Introduction

Controversy remains over the concept of ecosystem engineer (Jones et al. 1994, Wright and Jones 2006), although some signs indicate general resolution and acceptance (Stinchcombe and Schmitt 2006). The arguments for and against ecosystem engineering sometimes pertain to issues as trivial as whether or not engineer connotes motive (Power 1997a, Jones et al. 1997), with reservation of the term engineer to some entity having purpose (Power 1997b). One might assert that this level of semantic argument leaves ecology looking a bit pedantic, given that, say, physics discusses the flavors of quarks, including types called charm and strange. Even so, why should purpose be ascribed only to humans in their activities, and not, say, to beavers building dams?

More important arguments are that the ecosystem engineering (EE) concept has been so broadly defined as to include all species at all times in all situations (Reichmann and Seabloom 2002a, 2002b). If so, then the term becomes rather useless as an ecological concept, though some take this situation as a positive sign of its ubiquity (Wright and Jones 2006). After many discussions, however, the distinction between EE and abiotic interactions remains unclear.

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I have three objectives with this chapter. First, I briefly present historical precedents of the term abiotic interactions and early discussions of the feedback between organisms and their environment. In no way is this a definitive review, but hopefully it's representative of the intellectual development. Interestingly, I found a particularly poignant remark by Cooper (1926) regarding ecology's buzzwords: "It almost seems that the moment one formulates a concept and provides it with a name and a terminology the spirit of it flies away and only the dead body remains." The "Cooper principle" is holding in this case: The spirit of ecosystem engineering has been around for at least 80 years as "environment modification," but its recent naming as ecosystem engineering and the assertions of its importance and ubiquity might be revealing its skeletal remains. I hope this historical perspective helps bring back the spirit.

My second objective is exploring the concept in pictures and math, which, I think, helps clarify the many words written on the topic. Only a few theoretical treatments exist, which have all been good, and it is this type of development that I explore here. Focusing on logistic growth-like mathematical models, I uncover what I believe is the most interesting situation deserving the label ecosystem engineering as unique from environment modification. Briefly, it is the situation where an organism modifies its environment to promote its own growth.

Third, I try to relate previous definitions of ecosystem engineering to other concepts, primarily abiotic interactions, keystone species, and niche construction. I avoid old ground, primarily for the insufficient reason that the semantic arguments are difficult for me to follow given the mathematical wiring of my brain, not to mention that Wright and Jones (2006) reviewed the verbal arguments quite extensively.

Summarizing my conclusions, I show that the concept of feedback between organisms and their environments, including knock-on effects to other organisms, has a long history dating back at least to the 1920s, and would perhaps best be dubbed environment modification after Solomon (1949). I argue that environment modification is a more general idea that encompasses recent definitions of ecosystem engineering. However, I'll use the two terms somewhat synonymously until the very end. Using a series of linearized models, I demonstrate a connection between ecosystem engineering and the keystone species concept (e.g., Power et al. 1996). Essentially, there's the organism, the activities it performs, and the consequences of its activities. Keystone species are those with large consequences, whereas advocates of ecosystem engineering focus on the activities irrespective of the consequence's magnitude. Finally, exploring the consequences of feedbacks between organisms and their environment reveals a situation in which natural selection leads to runaway growth and extreme environmental modifications.

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