FACTORS REGULATING LOCAL VARIATIONS IN ABUNDANCE - FIELD-TESTS ON HEDGEHOGS, ERINACEUS-EUROPAEUS

This paper reports the results of a field experiment to identify factors regulating local variations in population sizes of a common insectivore. A low density population of hedgehogs Erinaceus europaeus was artificially increased in numbers while simultaneously a high density population was reduced. Subsequent dispersal, mortality and natality were monitored in complete censuses of the total populations, and compared to a control population of transplanted hedgehogs at unchanged density. Predation on hedgehogs was principally by badgers Meles meles which forage for substantially similar foods to hedgehogs, and was significantly higher in the expanded population during the first 2 wk after transplant than in the control, which nevertheless suffered a high overall mortality. Dispersal of male hedgehogs in particular from the expanded population contributed to reducing the density close to its original level within 1 month of transplant, after which the population remained relatively stable for a further 2 months of monitoring. Surviving hedgehogs at this site maintained closer proximity to residential buildings. which were avoided by badgers, than those at the control site, rates of weight gain by males were also superior to those in the control population. The artificially reduced population in an urban area containing no badgers was partially replenished within 1 month. through a decreased loss rate and an increased gain rate, initially of adults and later in the year of juveniles born after the transplant. The hedgehogs here showed seasonal variation in dispersion between fields, which were associated with the breeding cycle and the distribution of earthworm prey. Hedgehogs utilized mown grass fields in relation to the availability of earthworms Lumbricus spp., which was itself dependent on the age of fields. The study demonstrates that predation can influence the abundance of hedgehogs as well as their distribution in a heterogeneous environment. Residential and urban patches of land appear to represent a niche largely free from badger activity, where hedgehog populations maintain a high equilibrium density despite predation by domestic dogs which can contribute to a rapid population turn-over.