Introduction

There are both advantages and disadvantages of surveying invertebrates:

Advantages	Disadvantages
Easy to work with and rewarding	Invertebrates are huge group!
Abundant (providing large sample size)	97% of all animals and include wide range of animal phyla, many Arthropoda
Relative ease of capture (even in poor conditions)	Arthropods, particularly insects, usually need to be caught to be identified
Relatively simple equipment can be used for most surveys (nets, vessels, hand lens)	Different methods associated with each major habitat type

We will look at each habitat type in turn, outlining the appropriate methods for each. Some of these you may have used yourself, and others you may not have heard of at all. When planning our surveys, we still must make sure that they are representative of the area and standardised to avoid bias. We will talk about the different methods of standardisation as we go.

Techniques can be split into active and passive, where active techniques involve hunting the animal and trapping by hand, using nets or other physical means and passive techniques rely on setting traps. Relatively immobile animals will be caught less frequently in passive traps. This may be different even within one species, where males and females have differing behaviour.

Catch rate is also influenced by factors such as:

Feeding behaviour
Size
Ability to escape from trap (size, flight)
Ability to hide (size, crypsis)

It will be important to understand the animal’s ecology and behaviour, even at different times of the day or year, so you know the best times and places to survey (and when not to survey).

1. Aquatic invertebrates

We can use a variety of net types to capture aquatic invertebrates:

Dip net The most commonly used nets are dip nets that are ideal for catching invertebrates on the surface of the water or in the main water column. They can be using in running and still water. We move the net in a figure of eight motion to catch invertebrates and prevent them escaping. We can standardise this method by width and speed of sweep, number of sweep and depth.
	D - net Species living on the bottom of still or running water require more specialised techniques. D nets (named for the shape of the net) are used to sample invertebrates that live on the bottom of a pond, lake or stream. As the bottom of ponds and lakes tend to be muddy, we can't sample in the same way as for open water. Instead we jab the straight part of the net (along the bottom) into the mud a number of times, and then sweep above it. These actions will firstly disturb any invertebrates in the mud, and then will scoop them up in the net. In a similar way to dip netting, we can standardise by number of jabs and sweeps, and width of sweep. We can also use this net to sample invertebrates in streams. To do this we hold the net downstream to us, and kick the stones at the bottom of the stream. This is called kick sampling. It can be harder to standardise this method, so to do this more effectively, we use a Surber net shown below
Surber net A Surber net acts like a quadrat by standardising the area in which we sample. It is made up of a frame (right image) and a net that attaches (left image). For kick sampling, it allows us to standardise the area in which we kick. We would also standardise by force used and time. A net will attach to the frame and collect all dislodged invertebrates as they flow downstream.	<--net <--frame
	Drift, trawl and Hess nets There are a range of other specialised nets that may be used to sample invertebrates in large rivers or open sea. These are usually dragged by vessels, rather than be used by hand. The right net in the image is designed to sample animals living in the mud or sand, while the left is for those in open water.

2. Soil-living invertebrates

Burrowing animals will be harder to sample, but can be dug out of the substrate or extracted using a manual suction sampler. These work by placing them over the hole and a handle is drawn up to create a vacuum, pulling the animal out of the burrow. Coring can be used for muddy/sandy ecosystems especially in coastal or estuarine habitats - this is essentially just plastic/metal pipes with handle. Planning where you core will be determined by your aims, but you would need to consider the need for a random (or random-stratified) method determining where to sample in most cases.

Leaf litter can also be removed in a standardised way using a quadrat to delimit the area removed.

Berlese-Tullgren Funnel

Once soil is removed by coring or other techniques, invertebrates can be extracted. The most common method is the Tullgren (or Berlese-Tullgren) funnel. This uses a hot lamp above the sample to encourage animals to move away from light and heat down towards a collecting pot.

Image (right)

Berlese system A : liquid for conservation (alcohol at 70°) B : bottle C : filter D : ground, litter, leafs... E : funnel F : heat G : source of heat

By Self - Image:Berlese.png, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=3615984

3. Ground-active invertebrates

Many invertebrates live on the surface of the ground, in leaf litter, under logs or stones, in dung or dead wood. Hand searching can be efficient but time consuming, and standardisation by effort used (time spent per person). Logs and stones should be carefully returned to original position, and bark can be carefully lifted and replaced. This method is less biased towards more active species, although some animals may escape after disturbance.

The most commonly employed technique for collecting surface-active invertebrates is using pitfall traps. These can be plastic containers of various sizes, so are cheap and easy to use. Sometimes killing or preserving solutions may be placed in containers, although this method should be used only when absolutely necessary to prevent animals from eating each other!

It is vital that vertebrates not caught as they are mostly protected, and capturing them is likely to cause their death. You are also obliged to report any deaths you have caused as part of your experiment. To ensure this, a lid placed over trap, sometimes also with a layer of chicken wire. A 'mammal ladder’ should be included to enable e.g. shrews to escape as they are protected by law, and are likely to die quickly if trapped.

When setting up the traps, vegetation should be placed in the trap for cover and to prevent desiccation. Traps may be baited with dung or carrion. Traps should be arranged flush with the surface of the ground, and away from likely water runnels should it rain - you do not want your pots filled with water. Longer vegetation may need to be cleared around the trap.

Image (left) Pitfall trap

By Mnolf - Own work, photo taken in Kematen, Austria, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2534375

Sampling methods for pitfall traps

A set number of traps (per area) should be placed in a standardised pattern or line - this might depend on your habitat or what you are testing. For example if you are measuring along an environmental gradient, you would set the traps up int a line (a transect). It is a good idea to try to keep traps away from public footpaths to prevent human interference or damage.

