On The Purpose Meaning And Usage Of The Physical Ecosystem Engineering Concept

Clive G. Jones and Jorge L. Gutiérrez

1.1 - INTRODUCTION

There has been substantial growth of interest in the concept of physical ecosystem engineering by organisms since the publication of Jones et al. 1994 and 1997a. The concept has certainly catalyzed new case studies, methods, modeling, generalization, and synthesis (see reviews by Lavelle et al. 1997; Crooks 2002; Coleman and Williams 2002; Gutiérrez et al. 2003; Wright and Jones 2004, 2006; Boogert et al. 2006; Caraco et al. 2006; Gutiérrez and Jones 2006; Jouquet et al. 2006; Moore 2006; Hastings et al. 2007; also see Table 1.1). However, the concept has also generated controversy and uncertainty over meaning, usage, and purpose (e.g., Jones et al. 1997b; Power 1997a, 1997b; Reichman and Seabloom 2002a, 2002b; Wilby 2002), reflected in the following questions. Don't all organisms change the environment? Aren't all organisms therefore ecosystem engineers? If so, isn't the concept too broad to be useful? Don't engineers always have large or large-scale impacts? Shouldn't engineers be limited to species with large effects? Aren't engineers and keystone species the same? Isn't engineering equivalent to facilitation or positive influence? Isn't the approach overly reductionist? Why do we need the concept? How can we use it?

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TABLE 1.1 Illustrative usage of the physical ecosystem engineering concept.

Conceptual Application

References

Population dynamics

When survival depends on habitat modification Linked to dynamics of patch creation Invasion

Community organization

Consequences for community structure

Species interactions and altered resource availability or abiotic stress

Patterns of species distribution

Variation in species responses across abiotic gradients

Environmental heterogeneity and species diversity at patch and landscape scales

Parsing species effects into trophic (assimilatory-dissimilatory) and nontrophic contributions Structural legacies and community organization Species diversity in fossil communities Assessing effects on community organization Predicting patch-level richness effects

Ecosystem processes

Controls on material fluxes between ecosystems

General determinants of biogeochemical heterogeneity Integration with state factors

Conservation, restoration, and management

Global change scenarios for soil Persistence of endangered species Support of species diversity via habitat diversity

Conceptual models for management and conservation of threatened species Evaluation of abiotic restoration options

Gurney and Lawton 1996

Wright et al. 2004 Cuddington and Hastings 2004

Flecker 1996, Flecker and Taylor 2004, Gutiérrez and Iribarne 1999 Gutiérrez and Iribarne 2004,

Daleo et al. 2006 Escapa et al. 2004, Jouquet et al. 2004 Crain and Bertness 2005, Wright et al. 2006, Badano and Cavieres 2006b Wright et al. 2002, 2003, 2006; Lill and Marquis 2003; Badano and Cavieres 2006a, 2006b

Crooks and Khim 1999, Wilby et al. 2001

Gutiérrez and Iribarne 1999

Parras and Casadío 2006 Badano et al. 2006 Wright and Jones 2004

Caraco et al. 2006, del-Val et al. 2006, Gutiérrez et al. 2006

Gutiérrez and Jones 2006 Jones et al. 2006

Lavelle et al. 1997 Pintor and Soluk 2006 Bangert and Slobodchikoff 2006 Goubet et al. 2006

Byers et al. 2006

Uncertainty, misconstrual, and misunderstanding impede scientific progress, but since no concept is ever born fully developed, they also justify clarification. Concepts that cannot eventually be sufficiently unambiguously defined as to be made operational deserve to disappear. Further, while a concept is not a theory, it is a foundation upon which theory is built, and the foundation must be solid if one has any aspiration for theory development (Pickett et al. 1994). The questions outlined in Jones et al. (1994) clearly beg theory development.

Here we present a perspective on selected aspects of the purpose, meaning, and usage of the concept, including some new thoughts, some clarification, and some reification. We briefly describe the domain, general purpose, and components of the concept. We then define the two coupled, direct interactions comprising ecosystem engineering— the physical ecosystem engineering process responsible for abiotic change, and physical ecosystem engineering consequence that addresses biotic effects of abiotic change. We clarify the meaning of "ecosystem" in ecosystem engineer. We address causes of process ubiquity and how they lead to general expectations of consequence. We examine sources of context-dependent variation in engineer effect magnitude and significance and what needs to be known to predict effects. We define conditions for detectable engineering effects and the condition for large effects, all other factors being equal (i.e., ceteris paribus). We argue against unspecified conflation of process and consequence. We illustrate where explicit consideration of influential physical ecosystem engineering may or may not be needed, point out what the concept has been used for, and suggest general topics where it might be useful. We end with comments on how conceptual breadth relates to utility, and what perspective on species interactions is reflected in the concept. Our overall intent is conceptual clarification and amplification.

