Tracking-tunnel protocol?

[davidb]

Having done extensive monitoring for both the Auckland Regional Council and DoC I want to query the tracking-tunnel protocol for rodent tracking. Recently I have been working at Whakanewha Regional Park where there are 450 broadifacoum bait stations distributed over 270 hectares in anticipation of Mays bellbird release. I established 100 tracking tunnels on ten lines and followed the protocols of baiting with peanut butter and leaving the cards out for one fine night before bringing them in before repeating with rabbit for three fine nights for my mustelid monitoring data. Interestingly the three nights with rabbit bait yeilded a higher rat density in the park than the one night with peanut butter, however i am unsure wether the determining factor was the bait choice or the number of nights the cards were out (or a combination of the both). In one area for example, where there are a higher density of rat bait stations, I found a 0% rat density using PB for one night, but this same area had a 2% rat density using rabbit left out for three nights. Can anyone comment on this?

[Dave Houston]

Seems to me that simple maths solves your quandry.  By 2% I assume that 2/100 of your tracking tunnels were tracked by rats over the 3 nights.  If you divide 2 by 3 you get 0.666 which is close enough to the 0% you got in the one night tracking.  This, if  you got 1% for a one night rodent tracking I'd expect that you'd get 3% for the three nights.  You have to remember that this technigue measures relative abundance of your target species - not absolute abundance.

[davidb]

yes I understand the maths and that 0.66 is close enough to zero, I am just wondering why the rat tracking protocol is to leave the cards out for one night, rather than leaving for three nights and then dividing by three for better accuracy. To give you a more clear example, one area I have been monitoring has 50 tracking tunnels, I repeated the PB for one night twice and both times got no sign of rats in any of the tunnels. If I didn't do the stoat monitoring with rabbit for three nights, I would have no idea that there were still rats within the area.

[Dave Houston]

This excerpt from the protocol probably answers your question;

For monitoring rodents and mustelids, tracking tunnels only provide a coarse index of relative abundance; they are not a direct measure of population density, but a measure of activity. The technique is best suited for providing simultaneous comparisons of the relative abundance of rodents (particularly rats) or mustelids between similar habitat areas (e.g. treatment and non-treatment) or gross changes in relative abundance over time at a single site.

Your coarse index can be refined by running more tunnels over more nights, but is still a measure of relative abundance, not an absolute indicator of presence/absence.   Three nights are used for mustelid monitoring because they are harder to detect at low levels.

[Alicia Warren]

Dave Houston is right.  It seems from what you say you are wanting the tracking tunnels to detect the presence or absence of rodents (even at very low levels), but this is not actually the objective of the tracking tunnel protocol which is designed to give a relative index of abundance to allow comparisons between areas or within one area at different times.  If you are actually wanting to detect the presence/absence of rodents (even if present at very low levels) then you might want to use rodent surveillance techniques (such as used by DOC for island biosecurity) rather than rodent index of abundance techniques. Caution: Check objectives and don't make changes to techniques without a robust decision making process. 

[Alicia Warren]

Cont:  Worth looking at the posts on "Detection of rodents at low density". - http://www.wildlifemanagement.net.nz/index.php/topic,226.0.html

[Edit:  added link]

[davidb]

Ok thanks alot Alicia and Dave, that makes sense. Interestingly, the work I have done for DoC looking for evidence of rats on predator-free offshore islands has all been done using tracking tunnels as opposed to wax tags. We're at 5% threshold at 4% (repeated twice) using tracking tunnels, so looking good for the bellbird release in May

[Dave Houston]

We use both wax tags and tracking tunnels in the Chatham Islands to attemot to detect rodents on islands.  Both are unlikley to provide an early indication of rodent invasion, but if you don't look you won't find.  Ideally we would run tracking tunnels over several nights to increase the chances of a rodent finding one, however there are so many invertebrates there that the cards are nearly unreadable after one night.  The wax tags are out all the time and the only thing to bother them so far are parakeets.

[Andy Hutcheon]

I understood that one of the reasons for a one night protocol for rodents is that (at least for mice, at a 100m tunnel spacing) it is unlikely that the same animal tracks several tunnels in a single night. A trade of detection probability for a (presumably) more repeatable measure of activity.

If you just want to know if rodents are present, longer runs with tracking tunnels are more sensitive (cards can be readable reliably after a week if insects aren't too active).