Neutral Model

Question 1

If all species are identical, there is nothing to decide which species should be the winner

Answer 1

Consequently, if we add a bit of randomness to the model, we cannot predict the outcome…..

Question 2

Neutral competition is thus a non-equilibrium model.

Answer 2

In the end, in a finite environment we will end up with just one species unless we incorporate diversity-generating mechanisms

Question 3

Hubbell’s Neutral Theory Two scales of organisation:

Answer 3

Local Community: a set of species in a given location

Meta-community: a pool of species from which local communities are formed

: dynamics of the meta-community are much slower than those of the local community

Concept derives from Island Biogeography theory: think local community = island, meta-community = mainland

Question 4

Hubbell’s Neutral Theory (1) Model for a local community:

Answer 4

Think of the community as consisting of a series of ‘sites’
Each site is large enough to support one tree
J trees in the community: this number is constant, i.e. the community is assumed to be saturated with individuals
All individuals are ecologically identical: i.e. occupy the same space and have the same rates of death and reproduction
Competition occurs when individuals die
gap formation and competition for gap occupancy = gap dynamics

These are assumptions of the model. Suspend disbelief for the time being. The idea is to make simplifying assumptions and see where this gets us. Most alternatives are too complex to imagine or be mathematically tractable.

Question 5

Local community model

Answer 5

Neutral model for local community allows lots of species to persist for a long time……

….. but eventually stochastic extinction will lead to just one species remaining…….

….. so need a source of new diversity, hence metacommunity

Question 6

Hubbell’s Neutral Theory (2) Link from local to metacommunity:

Answer 6

Individuals migrate from the meta community to the local community
Provides an input of new species
This compensates for the loss of species in the local community through local extinction

Question 7

Evaluation of the Neutral Theory
Species are clearly not identical

Concept of metacommunity displaces the problem of explaining diversity

Answer 7

However neutral theory provides interesting comparisons with data, is mathematically tractable & is the first model to posit a role for speciation