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Kristiansson (1990) hedgehog population variables mortality causes

Title: Population variables and causes of mortality in a hedgehog (Erinaceus europaeus), population in southern Sweden, Masters Journal of Zoology, London 220; 391-404.  1990

Authors: H. Kristiansson

Country: Sweden

Background to study

A longitudinal study of the population dynamics (density, mortality & recruitment) of hedgehogs occupying rural habitat in southern Sweden.

Method

  • Between April/May and September/October, for a period of 8 years (1972-79), hedgehogs were captured during night time spotlight surveys within a small country village comprising of 250 residential dwellings and gardens, amenity green space, copses and grassland.
  • Each captured individual was weighed; their sex and age determined (juveniles as first summer, sub adults as second summer and adults after their third summer) and marked using a permanent ear tag.
  • Capture-mark-recapture data was used to estimate yearly density, recruitment, population structure and survival).

Key results

  • Between 21 and 57 non-juveniles and 15 and 39 juveniles were estimated as occupying the study area across the study period which equates to densities ranging from 0.3 to 0.78 individuals per hectare.
  • The average population growth rate was 0.7 and varied across years, ranging from -0.3 between 1977 and 1979 to 0.6 between 1975 and 1976.
  • There was no significant skew in the sex ratios of any age class across the study period.
  • The estimated number of juveniles recruited varied from 15 to 39 and was positively correlated to the non-juvenile density apart from 1978 when juvenile, but not non-juvenile numbers reduced.
  • The average number of juveniles per female (sub-adults and adults) was 1.43 (range: 0.61-2.44) and when only considering adult females, was 2.79 (range: 1.33-3.67).
  • Recruitment rate did not show any correlation with non-juvenile population density.
  • Average annual mortality was 47% and 34% of non-juveniles and juveniles respectively.
    • Summer and winter average mortality was 15% (range: 3-22%) and 33% (range: 26-33%) respectively for non-juveniles and 3% (range: 0-11%) and 33% (range: 6-94%) respectively for juveniles.
    • The main cause of summer mortality was road traffic and this was primarily sub adults and adults.
    • Juvenile and sub-adult (but not adult) mortality overwinter increased with increasing adult density.
  • Winter mortality of adult and sub-adult hedgehogs was significantly associated with temperatures that were much lower than normal and with increasing duration of cold weather conditions (< 0°C).
  • Juvenile mortality was not correlated to weather conditions.
  • Juvenile (J) and sub-adult (SA) hedgehogs that survived overwinter were significantly heavier in weight
  • (J: 710 g +5 g and SA: 1333 g + 161.5 g) than non-survivors (J: 665 g + 96.1 g and SA: 1179 g + 118.5 g) and when considering sex, surviving males, but not females were significantly heavier (J: 721 g + 99 g and SA: 1390 g + 145.6 g) than non-survivors (J: 671 g + 102.87 g and SA: 1170 g + 774.3 g).
  • Juvenile females had a significantly higher survival rate than juvenile males and both male and female survivorship decreased with increasing age.
  • Average life expectancy for juvenile males and females was 1.2 yrs and 1.3 yrs respectively and survival rate from birth to weaning was estimated to be 0.4 and 0.32 for males and females respectively.

Key messages to landowners and managers derived from these results

  • Increasing the amount and accessibility of hibernacula is recommended to reduce the risk of overwinter mortality and can include: Encouraging gardeners to establish shrubby areas and permeable fencing (holes or hedgerows) for hedgehogs or establishing more shrub boundaries and hedgerows in rural/urban areas.
  • Road crossings by hedgehogs are the primary cause of death during summer and an investigation into possible road crossing structures for hedgehogs is recommended.
  • Establishing structural connectivity between urban/rural green spaces and encouraging landowners to establish wide grassy margins on farmland will help to increase food abundance for hedgehogs to help them attain a suitable weight for surviving hibernation.

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