Food webs and ecosystem connectivity

What does Oksanen’s model propose?

• As potential primary productivity increases, the number of possible trophic levels increases

• Increases in productivity have alternating effects on adjacent trophic levels

Why doesn’t productivity have alternating effects on the biomass of adjacent trophic levels?

• Trophic levels are not homogeneous populations but are made up of a diversity of species

• An increase in potential productivity can lead to a shift from the better resource competitor to the more predator-tolerant species

• Shifts in species composition within trophic levels can influence the pattern of trophic-level responses

What is apparent competition?

Bottom-up effects on predator abundance can lead to top-down effects on alternative prey

Why are food chains too simple to predict food web dynamics?

• In the real world, energy can flow through either the green channel or the brown channel

• Energy flow through the 2 channels varies depending on the ecosystem, the identity of the primary producers, and the net primary productivity

What is the green channel?

The live consumer system

What is the brown channel?

The decomposer/detrital system

How does diversity compare between the 2 energy channels?

The brown energy channel is more species-rich

How does production compare between the 2 energy channels?

The green energy channel is more productive

How do interaction strengths compare between the 2 energy channels?

The brown energy channel has more links and weaker interactions

What are looped food webs?

Mobile consumers at the top of a food web can link the energy flow through the green and brown channels of the food web

What did McCann and Rooney propose about looped food webs?

• Detrital channels may be able to buffer higher trophic levels against fluctuations of energy flow through the green channel

• The species diversity within slow channels of food webs that result in skewed distributions of interaction strengths helps stabilize entire ecosystems

What did Ward propose about primary productivity in looped food webs?

• Responses to enrichment are the outcome of multi-channel control

• As primary productivity increases:

  • The proportion of the predator’s diet coming from the detritus channel goes up

  • The proportion of primary production consumed by herbivores goes down

What is an ecological subsidy?

The flux of organic matter or nutrients from a “donor” habitat to a “recipient” habitat

What is an ecotone?

The transition area between biomes or ecosystems

What does autochthonous mean?

Originating in the place (ecosystem) where it is found

What does allochthonous mean?

Originating from outside the place (ecosystem) where it is found

Why is the flow of energy and nutrients across ecosystem boundaries important?

• Allochthonous inputs are ubiquitous, link ecosystems, and can become dietary subsidies that support increases in the size of the user/recipient populations

• Subsidies to one trophic level can have indirect effects on trophic levels above and below the user of the subsidy

• Using resources from multiple ecosystems (marine and terrestrial) can stabilize predator populations in a rapidly changing climate