Veterinary Epidemiology and Zoonosis

DISTRIBUTION OF POPULATION

Vegetational zones

• Botanists were the first to note the division of earth into different vegetational zones.
• Early 18th century naturalists suggested that the world was divided into discrete formations of vegetations such as tundra, savannah and desert. These apparent neat formations were first explained by de Candole (1874) who argued that climate particularly temperature dictated vegetation. He drew the first vegetational map based on isotherms.
• At the beginning of the 20th century Koppen used de Candole’s classification as the basis for modern system, which provides good correlation between climatic and vegetational regions.

Biomes

• Zoologists attempted to classify different areas of the world according to types of animals and plants that were present because distribution of animals appeared to be related to vegetation.
• Merriam (1893) defined life zones in North America after studying the distribution of animals and plants at various altitudes on North American mountains.
• Merriam like de Candole thought that temperature governed the distribution of animals.
• These life zones are now commonly called biomes. Example: Tropical rain forest, savannah, tundra each with its own particular range of plants and animals.
• Distribution of infectious agents and their vectors and the disease may be limited by the environmental conditions of the biomes. E.g. Fungus Coccidioides immitis limited to areas characterized by hot summers, mild winter, and alkaline pH soil. Rift valley fever in West African ecological zones (Mosquito vectors in these zones)