Changes in Population Size

1. Changes in Population Size Text p. 660-669

2. 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

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

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

5. 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.

6. 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%

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

8. 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

9. 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

10. 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:

11. 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.

12. 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

13. 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.

14. 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.