Symbiotic relationships on the tundra include nematode parasitism on caribou, bacterial mutualism with pitcher plants and nematode commensalism on black flies. Symbiosis is long-term dependence of one species on another classified by the effect on the host. Parasitism causes injury, commensalism has no apparent effect and mutualism provides benefits.
Amensalism is a non-host form of symbiosis between two species injuring or destroying one organism and leaving the second unaffected. Competition occurs when a fitter organism monopolizes a food source or a niche. Antibiosis occurs when a secretion of one organism damages a second.
Pitcher plants provide nutrient-rich decaying carcasses for midges. Mosquitoes cannot process the same food until the midges break it down into smaller particles. Mosquito density does not affect midges, but increasing midge density improves the mosquitoes' chance of survival. This relationship is an example of commensalism, rather than competitive amensalism.
Pure commensalism is rare because typical examples relate to a non-host relationship. A wider definition of commensalism includes the adaptive scavenger behavior used by the arctic fox as it follows a polar bear to feed off the remains of its kill. The polar bear is not affected and the arctic fox benefits, but there is no long-lasting relationship between the two species. Another example is the widespread dispersal of lichens by mammals and birds. The carrier is not affected, but the population of lichens increases.
Commensalism often turns out to be parasitism or mutualism as newer research identifies a previously unknown injury or benefit to the host.