"If my decomposing carcass helps nourish the roots of a juniper tree or the wings of a vulture -- that is immortality enough for me. And as much as anyone deserves." -- Edward Abbey

I. Community Ecology: Energy Flow

1. You can divide organisms into three types, according to the roles they play in an ecosystem:
   1. Producers -- carry out photosynthesis. These are the ultimate sources of all living matter (biomass) and energy in an ecosystem. EXAMPLES: Plants; photosynthetic protists; photosynthetic bacteria.
   2. Consumers -- heterotrophs; obtain matter and energy from feeding on other organisms. EXAMPLES: Animals; many non-photosynthetic protists.
   3. Decomposers -- heterotrophs; obtain matter and energy from feeding on dead organisms, releasing nutrients back into the ecosystem. EXAMPLES: most fungi, many bacteria, some protists. (A.k.a. detritivores).
2. You can then look at energy flow through an ecosystem. As organisms feed on each other, etc., energy -- in the form of food -- flows through the ecosystem.
3. These pathways can be represented by a diagram called a food web.
4. And you can define several levels of energy flow, called trophic levels:
   1. Primary productivity -- the producers in the ecosystem.
   2. Primary consumers -- feed directly on the producers. A.k.a. herbivores (plant eaters)
   3. Secondary consumers -- feed on the primary consumers
   4. Tertiary consumers -- feed on the secondary consumers
   5. Quaternary consumers -- you guessed it -- feed on the tertiary consumers
   This is a bit of a simplification, because many organisms occupy several levels. EXAMPLES: A sparrow might eat both seeds and small fruits, and insects. Bears may feed on fruits and berries, grubs, fish, and carrion as available. Humans will eat almost anything.
5. If you stack the trophic levels, you get a trophic pyramid diagram.
   1. Think of the producers as forming the base on which all other trophic levels rest.
   2. It's a "pyramid" because the amount of biomass and energy at each level is smaller.
      1. Why? At each trophic level, some energy is lost. Not all of the matter an animal eats goes into building new biomass -- respiration isn't 100% efficient, some food is lost as waste, a lot of energy is lost as heat or used for "maintenance activities", and so on.
      2. The ecological efficiency of each trophic level averages about 10%, although in real life it varies quite a bit. (Typical beef cattle convert about 6% of their total food intake into beef; pigs are more efficient at about 14%, and broiler chickens can do about 21%.)
      3. Thus the rule of thumb is that each level of the trophic pyramid is only one-tenth the size of the next lower level.
   3. This leads to the phenomenon of biological magnification -- certain toxins which organisms can't easily break down or eliminate become more concentrated at higher levels of the pyramid. Examples: DDT and mercury, as discussed in lecture.

1. Ecological succession is a series of changes over time in the communities present in an ecosystem.
   1. Imagine a plowed field that's been abandoned. First you get small weeds growing there. . .
   2. . . . and then tall grasses. . .
   3. . . . and then shrubs and small trees. . .
   4. . . . and then large trees. . .
   5. . . . and, eventually, dense forest with no underbrush. This is an example of ecological succession.
2. Succession ends with a climax community -- in the example above, the climax community is dense forest.
   1. In some cases, repeated disturbances keep the community from ever continuing succession to the climax community. EXAMPLE: Periodic fires in prairie and chaparral communities keep them from being overgrown by forest.
3. Two types of succession: primary succession and secondary succession.
   1. Primary succession takes place in an environment that has never been colonized by organisms. EXAMPLE: growth of coral reefs on offshore drilling platforms or shipwrecks.
   2. Secondary succession takes place in an environment that has been colonized before, and has never been completely wiped out. EXAMPLE: growth of forests on abandoned farmland.
4. Keystone species
   1. Species whose presence in an ecosystem has major effects on overall diversity.
   2. Loss of a keystone species "ripples" through an ecosystem and causes loss of other species. (EXAMPLE: California sea otters)
5. Species may also enter very close, very specific relationships with each other, known as symbiosis.
   There are three possible types of symbiotic relationships:
   • Parasitism -- one partner benefits, the other is harmed
   • Mutualism -- both partners benefit
   • Commensalism -- one partner benefits, the other is neither helped nor harmed

III. Ecological Cycling -- happens on an ecosystem-wide or even global basis

1. Hydrologic cycle -- probably the simplest
   1. Major driving force: Solar energy, causing evaporation of water from oceans and other bodies of water
   2. Water leaves the atmosphere as rain and eventually returns to the oceans, either as surface runoff (e.g. rivers) or as groundwater (e.g. springs, seeps, etc.)
2. Carbon cycle
   1. Major driving force: Photosynthesis takes CO₂ out of atmosphere
   2. Respiration of consumers and decomposers eventually puts CO₂ back in
   3. Fires and volcanoesalso put CO₂ back in the atmosphere.
   4. Death and burial of once-living organisms takes CO₂ out of the cycle for millions of years. If these remains become fossil fuels (coal, oil, natural gas), the burning of fossil fuels releases that ancient carbon back into the atmosphere as CO₂.
3. Nitrogen cycle
   1. Major driving forces: Lightning, plus certain nitrogen fixing bacteria, convert nitrogen gas in the atmosphere into usable compounds (ammonia and nitrates).
   2. These are taken up by producers, which are taken up by consumers, etc.
   3. "Denitrifying" bacteria break down nitrogen-containing compounds and re-release nitrogen gas back into the atmosphere.
4. Phosphorus cycle
   1. Major driving force: Erosion of phosphorus-containing rocks
   2. Phosphorus moves to the oceans in runoff, where it may stay for millions of years in marine sediments
   3. Geologic uplift, forming new exposures or rocks on land, returns phosphorus to the cycle
   4. Phosphates in soil enter living organisms and cycle among producers, consumers, and detritivores / decomposers.