1.2 - ON THE DEFINITION

ON THE CONCEPTUAL DOMAIN, GENERAL PURPOSE, AND COMPONENTS

Physical ecosystem engineering as defined by Jones et al. (1994, 1997a; Table 1.2) is a particular form of abiotic environmental modification by organisms that often, but not invariably, has effects on biota and their interactions. Abiotic environmental change occurs as a consequence of the physical structure of organisms or via organisms causing changes in the physical structure of the living and nonliving materials. These abiotic changes can then affect biota, including the engineer. Biotic influence

6 I « HISTORY AND DEFINITIONS OF ECOSYSTEM ENGINEERING TABLE 1.2 Definitions of physical ecosystem engineering.

Jones et al. 1994: "Ecosystem engineers are organisms that directly or indirectly modulate the availability of resources (other than themselves) to other species by causing physical state changes in biotic or abiotic materials. In so doing they modify, maintain and/or create habitats. The direct provision of resources by an organism to other species, in the form of living or dead tissues is not engineering."

Jones et al. 1997a: "Physical ecosystem engineers are organisms that directly or indirectly control the availability of resources to other organisms by causing physical state changes in biotic or abiotic materials. Physical ecosystem engineering by organisms is the physical modification, maintenance or creation of habitats. Ecological effects of engineers on many other species occur in virtually all ecosystems because the physical state changes directly create non-food resources such as living space, directly control abiotic resources, and indirectly modulate abiotic forces that, in turn, affect resource use by other organisms. Trophic interactions, i.e., consumption, decomposition and resource competition are not engineering."

Physical ecosystem engineering process: Organismally caused, structurally mediated changes in the distribution, abundance, and composition of energy and materials in the abiotic environment arising independent or irrespective of changes due to assimilation and dissimilation.

Ecosystem engineering consequence: Influence arising from engineer control on abiotic factors that occurs independent or irrespective of use of or impact of these abiotic factors on the engineer or the participation by the engineer in biotic interactions, despite the fact that these can all affect the engineer and its engineering activities.

"Ecosystem" in Ecosystem Engineering: A place with all the living and nonliving interacting. Hence, ecosystem refers to the biotic on abiotic of the engineering process and the abiotic on biotic of engineering consequence.

For discussion see text and cited references.

encompasses organisms, populations, communities, ecosystems, and landscapes and can be integrated by thinking of physical ecosystem engineering as the creation, modification, maintenance, and destruction of habitats. The concept therefore addresses some but not all of the ways organisms can change the abiotic environment and the consequences thereof.

The concept was developed to encompass a variety of disparate and oft-ignored ecological phenomena not addressed by the historical focus of ecology on trophic relations (i.e., predation, resource competition, food webs, energy flow, nutrient cycling, and the like). Ecologists had long been familiar with many examples (see Chapter 2, Buchman). Some specialty areas in ecology and other disciples had emphasized some aspects (e.g., marine sediment bioturbation, mammalian soil disturbance, geomorphology). Nevertheless, as evidenced by omission from ecological textbooks, formal recognition and study of the general process and its consequences were not central to ecological science. So the primary purpose of the papers (Jones et al. 1994, 1997a) was to draw attention to the ubiquity and importance of this process and its consequences, to provide an integrative general framework, to lay out a provisional question-based research agenda, and to give it a name.

The concept addresses the combined influence of two coupled direct interactions. The first is the way organisms change the abiotic environment—the physical ecosystem engineering process. The second is how these abiotic changes affect biota—ecosystem engineering consequence. The distinction reveals important criteria of demarcation for what is and is not physical ecosystem engineering, exposes context dependency for effects that enhance prediction of effect magnitudes and significance, and helps clarify the purpose of the concept and how one might use it. In the following text we examine these two component interactions before briefly reintegrating them with the overall concept.

ON THE PHYSICAL ECOSYSTEM ENGINEERING PROCESS

The physical ecosystem engineering process can be defined as the following: Organismally caused, structurally mediated changes in the distribution, abundance, and composition of energy and materials in the abiotic environment arising independent or irrespective of changes due to assimilation and dissimilation.

