5 years ago

The Green Belt as a European Ecological Network strengths and gaps

The Green Belt as a European Ecological Network strengths and gaps

Andrew J. Gregory, Paul

Andrew J. Gregory, Paul Beier RESEARCHERS DESPERATELY SEEKING STABLE 50-YEAR-OLD LANDSCAPES WITH PATCHES AND LONG, WIDE CORRIDORS California, and 14 corridors in northern California (in progress). Each corridor (N = 72) is a swath of natural land 500 m to 80 km long identified for conservation so that it can support gene flow and demographic interactions between a pair of natural landscape blocks after “build-out” – i.e., after lands adjacent to the corridor and the natural landscape blocks have been converted to urban, agricultural, or industrial uses that are incompatible with wildlife movement. Despite the large body of research confirming that corridors promote wildlife movement [summarized by 7, 8], there is no strong evidence that these 72 corridors will promote demographic and genetic movement of plants and animals as intended. Evidence is lacking because these conservation corridors differ from the corridors studied by most ecologists in three critical ways, namely spatial extent, landscape context, and the response variables considered. First, almost all corridor research has concerned corridors < 150 m long, but conservation corridors are much longer. Second, in most ecological studies, a corridor is any narrow swath of land connecting two habitat patches where the patches and corridor share a land cover dissimilar from the surrounding matrix [e.g. 9]. This definition depends only on structural layout of the focal land cover type, regardless of adjacent land uses. In contrast, each conservation corridor is a swath of natural land that is (or eventually may be) embedded in urban, agricultural, or industrial landscapes. Although most conservation corridors are designated for conservation while the matrix is still in a relatively natural state, they are explicitly predicted to be useful for species conservation after build-out. With respect to response variables, almost all studies of corridor utility document only whether focal species were present in or moved through the corridor [7, 8]. Although presence and movement are necessary for corridor utility, these responses do not demonstrate that the corridor enhances demographic stability, gene flow, or recolonizations – which are ultimately the intended outcome of conservation corridors [10]. Genetic relatedness (reflecting effective connectivity among patches; [11] and long-term patch occupancy (reflecting demographic rescue and recolonization; [12]) are better response variables for assessing the effectiveness of conservation corridors. Thus, most corridor studies have not been conducted in a landscape context, at a spatial extent, or using response variables truly capable of assessing the utility of conservation corridors [8, 13, but see 14]. In this paper, we describe a study design to assess the utility of conservation corridors and identify conditions associated with successful conservation corridors. Our goal is to solicit readers to suggest appropriate landscapes for this study. AN IDEAL STUDY DESIGN Because corridors are intended to promote demographic persistence and gene flow, the two most appropriate response variables are probability of occupancy and genetic similarity. If a conservation corridor works, then the patches connected by corridors should have higher probability of occupancy than isolated patches. Inglis & Underwood [15] describe a study design to evaluate corridor effectiveness on the basis of patch occupancy. Although occupancy is a valid response variable, it has drawbacks. For the size of patches typically connected by conservation corridors, it may take a century or more to reach equilibrium among extirpation, demographic rescue, and recolonization. We therefore propose to use genetic similarity between populations in patches connected or not connected by corridors as the response variable. If a conservation corridor works, then genetic similarity between the 88

Andrew J. Gregory, Paul Beier RESEARCHERS DESPERATELY SEEKING STABLE 50-YEAR-OLD LANDSCAPES WITH PATCHES AND LONG, WIDE CORRIDORS patches connected by corridors should be higher than between isolated patches and about the same as between sampling sites in intact habitat (Figures 1 & 2). Although genetic divergence takes several to many generations [16], most populations exhibit genetic effects of isolation and fragmentation long before demographic stochasticity and extinction occur [17]. Moreover, genetic similarity is a better response variable than movement of individual animals because some movements simply do not result in reproduction or gene flow [1]. In contrast, genetic data reflect functional connectivity. Furthermore, gene flow among patches can occur in the absence of any movement by an individual animal between patches, such as when gene flow occurs incrementally across multiple generations. The best design would be a Before/After–Control/Impact (BACI) design [18]. This design would start by documenting genetic distances among focal populations inhabiting intact natural landscapes before fragmentation has occurred on the basis of samples collected in pairs of sampling sites that will become isolated patches, pairs of sampling sites that will become patches connected by corridors, and pairs of sampling sites that will remain part of a large area of natural land (Figure 1). Figure 1:. The spatial layout of an ideal design for the study of corridor effectiveness. White background and polygons indicate natural land cover; stippling indicates land converted to urban, agricultural, or industrial uses incompatible with movement by the focal species. A & B are sampling sites located in what will become patches connected by a corridor. M & N provide Reference Condition 1, namely a pair of sampling sites that become isolated patches. Y & Z provide Reference Condition 2, namely sampling sites that remain within a large expanse of natural land cover. The mapped distances AB, MN, and YZ are about equal. Note: In BACI parlance, the corridor [the conservation intervention] is the impact site and the other two conditions are controls; such terminology can be counterintuitive in a conservation context. Then the natural landscapes are fragmented, and the researcher waits 10-20 generations (time required increases with effective population sizes in the patches) for genetic patterns to respond to the new landscape configuration. Finally, genetic samples are again collected and analysed. Three possible outcomes of this design are illustrated in Figure 2. 89

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