The simple answer to this question is that invertebrates are to be found almost everywhere. They are the most successful living creatures on the earth and have heavily colonised all but the most severely cold habitats at the poles. Unfortunately this very success makes it difficult for those who wish to work with invertebrates, say to compile a species list for a site, as it means that a large number of differing habitat niches need to be addressed and a consequently large number of techniques may be applied to find the animals living in those habitat niches. As a start let us have a short look at a few 'common' habitats within which we are likely to look for invertebrates during our searches:
- Ground Layer:
The 'Ground Layer' is that area extending from a couple of inches below the surface of the ground (or soil) to a couple of inches above. Even this small layer of the environment is composed of many differing habitat types. These can include:
- dry soil or sand
All of these will harbour differing animals, each using the habitat niche to best advantage. Dry soils and sand contain the least number of invertebrates but even here small pieces of dead vegetable matter will support colonies of spring-tails (Collembola), mites, feather-wing beetles, and pseudoscorpions.
Wet sand or soil will see worms, woodlice, centipedes, millipedes, slugs, snails, beetles, nematode worms, and many more invertebrate types crawling through its environment (the increase is directly due to the water - many small invertebrates can not control water loss through their external surfaces and need to maintain a closely regulated humidity within their immediate environment).
A moss layer is often very productive for some of the mid-sized invertebrates as it tends to hold a constant relative humidity for much of the year. As a consequence many beetles, spiders and snails may be found in this habitat niche.
Deciduous leaf-litter provides a vast and nutritious food source for the detritovores. These animals break down the dead-leaves which fall in autumn and via their digestive processes convert the leaves into pieces small enough, wet enough and broken-down enough to be further reduced by bacteria and fungi. Consequently, this layer is the one in which to find the majority of millipedes, woodlice, detritivorous mites, collembola, earwigs and protura as well as a few of the species of detritivorous moth and beetle larvae. The very presence of the large numbers of detritovores also attracts other invertebrates which do nothing to break down the leaf-litter but which are carnivores and therefore hunt and eat the other animals living within the leaf-litter layer. These include, spiders, harvestmen, pseudoscorpions, carnivorous mites and centipedes.
- Low Herbage Layer:
The low herbage layer includes the plant layer from ground level, where the roots enter the soil, to about six to seven inches above the soil surface. This layer sees the most active growth area for the plants with dramatic alterations in structure as the plants sprout, grow and die during the spring, summer and autumn periods. This very rapid strange of structure sees a concomitant alteration in the species distribution within the layer as the growth cycle continues.
Some invertebrate species will be found merely due to the presence of particular plant or herb species as these will be used as food material by the invertebrates concerned. Others, particularly the carnivores, will be present due to certain structural conditions or conditions created by structural differences such as humidity levels, places to hide, the ability to build webs and traps (e.g. Linyphiid spiders), room to achieve a turn of speed to chase prey, etc., and obviously some invertebrates will be found only when the presence of their associated species is at a certain stage in the life-cycle (e.g. chalcid wasp parasites of certain moth and beetle larvae are only present when their hosts are in their larval stage - usually a short couple of weeks).
Dead timber is a very valuable resource for many species of invertebrate and as such is not one to be squandered by the concept of tidiness. Excessive tidiness leads to most of the dead timber stock being burned and this not only wastes a valuable resource but actively destroys all of the invertebrates using the timber at the time it is destroyed. Dead timber is used by large numbers of beetles, and flies as a breeding medium with the timber providing both homes and food for their larvae. In many cases it is not the timber itself which is so valuable as the micro-fungi which inhabit the timber during its breakdown process. Indeed the first couple of years following the death of a tree or a large branch are often the most productive in terms of invertebrate species as both the timber itself, which contains sugar loaded sap, provides food as do the fungi as they start to colonise and breakdown the cellular structure. The animals associated with this habitat are known as saprophagous or xylophagous invertebrates.
The dead wood habitat is a scarce resource which should not be over collected and restraint should always be exercised in its removal. Pieces which are removed can be placed in Owen Emergence traps (see relevant headed section) in which they can remain for upto three years and a succession of insect captured as they complete their life-cycles. Many wardens pile up 'log piles' as a means of tidying a site whilst still maintaining dead wood for colonisation. Unfortunately present research tends to indicate that this is of little use as the wood needs to be of a specific minimum water content to be of use and the higher pieces in such a pile are well below this minimum content level. If possible, as much wood as can be should be left in contact with the ground. Also, do not tidy all wood out of ponds and lakes. Large pieces of water-soaked wood sticking out of a pond, particularly large, whole branches, provide breeding sites for many of our rarer hoverflies and there is a national scarcity of such micro-habitat as the wood is dragged from such water bodies and either placed in log-piles or burnt.
