Download 07-Alternative Models of Succession

Alternative models of community
assembly
• Central question: understand mechanisms
controlling community assembly
Grime’s C-S-R theory
C – Competitive species:
high growth rate
efficient capture & utilization of
resources
C
S – Stress tolerant:
tolerant of abiotic stressors
low growth rates
S
R
R - Ruderal (weedy)
short-lived
high growth rate
high reproductive output
Productivity influence on succession
S1 = high productivity
S2 = moderate productivity
S3 = low productivity
C
•Early successional environments
colonized by ruderals
•Mid seral stages highly
competitive, so C species thrive
•Resources depleted at late seral
stages, so S species dominate
S
R
Productivity influence on succession
S1 = high productivity
S2 = moderate productivity
S3 = low productivity
•high-productivity, lowdisturbance habitats  C
C
•low-productivity habitats
S
S
R
•high-disturbance habitats
R
C-S-R Theory limitations
• Resources are not necessarily depleted over
time
• Competition can be important at low nutrient
levels
• Difficulty in testing  reduced predictive
power
Tilman’s R*
• Growth is related to resource availability
• R* = minimum resource level that supports
growth
Mechanism of succession
• Species compete for & alter resource
availability over time
• Species with the lowest R* will outcompete
others
Resource ratio hypothesis: 1 species
•A - species A
•dN/Ndt – growth rate
•R – resource level
•R*A – minimum
resource level for A
•mA – mortality
threshold of species A
Resource ratio hypothesis: 2 species
R*B < R*A 
species B
outcompetes
species A
Resource Ratio Hypothesis conclusions
• Resource niche is multi-dimensional  hard
to define
• Resources are heterogeneous in space and
time  R* will vary
• Experimental results show mixed support
• Succession not necessarily driven by resources
Hubbell’s Unified Neutral Theory of
Biodiversity
• Ecological equivalents – species with
functionally equivalent niches
• “chance and biological uncertainty may play a
major role in shaping the population biology
and community ecology “
• Functional roles of species more important
than species interactions
• Community assembly is unpredictable
Hubbell’s Unified Neutral Theory of
Biodiversity
Mechanisms
• Stochasticity
– Ecological drift
– Random speciation
• Stochastic processes, not biological interactions,
drive community structure and dynamics
• Strong departure from “niche-based” theories
• Non-equilibrium
• Neutral models are “wrong”, but also successful
Best news ever: none of this will be on
the exam!