Biol 208: Lecture 25 - Community Assembly

Define Ecological Networks

• Name the 3 types talked about

Set of ecological interactions within a community (>/= 2 species)

1. Trophic network (aka. Food web)

2. Host parasitoid network

3. Mutualistic network (width = frequency of interaction)

Food web/ Trophic Net work

• What does it summarize?

• Different levels

• What is a LIMITATION?

Summarize feeding relationship in a community

• Basal level = primary producers

• Level 2 = herbivores

• Level 3 = Primary + secondary predators

Limitation = can see interactions BUT cannot see abundance + density

What are functional groups that feed on organisms across all trophic levels in a food web? (2)

Detritivores

Pathogens + parasites

What is a BIOMASS PYRAMID

• How does it differ between aquatic + Terrestrial ecosystems?

Illustrates the abundance of trophic levels

• terrestrial = right side up

• Aquatic = Upside down

Where would: Rare species, Moderate species + Dominant/foundational species be placed on the graph if they were removed from the ecosystem

• Change in ecosystem function as a function of species biomass

Rare = Low biomass + Low effect/change

• Other species can perform the same functions. No vital role

Moderate = moderate biomass + moderate effect

Dominant/Foundational = High biomass + High effect

• Vital role in ecosystem

Define a KEYSTONE SPECIES + where it would place on the graph

• What is the main JOB/FUNCTION of a keystone species

Definition = Species that despite their LOW BIOMASS has STRONG effect on structure of community (Play vital role)

Job/Function:

• Keep #’s of PREY population BELOW K (carrying capacity)

***What are 3 results of the KEYSTONE species keeping prey populations below their carrying capacity/ what effect do they have?

1. Decrease Competition between prey species - Prey won’t out compete each other

   • Preferentially eats the strongest competitor prey species

   • Prevents that species from outcompeting and excluding others

2. Create ecological niches

3. Increase diversity within communities - Low competitive exclusion = high diversity

What are 2 examples of key stone species in the slides

• what specific effects do they each have?

1. Ochre Sea Star - One of the First keystone species identified

   1. Feeds on Barnacles + Urchins which consume kelp = ensure there is kelp for all the other species

2. Sea otter

   1. Feeds on urchins (consumes kelp) = also prevent sea urchin population from eating all the kelp

True or false: Key stone species are ALWAYS predators?

False

• often but not always

What is an example of a NON-PREDATOR Keystone species?

Elephant - herbivore

Key stone vs. Dominant

Both are vital to ecosystem and have HIGH IMPACT

• Keystone = LOW ABUDANCE

• Dominant = important because there is a high abundance

Does more predators increase or Decrease diversity?

Increase

Info dump on the Ochre sea star experiment that tested if more predators increase or Decrease diversity/ The importance of key stone species

• methods

• What was seen after 3 months

• What was seen after 1 year

• Why?

Removed Ochre sea star from the food web leaving only one predator

• After 3 months = Increase in Acorn Barnacles

• After 1 year = Only Bivalve + Goose neck barnacle remained

why?

• Chitons + Limpets are MOBILE so they LEFT the community because competition was too high + there was no food

• Bivalves + Gooseneck remained because they were SESSILE + ate all the primary producers in the area

  • Main resource in competition was SPACE

No Keystone species to keep prey population in check = competitive exclusion

WHY does more predators increase diversity?

Regulation of lower trophic level species population density Decreases interspecific competition for resources

• more species in lower trophic levels thus can coexist as predators are removing individuals instead of competitive exclusion

Info dump on the other experiment that tested the effects of a KEYSTONE species? - Mary power 1990

• Methods

• Which organism was keystone

• Conclusion? + how did we get there?

Keystone organism = Steel head (also primary predator)

• wanted to see effect of steel head on algae

EXCLUSION experiment:

• Enclosing steelhead or excluding it

Results:

• Enclosure of steelhead = Lower algae

• indicating a DECREASE in it’s PREY DENSITY (which eat the herbivores) + increase in herbivore density (which eat the algae)

Conclusion = Steelhead = keystone species in river ecosystem

Info dump on Wolf + elk + their effects

Removing wolf = changed ecosystem

• Low wolf = High elk = High consumption of primary producers = low willow abundance

After wolf introduction = willow abundance increased due to lower browsing pressure by elk

Ecosystem engineers

• define

• Contrast against Keystone species

Definition = species whose activity creates or fundamentally alters habitat (eg. Beaver)

Keystone = affects community through PREDATION

Engineer = Affects community by ALTERING HABITAT