An inclusive view of environmental engineering
As noted by Hastings et al. (2007), the recognition that organisms can profoundly alter their physical and chemical environments is far from new. Several nineteenth-century works of natural history, including Darwin's charmingly titled The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits (1881), explicitly investigated how animals and plants modify the soil they inhabit (see Sections 5.2 and 5.3). Frederic Clements's pathbreaking 1916 book on ecological succession examined how plants change their immediate abiotic environments in ways that regulate both their own persistence and the composition of their local communities (references in Hastings et al. 2007).
If attention to organismic impacts echoes an earlier insight, that is not in itself a conceptual weakness or an objection to renewed investigation. In this case, as with ecological development and norm of reaction studies (see Chapter 1, Section 1.3), a return to more inclusive, earlier approaches that better accommodate current data can constitute important progress. What is problematic, however, is whether it is meaningful to distinguish only those taxa that most strongly alter their environments as "engineers," given that any organism inevitably alters its environment to some extent (Reichman and Seabloom 2002). This conceptual difficulty is exemplified in published lists of "ecosystem engineers" that offer a seemingly arbitrary selection of examples out of countless potential cases (e.g., C. Jones et al. 1994; Odling-Smee et al. 2003). Is this separate category useful? What is the threshold between subtle environmental impacts that can presumably be safely ignored and modifications that lead to significant functional and selective feedback effects? This connects to a key dilemma in coevolution studies: while pragmatism dictates focusing on the subsets of a community's taxa that are engaged in strong (often pairwise) interactions, even those direct interactions are shaped by more diffuse, multispecies effects that must therefore be taken into account (Inouye and Stinchcombe 2001). Yet, practically speaking, how can environmental impacts be studied if they emanate from all species? In light of these questions, what strategies can be employed to identify, and ultimately understand, the ecological and evolutionary feedbacks generated by the activities of organisms?
One, often implicit, approach to refining the concept of "engineers" is based on scale: for instance, to recognize as "engineering" only those organismic effects that occur at a greater spatial scale, or last longer than, direct biotic effects such as predation (C. Jones et al. 1994; Hastings et al. 2007). Other authors do include immediate trophic effects (Od- ling-Smee et al. 2003) but consider as "engineering" only those environmental impacts that lead specifically to increased organismic abundance and diversity (references in Pringle 2008; Odling-Smee et al. 2013).
A third approach is to consider as habitat construction all effects of organisms on their environments, from the stunning chemical impact of global photosynthesis and the creation of vast coastal mangrove habitats, to the breaking of a tree's branches and the resulting formation of usable lizard refuges. Within that unified context, the type, extent, and duration of species' effects can be quantified and compared in various ecological settings. This inclusive approach may promote recognition of both ecologically important but less apparent environmental effects and combinatorial and context-dependent impacts (of species or groups of species) that would be missed by focusing solely on recognized "engineers." Starting with one such well-known case and proceeding to the less obvious, the following detailed examples show that, whether strong or subtle, cumulative or short term, the effects of taxa on their external environments can be studied in ways that enrich our insight into the organism-environment relationship. To date, however, studies of these environmental effects have seldom extended to their functional or evolutionary consequences (e.g., see Chapter 7, Section 7.4). Precisely how to encompass this causal reciprocity in empirical work and models of ecological interactions, ecosystem processes, and selective evolution remains a considerable challenge.