1. Community Interactions

2. Vocabulary:
A community consists of all the interacting populations within an ecosystem
An ecosystem is all the organisms and their nonliving environment within a defined area.
A niche is an organisms habitat, way of life, and physical environment.
An organisms habitat is were it lives.

3. Interactions
When populations interact with one another, they influence each other ability to survive and reproduce.
They are agents of Natural Selection.
Competition between different species is called inter-specific competition.
Competition between the same species is called intra-specific competition.
No two species occupy the exact same niche.
The competitive exclusion principle states if you put two species with the same niche together then one will out compete the other and kill it off.

4. Predator - Prey
Prey and predator populations both follow cyclical trends
When predators get too numerous, prey population drops and predator resources are thus depleted
When prey get too numerous, disease and other density-dependent factors decrease the population
During the population drops only the least fit individuals will die.
A change in the prey population illicit evolution in the predator population and vice versa.
This Process is
Co-Evolution

5. Predator-Prey population trends
Prey and predator populations both follow cyclical trends. Note that the predator lags behind the prey then overshoots, causing its population to crash.

6. Niche
This encompasses all the aspects of an organism way of life., it including:
the physical home or habitat
all the physical factors like temperature, pH, type of soil, etc.
how the organism gets its supply of energy
its predators, prey and interactions with other organisms.

7. Same Niche: Competitive Exclusion
Gause (1934): “If two organisms are competing for the same resources, one will be better at acquiring those resources.”
One organism will therefore win out over another if they are in the same niche. This is called competitive exclusion.

8. Seashore Gastropod Competition
Snails of the intertidal zone will eat algae. Niches overlap but they are never the same. Rough periwinkles are able to live high in the “splash zone” avoiding direct competition with common periwinkles and predation by crabs.
Both periwinkles can be observed in the same location >
Common periwinkles prefer the lower portions of the seashore.

9. Barnacle Competition

10. Other Interactions Symbiosis (“living together”)
Symbiosis is a close relationship between different species over an extended period of time.
Symbiosis (“living together”)
Parasitism (still predator and prey)
Predator much smaller than prey
Mutualism (everybody wins)
Partnerships with other species
Commensalism (at least somebody wins…)
One species gains, the other loses nothing

11. Parasitism Nematodes in cod fish tape worms on salmonids
Parasitism is a symbiotic relationship in which one organism benefits by feeding on the other.
Nematodes in cod fish
tape worms on salmonids
tapeworm larvae in snails
But what about evolution? Are parasites who damage or kill their host the parasites that are always selected for? Can you anticipate evolutionary change with the above parasites?

12. Newfoundland Example:
Tapeworm from salmonid gut.
Parasite under trout skin.

13. Mutualism
Mutualism is a symbiotic relationship in which both species benefit.
And...
Bacteria in our gut
Mitochondria
Nitrogen fixing bacteria
& legumes
Domesticated plants and animals
In fact, almost every known organism has some mutualistic relationship with at least one other organism...

14. Commensalism
Commensalism is a symbiotic relationship in which one organism benefits while the other is not harmed.
Newfoundland Example:
Barnacles on Whale Skin

15. Keystone Species
In some communities a certain species, called a keystone species plays a major role in determining community structure.
Removing a keystone species dramatically alters the community.
Eg. The predatory Star fish
It eats the mollusks that feed on plankton

16. Community Interactions
Have a look at the movies that
detail interactions on the seashore
and intertidal organisms.