4. MONITORING TECHNIQUES
4.3. Biota
4.3.3. Invertebrates
4.3.3.2. Trapping techniques
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taxa suggesting that spider indicator species are an acceptable surrogate for the conservation value of the total invertebrate mesofauna. Using Scott et al. (2006) short-survey protocol and stopping rules, they suggest that adequate indication of a good peatland site can be assumed when the naturalness index exceeds 0.5, the species quality index (SQI) is > 1.8, and the indicator species-area relationship gives a datum point on or above a trendline. However, they state that the usefulness of this protocol for year-to-year monitoring remains to be tested. Relys et al. (2002) stated that there was no turnover in the abundant spider species in consecutive years if the pitfall trap positions remained constant, although there were marked annual differences in individual abundances. Scott et al., (2006) concluded that there seems to be sufficient basis for accepting spiders as ecological indicators for peat bogs as they satisfy most of the criteria suggested by McGeoch (1998). Although simple observations of the invasion of the bog surface by grasses and trees can give an indication of deterioration of the biotope by lowering of the water table and/or eutrophication, spider surveys may signal other changes that stress the mesofauna and its constituent valued species. The presence of adequate numbers of indicator species at low density may identify those degraded and cut-over bogs that would respond to restoration attempts, e.g., at Holcroft Moss (see also Oxford and Scott, 2003).
Holmes et al. (1993) provides a similar method to Scott et al. (2006) for a survey of ground beetle fauna of Welsh peatland biotopes. Coulson & Butterfield (1985) surveyed invertebrate communities of peat and upland grasslands in the North of England and found that pitfall traps had the advantage over sweep-net sampling, vacuum sampling and extraction of soil samples that they collected large samples of invertebrates which produced markedly more species than the other methods. The catch included many adult insects which could be identified to species and also many of the nocturnally active species missing from sweep and vacuum samples.
Stoneman & Brooks (1997) provide the following guidelines for bogs:
All containers should be standard size and colour and spaced evenly along a transect.
All should contain the same solution.
Always mark the site of the traps well; they are surprisingly difficult to find again.
Use preservative in solution if the trap cannot be checked at least every two or three days.
An anti-freeze can be used in the winter months.
Pitfall traps are useful in survey and monitoring in conjunction with management where, for example, there are different grazing regimes on an extent of bog with the same hydrological regime or to monitor the change in invertebrate fauna before and after rewetting.
4.3.3.2.2. Water traps
Many flying insects are attracted to certain colours and can be caught in coloured water-filled bowls or trays (Fig. 8). Yellow bowls are the best for catching both flies and Hymenoptera (Disney, 1986).
However, whilst white and yellow traps attract the greatest number of individuals, other colours, particularly black, may attract different species (Stoneman & Brooks, 1997). When painting trays, Stoneman & Brooks (1997) recommend using enamel paints as these tend to be resistant to water.
The species composition varies with the elevation of the trap, thus it is recommended to set a number of traps at different elevations (Ausden, 1996). Conversely, if traps are being used to compare catches between sites, or at the same site over time, the height above the vegetation
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should be constant. Traps should be emptied once a week and therefore if sampling only occurs once or twice per year, the traps should be set up one week prior to sample collection. Invertebrates are removed from the water by pouring it through muslin into a bowl.
Figure 8 A water trap (from Stoneman & Brooks, 1997).
Water traps do not provide absolute population estimates, nor do they attract the full range of flying invertebrates. They are, however, very cheap and simple to use and can be useful in determining which are the most common species at any one time. They can be used in all habitats. The critical habitats for most species are associated with their requirements for breeding. It may not be possible to determine the breeding locations for almost any species as finding a winged adult (often the only life stage that can be identified) at a particular site does not prove that the species breeds at that site. Disadvantages are that they must be constantly checked, may be affected by grazing stock or birds which may drink the water, and cannot be fenced off due to long-term effects on vegetation that will affect invertebrate numbers. Obviously, the traps are also restricted to species prone to being caught in water traps and therefore not much use for species which rarely fly. The traps will be biased towards the attractiveness of the traps including the preservative used (see Ausden, 1996).
For water traps to be of use in a monitoring programme, Stoneman & Brooks (1997) advise to set the traps regularly through the spring and summer months in order to include a range of weather conditions. Setting the trap only once or twice a year and then repeating this on the same date in subsequent years may give misleading results, as the sample taken depends upon the weather on that particular day.
4.3.3.2.3. Light traps
Many night-flying insects, particularly moths, are attracted towards light, particularly that at the ultraviolet end of the spectrum. They can then either be actively caught, or encouraged to enter a trap (Fig. 9). The simplest light trap consists of a light on a cable hanging outside a building. Any bright white or bluish light is suitable, although a high-pressure mercury vapour bulb is best. Light traps can catch very high numbers of moths but are very variable according to weather conditions.
