chapter 17

Bio1100 Chapter 17

Ecosystems, communities, and species interactions

Ecology is the study of interactions of living organisms with one another (community) and their physical environment (ecosystem).
- An ecosystem comprises two components.
  - A biotic (living) environment of living organisms making up a community.
  - An abiotic (non-living) environment of chemical resources and physical conditions of the area, or habitat.
  Living organisms may also be ecosystems, providing habitat for other species.
- A mini ecosystem.
  This beetle (a kind of weevil) is covered by lichens (fungi and photosynthetic algae living together).
  The lichens in turn support a wide range of microscopic invertebrates and bacteria.
  Humans are also ecosystems, providing habitat to large numbers of microbes.
Ecosystems that occupy large areas are called biomes .
- Terrestrial biomes
  - Terrestrial biomes are defined by temperature and precipitation.
    1. Tropical forest thrives in high temperatures and abundant rain.
    2. Desert exists in hot and dry areas.
    3. Savanna is dominated by grass in tropical areas with moderate rainfall.
    4. Grassland is dominated by grass in temperate areas with moderate rainfall.
    5. Deciduous forest occurs in temperate areas with sufficient rain to support trees; a cold season causes trees to shed leaves.
    6. Chaparral (known as Mediterranean in Europe) is found in savanna-like coastal areas where winter rains support shrubs.
    7. Coniferous forest exists in cold areas that receive enough precipitation to support evergreen trees.
    8. Tundra is found in colder areas where the soil below 1 meter is permanently frozen into permafrost, and only short vegetation can grow.
    9. Polar ice is found near the poles with almost no precipitation. Freshwater is scarce; life is limited to coastal areas and its oxygen-rich cold waters.
- Aquatic biomes
- Aquatic biomes are defined by salinity, water movement and depth.
  1. Lakes and ponds have non-flowing fresh water.
  2. Rivers and streams have flowing fresh water.
  3. Estuaries have a mix of salt water and fresh water along coasts.
  4. Coral reefs are highly diverse regions in shallow, warm salt water.
  5. Open oceans have deep salt water.
Energy flows in one direction through trophic levels of an ecosystem, forming a complex food web .
- Trophic levels
  Light energy from the sun flows into an ecosystem and is used by organisms who occupy specific trophic levels defined by their energy source.
  1. Producers capture sunlight and transform it into food by photosynthesis.
  2. Primary consumers are herbivores that eat producers.
  3. Secondary consumers are carnivores that eat herbivores.
  4. Tertiary consumers are top carnivores, eating food at lower trophic levels.
  Other consumers include decomposers and detritivores, as well as parasites.
- Energy Pyramid
  Energy flow through an ecosystem can be shown as an energy pyramid, representing the biomass (kilograms per square meter) of trophic levels.
  Typically, 10% of the energy at each level is converted to biomass at the next higher level.
  Thus, producers yield
  - 10% of its energy to primary consumers
  - 1% of its energy to secondary consumers
  - 0.1% of its energy to tertiary consumers
  - Do you prefer to eat smaller fish like sardines, or larger fish such as swordfish and tuna?
    - Large fish are top-level consumers.
      Toxins such as mercury from lower trophic levels accumulate in fat tissue of predators and become more concentrated.
- These consumers can occupy any trophic levels above 1, recycling nutrients in the ecosystem.
  - Decomposers such as fungi and bacteria break down decaying organic matter at all trophic levels.
  - Detritivores such as dung beetles eat detritus (dead organisms and waste).
- Parasites are consumers that do not eat another organism, but depend on a host organism for energy.
  Ectoparasites (such as the bedbug) live on a host.
  Endoparasites (such as Trypanosoma brucei, which causes sleeping sickness) live inside a host.
- A food web
  The flow of energy in an ecosystem may not be a linear chain from small to large organisms, but may follow a complex web of consumption.
  Some consumers may have two-way trophic relationships with other organisms: some birds feed on snakes, while some snakes raid bird nests.
  These may be the same 2 species at different stages in their life cycle.
  - Can a snake eat a hawk?
    - Yes, especially when the hawk was an egg or chick.
Unlike energy, the physical components of ecosystems are reused in chemical cycles .
- Water cycle
  Water falls to earth by condensation and precipitation, and cycles back by evaporation and transpiration (loss of water from leaves due to photosynthesis).
  - What's missing from this picture?
    - Animals that pee.
- Carbon cycle
  1. Photosynthesis turns atmospheric carbon dioxide (CO₂) into sugar.
  2. Carbon returns to the atmosphere as CO₂ from cellular respiration by producers and consumers alike.
  3. When organisms die, incomplete decomposition over time converts some sugars into fossil fuels: coal, oil, natural gas.
  4. Fossil fuels played little role in the carbon cycle until humans began burning them for energy.