The water hyacinth story is about floating plants on a lake, but it is also about the human population on planet Earth. Thousands of fishermen are out of work, and the supply of fish has declined drastically. Parts of Lake Victoria are now so badly clogged with water hyacinth that fishing boats cannot move through the water. 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. 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? In fact, with hyacinth doubling every month, the lake will be completely covered after only three more months (see Figure 2.3).įigure 2.2 - Water hyacinth population after 30 monthsįigure 2.3 - Exponential growth of the water hyacinth population It took a long time to get this many hyacinths. Although there are not enough hyacinth plants to be a problem for the movement of boats, some people are worried. 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 Figure 2.2). Nobody pays attention to them because 17 million plants cover only 0.2 per cent of the lake. Two years pass, and the hyacinths have multiplied to 17 million plants. After another month the two plants have multiplied to four (see Figure 2.1), and the doubling continues month after month. After one month, this plant forms two plants. Then we introduce one hyacinth plant onto the lake. To start with, our lake has no water hyacinth. It takes eight billion hyacinth plants to cover a lake of this size completely. Imagine a lake that is 10 kilometres in diameter. It can cover the water so completely that it obstructs the movement of boats.įigure 2.1 - Growth in the water hyacinth population during the first four months after introducing a single hyacinth to a lake Water hyacinth is a floating plant that has spread from South America to waterways around the world. It also explains how negative feedback restrains the population of every species in a biological community within limits that the ecosystem can support.Ī simple story about exponential population growth can show why environmental problems sometimes appear so suddenly. This chapter explains how positive feedback causes populations to increase rapidly when there is a surplus of resources. People interact directly or indirectly with populations whenever they interact with ecosystems. Positive feedback is responsible for the sudden appearance of environmental problems and many other rapid changes in the world around us.Īn ecosystem’s biological community consists of populations of every species of plant, animal and microorganism in the ecosystem. All ecosystems and social systems have hundreds of negative feedback loops that keep every part of the system within the bounds necessary for the whole system to continue functioning properly. Feedback is the effect that change in one part of an ecosystem or social system has on the very same part after passing through a chain of effects in other parts of the system. Why do environmental problems sometimes appear so suddenly? The explanation lies with positive and negative feedback - powerful forces that shape the behaviour of all biological systems from cells to social systems and ecosystems.
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