Transcript Powerpoint

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 lifehistory 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 tradeoff 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 equivalence
The cornerstone of neutral theory
•
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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:
1.
2.
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)
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 shadetolerant 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…
Annual Survival (%)
•
(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 (shadeintolerant) 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.
1.
Combination of dispersal and recruitment limitation
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–
2.
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