In a habitat such as a wood or field the ground is covered by plants. Occasionally an area of bare soil appears. This could happen after rain when part of a grassy bank slips away from tree roots that were holding it in place. It also happens after fire has raged through an area of vegetation. However the bare soil appears it is good news for plants because it gives them a chance to grow.

Many plants are perennial they live and grow for many years. If they are at the edge of an area of bare soil they will slowly grow into it. Some plants have a quicker method of colonising bare soil. These plants are annuals. They have small light seeds which can be blown through the air. When the seeds land on the soil they take in water and germinate quickly. In fact compared to perennials everything about an annual is quick. It grows quickly, produces flowers and seeds and dies. The whole life of an annual is over in less than a year. Some plants only live a few weeks. The seeds these plants produce do not wait for the following year to germinate. They germinate quickly and the plants grow and complete their life cycle too. There may be many generations of these plants in a year.

You can think about how annuals and perennials compete in the following way. Imagine an area of bare soil surrounded by fifty perennial plants. If the bare soil was exposed early in the year the perennials may send out side shoots to slowly grow into it and produce seeds later in the year. Now imagine that at the time the soil was exposed the seeds of fifty annual plants were blown onto it. Each seed germinated quickly and in a few weeks the new plants grew to full size. Each plant produced twenty seeds which then fell into the soil. The total number of seeds from the annual plants is found by multiplying fifty by twenty. When you make this calculation you find that a thousand seeds have been released into the bare soil. If each one germinated and grew to full size you can see how the annuals would fill up the soil and keep the perennials out.

A disadvantage with the annual's life style is that the plants may grow too close together. When this happens the roots compete for the same amount of water and minerals in the soil and the leaves may overlap each other and not receive enough sunlight to make all the food the plants need. Each plant is competing with its neighbours for the things it needs to survive and this results in some plants losing out in the competition and dying. The space left by the dead annuals gives the perennials a chance to colonise the soil. In time there may be a mixture of annuals and perennials living in the soil.

Although annuals have to be strong competitors to survive there are many plants, such as the poppy, which have this life cycle and are found growing in a wide range of habitats.