What structures communities? Biotic & Abiotic factors PDF
Preview What structures communities? Biotic & Abiotic factors
Community structure What structures communities? Biotic & Abiotic factors Are communities predictable or stochastic (random)? • Are communities highly organized or just loose assemblages? – Do communities have a predictable number of trophic levels? – If a disturbance erases a community, will identical species recolonize? Will they have the same interactions with each other? Competing Hypotheses • H : Communities are stable & integrated 1 with predictable compositions (Clements) – Communities pass through predictable stages of development & species composition – Those stages end in a Climax Community – Determined by climate& species interactions – If disturbed, the same species will arrive & reconstitute the community Climax Competing Hypotheses • H : Communities are loose, unpredictable, 2 chance assemblages of species – The presence of each species is due to chance – History& abioticfactors determine which species are present – If disturbed, the chance of identical reconstitution is small Climax 1 Climax 2 Climax 3 Climax 4 Climax 5 Climax 6 Species interactions • Bob Paine (UW) • Intertidal ecology • How do communities respond to altered species interactions? – Predatory interaction between Pisaster& Mytilus – What effect does predator removal have on Mytiluspopulations Biotic interactions do structure communities • Removal of predator leads to: – increased prey densities – decreased species diversity • algae & invertebrates disappear • Mytiluspopulations explode • Keystone species – Has greater impact on surrounding species than its abundance suggests Historical data • Pollen record suggests that species composition at a given site changes through time (NA). – Some tree species appear and disappear together. – Others appear to come and go independently of each other. – Is community composition static or dynamic? – Is it predictable or dependent on history & chance? More experimental data • Pond communities! • Examined richness & diversity of planktonic species • 12 identicalponds – Same location, filled & sterilized @ same time • Prediction if H is 1 correct? • Prediction if H is 2 correct? Results • 60 possible species • Ponds contain ~ 34 species • Most species occur in most or all of the ponds, but NO assemblage is identical • Why? – Dispersal ability? – Occupied niches?
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