Neutral Theory

1
Neutral Theory

• Hubbell, S.P. (2005) Neutral theory in community
  ecology and the hypothesis of functional
  equivalence. Functional Ecology 19 166-172.

2
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.

3
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?

4
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?

5
Neutral theory and Occams Razor

• All things being equal, the simplest explanation
  is the best one.
• Is this true for ecological communities?

6
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

7
Implications of Gauses 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

8
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

9
Competition and the classical niche paradigm

• Levins (1968) multispecies community matrix
  theory
• Tilman (1980s) mechanistic theory incorporating
  the dynamics of resource supply and consumption
  along with the dynamics of the resource-dependent
  consumer species

10
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.

11
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-existencei.e. Bastows
  equalizing processes

12
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 authors work as described by Hubbell?

13
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)

14
Barro Colorado Island (BCI)

• 50 hectares, censused 5 times since 1980
• Old-growth tropical forest in Panama
• gt300 tree and shrub species, 230-240 thousand
  stems (gt1 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
  thats not the point
• This graph shows no distinct break that would
  indicate 2 functional groups

(Hubbell Foster 1992, Hubbell 2005)
15
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 conditionscausing
  species to converge on similar trait syndromes
  termed functional convergence

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• 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

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• This life history convergence is expected to lead
  to competitive exclusion and a loss of species,
  yet this doesnt 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 gt 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 plants neighbors
  was not quantitatively important or statistically
  significant, suggesting virtually every
  individual has a differs in the direction of
  selection imposed by interspecific competition