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 LightNutrients ABCD Nutrient or light availability Relative abundance

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

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

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).

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