An Introduction to Invertebrates:
Invertebrates is a very broad term which covers all animals which do not have backbones and consequently all those organisms which move, ingest food, respire and reproduce but which do not have an internal supporting skeleton. This gives a very wide coverage of the animals in the world ranging from single cell protozoa up to the more commonly known insects and crustaceans (i.e. moths and crabs), and including animals such as jellyfish and slugs. The size range covered is from approximately one one-hundredth of a millimetre right up to almost 2 metres across. All invertebrates and all mini-beasts are animals. They all have the same life processes and all have similar life requirements though obviously the habitat niches to which they have adapted are wide and varied.
(It must here be realised that the current use of the term 'mini-beast',
generated as a means of popularising the lower orders of animals, has
confused the issue somewhat as many of the mini-beasts known to children
are vertebrates (e.g. frog, toad, newt, lizard, snake, etc.). There
is a need to be careful when using the term 'mini-beast' to ensure that
the children are not confused by the 'vertebrate - invertebrate' distinction.
Vertebrate mini-beasts will not be further referred to in these notes
- for further information on these animals see the relevant Consult-Eco
document ('Teacher's Notes - Vertebrate Mini-Beasts')).
The basic groups/orders of invertebrates can be shown as follows:
Protozoa: Microscopic animals most of which live surrounded by water (either salt or fresh). There are a few which live within clumps of moss and encyst to prevent desiccation when drought arrives (e.g. water-bears (tardigrada)).
Cnidaria: This group encompasses the hydroids (hydrozoa), jellyfish (siphonophora) and sea-anemones (anthozoa). All are distinguished by having stinging cells called nematocysts. These are cells which propel poison barbs into their prey prior to ingestion.
Platyhelminthes: Flatworms which live in water, both salt and fresh, which move along the bottom with a gliding motion due to the action of thousands of cilia on the underside. The animals can change shape via muscle contraction and in most cases can regenerate new animals if the original is cut in two.
Annelids: The segmented worms. These include many marine worms (e.g. ragworm, lugworm, sea-mouse, etc.) as well as the those more commonly met by the children - the terrestrial worms (lob-worm, brandling, etc.)
Molluscs: These animals have occupied many niche habitats; marine, freshwater and terrestrial. They include cockles and mussels, whelks and winkles, limpets and chitons, sea-slugs, snails and slugs. The vast majority have an external shell made of secreted calcium carbonate (Chalk) but a few of the terrestrial slugs have a reduced internal shell and some have taken this evolutionary direction further by losing the shell altogether (sea-slugs have gone down the same route and have also lost their shells).
Cephalopods: These are all marine invertebrate animals. They include the octopuses, cuttlefishes and the squids. Some of these animals have an internal shell (e.g. the cuttlefish bone available in pet-shops for cage-birds is the internal shell of the cuttlefish).
Echinodermatids: The starfish, brittlestars and the sea-urchins; all are exclusively marine animals. They display radial symmetry of body form with a central mouth.
Arthropods: Crustaceans This grouping includes lobsters, crabs, shrimps, woodlice and barnacles and is characterised by the possession of two pairs of antennae and at least five pairs of legs. They have calcareous outer shells which are segmented.
Arachnids: These are the spiders, harvestmen, scorpions, king-crabs, mites and ticks. They are distinguished by the body normally being divided into two (a cephalothorax (head & thorax combined) and an abdomen) as with spiders. However, harvestmen have combined all three body parts (head, thorax and abdomen) into one single unit. There are four pairs of legs and no antennae.
Insects: An extremely successful group of life forms with somewhere approaching 2 million species world-wide and with an estimated further 2 million undiscovered; particularly within the tropical rainforest areas. They are distinguished by having a single pair of antennae on the head, three pairs of legs on the thorax (middle segment), an abdomen, and often one or two pairs of wings.
Diplopods: These are the millipedes which have a great number of body segments and each segment carries two pairs of legs and two spiracles (breathing openings) - The spiracles open into a divergent filamentous tubing array which allows oxygen to diffuse from the air they contain through the tube walls into the blood cavity of the invertebrate - invertebrates do have a heart to pump the blood (haemocele) around but it generally sloshes it around the body cavity rather than directs it through veins as in vertebrates). These animals are all either herbivores or detritovores.
Chilopods: These animals are the centipedes with which children
are usually familiar from their investigations. They are carnivores
with poison injecting fangs and have multiple body segments with one
pair of legs and one pair of spiracles to each segment.
