Impact of light grazing on the populations of P. eunomia and L. helle proved to be non-existent to moderate (< 30% loss) or heavy (> 50% loss) according to the grazing regime adopted and the species concerned, a periodic application generally appearing less harmful than a continuous one. This could be explained by the negative impact of livestock on (1) nectar flowers during the flight period of the butterflies and on (2) the bistort due to the trampling on rhizomes, and some direct impact on the early stages of the butterflies (Schtickzelle et al. 2007). However, butterfly losses amounting to > 70%, as reported by Schtickzelle et al. (2007) on P. eunomia in another Ardenne site (Prés de la Lienne), were only recorded on the site with continuous grazing regime (Martelange) in the present study. They were far more reduced on the other sites where periodic grazing was practiced. The regime applied in Prés de la Lienne was also continuous, but with heavier stocking rates (comprised between 0.23 and 0.42 Livestock Units ha-1 on an annual basis), even higher during the summer (0.54-0.74 LU ha-1), compared to the sites studied here.
The influence of tree/edge density on L. helle counts is readily explained by several aspects of adult life in this species. Indeed, scrubs and trees, from the vicinity of which they are rarely found away (Fisher et al. 1999; Turlure et al. 2009), are used as territorial, nectaring, and roosting places. First, males exhibit a perching territorial behavior along forest edges and bushes, engaging chasing flights after males and waiting for females to mate with. Moreover, both males and females exploit flowering bushes and trees in search of nectar and higher trees for roosting: at the end of the day, they reach the top of the trees, step by step, where they usually spend the night (Goffart and Waeyenbergh 1994; Goffart et al. 2001; Goffart unpublished data). L. helle appears to find an optimal habitat in sites with an intermediate density of edge and trees in and around wet grasslands.
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