Ecology: Lecture 17 Community structure. Vertical stratification in lakes.
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Transcript of Ecology: Lecture 17 Community structure. Vertical stratification in lakes.
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Ecology: Lecture Ecology: Lecture 1717
Community structure
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Vertical stratification in lakesVertical stratification in lakes
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River Continuum conceptRiver Continuum concept (pictures from Stroud Water Research Center)(pictures from Stroud Water Research Center)
Headwater Mid-sized stream Large river
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Biological StructureBiological Structure
Species dominance Numbers
Dominant groups in stream sample?
Impact Impact of introduced lampreys on fish Keystone predators such as Pisaster
ochraceus (type of sea star)
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Biological StructureBiological Structure
Species diversity Number of taxa (families or species for ex.)
First approximation
Shannon diversity index considers 2 factors: Number of taxa Evenness (or “relative abundance”) For a given number of families, the Shannon
index is higher when there are approximately equal numbers of individuals in each group (more even distribution of individuals)
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Rank abundance of speciesRank abundance of speciesMethod of examining evennessMethod of examining evenness
Pi values are on the y-axis
Groups placed in order of pi values
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Effects of population Effects of population interactions on community interactions on community
structurestructure Competition and community structure
Focus: Zebra mussels Invasive Freshwater filter-feeder Attach by byssal threads Encrust any submerged
hard surface
Envirozine (Canada)
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Zebra mussels: major Zebra mussels: major effectseffects
Outcompetes native clams for space (how?) zooplankton (efficient filter-feeding)
other filter-feeders or fish that feed on zooplankton
water clarity growth of benthic algae
Why so successful? Good “fit” with new
environment No native parasites
or predators
www.boatnerd.com
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Local example: Local example: Himalayan blackberryHimalayan blackberry
Note robust stem with heavy thorns
Grows tall and dense Outcompetes native
shrubs Blocks light for shade-
intolerant tree seedlings Net result: in
community diversity of both plants and animals (Why animals too?)
Also limits animal movement
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Two hypotheses for level of Two hypotheses for level of community diversitycommunity diversity
Bottom-up hypothesis Top-down hypothesis
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Bottom-up hypothesisBottom-up hypothesis
Defined: The abundance and diversity of primary producers (the “bottom”) affects the diversity at higher levels of the food web.
Stream-related hypothesis: Diversity in different parts of a river is a consequence of the diversity and abundance of primary producers.
Diversity: High diversity in the mid-reaches from multiple sources more feeding niches. Review: Sources of primary production along river
continuum Abundance: Higher total primary production increase
in food resources reduction of competition among taxa (i.e. reduces the chance of competitive exclusion)
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Top down hypothesisTop down hypothesis
Defined: Levels of diversity are controlled by predators
Classic example: Pisaster ochraceus, the ochre star (native inhabitant), is a keystone predator in the community (Bob Payne)
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Keystone predator:Keystone predator: Pisaster ochraceusPisaster ochraceus
Pisaster maintains (promotes) diversity Mussels would otherwise dominate mid-intertidal
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Pisaster ochraceusPisaster ochraceus predation predation and diversity (2 mechanisms)and diversity (2 mechanisms)
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Keystone predator Keystone predator (invasive)(invasive)
Non-native rainbow troutNon-native rainbow trout Keystone predator decreases
diversity in this case Understand the mechanism!
Step-by-step…