Further information provided here will deal only with the following
Have six legs, three body parts (head, thorax and abdomen), breath via spiracles along the sides of their abdomens, have one pair of antennae (or feelers) on their heads, and often, but not always, have either one or two pairs of wings depending upon the family to which they belong (e.g. flies (Diptera) have one pair of wings, whilst bees and wasps (Hymenoptera) have two pairs of wings).
They are split into various groups depending upon bodily construction
and lifestyles. These groups are:
Insects undergo two possible lifecycles. The more normal variety is the 'complete metamorphosis' whereby the insect develops from an egg (ovum), which hatches into a caterpillar (larva), the larva grows by eating and casting its skin several times. The skin needs to be shed as it is of a finite size and as the only growing period is as a larva (adult insects do not grow any bigger once they have achieved the adult form (e.g. a small beetle is not a 'baby' beetle and it will not grow any larger) the larva must make the most of the time available. Each new skin is several times larger than the one cast off.
When the larva reaches its full size, or occasionally when conditions become less than optimum, a case is constructed underneath/within the final skin. When the last skin is cast off this case hardens into a pupal shell. Often, particularly with the larvae of butterflies and moths, but also with some other orders, a silk cocoon is formed within which the pupa resides; this assists in providing protection from predators, parasites, and the elements (mainly water - flooding).
Inside the pupal shell the insect's body dissolves to a fluid of free cells and the genetic pattern causes a reconstruction cell by cell into the new adult form (e.g. butterfly, beetle, etc.). This stage is critical and many things can go wrong. Many insects have structural deformities due to problems at this stage (e.g. extra antennae, mixtures of sexual characteristics, colouration alterations in moths and butterflies wing scales, etc.). The adult (imago) will hatch out in due course, the correct timing being affected by:
- time itself (a certain minimum amount of time is needed for
the cellular conversion),
In many cases, by altering some of these parameters artificially, insects can be bred continuously on a large scale for research purposes and as a manufacturing process (e.g. silk production). Examples of insects which undergo complete metamorphosis are: butterflies, moths, beetles, flies, wasps, bees, sawflies, lacewings, dragonflies and caddis.
Examples of insects which undergo an 'incomplete metamorphosis' are:
cockroaches, grasshoppers, crickets, locusts, shield-bugs, plant-bugs,
plant-hoppers, booklice and earwigs.
Just about all of the possible feeding strategies are demonstrated by the insects and other invertebrates of the world. These include:
- polyphagous plant feeders (i.e. feeding on a wide variety
It must be pointed out at this stage that some insects do not feed in the adult stage at all; all of their feeding and growth being done in the larval stage (e.g. tiger moths). However, insects that do feed in the adult stage do so for two reasons; one to obtain energy for a longer life period or for a lifestyle which involves great energy expenditure (e.g. flight); or two, for development of reproductive organs and both eggs and sperm.
The strategy involving non-feeding in the adult stage has advantages
associated with reproductive capability based on the fact that the only
function of the adult is to find a mate, copulate and die. No distractions
from this primary function are available. Adult feeding insects gain
advantages of a longer life span which provides more mating opportunities
and the opportunity to find multiple food sources thus allowing more
eggs to be produced. However, the risk of predation during feeding periods
Reproduction within insects is fairly straightforward in its methodology though the life-stages of the resultant young may be extremely complex and varied.
Most insects have both male and female genders with the standard copulatory joining to pass sperm for the fertilisation of the eggs. This sperm is usually passed as a packet (a spermatheca) which is used by the female to release sperm to each egg as it is passed into her oviduct for laying. Parts of the reproductive organs of both male and female are constructed of the hard chitin which forms the horny exoskeleton of the insect. These are so designed to act as fairly complex 'lock and key' mechanisms thereby, usually, preventing copulation between different species and thus reducing the likelihood of hybridisation. This assists greatly in maintaining the species as a separate type from other insects of the same order or group.
Some insects, particularly in the wasps and saw-flies (Hymenoptera), and the aphids (greenfly - aphidae) have no males within their populations. Young are produced by direct division of the egg cells. This results in exact copies of the original female being produced. This mechanism reduces complexity and can mean a vast increase in the capability to reproduce in times of habitat danger. However, it also means that the chance alterations in individuals normally due to the constant mixing of the gene pool provided by normal male/female progeny are lost. This lack of change is good while the insect matches its living conditions but should such conditions change the insect may die out due to a lack of variability and its inherent flexibility.
