A COBRA COOKBOOK
At each site to be studied, a sampling plot with 50×50 m should be delimited. All the sampling methods will be made inside this plot and collectors are allowed to freely roam inside it. The shape can be adapted for linear habitats (e.g. river margins) as long as the area (0.25 ha) is kept.
A semi-quantitative sampling design, with a sampling unit defined as one person-hour of effective fieldwork, is to be followed. For the purpose, collectors should use a stopwatch to control the time that is spent collecting, and to be able to stop the counter when occupied with other activities (e.g. photography, maintenance of tools, eating, etc.; one hour of collecting can take up to 10 or 20 minutes more).
Previous work has demonstrated that each combination of method and time of the day can be considered as a different method itself. For example, sweeping low vegetation during the day samples a different fraction of the spider community than sweeping that same vegetation during the night. Therefore, these should be regarded as different methods.
The proposed methods were chosen due to their proved efficiency to sample spiders, and also because they target different fractions of the sampled community, even if with some overlap in a few cases:
- Aerial – this method consists in collecting all spiders found above knee-level by hand, forceps, pooter or brush and immediately transferring them into alcohol. All the time spent searching is to be accounted for.
- Ground – Similar to the aerial method, but directed towards spiders seen below knee-level, including species in hidden sites such as below stones or inside hollow trunks.
- Beating – A one by one meter square sheet with a frame should be used as a drop-cloth and a wooden pole used to beat tree branches, as high as possible. The effective time includes all the time spent in the activity, like beating, searching for fallen spiders on the sheet and transferring them to alcohol.
- Sweeping – A round sweep net with an opening diameter of 40 cm is used to sweep bushes and tall herbs. All time spent sweeping or searching for dislodged spiders is accounted for.
- Pitfall – Pitfall traps are placed next to the delimited plot (not inside to avoid interference with collectors). Each trap is placed 5 meters apart from the nearest traps. A sample is a group of 4 contiguous pooled traps. The clumping of traps makes individual sampling effort reasonably comparable with time-based samples, as the effort applied to rig and collect four traps is calculated to be roughly equivalent to one person-hour of work. Traps should be left in the field for two weeks. Preference should be given to standard 33 cl plastic cups, 8 cm wide at the top and 12 cm high. Two-thirds of each cup is filled with a preservative liquid, either ethylene or propylene glycol and a drop of detergent to break surface tension, covered with a square wooden plate placed about 2 cm above the ground.
Other factors like collector experience have been tested and only occasionally found to influence the results. It is therefore unnecessary to strictly control the experience of collectors, although I recommend that at least one of the collectors is experienced in this type of sampling and proficient with all the methods, so that consistency between different teams is maximized.
With these methods, and using an iterative optimization procedure, three nested (sub-)protocols were reached (Table 1). The low-effort protocol is intended to capture around 50% of the existing species, regardless of the habitat type, and is part of the medium-effort protocol, which requires four times more effort to capture 70% of the richness. Concurrently, this medium-effort protocol is part of the high-effort protocol. This way, the flexibility is guaranteed, as different sampling teams may have different objectives and resources, but at the same time all sites are comparable, even if sampled with different effort.
Table 1. Proposed nested (sub-)protocols. Ad/An – Aerial searching day/night; Bd/Bn – Beating day/night; Gd/Gn – Ground searching day/night; Sd/Sn – Sweep day/night; Pf – Pitfall trapping (each sample being comprised by four pitfall traps as explained in text). Numbers of samples in parenthesis refer to alternative protocols when sites do not have arboreal cover. %S – expected % of species captured.
|24||50||0||4 (0)||2 (0)||2 (0)||0||0 (4)||2 (4)||2 (4)||12|
|96||70||0||16 (0)||8 (0)||8 (0)||0||0 (16)||8 (16)||8 (16)||48|
|320||90||32 (0)||32 (0)||32 (0)||32 (0)||32 (64)||32 (64)||32 (64)||32 (64)||64|
For tropical forests we found that these sampling completeness levels require higher effort to reach. Therefore, for these habitats additional methods and samples were added to the COBRA50 protocol (COBRA-TF; Table 2).
Table 2. Proposed protocols for tropical forests (COBRA-TF).