- Tall Herbage Layer:
The tall-herbage layer extends from about 6 inches above ground level up to approximately three feet above ground level and it is in this layer that most of the flowering plants, flowering grasses and tall herbs can be found. Again, this layer is excellent in both content and structure for many invertebrates. Many of the lepidopterous larvae of butterflies and moths inhabit this layer and whilst a few are polyphagous (feed upon a variety of plants) many more feed upon only one species of plant or on members of one family of plants (e.g. the Rosacae). Beetle larvae and herbivorous adults (e.g. weevils) can also exhibit the same tendencies. The very structure of the plant community in this area traps pockets of heat, plant material and air warmed by the sunshine, and these warm pockets allow the metabolism of the invertebrates living here to rise resulting in speedier movement, faster feeding, sorter life-cycles and dramatically increased rates of reproduction. This is the area to find moth larvae, butterfly larvae, sawfly larvae, weevils, flower beetles, wasps and bees, ants, aphids, ladybirds, orb-web spiders and many species of flies.
- Bush / Scrub Layer:
The bush or scrub layer is comprised of plants which can grow up to 12 to fifteen feet in height with small central trunks and twiggy closely set branches which have a tendency to form thick impenetrable thickets. Many of these species have been used by man to form stock-proof hedges, the very self-same habitat which is now fast disappearing as more mechanised farming is ever more frequently used and man-power usage is reduced. All of these scrub bushes flower and all of them develop a complete leaf coverage during the spring and summer months. As a consequence they provide a habitat with many similarities, particularly in structure, to that provided by the tall herb layer but with significant differences in temperature, protein and sugar content of the leaves, toxin content of the leaves, humidity levels and predators. The animals using this layer have developed a number of differing strategies to cope with the hazards of life. Although many live and feed quite openly in the same manner as in the tall herb community others live quite secretively either by becoming nocturnal, coming out at night to feed and breed, or by surrounding themselves with living plant materiel as a protection mechanism. In this latter group are animals such as the larva of the Leopard moth which lives as a caterpillar for two years slowly eating a tunnel through the wood of a trunk or branch of a hawthorn or fruit bush (e.g. apple, pear, etc.). It takes so long to complete its growth due to the poor food value of the wood but does have a very secure habitat away from the majority of predators. Another example are many species of micro-moth whose larvae live between the upper and lower surfaces of a leaf. These feed on the parenchyma cells between the leaf surfaces, slowly eating out a channel which is described as a leaf-mine (some fly and a few beetle larvae also live in the same manner).
- Mid-Canopy Tree Layer:
This layer has the fewest species associated with it as it is the area where there are the fewest leaves to act as food for developing larvae and it is also the area where there is the least available energy as much of the sunshine is shaded out by the leafy canopy layer above. What it does posses is a relatively stable humidity and temperature gradient and this situation is preferred by many of the adult flies, particularly those associated with feeding at flowers, dung and/or hunting other flies as food.
- Upper-Canopy Tree Layer:
This is one of the richest areas for invertebrates, particularly for insects but due to its lack of easy access it is also one of the least studied or understood habitats even in Britain where we have a long history of invertebrate associated work. This area provides vast amounts of food material for herbivorous insects and their larvae in the form of tree leaves, especially newly opened buds with heavy sugar loads and low toxin yields. Many trees carry their flowers up high in the canopy layer and this provides nectar for adult flies, moths, beetles, wasps and bees. All of this is assisted by the highest degrees of available energy from the sunshine which is caught and reflected by the leafy canopy causing very high temperatures to exist in the upper canopy with a resulting speeding up of the invertebrate, cold-blooded, life-cycles. This can mean that a number 2, 3 or even 4, complete generations can be brought into existence during a long, warm summer season.
This upper canopy layer is extremely difficult to sample for invertebrates but a number of methods have been, and are being, developed. These include aerial walkways from which ordinary netting techniques can be employed, aerial trapping techniques (including light-traps, baited traps, malaise traps and interception traps, and the notorious 'fogging' - for information on any of these techniques please see the relevant chapter or sub-chapter in this document).
Water-bodies, small or large, are excellent sites to find invertebrates
as these site remove various constraints imposed on the invertebrate
structure by gravity, the need to control humidity and temperature,
and a potential food supply. Much food is supplied in the form of rotting
vegetation and the accompanying bacterial swarms and this attracts and
sustains a whole food-web of invertebrate and vertebrate life forms.
Methods of extracting these from their environment are given elsewhere.