Design Principles for Ecological Engineering

Ecological Engineering 18 (2001) 201–210 www.elsevier.com/locate/ecoleng

Design principles for ecological engineering

Scott D. Bergen a,1, Susan M. Bolton b,*, James L. Fridley a

a Forest Management and Engineering Di6ision, College of Forest Resources, Box 352100, Uni6ersity of Washington, Seattle, WA 98195-2100, USA b Center for Streamside Studies, Box 352100, Uni6ersity of Washington, Seattle, WA 98195-2100, USA

Received 22 August 2000; received in revised form 18 December 2000; accepted 16 February 2001

Abstract

The emerging discipline of ecological engineering is a response to the growing need for engineering practice to provide for human welfare while at the same time protecting the natural environment from which goods and services are drawn. It recognizes that humanity is inseparable from and dependent on natural systems, and that the growing worldwide population and consumption have damaged, and will increasingly stress, global ecosystems. Ecological engineering is the design of sustainable systems, consistent with ecological principles, which integrate human society with its natural environment for the benefit of both. It recognizes the relationship of organisms (including humans) with their environment and the constraints on design imposed by the complexity, variability and uncertainty inherent to natural systems. Successful ecological engineering will require a design methodology consistent with, if not based on, ecological principles. We identify five design principles to guide those practicing ecological engineering. The principles are: (1) design consistent with ecological principles, (2) design for site-specific context, (3) maintain the independence of design functional requirements, (4) design for efficiency in energy and information, and (5) acknowledge the values and purposes that motivate design. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Ecological engineering; Ecological design; Design principles

1. Introduction recognizes that humanity is inseparable from and dependent on natural systems, and that the grow- The emerging discipline of ecological engineering worldwide population and consumption have ing is a response to the growing need for engineer- damaged, and will increasingly stress, global ing practice to provide for human welfare while at ecosystems. Sustaining human society requires the same time protecting the natural environment engineering design practices that protect and enhance the ability of ecosystems to perpet- from which goods and services are drawn. It uate themselves while continuing to support humanity. * Corresponding author. Tel.: +1-206-6857651; fax: +1- Ecological engineering is the design of sustain- 206-6853091. E-mail address: [email protected] (S.M. Bolton). able systems, consistent with ecological principles, 1 Present address: William McDonough+Partners, 410 East which integrate human society with its natural Water Street, Charlottesville, VA 22902, USA. environment for the beneﬁt of both (see Mitsch

