Trampling affects the distribution of specialised coastal dune arthropods
Introduction
Both natural and anthropogenic disturbances are known to affect the viability of populations (Hansen & Clevenger, 2005; Pascual & Guichard, 2005). Many studies dealing with disturbance as a driving factor in metapopulation dynamics consider local disturbance as an agent that is directly responsible for local extinction events, so-called “catastrophes”. In these cases, metapopulation extinction is directly affected by the degree of spatial correlation in disturbance processes and habitat geometry (Kallimanis, Kunin, Halley, & Sgardelis, 2005), with spatially correlated disturbances eventually leading to metapopulation extinction in the absence of long-distance dispersal. Subsequently, deterministic disturbance events may affect patch connectivity by reducing the number of populations within the dispersal range of an organism. The degree to which populations are affected by the magnitude of local disturbance and its interaction with patch geometry, however, remains largely unanswered, although recent theoretical work highlights the importance of habitat availability, disturbance regime and dispersal properties (Kallimanis et al., 2005). If the magnitude of local disturbance is inversely related to habitat quality, then source-sink models predict that patches with increasing local disturbance remain occupied only when connected to patches with a demographic excess (Kawecki, 2004).
Within a conservation framework, species’ tolerance towards comparable disturbances regimes is often generalised (White & Jentsch, 2001). However, variation due to different spatial scales and underlying processes (Denny, Helmuth, Leonard, Harley, Hunt et al., 2004; Hobbs & Yates, 2003) is hardly addressed. Earlier theoretical (Henle, Davies, Kleyer, Margules, & Settele, 2004) and empirical work (e.g. Brouat, Chevallier, Meusnier, Noblecourt, & Rasplus, 2004; Dennis, Hodgson, Grenyer, Shreeve, & Roy, 2004; Krauss, Steffan-Dewenter, & Tscharntke, 2003; Steffan-Dewenter, 2003) revealed that distribution patterns are not only affected by differences in dispersal ability (which determine connectivity), but also by (often co-varying) traits related to habitat specialisation. In particular, species with different life-histories do not have the same resource demands and may considerably differ in their mode of habitat use (Dennis, Shreeve, & Van Dyck, 2003). As a consequence, research of multi-species responses towards environmental changes may yield complementary insights for conservation and management purposes as opposed to single-species studies (e.g. Lambeck, 1997; Maes & Bonte, 2006; Maes & Van Dyck, 2005; Simberloff, 1998). If a species from a particular ecosystem is at high risk, comparison of its response towards a suite of environmental parameters may enable local managers to predict how conservation efforts targeted at this single-focal species can affect other species that are at less risk. However, when multi-species studies reveal contrasting patterns, it is difficult to set management priorities as beneficial actions for one species may be detrimental for another one. Instead, we argue that a compromise which generates insights into the average reaction of the species-group (the assemblage from which representative species are chosen) is more practical and that patterns revealed by the latter can be translated to management priorities that are beneficial to an entire group of species. This compromise can be expected to be a strategy for an optimal and sustainable conservation of local species richness, and not of one specific target species. Consequently, the delineation of management actions based on average reactions of species towards a particular disturbance regime may provide complementary information to the ‘umbrella’ approach, in which conservation actions are based on responses of one species that is assumed to be representative of an entire species assemblage (Fleishman, Blair, & Murphy, 2001; Fleishman, Murphy, & Brussard, 2000).
In coastal dune ecosystems, trampling is often tolerated because of the assumed adaptation of species towards natural disturbance, i.e. aeolian dynamics (displacement of sand by wind; Bonte, Maelfait, & Lens, 2006). Maes and Bonte (2006) focussed in detail on the habitat demands of the selected species and landscape-related factors that influence their colonisation and extinction dynamics. We here adopt the earlier delineated ‘compromise’ approach to study the average response of five arthropod species towards trampling variation (intensity and mode) by controlling for these earlier revealed landscape-related variation (see Maes & Bonte, 2006).
Section snippets
Studied species
The set of study species comprises two butterfly species, the Grayling Hipparchia semele (Linnaeus 1758) and the Queen of Spain Fritillary Issoria lathonia (Linnaeus 1758), the Blue-Winged Grasshopper Oedipoda caerulescens (Linnaeus 1758) and the spiders Xysticus sabulosus (Hahn 1832) and Alopecosa fabrilis (Clerck 1757). These species are restricted to dynamic grey dunes in the coastal area (Provoost & Bonte, 2004). Local distribution patterns in coastal dunes are affected by patch size, patch
Methods
The study was carried out in 133 patches of dynamic grey dunes along the coast between Nieuwpoort – Belgium (51°08′ N, 2°43′ E) and Bray-Dunes – France (51°05′ N, 2°33′ E; see Supplementary Appendix A: Fig. S1). Like other coastal dunes in Western Europe, they comprise semi-natural landscapes that have been influenced by agro-pastoral use prior to the 1950s. Owing to the withdrawal of agro-pastoral activities, the rise of tourist activities and the dramatic decline in rabbit populations due to
Results
Patch area, trampling intensity and species-specific connectivity measures only show weak (r2<0.20) correlations (see Supplementary Appendix A). Most important is the positive relationship between patch area and connectivity for all species, mainly due to the absence of small patches with high connectivity. Fragments in sites subjected to recreational trampling are smaller than those subjected to cattle (Table 1) and those excluded for both cattle and vacationers. Mean trampling intensity was
Discussion
Our selected species clearly show different reactions towards landscape–ecological features and disturbance, as reflected in the high variation components of the random factors. Threshold values to ascertain the presence of these species have been documented in Maes and Bonte (2006). We here show that average occupancy patterns of dune arthropods clearly depend on the landscape structure, but also on its interaction with characteristics of local trampling disturbance. The strength of the
Acknowledgements
Ward Vercruysse provided assistance during fieldwork. We are very grateful to Roger Dennis and Luc Lens for valuable comments and linguistic streaming and to Sam Provoost for his permission to use his digital orthophotographs. Koen De Smet granted us permission to study the legally protected O. caerulescens (license number AMINAL/NAT/DI.832.172/2275). We are also grateful to three anonymous referees for valuable suggestions to improve the quality of this manuscript. Dries Bonte holds a post-doc
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