Habitat Choice and Demography

Demography and habitat choice of C. variolosas were investigated in a 2 year mark-recapture study at two representative relict populations of C. v. nodalosas in NorthWestern Germany. Parameters determining the habitat preferences of adult beetles were estimated from habitat suitability models (e.g. Matern et al. 2007). These studies have highlighted and further refined which specific requirements determine habitat associations (Matern et al. 2007). Key habitat variables governing the occurrence of the adult beetles were found to be: short distance to water, high soil moisture, open woodland vegetation cover, a near-neutral pH of the soil and the lack of ground cover. Accordingly, the extent of suitable habitat was rather restricted (Fig. 1). Continuous observations of individual beetles confirmed their confinement to regions of wet soil and close to, or covered by, water (Drees et al. 2008).

Mark-recapture data showed C. variolosus to be a spring breeder in the sense of Larsson (1939), i.e. reproduction took place after the emergence of overwintering adults between April and June. Estimates of population sizes at both sites were very low (Matern et al. 2008). During the spring activity period in 2004 the total number of different adults captured amounted to 63 (Site 1) and 161 (Site 2) individuals, corresponding to a maximum of 150 (Site 1) and 215 (Site 2) individuals when corrected using the Jolly-Seber estimate (Jolly 1965; Seber 1965). Population densities, however, estimated at 1.75 (Site 1) and 0.85 (Site 2) individuals per 10m2, were normal to fairly high in comparison to more common species of Carabus

Fig. 1 Habitat preference of two C. variolosus nodulosus populations in the Forest of Arnsberg, Northwestern Germany. Occupied habitat is represented in grey. Figure 2 shows the location of these sites. (a) Population 1, (b) Population 2

(Matern et al. 2008). Hence, the total population sizes appear to be limited by the extent of suitable habitat, rather than low density. This illustrates the imminent threat to population survival from mechanisms operating in small habitat patches (extinction vortex, Gilpin and Soule 1986). However, adults were found to be long-lived and almost 30% of the reproductively active individuals were at least in their second breeding period (Matern et al. 2008). This reduces risks of reproductive failure from climatic variation or other stochastic factors in any given year.

Was this article helpful?

0 0

Post a comment