Measuring Terrestrial Invertebrates

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This is important and useful because invertebrates outweigh all other terrestrial species with an estimated 20,000 – 30,000 species in NZ. Close relationships between invertebrate populations, birds and introduced predator populations exist.There are 2 widely used methods to monitor invertebrate populations:

Pit Fall Traps

Small containers are dug into the forest floor so invertebrates fall into them for later collection, identification and counting. The containers are sheilded from the rain so they don”t fill with water, and have preservative solution in them to preserve insects when traps are left for an extended period. A small squeeze of detergent is added to break surface tension on the liquid, thus preventing insect escape. A tablesppon of salt can also be added as a further preservative.

Equipment:

  • Plastic cups with small holes pierced at the top (allows water overflow) & a PVC drainpipe segement to encase/protect the cup
  • Trowel to dig holes approx. 80mm diameter
  • 100ml diluted anti-freeze (30/70 water) per cup
  • Deteregent
  • Salt
  • Covers e.g. a plastic plate with wire simply inserted

4 equally spaced pit fall traps can be created in the forest floor, North, East, South, West. Traps are left out for a month and then gathered in. An insect identification then needs to be carried out. Insect ID list – Landcare Research.

Malaise Traps

These are designed to capture low-level flying invertebrates, which are herded into a container for identification and counting. A small tent like structure is erected to trap invertebrates emerging from the ground as well as those flying within about a metre of the ground. This method takes a bit more effort than pit fall trapping, but it samples a greater range of invertebrates.