EBIO 1220 Lecture 34: 4/26/17 - Population Ecology II
University of Colorado - Boulder
Ecology & Evolutionary Biology
CLICKER QUESTION → Which of the following best accounts for the niche separation of the two
barnacles Balanus and Chthamalus?
● Competitive exclusion → if 2 species have the same niche they can’t live in the same place
Primary Research Article → Does energy input determine food chain length?
● Field experiment
● Study System
○ Tree Hole Communities
■ Mosquito larvae (filter feeders)
■ Predatory midge larvae
■ Detritivorous midge larvae
■ Predatory mites
■ Detritivorous mites
■ Detritivorous beetle larvae
● Experimental Design
○ Soda bottles → leaf litter and lid
■ Natural rate (high)
■ Medium rate
■ Low rate
■ Number of trophic links between species (food chain length)
○ Support the energetic hypothesis because the container with more lead litter (high rate
of litter fall) had more trophic links.
○ Conclusion → Reducing energy input reduces food chain length.
CLICKER QUESTION → What do you predict they observed?
● Containers with most leaf litter had most trophic links
○ More energy into the bottom of the food chain (energetic hypothesis), food chain will
Community Ecology Learning Goals ●
● How does food chain length or energy input influence community ecology?
Otter Case → How does adding killer whales (community perspective) effect the ecology of the otters?
● Trophic Cascade → example of how feeding relationships structure communities
● Shows us that early on in the 1970’s this food chain did not include orcas (it stopped at otters)
so there were large numbers of otters and small numbers of sea urchins. Orcas were introduced
in about 1990 and what happened was the usually prey of the orcas (seals) were competing with
humans for fish and they became less common and the orcas needed to find a new food source
(switched to sea otters) and became part of the food chain.
● When orcas were added to the food chain → otters declined and urchins increased
○ This is because there were less otters due to orcas eating them so therefore less urchins
were being eaten.
● Introduce orcas → feed on otters → otter population goes down → urchin population goes up
→ kelp population decreases → kelp forests become more abundant
● Introduction of orcas to the food chain led to the disappearance of kelp forests.
CLICKER QUESTION → Predict the effect of Orcas on kelp forests.
● A. Kelp forests disappeared because more urchins were eating them.
● What happened in the northern part of the otters’ terrain (Alaska)?
Think-Pair-Share → This is an example of a trophic cascade.
● My definition → When something is introduced to a community and the food chain is shifted
completely (like a domino effect).
● Student definition → A disturbance to a food chain in response to adding or removing
something to a food chain.
● A change at one level of a food chain that cascades to the other levels.
● Effects cascade down the food chain in alternating +/- effects. ●
What other kinds of things determine community structure?
● So far:
○ Species niches
○ Energy flow in food chains/webs
○ Feeding relationships: predation, herbivory, parasitism
Species with a large impact:
I. Dominant Species
○ High abundance