D. Survivorship and Selection Type

1. Survivorship curves:

tell us how long individuals survive in a population

type I

species with a high survival rate of their young

most individuals are expected to die only when old

eg. humans

type II:

- species in which individuals die at a constant rate from hunting, disease, etc.

- eg. squirrels, bees, most reptiles

type III:

- species with most individuals die when young

- many babies are born, but few survive very long

eg. tobacco, salmon, oysters

 

                   

                   

                   

                   

                   

                   

  

1000

100

10

0

Num

ber of survivors [log scale]

0 20 40 60 80 100 Percent of life span

Type I

Type II

Type III

2. ‘r’ and ‘K’ selection

a. ‘r’ refers to species whose population is well below the carrying capacity and so can still grow exponentially with a rate ‘r’

b. ‘K’ refers to populations that are almost at the carrying capacity [K]

c. ‘r’ and ‘K’ refer to different strategies that will ensure the survival of the species

d. the environment, size of the organism, number & size of offspring, maturity, life expectancy and frequency of reproduction all influence this

‘r’ strategy

unstable environment, density independent

small size of organism

energy used to make each individual is low

many offspring are produced

early maturity

short life expectancy

each individual reproduces only once

 

                   

                   

                   

                   

                   

                   

  

1000

100

10

0

Num

ber of survivors [log scale]

0 20 40 60 80 100 Percent of life span

Type III

‘K’ strategy

stable environment, density dependent

large size of organism

energy used to make each individual is high

few offspring are produced

late maturity, often after a prolonged period of parental care

long life expectancy

individuals can reproduce more than once

 

                   

                   

                   

                   

                   

                   

  

1000

100

10

0

Num

ber of survivors [log scale]

0 20 40 60 80 100 Percent of life span

Type I

Type II

handout;

work on parts B and C