《森林生态学》课程教学资源（生态学热点）Issues in ecology - 16 景观连通性在规划和实施保护中的作用 The Role of Landscape Connectivity in Planning and Implementing Conservation and Restoration Priorities.pdf_大学文库

© The Ecological Society of America • esahq@esa.org esa 1 ISSUES IN ECOLOGY NUMBER SIXTEEN FALL 2012 The Role of Landscape Connectivity in Planning and Implementing Conservation and Restoration Priorities SUMMARY L andscape connectivity, the extent to which a landscape facilitates the movements of organisms and their genes, faces critical threats from both fragmentation and habitat loss. Many conservation efforts focus on protecting and enhancing connectivity to offset the impacts of habitat loss and fragmentation on biodiversity conservation, and to increase the resilience of reserve networks to potential threats associated with climate change. Loss of connectivity can reduce the size and quality of available habitat, impede and disrupt movement (including dispersal) to new habitats, and affect seasonal migration patterns. These changes can lead, in turn, to detrimental effects for populations and species, including decreased carrying capacity, population declines, loss of genetic variation, and ultimately species extinction. Measuring and mapping connectivity is facilitated by a growing number of quantitative approaches that can integrate large amounts of information about organisms’ life histories, habitat quality, and other features essential to evaluating connectivity for a given population or species. However, identifying effective approaches for maintaining and restoring connectivity poses several challenges, and our understanding of how connectivity should be designed to mitigate the impacts of climate change is, as yet, in its infancy. Scientists and managers must confront and overcome several challenges inherent in evaluating and planning for connectivity, including: •characterizing the biology of focal species; •understanding the strengths and the limitations of the models used to evaluate connectivity; •considering spatial and temporal extent in connectivity planning; •using caution in extrapolating results outside of observed conditions; •considering non-linear relationships that can complicate assumed or expected ecological responses; •accounting and planning for anthropogenic change in the landscape; •using well-defined goals and objectives to drive the selection of methods used for evaluating and planning for connectivity; •and communicating to the general public in clear and meaningful language the importance of connectivity to improve awareness and strengthen policies for ensuring conservation. Several aspects of connectivity science deserve additional attention in order to improve the effectiveness of design and implementation. Research on species persistence, behavioral ecology, and community structure is needed to reduce the uncertainty associated with connectivity models. Evaluating and testing connectivity responses to climate change will be critical to achieving conservation goals in the face of the rapid changes that will confront many communities and ecosystems. All of these potential areas of advancement will fall short of conservation goals if we do not effectively incorporate human activities into connectivity planning. While this Issue identifies substantial uncertainties in mapping connectivity and evaluating resilience to climate change, it is also clear that integrating human and natural landscape conservation planning to enhance habitat connectivity is essential for biodiversity conservation. Cover photos: Examples of ways different species move through landscapes and depend on connectivity. Clockwise starting on the upper left: a) The interconnection of ocean surface current patterns provides pathways for dispersal of larvae between coral reefs. b) A network of riparian corridors used by wildlife to move through an agricultural landscape. c) Continuous grasslands are used by migrating wildebeest in eastern Africa. d) Intact lowland forest is used by endemic forest birds for dispersal between mountain ranges. Photos credits: a) NASA Goddard's Scientific Visualization Studio. b) Adina Merenlender. c) Flickr user Abeeeer. d) Flickr user Daniel Lane

2 esa © The Ecological Society of America • esahq@esa.org Introduction What Is Landscape Connectivity and How Does It Affect Conservation Objectives? Connectivity is the extent to which movements of genes, propagules (pollen and seeds), individuals, and populations are facilitated by the structure and composition of the landscape. A landscape’s connectivity is defined relative to the requirements of the organisms that live within it and move through it. Therefore, connectivity is species and context dependent. Consider the interconnection of ocean surface current patterns providing pathways for dispersal of larvae between coral reefs; a network of riparian corridors used by wildlife to move through an agricultural landscape; the continuity of grasslands used by migrating