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 Purves 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 McKane, Alonso Etienne MacArthur MacArthur 1970). The principle also implied that competitive exclusion should be a commonplace observation in nature, or at the very least, that there should be widespread evidence of character displacement in resource use when very similar species co-occur. The empirical evidence, in general, has not borne out these predictions (Grant 1972, 1975), particularly in plant communities. There are only a few well documented examples of character displacement, all of which are in animals; for example, the evolution of beak size in competing Darwin's finches (Grant 1986), similar niche separation in seed-eating sparrows (Pulliam 1975) or in body size in desert rodents (Brown 1975). Most of these cases involve low-dimensional resource competition. However, I know of few examples of proven character displacement in plants, and the list of species whose actual disappearance from a community can be attributed to competitive exclusion is vanishingly short. Hutchinson's (1957) niche hypervolume was a more general concept of niche than one defined solely by limiting resources, and included, for example, the tolerance ranges for physical variables of the environment. His ideas about the fundamental and realized niches invoked competition to explain why species only occupy a subset of the niche space that would be suitable for them in the absence of competitors. However, if niches evolve, and if species are always restricted to smaller, realized niches in equilibrium communities, then how do we explain the persistence of adaptations for parts of fundamental niche space that are never occupied? I will return to this question shortly. The theory for community assembly based on the competitive niche paradigm became highly developed, first with multispecies community matrix theory (Levins 1968), which was developed on the foundation of the Lotka–Volterra equations, and then with more mechanistic theory, which explicitly incorporated the dynamics of resource supply and consumption along with the dynamics of the resource-dependent consumer species (Tilman 1982, 1987). This resource-based theory gave new impetus to testing community assembly rules through direct experimental manipulations of resource availability, most notably in plant community ecology. Perhaps the most significant conceptual spin-off of resource-based theory, however, was broader recognition of the fundamental importance of physiological and life history trade-offs (e.g. Tilman 1982, 1988; Kneitel Goldberg Shmida Hubbell 2005). As a result, there is not much development of theory in community ecology for the assembly of functional groups. Does a limiting niche similarity for species in functional groups exist? How many coexisting species can be packed into a functional group? I believe that the answer to the first question is no (at least in plants), and the answer to the second question is essentially arbitrary (again, at least in plants). Loreau (2004) has recently argued that functional equivalence does not really exist. However, in a companion paper (Hubbell 2005), I demonstrate that functional equivalence can evolve easily and often, under selective circumstances that should be commonplace in nature, especially in species-rich communities. My ideas on functional equivalence have developed slowly over the past quarter century. Since 1980 my colleagues and I have been studying the dynamics of a large (50 ha) mapped plot of old-growth tropical forest on Barro Colorado Island (BCI), Panama, attempting to understand the assembly rules of the BCI tree community (Hubbell et al. 2005). This result that the of by competition at the level of virtually every There is little or no in the of at the species level to character displacement in one or the for beak in Darwin's I that this is a major that tropical such of variation and & would that most BCI species are and this does to be the (Hubbell & Foster et al. 2001). the species of For example, consider the variation in growth among of the The variation in growth in this single species virtually the entire of growth of both the and the functional this we how much of this variation is but it does that there be problems with species-level trade-off between and growth rate that we ecologists have argued that such and implies that ecological communities are very (e.g. et al. 2001). However, if species overlap because the is far than the then such as neutral theory, not be such a approximation to of the in growth of the species, represents a of that were cm in that the The of years was from cm a tree that at all to a of cm This in just the entire of variation in growth the entire BCI tree community, including species. This is not and that BCI overlap in the distribution of their vital rates. It that the concept of limiting similarity has little to traits of BCI also the trade-off to The hypothesis of functional equivalence can only be so but the main point of this paper is that there are both empirical and theoretical for taking the hypothesis at least in the tropical tree communities I have It is almost the that the hypothesis will to some communities and functional groups than I it will be often true in communities of but this question will be for the on those communities to What to however, is how far one can with a neutral theory based on the assumption of functional Although have that theories can also many of the same to me this is not the main point of neutral theory. – but also more complex – theories to the if the what or is me the most and question of all is why neutral theory as well as it given the it For neutral theory has been a conceptual because it is a theory of that from fundamental in For example, it is both and that one can the most of species et al. very and from the theory et al. 2003), and that this number out to be to the It is this that my that neutral theory is fundamentally true about the aggregate of on will if my is