Influence of land mosaic composition and structure on patchy populations: the case of the water vole (Arvicola sapidus) in Mediterranean farmland

Abstract

The ability of patchy populations to persist in human-dominated landscapes is often assessed using focal patch approaches, in which the local occurrence or abundance of a species is related to the properties of individual patches and the surrounding landscape context. However, useful additional insights could probably be gained through broader, mosaic-level approaches, whereby whole land mosaics with contrasting patch-network and matrix characteristics are the units of investigation. In this study we addressed this issue, analysing how the southern water vole (Arvicola sapidus) responds to variables describing patch-network and matrix properties within replicated Mediterranean farmland mosaics, across a gradient of agricultural intensification. Patch-network characteristics had a dominant effect, with the total amount of habitat positively influencing both the occurrence of water voles and the proportion of area occupied in land mosaics. The proportions of patches and area occupied by the species were positively influenced by mean patch size, and negatively so by patch isolation. Matrix effects were weak, although there was a tendency for a higher proportion of occupied patches in more intensive, irrigated agricultural landscapes, particularly during the dry season. In terms of conservation, results suggest that water voles may be able to cope well with, or even be favoured by, the on-going expansion of irrigated agriculture in Mediterranean dry-lands, provided that a number of patches of wet herbaceous vegetation are maintained within the farmland mosaic. Overall, our study suggests that the mosaic-level approach may provide a useful framework to understand the responses of patchy populations to land use change.

Figure 1. Location of the study region and sampling sites (land mosaics) used to investigate water vole occupancy according to patch-network and matrix characteristics.

Examples of four land mosaics with different patch-network and matrix characteristics are also presented. Triangles, circles and squares represent sampling sites surveyed respectively in 2006 (n = 20), 2007 (n = 37), and 2008 (n = 18). Colours indicate the sampling season of surveys: dry season (black, n = 38) and wet season (grey, n = 37) (see text for details).

Figure 2. Redundancy Analysis (RDA) relating patch-network and matrix gradients performed for the 69 land mosaics including suitable habitat for water voles.

Bi-plot of the first two canonical axes of patch-network (H1, H2, H3) and matrix gradients (M1, M2, M3). Patch-network variables and sites were scaled symmetrically by the square root of eigenvalues. Eigeinvalues for axis 1 = 0.304, and axis 2 = 0.059. Habitat-matrix correlations for the first two axes were 0.996 and 0.942. Explained variation was 0.37, pseudo-F = 3.01, p = 0.01. Effects of matrix characteristics on patch-network structure were significant in respect to irrigated agriculture (M1, p<0.01) and pasture intensification (M2, p = 0.02), but not significant regarding forest plantation (M3, p = 0.827).