Your results are reliable if you placed sufficient traps and used the correct statistical tests to analyse the data. So, assuming the results are valid, you have to attempt to explain them. Rather than 'assume' why you got the results you did, you should retrospectively formulate a hypothesis that you were testing and see if the results support it or cause it to be rejected. Then you can try to explain the results and why they seem to contradict your expectation, using other research to support your findings. I think you need to trust your data even though they seem to contradict your expectations. Don't keep experimenting just so you might eventually get the results you expect! lol :)

Have you tried unbaited traps as a control?  

Sometimes it is useful to add a second sampling method, and combine the results from both. So if you were to do transects (like in the "distance sampling" method) you would encounter also dung beetles busy working on the dung piles of the bisons.

That's true Amos – if you don't sample where the beetles are, you won't record them at all even though they're there lol :)

Simon Robert Leather · and Amos -

Useful answers - do you have experience with calibrating such passive sampling to trap yields?  I'd not too sure that distance sampling would really work with a non-random distribution - neither the dung or the beetles using the resource will be randomly distributed across the habitat. 

Hi John,

I think you are right in that the baited traps in the bison-populated field are not having the same level of 'attractiveness' as they do in the bison-free field, as the bison are providing plenty of natural resources.

As Simon L suggested, using unbaited traps would allow you to ascertain the effect of the baits in the bison-free area.  But the lack of beetles in the traps in the bison-populated area suggests the beetles aren't moving around much, as you might still have expected  to get a few specimens just falling in to the traps (regardless of the bait). 

You could maybe try some form of random quadrat sampling method.  As the animals are highly spatially clustered, this would probably produce lots of zero counts, and a direct hit on dung patch many, many specimens, so could maybe just score binary presence-absence data to speed things up. The interesting part might be in the bison-free paddock: your pitfalls are telling you the beetles are present in the field, but are they still recorded by quadrat sampling?

Good luck!

Simon

Thanks - we will try and adapt this strategy and see what happens.

If you have baseline data of the beetle community/abundance prior to the addition of bison, you could sample after allowing the bison time in the landscape. Removing the bison and then re-sampling after they are removed my decrease resource abundance enough for baited traps to work. Allowing the bison some time (maybe a year or more) in the pasture would probably provide enough of a change in the beetle community before limiting the resources out to them again.

We don’t have baseline data – and moving bison sounds easier than it really is in practice. Like we would need to invest a few hundred thousand dollars in bison resistant fencing at a new site….. We convinced donors to pay for one fence – a second pasture would be an impossible sale. We have implemented an un-baited calibration trap at each sample site, which may allow us to standardize our catches (assuming that the unbaited traps catch enough animals to be relevant).

Hi John, in Brazil several species of dung beetles are attracted to light traps; I wonder if light traps may be used as a source of specimens more or less independent of the amount of "resources" (dung) in the grasslands.

Hi Fernando - I had not thought of that. Perhps we will pilot this now (end of our growing season) and to see if it is worth pursuing. Say hi to everyone in Curitiba for me.

There are many dung beetles present in our pastures with bison, so unlike the first response I agree with you in that it is the sampling method that is not effective. Your results seem to answer the question "how does the attraction of dung beetles to bait change as the amount of dung changes?"

I like the idea of the transects, but you would probably want to walk and take samples of the bison dung in order to extract the dung beetles present in the bison pasture. However, I'm not sure how you would do a transect in a pasture without any grazer dung. This method seems ineffective for the ungrazed pasture. If you were comparing bison and cattle dung I think the transect would be perfect! The light traps seem most effective as it seems the answer is using a bait that is not affected by the amount of dung around.

Light traps are a great tool, but you have to remember they introduce a bias - not all species are attracted equally. Species that are diurnal, for example, are likely to be trapped in a much smaller proportion then their proportion in the community, if they will be trapped at all. So if the goal is to record changes in the community, not all changes may be documented by light traps. Thus the light traps can be an addition to a sampling regime of line transects, for example, but maybe not alone. We had a similar problem when sampling reptiles in sand dunes with or without treatment (treatment = woody plants removal). There isn't any trapping method that would be efficient in documenting all species that might occur there, so we use a combination of three sampling methods: pitfalls + transects + tracks counts. Often different species are documented by the different methods.