Measuring and Modelling Population Change

1. Measuring and Modelling Population Change Chapter 22.2 Page 742-750

2. Part 2. Population Growth and InteractionsIII) Population Growth • there are four processes that can change the size of a population: • the number of individuals (N) • Births (b) • Immigration (i) • movement into a population • deaths (d) and emigration (e) • movement out of a population

3. Part 2. Population Growth and InteractionsIII) Population Growth • the change in population size can be calculated using: ΔN = (b + i) – (d +e)

4. Part 2. Population Growth and InteractionsIII) Population Growth • the rate of population growth measures the speed at which a population changes size. • a rapid increase in the size of a population is called a population explosion. • a rapid decrease in the size of a population is called a population crash. • a populations growth rate (gr) can be calculated using the equation gr= ΔN/ Δt

5. the calculation of growth rate does not take into account the initial size of the population. • if there are no limiting factors a large population can increase in size much faster than a smaller one.

6. Part 2. Population Growth and InteractionsIII) Population Growth • to compare populations of the same species that are different sizes or live in different habitats you use per capita growth rate (cgr) cgr= ΔN/N or cgr= (Nfinal –N)/N example: • in a town of 1000 there are 50 births and 30 deaths with no immigration or emigration cgr= ΔN/N remember ΔN = (b + i) – (d +e) cgr = ((b + i) – (d +e) )/N cgr = ((50 + 0) – (30 + 0) )/1000 cgr = 20/1000 = 0.02

7. Example problem • Complete the following table 20 10 0.33 -10 -2.5 -0.05 580 150 0.005 2 10 0.01

8. Part 2. Population Growth and InteractionsIV) Factors that Affect Population Growth • both abiotic (non-living) and biotic (living) factors limit the growth of a population. • each species has an intrinsic rate of growth if they had unlimited resources and ideal conditions. • the highest possible per capita growth rate (cgr) for a species is called its biotic potential (r).

9. Part 2. Population Growth and InteractionsIV) Factors that Affect Population Growth • factors that affect biotic potential (r) include: • the number of offspring per reproductive cycle. • the number of offspring that survive long enough to reproduce. • the age of reproductive maturity and the number of times an individual reproduces in a life span. • the life span of the individual.

10. Part 2. Population Growth and InteractionsIV) Factors that Affect Population Growth • a species growing at its biotic potential would grow exponentially. • an exponential growth pattern starts with a lag phase followed by a steep increase in the growth curve (J-shaped curve)

11. Part 2. Population Growth and InteractionsIV) Factors that Affect Population Growth • in the beginning the growth rate is slow because there is only a few individuals to reproduce • this is called the “lag phase” • a rapid growth rate follows because the birth rate is greater than the death rate. • under natural conditions this rapid growth cannot be sustained. • when the death rate equals the birth rate the population is in a stationary phase.

13. Part 2. Population Growth and InteractionsIV) Factors that Affect Population Growth • this pattern of growth rate is called S-shaped (sigmoidal) cure and represents a logistic growth pattern. • the straight line running through the curve is the carrying capacity (K) of the habitat • the carrying capacity is the theoretical maximum population a habitat can sustain over an extended period of time.

14. Part 2. Population Growth and InteractionsIV) Factors that Affect Population Growth • the factors that limit a habitats carrying capacity can be: • density-dependent factors • biotic factors • predators, food, water, disease • density-independent factors • abiotic factors • weather, forest fires • combined the factors are called environmental resistance and it prevents a population from growing at is biotic potential.

15. Part 2. Population Growth and InteractionsIV) Factors that Affect Population Growth • organisms have adapted life styles that are suited to the availability of resources in their habitat. • in an unstable environment organisms can reproduce close to their biotic potential (r) • this is called r-selected strategies • species have short life spans • early reproductive ages • reproduce large broods of offspring that have little or no parental care. • this strategy takes advantage of favourable but not long lasting environmental conditions.

16. the other end of the spectrum is k-selected strategies. • few offspring are produced per reproductive cycle. • one or both parents take care of the offspring • offspring take a relatively long time to mature and reach reproductive age • offspring have relatively long life spans and larger bodies. • k-selected strategies are useful for organisms that live close to the carrying capacity of the habitat. • long-term sustained availability of resources.

17. Part 2. Population Growth and InteractionsIV) Factors that Affect Population Growth • most populations have a combination of k-selected and r-selected strategies. • populations can only be described as k- or r-selected by comparing it to another population.