July 2022

As in the plant world, there are many forms of succession among animals. We often hear about the pyramid of numbers. The green world of plants supports a myriad of insects, such as aphids, grasshoppers and caterpillars. Their numbers go into the quadrillions on the planet. These in turn are eaten by predatory invertebrates such as ladybirds, wasps, spiders, and also vertebrates such as insectivorous amphibians, reptiles, birds and mammals. All invertebrates that fly are prey for animals that never land to feed such as dragonflies, swifts and bats. These in turn are food for predatory birds such as falcons which can be considered as top predators at the peak of this pyramid whose numbers are normally regulated by by food abundance. However, disease and sometimes poisoning or shooting of raptors by humans can disrupt this regulation.

It is generally believed that animal numbers are naturally controlled by food abundance and not by predators. However, if predators are removed by artificial human tactics, the prey organisms can experience a population explosion such as can occur if too many foxes are hunted, resulting in an abundance of rabbits. This in turn results in rabbits damaging farmers food crops, and also important species rich plant communities such as the chalk grassland of our own South Downs National Park. This is called a trophic (feeding) cascade which runs through and disrupts different levels of the pyramid of numbers.

A balanced ecosystem is one where predators and prey live along-side each other, just as humans live alongside chickens, sheep, pigs or cattle. The lions of the Serengeti in Africa live alongside abundant antelope and only eat when hungry, and then usually take weakened individuals that are easy to catch. In my garden, grass snakes live alongside the frogs that they eat, which mainly feed on insects and reproduce in my pond.

In Britain, we do not have sufficient balanced ecosystems to maintain our indigenous biodiversity. Habitat loss and degradation due to over exploitation for recreation or commercial gain disrupts too many components of our pyramids of numbers. We can contribute to reversing this trend by campaigning, or managing, our own local environments.