Biology Fieldwork

A Level

Method

Uaing a pooter to collect invertebrates

2. Method

The flow of energy through a terrestrial ecosystem can be studied by sampling terrestrial invertebrates by sampling within a grassland or woodland area. A comparison study between two different woodland types could also be carried out. Mobile organisms can be captured in soil and leaf litter, or on the stems and leaves of plants, or using mark-release-recapture.

Equipment

  • A quadrat (choose a size appropriate to the species present).
  • 2 tape measures
  • Identification guides, such as FSC Bugs on Bushes fold-out chart
  • Method of generating random numbers e.g. calculator or random number table
  • Equipment for measuring abiotic factors (e.g. humidity, light, soil depth, soil temperature, soil pH, soil compaction, canopy cover)

Safety

Care should be taken when working outdoors. Wear long sleeves and trousers, as woodland plants such as brambles and nettles can be prickly or stinging and in some grassland areas ticks may be a concern. Beware of branches and twigs in the eye. Wear sturdy footwear and beware of rabbit holes, roots and other uneven ground. The site should be visited by a member of staff to check it is safe to use.

Procedure

  • Carry out sampling of invertebrates using the most appropriate techniques for mobile organisms
  • Identify invertebrates to the lowest taxonomic level as possible.
  • The number of individuals of each species needs to be recorded for each area.
  • Information about the feeding type and biomass of each species can be obtained using secondary sources.

Invertebrates in leaf litter and soil

Invertebrates in leaf litter and soil generally move by running or crawling, although some may also be able to fly. Be gentle when handling live animals. 

Pitfall trap

Pifall traps are containers, such as glass jars and plastic cups, sunk into the soil for animals to fall into. They work for running or crawling invertebrates. Place a cover, raised on stones, above the trap to keep out the rain.

To avoid predatory animals such as spiders from eating the rest of the catch, you can place a small amount of washing-up liquid in water in the bottom of the trap to kill the invertebrates. Pitfall traps can be baited - for example, beer attracts slugs and snails, while fermenting fruit attracts flies.

Chemical irritants

Deep-burrowing earthworms can be brought to the surface by watering the ground with a dilute solution of potassium permanganate. A non-toxic alternative is a dilute solution of mustard powder in water.

Tullgren funnel

The Tullgren funnel is used to extract invertebrates from samples of soil or leaf litter brought back from the woodland to the lab. The sample is placed in a funnel above a perforated disc and the whole apparatus placed under a low-powered electric light bulb. The rise in temperature and the drying effect encourage the animals to move away from the source of heat, downwards through the holes in the perforated disc, into the funnel and so to the collecting vessel underneath. Leave the funnel for at least 2-3 days.

Invertebrates on trees and bushes

Invertebrates on leaves and stems can move by running or crawling or flying. Be gentle when handling live animals. 

Pooter

A pooter is a flexible tube that can be used to catch invertebrates directly from leaves. The user sucks one end, which draws the invertebrate upwards to a trap area. A barrier, such as a layer of muslin, prevents the user from ingesting the invertebrate. Several designs of pooter are available, but the principle is the same throughout.

Once you have finished collecting invertebrates, you will need to sort the catch, which is much easier if the animals have been anaesthetised or killed. Larger invertebrates can be temporarily stunned by placing the collecting tube in a fridge for 30 minutes. Earthworms cannot be reliably identified to species level in the field, so they will need to be killed.

Kite nets and sweep nets

Kite nets are lightweight nets used for catching insects flying above the vegetation layer. In woodland, they can really only be used in open areas, such as tracks and woodland rides, or above low-growing plants. Use a figure-of-eight motion to move the net, and keep clear of any brambles and thorns that may make holes in the net.

Sweep nets are more robust nets that can be used to disturb non-woody vegetation such as grassland and meadow plants. Use a figure-of-eight motion to move the net just on top of the vegetation. 

Beating tray

Place a white sheet on the ground below the branches of a bush. Use a strong stick to shake the leaves and branches of the bush so that invertebrates fall onto the white sheet. They can be removed from the sheet using a pooter.

The size of populations of invertebrates or small mammals in an area can be estimated using mark-release-recapture technique. This technique is particularly useful for animals with shells, such as snails and limpets or invertebrates with exoskeletons such as woodlice.However it can also be carried out on small mammals using Longworth traps.

A sample of animals is captured, counted and marked in some way. These are then released and allowed to re-mix with the population. A second sample is taken later. The population can be estimated from the proportion of marked animals in the second sample.

Mark-release-recapture

You will need:

Apparatus for collecting invertebrates, such as pooters or pitfall traps

Nail varnish or similar for marking captured invertebrates

  • Take a sample from the population. For snails, pitfall trapping can be used (as long as a non-lethal option is chosen) or you can sort through leaf litter. Count them and mark them in some way. Since snails have a hard shell, nail varnish or correcting fluid is ideal.
  • Release the animals back into the population. You now know how many marked animals there are in the population.
  • Leave the marked animals to mix randomly with the rest of the population. Normally the population is left overnight.
  • Take a second sample in exactly the same way as the first. Record (a) the total number of animals in the second sample, and (b) the number of marked animals in the second sample (recaptures)
  • You can now calculate a population estimate. If the animals have been mixing randomly, then the proportion of marked to unmarked animals in the population should be the same as the proportion of marked to unmarked animals in the second sample.

Lincoln Index

The Lincoln Index is used to estimating population size.

`"Population" = ("total number animals in 1st sample" xx "total number of animals in 2nd sample") / "number of marked animals in 2nd sample"`

Assumptions

Several assumptions made when estimating the size of population by mark-recapture. Some are more realistic than others:

  • The population is closed. In other words, there is no migration into or out of the population. In addition, it assumes there are no deaths or births. Births and deaths can be either real or virtual. A real death happens when the animal actually dies while a virtual death means the animal is no longer available for sampling. Perhaps it has gone into hibernation or like a caterpillar has turned into a pupa.
  • All members of the population mix randomly. In other words, each member of the population has an equal chance of capture. However, this may not be the case for everything. For example, male mice and voles are inquisitive and more likely to enter a trap then females. Lactating females are going to stay close to their nests, so you may effectively only be sampling from half the population.
  • The marks are not lost between samples.
  • The mark does not harm the animal. Does painting nail varnish onto a snail's shell make it more obvious to a predator like a bird?
  • The marks do not affect the chances of recapture.