Changes in Population Size Text p. 660-669

Population Dynamics Populations always changing in size Deaths, births Main determinants (measured per unit time): Natality = number of births Mortality = number of deaths Emigration = # of individuals that move away Immigration = # of individuals that move into an existing population

Effect on Determinants The determinants vary from species to species Environmental Conditions Fecundity Potential for a species to produce offspring in one lifetime vs.

Limits on Fecundity Fertility often less than fecundity Food availability Mating success Disease Human factors Immigration/Emigration

Calculating Changes in Population Size 1. Population Change (∆N) = [(births + immigration) – (deaths + emigration)] - just plug in the values as determined. 2. To calculate the rate of population growth: gr = ∆N/∆t eg. Snail population in Banff Springs is 3800 in 1997, but falls to 1800 in 1999. Calcluate the growth rate. gr = 1800-3800 2 years = - 1000 snails/year the population declined at a rate of 1000 snails per year.

Calculating Changes in Population Size 3. Calculating the per capita growth rate (cgr): ∆N/N (N= initial pop. size) cgr = [(birth + immigration) – (deaths + emigration)] x 100 (%) initial population size (n) Positive Growth: Birth + Immigration > Death + Emigration Negative Growth: Birth + Immigration <Death + Emigration Calculate: a) the change in a population in which there were 20 births, 25 immigrants, 10 deaths, and 5 emigrants. b) What is the per capita growth if the original population was 75?. + 30 + 40%

Open/Closed Population Growth can depend on type of population Open: influenced by natality, mortality and migration Closed: determined by natality and mortality alone

Population Growth in Unlimited Environments Under ideal conditions – no predators and unlimited resources… A species can reach its biotic potential - the highest possible per capita growth rate for a population. Factors that determine biotic potential are related to fecundity: # of offspring per reproductive cycle # of offspring that survive to reproduce The age of reproductive maturity # of times that an individual reproduces in its life span the life span of the individuals

Population Growth Models 1) Exponential Population Growth: Population grows at its max. biotic potential Starts with a lag phase and forms a J-shaped curve. Really only see in lab conditions. Hypothetical model

Pop. Growth in Limited Environments If one bacterium divided every 30 minutes for 4 days – the mass would be greater than that of the earth. Doesn’t happen – resources run low and growth rate slows. Eventually, the habitat reaches its carrying capacity: the maximum number of organisms that can be sustained by available resources. This is shown by a logistic growth curve:

Logistic Growth Curve S-shaped curve (sigmoidal) 3 phases: Lag (slow growth) Log (rapid growth) Stationary (no growth) At stationary phase, population is in dynamic equilibrium, it will fluctuate around the carrying capacity of its habitat. carrying capacity changes in response to environmental conditions – resource supply, predation, limited space, disease etc.

Population Growth Models and Life History: Organisms use strategies to maximize the # of offspring that survive to reproductive age. Theoretically, the best idea is reach sexual maturity early have a long life span produce large #s of offspring provide them with high quality care until they can reproduce. Not realistic… There are different life strategies which relate to the environment in which the organism lives. These are known as r-selected or K-selected strategies

r-selected strategy Species that have an r-selected strategy live close to their biotic potential. In general, these organisms: Have a short life span Become sexually mature at a young age Produce large broods of offspring Provide little or no parental care to their offspring. Insects, annual plants and algae are examples – take advantage of favorable conditions in the summer – reproduce exponentially, but die at the end of the season.

K-selected strategy: Organisms with a K-selected strategy live close to the carrying capacity of their habitats. In general, these organisms: Have a relatively long life span Become sexually mature later in life Produce few offspring per reproductive cycle Provide a high level of parental care Mammals and birds are examples of organisms that use this. Many populations are in-between these strategies and describing a population usually involves a comparison – rabbits could be described as K-selected…compared to mosquitoes. Compared to bears, rabbits are r-selected.