E7 Summary

Stage 2 Biology – Ecosystems Notes

 

E7. Within communities different species use different reproductive strategies

 

·         Not all species have the same capacity for reproduction

o   Some bacteria have a generation time of 20 minutes, and can give rise to 16 million offspring in 8 hours = exponential growth rate

o   If bacteria grow continually at this rate they would take over the Earth, however their growth is limited by resources – food, nutrients, water, oxygen and accumulation of wastes

o   Also temperature, predators and fire can affect populations

o   Reproductive strategies are a result of natural selective pressures acting over long periods of time

·         Range of reproductive strategies

o   r-selected

§  Opportunistic strategies

§  Grasses, weeds, sea stars, oysters, cockroaches

§  Short life cycle (ie. germinating, growing and setting seed quickly to maximise chance of survival with infrequent rainfall)

§  Put a lot of effort into reproduction

§  Exponential growth rate

§  Growth rate is not sustainable

§  Resources in the environment control their numbers – when conditions are right they flood the habitat with millions of offspring

§  Produce many small young that mature rapidly and are given minimal care

§  Often occur in disturbed communities

§  Often little competition

§  Tend to boom or bust – flourish and exploit favourable conditions and then die back

o   K-selected

§  Equilibrium strategies

§  K stands for carrying capacity – maximum population density that the environment can support indefinitely; depends on resources and interactions with other species

§  Have adaptations to increase their ability to maintain the population size close to K

§  Each couple only has a small number of offspring that are nurtured for long periods of time

§  Young are larger and they have slower maturation periods

§  Reproduce over a longer timespan

§  Occur as communities stablise

§  Examples – vertebrates such as lizards, kangaroos, koalas

§  In good times, the numbers rise above K, but fall back as the numbers are kept in check by the environment

o   On the continuum

§  The two reproductive strategies are extremes, and most organisms fall somewhere between the two

§  Eucalyptus and acacias produce large quantities of seed, but wait until they are relatively well established before beginning reproductive cycles