"Organismally caused" distinguishes the process from purely abiotic forces (i.e., climatic and geologic processes) that are functional analogs when they change the same abiotic variables. Wind and elephants both uproot trees creating tip-up mounds. Organismal causation also invokes potential for spatial and temporal differences in the resulting abiotic environment compared to purely abiotic forces, even when the mean abiotic change is the same (cf. Reichman and Seabloom 2002a). Elephants and wind both may knock over trees, but different factors are needed to predict when and where such events might occur (Pickett et al. 2000).

"Structurally mediated changes" reflects the requirement for abiotic change to arise via structural change (i.e., physical state changes, Jones et al. 1994, 1997a). This can occur autogenically where the living organism is the structure, or allogenically where the organism makes the structure from living or nonliving materials (Jones et al. 1994). Thus if there is no structural change there is no physical ecosystem engineering process. This requirement distinguishes this process from other ecological processes that may have the same abiotic effect (e.g., increased nitrogen in aquatic invertebrate burrows can result from invertebrate excretion and from increased oxygen supply that controls microbial mineralization, Aller 1988), or the same overall biotic response (e.g., increased macrophyte growth in the presence of burrows, Bertness 1985).

Inherent in structural mediation but not explicit in the definition is recognition that structures have some degree of persistence. Dead auto-genic engineers and allogenic engineering leave structural legacies with concomitant abiotic effects, with the persistence of legacies being a function of construct durability and the abiotic and biotic forces causing their disappearance (Jones et al. 1994, Hastings et al. 2007).

"Changes in the distribution, abundance, and composition of energy and materials in the abiotic environment" is the most general possible description of abiotic influence. Such effects are not unique to the engineering process. Geomorphic structures can have similar abiotic effects (e.g., rocks and trees both cast shade), and as discussed in following text, organismal uptake and release of materials can bring about comparable abiotic changes. However, within a structural context, ecosystem engineering encompasses organismally changed structure (e.g., a burrow, leaves tied by caterpillars, earthworm litter burial), interactions of structure with various forms of kinetic energy (e.g., hydrological attenuation by beaver dams), abiotic consequences of such kinetic interactions (e.g., sedimentation behind the dam), and interactions of organismally made structures and kinetic energy imparted by organisms (e.g., burrowing polychaetes pumping water by body movement, Evans 1971). For further discussion of some of these relationships, see Gutiérrez and Jones 2006.

Finally, the requirement that abiotic change occur "independent or irrespective of changes due to assimilation and dissimilation" distinguishes the engineering process from changes caused by the universal processes of organismal uptake (light, water, nutrients, other minerals, O2, CO2, trace gases, organic compounds) and release (carbon and nutrients in litter, woody debris, feces, urine, and carcasses; water, O2, CO2, trace gases, H+, other organic and inorganic chemicals). Since the physical ecosystem engineering process can result in altered energy and material flows (e.g., water kinetic to potential energy in a beaver impoundment and sedimentation of suspended materials), and these can involve chemical changes (e.g., redox effects on beaver pond sedi ment geochemistry due to reduced water column oxygen exchange), this part of the definition is a necessary and important qualifier for the non-assimilatory and nondissimilatory (or "nontrophic") basis of any abiotic effects.

It is worth further exploring what we mean by "independent or irrespective," since it informs where the engineering process begins and ends. "Independent," in the context of our definition, means that there are many other life processes unrelated to or only very distally related to assimilation and dissimilation that can result in changes to structure and the abiotic environment—growth, predator and stress avoidance, and movement, to name but a few. Examples include wind attenuation by trees, nests and dens that shelter animals, and the hoofprints and trails made by large animals.

"Irrespective," in the context of our definition, means that many organismal activities associated to varying degrees with assimilatory and dissimilatory transfers also have structural influences whose effects on the abiotic occur regardless of any influence of the transfers. For example, leaf litter affects soil-gas exchange and rain splash impact irrespective of its role as a resource for decomposers (Facelli and Pickett 1991). Trees cast shade, in part because they assimilate photons (uptake) and in part because, like any physical structure, they absorb and reflect photons (engineering). Desert porcupines always dig soil to feed on bulbs (Shachak et al. 1991); soil effects occur irrespective of consumption but are always associated with it. Effects of insect defoliation on the under-story physical environment (e.g., Doane and McManus 1981) depend upon consumption amount (along with extant canopy structure and extrinsic abiotic conditions) but occur irrespective of effects on trees or caterpillars or altered nutrient cycling via frass. The central point is not that assimilation-dissimilation must always occur separately from the engineering process, although as noted in preceding text it is often independent, but that any co-occurrence requires the distinction if we are to invoke either engineering or assimilation-dissimilation as a causal explanation for abiotic change.

