Introduction

In Europe, and particularly in Belgium, butterfly diversity has strongly decreased during recent decades (Van Swaay et al. 2006; Maes and Van Dyck 2001; Goffart et al. 1992; Fichefet et al. in press); specialist species have been particularly affected. Of prime importance in this context is the conservation of remnant relict butterfly species. The wet meadows and peat bogs of the Belgian Ardenne high plateaus still offer ecological conditions close to those of Northern Europe, allowing populations of relict butterfly species to persist.

However, these populations, at the Southern margin of their distribution range, have a sparse and disjunct distribution (Kudrna 2002). Moreover, they are spatially trapped: they cannot shift their distribution range either to the North, because there is no suitable habitat patch between this region and other populations widespread in Northern Europe (Parmesan et al. 1999; White and Kerr 2006), or uphill because the upper altitudinal limit is reached in the region (Konvicka et al. 2003).

The bog fritillary Proclossiana eunomia and the violet copper Lycaena helle are two remarkable relict butterflies of the Ardenne massif. Their Belgian populations are amongst the most important of Western Europe and are genetically unique (Meyer 1980, 1981a,b; Barascud and Descimon 1992; Neve et al. 2000; Descimon et al. 2001; Finger et al. 2009). Moreover, they are in decline and threatened (Fichefet et al. 2008), as is the case in other parts of Europe too, where L. helle is classified as "vulnerable" (van Swaay and Warren 1999) and recently added to the Appendices II and IV of the European Habitat Directive (92/43/CEE). Their preservation is, therefore, highly desirable.

All remnant populations of relict species in the Ardenne region inhabit pioneer more semi-natural biotopes. Without any disturbance, these biotopes are progressively invaded by shrubs and trees, turn to forest, and ultimately disappear. In extended ecosystems, natural disturbances, like fires or storms, can rejuvenate the biotope by converting it back to open vegetation (Bullock and Pakeman 1997; Svenning 2002). This is no longer the case anywhere in the Western Europe today because of the small area of such remnants. Therefore, human actions are clearly needed to mimic the effects of natural disturbances, through the use of management techniques such as mowing, grazing, and tree cutting (Sutherland 2002). Such management often positively affects the ecological processes that are crucial for biodiversity, by breaking the dominance of highly competitive plants, reducing bush encroachment, or making vegetation structure more varied (Ausden and Treweek 1995). Although these management methods are widely and more or less efficiently implemented on biotopes (WallisDeVries et al. 1998; Krebs et al. 1999), little is known about the detailed effects on butterfly populations and their resources.

In this context, our objectives were to assess the impact of two management techniques, mowing and grazing, on populations of P. eunomia and L. helle, which inhabit wet meadows and peat bogs in the Belgian Ardenne. Mowing was traditionally applied with non-mechanical means in the Ardenne alluvial meadows (Lambert 1963). Grazing was not used in Ardenne valleys, but is currently becoming popular among nature managers (Delescaille 2002), because of its ease of implementation.

We assessed for both species (1) the impact of mowing on larval stages (through the number of emerging adults) and on the attractiveness of mown biotopes for adults and (2) the impact of three distinct grazing regimes on the adult density. As resources and life history traits of the two species are different, we expected to find contrasted, perhaps even completely different, impacts of both management techniques according to species. Indeed, even if caterpillars of both species feed only on P. bistorta leaves, nectar use by adults is different: P. eunomia adults only feed on P. bistorta flowers, while L. helle adults use a wider range of nectar resources, from both herbaceous and woody species. Moreover, overwintering strategies are distinct (Barascud and Descimon 1992): L. helle spends the winter as a pupa, while P. eunomia spends the winter as a caterpillar. The related applied question is whether there is an efficient management regime for these two relict butterfly species at the same time.

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