Relationships Between Organisms

All organisms in an ecosystem depend on other organisms for survival. Any organisms which share long term interactions have a “symbiotic relationship”.

Parasitism is a symbiotic relationship that benefits one organism and harms another. Most parasites don’t kill their hosts, but weaken them. Ex. Tapeworms harm their host by stealing important nutrients, causing malnutrition and intestinal blockages.

Mutualism is a symbiotic relationship that is beneficial to both organisms. Ex. A bright clownfish attracts prey to the stinging tentacles of a sea anemone. The clownfish is immune to the sting and receives shelter.

Commensalism is a symbiotic relationship in which one organism benefits and the other is unaffected. Ex. A remora (small fish) attaches itself to the underside of a shark and feeds on pieces of food left over from the shark’s meals.

Predation is a symbiotic relationship in which one organism (predator) hunts and kills another organism (prey) for food and energy. Ex. Some bears hunt and kill salmon for food.

Predation is a limiting factor on population growth. As the prey’s population increases, so does the predator’s population because of the larger food supply. However, with increased predation, the prey population decreases because there is more prey being eaten. As the prey population decreases, so does the predator population because they’ve run low on food. As the predator population decreases, the prey population has a chance to increase once again. The relationship is a continuous cycle.