Population size and ecosystems

Question 1

How are ecosystems dynamic?

Answer 1

• The intensity of energy flowing through the ecosystem varies
• Biological cycles, such as the nitrogen cycle, vary the mineral availability
• Habitats change over time as succession occurs
• New species arrive and some species are no longer present

Question 2

What does the size of a population at a particular time determined by?

Answer 2

• Birth rate, or natality, which also refers to any ways living organisms increase their numbers
• Death rate, or mortality
• Immigration
• Emmigration

Question 3

Strategies for population growth used by fugitive species (those that are poor at competition)

Answer 3

• They rely on a large capacity for reproduction and dispersal to increase their numbers
• They invade a new environment rapidly, e.g. algae colonising bare rock, rose bay willow herb colonising soil cleared by fire

Question 4

Strategies for population growth used by equilibrium species

Answer 4

• Control their population by competition within a stable habitat
• Their usual pattern of growth is a sigmoid (S-shaped) curve called the one-step growth curve
• It is seen, for example, when bacteria are put into fresh nutrient solution or when rabbits are newly introduced to an island

Question 5

One step growth curve:

1. The lag phase

Answer 5

• Initially, the population does not increase but then there is a period of slow growth
• In bacteria, it may last from a few minutes to several days
• It is a period of adaptation or preparation for growth, with intense metabolic activity, especially enzyme synthesis
• In sexually reproducing organisms, such as rabbits the lag phase represents the time for individuals to reach sexual maturity, to find a mate and gestate their young