0925-8574/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8574(01)00078-7 202 S.D. Bergen et al. / Ecological Engineering 18 (2001) 201–210 and Jørgensen, 1989; Mitsch, 1996; Bergen et al., define the practice as ‘‘the design of human soci- 1997). ety with its natural environment for the benefitof Many scientists working in environmental fields both’’. This definition was slightly refined to read are, in fact, practicing engineering as they take ‘‘the design of sustainable ecosystems that inte- scientific principles and use them to address spe- grate human society with its natural environment cific problems. However, very few scientists have for the benefit of both’’ (Mitsch, 1996). Mitsch had any engineering training, and there is little suggests that the goals of ecological engineering evidence of accepted engineering design methods are the restoration of human disturbed ecosystems being followed in applied ecology. Additionally, and the development of new, sustainable ecosys- engineers are increasingly undertaking design tems that have human and ecological value. In the problems in which a solid scientific understanding latter case, ecosystems are designed and created to of natural systems is needed. We do not propose solve human problems. adding a little ecology to engineering, or a little Harnessing the self-design or self-organizational engineering to ecology. Rather, we envision a new properties of natural systems is an essential com- engineering discipline with ecological science as its ponent to ecological engineering (Odum, 1989; basis. In other words, the practice of design with Mitsch, 1996). In a constructed ecosystem, hu- an appreciation for the relationship of organisms mans are likely responsible for providing the ini- (including humans) with their environment and tial components and structure of the system, as the constraints on design imposed by the com- well as for influencing the larger environment to plexity, variability and uncertainty inherent to which the ecosystem connects. Once created, how- natural systems. This approach could lead to a ever, nature takes over and the composition and new paradigm for engineering design in general. structure become those best suited to respond to Ecological engineering has been defined in a the condition imposed on the system. Humans do number of ways, so we begin this paper with a not need to add matter or energy to maintain a look at past definitions and then propose our own particular ecosystem state. definition. We discuss the current and potential In Section 1, we presented a definition for scope of ecological engineering applications. Fi- ecological engineering that is a modification of nally, we identify five design principles to guide Mitsch (1996), i.e. ecological engineering is the those practicing ecological engineering. The prin- design of sustainable systems, consistent with eco- ciples are a distillation of our own ideas and those logical principles, which integrate human society of other authors who have written on engineering with its natural environment for the benefitof and ecological design. both. This definition has a number of important elements that should be in any definition of the discipline: 2. Ecological engineering defined 1. that the practice is based on ecological science, 2. that ecological engineering is defined broadly As a relatively new field, effort continues to be enough to include all types of ecosystems and spent in defining the scope and purpose of ecolog- potential human interactions with ecosystems, ical engineering. Several authors have put forward 3. that the concept of engineering design is in- definitions for ecological engineering, and these cluded, and definitions reflect the particular aspects of the 4. that there is an acknowledgment of an under- practice they feel are critical. The term itself is lying value system. attributed to H.T. Odum, who defined ecological The first point is the most fundamental. A branch engineering as ‘‘environmental manipulation by of engineering can be defined by its science base, man using small amounts of supplementary en- by its application, or by both. We suggest that ergy to control systems in which the main energy unlike civil engineering, which is more clearly drives are still coming from natural sources’’ defined by its applications than the science that (Odum et al., 1963). Mitsch and Jørgensen (1989) informs it, ecological engineering, to be truly a S.D. Bergen et al. / Ecological Engineering 18 (2001) 201–210 203 unique engineering discipline, must be based on values in the definition of ecological engineering ecology. Applications for ecological engineering will be most accepted if it appeals to a plurality of may stretch beyond working with ecosystems and value frameworks (Miller, 1995). influence all engineering practice, representing a new paradigm for engineering design. However, the question of whether there is a way of practic- 3. Scope of application ing engineering that is significantly different from current practices, and is based on an understand- We have defined ecological engineering broadly ing of ecology, remains. This has not been shown, and advocate its application to a number of prob- but we believe the answer is yes. lem areas. Potential applications include: The second element relates to application. 