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Population size and limiting factors
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Anything that constrains a population's size and slows or stops it from growing is a __________ factor.
Somewhere in the ocean, there is a small island. It’s warm, the sun shines there all the time, and there are many rivers full of clean fresh water. Plants bloom all year round, producing fruit and seeds, and no big animals live here. It is truly a paradise for this family of mice! Because they are the same kind and live together in one area, they form a POPULATION.
These mice really enjoy living here. They have a lot of food and feel safe. That’s why their population starts growing... Quite rapidly, in fact. Mice can have several pups every few weeks.
How will they all fit on this island?! No population can grow forever because that could destroy the balance in nature. To keep that balance, the natural world regulates itself. There are things that limit the number of individuals that are born and survive - These are called LIMITING FACTORS. Limiting factors can be weather, water, soil, or sunlight.
Even fires or droughts. These are non-living or ABIOTIC FACTORS. Living things affect population growth too. Examples of these are other plants and animals, or organisms which cause diseases. These are BIOTIC FACTORS. [...] If the sun shines too much on the mice island, and it doesn’t rain at all, it won’t be a paradise anymore.
Plants will dry out, and the mice will have much less food. Some of them will starve, and some will be too weak to have pups. The population will decrease, and only a very few mice will survive. When it starts to rain, the plants come back to life, so the mice can eat. The population starts growing again.
The population of mice depends on how much food there is. This is called BOTTOM-UP population regulation. There is also another type of regulation. If there is a lot of mice on the island it will attract birds like buzzards. The buzzards eat other animals - they are predators.
They will hunt and kill mice, so the population growth will slow down. When predators affect the population of the species they eat, we talk about TOP-DOWN population regulation. Usually, more than just one factor regulates the number of individuals. It’s a combination of different factors, and populations are affected both from the TOP and from the BOTTOM. For example, if too many mice live on the island, they eat up all the food and there won’t be enough left.
At the same time, it will also be easier for buzzards to hunt the mice. Then, the population of mice will drop, and the buzzards are going to fly away to look for other prey. With fewer mice on the island, plants will have a chance to grow again and produce more food. The way these factors balance each other regulates our planet, so that it can be a paradise for many different species.