The social insects have taken a slightly different route by abrogating
the female responsibilities for reproduction to a single member within
each colony. The main group within which this method occurs is the Hymenoptera
(Ants, Bees and Wasps). Here there are three sexes: the females, the
males and the neuters. Males have a short life, do not have a sting,
and their main function is to inseminate the females on a nuptial flight
in the spring. The females following their mating flight disperse to
form their own 'new' colonies. Each colony has a single female; usually
the colony founder, now termed 'the queen', and it is her function for
the rest of her life to lay eggs. These turn into neuters (workers)
to serve the building of the colony and to allow the queen to lay more
female eggs which will then go on to found further colonies carrying
the genes of the original queen. Ants perform in exactly the same manner
but the females, which are about to become new queens, bite off their
wings immediately they have been mated and have landed following the
The molluscs associated with land and freshwater within Britain all fall within two distinct classes; these being:-
- Bivalves - these have two shells
which are joined by a hinge (e.g. swan mussels),
There are few bivalves with which we need to be familiar. Indeed apart from a few freshwater mussels this group tends to be restricted to a marine habitat where, as anyone who has walked down a coastal beach can testify, they have obviously found their niche environment.
Consequently, we will concentrate now upon the gastropods. Snails, familiar animals to all, have a hard calcareous, more or less spirally coiled shell, into which the animal can withdraw its body. In a few of the snails the shell has become completely uncoiled and a mere flattened cone shape remains. This is exemplified by the freshwater limpets of our rivers and streams. Slugs have in most cases taken the shell within their body where it is wrapped around the vital organs as a mode of protection - this reduces the burden of a totally enclosing shell thereby reducing the energy requirements necessary for carrying it around. A few slugs have gone the whole distance and have dispensed with a shell altogether relying upon a nocturnal lifestyle and the capability to produce large quantities of sticky foul tasting slime as a means of protection.
The group is not very well adapted for land conditions as it requires the maintenance of a moist skin at all times and uses, for its size, vast quantities of moisture in the production of the slime which it needs for mobility. Basically, the animal slides on a pool of slime which it constantly generates, but then leaves this valuable moisture behind it - not a particularly efficient manner of locomotion. During drought conditions these animals will become dormant to reduce water loss. Snails will seal themselves into their shells and slugs will hide away in dark damp places; finally encysting themselves should the dampness of the hideaway dry out. They will remain in this condition for upto three or four years awaiting the return of damp conditions.
Snails and slugs feed mainly upon decaying vegetation (e.g. dead leaves) but will attack living plants particularly in dry conditions or when the plant is putting out new growth which is soft and succulent.
Gastropod reproduction is unusual as the animals are hermaphrodite
(i.e. they are both male and female at the same time). The molluscs
will normally have quite a prolonged courting ritual incorporating extensive
caressing and nibbling with the rough tongue (radula); this process
is accompanied by an increased production of slime. As part of the stimulation
process leading to insemination they will each fire off 'love-darts'.
These are small barbed darts made of a calcareous material (chalk) which
are fired, by muscular contraction, into the opposite members body.
Mating then concludes with the passage of sperm from each individual
to the other. Following mating both snails will lay a batch of eggs
which will be covered with soil or decaying leaves or they may be placed
in a damp hollow under a stone or log. No further parental care will
These animals belong to three separate orders, these being:
Millipedes have approximately 200 to 250 legs, but to be a millipede each body segment must have 4 legs (2 pairs on each segment). Millipedes eat dead leaves and are one of the main agents for breaking down dead plant material for recycling in the food-chain (i.e. often thought of as herbivores, they are more accurately described as detritovores).
Centipedes usually have between 30 and 80 legs. They are fierce hunters being totally carnivorous. Their thin flat shape with many segments allows them to slip between soil grains and plant roots in their search for prey. They inject a poison through the fangs (chelicerae) which immobilises the victim allowing the jaws to macerate the body and the fluids to be extracted. These animals are carnivores.
Woodlice have 7 pairs of legs (14 legs in total) which are known as
periopods. They are crustaceans (i.e. related to crabs and lobsters)
with a calcareous shell made up of segments covering their body. They
rely on damp external conditions as they still need a moist atmosphere
to breath and constantly run the risk of dehydration. Woodlice feed
on dead vegetable and animal material and, like millipedes, assist in
recycling this dead material for re-use as fertiliser by plants and
consequently form the last link in the food-chain prior to this re-use.
Woodlice, like millipedes, are detritovores.