ON ECOSYSTEM ENGINEERING CONSEQUENCE

Abiotic changes due to the engineering process are the starting point of consequence. While worthy of study alone (e.g., erosion, hydrology, sedimentation, pedogenesis, heat balance, physical gas exchange, etc.), they necessarily underpin all consequences for biota and their interactions on which we now focus. We can broadly define consequence as the following: Influence arising from engineer control on abiotic factors that occurs independent or irrespective of use of or impact of these abiotic factors on the engineer or the participation by the engineer in biotic interactions, despite the fact that all these can affect the engineer and its engineering activities.

"Control" (modulation is equivalent) is analogous to a faucet on a pipe; flow is regulated independent or irrespective of water use. Thus beaver dams control hydrology and flood and drought impact (Naiman et al. 1988), while dead mollusk shells control living space, enemy-free space, and abiotic stress (Gutiérrez et al. 2003). The term control helps distinguish engineering effects on biota and their interactions from any other influence of the engineer via other types of ecological interactions (e.g., abiotic resource uptake and direct resource competition; role as predator, prey, pollinator, or disperser).

"Abiotic factors" is shorthand for the large number of abiotic influences on biota and their interactions very familiar to ecologists. All that differs here is recognition that an organism is responsible for abiotic change via structural change, but the kinds of abiotic variables are no different. They are the following: consumable energy and materials (e.g., light, nutrients, water); nonconsumable resources (e.g., living space, enemy- or competitor-free space); and abiotic constraint or enablement including direct abiotic influences on organisms (e.g., temperature, salinity, wind, redox) and influences on information exchange or cues used by organisms (e.g., sound attenuation or amplification, temperature, light quality).

This first part of the definition ("influence arising from engineer control on abiotic factors") contains an important, unstated but implicit recognition that since species and their interactions vary in their sensitivity to the abiotic, engineer effects will be context dependent on the degree of abiotic change caused by the engineering process and the degree of abiotic limitation, constraint, or enablement experienced by species. Such context dependency applies to direct abiotic effects on species (e.g., trapped runoff water on plant growth, Eldridge et al. 2002) and abiotic influences on species interactions (e.g., how engineer-altered resources influence plant competition, Shachak et al. 1991; how refugia may affect predator-prey interactions, e.g., Usio and Townsend 2002).

The latter part of the definition ("that occurs . . . engineering activities") recognizes the potential importance of engineering feedbacks to the engineer and effects of other biotic interactions on engineering activities. It also emphasizes that the relationship between the engineer and its engineering effects is fundamentally no different from the effect of the engineer on other species, i.e., effects arise via control on abiotic factors. Again, it excludes any other types of ecological interactions that the engineer may have with other biota, while recognizing that if these other interactions affect engineer density, engineering activities, and structural change, they can then affect the degree and type of abiotic change.

ON COMBINING ENGINEERING PROCESS AND CONSEQUENCE

Given a suitably broad construal of habitat encompassing all relevant abiotic aspects of place along with some biotic effect, process and consequence can be usefully combined into the recognition that physical ecosystem engineering is organismal, structurally mediated habitat change, conforming to the definition of Jones et al. (1994, 1997a).

We think the definitions of ecosystem engineering process and consequence enhance the overall definition of physical ecosystem engineering, helping provide clear criteria of demarcation as to what it is and what it is not. There is no fundamental change in either the intent or meaning of the concept, hopefully just illumination. As we show later, this collectively informs expectations for effect magnitude and significance, and how to use the concept.

