Neutral theory in community ecology and the hypothesis of functional equivalence

Probably no ecologist in the world with even a modicum of field experience would seriously question the existence of niche differences among competing species on the same trophic level. The real question, however, is how did these niche differences evolve, how are they maintained ecologically, and what niche differences, if any, matter to the assembly of ecological communities? By ecological community I refer to co-occurring assemblages of trophically similar species. By assembly I mean which species, having which niche traits, and how many species, co-occur in a given community. In my judgement, despite a long and rich tradition of research on these questions in community ecology (Chase & Leibold 2003), we are still far from having definitive answers. Several years ago, in an attempt to provide a fresh approach for tackling these questions, I introduced a formal neutral theory for ecology (Hubbell 1997, 2001). The traditional strategy has been to assume that ecological communities are inherently high-dimensional sensu Hutchinson’s (1957) niche hypervolume for each species, and then to build rather complex models from the outset, incorporating as many of the details of the growth and interactions of each and every species and with their physical environment as possible. Neutral theory, however, adopts a fundamentally different strategy, taking virtually the opposite tack. It begins with the simplest possible hypothesis one can think of – for example, the functional equivalence of species – and then adds complexity back into the theory only as absolutely required to obtain satisfactory agreement with the data. In this approach, one does not assume from the start that ecological communities are high-dimensional. Indeed, the fundamental question now becomes: What is the minimum necessary dimensionality of the theory required to characterize a given ecological community to a desired level of realism and precision? The hypothesis of functional equivalence is the cornerstone of neutral theory. It states that trophically similar species are, at least to a first approximation, demographically identical on a per capita basis in terms of their vital rates, of birth, death, dispersal – and even speciation. Elsewhere I have also used the term symmetry to describe such species (Hubbell 2005). Under the hypothesis of functional equivalence, species can still differ in a great many ways – including characters that enable taxonomists to recognize them as good species – so long as species do not differ in their per capita vital rates. All species in all trophically defined communities violate this assumption to some degree, but the question is, how good an approximation is it? In ecology, there are often multiple, sufficient explanations for the same phenomenon (Chave, Muller-Landau & Levin 2002; Purves & Pacala 2005), and this is especially true of approximate explanations. The strategy of neutral theory relies heavily on Occam’s Razor; but whether the assembly rules of ecological communities in nature obey this maxim – to always choose the most parsimonious explanation – is not yet clear. However, as Harte (2003) pointed out, one doesn’t necessarily discard theories just because they are just approximations. Physicists still use Boyle’s Law, PV = nRT (pressure P times volume V is a linear function of absolute temperature T ), even though there are no perfect gases that follow this relationship exactly. Similarly, we continue to teach the Lotka–Volterra equations in ecology, even though no ecologist today would defend them as precise or mechanistic descriptions of nature. Much has been written about the unified neutral theory (UNT) since my book appeared (Hubbell 2001), some of it critical, and there have been several major technical and conceptual advances in the theory since then (e.g. Volkov et al . 2003; Vallade & Houchmandzadeh 2003; Houchmandzadeh & Vallade 2003; McKane, Alonso & Sole 2004; Etienne & Olff 2004). However, my main focus for this forum on neutral theory will not be on the new developments in the theory or on the criticisms. Rather, I wish to highlight the empirical observations that led me to question my own long-held beliefs about the assembly rules for ecological communities in the first place, particularly in plant communities. Before discussing these observations, however, I should sketch out a few of the major milestones in the intellectual history of the classical paradigm of competitive niche assembly theory.