Linking the Impact of Mowing with Habitat Requirements

The impact of mowing was clearly more pronounced on P. eunomia than on L. helle. The disastrous impact of mowing, both when done in summer and in autumn, on P. eunomia larvae is easily explained by the usual location of caterpillars,

Fig. 2 Impact of mowing treatment on the attractiveness of the area for the two butterfly species. Shown are the numbers and associated percentages of adult butterflies of each species counted in control and mown plots

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P. eunomia L. helle which find shelter preferentially in grass tussocks, from the beginning of summer until the next spring (Goffart et al. 2001). Indeed, grass tussocks appear to play a crucial role in caterpillar thermoregulation (Turlure unpublished data), and probably also allows them to escape from parasitoids, predators, and winter flooding (Joy and Pullin 1997; Severns et al. 2006). These tussocks are cut during mowing, and caterpillars very likely removed with the hay, causing direct mortality. Concerning the avoidance of mown areas - or at least the lower attraction towards the mown areas - displayed by P. eunomia adults, it may be related to the change of biotope physiognomy induced by mowing: grass-tussocks are cut and relief becomes almost flat, giving a less attractive vegetation facies for males and females (Turlure et al. 2009). This has been confirmed by another experiment, in which the same plots were manipulated by sequentially placing and removing grass tussocks during a single day, and comparing attractiveness for P. eunomia (Goffart unpublished data). Both sexes indeed actively search for tussocks: females find appropriate locations for egg-laying and males find a higher probability to encounter virgin (unmated) females at the precise location of their emergence.

For L. helle, the period of mowing proved to be crucial for the survival of larvae. The contrasted responses to summer and autumn mowing can be understood in the light of the butterfly life history: caterpillars, hanging under the leaves in summer, are likely to be carried away with the hay and suffer direct mortality, whereas the overwintering pupae, settling on the top soil in autumn, escape from later mowing. Survival of the pupae (and hence, emergence success) might be even higher in the mown areas, because the shorter vegetation height and associated change in micro-climatic conditions (Thomas 1991; Bourn and Thomas 2002) induced by mowing could be more favorable for them. Furthermore, L. helle adults seemed to be attracted towards the mown areas. This was clearly linked to the higher density of some nectar flowers like Cardamine pratensis (Goffart et al. 2001), as the majority of adult butterflies were observed feeding on them. L. helle adults being attracted to mown areas where their offspring suffer from elevated mortality due to mowing actions, an "ecological trapping effect" (Shlaepfer et al. 2002; Battin 2004) might occur if mowing is applied every year in the stipulated period, i.e., in summer.

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