There are over 600 species of spider in Britain with our largest bodied being Araneus quadratus at 20 millimetres across. Some species have longer legs which makes them look larger (e.g. the common house spider which people find in the bath, Tegenaria domestica, can be up to 60mm leg-span). Spiders are true carnivores.
All spiders are carnivorous. There are a number of different families in Britain and these can be distinguished by the number of eyes on the head. The spiders in some of these families spin webs, with sticky threads incorporated, in which to catch their flying prey insects. Other families do not spin webs but actively chase their prey as hunters. These latter have specially developed binocular eyes to enable them to be able to judge distances just like humans. The male spider is usually much small than the female and this can lead, particularly in a number of the families who spin webs, to the male being eaten by the female following or even during copulation.
Spider silk is actually, thickness for thickness, stronger than steel wire and experiments are currently going ahead (as of January, 1995) in America to create bullet-proof vests from spider-silk.
Spiders have, over the past ten or so years (1985 - 1996), become extremely
popular as pets in the form of the 'tarantula'. In truth this name has
been misapplied as the real tarantula is a small spider from central
Europe. These large spiders being kept as pets are bird-eating spiders
and their relatives. Some are fairly docile though others are distinctly
aggressive and not for the beginner. They should not really be handled
as they have urticating (irritating) body hairs which can cause arthritis
and all of them can bite with half centimetre long fangs which deliver
a poison injection. Many of these bites are no worse than a wasp sting
but even so this handling should not be encouraged as some people can
have allergic reactions to the venom.
One of the largest groupings of invertebrates are the Insects or 'Insecta' of which there are almost one and a half million known species in the world. It is expected that with the many species that remain to be discovered, and those which will never be discovered as they are becoming extinct faster than we can find them, the final total would be well in excess of two million different species. The insects have solved the problems associated with living without an internal skeletal structure by developing a hard external skeleton which covers the whole animal like a shell but which has special joints in the cover to allow the animal to move and for its appendages (legs, antennae, etc.) to articulate for specific purposes (e.g. movement, swimming, feeding, etc.). The external skeleton (exoskeleton) is composed of a material called chitin which is a pale-brown, hard, strong, proteinaceous material which is relatively light for its strength and which is often honeycombed with internal spaces when in use by the insects as a means of still further increasing the strength to weight ratio. All insects have six legs and three body-parts (head, thorax and abdomen); some insects have wings and some do not. Examples of insect groups include the following:
Coleoptera: examples of which are:
Diptera: examples of which are:
Other invertebrate groups with which we shall be dealing in this volume include:
- the Arachnida - the spiders, harvestmen, pseudo-scorpions,
and mites. All of these creatures posses a similar chitin based exoskeleton
to the insects but all have, at least in their adult phase, eight legs.
All of the above creatures have differing habitat requirements and
have adapted themselves to specific habitat niches. Indeed, even within
a single grouping the variety of habitat niches for which the individual
species have evolved cover as wide a range as it is possible to find
on the planet. As a consequence finding the different species can mean
targeting specific habitats, even specific micro-habitats, and each
of these habitat niches or species groupings need specialised techniques
to find them in the places they have chosen to live.
Before we begin to look at the ways in which the various invertebrates can be captured let us look at some of the structural anatomical differences between these groups of animals and discuss how or why these differences in basic structure may affect the choice of habitat, or other 'life-style' requirements.
The insect body is divided into three main parts; these being the Head, the Thorax, and the Abdomen. All insects have six legs which are attached to the animals thorax. Due to the high degree of specialisation which evolution has impressed upon many insect species the variability in form and structure beyond the basics here given is extremely high and complex.
This group of animals comprises the spiders, harvestmen, pseudo-scorpions, mites, ticks and scorpions; this latter group being included to cover the introduced and naturalised species now found on the south coast of Britain. They have various number of body parts (e.g. spiders have two parts: the Cepahlothorax and the Abdomen, whilst Harvestmen have only one body part) but all have eight legs in their adult phase. All are carnivorous to some degree and many are active predators. The least carnivorous are some of the species of harvestmen but even these will consume recently dead small invertebrates and anything they can overpower as well as small amounts of rotting vegetation.
