RHS Journals
The Garden
May 2008
Predators and parasites
Life-and-death dramas are played out daily in your garden, where a multitude of predators, particularly insects, stalk their unwary prey. Ken Thompson goes to the front line in the fifth part of his series on gardens as ecosystems
They may not, in terms of numbers of individuals, be as numerous as the decomposers or plant eaters, but predators are the most diverse group of animals in the garden ecosystem. Catching other animals to eat is (usually) more difficult than feeding on static plants, but the rewards are great. Animal tissue is protein rich, high energy and much easier to digest than plants, so a wide range of species have adapted to exploit other animals, either by catching and eating their prey (predators) or by ‘siphoning off’ nutrients while the host is still alive (parasites). The victims of both types in any ecosystem are usually the far more numerous herbivores and detritovores, but small predators themselves are often eaten by larger ones.
The majority of garden vertebrates (animals with backbones) are predators, including frogs, toads, slowworms, hedgehogs, foxes, moles, bats and most birds. The majority are unfussy, opportunistic feeders: if it moves and will fit in their mouths, it is regarded as prey. Should your garden support thriving populations of some of these larger animals, it is a sure sign that the rest of the ecosystem is functioning smoothly.
Much has been written elsewhere on garden vertebrates, so here I want to concentrate on the smaller, more numerous invertebrate predators.
Small but deadly The invertebrates (animals without backbones), especially insects, show the real diversity among garden predators. The needle-like mouthparts of bugs are just as good for sucking the juices out of animals as plants, and many bugs are predators, including the wonderfully named ‘assassin bugs’. Common flower bug (Anthocoris nemorum) is the most abundant predatory bug in gardens, feeding on aphids and spider mites.
Terrestrial bugs are easily missed, but not aquatic ones: pond-skaters patrol the water surface, attacking anything careless enough to fall into the water, while water boatmen are fearless predators below the surface, tackling anything up to small fish. Both groups can rapidly colonise a new pond as, although they are aquatic, they both fly well.
Ponds are often one of the most productive parts of the garden, which is reflected in the abundance of predators that can often be found in even a small pond. Some are more or less permanent residents, but others, such as dragonfly nymphs, are present only as juveniles.
Few garden visitors are as charismatic as adult dragonflies, but most of their life cycle is spent underwater as aquatic nymphs; like the adults, these are large (several centimetres long) and fiercely predatory. Tadpoles and small fish are among their favoured prey.
Great diving beetles reach 4cm (1 1/2in) long with large larvae known as water tigers, and either will eat anything that moves, including frogs, newts and fish much larger than themselves. Their land-based relatives, carabids or ground beetles, are among the most important predators in the garden, both adults and larvae. Rove beetles (staphylinids) and ladybirds are also common predatory beetles.
As with herbivores, it is the larvae of predators that do most of the growing, and so do most of the eating. Learn to recognise the larvae of predators such as ladybirds and hoverflies: they eat aphids and other pests so should be welcomed.
Striped assassins
Wasps are another group of largely predatory insects. Like bees, wasps may be social or solitary. Mainly we notice the larger, social species, but there are huge numbers of solitary wasp species. Social wasps can be an annoyance but they are important predators, particularly of other insects. Some solitary wasps look like social species, but most are small and less-brightly coloured.
Most adult wasps eat pollen and nectar, but they catch prey to feed to their young. Social wasps will catch anything, but solitary wasps tend to specialise on one group – caterpillars, beetles, moths, flies, aphids, spiders, bugs… even other wasps. Nests are made in the same places as solitary bees (hollow plant stems, dead wood or in the ground). They are stocked with prey the wasp’s sting has paralysed and then an egg is laid on top.
Some wasps are ‘kleptoparasites’ (literally, parasites by theft) and lay their eggs in the nests of other solitary bees or wasps. On hatching, the young parasite eats the rightful owner, and usually its stored food, too.
As varied, odd and fascinating a bunch as garden predators already are, there may be even more diversity in future. Not far away in Europe lurk species of scorpion and praying mantis; climate change may mean one day we will have them here, too. If that sounds disquieting, remember even the few predators that can bite or sting us usually do so only if threatened. Garden predators do a fine job, keeping the ecosystem in balance by controlling the populations of herbivores trying to make a meal out of our plants. They are to be valued.
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