Foodwebs and feeding roles

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 plants as primary producers, herbivores which eat plants, predators or carnivores which eat animals and the vital decomposers which return dead material back into the food web. 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.

What passes through a food web is both biomass - the organic material of which all life is made, and energy.  Energy is created from the breakdown of digested and metabolised biomass, and is used to move, keep warm, and make the chemical reactions of life happen.  Surplus biomass not broken down for energy is used to build the living body (or offspring) or can be stored as reserves (usually fat) for hard times.

Food chains and webs

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.
Foodwebs and feeding roles

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 plants as primary producers, herbivores which eat plants, predators or carnivores which eat animals and the vital decomposers which return dead material back into the food web. 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.

What passes through a food web is both biomass - the organic material of which all life is made, and energy.  Energy is created from the breakdown of digested and metabolised biomass, and is used to move, keep warm, and make the chemical reactions of life happen.  Surplus biomass not broken down for energy is used to build the living body (or offspring) or can be stored as reserves (usually fat) for hard times.

Food chains and webs

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 living together in gardens, working out their food webs is immensely difficult and few people have attempted to do so. Find out more about food webs in our leaflet Garden Food Webs


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 chemicals of life.  The most important nutrients are nitrogen, phosphorus and potassium, followed by calcium, sulphur and magnesium, and many other trace elements needed in tiny quantities.

Using energy, plants build their bodies, creating living material or 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 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 such as 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.
With so many plant and animal species living together in gardens, working out their food webs is immensely difficult and few people have attempted to do so. Find out more about food webs in our leaflet Garden Food Webs


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 chemicals of life.  The most important nutrients are nitrogen, phosphorus and potassium, followed by calcium, sulphur and magnesium, and many other trace elements needed in tiny quantities.

Using energy, plants build their bodies, creating living material or 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 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 such as 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 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 - in fact the majority - have a relationship with mycorrhizal soil fungi in their roots, which make them much more efficient at extracting water and nutrients.

Find out more about garden plant ecology in our leaflet Garden Plant Ecology and find lots of advice about using plants in the garden in our Plants and Planting section. Gardens contain wild plants too, and you can find more about them in the Plants section of Garden Wildlife.

Herbivores

It’s clear that herbivores don’t 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 and find out about individual groups of herbivores in the Garden Wildlife section of this website.

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 and 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: Many garden vertebrates are predators or take at least some animal food.
  • Predatory true bugs: Not all bugs suck plant juices, many do the same to herbivores and can be useful pest controls.
  • Beetles
  • Centipedes
  • Wasps
  • Dragonflies
  • Spiders
  • Parasitoids: Crucially important and diverse insects that lay their eggs in other insects, and provide natural (and commercially available) pest control.


See the leaflet Garden Predators for an introduction to these animals, and look in Garden Wildlife 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 crustaceans
  • Beetles and flies
  • Earthworms

Find out more about these important creatures in Detritivores and Decomposers, and in the Garden Wildlife section.  Look at the Compost and Fertility page to see how gardeners can make good use of their garden decomposers.

pdf leaflets you can download related to this page:

    Garden food webs
    Garden plant ecology
    Garden herbivores
    Garden predators
    Detritivores and decomposers

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 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 - in fact the majority - have a relationship with mycorrhizal soil fungi in their roots, which make them much more efficient at extracting water and nutrients.

Find out more about garden plant ecology in our leaflet Garden Plant Ecology and find lots of advice about using plants in the garden in our Plants and Planting section. Gardens contain wild plants too, and you can find more about them in the Plants section of Garden Wildlife.

Herbivores

It’s clear that herbivores don’t 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 and find out about individual groups of herbivores in the Garden Wildlife section of this website.

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 and 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: Many garden vertebrates are predators or take at least some animal food.
  • Predatory true bugs: Not all bugs suck plant juices, many do the same to herbivores and can be useful pest controls.
  • Beetles
  • Centipedes
  • Wasps
  • Dragonflies
  • Spiders
  • Parasitoids: Crucially important and diverse insects that lay their eggs in other insects, and provide natural (and commercially available) pest control.

See Garden Predators for an introduction to these animals, and look in Garden Wildlife 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 crustaceans
  • Beetles and flies
  • Earthworms

Find out more about these important creatures in Detritivores and Decomposers, and in the Garden Wildlife section.  Look at the Compost and Fertility page to see how gardeners can make good use of their garden decomposers.

Leaflets related to this page:

    Garden food webs
    Garden plant ecology
    Garden herbivores
    Garden predators
    Detritivores and decomposers

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