1. The design of ecological systems (ecotechnol- While ecological engineering may represent a new ogy) as an alternative to man-made/energy-in- paradigm for design, its most obvious application tensive systems to meet various human needs is to engineering as it relates to human interaction (for example, constructed wetlands for with ecosystems. The literature focuses primarily wastewater treatment). on created and restored ecosystems, but leaves out 2. The restoration of damaged ecosystems and the broad and important area of societal interac- the mitigation of development activities. tion with existing, and not necessarily degraded, 3. The management, utilization, and conserva- ecosystems. tion of natural resources. Explicitly using the word design in our defini- 4. The integration of society and ecosystems in tion makes it clear that it is the primary activity built environments (for example, in landscape of engineering. Successful engineering design re- architecture, urban planning, and urban horti- quires, in part, adherence to a formal methodol- culture applications). ogy. Much has been written about engineering Methods currently exist for dealing with all the design as it relates to engineering in general. It applications listed above. We feel, however, that remains to be shown if traditional engineering ecological engineering can offer a unique ap- practice can solve ecological engineering prob- proach to each. The first application, ecotechnol- lems. Given that traditional practices contributed ogy, is the most thoroughly discussed to date. to environmental degradation, methodological is- Ecotechnology has been described as a means for sues should be considered. We will discuss design environmental management (Strasˇkraba, 1993) principles in more detail later. and as ecological solutions to environmental engi- The last element, regarding values, raises two neering problems (Mitsch, 1996). The most preva- important issues. The first is whether the defini- lent example of the latter is the treatment of tion of an engineering discipline should include a various forms of waste products. Environmental statement of values. We believe it should because engineering solutions to waste management focus it is naive to assume that we can separate our on energy-intensive processes such as sewage motivation for practicing engineering from our treatment plants, settling tanks and scrubbers. actions. Therefore, the motivations should be Ecological engineering addresses the same prob- made explicit. Given that engineering and other lem with systems that rely on ecological processes professional societies generally adopt a code of that require minimal energy input from humans ethics to guide their membership, there are prece- (essentially solar-powered). dents for the stating of values. Ecological restoration and development mitiga- The second issue is, if we decide to include a tion currently fall under the domain of applied or statement of values in the definition, what values restoration ecology. Ecological engineering can should we express? This is a contentious topic, but add to these activities by providing a more formal concepts such as human benefit, sustainability, and structured design method. Attention to the and ecological health and integrity are often men- process of design in reports on applied ecology is tioned in the literature. For now, the statement of often missing. Using repeatable design procedures 204 S.D. Bergen et al. / Ecological Engineering 18 (2001) 201–210 in ecological restoration would facilitate learning of principles, we have combined our own ideas how to improve future projects. with ideas from other authors who have written The third application area relates to the man- on engineering and ecological design. In another agement of natural resources. The goal for man- paper we discussed how the two design axioms aging existing systems would be to harvest some proposed by Suh (1990) may be applied to forest benefit from the ecosystem while preserving the engineering (Bergen and Fridley, 1994), and will health or integrity of the system, not compromis- do the same here for ecological engineering. ing the production of ecological services, and not Odum (1992) proposed 20 ecological concepts inducing unexpected changes in the system. Ex- from which design implications may also be amples of such systems include forest and range- drawn. Strasˇkraba (1993) described seven ecosys- land ecosystems and fisheries. In the case of forest tem principles and 17 rules for practicing ecotech- management we may be interested in harvesting nology. Mitsch (1992) presented eight principles timber from an ecosystem without diminishing the for wetland design. Todd and Todd (1994) pro- ability of the forest to regenerate, to provide clean pose nine precepts and Van der Ryn and Cowan water and air, and provide habitats for a range of (1996) propose five principles for ecological de- plant and animal species: essentially, use of the sign. Holling (1996) also details ecosystem charac- forest with minimal impact to the ecosystem. In teristics that have implications for design. the case of fisheries, we may want to harvest some Jørgensen and Neilsen (1996) proposed 12 princi- fish without depleting stocks beyond recovery. ples for ecological applications to agriculture. Za- This would signal a shift toward living off ecolog- lewski (2000) identified three principles for the ical ‘interest’ and not depleting natural ‘capital’ study of ecohydrology. From the above material, (Cairns, 1996). Our extraction level would fall we have distilled five general principles to guide within the noise level of the natural variation of those practicing