realization A associative aquatic plant biomass context plants C A Energy for beaver; food Typology depletion Essence of of realization Harvest, store function: obtain C and consume access to food B Context for woody browse realization B Context for essence of function D Ultimate move upscale forces change in Essence E

Fig. 8. At this scale, the ecosystem-speciﬁed essence of function is to safely obtain energy from plant biomass. As in Fig. 6, they can pursue different typologies of realization strategies, dependent upon high (A) or low (B) quality resources. The realization again changes the context (C), affecting future realizations (D), and eventually feeding back to the large scale context of site organization when energy can no longer be obtained within the current site structure (E).

In adaptive systems, in the end it is nearly impos- or otherwise context-changing events occur, who will sible to achieve maintenance of the right mapping be better able to rebuild, the pig who has lost a house between essences of functions and typologies of re- of bricks or the one with a house of sticks? One could alizations. This is because of parallel changes of argue that beavers would be more successful if they contexts over different scales. Truly unpredictable “worked smarter, not harder,” but their unpredictably surprises always emerge, forcing resort to damage changing context means that “smarter” can never be mitigation. Damage control involves seeking a proper smart enough. At times, humans build things with investment scale to achieve long-term dynamic sta- intention of withstanding ecological change. Beavers, bility. Beavers often build dams and lodges which by contrast, build things with acceptance of change. then wash away when the calm, tranquil river context This is what we mean by the open-ended process of in which the dams were constructed changes to the realization. raging river context that disallows dams (Barnes and Human-engineered impoundments are an excellent Mallik, 1997). Beavers can afford to live this way example of failure to endure in a changing context because little investment is made in the making of coming from a temporal move upscale. These con- each dam. Dams are initially constructed from sticks, structs are very effective at supplying electricity in the not from rock. In the story of the Three Little Pigs, short-term. However, in the long term dams become he who builds the house of sticks is soon eaten by unsustainable, filling with sediment that eventually the huffing, puffing wolf, while the pig in the house buries the gradient used to create electricity. The real- of bricks remains safe. The strategy of traditional en- ization (the dam) of the essence (use water to create gineers is to build a house of bricks, counting on the power) itself is responsible for effecting the eventual threats of predation by wolves, wind stress, even fire, change. The same phenomenon of sedimentation oc- and insuring against them by creating structural sta- curs in beaver ponds, but beavers respond to imperma- bility. However, when the unexpected flood, tornado, nence by changing their goals. Beavers shift amongst T.F.H. Allen et al. / Ecological Engineering 20 (2003) 389–407 405 territories, sacrificing short-term structural stability so Most wetland managers will agree that introducing as to achieve long-term dynamic stability. A possi- beaver to a site is dangerous. They deplete their own ble strategy for attaining sustainability in human-made resource base, and eventually spread onto neighboring impoundments might be to engineer short-term in- lands, generating unpredictable consequences. stability into the system. This instability could come in the form of occasionally opening the flood gates to reduce sediment accumulation. Schneider and Kay 5. Conclusion (1994) suggest that structure/order emerges to destroy an exergy gradient. The entropy produced must go Ecological engineering, if it wishes to achieve somewhere, and it is transferred to the larger-scale sys- long-term structural stability, must focus on creating tem, which is the context of the structure (Schneider things reinforced by their changing contexts. This and Kay, 1994). Allen and Hoekstra (1992) are ex- amounts to evolution, or rather co-evolution, with a plicit in their assertion that engineering inert material living context. We have already pointed to the world so as to have it be predictable exports the surprise of biology as being populated by things that have their to the upper level context. The example they use is own models, and that a biological entity knows itself Freon, a class of refrigerants, which is responsible for best through the models of it belonging to the other the hole in the ozone layer. Surprise upscale can en- biological entities in its context. Buildings can evolve tail unpleasant consequences, and it is exactly here from their community surroundings, but in order to that the issue of modulating the trade of lower-level increase the probability that they remain sustainable, stability for upper level instability comes into play. In they are better built with the recognition that the order to achieve upper level stability, we must retain community will change. The community will change some of that entropy at lower levels by engineering due to the influences of the building, and also of its instability. own accord. The purpose of some buildings resonates One might point to giant sequoias (Sequoiadendron with long term human activity, such as religious faith, giganteum) as an example of long-term structural sta- and being always useful they remain sustainable. Of bility. They remain stable structures as long as the course, changes in faith may create tensions, as in the ecosystem that produced them remains, but they do not Holy City, but as a counter example, the buildings fair well when human exploitation has changed their of Moorish Spain remain fully viable at the heart context. Beaver also have long-term structural stabil- of a now Christian landscape. For more ephemeral ity, even though the realizations of individual organ- secular purposes, the type of building needed by the isms turn over within the essence of beaverness. The community will change with time (Allen, 2002). Who ecological and evolutionary capabilities of the essence can predict what the community will become in the of the beaver are resilient compared to the environ- future—a ghetto, a center of urban revival, or a ghost ment that it organizes. But this is possible exactly be- town? These are the dilemmas of ecological engineer- cause the identity of that species (the set of essences ing. We are a long way from predicting urban social of functions and typologies of realizations on differ- ecosystem behavior, and yet we are creating entirely ent scales) reflects the dual goal of: (i) reproducibil- new types of urban ecosystems. Can we plan ahead ity of individual members of the species at the local sufficiently with regard to the fabrication of these new scale and (ii) reinforcement of contextual essences at types of ecosystems? They are ecosystems in which the scale of the whole ecosystem (Fig. 3). human generated essences and ecologically generated Using beavers and their created impoundments to essences are fighting to get due recognition in the reduce erosion is a restoration strategy that has re- phase of fabrication. The authors here doubt we can cently surfaced in the riparian ecosystem literature achieve this degree of self-reference in the detailed (Parker et al., 1985; Naiman et al., 1988). This is essen- modeling that must be part of fabricating engineered tially treating beavers like machines that do work for structures. humans by slowing water flow and retaining sediment. After acknowledging the unavoidable existence of This human strategy is only feasible if the context that uncertainty, scientific ignorance and legitimate con- allows and creates the beaver realization is preserved. trasting views found when dealing with a messy real 406 T.F.H. 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