Neutral Theory Hubbell, S.P. (2005) Neutral theory in community ecology and the hypothesis of functional equivalence. Functional Ecology 19: 166-172.
Hubbell asks… “…how did niche differences evolve, how are they maintained ecologically, and what niche differences, if any, matter to the assembly of ecological communities?” “…which species, having which niche traits, and how many species, co-occur in a given community.”
How to answer these questions? • Assume ecological communities are complex, high-dimensional entities • Start from the “simplest possible hypothesis one can think of” and add complexity from there • Ex: the functional equivalence of species • 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?”
Functional equivalence • “…trophically similar species are, at least to a first approximation, demographically identical on a per capita basis in terms of their vital rates…” • Species in communities violate this assumption, but how by how much? Is this a good “first approximation?”
Neutral theory and Occam’s Razor • All things being equal, the simplest explanation is the best one. • Is this true for ecological communities?
Competition and the classical niche paradigm • Gause (1934) and the competitive exclusion principle; no two species with identical niches can coexist indefinately • Laboratory experiments with Paramecium modeled by Lottka-Volterra equations • framed the discussion of coexistence and community assembly in terms of competition
Implications of Gause’s work • Limiting similarity between the niches of coexisting similarity • Competitive exclusion should be observed in the natural world • Otherwise, at least, we should observe character displacement in resource use when similar species DO co-occur • Hubbell notes that there are few examples of character displacement or competitive exclusion
Niche hypervolume • Hutchinson (1957) • Competition results in species occupying only their realized niche, as opposed to their fundamental niche • Hubbell wonders how, then, should we explain the persistence of adaptations for parts of fundamental niche space that are never occupied
Competition and the classical niche paradigm • Levins (1968): multispecies community matrix theory • Tilman (1980’s): mechanistic theory incorporating the dynamics of resource supply and consumption along with the dynamics of the resource-dependent consumer species
Competition and the classical niche paradigm • Resource-based theory lead to realization of the importance of physiological and life-history trade-offs • However, “…if there was a strict transitive trade-off between competitive ability (site tenacity) and dispersal ability then, in principle, any arbitrary number of species could coexist.”
Competition and the classical niche paradigm • Hurtt and Pacala (1995): relax strict trade-off assumption, coexistence possible via strong dispersal and recruitment limitation • If a dominant species is recruitment limited, inferior species will be able to win some sites by forfeit • Non-equilibrium co-existence…i.e. Bastow’s equalizing processes
Competition and the classical niche paradigm • Is there anything left out in this time-line of the development of niche paradigm? • Do we agree with the conclusions and implications of each author’s work as described by Hubbell?
The hypothesis of functional equivalenceThe cornerstone of neutral theory Functional equivalence is assumed at the entire community level…(for all species?), a broader view than the aggregation of similar species into functional groups “Recognizing functional groups implies that niche differences among these groups are believed to matter to the assembly, stability and resilience of communities to disturbance” Hubbell poses two questions regarding the assembly of functional groups (his answers are in parenthesis in italics) worth discussing: Does a limiting niche similarity for species in functional groups exist? (no, at least in plants) How many coexisting species can be packed into a functional group? (arbitrary, again plants)
Annual Survival (%) Barro Colorado Island (BCI) • 50 hectares, censused 5 times since 1980 • Old-growth tropical forest in Panama • >300 tree and shrub species, 230-240 thousand stems (>1 cm dbh) • Figure 1 shows a dominating axis of niche differentiation, light availability. • Concentration of species at the shade-tolerant end, upper left can be interpreted as evidence for a trade-off between survival under shade stress and maximum growth rate in full sun. • Shade-tolerant species are more abundant, this should make sense, its an old growth forest, but that’s not the point… • This graph shows no distinct break that would indicate 2 functional groups… (Hubbell & Foster 1992, Hubbell 2005)
Figure 1 discussion cont.. • Can classic niche theory explain this clustering of species? • Can classic niche theory explain why niches are more finely partitioned under low-light conditions than under high-light conditions? • Hubbel has an alternate hypothesis: “…species are selected to exhibit the syndrome of traits that adapt them for growth and survival under the commonest environmental conditions…causing species to converge on similar trait syndromes…” termed functional convergence
So a functional convergence hypothesis would claim that species with similar niche requirements are sorted into similar habitats.. • Applied to the BCI example, the scarcity of pioneer species (shade-intolerant) is a reflection of the lack of gap habitat compared to shady habitat, over evolutionary time • species richness of shade tolerant and gap species is determined by the “richness of the regional species pool and the abundance of shady and gap habitats in the metacommunity over long periods of time”
This life history convergence is expected to lead to competitive exclusion and a loss of species, yet this doesn’t seem to be occurring in BCI; Hubbell cites two reason why this may be. • Combination of dispersal and recruitment limitation • Only 12/260 species dispersed at least one seed to ½ or > of traps • Pervasive density dependence affecting seedling germination • **”dispersal and recruitment limitation are sufficiently strong to prevent competitive exclusion among species sharing life history traits for the most common environments of the forest.” • Heterogeneity of biotic and abiotic microenvironments of individuals of each species. • “in species-rich communities, the opportunities for directional character displacement among a large number of competing species would be low” (Hubbell & Foster 1986b) • BCI species identity of focal plant’s neighbors was not quantitatively important or statistically significant, suggesting virtually every individual has a differs in the direction of selection imposed by interspecific competition