ecological engineering in any con- the system. Moving away from spending natural text or ecosystem. capital given current and probable future pres- Stating first principles is a challenging exercise sures on ecosystems is a monumental problem. for an emerging field. It is an important exercise The goal of ecological engineering is to better because we believe design solutions that adhere to integrate society with its supporting environment. the following principles will have the best chance Creating integrated urban and other built envi- of success. Successful designs, in the terminology ronments is a potential application for ecological of Suh (1990), efficiently meet their stated func- engineering. Endemic ecosystems are often com- tional requirements without violating constraints. pletely destroyed when dense human populations What follows, while we call them principles, is arrive in an area. Increasing calls for ‘greening’ more a combination of axioms, heuristics and urban environments, allowing for more of a con- suggestions. The boundary between some of the nection between place and nature in built environ- principles is fuzzy, implying that more work needs ments, will require design that includes ecology to be done to distill and clarify them. Most of our and engineering. Traditional landscape architec- proposed design principles contain more than one ture, urban planning, and urban horticulture ap- unique idea that we have grouped together be- proaches can be augmented by ecological cause of certain similarities. In addition, some engineering. apparent contradictions need to be resolved, aris- ing from our attempt to merge ecology with engineering. 4. Ecological engineering design principles 4.1. First principle — design consistent with We have mentioned a number of ideas, such as ecological principles utilizing the self-designing capacity of ecosystems, which can serve as design principles for ecological Designs produced with regard for, and taking engineering. To identify a more comprehensive set advantage of, the characteristic behavior of natu- S.D. Bergen et al. / Ecological Engineering 18 (2001) 201–210 205 ral systems will be most successful. When we equilibria define functionally different states, and include and mimic natural structures and pro- movement between states maintains structure and cesses, we treat nature as a partner in design, and diversity’’. The structure and diversity produced not as an obstacle to be overcome and dominated. by the large functional space occupied by ecosys- We are all familiar with the second law of tems is what allows them to remain healthy, or to thermodynamics and the concept of entropy. Per- persist. haps the first law of biology is that life is a The large functional space required for sustain- negentropic process. Life causes local decreases in able ecosystems is directly at odds with traditional entropy by producing order out of chaos. The engineering design practices that create systems second law is not violated because the energy that operate close to a single, chosen equilibrium expended to produce order results in more en- point. Holling (1996) uses this idea to distinguish tropy overall. The practical implication, however, between what he terms engineering resilience and is that ecosystems have the capacity to self-orga- ecological resilience. Engineering resilience mea- nize. Mitsch and Jørgensen (1989) state that it is sures the degree to which a system resists moving this ‘‘capability of ecosystems that ecological engi- away from its equilibrium point and how quickly neering recognizes as a significant feature, because it returns after a perturbation. Ecological re- it allows nature to do some of the ‘engineering.’’ silience reflects how large a disturbance an ecosys- We participate as the choice generator and as a tem can absorb before it changes its structure and facilitator of matching environments with ecosys- function by changing the underlying variables and tems, but nature does the rest.’’ processes that control behavior. The equilibrium Self-organization is manifested through the pro- conditions discussed above for ecosystems exist cess of succession in ecosystems. Todd and Todd within the range of ecological resilience. (1994) discuss how as ecosystems mature, connec- The distinction between the two types of re- tions between components become more numer- silience is important because management policies ous and complex, with the system becoming more that force ecosystems to function in a state of diverse and resistant to perturbation. They de- engineering resilience lead to a loss of ecological scribe current design practices as ‘‘early succes- resilience. Systems managed to produce a consis- sional’’, with simple linkages and patterns, and no tent, high yield of a single variable (such as timber room for maturation. Designs are then more sus- or fish) lose the functional and structural diversity ceptible to disturbance and failure. Kangas and required to remain ecologically resilient. The sys- Adey (1996) propose that mesocosms (scale range tem is then more susceptible to ‘failure’, where it m2 to ha) most clearly express the self-organiza- may lose the ability to produce the same outputs tion of ecosystems and provide experimental units in the future (Holling, 1996). that will be critical for ecological engineering and Diverse systems are more ecologically resilient restoration ecology. and able to persist and evolve. Diversity can The key ecosystem attributes that allow