Hidden creatures: animals of the leaf litter
Leaf litter is a world of living things, a place of dampness and darkness inhabited by a host of insects, arachnids, mites and other animals. Here we look at those dwellers in the debris of the woodland floor that are easily visible.
Look at the debris on the floor of a well developed piece of mixed deciduous woodland containing oak. Hazel and birch, with perhaps a little rowan and sycamore. In such a wood, on a medium loamywith reasonable and no waterlogging, and with not too much disturbance, you stand a good chance of finding a rich and fascinating assemblage of animals.
The upper few centimetres of the woodland floor contain the most recently accumulated leaves, most of them still easily identifiable. This is the litter layer. The layer, or horizon. Below has much more compacted leaves, bits of which are still recognisable, but most of which are already rotting. This fermenting layer, as it is called, may be several centimetres thick. Under it is a dark brown or blackish horizon-the humus-consisting of fully rotted plant, and some animal, remains. The distinction between the mineral soil and the three organic layers above it is sometimes blurred by plant roots and bits of twigs and stones.
Normally the three layers of the leaf litter harbour most of the visible plant life, but this area is also inhabited by millions of microscopic organisms-bacteria, one-celled animals, plants and fungal threads. These are the living things you cannot see, except with the help of a microscope. Here we describe some of the animals you can see with the naked eye.
Types of leaf litter
The soil below leaf litter, and the trees above it, are both important factors in determining what kind of life can live within it. A well drained sandy soil is drier than one with a lot of clay, and is often acid, harbouring little wildlife in or on it. Under Scots pine on heathlands, for example. The needles may take up to nine years to decay because the acidity prevents the rotting agents, chiefly fungi, from doing their job properly. But a chalky soil, on the other hand. Favours rapid bacterial action, and a great number of small animals can live in it.
The types of leaves are also important. The needles that drop from most conifers give a rather acid kind of humus, but oak, ash and sycamore leaves produce a much less acid medium. Beech, however, which favours a chalky soil, gives a rather acid humus, especially if it accumulates in large masses. However, if it is mixed with the top layers of soil and contains some chalk, it then forms a moister, better rotted type of humus.
Leaf litter hunters
You can usually find a fair number of brown spiders of the Lycosidae family (hunting or wolf spiders) in leaf litter. They do not make webs, instead running down their prey by speed alone. They are often numerous on the floors of drier woodlands, and on warm days can be heard running about over the leaves. Some small web-spinning spiders also make their home in woodland leaf litter.
Only a few species of harvest spiders (harvestmen or Opiliones) – also arachnids – live in litter. Oligolophus tridens is one of the commonest and can be recognised by its dark brown body-saddle, sharply cut off at the back. Under the litter and among the humus, close to the earth, are two or three other species – all with the short legs that are an adaptation to the habitat. Harvest spiders are mainly carnivorous and feed on any small creatures they can capture, or on freshly dead bodies and animal droppings.
There is normally a vast population of mites in leaf litter. Most are visible to the naked eye, but young ones are extremely small. They live throughout all the organic layers. Many feed on fallen leaves and plant debris and thus initiate the process of decay. In this they are helped by such animals as springtails. The process is finished by bacteria and fungi, by which time the material has been covered by new leaf fall and is approaching the state of humus. Eventually, chemical plant foods are released from this decayed matter into the mineral soil and taken up again by plant roots. This recycling is vital to the economy and successful functioning of the ecosystem.
Leaf litter insects
Only a small number of our insect species are true litter dwellers, but one or two kinds can be very numerous there. Among these are some of the most primitive and least known of all the insects, none of them having wings or undergoing complete metamorphosis. They are the springtails (Collembola), proturans (Protura), two-pronged bristle-tails (Diplura) and three-pronged bristle-tails (Thysanura).
Springtails, living in the upper part of the leaf litter, are very active insects, rarely more than 5mm (1/2in) long and with grey or yellowish hairy bodies. At the end of its body a springtail bears a forked tail usually held underneath the abdomen by a small hook. When this tail is released and suddenly moved downwards and backwards, the insect springs vigorously forward-hence its common name. Springtails also have long antennae. They eat fallen plant material on the woodland floor.
Proturans are minute, pale, 1-1.5mm long insects that have no antennae. Their forelegs are held upwards and forwards and perform the same sensory functions as the antennae of other insects. The Diplura, or two-pronged bristle-tails, are also small, normally only 6-8mm long. They are white with long antennae but no eyes. The Thysanura, or three-pronged bristle-tails, have long antennae, compound eyes and scaly bodies. These three latter groups are thought to be mostly saprophytic in habit. That is, they feed on dead and decaying plant remains.
Beetles are also found in leaf litter. Some, like the pine weevil, use this habitat as a hibernating place, as do some ladybirds. Ground beetles and rove beetles, on the other hand, are permanent residents. Some are ‘carnivorous while others feed on decaying plant remains or on roots and fruits. One of the most conspicuous carnivores is the violet ground beetle which is 22-28mm (about lin) long and dark violet-black. This insect is also frequently seen in gardens.
Flies, and especially their legless larvae, frequently appear in leaf litter. Most are very small and live in the damper parts, usually in the less acid types of leaf litter. They are an important part of the food of such predators as beetles and centipedes.
Other litter dwellers
Woodlice. Despite their name, normally have little association with wood: they feed on leaf litter, seedlings, fungi and occasionally ‘dung, and are decomposers helping, along with millipedes and other organisms, to break down dead plant remains.
Millipedes, familiar as long-bodied, many-legged creatures, also make their homes in leaf litter. They are exclusively vegetarian and most species probably prefer living to dead plant material. Centipedes, on the other hand, are carnivorous and eat any living creature they can catch. Neither they, nor the wood-lice and millipedes, have a waxy layer in their skins as true insects do and so they are liable to dry up. They must therefore remain in fairly moist places, emerging only at night when the air is damper.