New Zealand Journal of Zoology abstracts
Population dynamics of the ship rat Rattus rattus L. in
the Orongorongo Valley, New Zealand
M. G. Efford*
Landcare Research
Private Bag 1930
Dunedin 9054, New Zealand
murray.efford@stonebow.otago.ac.nz
B. M. Fitzgerald
Ecological Research Associates of New Zealand
PO Box 48147
Silverstream
Upper Hutt 5142, New Zealand
bmfitzgerald@ezysurf.co.nz
B. J. Karl
Landcare Research
Private Bag 6
Nelson Mail Centre
Nelson 7042, New Zealand
karlb@landcareresearch.co.nz
P. H. Berben
Landcare Research
Private Bag 11052
Manawatu Mail Centre
Palmerston North 4442, New Zealand
berbenp@landcareresearch.co.nz
*Present address: Zoology Department, University of
Otago, PO Box 56, Dunedin 9054, New Zealand.
Abstract The population dynamics of ship rats (Rattus
rattus L.) were studied in podocarp/hardwood and hard beech (Nothofagus
truncata) forest in the Orongorongo Valley, near Wellington, from
1971 to 1998. Rat density (indexed by captures in 116 snap traps over 3
nights per quarter) varied up to five-fold between consecutive years.
Fruiting of hinau (Elaeocarpus dentatus) and hard beech did not
drive the fluctuations as had been suggested. Average density increased
over the first 15 years of the study (from 2.3 ± 0.3 to 7.5
± 0.8 per 100 trap-nights (mean ± SE)), and then
declined; this mirrored a trend in cat density driven by external
factors. Breeding was largely restricted to spring (November) and
summer (February). Breeding rate (proportion pregnant or lactating) was
density-dependent, particularly in older rats. Litter size (based on
live embryos and new uterine scars) increased between spring (4.4
± 0.4) and summer (5.5 ± 0.3) (mean ± SE), but did
not vary with density or age. Winter breeding was rare (2% pregnant or
lactating in August). Hinau fruitfall did not predict the proportion
breeding in autumn (May), or survival in winter-spring (i.e.,
population change between August and November). Varying predation
intensity was the most likely explanation for the population trend. We
review other New Zealand studies and question the assumption that ship
rat populations are “eruptive”, except possibly when predation is
eliminated.
Keywords Rattus rattus; Elaeocarpus
dentatus; Nothofagus; frugivory; predation; phenology;
seasonal breeding; masting; feral cat; stoat; eruptive dynamics
Z06019; Online publication date 20 October 2006 Received 3 June
2006; accepted 25 September 2006
New Zealand Journal of Zoology, 2006, Vol. 33: 273–297
0301–4223/06/3304–0273 © The Royal Society of New Zealand 2006
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