Written by Ken Thompson
Reviewed by Steve Head
Gardens are no different from any other ecosystem. Plants, and the animals that eat them (and each other) are linked in complex pathways of energy flow between the primary producer plants, herbivores, predators or carnivores and decomposers. These roles of producer, herbivore, carnivore and decomposer are the basic trophic levels that can be identified in all ecosystems, although they can be hard to pin down for some species. Humans for example are herbivores or primary consumers when we eat plants, secondary consumers when we eat herbivores like sheep or chickens, and tertiary consumers when we eat predators, such as fish and crabs.
The way in which energy or biomass from plants pass to herbivores, then small and large predators is often termed a food chain. So your cabbages are eaten by butterfly caterpillars, which are then eaten by birds, which may fall prey to the cat. In practice, these connections are far from simple and interconnect at several levels, so the term food web is more appropriate. This hugely simplified diagram gives just a hint of this complexity.
With so many plant and animal species in gardens, working out food webs is immensely difficult and few people have attempted to do so. Download our leaflet "Food Webs in Gardens" to find more about this fascinating topic.
Energy and biomass flow
The primary producers are plants. Almost all life on earth depends on the photosynthesis of plants, which capture light energy from the sun and carbon dioxide from the air, and make them available to themselves and herbivores in the form of carbohydrates. Plants also need nutrients, the inorganic chemicals that provide other key elements needed to build proteins and all the other chemical of life. The most important nutrients are nitrogen, phosphorus and potassium, followed by calcium, sulphur and magnesium, and other elements needed in tiny quantities. Using energy, plants build their bodies, creating living material often termed biomass.
Plants are rarely able to capture more than 5% of the light energy that falls on them, and this is the first loss of energy in the ecology of gardens. Plants are eaten by herbivores, the primary consumers. Some are large mammals and birds, but in gardens the most important are insects and other invertebrates. However, only a small proportion - about 10% of the biomass and energy captured by plants is taken up by herbivores, and only another 10% of the herbivore biomass is taken up by secondary consumers, the predatory carnivores. So for every 1000 units of plant biomass like broad beans, only 100 gets used by herbivores like aphids, and only 10 units get into predators like ladybirds or lacewings. The next level predators like the birds that eat lacewings get only 1 unit. Sparrowhawks, which eat these birds are top level predators, and get only 0.1 units.
This diagram shows the drop in biomass (or energy) between trophic levels in what is often called a trophic pyramid. It also brings in the links that complete the chain. All creatures die, and all animals produce waste products, and this now non-living material is consumed and broken-up by detritus eaters like woodlice, millipedes and many insects. Decomposers such as bacteria and fungi complete the process of breakdown, turning waste matter back into the nutrients that plants turn back into biomass.
Primary producers - the plants
Plants are the powerhouse of your garden, and generally you can choose the species that grow in your garden. All plants need light, air, water and nutrients but they come in a huge variety. As well as trees, shrubs, perennials and annuals, there are algae growing in your pond and on shaded tree trunks, and lichens on walls and trees. Some plants, such as legumes have symbiotic nitrogen-fixing bacteria in their root nodules, and so can fix atmospheric nitrogen and increase their nutrient supply. Many plants - possibly the majority, have a relationship with mycorrhizal soil fungi in their roots, which make them much more efficient at extracting water and nutrients.
Itís clear that herbivores donít destroy all plant material in gardens (with my cabbages a possible exception). Most plant primary production is recycled as dead material, such as fallen leaves, in the soil and in the compost heap. Nevertheless, there is a great diversity of garden herbivores, many of which are definitely not a problem, and even beneficial. The principal groups of herbivores are:
∑ Slugs and snails
∑ Sucking insects: True bugs, including aphids, spittle-bugs and shield bugs
∑ Chewing insects: Beetles, sawflies and caterpillars of butterflies and moths.
∑ Leaf miners and galls: insects living inside leaves or within plant overgrowths.
∑ Pollinators: Bees, hoverflies and other insects that take nectar (and pollen) are beneficial herbivores, although some (like aphid-eating hoverfly larvae) may have a different way of feeding as larvae.
Read more about the range of herbivores in our leaflet Garden Herbivores (download on the left)and find out about individual groups of herbivores in our pages on Garden Wildlife.
Predators and parasites
Garden predators are high up the trophic pyramid, so that they are less numerous and add up to less biomass than herbivores or detritus eaters, but they are by far the most diverse group of animals in the garden. By eating other animals, predators get very concentrated high quality food, and can afford to put a lot of energy into getting it, or surviving without feeding frequently. Many predators, have evolved very specialist cunning techniques to catch food. This perhaps explains the diversity of predators, compared with herbivores that have to spend most of their time eating, because plant leaves are not very nutritious. The main groups of garden predators are:
∑ Vertebrates: Most garden vertebrates are predators.
∑ Predatory bugs: Not all bugs suck plant juices, many do the same to herbivores and can be useful pest controls.
∑ Parasitoids: Crucially important and diverse insects that lay their eggs in other insects, and provide natural (and commercially available) pest control.
Download or Garden Predators leaflet (on left) for an introduction to these animals, and look in the Garden Wildlife pages for more about individual groups.
Detritivores and decomposers
Plants may be the primary producers, but gardens and the whole world would be knee-deep in dead matter without the countless usually tiny organisms that break down dead material and waste. As ever, the boundaries between trophic levels can be blurred, and some herbivores, such as snails and slugs eat dead or dying plant material as well as living. The main groups of detritivores and decomposers are:
∑ Bacteria, fungi and actinomycetes - which do the bulk of decomposing
∑ Nematodes and springtails
∑ Woodlice - not insects but terrestrial crustacean
∑ Beetles and flies
Find out more about these important creatures in our leaflet Detritivores and decomposers, and in the Garden Wildlife section. Look at Fertility and Compost to see how gardeners can make good use of their garden decomposers.