ON "ECOSYSTEM" IN ECOSYSTEM ENGINEERING

We will not go into the meaning of the word engineer. It is certainly neither defined nor treated tautologically in the concept, and this issue has been adequately discussed (Power 1997a, 1997b; Jones et al. 1997b; Wright and Jones 2006). However, we will make a brief comment on "ecosystem" in ecosystem engineer. Some have construed the meaning as large scale or extensive. However, the meaning derives from Tansley (1935). His definition of ecosystem was size independent. An ecosystem can be large or small, but it is always a place with all the living and nonliving interacting (Likens 1992, Pickett and Cadenasso 2002). Thus here "ecosystem" refers to the biotic-abiotic-biotic interactions representing the engineering process (biotic on abiotic) and consequence (abiotic on biotic). Certainly, some engineers can affect the functioning of large areas (e.g., oyster reef influences on estuarine flows and sedimentation, Ruesink et al. 2005; tsunami attenuation by mangrove forests, Kathiresan and Rajendran 2005), but they often have local effects (e.g., animal burrow, woodpecker hole, phytotelmata, birds nest). So, although the spatial scale of engineering is an interesting and important topic (e.g., see Hastings et al. 2007), it is neither a defining feature of the concept, nor the meaning of the word ecosystem in the concept.

1.3 - ON PROCESS UBIQUITY

Are all organisms capable of the physical ecosystem engineering process? Based on the definition of the process and first principles of physics, the answer is almost certainly yes for all free-living organisms, although this clearly cannot be empirically proven. All physical structures interact with kinetic energy (i.e., radiant as light, heat, sound; energized fluids as water, air, and other gases). The inanimate and animate do not fundamentally differ in this regard. All free-living organisms have physical structures (autogenic). Many alter the physical structure of their surroundings (allogenic). Some, such as bioturbators, also generate kinetic energy in their structurally modified surroundings (allogenic). All these structures are inserted into abiotic kinetic energy flows. Physics tell us that these structures must affect and be affected by those flows, resulting in some degree of energy transformation and the redistribution of energized fluids and the materials they may contain. Given sufficiently accurate and diverse measurement instrumentation, it is a reasonable bet that all structures will result in some detectable change in one or more abiotic variables. A bird's nest affects local turbulent airflow, and mobile animals cast temporary shade, even though these almost certainly have no broader significance. So in this sense the physical ecosystem engineering process is an extended property of life. This should not be a blinding revelation, but then, nor is the fact that all free-living organisms also necessarily change the abiotic environment via the uptake and release of energy and materials.

Organisms therefore cannot be physically engineering unless they directly cause structural change within an abiotic milieu. So, ignoring the obviously trivial (e.g., shade cast by moving animals), it follows that if they are not causing such changes they are not engineering; and if they are not free-living they cannot engineer (cf. Thomas et al. 1998). We might expect greater capacity for influence when organisms are or make persistent rather than ephemeral structures (Jones et al. 1997a). Organ-ismally created structures that are large relative to the abiotic environment experienced by other biota might be more influential (e.g., forests, Holling 1992; impoundments in tree holes or phytotelmata, Fish 1983; leaves tied by caterpillars, Lill and Marquis 2003) than those that are relatively small (e.g., effects of herb shade on large mammals). Small-bodied autogenic engineers likely have to be numerous (e.g., algae, Townsend et al. 1992) or aggregated into larger structures (e.g., microbial biofilms, Battin et al. 2003) to have large abiotic effects. It seems reasonable to suppose that small allogenic engineers will either have to have large per capita effects (e.g., earthworms, Darwin 1890, Lavelle et al. 1997) and/or be numerous (e.g., termites, Dangerfield et al. 1998, Jouquet et al. 2006) to cause substantive abiotic change (Jones et al. 1994, 1997a).

While the preceding is somewhat informative, it is clearly insufficient to predict what abiotic changes will occur, how large they will be, or what the biotic significance may be—issues we turn to next.

1.4 - ON EFFECT MAGNITUDE AND SIGNIFICANCE

Ubiquity of a life process does not equate to universality of importance. We should expect that the physical ecosystem engineering process may often have little consequence, in the same way that energy and material uptake and release by many of the organisms in an ecosystem are not central to understanding energy flow, nutrient cycling, or food web dynamics. Nor for that matter is ubiquity a cause for phenomenological dismissal. Some physical engineering is significant, just as the uptake and release of energy and materials by some organisms is important. The challenge is to determine what makes the difference between the significant and insignificant.

