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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 β £ Δ Figure 2: Possible outcomes of the study design illustrated in Figure 1. The dotted line indicates genetic distances between patches A and B, the solid line indicates genetic distances between patches M and N, and the dashed line indicates genetic distances between Y and Z. Figure 2β: results supporting the conclusion that the corridor worked. Genetic distances between all pairs of patches were similar before build-out, but over time the genetic distance between isolated patches increased while the genetic distances between patches connected by corridors remained low and similar to genetic distance between sampling locales in intact habitat. Figure 2£: Results suggesting a partially successful corridor. The corridor promoted more gene flow than occurred between isolated patches, but less gene flow than occurred across intact habitat. Figure 2Δ: an outcome consistent with failure of the corridor. The patches linked by a corridor became as genetically dissimilar as isolated patches while gene flow was maintained between sampling locales in intact habitat. We believe that this study should be conducted and we advocate that conservation scientists and management agencies collect DNA samples from appropriate species in a variety of landscapes, and store the samples for future analysis. Appropriate sampling sites can be identified most reliably in areas where large-scale conservation plans, such as Habitat Conservation Plans or other large-extent, ecosystem-based conservation plans [19, 20], are being initiated. Unfortunately, it will take between 30-100 years for data from this design to be useful for conservation planning because the fully controlled BACI design requires protecting swaths of land as conservation corridors, waiting 10-30 years for build-out to destroy most of the remaining habitat, and waiting an additional 20-50+ years (10-20 generations) for genetic consequences of isolation and corridors to manifest. MODIFYING THE STUDY DESIGN FOR FASTER RESULTS To get information without waiting several decades, we propose to use a space for time substitution by identifying and studying landscapes containing corridors that resemble conservation corridors (even if they were not designed as such) that have been stable for 20- 50 years. We call these “de facto” conservation corridors to distinguish them from corridors explicitly designed as conservation interventions. This design is the same as illustrated in Figures 1 & 2, except that it lacks any genetic samples prior to build-out. A crucial assumption is that contemporary gene flow among sampling locales within intact habitat is similar to the gene flow that occurred between all pairs of patches in the landscape before humans altered it. In other words, the landscape prior to major human disturbance resembled 90

Andrew J. Gregory, Paul Beier RESEARCHERS DESPERATELY SEEKING STABLE 50-YEAR-OLD LANDSCAPES WITH PATCHES AND LONG, WIDE CORRIDORS the landscape in the Before panel of Figure 1, such that pre-disturbance genetic distances probably were similar between all pairs of sampling sites. So far, only two or three published studies have used this design. Mech & Hallett [21], studied red-backed voles in a matrix of 20-year old clear-cuts in Oregon. The genetic distance between the patches connected by corridors was lower than that between isolated patches (leading them to conclude that the corridors worked) but higher than between sampling areas in intact habitat; suggesting that corridors provided only limited connectivity, as in Figure 2£. Horskins et al. [22] studied two woodland rodents in an 85-year-old landscape containing one corridor in a pasture matrix. The genetic distance between populations in the connected patches was similar to the distances between completely isolated patches, and much greater than the genetic distance between sampling locales within nearby intact woodland as in Figure 2Δ. They concluded that the corridor failed. Banks et al. [23] evaluated the extent to which exotic pine plantations restricted dispersal of a small marsupial between remnant patches of native forest in Australia. The genetic dissimilarity between isolated patches was greater than genetic dissimilarity between individuals sampled at similar distances within a linear “corridor” (which differed from conservation corridors in that there were no terminal patches) or between individuals sampled at similar distances in continuous native forest. They concluded that the corridors likely facilitated natural levels of gene flow. These three studies demonstrate that some corridors may facilitate gene flow, whereas others do not. Unfortunately, each study contained only one landscape, so we cannot infer why the corridors in each landscape succeeded or failed. We speculate that the corridor through the Australian pasture failed because it was too narrow (50-300 m wide along its 4.5km length). Strong inference about the effect of corridor width or other features requires replicating this study design across many landscapes that vary with respect to these features. We propose to do just that. DESPERATELY SEEKING STABLE, 50-YEAR-OLD LANDSCAPES WITH PATCHES AND LONG, WIDE CORRIDORS It is the authors’ good fortune to live in the south-western U.S.A., an area with many large and intact natural landscapes; a place where fragmentation has occurred too recently for genetic response to be manifest. We need help from readers and colleagues to identify appropriate study systems, where each study system consists of a landscape and focal species. Interested persons can provide information at www.docorridorswork.org. We encourage readers to direct potential informants to the website. Small honoraria are available for informants who provide leads to study systems that become part of our study. Each study system should meet all seven of the following criteria: Historically continuous habitat.--Prior to human alteration of the landscape, the natural cover types used by the focal species must have been widespread and relatively continuous, as in the “Before” panel of Figure 1. In other words, we will not study landscapes in which the patches are naturally isolated, such as a group of naturally disconnected marshes. Focal species restricted to natural matrix and dependent on corridors for connectivity.-- There must be at least one mammal, reptile, amphibian, sedentary bird, or flightless arthropod that is expected to occur in natural patches, but probably cannot disperse through habitat in the matrix. Although bats and flying birds have been shown to travel along linear habitat features, most of them are not suitable focal species because they can maintain demographic and genetic flows without corridors. Focal species will differ among landscapes, and we 91

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