Community Interactions

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.

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.

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

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.

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.

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.

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.

Barnacle Competition

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

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?

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

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...

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

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

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