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08. NOTES: Ecosystem Interactions (symbiosis, predation, competition)

posted Nov 5, 2010, 9:44 AM by Ms. Koenig   [ updated Nov 9, 2010, 1:30 PM ]
There are 3 interactions you need to know about:
- Predation (predator & prey)
- Competition (also "The Competetive Exclusion Principle")
- Symbiosis (mutualism, commensalism, parasitism)

Predation - when one organism (the predator) captures and kills another organism (the prey) for food
  • Example: Lions capture and kill zebras for food. The lion is the predator; the zebra is the prey.
  • Non-example: Deer eat grass and the leaves off of trees. However, they are not trying to kill the grass or the trees. They eat the leaves, and move on to eat somewhere else. The tree has time to grow back the leaves. The deer only eat part of the grass (like a lawnmower), then they move on and the grass grows back. Therefore deer are NOT predators.
  • Example: Seed-eaters, like cardinals and squirrels, eat and digest the seeds of plants. The seed is the plant's child. That whole seed was its own organism, a baby plant that would have sprouted and grown up. The cardinal or squirrel take the seed from the parent plant and eat it. Therefore they are predators, and the baby plant is their prey. Now you know, a squirrel is a predator!
  • Non-example: When a fruit-eating bird, like a toucan, eats a fruit, the seed inside is also swallowed. However, the bird's digestive tract digests the fruit but not the seed. The seed passes out of the bird unharmed. The seed can sprout where it landed.
*Part of a predator's definition includes its impact on the prey population. Squirrels are predators because they are removing trees (potential trees) from the tree population. A detritivore/scavenger is not a predator, because it does not cause the dead organism to die. So a vulture who eats dead bunnies is not a predator because he is not affecting the bunny population size.

Competition - competition will happen when 2 (or more) organisms both try to use the same limited resource
  • *Competition does not have to result in a physical fight!
  • *Intraspecies Competition - occurs in the same species (cardinal vs. cardinal)
  • *Interspecies Competition - occurs between different species (cardinals vs. bluejays)
  • Example: Two male deer fight using their antlers. They are competing for mates. (Intraspecies)
  • Example: The cardinal and the bluejay both eat seeds. They are competing for food. (Interspecies)
  • Example: A tree and a vine both need their leaves to catch sunlight. The leaves on top will catch the most light. (Interspecies)
*If two species are competing closely in their roles (they are trying to fill the same niche), one will be "better" and the "loser" will be forced out of the niche (adapt or die). This is called Competitive Exclusion Principle - "No two species can fill the same niche in the same place at the same time."

Symbiosis - when two (or more) species live and interact closely together
*it does NOT mean they are just "near" each other
*it does NOT include predator/prey relationships
*it does NOT include "weird" examples or "one time ..." stories; it has to apply to the whole species
*it does NOT include one animal helping another if they are the same species (they have to be different species)

3 Kinds:
  • Mutualism - both species benefit
    • bees and flowers - the flowers give the bees food, the bees help pollinate the flowers
    • fungi and algae in lichens - the fungi give the algae shelter and nutrients, the algae gives the fungi food/sugar
  • Commensalism - one species benefits, the other is neither helped nor harmed
    • orchids and tress - orchids are flowers that live up in trees, the flowers get more protection and light, the tree doesn't care
    • barnacle and whale - the barnacle lives on the whale and gets a free ride - which means more food, but the whale isn't affected
  • Parasitism - one species benefits (parasite), the other is harmed (host)
    • *This also requires that the parasite lives on or inside the host.
    • ticks and dogs - the tick gets food and lives on the dog, the dog is hurt by the tick stealing its blood
    • tapeworm and human - the tapeworm gets free food and a place to live, the human is hurt by the tapeworm stealing the human's food