Wildlife Research Wildlife Research Society
Ecology, management and conservation in natural and modified habitats
RESEARCH ARTICLE

Primary and secondary resource pulses in an alpine ecosystem: snow tussock grass (Chionochloa spp.) flowering and house mouse (Mus musculus) populations in New Zealand

Deborah J. Wilson A B and William G. Lee A
+ Author Affiliations
- Author Affiliations

A Landcare Research, Private Bag 1930, Dunedin 9054, New Zealand.

B Corresponding author. Email: wilsond@landcareresearch.co.nz

Wildlife Research 37(2) 89-103 https://doi.org/10.1071/WR09118
Submitted: 9 September 2009  Accepted: 13 January 2010   Published: 16 April 2010

Abstract

Context. Rodent populations in many parts of the world fluctuate in response to resource pulses generated by periodic high seed production (masting) by forest trees, with cascading effects on predation risk to other forest species. In New Zealand forests, populations of exotic house mice (Mus musculus) irrupt after periodic heavy beech (Nothofagus spp.) seedfall. However, in alpine grasslands, where snow tussock grasses (Chionochloa spp.) also flower and set seeds periodically, little is known about house mouse population dynamics.

Aims. Our primary objective was to test for an increase in alpine mouse density following a summer when snow tussocks flowered profusely. We also estimated mouse density in adjacent montane forest over 2 years, and assessed mouse diet, to predict their potential impacts on native species.

Methods. Flowering intensity of Chionochloa was assessed by counting flowering tillers on permanent transects (2003–06). Mouse density was estimated with capture–mark–recapture trapping in alpine (2003–07) and forest (2003–04) habitats. Mice were also collected and their stomach contents analysed. Flowering or fruiting of alpine shrubs and herbs, and beech seedfall at forest sites, were also measured.

Key results. Chionochloa flowered profusely in austral summer 2005/06. Between autumn (May) and spring (November) 2006, mean alpine mouse density increased from 4 ha–1 to 39 ha–1, then declined to 8 ha–1 by autumn (May 2007). No mice were captured in 768 trap-nights during the following spring (November 2007). Prior to the mouse irruption, mouse density was consistently higher at alpine (0.4–4.0 mice ha–1) than at montane forest (0.02–1.8 mice ha–1) sites (in 2003–04). Alpine mouse diet was dominated by arthropods before mast flowering, and by seeds during it.

Conclusions. The density and dynamics of alpine mice in relation to intensive snow-tussock flowering were similar to those in New Zealand beech forest in relation to beech masts.

Implications. We predict the timing and duration of periods of heightened predation risk to native alpine fauna, as the result of pulses in mouse density and likely associated pulses in the density of stoats (Mustela erminea), a key exotic predator.


Acknowledgements

We thank Gary McElrea, Lisa McElrea, Peter Lei, Rachel Peach and Mike Perry for organising field trips, and many Landcare Research staff and students for fieldwork and laboratory work. Murray Efford, Mike Fitzgerald, Chris Gillies, Kelvin Lloyd, Elaine Murphy and Clare Veltman gave scientific advice. Corinne Watts, John Dugdale, Leonie Clunie and Robert Hoare assisted with invertebrate identification. Southland Conservancy, DOC, supplied research permits, and the Borland Lodge provided accommodation and other assistance. Craig Briggs prepared the base map for Fig. 1. Christine Bezar, Dave Kelly, C. M. (Kim) King, Adrian Monks and Des Smith gave very valuable comments on earlier versions of this paper. The research was done with the approval of the Landcare Research Animal Ethics Committee (AEC 03/01/01 and 04/12/03) and was funded by the New Zealand Department of Conservation, the Miss E. L. Hellaby Indigenous Grasslands Research Trust, and the New Zealand Foundation for Research, Science and Technology.


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Appendix 1.  Number of seeds per square metre collected at three montane beech forest sites in the Borland Valley in 2003–06
Collections identified by bold type were used in between-year comparisons of autumn (Feb.–May or Mar.–May) seedfall. Autumn collections from 2003 and 2003/04 were also compared with winter (May–Nov.) collections from the same years. The start date used for each period is the final day of the field trip when seed trays were established or were last emptied, although collection in individual trays may have begun several days earlier
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Appendix 2.  Numbers of captures, density (WR09118_E1c.gif) and spatial detection parameters (WR09118_E1d.gif and WR09118_E1e.gif) estimated by program Density for each capture session of house mice in the Borland Valley, February 2003 to November 2007, at three sites in beech forest (F1–F3) and three sites in alpine tussock grassland (A1–A3)
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Appendix 3.  Alternative models of the relationship between flowering of snow tussocks (Chionochloa spp.) and alpine shrubs and herbs (2002/03 to 2005/06), and mouse abundance the next summer (2004–2007)
Table shows alternative fixed-effect models, where Int = Intercept; Tussock = all Chionochloa species combined (loge-transformed); other species names as in Fig. 2b (arcsin(square-root)-transformed). Correlations between variables show ranges of the correlation coefficients r over the three sites. Estimates and standard errors represent the coefficients fitted for the Tussock term and/or the other flowering variable fitted in each model. All models also included a random Site term
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