Representación de la Encuesta realizada a la Población del Instituto Manuel

The problems highlighted with the various rodent density indices tested suggest that caution must be taken when choosing an appropriate density index. The research or management questions posed will have a large influence on the type of index selected. While there are a number of advantages in using tracking tunnels (they are easy to set and service, they do not remove individuals from the population, and they can cover large areas quickly; King and Edgar 1 977) they are susceptible to changes in activity and rodent abundance. Consideration of these limitations on tracking tunnel use will greatly increase the accuracy and reliability of the index. The effect of activity on the index can be countered in a number of ways:

1 ) Tracking tunnels should be used to directly compare populations in the same area, or in the same habitat type only. By running tracking tunnels w ith a consistent protocol, by running tracking tunnels in treatment and non-treatment areas on the same night(s) to account for activity, and by only comparing the same habitat types, the density index will more closely reflect abundance, rather than activity.

2) S ufficient numbers of tunnels should be run to allow detection of treatment effects. If individual animals are not marked, tracking tunnels can only be scored as tracked or untracked. If, for example, ten tracking tunnels are used, then a difference of

one tracking event will alter the index by 10%, which may mask small, but significant, treatment effects.

3) The underlying behaviour of the study animals should be considered. A variety of tracking tunnel protocols have been presented that use different trap layouts and spacing (King and Edgar 1 977; Innes et al. 1 995; Brown et al. 1 996). Studies of ship rat home range use in broadleaf-podocarp forest in New Zealand have

highlighted average home ranges of around 1 ha for males, and smaller for females (Hooker and Innes 1 995). A tracking tunnel spacing of 50 m will therefore be susceptible to contagion of the index, through multiple tracking of tunnels by the same individual. In this case, a tracking tunnel spacing of 1 00 m would lower the number of tunnels, but increase the reliability of the index.

4) The tracking tunnel index should always be complemented by a second density measure. The first experiment presented here showed that tracking tunnels and snap traps were highly correlated on a trapping grid, while tracking tunnels and Fenn traps were significantly correlated on the tawa-podocarp forest trap-lines. The use of more than one index allows the calibration of the density index and increases the confidence in observed population trends. The use of a second, separate index will also increase the quality of information gained. For example, the use of a kill-trapping index can allow the collection of morphometric and bionomic information from the population, greatly increasing the understanding of the population under observation.

In conclusion, tracking tunnels are an efficient and generally reliable small-mammal index. They can be used to record relative changes in population density and

variations both within and between areas. Confidence in the accuracy of the index can be increased by comparing population dynamics within similar habitat types, by paying attention to the influence of sampling effort and target species behaviour, and by calibrating density estimates obtained from tracking tunnels with those of a second index.

2.5 References

Chapter 2. Calibration of density indices 46

Brown, K., Moller, H., Innes, J., and Alterio, N. ( 1996). Calibration of tunnel tracking rates to estimate relative abundances of ship rats (Rattus rattus) and mice (Mus musculus) in a New Zealand forest. New Zealand journal of Ecology. 20:27 1 -

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Caughley, G. ( 1 977). Analysis of vertebrate populations. John Wiley and Sons, London.

Cunningham, D . M., and Moors, P . J. 1983 . A guide to the identification and collection of New Zealand rodents. New Zealand Wildlife Service.

Daniel, M. J. ( 1 972). Bionomics of the ship rat (Rattus r. rattus) in a New Zealand indigenous forest. New Zealand Journal of Science. 15: 3 1 3-34 1 .

Daniel, M . J. (1 978). Population ecology o f ship rats and Norway rats in New

Zealand. In P. R. Dingwall, I. A. E. Atkinson and C. Hay (eds.), The Ecology and Control of Rodents in New Zealand Nature Reserves. Department of Lands and Survey, Wellington.

Ewer, R. F. ( 1 971). The biology and behaviour of a free-living population of black rats (Rattus rattus). Animal Behaviour Monographs. 4: 1 27- 1 74.

Hooker, S., and Innes, J. G. ( 1 995). Ranging behaviour of forest dwelling ship rats, (Rattus rattus) and effects of poisoning with brodifacoum. New Zealand Journal of Zoology. 22:29 1 -304.

Innes, J. G. ( 1 990). Ship Rat. In C. M. King (ed.), The Handbook of New Zealand Mammals, pp. 206-225. Oxford University Press, Auckland.

Innes, J. G., and Skipworth, J. P. ( 1 983). Home ranges of ship rats in a small New Zealand forest as revealed by trapping and tracking. New Zealand Journal of Zoology. 10:99- 1 1 0.

mnes, J. G., Warburton, B . , Williams, D., Speed, H., and Bradfield, P. ( 1995). Large­ scale poisoning of ship rats (Rattus rattus) in indigenous forests of the North Island, New Zealand. New Zealand Journal of Zoology. 19:5-1 7.

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King, C. M., and Edgar, R L. ( 1 977). Techniques for trapping and tracking stoats (Mustela erminea); a review and a new system. New Zealand Journal of Zoology. 4: 1 93-2 1 2 .

King, C. M . , mnes, J. G . , Flux, M., Kimberley, M . 0 . , Leathwick, J. R, and Williams, D. S. ( 1 996). Distribution and abundance of small mammals in relation to habitat in Pureora Forest Park. New Zealand Journal of Ecology. 20:2 1 5-240.

Korpimaki, E., and Norrdahl, K ( 1 998). Experimental reduction of predators reverses the chrase phase of small-rodent cycles. Ecology. 79:2448-2455.

Korpimaki, E., Norrdahl, K, and Rinta-Jaskari, T. ( 199 1 ). Responses of stoats and least weasels to fluctuating food abundances: is the low phase of the vole cycle dus to mustelid predation? Oecologia. 88:552-5 6 1 .

Lebreton, J. D., Burnham, K P., Clobert, J., and Anderson, D. R. ( 1 992). Modeling survival and testing biological hypotheses using marked animals: A unified approach with case studies. Ecological Monographs. 62:67- 1 1 8 .

Lidicker, W. Z . ( 1966). Ecological observations on a feral house mouse population declining to extinction. Ecological Monographs. 36:27-50.

Miller, C. J., and Miller, T. K ( 1 995). Population dynamics and diet of rodents on Rangitoto Island, New Zealand, including the effect of a 1 080 poison operation. New Zealand Journal of Ecology. 19: 1 9-27.

Pollock, K H., Nichols, J. D., Brownie, c., and Hines, J. E. ( 1 990). Statistical inference for capture-recapture experiments. Wildlife Monographs. 107: 1 -97.

Chapter 2. Calibration of density indices ₄₈

Reid, D. G., Krebs, C. J., and Kenney, A. ( 1 995). Limitation of c ollared lemming population growth at low densities by predation mortality. Dikos. 73: 387-398.

Sarrazin, J. P. R., and Bider, J. R. ( 1 973). Activity, a neglected parameter in

population estimates - the development of a new technique. Ecology. 54:369- 382.

Sheppe, W. ( 1 965). Characteristics and uses of Peromyscus tracking data. Ecology.

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Tanaka, R. ( 1 960). Evidence against the reliability if the trap-night indez as a relative measure of population in small mammals. Japanese Journal of Ecology.

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Chapter 3. The diet and dietary selection of ship rats ₄₉

Chapter 3: The diet and dietary selection of the introduced ship rat