ecology: lecture 17 community structure. vertical stratification in lakes

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…