Invertebrate Surveys

Introduction

As a result of their small size, invertebrates are able to exploit a wide range of habitats within the environment, including very small and specific features, which are known as microhabitats. In order to survey invertebrates, a wide range of different methods is therefore required.

It is beyond the scope of this guide to include details of all possible methods to survey such a disparate group. Instead, a number of different methods are described. Using a range of methods should ensure that a significant proportion of the species present at a site can be identified.

In order to survey particular groups or species, it is recommended that relevant literature or experts in the field are consulted. The methods below are based on those described by Ausden (pp. 139-177)

Personal Safety

Please read this safety information before carrying out a survey.

Seasonality

It may be necessary to sample on a number of occasions throughout the year, in order to obtain a representative selection of species present. Conversely, when comparing invertebrates between sites, or at the same site over time, it is necessary to ensure that the fauna of similar microhabitats are being compared, and that they are sampled at similar times of year. If you are surveying specific species you should ensure all visits are carried out at the appropriate time of year. If in any doubt consult literature about the species or an acknowledged expert before deciding when to visit.

Monitoring Frequency

The aim is for at least one standard count each month. More frequent counts can be accommodated, and will provide a more precise measure of population changes. It is more important, however, that the counts continue over a long series of years than that they are numerous. Missed months can be ‘imputed’ during analysis, and so gaps are not fatal to the purpose, but the aim should be to count as many months as possible.

If the walk is on a site that is subject to the Wetlands Birds Survey, then it is convenient to use the WeBS count dates and include the WeBS data on the standard walk form.

Methodology

There are a large variety of methods, with the most frequent being described. The table below lists some of the more common groups of invertebrates and give comments about the recommended survey methods for each group. To achieve a full list of species present at a site more than one method will often need to be used.

Methods

Spiders & harvestmenDirect searching (Method 1)
Pitfall traps (Method 4)
Beating and sweep netting (Method 7)
Centipedes and millipedesDirect searching (Method 1)
Soil and litter sampling (Method 6)
Odonata (Dragonflies and Damselflies)The recommended method for surveying adults is standard walks (Method 2).
Note: certain species are difficult to identify and a photograph is requested for verification purposes - see list of difficult species here
Larvae can be surveyed by using pond nets (Method 8)
Orthoptera (grasshoppers, Crickets and allies)Direct searching (Method 1)
Sweep netting (Method 7)
Hemiptera (bugs)
Bugs use a wide variety of habitats and therefore need to be surveyed using a variety of techniques. The main ones are
Beating and sweep netting (Method 7)
Pond netting (Method 8)
Direct searching (Method 1)
Pitfall traps (Method 4)
Lepidoptera (Butterflies)Butterflies are discussed separately in Ôgarden butterfly surveyÕ
Lepidoptera (Moths)The most commonly used method to survey moths is using light traps (Method 3).
Trichoptera (Caddis flies).The recommended method to survey caddisflies is to survey the larvae using pond nets
Hymenoptera (Ants, Wasps, Bees etc)The recommended method for surveying hymenoptera is through direct searching (Method 1).
Bees could also be surveyed using standard walks (Method 2).
Coleoptera (Beetles)Beetles make up around 40% of all described insect species and can be found in almost all habitats including aquatic habitats. As a result any of the methods described could be used depending on the target species. A survey aimed at producing a list of species present at a site would need to use several different methods.
Snails and SlugsDirect searching (Method 1)

Method 1 – Direct searching of an area (Ausden pp145-148)

Many invertebrate species require specific microhabitats and can be found by direct searching of an area (e.g. looking under stones, logs and bark, around the bases of plants, in crevices on walls and rocks, and in leaf litter, nests, strandline litter, dead and decaying fungi, dung and carrion).

Where possible, any items that have been disturbed should be returned to their original position. For ongoing monitoring, it is important that the area is clearly mapped or ideally marked on site so it can be relocated and searched at future dates. It is also important that observer effort (i.e. amount of time searching) is recorded so that this does not vary significantly between visits.

