Grasshopper Abundance/Survey Question

[kita]

Hi,

I am planning to research abundance of grasshopper species, using the ecosystem of a local pond that has been dedicated to student research. My teacher suggested that I keep in mind the context of the relationship between abundance and temperature. I plan to have a large sample size and to use sweep nets. I have had some trouble coming up with a reliable way to correlate abundance and temperature (obviously just measuring density at different temperatures would not work; what I really need is a longterm study of population over a few years). So instead I came upon the idea of measuring abundance with proximity to the water being my independent variable (and also possibly measuring humidity).

A few concerns that I can't sort out:

If I re-release the grasshoppers after I capture and ID them, how can I mark them without harming them and also to withstand molting?

Should I try samples from different areas around the pond, or would it be more accurate to thoroughly survey one particular area?

I know it's a lot to ask, but could anyone offer tips on fine-tuning my design? Or suggestions on how I COULD include temperature into my study, if possible?

Thanks!

[Walker]

Hi Kita,

Your best bet for marking insects after capture is probably just to put a small dot of paint on them. I can't think of any non-invasive way of labeling an insect that will endure beyond molting, however!

For your study design investigating water proximity and grasshopper abundance, it will be very important for you to sample several different sites around the pond. If you do a thorough study of, say, one area close to the water and one far from it, at the end of the study you will be able to say that there are more grasshoppers in one area than in the other, but it would be a mistake to interpret those data as meaning that water proximity was the determining variable, as those two areas probably also differ in many other potentially relevant ways (e.g., vegetation types and abundance, predator populations, etc.), not just in their proximity to water. To find out if water proximity is really important, you must sample many areas in an unbiased fashion. To protect against bias, you'll want to randomize your sampling; for example, you could divide the study area into numbered sectors (along lines of a given distance from the pond) and roll dice to determine which ones you collect insects from.

To address temperature differences over a short time scale and a small geographic area does present challenges. You might be able to ask a question having to do with temperature, though. For example, if you have regions of your study area that vary greatly in sun exposure (say, a bare rock field exposed to the sun, a cool shady canyon, and a grove of trees by the water), then the distribution of grasshoppers across those microhabitats might be different on hot sunny days compared to cold cloudy days. Grasshoppers might avoid the rocks on hot days but seek them out on cold days, for instance. This is something you could test -- if the weather were kind to you and if you have the time to sample on so many different days!

Hope this helps. Good luck in your project!

[kita]

That helps quite a bit, actually! Your point about eliminating bias and false correlation makes perfect sense. Thank you very much.

I know it's a fairly straightforward project, but I still hope to make it as accurate and sophisticated as possible. So if anyone else can provide additional information on my problem of labeling, that would fantastic!

[deleted-71536]

Hi Kita,

You have already gotten some great advice on a well-designed project. I just wanted to let you know about a little money-saving trick that field biologists use to mark animals. For the "paint," we often use nail polish. It's an inexpensive way to mark the animals, dries quickly, and does not harm them.

Good luck with your project!

Heather