Populations and Interactions

population density - the number of individuals of the same species (ie population density - the number of individuals of the same species (ie., a population) that occur per unit area or volume population dynamics - changes in population characteristics determined by natality (birth rate), mortality (death rate), immigration (number of new members to a population) and emigration (number of members that have left a population)

Each population has a: minimum viable size- the smallest number of individuals needed for a population to continue for a given period of time carrying capacity - the maximum number of organisms that can be sustained by available resources over a given period of time

Some populations can affect their survival rates through defence mechanisms: Mimicry - the similarity of one species to another to protect one species or both. The similarity can be appearance, behaviour, sound or scent. Frogs on top are poisonous. Frogs on bottom are not.

“Red next to yellow kills a fellow “Red next to yellow kills a fellow. Red next to black is a friend to Jack”

https://www.youtube.com/watch?v=bIp7sRWEt_w A video on mimicry

Cryptic colouration - colouring that conceals or disguises an organisms shape https://www.youtube.com/watch?v=GFUiCsUSzyw

Some organisms play dead: A butterfly masquerades as a dead leaf Others stand in herds for protection And some shed body parts to stay alive:

Each population is affected by: fecundity - the potential for a species to produce offspring in one lifetime density dependent factors - influences population size. Impact of factor depends on population density (space, competition, predation, disease, etc)

density independent factors - factors that influence population size regardless of population density (Sunlight, water, soil, air, floods, drought, fires, storms)

Population growth is also affected by biotic interactions.

Predation - the relationship between two different species in which one species, the predator, catches, kills and feeds on another, the prey

competition - when two or more organisms compete for the same resource in the same location at the same time.

symbiosis - the close interaction between two different species in which members of one species live in, on, or near members of another species Includes: mutualism, commensalism and parasitism

mutualism - both species benefit from the relationship mutualism - both species benefit from the relationship. (a +/+ relationship) Eg.Crocodiles and Dikkops. Crocs try to eat most nearby birds. But the Water Dikkops eat insects that bother crocodiles. Easy meal for the bird, and the croc isn’t bothered by the pests. So the bird doesn’t get eaten!

Commensalism - one species benefits from the relationship without harming or helping the other species (a +/0 relationship) Eg. A bird building a nest on a branch of a tree, where the nest does not harm or help the tree) Cattle egrets (birds) that feed off insects that get stirred up by grazing cattle.

Parasitism - one species benefits from the relationship at the expense of the other species. Parasites live on or inside the host species and obtain some or all of their nutrition from the host. (A +/- relationship) (Eg. Ticks that live on the bodies of mammals, and feed on the hosts blood making the host sick.) do not usually make the host die because then they lose their source of nutrients.

Population Cycles Predatory-prey interactions and density dependent factors can result in population cycles - regular fluctuations in the size of a population.

Eg. lemmings can have a high mortality due to overcrowding which causes a sudden decline in the population - due to competition for food, space, air and water. Gradually, the reduced population will recover and build up again as competition decreases and accessibility required factors increases.

In Northern Canada, there is a pattern between the population cycles of the lynx and snowshoe hare. As the prey increase in number, the predator increase as well. Eventually, the prey numbers will drop and get so low that the lynx population cannot be sustained - so it declines.

This gives the prey population a chance to recover and increase - only to provide more food for the lynx which will increase in population. And so the cycle continues.