Method 2 – Transects/Standard walks

This method is suitable for monitoring some of the larger, more visible invertebrates such as butterflies, dragonflies and damselflies or bees. The method is described in detail for butterflies can be adapted as required for the other groups.

Method 3 – Light traps (Ausden pp153-154)

Many night flying insects, especially moths, are attracted to light and can be caught by use of a light trap. A variety of traps is available and should be operated according to the instructions. For ongoing monitoring, it is important that the location of the trap is recorded, and that the trap is used at the same time and approximately the same date as in previous years.

Method 4 – Pitfall traps (Ausden pp. 162-164)

Pitfall traps are a good method for catching large numbers of invertebrates with a minimum of effort. Pitfall traps consist of straight sided containers sunk level with the surface of the ground into which invertebrates inadvertently fall and become trapped. Various preservative solutions can be put into the trap to prevent invertebrates from eating each other and arrest decay. Pitfall traps also can be baited in order to attract beetles. Pitfall traps are not necessarily a good method for comparing numbers over time or between sites as catch rates vary with the nature of surrounding vegetation which can vary from year to year. It is also important to be aware that pitfall traps tend to catch proportionally more large invertebrates (>3mm long).

Method 5 – Other traps (Ausden pp. 150-158, pp165-166)

There are a variety of other traps that can be used for different target species, including water traps (flying insects), flight interception traps (flying insects), sugaring (for moths), aerial attractant traps (flies and butterflies), emergence traps (flies, mayflies and caddis flies) and suction sampling (invertebrates in low vegetation). Details of each of these methods can be found in Ausden.

Method 6 – Soil and litter sampling (Ausden pp158-162)

Soil and litter sampling can be undertaken to monitor invertebrates such as centipedes and millipedes, earthworms and springtails. A known volume of soil is taken by using a quadrat or a corer. Sieving can be used to separate invertebrates from soil or litter (normally more than one size of sieve would be used). ‘Some invertebrates such as beetles and pseudoscorpions become motionless when disturbed, so it is best to wait a short time before discarding sorted material. It is also useful to sieve under a strong light. Not only does this make it easier to see, but as some groups are negatively phototactic and are also stimulated by heat, they become more active under a strong light and hence are easier to find’. (Ausden p160)

Method 7 – Beating and Sweep netting (Ausden pp. 164-165)

Beating and sweeping are two techniques that are used to monitor invertebrates that live on foliage and in low vegetation. Beating involves tapping branches with a stick and trapping invertebrates in a beating tray held underneath. Sweeping involves passing a sweep net back and forth through the vegetation to capture invertebrates. Further information on these two methods can be found in Ausden.

Method 8 – Methods of surveying water dwelling invertebrates (Ausden pp. 167-175)

There are a number of methods of surveying water dwelling invertebrates described in Ausden which could be used depending on the target species. Pond nets and tow nets are one such method that can be used to quickly catch large numbers of aquatic invertebrates, particularly the larger species.

‘When surveying invertebrates, it is a good idea to try a variety of techniques: moving the net in a figure of eight, just above the bottom of the water, so that invertebrates on the substrate (e.g. bugs) are stirred and caught as they swim away, pressing the net rim against mossy stones to catch tightly clinging nymphs, and moving the net at different speeds and depths through open water and patches of aquatic vegetation. The net should be twisted at the end of its movement through the water, so that the net bag flips over the frame and so prevents its contents from escaping.’ (Ausden p 167)

Netting can be standardised across sites and over time at the same site, by recording the number of times each technique is used, and repeating the same steps on subsequent visits or visits to other sites.

References

Ausden, Malcolm ‘Invertebrates’ in Sutherland, William J. (Ed) ‘Ecological Census Techniques: A handbook’, Cambridge University Press, 1996