2. The consensus view and Millstein’s challenge

The consensus view referred to above supports a negative thesis: ecological functions cannot generally be interpreted as selected-effects functions. Its proponents differ as to the positive theses they defend. Some advocate an account along the lines of Cummins’s (Reference Cummins1975) causal role theory of function (Maclaurin and Sterelny Reference Maclaurin and Kim2008; Odenbaugh Reference Odenbaugh and Hazlett2010); others promote an account derived from Mossio et al.’s (Reference Mossio, Cristian and Alvaro2009) organizational theory (Nunes-Neto, Moreno, and El-Hani Reference Nunes-Neto, Alvaro and El-Hani2014); still others support an account that extends Bigelow and Pargetter’s (Reference Bigelow and Robert1987) contribution to fitness theory to ecosystems (reinterpreting Darwinian fitness as a propensity to persist) (Dussault and Bouchard Reference Dussault and Bouchard2017); and Lean (Reference Lean2020) adopts a pluralistic view. This variety of accounts implies divergences on key issues. One important divergence is whether ecological functions should or should not be conceptually dissociated from evolutionary considerations (the causal role and organizational accounts advocate such dissociation, while the contribution to fitness account preserves a connection between functions and Darwinian fitness—reinterpreted as persistence). Related issues are whether communities and ecosystems may legitimately be regarded as functionally organized entities or systems and the extent to which our understanding of their functional organization should be based in evolution. Proponents of the consensus view also differ over whether their accounts are focused on communities (Maclaurin and Sterelny Reference Maclaurin and Kim2008; Lean Reference Lean2020), or on ecosystems (Odenbaugh Reference Odenbaugh and Hazlett2010; Nunes-Neto, Moreno, and El-Hani 2014; Dussault and Bouchard Reference Dussault and Bouchard2017; Dussault Reference Dussault2018).

Nevertheless, these versions of the consensus view share two key ideas:

Millstein challenges the conceptual claim.Footnote ² She starts by considering two commonsensical broadly defined ecological functions (or roles): predation and parasitism. Drawing on John Thompson’s (Reference Thompson1994; Reference Thompson2005) account of coevolution, she argues that such broadly defined ecological functions or roles do not accurately reflect how specialized ecological interactions that occur within ecological communities tend to be. As Thompson highlights, although generalist species exist, “most species are specialized to interact with only a few other species” (Thompson Reference Thompson1994, 121; quoted in Millstein Reference Millstein2020, 1111). Through coevolutionary interactions with other species, most species have acquired traits that constrain them to feed upon, compete against, and seek to avoid a very restricted number of species. Similar specialization also commonly occurs among different populations of the same species.

According to Millstein, the fact that most species are specialists entails that broadly defined ecological functions or roles like predator and parasite are misleading. Instead of focusing on such broad roles, we should focus on more specialized ones such as “predator of type of organism X” or “parasite of population Y.” These are the roles organisms actually perform; for instance, what qualifies the blister beetle in general as a parasite is the fact that the Oregon blister beetle is a dune silver bee parasite, and the Mojave Desert blister beetle is a white-faced bee parasite.

Millstein contends that when ecological functions are defined with this higher degree of specificity, their selected-effects character and their rootedness in past coevolution become obvious:

Hence, a particular species S ₁ or population P ₁’s being the predator or parasite of another species S ₂ or population P ₂ is a selected effect. It is an effect of selection on members of S ₁ or P _1, favoring traits that enable them to (more efficiently) prey on or parasitize S ₂ or P ₂.Footnote ³

3. Two notions of ecological function or role

In the ecological literature, the terms “function” and “role” are often used ambiguously. Jax (Reference Jax2005, 641–42) identifies four function-related concepts used by ecologists: (1) function as an interaction between two species; (2) the functioning of a complex system; (3) function as the role of an organism, species, or abiotic item in a community or ecosystem; and (4) ecosystem services.

Jax’s third notion, however, remains ambiguous, and, I submit, can mean at least two different things: either (3a) a type of activity that occurs across communities or ecosystems (e.g., predation, parasitism); or (3b) the contribution of such an activity to the maintenance and/or functioning of the community or ecosystem within which it occurs.Footnote ⁴ With respect to general uses of the term “function,” 3a is an ecological version of the notion of function involved in the structure/function or form/function distinction (on this notion, see Wouters Reference Wouters2003, sec. 2.1; Gayon Reference Gayon, Gayon and de Ricqlès2010, 127–29). Roughly, “function” in this sense distinguishes what an item does or is capable of doing from what it is made of, what it looks like, or where it originates. Equating “function” in this sense with “role” involves a notion of role as a position that something may occupy in an abstract model, and which might (at least in principle) be occupied by more than one type of thing. “Ecological role” is sometimes used in this sense by biologists and philosophers of biology in discussing ecological niches as “places” that can (in principle) be occupied by different species or populations in an abstract model of communities, similar to “roles” in a play, which can be taken on by different actors in different circumstances (e.g., Hull Reference Hull1987, 179; Colwell Reference Colwell, Keller and Lloyd1992, 242–43; Sterelny Reference Sterelny2001, 153–54).Footnote ⁵ Importantly, this notion of ecological role does not imply any idea of contribution to the activities or capacities of a higher-level system. What it characterizes is simply how a type of organism is disposed to interact with other types of organisms, and, as a result, what place it occupies within the interactive network that constitutes its community.

