The animal kingdom can be split into two main groups: vertebrates and invertebrates. Vertebrates such as mammals, fish, birds, reptiles and amphibians all have a backbone, whereas invertebrates, such as butterflies, slugs, worms, and spiders, don’t. Approximately 96% of all known species of animals are invertebrates. The world as we know it couldn't function without invertebrates. They pollinate plants, help break down and recycle organic material, and provide an important food source for lots of other animals.
Invertebrates can be found in land, marine and freshwater habitats, and live on every continent. They are Earth's biggest source of biodiversity - there are over 1.3 million known species of invertebrates, and potentially millions more yet to be found.
Are most invertebrates insects?
It is a common misconception that all land invertebrates are insects. Lots of different kinds of animals are invertebrates – insects are just one type, or class. There are more than 27,000 species of insect in Great Britain, including bees, butterflies, beetles, flies and dragonflies.
Insects are part of a higher classification group of invertebrates known as the arthropods. Arthropods have segmented bodies, jointed legs and a tough outer exoskeleton. There are well over 3,000 known arthropod species in Great Britain that are not insects. Examples of arthropods that are not insects include spring tails, woodlice, centipedes, millipedes and spiders.
There are also many other invertebrates that are not arthropods and have very different types of bodies. These include snails, slugs and worms that all lack segmented legs and usually have soft bodies.
What do invertebrates do?
What is pollination?
Plants often rely on animals for pollination. Pollination is the process by which pollen is transferred from the stamen (the male part) to the stigma (the female part) of a flower, resulting in the fertilisation of the plant. For many plants, this is how they complete their reproductive cycle, enabling the next generation of plants to be produced. Pollination that occurs within the same plant is known as self-pollination. Although if the pollen is transferred to another plant this is known as cross- pollination. Cross-pollination is generally preferable as it produces plants that are more resistant to disease.
What do pollinators do?
Pollen can be transferred between plants by animals that are in search of food and are attracted to flowers by their bright colours and sweet scents. Whilst feeding they rub against the stamen, allowing pollen to stick to them. When they move on to another flower, the pollen is rubbed off again onto the stigma. Over 200,000 species of animals can act as pollinators. About 5% of this number is accounted for by birds, bats and other small mammals. The rest are all invertebrates!
- Invertebrate pollinators are mainly insects, and include bumblebees, solitary bees, butterflies, moths, flies, wasps and beetles.
- There are at least 1500 species of insect pollinators in the UK.
- One third of the species of crops cultivated for food are insect pollinated; insects are responsible for the production. of many of our favourite foods including strawberries, chocolate, tomatoes, apples and much, much more.
- In addition to food crops, 90% of all wildflowers would become extinct if there were no insects left to pollinate them.
- Pollinators add an estimated $217 billion to the global economy.
- Worryingly, insect pollinators are experiencing a worldwide decline, as the rise in habitat loss, pesticide usage and the changing climate make survival increasingly difficult.
What is the nutrient cycle?
What goes in must come out! The nutrient cycle describes how essential nutrients are continually used, exchanged and recycled between living organisms and the physical environment, which together form an ecosystem.
The nutrient cycle is one of the most fundamental ecosystem processes. Important elements such as nitrogen, phosphorus, hydrogen, oxygen and carbon need to be continually recycled for life to exist. Dead organic matter such as leaves and animal carcasses accumulate on the ground and are broken down by organisms known as detritivores and decomposers. Invertebrates fall into the detritivore category, whereas the term decomposer refers to the smaller fungi and bacteria. The work of these two groups releases nutrients back into the soil, where they can be taken up by plant roots and used again for growth.
Many invertebrates are important detritivores, including earthworms, millipedes, slugs, woodlice, beetles, and springtails.
- Earthworms eat decaying plant and animal matter in the soil. By breaking down this organic material and burrowing through the soil they aerate and improve its composition. An abundance of earthworms is a good sign and means you have a healthy garden.
- Dung beetles feed on the faeces of other animals. Because of this they are very important in agriculture. Consuming dung reduces the level of flies and other pests found on farms, which helps to protect livestock.
- Springtails are the most abundant arthropods on earth: one square meter of soil can contain 60,000 individuals! Springtails commonly help with the breakdown of fungi and dead plants. As their name suggests, springtails are able to jump to great heights when disturbed.
- Woodlice are terrestrial crustaceans, closely related to shrimp and lobster. Like many detritivores, they are usually active at night.
Some invertebrates prey, parasitise, or outcompete species of plant or other invertebrates that we consider pests. By inadvertently helping us to control these populations, invertebrates can remove the need for chemical pesticide usage.
- Many plants suffer from aphids, small bugs that feed on the sap of plants, potentially spreading viruses and harming plant growth in the process. Ladybirds, hoverfly and lacewing larvae all love to eat aphids – a single ladybird can eat up to 50 aphids a day! By releasing ladybirds into greenhouses, growers can help control aphid infestations.
- You might not be keen on sharing your home with spiders but keeping these friendly invertebrates around can help reduce the number of flies in your house.
- Wood ants help protect our trees. In the north of England and Scotland these busy ants predate pest species such as the pine looper moth (Bupalus piniaria) and sawfly (Neodiprion sertifer) that feed on pine needles. By keeping these potentially harmful species in check, the ants help maintain healthy forests.
Food chains and food webs
All organisms need energy to live. A food chain shows the ways that organisms use and transfer this energy between each other within an ecosystem. Organisms can be grouped together according to the different feeding roles they play within a food chain, called trophic levels. Energy flows up a food chain from lower to higher trophic levels. Within an ecosystem there will be many food chains that overlap and interconnect, forming a complex food web. The complexity of food webs means that they can be very fragile and need to be maintained in order to stay balanced.
Invertebrates are a critical part of food webs, representing a source of food for many animals including birds, frogs, fish and hedgehogs. They occupy several different trophic levels and form a vital link connecting plants and leaf litter with larger animals.