for manifest in terms of the number of species, ge- self-organization are complexity and diversity. netic variation within species, and as what Holling Ecosystems can be complex structurally and in the (1996) calls functional di6ersity. Functional diver- temporal and spatial scales of processes. Signifi- sity is another way of saying redundancy, where a cant ecological change is episodic, and critical number of species or processes in the system can processes occur at rates spread over several orders perform similar functions. If one is impaired then of magnitude, but clustered around a few domi- others fill the void contributing to the ecological nant frequencies (Holling, 1996). Ecosystems are resilience of the system. The implication here is to heterogeneous, displaying patchy and discontinu- maintain diversity in managed systems and al- ous textures at all scales. Ecosystems do not func- ludes to the classic quote from Leopold (1949), tion around a single stable equilibrium. Rather, ‘‘To keep every cog and wheel is the first precau- Holling states that, ‘‘destabilizing forces far from tion of intelligent tinkering’’. Protecting diversity equilibria, multiple equilibria, and absence of also provides insurance against uncertainty, which 206 S.D. Bergen et al. / Ecological Engineering 18 (2001) 201–210 we will discuss further as part of the third 3. What will nature help us to do here?’’ principle. Knowledge of the place also allows for more Another important characteristic of ecosystems holistic designs. Todd and Todd (1994) refer to is that the outputs of one process serve as the the Gaia hypothesis that the Earth is a complex, inputs to others. No waste is generated and nutri- living organism with all its components intercon- ents are cycled from one trophic level to the next. nected. Ecological design considers both the up- The field of industrial ecology is principally based stream and downstream affects of design decisions on this concept. — upstream in that we consider what resources A final characteristic of natural systems is that must be imported and appropriated to create and they tend to function near the edge of chaos or maintain a solution, and downstream in our con- instability (Cairns, 1996; Holling, 1996). Systems sideration of the site-specific and off-site impacts operating near the edge can take better advantage of the design on the environment. of e6olutionary opportunities. Cairns notes that In addition to the physical context of a design, our current technological systems have co-evolved knowledge of the cultural context is important. with ecosystems and that introducing chaos into Designs are more likely to succeed and to be one system will likely lead to chaos in the other. accepted by the local community when the people Designing systems to include ecological charac- who live in a place are included in the design teristics departs from common engineering prac- process. They bring knowledge of the particulari- tice. Designing for ecological rather than ties of a place and are empowered through direct engineering resilience means encouraging diversity participation in shaping their environment. and complexity and allowing systems to self-orga- Attention to group dynamics and conflict media- nize, mature, and evolve. How to design systems tion is important for successful stakeholder to perform like ecosystems and still function as participation. desired is explored in the remaining principles. 4.3. Third principle — maintain the independence 4.2. Second principle — design for site-specific of design functional requirements context Ecological complexity adds high and often irre- The complexity and diversity of natural systems ducible levels of uncertainty to the design process. cause a high degree of spatial variability. While Even under conditions of certainty, the amount of the ecological characteristics discussed above are relevant information we possess may be over- generally applicable, every system and location is whelming and unmanageable. We want to keep different. The second principle can be stated in a solutions simple and workable. A strategy for number of ways, but boils down to the idea of dealing with uncertainty is to set the tolerances on gaining as much information as possible about the our design functional requirements as wide as environment in which a design solution must possible. function. Spatial variability precludes standard- The third principle is a restatement of the first ized designs, so solutions should be site-specific design axiom of Suh (1990). In the realm of and small-scale (Van der Ryn and Cowan, 1996). mechanistic engineering design, where this axiom Standardized designs imposed on the landscape originates, it appears very straightforward and without consideration for the ecology of a place easy to grasp. Functional requirements (FRs) are will take more energy to sustain (see Section 4.4). the specific functions that we wish a design solu- Berry (1987) sums up this principle succinctly: tion to provide. Design parameters (DPs) are the ‘‘There are, I think, three questions that must be physical elements of the solution chosen to satisfy asked with respect to a human economy in any FRs. Best designs are those that have independent given place: (not coupled) FRs and one and only one DP to 1. What is here? satisfy each FR. When modifying one DP affects 2. What will nature permit us to do here? more than one FR, a design is coupled. S.D. Bergen et al. / Ecological Engineering 18 (2001) 201–210 207