In contrast, 3b is an ecological version of the notion of function involved in characterizations of how the traits and parts of a biological system (typically an organism) contribute, or are supposed to contribute, to the capacities or activities of this system. This notion is the one that has been the focus of philosophical discussions of function (for reviews, see Wouters Reference Wouters2005; Lewens Reference Lewens, Matthen and Stephens2007; Garson Reference Garson2016). The various theories offered by philosophers of biology differ mainly over what they take to be the relevant systemic activities or capacities in relation to which functions should be defined, and over whether they take functions to be actual contributions of traits and parts to those systemic activities or capacities, or rather contributions they are supposed to achieve. For instance, the causal-role theory sees any systemic activity or capacity as potentially relevant and focuses on the actual contributions of traits and parts (Cummins Reference Cummins1975), whereas the selected-effects theory restricts relevant activities and capacities to those that are products of selection, and focuses on contributions that traits and parts are supposed to achieve (e.g., Millikan Reference Millikan1989; Neander Reference Neander1991; Garson Reference Garson2017; see also Wright Reference Wright1973). Equating “function” in this sense with “role” involves a notion of role that is bound up with biologists’ thinking about the part-whole organization of living entities. A role, in this sense, is something that a system’s part does (or is supposed to do) in the context of that system, and which, collectively with other roles fulfilled by other parts, realizes overall capacities or activities of this system. “Ecological role” is used in this sense by ecologists and philosophers of ecology when they discuss roles fulfilled by organisms, species, or abiotic items within communities and/or ecosystems conceived as functionally organized systems (analogously, though only weakly so, to individual organisms) (e.g., Odum Reference Odum1971; Schulze and Mooney Reference Schulze, Mooney, Schulze and Harold1993; Naeem Reference Naeem, Harold and Josep2002). The “ecological roles” of organisms, species, or abiotic items here consist (roughly) in their contributions to the maintenance of their communities and/or the functioning of their ecosystem (i.e., ecosystem-level processes such as nutrient cycling and energy flow, Jax’s concept (2) identified above).

I submit that proponents of the consensus view against selected-effects ecological functions have been concerned with ecological functions or roles in the 3b sense. What they deny is the general possibility of a selected-effects account of the notion of ecological function that denotes organisms’, species’, and abiotic items’ contributions to the capacities or activities of communities and/or ecosystems. This is the notion that they are concerned with when they claim that a selected-effect account of ecological functions would implausibly require the general occurrence of community- or ecosystem-level selection. To use Sober’s (Reference Sober1984) well-known distinction, without community- or ecosystem-level selection, there could be selection of traits that lead organisms or populations to contribute to the maintenance of their community or the functioning of their ecosystem, and therefore to fulfill ecological functions in the 3b sense, but these traits could not be selected for their 3b ecological functions. The reason for this is that only selection at community or ecosystem levels seems capable of selecting organismal traits for how they contribute to community maintenance or ecosystem functioning (as opposed to selecting them for effects that are beneficial to the organisms themselves). Hence, ecological functions as contributions to community maintenance or ecosystem functioning seem better construed as non-selected-effects functions, functions that organisms, populations, and abiotic items fulfill without necessarily having been shaped by natural selection to fulfill them. They are functions along the lines of alternatives to the selected-effects theory, such as the causal role, the organizational, or the contribution to fitness theory (see Maclaurin and Sterelny Reference Maclaurin and Kim2008; Odenbaugh Reference Odenbaugh and Hazlett2010; Nunes-Neto, Moreno, and El-Hani Reference Nunes-Neto, Alvaro and El-Hani2014; Dussault and Bouchard Reference Dussault and Bouchard2017; Lean Reference Lean2020).

That what proponents of the consensus view deny is the general possibility of selected-effects ecological functions in the 3b sense, as opposed to 3a, is apparent in their discussions. This is particularly so in Odenbaugh’s (Reference Odenbaugh and Hazlett2010, 250–51) discussions of the ecological functions of fungi and Rhizobium. What Odenbaugh denies is not that the activities by which fungi and Rhizobium achieve their ecological functions—namely, the decomposition of woody product and the fixation of nitrogen—are selected effects, but that those activities have been selected for their contributions to the ecosystem level processes (respectively, the carbon and nitrogen cycles). It is also apparent in MacLaurin and Sterelny’s (Reference Maclaurin and Kim2008, 114–15) discussions of the ecological functions of eucalypts in Australian woodlands and of keystone predators like starfish. They are not concerned with whether eucalypts’ flammability and starfish’s predation on mussels are selected effects, but with whether those effects have been selected for their contributions to the overall behavior of the eucalypts’ ecosystem and the maintenance of diversity, respectively. Nunes-Neto et al. (Reference Nunes-Neto, Alvaro and El-Hani2014), Dussault and Bouchard (Reference Dussault and Bouchard2017), and Dussault’s (Reference Dussault2018) general focus on the use of the function concept to denote the contributions of biodiversity items (e.g., traits, populations, functional groups) to ecosystem-level processes, also makes clear that their rejection of selected-effects ecological functions concerns functions as contributions to the capacities or activities of a higher-level system (i.e., 3b).

