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Lecture 7

Biology Lecture 7 Article 3.docx

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Scott Browning

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BIOLOGY LECTURE 7 Species with a Large Impact on Community Structure (Summary) o Predators can have a huge effect on composition of communities by eating lower trophic lvls + changing B/ habitat selection of prey o Predators are in decline, and conservation of these important species needed for long-term stability of freshwater, marine + terrestrial ecos Predators help to maintain a balance among orgs by eating prey + altering prey B and habitat selection Some Species Have a Greater Impact than Others on Community Structure *Dominant species: - most abundant species, strong influence over the occurrence + distribution of other species *Key stone species: - Effects on communities that far exceed their abundance - Importance isn’t predicted based on their occurrence - Dominant + Keystones influence the presence + abundance of their organisms through their feeding relationships aka “Tropic Interactions” *Foundation species: - Influence on community isn’t through trophic interactions, done by causing physical changes in the environment - Alter the environment through their B or their large collective biomass - Can also be dominant Predation: - effects prey pops + community structure - increase diversity in communities by 1) preying on competitive dominants 2) Reducing consumer pressure on foundation species Ex) Intertidal systems: mussels (dom) are superior competitors, starfish (keystones) eat them and free up space for others and increases biodiversity. Ex) Kelp forests: giant kelp (dom/foundation) and can be eating by sea urchins which can make a barren area. Urchins are eating by sea otters (keystones) and keep their #s low to help kelp forest Communities Can Be Structures by “Bottom-Up” or “Top-Down” Forces Bottom Up:  Lower to high trophic levels of the food chain  Ex) Nutrient lvls rise, stimulate growth of plants, higher trophic lvls increase in biomass Top Down: Predation, consumption  Predation, cause effects of predators start at top of food chain and cascades down  Trophic cascades when predators indirectly affect the abundance of orgs more than 2 trophic lvls down Ex) Otter-Urchin-Kelp = Trophic Cascade Ex) Reintroduce Wolves in Yellowstone: increased plants because wolves (top pred) ate elk (intermediate consumer). Eating elk alleviates grazing by elk and increases amount of primary produces  “PREDATION BY – ON”  Consumers have a (-) effect on trophic level immediately below, but (+) effect 2 trophic lvls down by removing consumer pressure  Can occ
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