These animals are the producers of the shells which children collect
on beaches all over the world. However, not all molluscs are marine,
some live in freshwater and some have managed the transition to dry
land though all tend to colonise fairly damp habitats. Molluscs are
split into two groups these being bivalves (e.g. the mussels and cockles
having two shells joined by a hinge) and the gastropods (e.g. the snails,
winkles, slugs, etc.). Most slugs have evolved and have lost their calcareous
shell though some species still carry a rudimentary shell and others
have a small internal shell. All of the molluscs are herbivorous, or
at least detritivorous in nature. All have only one body part and a
single 'foot'. All secrete a sticky mucous either as a means of assisting
their mobility, as an adjunct to their sex lives (e.g. some slugs will
climb trees and descend from branches on ropes of slime to mate in mid-air)
or as a defence mechanism.
Some of the other invertebrate animals to be found in Britain are: millipedes (Diplopoda), centipedes (Chilopoda), and woodlice (Isopoda).
The millipedes are elongate arthropods with two pairs of legs to each body segment and somewhere in the region of 20 to 120 body segments depending upon the species and the age. They are relatively slow moving animals with body-segments which are mainly circular in cross-section, However, one family of millipedes, the Polydsemids, are called 'flat-backed' millipedes and appear to have flattened body segments which are actually circular with flat extensions on the sides.
Centipedes are elongate, predatory arthropods with one pair of legs to each body-segment and having the hind pair of legs elongated and stretched out at the rear of the animal as sensory organs. The number of body segments ranges from 12 to 80. Being predatory they are equipped with poison claws at the head. Most species can move fairly swiftly. They tend to live in leaf litter, under bark and under stones though some of the thinner geophagous species live and hunt deep within the soil.
Woodlice are land-living crustaceans with 30 species native to Britain.
They have seven pairs of legs and are extremely vulnerable to desiccation.
As a consequence they tend to live in damp habitat niches.
Use of Binomial Nomenclature versus English Names:
The use of binomial or so-called 'scientific names' for plants and animals was first started by Linnaeus in the 16th Century (1700's) and comprises a genus name (the first name) as well as a specific name (the second name). It was invented as a form of shorthand, as prior to that time, all living things were named by their full description and this was written in Latin, the language of science at the time. Can you imagine having to talk about "the butterfly whose wings are orange-brown with blue-spots around the outer edge and dark brown spots on the costal edge, which hibernates over winter and comes out again to mate in March laying its eggs on nettle and whose larvae are black with small yellow markings and initially live gregariously" rather than merely Aglais urticae L., or even the Small Tortoiseshell? People interested in British butterflies, flowering plants and birds have promulgated the use of English names as being somehow 'easier' to remember than scientific names but apart from unfamiliarity with the binomial names there is no reason why they should be more difficult to remember. In fact with much usage of the scientific names it tends to be the English names which are forgotten.
Many a starting naturalist wonders why we still use scientific names when so many of the animals and plants have English names ascribed to them. The answer has a number of strands to it. Firstly, there are often many English names which can be ascribed to one species and these are often local to a certain part of the country - by which English name should the species actually be known and will confusion arise by the use of the 'wrong' one? Secondly, the name used as the English name is not the same when one moves onto the continent - for example, the Camberwell Beauty butterfly is known as the Mourning Cloak in Scandinavia - there are 'French' names, German names, etc. etc. Which should be used to prevent confusion? Thirdly, the only constant in all of this turmoil and difference lies in the scientific name which is subject to specific rules as laid down by the Zoological Societies of the World. This means that Aglais urticae in England is known by the same name in America, Japan, Germany and everywhere else on the planet. A much more satisfactory state of affairs and one which aids both understanding and also communication. Please if you must use the English name then do so but do add the scientific binomial name as well when writing reports or articles to ensure that the communication is not restricted to those who know the 'common' name but is open to invertebrate workers the whole world over.
Having just made a plea for inclusion of the scientific name in all
communications there are definitely times when both names should be
used, where possible. These are firstly when dealing with the general
public as they can not be expected to be as knowledgeable as someone
who is deliberately looking at invertebrates; and the second is when
sending reports or lists of species found to site managers, wardens,
and rangers. These people tend to be generalists and even if they do
specialise is a particular area they can not be expected to be knowledgeable
about all of the British flora and fauna. As a consequence they will
often know the scientific names of a particular group but will be completely
at sea in other areas. It helps distinctly to use both names here as
it allows the readers to increase their own knowledge, it increases
the likelihood of understanding by the layman (in relative terms), it
increases the chance of the information being used to good effect for
conservation, and finally, allows better understanding by other 'experts'
should they be brought in to provide verification of the data at a later
date or to aggregate the data at some time in the future.
