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