Olesen, C.R & Asferg, T. 2006. NERI Technical Report No. 600. 32 p.
Recognising that multiple factors are interacting in the case of the declining European hare populations, modelling integrating resource availability, ecology, behaviour and the environment is required in future research.
Despite considerable research efforts populations of the European brown hare (Lepus europaeus ) have been declining throughout Europe since the early 1960s. The hypothesised causal factors for the decline are numerous and act via resource availability, reproduction and survival. Based on a systematic review of the literature the potential causes are discussed, a hypothesis for the underlying cause of the decline is suggested, as are future research priorities.
Studies indicate that food shortage in the modern cereal dominated agricultural landscape, possibly caused by the introduction of herbicides reducing weed biomass ten-fold, may result in a summer food bottleneck. If true, this change has postponed minimum resource availability from the winter period to the central reproductive period of the hare with distinct effects on population dynamics. Previously, seasonal variation in resource availability with summer-high and winter-low was well adjusted with the main reproductive output through spring and summer allowing high density in autumn before onset of the hunting season. In this situation the majority of mortality factors, hunting inclusive, were compensatory, only taking the surplus and thus reducing density according to the pre-winter drop in resource availability. In the modern landscape the situation may be reversed with low resource availability in summer and high during winter. Hence, the food resources available cannot support the new generation of hares resulting in low survival and eventually autumn densities far lower than the resource availability allows in this period. As a consequence most mortality during autumn and winter including hunting act as additive factors, with an elevated risk of reducing the breeding population of the following year.
Predation can not unequivocally be ruled out in having an effect on hare population dynamics, but since fertility of female hares even at reduced population densities seems far beneath the physiological maximum, predation does not appear to be the primary cause of decline.
An average number of litters per female as low as 2.3 in hare populations in intensive agricultural landscapes strongly support the hypothesis of resource limitations during the breeding season. Apparently female hares experiencing limited resources postpone ovulation until a minimum level of internal energy stores are rebuilt, thus limiting the possible number of litters during a reproductive season.
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