Light traps have the advantage of catching insects alive and are therefore ideal for survey work. They can be useful for monitoring changes in the population of night flying moths in conjunction with
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management practices. They are biased towards species attracted to light and therefore only reflect the activity of these attracted species (see Ausden, 1996). Stoneman & Brooks (1997) suggest using the same trap in the sam place when monitoring population change from year to year. Also, always release the insects caught in the trap back to the same site. Scatter individuals over an area in and among undergrowth to prevent predation from birds.
Figure 9 A light trap can be used to collect night-flying insects. There are a number of designs in common use, of varying power, size and portability (from Stoneman & Brooks, 1997).
4.3.3.2.4. Flying interception traps
Flight interception traps work by blocking flying insects with a screen of fine black netting. Blocked insects then drop down into collecting trays laid beneath the netting, or are guided upwards into a collecting bottle (Malaise traps) (Fig. 10). Malaise traps are designed to sample large numbers of flying insects, especially flied (Diptera) and wasps (Hymenoptera). Traps are relatively inexpensive and have the advantage that they do not require any power source and can, therefore, be taken anywhere. If the time and expertise is available to identify all the invertebrates sampled, then the traps become a very useful tool for examining populations and communities of winged invertebrates.
Stoneman & Brooks (1997) recommend that malaise traps be used on small sites (a good example is a small lowland raised bog), as this could have a detrimental effect on local populations of invertebrates associated with, or adapted to, those sites. Similarly, malaise traps should not be used where an endangered species is known to occur. Traps are rarely used to compare numbers of insects between sites or at the same site over time, because their size tends to make replication impractical (see Ausden, 1996). Stoneman & Brooks (1997) provide the following guidelines:
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Change the bottle at least every two days in the summer months, as sampled invertebrates soon begin to decompose. If the site cannot be visited every two days, use alcohol to delay decomposition for approximately one week.
Where there is scrub or woodland on the bog, place the trap at 90° to the edge of a block of trees – this catches the invertebrates hawking along woodlan edges.
Loosely place some vegetation or tissue paper in the collecting bottle along with the killing agent – this helps to prevent antagonism between individuals and increases the surface area within the bottle.
Figure 10 A malaise trap. Flying insects hit a vertical wall of netting. They move upwards towards the light and are funnelled through a small hole into a collecting chamber (from Kirby, 1992 in Stoneman
& Brooks, 1997).
4.3.3.2.5. Aerial attractant traps
Flies can be attracted into containers holding suitable baits and then trapped within these or guided upwards into a collecting bottle. A wide range of baits can be used: rotting fruit for fruit flies, dung for dung flies, fungi, fish, rotting eggs, etc. To obtain the widest variety of species when using baits that decay, such as meat, it may be worthwhile leaving bait in different traps for varying periods of time since different fly species are attracted to meat in different stages of decay. For this reason, catches are biased towards certain flies, and flies already caught may attract other flies (Cragg &
Ramage, 1945).
4.3.3.2.6. Emergence trapping
Insect groups which have aquatic larval phases and which swim up to emerge at the water surface, include non-biting midges (chironomids), biting midges (ceratopogonids), some caddis and mayflies.
Emerging flies can be caught in floating mesh-boxes buoyed up by polystyrene floats and emptied of their contents. To catch the flies, they need to be sprayed with dilute alcohol and grasped with tweezers, which can prove extremely difficult. Placing the trap quickly in a large polythene bag
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increases the efficiency of the operation. Clearly, these techniques depend on the habitat such as fens where there is standing water.
4.3.3.2.7. Suction sampling
Suction sampling involves the sucking up of invertebrates from a known area of vegetation into a net.
The most commonly used purpose-built suction sampler is the D-vac, which is a large piece of apparatus, carried on a person’s back. Two methods of monitoring invertebrates can be used. The collecting nozzle of the sampler can be pushed vertically downward into the vegetation and held there for a standard length of time (e.g. ten seconds) to suck up invertebrates from an area of vegetation the size of the sampler’s nozzle. This can then be repeated many times. Alternatively, a known area of vegetation can be defined and enclosed, and the collecting nozzle used to suck up the invertebrates from it for a standard length of time. After the sample has been taken, the net bag containing the invertebrates should be sealed and placed in a killing bottle and its contents then removed and preserved.
Suction sampling is only effective in vegetation less than 15 cm high, which has not been flattened by wind, rain, or trampling. Like sweep netting, it cannot be used if the vegetation is damp and thus is probably not recommended for peatlands during wet conditions. Suction sampling collects fewer invertebrates per unit time spent in the field than sweep netting does. However, although extraction efficiency varies to some extent in differently structured vegetation, this is usually less of a problem than it is with sweep netting. Hence suction sampling may often be the preferred option for monitoring invertebrates in low vegetation and sweep netting the preferred option for monitoring them. Suction sampling under-records large invertebrates (>3 mm long) that can take shelter (e.g.
hunting spiders) or that are firmly attached to the vegetation, e.g. Lepdiopteran larvae. They will also probably under-record species living low down in tall vegetation, and species that can take action when they sense the noisy sampler approaching (Ausden, 1996).