In circumstances where there is functional cou- Similar to the flow of energy, the second design pling, wide tolerances on FRs can make the de- axiom proposed by Suh states that we want to sign essentially uncoupled. Wide design tolerances minimize the information content of a design. The allow a larger functional range for a system while ideas and principles we have discussed so far all the outputs remain within acceptable ranges. This relate to minimizing information, or making de- is another important aspect of designing for eco- signs simple yet successful. When we cooperate logical rather than engineering resilience. Systems with natural processes and allow systems to self- designed for engineering resilience often have organize, it requires less energy and information tight tolerances. to implement and maintain a design (Kangas and When interacting with ecological systems, how- Adey 1996; Odum, 1996). Meeting wide tolerances ever, the concept of functional independence be- requires less information. In the case of stream comes a lot less clear. Ecosystems are complex restoration, high-energy inputs to control system with many levels of interconnection between com- structure or function are counterproductive to the ponents. Many elements of the system may be ecological resilience and performance of the non- involved in more than one process. We must not emphasized functions of the system. For example, confuse ecosystem functionality with design FRs. the energy input needed from humans to restrict a Ecosystems can function and provide benefits to stream channel to a confined space tends to be society without human intervention. We under- high and ultimately unsuccessful when a large take the process of design to satisfy unmet human flood occurs. A better design would recognize the needs, and the FRs for design follow from the expected variability in stream flows and design the statement of these needs. Ecosystem processes system to withstand large variations in flow (wide that presently exist that we wish to preserve while tolerance) and still maintain its ecological and we design for unmet needs act as constraints on engineering functions. design. The independence principle states that we Minimizing information content appears con- are more likely to have successful designs when trary to encouraging diversity and complexity in we can keep the FRs uncoupled in the solution. In design solutions. The extra information required, reality, however, it would be foolish not to take however, is balanced by utilizing self-organization advantage of the multiple, coupled services an and wide tolerances. We can consider it an up- ecosystem can provide. front capital investment in diversity to gain overall efficiency later through reduced energy 4.4. Fourth principle — design for efficiency in requirements and a reduced risk of failure. Diver- energy and information sity provides insurance against uncertainty in addition to contributing to ecological resilience, as The fourth principle follows from taking ad- discussed in the first principle. In the case of an vantage of the self-organizing property of ecosys- engineered wetland, for example, a wide range of tems. To let nature do some of the engineering species may be included in the initial construction, means that we should make maximum use of the but natural processes are allowed to select those free flow of energy into the system from natural best suited for the imposed environment (Mitsch, sources, primarily the Sun. Conversely, we want 1996). Similarly, the first and second principles to minimize the energy expended to create and advocate an up-front investment in knowledge of maintain the system directed, by design, from the design context to minimize uncertainty and to off-site sources, such as fossil fuels, large-scale allow less information to be transferred during hydroelectric sources, etc. While utilizing free design implementation. flowing energy, however, it is important to follow where the energy would go without intervention, 4.5. Fifth principle — acknowledge the 6alues to make sure that it is not more critically needed and purposes that moti6ate design downstream and that there is minimal adverse impact. The definition of ecological engineering we ad- 208 S.D. Bergen et al. / Ecological Engineering 18 (2001) 201–210 vocate states that design is practiced for the proaches, safe-fail solutions acknowledge that our benefit of both society and the natural environ- original functional requirements for a design may ment. Most engineering codes of ethics state at not be met or that there may be unexpected least that engineers have a responsibility to serve results. Failure in this case is not catastrophic. and protect society. We have explicitly broadened Costanza (1996) advocates selecting design alter- that responsibility to include the natural systems natives that have the best worst-case outcome. that support life. Regardless of specific ideology, The precautionary approach has also been ex- however, design practices that acknowledge the pressed as shifting from minimizing type-I error motivating values and purposes will be more to minimizing type-II error (Shrader-Frechette, successful. 1994; Lemons and Westra, 1995). It is the scien- Proponents of an ecological approach to design tific norm to achieve high levels of confidence in a are passionate in their arguments, relying as much hypothesis before it is accepted (minimizing type-I on scientific observation as on ideology, morality, error). When applied to environmental manage- ethics, and spiritual beliefs. Three of the nine ment this means that we would need almost com- precepts proposed by Todd and Todd (1994) plete certainty in a hypothesis of ecological are value statements. Ecological design invites damage resulting from engineering activity before and embraces the qualitative, the uncertain, and we would accept the hypothesis. Minimizing type- the non-rational aspects of human nature. II error would shift the burden of proof to the Goals such as connection to place, equity, sustain- hypothesis that damage is not occurring. Shrader- ability, and esthetics are as important as material Frechette (1994) spells out a number of reasons output. why the choice of minimizing type-II error is an While those writing about ecological design ethical preference. The reasons include concepts hold a variety of values, there is agreement at of intergenerational equity, equitable distribution least on how to respond to risk and uncertainty. of risk, and concern for non-human species. When dealing with the natural environment, many engineering decisions result from what can best be characterized as hubris. The term hubris 5. Conclusions seems most fitting because it implies not only overconfidence, but also that retribution may oc- Ecological engineering is emerging as a distinct cur as a result. Herman (1996) uses the term engineering discipline. As a new field, there is re6enge (after Tenner, 1991) to describe how our danger of confusion from multiple and competing attempts to manage complex systems always seem visions of what ecological engineering is, and of to produce unexpected and unwanted side effects. the scope of its application. We have attempted to Costanza (1996) warns that the worst form of provide an inclusive and broad definition, and ignorance is misplaced certainty. suggest potential applications where an ecological The third principle recommends using wide tol- approach to engineering design can augment the erances under conditions of uncertainty. From a efforts of other professionals to solve complicated value standpoint, we also recommend adopting a and pressing problems. precautionary approach for ecological engineer- Ecological engineering represents the marriage ing. A precautionary approach will act as a form of ecology and engineering design, and as such of insurance against unpleasant surprises in the can perhaps have its greatest contribution in future (Perrings, 1991; Costanza, 1994; Ehrlich, changing how design is practiced in all disciplines. 1994). Engineering would be applied sparingly, From the literature and from our own work, we and complex solutions avoided where possible have proposed five design principles for ecological (Herman, 1996). engineering. Each principle needs to be more fully To avoid catastrophic failures, design solutions developed through further research, interdisci- that are both fail-safe and safe-fail should be plinary dialogue and experimentation. Some of pursued. As opposed to traditional fail-safe ap- the most pressing research issues, we believe, are: S.D. Bergen et al. / Ecological Engineering 18 (2001) 201–210 209

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