Bio1110 Chapter 9 Community Ecology
  1. Energy in an ecosystem flows through organisms in a trophic pyramid.
    • Organisms in an ecosystem occupy trophic levels defined by energy use.

      Light energy from the sun flows into an ecosystem and is transformed into other forms through organisms.

      Producers occupy trophic level 1: they capture sunlight and transform it into chemical energy by photosynthesis.

      The transformed energy is then available for consumers at higher trophic levels: primary consumer, secondary consumer, etc.

      Most energy is lost at each trophic level; only about 10% is available to the next level on the trophic pyramid.

      After organisms die, the remaining energy is available to decomposers, which recycle nutrients in the ecosystem.

    • 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.

      Producers such as Periphyton algae in the Florida Everglades are at the base of all food webs.

      Some consumers may have two-way trophic relationships with other organisms: alligators are the apex predator here, but young gators can be prey to other predators.

  2. Species diversity depend heavily on habitat.
    • Species diversity of an area consists of 2 components.

      • Species richness is the total species count.

      • Species evenness compares the relative population size of each species.

      In comparing two areas with the same species richness (25), the one with more even distribution of species has higher species diversity, and more resilient to environmental changes.

    • Ecotones are boundaries between 2 different ecosystems, and may be zones of rich biodiversity.

      Some edge species require resources provided by both ecosystems to survive.

  3. Complex interactions form among organisms as energy flows through them.
    • Symbiosis is the interaction between species in an ecosystem.

      • Mutualism is a symbiotic relationship when both species benefit. An example is a moth obtaining nectar from a flower and carrying pollen for the plant.

      • Parasitism occurs when one species benefits and the other (host) is harmed. Leeches are parasites that drain blood from their hosts.

      • Commensalism occurs when one species benefits and the other is unaffected.

      A healthy ecosystem is a complex web with countless symbiotic relationships.

    • Species interactions shape the mix of organisms found in a community.

      • Predation occurs when one organism (predator) consumes another (prey) for food.

      • Competition can occur both between and within species for resources.

      • Resource partitioning reduces competition between species when different species utilize different parts of the habitat for similar food sources.

    • • The American alligator is a keystone species in the Florida Everglades.

      Gator holes provide mini wet habitats during the dry season.

      Some birds such as wood storks prefer to nest in trees surrounded by alligators; without gators, raccoons will raid nests for eggs and chicks.

  4. Ecosystems often undergo ecological succession as community composition changes over time.
    • Ecological succession occurs when a new area is colonized or an existing area is disturbed.
      Lichens and mosses first colonize bare rock, breaking down rock particles and leaving organic matter.
      Small grasses and shrubs move in as soil builds up.
      Further soil accumulation allow sun-loving, pioneering trees to grow.
      Eventually a mature, climax community forms, dominated by large trees that shade out pioneers.