The answer is it depends on context, and we think the separation of physical ecosystem engineering into process and consequence helps address this context dependency. First, from the definition of process, there can be no abiotic effect, hence no biotic consequence, without structural change. Second, given structural change, depending on the abiotic variable(s) of interest selected and baseline abiotic conditions (i.e., the structurally unmodified state), measurable abiotic change may or may not occur, depending upon structural form and abiotic milieu. The physical properties of structures and the physics of their interaction with kinetic energy are central to predicting this effect. Third, given some detectable abiotic effect, changes may be the same as, or larger or smaller than, those caused by other forces (i.e., purely abiotic or assimi-latory-dissimilatory). Further and as noted earlier, the spatial or temporal dynamics of such abiotic effects may be the same as or different from those due to other forces. Thus we can judge the importance of the engineering process in terms of abiotic change relative to the effect magnitudes and dynamics due to these other forces acting on the same abiotic variable(s). Fourth, given some abiotic change, we should then expect that whether or not there will be biotic consequence will depend upon the degree of abiotic change (magnitude and direction) and the sensitivity of the biota or their interactions to this abiotic variable in terms of limitation, constraint, or enablement. An understanding of species sensitivities relative to baseline abiotic conditions can be used to predict the particular response. Finally, given some abiotic effect on some biotic response variable of interest, we can judge the relative import of the engineering in comparison to other forces (abiotic or other types of biotic interactions) affecting the same biotic response variable.

The preceding dependencies allow for a very precise definition of when physical ecosystem engineering will have a biotic effect. If an organism causes structural change that results in an abiotic change that is larger than or different from that caused by other abiotic or biotic forces; and if biota are sensitive to that degree or type of abiotic change; and if the biotic responses to these abiotic changes are greater than those due to other biotic forces acting on the same biotic response variable; then there will be a detectable engineering effect. If any one of those conditions does not hold, there will be no detectable effect. It follows that physical engineering by organisms that causes large abiotic changes affecting highly sensitive biota where there is no other influence (i.e., ceteris paribus) will have large effects.

While the preceding analysis identifies the primary sources of context dependency and how to address them, it is clear on both theoretical and empirical grounds that we should expect that, overall, physical ecosystem engineering by organisms can have no effect, or positive or negative effects; and that any effects will vary from small to large (Jones et al. 1994, 1997a). Such considerations indicate that it might be unwise to conflate process and consequence without clear accompanying statements of conditionality.

As ecologists we seek to predict and explain the significant. We doubt anyone could get a paper published on the lack of effects of turbulence due to bird's nests on canopy gas exchange, or the lack of effects of shade cast by mobile animals on plant growth. Scientists know how to avoid the trivial, so we are not concerned that the literature will be overwhelmed by such papers. We are, however, very much concerned about the opposite tendency, that of merging engineering process and consequence into statements that are solely about the significant without appropriate statements of conditionality.

We note an unhealthy tendency in the literature for such unspecified conjunction, and we think this a dangerous deviation from the meaning and intent of the concept that seriously weakens its value. Thus we are not at all enamored of statements that can be construed as saying the equivalent of the following: All engineers have large effects; or engineers ought to be restricted to those that have large effects; or keystone species and engineers are the same; or engineers have mostly positive or facilita-tive effects. Based on the original papers that discussed these issues

(Jones et al. 1994, 1997a), other papers pointing out the same problem (Boogert et al. 2006, Gutiérrez and Jones 2006, Wilby 2002, Wright and Jones 2006), and the preceding considerations, we think such statements are scientifically indefensible on both empirical (e.g., Wright and Jones 2004) and theoretical grounds unless they are accompanied by clear statements of conditionality. Such unconditional statements are episte-mologically equivalent to saying that predation always has large effects on prey density; or we will only call it a predator if it has a large effect; or that a predator invariably negatively affects prey density—statements we know not to be universally true (e.g., Adams et al. 1998, Strauss 1991, Wooton 1994).

Physical ecosystem engineering is a process that may have significant consequence given certain conditionalities outlined in preceding text. We are as concerned as anyone with being able to predict which species will be important engineers and what and how big their effects will be; it is the central theoretical challenge to which the concept can contribute. We already know that organismal activities that change structure vary, that structures vary, that baseline abiotic environments vary, that resulting abiotic change varies, and that species vary in their sensitivity to abiotic factors. We do not think this challenge can be met by unspecified conflation that thereby eliminates the very sources of variation in cause and effect. Ecological outcomes are often context dependent. Little is to be gained by ignoring this in our quest for general understanding.

That a concept exists and is used by some should not obligate others to use it, nor should the fact that it is unnecessary in some situations preclude consideration of its utility elsewhere. Nor should we, as authors, attempt to proscribe usage; this is anathema to creativity and assumes omniscience we lack. Instead, we will illustrate some situations when explicit consideration of physical ecosystem engineering may not be needed even though it may be influential, briefly point out what the concept has been used for, and make a few suggestions for general topics where it might be particularly useful.

