Table 1.
Neutral theory and species abundance distribution terminology
| Term | Definition |
|---|---|
| Emergent neutrality (EN) | A model in which neutrality is the outcome of community evolution. According to the model competing species self-organize into groups of species with similar traits |
| Fundamental biodiversity number | A dimensionless parameter in neutral models which describes various characteristics of the metacommunity. The parameter (θ) is given by  , where Jm is the number of individuals in the metacommunity, and v is the probability that a deceased individual is replaced by a new species through speciation (per capita speciation rate) |
| Fundamental immigration number | A parameter (I) given by  , where J is the number of individuals in the local community and m is the probability that a deceased individual in the local community is replaced by an immigrant from the metacommunity. It is often used in neutral models as a measure of dispersal limitation as the parameter m can be difficult to interpret. “I” is also independent of sample size and can be seen as measure of community isolation |
| Likelihood surface | The value of the likelihood, usually displayed in graphical form, as a function of a number of parameters (generally two or three). Multiple local maxima refer to the situation in which the likelihood surface has more than one peak |
| Local community | Generally relates to the local community in Hubbell's spatially implicit neutral model, in which it is a set of individuals that live in the same smaller sample/island distinct from the larger metacommunity/mainland. A dead individual is immediately replaced either by an offspring of another individual (of any species) in the local community, or by an immigrant from the metacommunity (determined by m; the probability that a deceased individual in the local community is replaced by an immigrant from the metacommunity). As long as m is greater than 0 the local community receives a certain amount of immigrants from the metacommunity. The number of individuals in the local community is predicted to be too small for speciation to occur |
| Lognormal distribution | A probability distribution of a random variable whose logarithm follows a Gaussian distribution. In relation to species abundance distributions, the lognormal distribution characterizes a sample with relatively few very abundant or very rare species |
| Logseries distribution | A probability distribution which results from the Poisson sampling of a gamma distribution after a certain relevant limit is taken, and conditional presence is considered, that is, it gives the conditional probability of attaining a certain abundance level given that the species is present. In relation to species abundance distributions, the logseries distribution characterizes a sample in which the most common abundance category is a single individual |
| Metacommunity | In the context of neutral theory, the metacommunity generally relates to Hubbell's spatially implicit neutral model, in which it is the source pool of individuals. A dead individual is immediately replaced either by an offspring of another organism in the metacommunity, or by an individual from a new species (speciation). Offspring of individuals may disperse to the local community (above) |
| Multimodal species abundance distribution | A species abundance distribution with multiple modal abundance values or octaves. The majority of published species abundance distribution models are unimodal, but it has become increasingly apparent that many empirical abundance distributions exhibit multiple modes |
| Spatially explicit neutral model (SENM) | A neutral model that incorporates an explicit spatial structure, which enables the model to predict the exact location of each individual in space |
| Spatially implicit neutral model (SINM) | A neutral model that incorporates a restricted consideration of spatial structure. Hubbell's (2001) classic neutral model is spatially implicit as it only focuses on two scales of community organization, that is, the metacommunity and the local community |
| Speciation mode (within neutral models) | The manner in which speciation is modelled in neutral models. In Hubbell's classic SINM, speciation occurs via the point mutation mode whereby speciation is an instantaneous process. Neutral models incorporating alternative speciation modes have since been developed; for instance, whereby speciation is a gradual, drawn out process (protracted speciation) |
| Species abundance distribution (SAD) | The typical univariate SAD gives the expected frequency of species at each abundance level, either in terms of relative frequencies or simply by the average number of species at each abundance level. The multivariate SAD gives the whole multidimensional distribution: the abundance of all species observed within a sample of an ecological community |
| Species–area relationship (SAR) | The relationship between the area of a sample or island and of the number of species in that area |
| Stochastic niche theory | A theory of community structure which combines niche apportionment with stochastic processes |
| Zero-sum assumption | An assumption of many neutral models, including Hubbell's (2001) SINM, stipulating that when an individual dies it is immediately replaced by another individual, that is, resources are fully saturated at all times |
| Zero-sum multinomial distribution (ZSM) | The species abundance distribution predicted for the local community in Hubbell's (2001) SINM (above) |