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Community Dynamics
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Colonization ARRIVAL ESTABLISHMENT
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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.
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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
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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.
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Resource - Ratio LightNutrients ABCD Nutrient or light availability Relative abundance
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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.
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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.
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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
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Species diversity over time
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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
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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
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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
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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
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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).
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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
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