Lesson 1 What affects Population Size?

1. Lesson 1 What affects Population Size?

2. Explain the significance of limiting factors in determining the final size of a population. • Explain the meaning of the term carrying capacity. • Describe predator-prey relationships and their possible effects on the population sizes of both the predator and the prey.

3. A fable: • A ruler of India was so pleased with one wise man, who had invented the game of chess, that he offered him a reward of his own choosing.

4. The wise man, who was also a mathematician, told his master that he would like just one grain of rice on the first square of the chess board, double that number of grains of rice on the second square, and so on: double the number of grains of rice on each of the next 62 squares on the chess board.

5. This seemed to the ruler to be a modest request, so he called for his servants to bring the rice. How surprised he was to find that the rice quickly covered the chess board, then filled the palace! How many grains of rice is this? • The number of grains of rice on the last square can be written as "2 to the 63th power", or 263 Which can be written as approximately: 18,446,744,070,000,000,000

6. A grain of rice is approximately 4 mm long. The grains of rice, placed end-to-end, would stretch approximately 60,000,000,000,000 miles. How far is that? Alpha Centauri, the nearest star, is located 25,000,000,000,000 miles from Earth. Placed end to end, these grains of rice would reach farther than from the Earth, across space to the nearest star, Alpha Centauri, and back to Earth again!

7. How does this apply to living populations? • Organisms cannot go on multiplying at the same rate as the grains of rice on the chess-board, because of limiting factors. • The growth of a bacterial culture can be represented by a curve that consists of four stages or phases: • Lag phase - growth and reproduction are just beginning. • Log phase - reproduction is occurring at an exponential rate. If this continued, like the grains of rice on the chess board there would soon be astronomical numbers. • Stationary phase - environmental surroundings and food supply cannot support any more exponential growth. Death and reproduction rates balance out. • Death phase - when all of the nutrients have been exhausted, or toxic waste products reach sufficiently high levels, the population dies off.

8. Exponential Population Growth • A simple story about exponential population growth can show why environmental problems sometimes appear so suddenly. • Water hyacinth is a floating plant that has spread from South America to waterways around the world. It can cover the water so completely that it obstructs the movement of boats.

9. Imagine a lake that is 10 kilometres in diameter. It takes eight billion hyacinth plants to cover a lake of this size completely. To start with, our lake has no water hyacinth. Then we introduce one hyacinth plant onto the lake. After one month, this plant forms two plants. After another month the two plants have multiplied to four (see graph 1), and the doubling continues month after month. Two years pass, and the hyacinths have multiplied to 17 million plants. Nobody pays attention to them because 17 million plants cover only 0.2 per cent of the lake. • Six months later, 30 months after we put the single plant on the lake, there are one billion hyacinth plants, which cover about 13 per cent of the lake (see graph 2). Now people notice the hyacinths. Although there are not enough hyacinth plants to be a problem for the movement of boats, some people are worried. Other people say, ‘Don’t worry. It took a long time to get this many hyacinths. It will be a long time before there are enough to cause a problem.’ Which people are right? Is the problem a long time in the future, or will there be a problem soon? Author: Gerald G. MartenPublisher: Earthscan Publications

10. In fact, with hyacinth doubling every month, the lake will be completely covered after only three more months. Author: Gerald G. MartenPublisher: Earthscan Publications

11. This is a true story. Water hyacinth has become an uncontrollable nuisance in many places, including the world’s second largest lake - Lake Victoria in East Africa - where fish from the lake are a major source of animal protein for millions of people. • Parts of Lake Victoria are now so badly clogged with water hyacinth that fishing boats cannot move through the water. • Thousands of fishermen are out of work, and the supply of fish has declined drastically.

12. Populations are dynamic. Two events increase populations: • Birth + Immigration Two events decrease populations: • Death + Emigration

13. The balance between population growth and decline is determined by biotic and abiotic factors.

16. Predation • Predator: an animal which hunts other animals (prey) for food.

17. All forms of organisms which feed on living things can be considered predators, although they don’t all hunt and kill their prey: • Carnivores • Herbivores • Omnivores

18. Parasites • Pathogens Exceptions include scavengers, detritivores, and decomposers (which feed on dead things)