wildebeest; intact lowland forest through which endemic forest birds move (see cover photos). Each of these examples demonstrates that connectivity is measured relative to the ease or difficulty with which a particular species is able to move across a particular land or seascape. Connectivity has both structural and functional components. Structural connectivity describes the physical characteristics of a landscape that allow for movement, including topography, hydrology, vegetative cover, and human land use patterns. Functional connectivity describes how well genes, propagules, individuals, or populations move through the landscape. Functional connectivity results from the ways that the ecological characteristics of the organism, such as habitat preference and dispersal ability, interact with the structural characteristics of the landscape. The examples provided on the cover depict the ways that different species move through and depend on the landscape and demonstrate both functional and structural connectivity, whereby ecological requirements of individual species interact with the composition and configuration of the landscape. These interactions influence the ability of individuals and populations to move among locations to find key resources, such as food, water, appropriate substrates for sessile organisms, or breeding partners. The destruction and degradation of natural habitats on which all organisms rely – including humans – is occurring at an unprecedented rate across most regions of our planet. As humans convert land for resource extraction and for urban and agricultural uses, and as our impacts on global climate continue to grow, we profoundly change the physical, chemical, and biological character of these landscapes. Land use changes may reduce the amount of a habitat or fragment it, breaking it up into smaller or differently arranged units. This process changes not only the size of habitat patches but also other landscape features, such as patch geometry or the amount of edge habitat, that may be of fundamental importance to species, communities, and ecological functions. Because human-caused disturbances often occur in shorter timeframes and over larger areas than do natural disturbances, ecological communities face challenges of how to adapt and respond to novel rates and scales of disturbances that are quite different from those with which they may have evolved. Fragmentation, habitat degradation, and habitat loss are the dominant mechanisms by which connectivity is reduced or lost, and are widely recognized as major drivers of the present global biodiversity crisis. Fragmentation, the subdivision of habitat into smaller or more isolated remnants, can directly impact species persistence and accelerate local extinction rates. Habitat fragmentation is frequently associated with habitat loss. However, fragmentation can also eliminate dispersal or gene flow without causing impacts on a population’s core The Role of Landscape Connectivity in Planning and Implementing Conservation and Restoration Priorities Deborah A. Rudnick, Sadie J. Ryan, Paul Beier, Samuel A. Cushman, Fred Dieffenbach, Clinton W. Epps, Leah R. Gerber, Joel Hartter, Jeff S. Jenness, Julia Kintsch, Adina M. Merenlender, Ryan M. Perkl, Damian V. Preziosi, and Stephen C. Trombulak ISSUES IN ECOLOGY NUMBER SIXTEEN FALL 2012

© The Ecological Society of America • esahq@esa.org esa 3 ISSUES IN ECOLOGY NUMBER SIXTEEN FALL 2012 habitat – for example, a highway bisecting a movement corridor. For any given species, some parts of the landscape provide better opportunities than others to fulfill its ecological requirements, such as food, breeding habitat, or refuge from predation. Fragmentation and degradation can further increase the patchiness of the landscape in terms of meeting a species’ needs. Conserving connectivity in this context requires identifying, maintaining, and possibly enhancing the linkages between suitable patches of habitat in the landscape. Corridors, which are generally linear spaces that facilitate movement between patches, are frequently used as a tool for conserving or enhancing linkages. The creation or protection of corridors can maintain connectivity for mobile species, such as ungulates or large felines that typically have large territories. Corridors provide structural connectivity and are consistent with the functional connectivity needs of animals that can take advantage of linear spaces to move among disparate habitat patches. However, landscape connectivity is highly diverse and speciesdependent, and other forms of connectivity may be more relevant to other types of organisms; for example, a linked mosaic of small wetlands for breeding populations of amphibians, continuity of vegetated intertidal rocky substrate along a coastline for a marine snail, or a heterogeneous assemblage of meadow plant communities with different flowering times for a population of pollinators. The challenge of matching connectivity