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IBYAMBARWA
Sampling Invertebrates: Pitfall Traps and the Beating Tray
Most of the animals in any habitat are small, numerous and out of sight. You cannot quadrat them because they move, and you cannot mark-recapture most of them because there are too many — so you sample the ways they move instead.
A pitfall trap is a container sunk level with the soil that ground-active animals fall into as they walk. A beating tray is a pale sheet held under a branch that is struck sharply, so everything clinging to it drops onto a background where it can be seen. Between them they cover most of the invertebrates you will meet.
Neither gives you a population. A pitfall catch measures ACTIVITY-density — how many animals walked past — so a fast-moving beetle is over-represented and a sedentary one barely appears. Knowing exactly what your trap is measuring is the whole difference between a result and a pile of animals.
Utangiye
3 days
Amabwiriza
1
1
Set the pitfall rim exactly level with the soil
Set the pitfall rim exactly level with the soil
Dig a hole, sink a smooth-sided container so its rim is EXACTLY flush with the surface, and firm the soil back against it so there is no gap or lip.
A rim standing a few millimetres proud is a wall that most ground beetles will simply walk around, and one set low creates a moat that funnels others in. Either way the catch reflects your digging rather than the fauna. Use two nested containers so the inner one can be lifted out without disturbing the hole.
Ibikoresho by'iyi ntambwe:
Icupa ry'Ikirahure10 ibiceIbikoresho bikenewe:
Ikayeri yo mu busitani
Umurongo w'ipima2
2
Cover it, and decide whether to kill
Cover it, and decide whether to kill
Set a flat stone or a tile on small pebbles a few centimetres above the trap to keep rain and birds out while leaving the gap open at ground level.
A dry trap catches animals alive, which is kind and lets you release them — but the predators eat the rest before you return, so you must check it every few hours. A preservative catches everything and can be left for days. For a first survey, use dry traps checked morning and evening, and only take specimens you genuinely cannot identify alive.
Ibikoresho by'iyi ntambwe:
Impapuro Zikomeye10 ibice3
3
Lay a grid of traps, not one trap
Lay a grid of traps, not one trap
Set at least five traps in a regular grid or along a transect, several metres apart, and treat each one as a separate sample.
Invertebrate catches are extremely variable — one trap beside an ant nest can hold more animals than the other four together — so a single trap tells you almost nothing. Several traps let you quote a mean AND a spread, and the spread is what tells you whether the difference between two habitats is real.
Ibikoresho by'iyi ntambwe:
Icupa ry'Ikirahure10 ibice
Ibikoresho by'Ibendera n'Insinga byo Gupima Ubutaka1 ibikoreshoIbikoresho bikenewe:
Umugozi wo GupimaIkayeri yo mu busitani4
4
Use a beating tray for everything above ground
Use a beating tray for everything above ground
Hold a pale sheet or an inverted pale umbrella under a branch and strike the branch sharply three times with a stick. Do not shake it.
A sharp jar dislodges animals that are clinging; shaking gives them time to grip harder or fly. Standardise the number of beats and the branch size and write them down, because 'a good shake' is not a unit. A pale background is what makes small dark animals visible, and it is why a white sheet outperforms a better net.
Ibikoresho by'iyi ntambwe:
Umwenda wa Musulini w'Ipamba2 metre
Umusumari w'igiti2 ibiceIbikoresho bikenewe:
Indorerwamo Yagura
Utwuma two gufata5
5
Sort, count and release promptly
Sort, count and release promptly
Tip the catch into a shallow tray, sort into groups with a soft brush or forceps, count, and release near the capture point.
Sort to whatever level you can defend — 'ground beetles', 'spiders', 'woodlice' is far more useful honestly recorded than uncertain species names. Keep the animals cool and shaded while you work; a plastic tray in the sun cooks a catch in minutes, and a survey that kills its subjects unnecessarily has failed at something more important than counting.
Ibikoresho bikenewe:
Utwuma two gufataIndorerwamo Yagura
Ikayi yo mu murima6
6
Activity-density, diversity and comparing two habitats
Activity-density, diversity and comparing two habitats
Gupakira ikaye ya Jupyter…
Ibikoresho bikenewe:
Ikayi yo mu murima7
7
Take the traps out when you have finished
Take the traps out when you have finished
Lift every trap, fill the holes, and remove the covers.
An abandoned pitfall trap goes on catching and killing for months, and an open hole is a hazard to small mammals. Count your traps out and count them back in. This is the least interesting sentence in the batch and the one most often ignored.
Ibikoresho bikenewe:
Ikayeri yo mu busitaniIkayi yo mu murimaIbikoresho
5- 10 ibiceUmwanya
- 10 ibiceUmwanya
- Umwanya
- 2 metreUmwanya
- 2 ibiceUmwanya
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