BIL 160 - Lecture 23

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COMMUNITY ECOLOGY: How do populations interact with one another?

All the populations of different species living in a particular area comprise that area's COMMUNITY--the living portion of the ecosystem. The number of different species found in an ecosystem comprise that system's SPECIES DIVERSITY. Diversity varies greatly among ecosystems, and hence, so do the interactions among populations in those ecosystems.

A COMMUNITY is any assemblage of populations in an area or habitat.

Individualistic Hypothesis (H.A. Gleason)
Interactive Hypothesis (F.E. Clements)

SYMBIOSIS - This term (from the Greek sym, meaning "together" and bios, meaning "life") refers to the members of two different species (i.e., two populations) having some sort of ecological interaction that affects both populations. Here are some of the theoretical types of interactions that can evolve over many generations. When two species evolve in response to each other's activities, the process is known as coevolution.

"+" means that the population benefits from the interaction

"-" means that the population is harmed by the interaction

"0" means that the population is not affected by the interaction

type of interaction	pop'n A	pop'n B	nature of effect
mutualism	+	+	obligatory; both populations benefit
EXAMPLES:
protocooperation	+	+	NOT obligatory; both pop'ns benefit
EXAMPLES:
competition	-	-	populations inhibit one another
EXAMPLES:
neutralism	0	0	populations don't affect one another
EXAMPLES:
predation	+	-	predator (A) kills & consumes prey (B)
EXAMPLES:
parasitism	+	-	parasite (A) exploits the host (B), but does not kill it outright
EXAMPLES:
parasitoidism	+	-	parasitoid (A) eventually kills host (B)
EXAMPLES:
commensalism	+	0	commensal (A) benefits; host (B) not affected
EXAMPLES:
amensalism	-	0	A inhibited; B unaffected
EXAMPLES:

Some of the results of predation...

Crypsis - camouflaging coloration

Aposematism - warning coloration (poisonous or venomous species)

Mimicry - a species has evolved the superficial appearance of something else

Batesian Mimicry - a harmless mimic looks like a poisonous model.

Mullerian Mimicry - several poisonous/distasteful species resemble one another.

ECOSYSTEM ECOLOGY

You've all seen something like THIS, and know what TROPHIC LEVELS are.

The Food Web reflects the flow of ENERGY and NUTRIENTS through ecosystems.

Energy Flow begins with PRIMARY PRODUCTIVITY, the amount of light energy converted to chemical energy (organic molecules) by an ecosystem's autotrophs over a given period of time.

GROSS Primary Productivity (GPP) - Total primary productivity
NET Primary Productivity (NPP) - Total primary productivity - energy used for respiration

NPP = GPP - R
Primary productivity (always expressed as a RATE) can be expressed as
Energy per unit area per unit time (e.g., Joules per square meter per year)
or
BIOMASS, the dry weight of vegetation added to an ecosystem per unit area per unit time (e.g., grams per square meter per year).
STANDING CROP BIOMASS is the dry weight of vegetation in an ecosystem at any given "snapshot moment" in time. It is not the same as primary productivity: it's not a rate!
Energy flow is never 100% efficient, and this results in the classic Pyramid of Productivity.
The efficiency with which trophic levels convert energy from the previous trophic levels varies greatly with ecosystem, and usually range between 5% - 20% (What does this mean in terms of how much is LOST at each trophic level?), as reflected by the STANDING CROP BIOMASS of each level.
In a few ecosystems, there are some surprises!

Biogeochemical Cycles
A generalized cycle...

...can be superimposed on Biogeochemical Cycles for many different nutrients
- The Water Cycle
- The Carbon Cycle
- The Nitrogen Cycle
- The Phosphorus Cycle
- ...and any other element.
Your activities any day have profound effects on these cycles, and on energy cycling in the biosphere. To test how much impact you have on our biosphere, take the following test of your Ecological Footprint.
Go forth and spread the word.

COMMUNITY ECOLOGY: How do populations interact with one another?

ECOSYSTEM ECOLOGY

NPP = GPP - R

Biogeochemical Cycles