Trophic Cascades and Food Webs

Learning goals:

* understand how changes in trophic interactions at one level can indirectly influence energy flow and  other trophic relationships in ecosystems

* know the consequences of trophic interactions for the accumulation of toxins in ecosystems

* be able to describe what food webs are, their limitations, and how they help to understand properties of energy flow and the stability of ecosystems

Two general controls on NPP: Resource supply influences NPP, which in turn determines how much energy flows through the ecosystem = “bottom-up control” or energy flow may be determined by rates of consumption by predators at the highest trophic level, which influences abundance and species composition of multiple trophic levels below them = “top-down control”

A trophic cascade is a series of trophic interactions that result in change in species composition, and indirectly in a change in NPP. 
Greater complexity in terrestrial ecosystems is thought to limit the existence of trophic cascades

What determines the number of trophic levels?
There are four basic, interacting controls:
1. Dispersal ability may constrain the ability of top predators to enter an ecosystem.
2. The amount of energy entering an ecosystem through primary production.
3. The amount of habitat available/ size of the ecosystem
4. The frequency of disturbances or other agents of change can determine whether populations of top predators can be sustained.

Accumulation of toxins and trophic interactions
Some chemical compounds, particularly organic pollutants and heavy metals, can become concentrated in the tissues of organisms at higher trophic levels.
They may not be metabolized or excreted for a variety of reasons, and become progressively more concentrated over the organism’s lifetime: bioaccumulation.
Biomagnification occurs when the concentration of these compounds increases in animals at higher trophic levels, as animals at each trophic level consume prey with higher concentrations of the compounds.

Food Webs
Food webs are diagrams showing the connections between organisms and the food they consume.
interaction strength: measure of the effect of one species’ population on the size of another species’ population.
energy flow and community structure might be controlled by a few key species (e.g. keystone species).
Indirect effects (e.g. competition, facilitation among prey species) add to the net interaction among species

Complexity of food webs can influence stability- although more simple food webs may seem less stable, reinforced population oscillations in complex food webs may make them less stable.  Stabilization by indirect effects may offset these potentially destabilizing effects