A Correlation Analysis of Weather versus Snowshoe Hare Population Parameters

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Computer facilities were utilized to examine five snowshoe hare (Lept4s americanus) population parameters for correlations with 12 weather factors in a wide array of time periods. Intensity of illumination in midwinter, as measured by cloud cover, was highly significantly correlated with the date of onset of breeding the following spring. No significant correlations between weather and the date of testis regression were disclosed. Between-year differences in litter size were significantly correlated with temperatures and snow depth. The colder the temperatures in the 250 days preceding mid-February, and the deeper the winter's accumulation of snow, the larger the litters the next spring. Adult survival in the period January through April 25 was highly significantly correlated with both temperatures and snowfall; the critical periods were the 8() days preceding April 2S, and January 5 to March 26, respectively. Colder temperatures and deeper snow in these periods correlated with poorer adult survival. Relative survival of first-litter young during their first 45 days of life was highly significantly enhanced by brighter days and warmer temperatures throughout the 45-day survival period. Relative survival of second-litter hares was highly significantly negatively correlated with rainfall. Both total rainfall and the number of days with rainfall throughout almost the entire 45-day survival period were involved. Relationships expressed by the correlations were compared with the findings of other investigators; possible biological mechanisms operative in the correlations were outlined. In a previous paper ( Meslow and Keith 1968) we documented demographic changes in a snowshoe hare population between 1961 and 1967 at Rochester, Alberta. Fluctuating rates of adult survival, juvenile survival, and birth all contributed to observed numerical changes. The present paper examines, through correlation analysis, relationships between weather and elements of the previously documented population parameters. 1 A contribution of the Department of Wildlife Ecology, University of Wisconsin, and the Research Council of Alberta; study supported by the Research Council of Alberta; the College of Agricultural and Life Sciences, University of Wisconsin; the National Institutes of Health ( Grant A104725); the Ford Foundation (Grant 63-505); and the National Science Foundation ( Grant GB-7744 ) . 2 Present address: Department of Zoology, North Dakota State University, Fargo. Incorporation of unpublished hare data from 1968 gave us population statistics spanning 6 to 8 years. We limited investigations of weather factors to those standard observations of temperature, precipitation, and cloud cover recorded by most climato logical reporting stations. An IBM 1620 computer facilitated the correlation of population statistics versus weather data. While the resulting correlations may imply cause and effect relationships, in themselves they prove nothing. We used the data-processing facilities of the College of Agricultural and Life Sciences, University of Wisconsin, and gratefully acknowledge Arden Hardie, Department of Dairy Science, for computer programming and James Torrie, Department of Agronomy, for statistical advice. This content downloaded from 207.46.13.124 on Wed, 22 Jun 2016 06:03:21 UTC All use subject to http://about.jstor.org/terms a Based on population estimates and acreages of the study areas (Meslow and Keith 1968, and unpublished). b Date on which the average testis weight fell below 4 grams. e Based on the ratio of juvenile/adult captures; corrected for litter size. dHare density estimated as greater than the densities of 1960 and 1962 (Meslow and Keith 1968:815). e Mean date of birth. 2 Journal of Wildlife Matlagement, Vol. 35, No. 1, January 1971 for correlution with weather factors. Table 1. Population porameters of Rochester, Alberta snowshoe hares used, RELATIVE JUVENILE ADULT SURVIVAL RATE SURVIVAL (BIRTH TO AGE 45 DAYSC) (JAN.-APR. ) Litter 1 I.it:ter :> HARES/1SOOO ACRES IN YEAR SPRINGa 1961 Peak Populationd