Lecture 15

What is an ecological community? What does a community ecologist study?

A community consists of all organisms of all the species that inhabit a particular area. Community ecology examines the interactions between populations, and how factors such as predation, competition, and disease affect community structure and organization.

What is the difference between direct and indirect interactions in a community? Give a real example of each.

A direct interaction deals with the direct impact of one individual on another when not mediated or transmitted through a third individual. Ex, a cheetah capturing a gazelle or a bee pollinating a flower. An indirect interaction is the impact of one organism or species on another that is mediated or transmitted by a third. Ex. if a bird is eating a caterpillar, then the caterpillar is not eating the plant and thus the plant can grow more successfully.

What is a keystone species? Give an example of one in a real ecosystem

A keystone species is any species that has a disproportionately large impact on the diversity of its community, relative to its own abundance. Ex - starfish

What is a mycorrhyzal association? Why is it important ecologically? Describe the 3-way association I described in lecture. Name all three participants, and describe how they interact.

A mycorrhyzal association is a symbiotic relationship between fungus and plant in which the fungus interpenetrates the roots, mobilizing soil nutrients for the plant and absorbing complex organics produced by the plant. This is important because it helps the tree get the nutrients so that it can grow successfully.

What is the difference between a producer and a consumer?

A producer is any organism that converts energy from inorganic sources to organic material. A consumer is any organism that gets its energy only by eating organic material.

Why is the decomposer food web so critical to ecology?

Because it returns nutrients back into the abiotic environment and living things can then use these raw materials. This is a critical component of ecosystem function.

What are the seven different kinds of interactions among species that I listed in class? Could you describe a couple of them?

Herbivory - organisms that directly eat producers to get energy Predation - consumers that eat other consumers Competition - different species can compete for any resource (e.g. shelter, food, sunlight, nutrients, etc) Decomposition - many invertebrates, bacteria, and fungi eat dead material, which returns nutrients back into the abiotic environment Parasitism - A parasite is physiologically dependent upon its host for nutrition Commensalism - occurs when one organism is positively affected by the relationship while the other organism is not affected, either negatively or positively, by the interaction. Mutualism - both partners benefit from the relationship

How did the killer whale affect the marine shoreline community when it invaded? What is a trophic cascade?

Trophic cascade occurs when a predator over-crops its prey, which then has a "cascading" effect on other trophic levels. Killer whale reduces the numbers of otters; free from otter predation, sea urchins multiply; urchin grazing depresses populations of kelp.

How, in general, is the effective trophic level of an organism calculated? What two pieces of information about the organism's biology do you require?

What it eats (literally). So for each, you list the prey types. Take the trophic level of the food and multiply it by the percentage of food it makes of that organisms diet. Everything is one trophic level above its food supply.

Describe the 2-part experiment that a famous scientist (Gause) performed using three species of a little creature called Paramecium. What important ecological insight did this experiment provide?

When two species compete for the same, limited vital resource, one will always drive the other to local extinction, this is competitive exclusion at work. When Gause put P. Aurelia together with P. bursaria, the two species divided up the habitat and both survived, which was a demonstration of resource partitioning. Both species with identical needs won't be able to coexist, or they divide up the resources and the populations of both decrease.