Hello. I'm currently writing a proposal for an undergraduate dissertation on Silphidae and woodland fragmentation. I've decided to do pitfall trapping in 3 woodland sites deemed large, medium and small in addition to experimental carcass placement to assess interment rate (similar to Wolf & Gibbs (2004)). The study period will be 1 month. I've got everything else covered, I believe, but I'm struggling to determine how many pitfall traps I'd need for each site and the sampling strategy to be used. My best guess would be a stratified random grid to allow for interspersion and independence, however I'm not at all sure about number. I assumed number of pitfalls would be the same, however as the woodlands will differ in size wouldn't this cause scaling issues? Wouldn't the grid need to be different sizes to account for the woodland size? I'm not sure if I'm over thinking it or not.
Edit: furthermore, Silphidae are obviously going to be drawn toward bait and will intentionally seek it out, so the layout must be different from other studies involving Carabidae and so forth. I just can't envisage it at the moment.
Any help is greatly appreciated.
Hi Taylor,
sounds an interesting project, but you haven't really described what you want to measure.
are you measuring silphidae abundance or species richness, or both?
or comparing the effectiveness of different baits?
how small is your 'small' woodland? is there any chance of depletion of the population(s) if you have too many traps, or trap for too long a period in one location?
how often do you intend to check the traps?
and do you intend to leave them in one place over the whole study?
could you do a pilot study in the large woodland to see how many pitfalls are needed before you have collected all the species present? or how many days are required before the species number levels off? a kind of species by sampling effort accumulation curve?
I realize I haven't answered your question here, just given you more to answer!
Good luck!
Simon
Apologies, Simon. I've been changing my proposal a lot recently, so things haven't been too concrete. However, after a lot of discussion with various people I've decided to look into woodland fragment sizes. I will select 2 large sites and 2 small sites of roughly the same size (to allow me to compare) and place baited pitfall traps in each. I was originally going to add 2 medium sites, but I think it might be difficult enough to find 4 sites to begin with and it might be unnecessary to have 6 sites. In discussion with someone from the Silphidae Recording Scheme, I found out that the arrangement of traps doesn't particularly matter because of the deliberate location of the bait and the bias effect. Density of traps will be fairly low, around 6 in each woodland or so.
Something I'm also thinking about is doing an interment study by placing carcasses at the other end of the sites, one per site per week in order to see if, as hypothesised, larger woodlands will result in fewer incidences of scavenging and thus greater opportunity for carrion utilisation. This is still a work in progress though as I'm not sure how to set it up so that it doesn't interfere with my pitfall traps.
I have a question, but it's particularly basic. However, I can't quite get my head around this. I've read that pitfall trapping can't be used to measure abundance if done over a short period of time. The problem is I don't know how to word my null hypothesis if I can't use the word abundance. Surely I'd still be able to look at how many species I catch in each site, which is technically abundance. Could I just say "relative abundance". My plan was to look at species richness but also abundance, but apparently abundance wouldn't be suitable. When I think of abundance I think of the quantity of the catch. Is that not correct? Not sure why I couldn't measure abundance.
Hi Taylor,
the catches of beetles in pitfall traps are a function of their abundance and their activity over the ground surface. So, it is generally OK to compare catches of a single species as it can be assumed the activity is the same (as long as ground conditions are the same!). When comparing species then we don't know if different numbers occur because of differences in abundance or differences in activity. So, you are correct in saying you have to modify how you describe your 'abundance': something like 'catch', 'numbers', 'records', 'specimens' or 'count' usually does.
I think six baited traps per site would be OK, as long as the traps were fairly well spaced out. You could compare individual species counts, total count, species richness per trap among the four sites, or use some form of nested design. Or maybe compare the composition of the catches in each trap using some form of multivariate analysis, to see if the samples from the large fragments (and those from the small fragments) are similar and form clusters.
Cheers
Simon
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