One characteristic of living things is that they can reproduce to create new organisms similar to themselves and thereby continue as a species through time even though individuals die. Without the ability to reproduce, the species would become extinct.

Asexual reproduction
Some simple organisms like bacteria can reproduce asexually. This involves the growth of the cell, duplication of the nucleus (see Animal cells) and then division into two new cells. Some changes (mutations) can occur to the genetic information during nuclear division, but generally the new individuals are the same as the "parent".

Growing plants from runners or cuttings is a form of asexual reproduction as only one "parent" is involved, and no gametes are produced. The "offspring" are the same as the parent.

Similarly, cloning uses genetic information from only one parent, so the offspring will be identical to that parent.

Sexual reproduction
Most organisms reproduce sexually. This always involves the fusion of two different types of gametes (eggs and sperm).

The offspring therefore carry a mixture of characteristics from both parents (see Inheritance). This increases biodiversity, or the number of different types in a population, some of which may survive better. If an individual survives better, it has a better chance of reaching reproductive maturity and having more offspring which will also inherit this advantageous feature (see Species and Evolution).

Number of young
Humans generally only have one or two babies at a time. This reflects the amount of care needed before the child is capable of surviving alone.

Fish, however, produce thousands of eggs and babies, but the chance of any one of these surviving is very low. Without parental protection, they are likely to be eaten by predators, hence large numbers of young are required to ensure enough survive to reproduce.

Reproductive rate
Many animals like kangaroos and rabbits will not reproduce if the environmental conditions are poor, such as when there is no food due to drought. This reduction in reproduction obviously helps the survival chance of the young.

Endangered?
If the life cycle is interrupted so that the birth rate falls below the death rate, then the population numbers will drop. This could be caused by reduction in nesting sites due to logging, reduced food, disease, increased predator numbers, or poor environmental conditions. The following terms reflect the degrees of population decline.

• Vulnerable: Population likely to decline if environmental conditions change unfavourably.
  
  
• Threatened: Population likely to continue to decline due to altered environmental conditions.

Endangered: Numbers in nature considered too low to avoid extinction without Man's intervention through conservation management, for example, tiger quoll, helmeted honeyeater, giant panda. Extinct: No known individuals still alive of a species once present on Earth, e.g. thylacine, dinosaurs (see Too late: Extinctions).