Many ecological questions about abiotic environmental effects can be answered by taking the abiotic as a given or treating it as stochastic variation. We do not need to consider the engineering if the abiotic is measured as an independent variable, and we make no inference about causation. If the abiotic is not measured, any assumptions about and conclusions based on independence in abiotic state or dynamics, or treatment as stochastic abiotic variation, are violated if it is engineered. This is because the spatial and temporal dynamics of the abiotic environment will, in some way, reflect the factors influencing the engineer and its engineering activities. If the engineering can legitimately be treated as an externality (i.e., no engineer feedback), the abiotic still can be taken as a given, even though it is "made" by the engineer, again provided it is measured and provided no assumptions are made that its dynamics are independent of biota. If the engineer is not an externality to the system, then whether or not the engineering has to be explicitly considered will be a function of the degree to which engineering feedbacks to the engineer and structural legacies alter dynamics. For example, if the abiotic is always changed the same way and to the same degree over the same space and time scales as the presence of the engineer, then the engineering could be collapsed into presence-density of the engineer.

Parsimony suggests that other extant models or concepts may serve as well or better than engineering in some circumstances, even when engineering is responsible for observed effects. For example, plant shade is, in part (see earlier text), an engineering process controlled by canopy architecture, leaf area index and photon absorption, and reflection properties of leaves; however, simple light competition models often suffice (e.g., Canham et al. 2006). Such models are not appropriate for understanding habitat creation for understory plants, since this is not competition; either nonmechanistic facilitation models or engineering models could be used. If we are interested in how variation in light quantity and quality within a forest creates habitat diversity for understory species, we may need to measure some of the preceding physical engineering variables across species. But perhaps we might also collapse this into light quality neighborhoods associated with certain tree species, taking the underlying engineering processes as given.

One might imagine that consideration of engineering would be de rigueur in studies on the population dynamics of obvious, significant ecosystem engineers. However, we may not have to explicitly expose the engineering under all circumstances. To date, modeling and theoretical studies indicate that explicit consideration is required under five basic circumstances: When engineering feedbacks affect density-dependent regulation (Gurney and Lawton 1996, Wright et al. 2004; also see Chapter 3, Wilson); when structural legacies created by engineers introduce lagged environmental decay (Gurney and Lawton 1996, Wright et al. 2004, Hastings et al. 2007); when mobile engineers exhibit differential preference for various engineered environmental states (Wright et al. 2004); when engineering is optional and dependent on environment state (Wright et al. 2004); and when the engineering has spatial dimensions that do not simply relate to the presence of the engineer (e.g., extensive influence, Hastings et al. 2007).

So, in general, if we seek causal explanation of abiotic change, including its dynamics, we may often, but not invariably, invoke physical ecosystem engineering, but this does not mean that all the underlying details always require exposure. Clearly, understanding when explicit consideration is de rigueur would be of considerable value, and modeling can do much to help answer this question. Perhaps the easiest answer to the usage question is just to point out where the concept seems to have been useful over the last 12 years. Table 1.1 illustrates some of the diversity of ecological questions that have substantively made use of the concept in population, community, and ecosystem ecology, and in conservation, restoration, and management.

We end this section with some eclectic suggestions of general topic areas where we think consideration of the ecosystem engineering dimensions may be particularly worthwhile: abiotic heterogeneity, its consequences and context dependency; explanation of indirect, legacy, keystone, foundation, and facilitative species effects; assessing relative contributions of species to multiple processes; understanding species effects at various levels of organization, especially comparative studies; habitat creation, maintenance, and destruction by species; understanding human environmental impacts; and using species to achieve conservation, restoration, and environmental management goals.

1.6 - ON BREADTH AND UTILITY

We have periodically heard comments that the ecosystem engineering concept is too broad to be useful. Certainly the concept is broad, but we do not understand this reasoning. Many ecological concepts are at least as broad in scope and are very useful (e.g., the ecosystem, predation, competition [as a process], nutrient cycling, energy flow, dispersal). Some concepts are broad and still under debate as to their utility (e.g., keystone species, intermediate disturbance, ecological thresholds, functional groups). Some broad concepts have been abandoned as not being particularly useful (e.g., Clemensian superorganism, balance of nature, phytosocial sintaxa). Breadth is determined by the variety of phenomena encompassed by the central idea. Conceptual value is judged by the degree to which it affords better scientific understanding, given sufficient time for a community of investigators to further develop and assess it. We leave it to the community to judge whether the concept has been useful and still can be useful based on the literature and our preceding discussion.