4. Plant-associated invertebrates

Many invertebrates feed and live on plants, or use them for shelter. We can capture invertebrates from the different parts of the plant using a number of methods. It is important that we do not attempt to capture them while the vegetation is wet as this will result in many dead bugs stuck the sides of our equipment :/

Suction samplers

Invertebrates that are found low down on the plant may be harder to extract than those towards the ends of leaves and stems. Suction samplers may be specialist machines or modified household vacuum cleaners. They are useful in homogeneous vegetation, and are more likely to collect the smaller and less active species that are unlikely to be caught by pitfall traps. Therefore, they will show a more accurate reflection of the immature invertebrate population. Standardisation will be by time, biomass and/or area sampled.

Pooters or aspirators

These are used to collect small arthropods that are found using visual searches. One tube that is sealed off with muslin or gauze is applied to mouth, while the other tube is directed towards the arthropod. This method can also be standardised by time, biomass and/or area.

Suction sampler

Pooter

Sweep netting

This method is only useful on long vegetation (> 30 cm) and is used to collect invertebrates from the canopy of grasses, herbs and tall ferns. The nets used have strengthened rims and strong handles to allow them to be moved through the vegetation. A figure of eight motion (as for dip netting in water) is used to collect animals on fore- and back-hand of sweep. This method can be standardised in a number of ways, keeping (type of) net used the same:

- Large blocks/quadrats of vegetation swept entirely of insects
- Number of sweeps used
- Line transects walked, sweeping at each step at constant speed
- Timed sweep of an area, keeping walking speed and sweep style constant

This method is quick and easy, and can be used with other methods, e.g. suction sampling, to collect insects lower down in grass canopy

Beating

Beating is used to collect invertebrates from branches of trees and shrubs. The wood part of the plant can be hit repeatedly with a stick while a tray or sheet is placed underneath to catch invertebrates as they fall.

The catches may be standardised by the area of collection and by the number and force of the beats.

There will of course be bias against insects that may be able to fly to escape.

5. Airborne invertebrates

Insects are the only invertebrates that are able to fly, but some groups that use passive flying techniques may be caught. This includes small spiders that use the wind to move, and other species of aerial plankton (click for more).

Nets can be used to capture insects while in the air, but it takes skills and practice. There are a variety of specialised nets for different groups of insects (right).

Sticky traps can be quite successful and include things like sticky fly-paper that can be hung from tree or poles (below left). There have been reports of bats being caught by these (below centre), so it is important to consider other species that may be affected. Bats are fully protected by law, so you would face charges if you caused their deaths even if it was unintentional.

You can also use a simple bottle trap (below right), made by cutting off the top of a bottle and inverting the top into the bottom

Flight interception traps capture insects as they fly through open areas. They can be made from fine black netting or a clear plastic sheet suspended across a suitable area, for example a wide path in woodland. The insects hit the net or sheet and fall into trays or guttering below.

An alternative to this method is a malaise trap (right) that works in a similar way but is more suitable for open areas. It is essentially a tent with no sides but a middle partition. Insects hit the partition as they fly, and will fly upwards to escape. In the highest point is a hole leading to a collection vessel.

Light traps are used to collect night-flying insects. This method will be very weather dependent, best used on warm nights with little wind and low moonlight. Some traps require mains electricity and some can work from a car battery, making them more portable (although batteries are heavy!). The electricity source will determine which bulb type you can use to some extent - the most effective types requiring mains electricity. Bulbs used usually emit UV light, so care must be taken when setting up as they can cause eye damage - glasses are available to protect eyes.

Direct observation can be used if the insects are large and easy to identify. The UK Butterfly Monitoring Scheme uses a standardised visual visual survey method to record butterfly numbers along transects. For this method, the observer counts butterflies as they walk at a steady pace. They imagine a moving cube of 5 m in front of them, 2.5 m either side of them and 5 m above them (left A), and count all butterflies within this space. It is essential that the observer can identify species by eye as they move along.

The transects are fixed routes that are established at a site on which butterflies are recorded along the route on a regular (weekly) basis under reasonable weather conditions for a number of years. Care must be taken in choosing a transect route as it must then remain fixed to enable butterfly sightings to be compared from year to year. Transects are typically about 2-4 km long, taking between 45 minutes and two hours to walk, and are divided into sections corresponding to different habitat or management units.

As butterflies are so affected by weather conditions:

Transects are not walked when the temperature is below 13°C (in northern upland areas this may be reduced to 11°C)
Between 13-17°C, a transect may be walked providing there is at least 60% sun
Above 17°C, a transect may be walked in any conditions, providing it is not actually raining
When wind speeds are above 5 on the Beaufort scale, transects should not be walked

Case Study: Marsh Fritillary

The Marsh Fritillary (Euphydryas aurinia) was once widespread in Britain and Ireland but has declined severely over the twentieth century; it is now protected throughout the UK and Europe. The Marsh Fritillary requires large areas of habitat with good connectivity, many of these habitats being lost to development or through fragmentation.

Its main food plant is Devil's-bit-Scabious (Succisa pratensis), and this plant is used as the focus of the survey methods. In small sites you search all the host plants present for larval webs (left image). In larger areas you set up a Z-shaped transect that is representative of the habitat. As you walk the transect, you look 1 m either side and record any larval webs seen.