Veterinary Epidemiology and Zoonosis

REGULATION OF POPULATION SIZE

Balance of Nature

• Stability of animals and plant populations.
• Populations growth reach a certain size and then stop growing.
• The population becomes stable and balanced with the rate of reproduction equaling the death rate.

Control of population size by competition

• The populations are brought into balance by competition for the resources of the habitat especially food.
• Competition therefore is density dependent. Example: Reproduction of Ascaris species is density dependent.

Dispersal

• In some parts of the world, there may be dramatic variations in climate.
• An Australian species of grasshopper overwinters in its egg. The warmth of spring causes the eggs to hatch. The adults that develop then lay eggs as long as the weather is wet. A drought kills all of the adults. This is not density dependent. It occurs long before the competition occurs. Such insects survive only by dispersal over large areas to different climates so that at least some are in an area that are wet.

Predation

• Predation has a role in controlling the size of the population. This is particularly true in the case of small animals.
• Small predators notably insects are efficient controllers of population.
• Predators have been used to control insect pests. Example: Lady birds used to control cotton cushony-scale insects.

Infectious disease

• Infectious diseases are determining and regulating the size of the population. Example: Epidemics.
• Infectious agents can be divided into two groups according to generation dynamics, as Microparasites and Macroparasites.
  • Microparasites: Multiply directly when inside the host increasing the level of parasitism. Example: Viruses, bacteria and protozoa.
  • Macroparasites: Do not increase the level of parasitism, grow in the host but multiply by producing infective stages which are released from the host to infect new hosts. Example: Helminths and arthropods.

Home range

• Certain animals have natural restriction to the area over which they roam, i.e. is their home range.
• This may control the population and has implications for the transmission of infectious diseases.
• Infected animals may transmit infections over their home range but no further. Example: Scrub typhus.
• The small home range of rats (maintenance host) results in mite’s (vectors) life cycle restricted to small area called 'mite islands'. When mites are infected with this rickettsial disease, localized endemic areas of scrub typhus associated with mite islands occur.

Territoriality

• The part of an animal’s home range that defends aggressively from invaders is the animal’s territory. This behavioral response is territoriality.
• Territoriality may control the population.
• The size of the territories varies for the same and for different species.

Social dominance

• In the 1920's, a social hierarchy called the 'peck order' was discovered among birds.
• Some gregarious species especially rodents inhabit at favourable places.
• When crowding occurs, socially weaker sections are forced out. This may be a population control mechanism.

The “Wynne – Edwards” hypothesis

• The population control consequence of territoriality, social hierarchy and behaviour may be just a side effect.
• The Aberdeen zoologist, Wynne - Edwards suggested that population control was the main purpose of group behaviour, which sometimes caused physiological stress.
• The crowding of rats results in associated fighting, cannibalism and reduced fecundity.