Populations Interactions between and within populations influence patterns of species distribution and abundance. Interactions between populations affect the distributions and abundance of populations. Competition, parasitism, predation, mutualism and commensalism can affect population dynamics. Relationships among interacting populations can be characterized by positive and negative effects: Predation: (+/-) Parasitism: (+/-) Mutualism: (+/+) Commensalism: (+/0) Predation: (+/-) Parasitism: (+/-) Mutualism: (+/+) Commensalism: (+/0) Relationships among interacting populations can be can be modeled mathematically. (Examples: predator/prey, epidemiological models, invasive species). Moose and Wolf populations on Isle Royale Many complex symbiotic relationships exist in an ecosystem, and feedback control systems play a role in the functioning of these ecosystems. A population of organisms has properties that are different from those of the individuals that make up the population. The cooperation and competition between individuals contributes to these different properties. Factors that affect species distribution and abundance: Species-specific catastrophes Environmental catastrophes Geological events Sudden influx/ depletion of abiotic resources Increased human activities Species-specific catastrophes Environmental catastrophes Geological events Sudden influx/ depletion of abiotic resources Increased human activities Example: A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. The sea otter is an example of a keystone species. They feed on sea urchins, thereby controlling their population. Without otters to keep their numbers in check, sea urchins would destroy the kelp forests. Kelp is a major source of food and shelter for the coastal ecosystem.