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General Discussion

JANE. Do you think it is practical to use pellet counts in exotic forests when the frequency of pellet counts is about five percent?

D.J. BELL. No. At that level the errors probably make the estimate rather meaningless.

JANE. I have experienced a considerable bias in spotlight counts in estimation of density. Have you found this?

D.J. BELL. No. The results I have seen tend to indicate that you can achieve some sort of stability if you select your nights and maintain them over a period of time. I can't really comment on what bias is involved - it would depend partly on the type of control operation used.

JANE. No it is not so much the type of control operation. Rather it is due to variations in the area that is being observed with the spotlight. Where you have a number of tall stands with understorey your search area or search distance is short; in younger more open stands your search distance can increase ten-fold.

D.J. BELL. I pointed out spotlight counting was a technique with high potential for use in pastoral areas. One usually repeats the same route on a trial-bike each night so the intensity of search does not vary much from one count to another. When you get into forest areas there are many difficulties.

WODZICKI. The decay rate of pellets must vary between different environments, and are you aware of the work by Taylor and Williams^* on rabbit pellet density estimates?

D.J. BELL. Actually Taylor and Williams' method is the basis for our pellet count work on possum populations. Pellet disappearance or decay is a very important factor.

WODZICKI. But have you measured decay rate in a range of possum environments?

D.J. BELL. We run a series of plots on a transect through the habitat which we want to sample and we measure the density of pellets on these plots and we also mark pellets on these very plots for decay. So on exactly the same ground that we are measuring pellet density, we are measuring their decay rate. We do this twice to get one estimate of recruitment rate, which is directly attributable to the number of animals.

KEBER. Have you ever tried calibrating these indirect methods of population assessment by say, laying cyanide out over successive nights, collecting dead animals and then calibrating the recorded fall-off in population numbers with changes in pellet density or bait interference?

D.J. BELL. Not yet. We are planning a large kill at the end of the Haupiri study when detailed information on the composition, movements, and density of the possums is available. We will probably try out such experiments there. Trap counts and pellet counts have been compared in the past and the results tend to agree fairly well.