ecology: lecture 17 community structure. vertical stratification in lakes
Post on 21-Dec-2015
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Ecology: Lecture Ecology: Lecture 1717
Community structure
Vertical stratification in lakesVertical stratification in lakes
River Continuum conceptRiver Continuum concept (pictures from Stroud Water Research Center)(pictures from Stroud Water Research Center)
Headwater Mid-sized stream Large river
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)
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)
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
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)
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
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
Two hypotheses for level of Two hypotheses for level of community diversitycommunity diversity
Bottom-up hypothesis Top-down hypothesis
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)
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)
Keystone predator:Keystone predator: Pisaster ochraceusPisaster ochraceus
Pisaster maintains (promotes) diversity Mussels would otherwise dominate mid-intertidal
Pisaster ochraceusPisaster ochraceus predation predation and diversity (2 mechanisms)and diversity (2 mechanisms)
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…