Millstein’s discussion, in contrast, as she herself highlights (Millstein Reference Millstein2020, 1109n1), is concerned with functions as activities, and hence with ecological functions in the 3a sense. Strictly speaking, predation and parasitism and their more specialized subdivisions (Millstein’s paradigmatic examples of coevolution-based selected-effects ecological functions) are not ecological roles in the sense of contributions to capacities or activities of communities or ecosystems (3b). They are types of feeding activities that are realized by many types of organisms, and which determine the types of ecological interactions they will be involved in and the “place” that they will tend to occupy in communities (3a). Those feeding activities will indeed have community- and/or ecosystem-level effects and will therefore engage the organisms that realize them in the fulfillment of ecological functions in the 3b sense. Typically, predators and parasites will contribute to the circulation of nutrients and energy through their ecosystems and to the regulation of the populations that they consume. However, contributions to nutrient and energy circulation and to population regulation are not equivalent to predation and parasitism; these contributions are the community- and ecosystem-level consequences of these activities and are not themselves selected for under predator-prey or parasite-host coevolution.Footnote ⁶ Hence, predation and parasitism, Millstein’s paradigmatic examples of coevolution-based selected-effects ecological roles, illustrate the notion of function as an activity and the notion of ecological role as a position that a species or a population occupies in a model of a community (i.e., 3a).

Millstein and proponents of the consensus view are thus concerned with two distinct notions of ecological function, and their arguments do not, therefore, contradict each other. Hence, Millstein’s defense of the possibility of coevolution-based selected-effects ecological functions in the 3a sense does not invalidate the consensus view. Selected-effects ecological functions in the 3b sense still (implausibly) require the general occurrence of community- or ecosystem-level selection, because only selection at those levels can favor some organisms’ traits on the basis of their contribution to their community or ecosystem.

4. Guilds versus functional-effect groups

The two notions of ecological function distinguished in the previous section essentially correspond to two concepts that underlie functional classifications used by ecologists, in relation to which they use the term “functional role”: guilds and functional-effect groups (Blondel Reference Blondel2003; Stroud et al. Reference Stroud, Bush, Ladd, Nowicki, Shantz and Jennifer2015, 4762).Footnote ⁷ Guilds are groups of organisms that use similar resources in similar ways, for instance, predators, grazers, seed eaters, and their more fine-grained subgroups (Root Reference Root1967; Simberloff and Dayan Reference Simberloff and Dayan1991; Blondel Reference Blondel2003; de Satgé, Teichman, and Cristescu Reference Satgé, Kristine and Bogdan2017). The classification of organisms into guilds is primarily tied to the aim of studying competition and coexistence among species using similar resources as well as the coevolutionary specialization that typically results from competition among guild members. Functional-effect groups, in contrast, are groups of organisms that contribute similarly to some important ecosystem process or community property, for example, producers, primary and secondary consumers, seed dispersers, and their more fine-grained subgroups (Naeem Reference Naeem, Harold and Josep2002; Blondel Reference Blondel2003; Thornhill et al. Reference Thornhill, Jeremy, Hill, Robert, David, Wood, Gee, Mark and Christopher2018). The classification of organisms into functional-effect groups is tied to the epistemic aim of explaining and predicting the effects of changes in the species composition of communities on ecosystem processes and on their ability to maintain themselves. The guild concept thus essentially corresponds to 3a ecological functions, whereas the functional-effect group concept essentially corresponds to 3b.

The contrast between guilds and functional-effect groups has proven important for ecological research, because organisms that belong to the same guild need not belong to the same functional-effect group and vice-versa (see Blondel Reference Blondel2003, 228; Stroud et al. Reference Stroud, Bush, Ladd, Nowicki, Shantz and Jennifer2015, 4762). For instance, birds that consume fruits and seeds, and thus belong to frugivore and granivore guilds, need not all belong to seed-dispersers functional-effect groups. Only those that do not fully digest the seeds (and defecate or regurgitate them elsewhere) disperse them. Conversely, seed dispersers need not be consumers of fruits and seeds. Some birds passively disperse seeds by catching them in their plumage and dropping them elsewhere. A model that conflates guilds and functional-effect groups can therefore have limited explanatory and predictive power.

The contrast between guilds and functional-effect groups indicates that the above distinction between 3a and 3b ecological functions is important for ecological research, and that the 3a notion on which Millstein focuses is one that ecologists actually use. Hence, a significant import of Millstein’s discussion is that it draws attention to a notion of ecological function or role that has been neglected in philosophical discussions, but is central to some ecological research programs.