models for coexistence and diversity (lecture 7)

Question 1

What are Lotka-Volterra models?

Answer 1

• used to understand when competitive exclusion occurs
• conditions for equilibrium co-existence mathematically and ecologically restrictive
• basically require that species are ecologically different and exploit different niches

Question 2

What is the neutral theory model for diversity?

Answer 2

• competitive exclusion says if two species are similar one will outcompete the other
• the more similar the longer this takes
• if identical outcompeting will take infinite time in theory
• in finite stochastic world a long time
• neutral theory
• all species are identical
• species elimination due to random chance e.g. stochastic events

Question 3

What are problems with the neutral theory model for diversity?

Answer 3

• doesn’t explain diversity in itself
• only that rate of elimination slow
• therefore a non equilibrium model
• in finite environment eventually end up with one species unless diversity-generating mechanisms incorporated

Question 4

What are the scales of organisation in Hubbell’s neutral theory?

Answer 4

local community
- set of species in given location

meta-community
- pool of species local communities are formed from

• different processes operate to determine species dynamic at each scale
• meta-community dynamics much slower
• derives from island biogeography theory
• local = island
• meta = mainland

Question 5

What are the assumptions of Hubbell’s local community?

Answer 5

• community = series of sites
• site = large enough to support one trees
• number of trees in community = J
• J = constant, community assumed to be saturated
• all individuals ecologically identical
• occupy same space, same death and reproduction rates
• competition occurs when individual dies
• gap dynamics: formation and competition for occupancy

Question 6

Why is the metacommunity needed in Hubbell’s neutral theory?

Answer 6

• neutral model for local community allows species to persist a long time
• eventually stochastic extinction = one species remaining
• need diversity source hence metacommunity

Question 7

What function does the metacommunity in Hubbell’s neutral theory serve?

Answer 7

• individuals migrate from metacommunity to local community
• new input of species
• compensates for local extinctions causing community species loss

Question 8

What is the model for the metacommunity in Hubbell’s neutral theory?

Answer 8

• wider community from which species may immigrate to local communities
• size = Jm individuals
• dynamics of species pool governed by speciation-extinction dynamics
• speciation rate = v

Question 9

Why is neutral theory interesting?

Answer 9

• interesting comparisons with data
• mathematically tractable
• many comunities contain similar species
• first model that posits a role for speciation
• attempts to link togethr niche/neutral theory in same framework e.g. niche neutral model, emergent neutral model

but

• species obviously aren’t identical
• assumptions probably wrong
• displaces problem of explaining diversity with metacommunity