If ecosystem engineering encompassed only beaver or only gophers, it would be so narrow that it would be just a species description and neither interesting nor useful. If, based on its definition, the concept attempted to encompass all types of abiotic change by all organisms, then it would be incorrect, an impediment, and too broad. The conceptual domain is, however, very specific. It refers only to organismally caused, structurally mediated abiotic change and its biotic effects. The breadth arises from the fact that many organisms do this to some degree. While we can recognize subclasses within (e.g., autogenic, allogenic), we cannot arbitrarily include some organisms that fit the definition, while excluding others that also fit the definition. This is another reason why we consider that defining an ecosystem engineer as such only when it has a large effect is a fundamental deviation from the purpose of the concept. Such a deviation would force us into confronting the same insoluble problem facing the keystone species concept: how to universally define species importance in a context-dependent world with variable outcomes.

1.7- ON THE UNDERLYING PERSPECTIVE

The ecosystem engineering concept has certainly led to a wider appreciation of the ubiquity of organismally caused, structurally mediated abiotic change and its effects on organisms, populations, communities, ecosystems, and landscapes. We think it helps provide a broader view of nature, one extending beyond the dominant trophic perspective. Nevertheless, it is also a perspective. It is just a way of looking at certain things organisms do that affect the way they interact with the abiotic environment and hence each other.

It is a mechanistic rather than a phenomenological view. The ecosystem engineering process and organismal abiotic sensitivity both must be considered to predict outcomes. To some who consider outcomes the Holy Grail, in ecology—we agree that predicting outcomes is a Grail— such a mechanistic, context-dependent perspective may seem insufficiently phenomenological. On the other hand, as pointed out by Wright and Jones (2006), many process-based concepts have ultimately turned out to be more useful than outcome-based ones, perhaps reflecting their greater suitability for addressing context dependency.

To others, the abstraction of organismal features relevant to engineering may seem like reductionism or atomization. Yet the focus on relevant organismal features has been of great value in other areas of ecology

(e.g., predation, direct resource competition, vectoring). It does not preclude recognition of multiple roles of species, nor their integrated total effect. So akin to these other areas, identifying organismal attributes relevant to engineering can contribute to our understanding of context-dependent species effects, while facilitating cross-species and cross-system comparisons (for excellent examples, see Crooks and Khim 1999, Wilby et al. 2001).

1.8 - A CONCLUDING REMARK ON CONCEPT AND THEORY

As pointed out in this chapter's introduction, a concept is not a theory, but it is a foundation upon which theory is built. This foundation must be solid. We hope that our discussion helps provide some solidification with a concomitant reduction in uncertainty, misconstrual, and misunderstanding. We do think the concept can be built into more fully developed theory. Indeed, we see clear signs that this is happening. Many of the examples of use of the concept (Table 1.1) involve general hypotheses, frameworks, methodologies, models, and applications that all contribute to theory development. There is, however, much to be done before we would call physical ecosystem engineering a developed theory; not least, demonstrating that the concept can help predict which species will have what magnitude of engineering effects, on which abiotic variables, with what biotic consequence, in which types of abiotic environments.

ACKNOWLEDGMENTS

We thank Bob Holt for pointing out abiotic information use by organisms and that engineering also can control signals made by them; Brian Silli-man for urging us to address the question "Are all organisms ecosystem engineers?"; colleagues in the NCEAS working group, "Habitat modification in conservation problems: Modeling invasive ecosystem engineers" (Jeb Byers, Jeff Crooks, Kim Cuddington, Alan Hastings, John Lambrinos, Theresa Talley, and Will Wilson) for valuable discussion; and countless authors, colleagues, and students for their writings and comments on the concept over the last 12 years. We thank the Andrew W. Mellon Foundation and the Institute of Ecosystem Studies for financial support. CGJ thanks the state and region of the Île de France for a Blaise Pascal International Research Chair via the Fondation de École Normale Supérieure. This chapter resulted from a working group at the National Center for Ecological Analysis and Synthesis, a center funded by NSF (Grant No. DEB-94-21535), the University of California at Santa Barbara, and the State of California. This is a contribution to the program of the Institute of Ecosystem Studies.

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