Community Dynamics
Colonization
ARRIVAL
ESTABLISHMENT
Initial floristic composition model Proposed by Egler (1954)
All the plants arrive or germinate near the same time.
Second stage of succession: the perennials overtop the annuals.
Third stage of succession: shrubs overtop the perennials
Fourth stage: trees overtop, etc.
Connell and Slatyer
Facilitation– invasion depends on conditions created by earlier
colonists Tolerance
– later successional species probably competitor-stress tolerators
– help reduce the level of resources to where only they can tolerate it
Inhibition– species displaced only by death or damage by
factors extrinsic to competition
Tilman: resource-ratio hypothesis Successional sequence depends
primarily on three things:
– 1) interspecific competition for resources;
– 2) long-term pattern of a supply of limiting resources, especially nutrients and light;
– 3) a small group of other life history factors.
Early in sequence Light is high and nutrients low. These factors have reversing trends over time.
Resource - Ratio
Light Nutrients
A B C D
Nutrie
nt o
r light a
vaila
bility
Rela
tive
ab
undance
Probabilistic view of species replacement Plots were mapped on glacial outwash in Alaska:
617 plants were initially recorded
417 died during a 5 year period
535 new plants became established during 5 years
Density increased by 20%, but more than 60% of the original individuals were replaced by others.
Horn: replacement probabilities
Studied saplings under a mature tree. If 60% of the saplings under aspen trees were beech, the probability of a beech replacing an aspen would be .6
Forest studied: beech replacement of aspen= .6 and aspen had only a .03 chance of replacing itself. Beech had a .8 probability of self replacement.
General Postulates - succession
Biomass– As supported biomass increases, net productivity
decreases– total resources available to higher trophic levels are about
the same Nutrients
– increasingly tied up in biotic pools. Rate of loss decreases Life history
– r-selected species are replaced by K-selected Species diversity
– increases (dominance declines) until just before climax growth, then reverses some
Species diversity over time
General Postulates - succession Biomass– As supported biomass increases, net productivity
decreases– total resources available to higher trophic levels are
about the same, but heterogeneity is increasing Nutrients
– increasingly tied up in biotic pools. Rate of loss decreases as they are held in standing biomass
Life history– r-selected species are replaced by K-selected ecological
specialists as resources become more limiting Species diversity
– increases (dominance declines) until just before climax growth, then reverses some
Stability - to ecological perturbations increases
Alternation of species
James Fox– Amer. Nat. 111(977):69-88
Kerry Woods (reciprocal replacement)– Saplings (not suckers) counted: maple more
common under Beech and vice versa– Mature trees within 10m of a sapling counted,
measured
Alternation of species
James Fox (1977)– Amer. Nat. 111(977):69-88
Kerry Woods (reciprocal replacement)– Saplings (not suckers) counted: maple more
common under Beech and vice versa– Mature trees within 10m of a sapling counted,
measured
b = beach; m = maple
I =dbh
distance1
b
∑ −dbh
distance1
m
∑
Alternation of species
James Fox (1977)– Amer. Nat. 111(977):69-88
Kerry Woods (reciprocal replacement)– Saplings (not suckers) counted: maple more common
under Beech and vice versa– Mature trees within 10m of a sapling counted,
measured
b = beach; m = maple
– Found I>0 for high frequency Maple, Therefore Beech has greatest canopy influence
– More dead saplings under conspecific canopies
I =dbh
distance1
b
∑ −dbh
distance1
m
∑
Alternation of Generations
Larry Forcier
– Found maple with many beech saplings, but not the reverse. He found a trend towards beech dominance.
– Replacement tended to be in a series when a gap was created (minor disturbance).
Beech
Maple
Yellow Birch
Gap
Reasons for Alternation
Competition– Intraspecific stronger than Interspecific
Chemical antagonism– Allelopathy: canopy and root exudates
Seed predation Microhabitat differences
– relates to “facilitation” fungal pathogens
– saplings the most susceptible to attack - most mortality is related to this
– most easily obtained near the host species