Invertebrates are the most successful animals in the world if we take sheer numbers as a measure of success. They are more numerous both in terms of species and in terms of individuals than any of the vertebrates. Their very numbers do both enhance their usefulness in conservation terms whilst at the same time making the accumulation of knowledge regarding their biology, and distribution difficult.
How do the sheer numbers of species enhance their conservation worth? This is purely a matter of statistics in that large numbers of species combined with very few invertebrate workers, other than for the 'pretty' animals such as butterflies means that very little is known about the biology or distribution of most of the species involved. This lack of complete knowledge with regard to most of the species has led to many of them being described as Local, or even more scarce whereupon they enter the realms of Nationally Notable or even Red-data Book (RDB) status. This provides opportunities for site managers and wardens to increase the profile of their sites in two ways. These are:
- by using the large numbers of species which can be found to boost the overall species list for the site. This can look impressive both to the public and also to Local Authority Councils when dealing with planning applications.
- the scarcer animals, from Local status up to RDB-1 status,
can be used as the basis to apply to English Nature to have the site
re-graded as a Site of Special Scientific Interest (SSSI) whereupon
it gains a degree of legislative protection. Where sites have lesser
numbers of scarce invertebrates or invertebrates with a lower status
rating (i.e. mainly those of a 'Local' nature) they can be used in an
approach to the county Wildlife Trust or to the county Council to have
the site graded as a Local Nature Reserve (LNR) or as a Site of Biological
Importance (SBI). Whilst these latter two designations do not carry
any formal legislative protection they do raise the profile of the site
with the public and thereby attract more 'friends' of the site and increase
the numbers of those who will 'fight' to protect the future of the site.
Position in food-chains for vertebrates:
Most sites that are designated as being a form of nature reserve have at some time in their past had a Phase-1 habitat survey conducted. Apart from the major habitat types which are discovered and mapped out this form of survey concentrates on the botanical aspects of the site. This is the correct place to start as the plants form the major providers at the bottom end of the food-chain for all of the fauna of the site.
However, the next stage up in the food-chain, both as direct
herbivores and also at the opposite end of the chain as detritovores,
are the invertebrates. They provide the food supply for many of the
larger mammals and birds as well as for large numbers of other carnivorous
invertebrates. As primary consumers and as secondary providers these
invertebrates form major items in the food-chain and are extremely important
to a site.
Management for botanical and invertebrate aspects assures food chains for higher species:
It must also be born in mind that invertebrates play the major role in pollination and consequently need to be encouraged if only to maintain the viability of the botanical interest of the site. They also have a profound effect on the structure of the site, particularly over time. This can be seen in many ways some of which are:
- destruction of certain plant species by population explosions
As can be seen, invertebrates can have a large effect on the visible
features of a site as well as effects which are not immediately noticeable
and as a consequence attempts should be made to understand their distribution
and biology and to apply conservational measures to their protection.
Looking after the botanical and invertebrate diversity at a site will
virtually ensure that the requirements of higher animals are met, including
the mammals and the birds.
If you don't know what is there how can you manage for it?
One of the greatest problems for any site manager is that of deciding how to manage a site and then how to know that his/her management decisions are working to assist and improve the site rather than being to its detriment over a lengthy time period. A true asset which can help a manager to decide the regime by which the site will be controlled and 'looked after' is a complete and regularly updated species list broken down by compartment. This species list enables the manager to begin to understand the reasons behind the usage of a compartment by the specific plants and animals which are present and this can guide how the alterations or control of the compartment is organised and undertaken. For example, thinning a small woodland copse composed of mixed tree species may be done haphazardly but if it is known that White-letter Hair-streak butterflies exist on the only two Wych Elm trees in the copse it would be prudent to leave those and to thin out a couple of Sycamores instead (this a true example where a local authority took out the Wych Elms and left the Sycamores - resulting in the end of the butterflies through lack of knowledge and the consequent result of mismanagement).
To manage a site for its bio-diversity requires a base-line knowledge
of the flora and fauna present. To understand how management decisions
affect the bio-diversity and whether they have been good or bad for
the site/compartment needs regular monitoring via survey to be undertaken
(i.e. this surveying, especially of invertebrates, needs to be done
on a 3 to 5 year rotational basis). Certain species groups and habitat
types will need specialist monitoring expertise and this can be provided
in a number of ways though to be sure of quality of survey and identification,
particularly if considering applying for grading to SSSI status, consultancy
expertise will almost certainly be the only route to follow.