patterns to ecological requirements becomes even greater when we expand our thinking to consider maintaining or restoring connectivity for multiple species or entire communities. Many populations and ecosystem functions are dependent on extensive, well-connected habitats; however, understanding the factors that contribute to landscape connectivity for specific populations, species, or communities is challenging. This Issue reviews the importance of habitat connectivity, summarizes current science-based strategies for mitigating the negative ecological effects of fragmentation, explores data gaps and limitations of connectivity models, and describes obstacles and opportunities for developing policies and management approaches that improve connectivity and reach conservation goals. Case Study 1. Managing for Marine Connectivity: Marine Protected Areas in the Gulf of California, Mexico The growing movement toward ecosystem-based management, including networks of no-take zones in marine ecosystems (marine protected areas, or MPAs) requires that these conservation areas be deliberately and adequately spaced to allow for connectivity. The performance of a network of sites designed with the two-fold purpose of protecting commercial species and allowing for spillover effects (movement of organisms from protected areas into harvestable areas) will largely depend on whether sites in a network are functionally and structurally linked to each other by both biological (e.g., dispersal of organisms) and physical (e.g., currents) processes. Although the number and extent of MPAs has increased recently, studies have shown that, on a global scale, average distance between neighboring MPAs exceeds the distance of reef organism propagule dispersal. This distance suggests that some taxa could become genetically isolated if populations cannot reach each other, undermining the viability of populations in the MPAs. The conservation of species, habitats, and ecoregions depends on developing practical, efficient, and effective planning strategies. This is especially true in the marine realm, where threats are diffuse and difficult to both identify and quantify. Well-designed networks should include MPAs and other conservation and management areas that support each other by taking advantage of oceanic currents and movement/migration capabilities of species. They also provide much-needed resilience against a range of threats. Because establishment of isolated marine reserves may not alone suffice for the conservation of biodiversity, identifying the level of connectivity between the areas is a critical aspect in network design. In the Gulf of California, Mexico (GOC), two organizations, Comunidad y Biodiversidad and The Nature Conservancy, recently completed a marine ecoregional assessment to identify priority conservation sites and establish a network of conservation areas. This analysis identified 54 conservation areas that are deemed critical to marine conservation objectives, which cover 26% of the ecoregion. An important step towards implementing the assessment will be to account for connectivity between putative sites. To move from connectivity assessments based exclusively on structural attributes of connectivity to a detailed assessment of actual connectivity, models are used to track species’ dispersal from site to site as well as movement through the matrix (for example, satellite tracking and the development of oceanographic models for the entire ecoregion). Pop-up satellite archival tags are being used globally for many marine species (e.g., the Tagging of Pacific Predators Program, http://www.topp.org/) and can greatly enhance knowledge of the dispersal of focal species. For example, sea turtle, cetacean, and whale shark tagging programs are already underway in the ecoregion, and expanded versions of these programs are expected to provide a more complete understanding of connectivity throughout the ecoregion. Integrating data from these tagging programs into the GOC ecoregional assessment is an important priority in understanding the role of connectivity in marine spatial planning

ISSUES IN ECOLOGY NUMBER SIXTEEN FALL 2012 How Fragmentation Affects Movement: From Genes to Species Landscape fragmentation affects ecological communities at multiple levels of organization. Here, we briefly explore these effects, ranging from the movement of individuals and gene flow within and between populations to shifts in species range and species persistence. Landscape connectivity is important for dispersing or migrating individuals. Dispersal increases resilience to disturbances by allowing organisms to track their shifting habitats, and it promotes the spread and expansion of populations. In some species – for example, wildebeest in Africa, bison in North America, a wide variety of bird species – seasonal migration has evolved as a means of maximizing access to critical resources as ecological conditions change throughout the year. Habitat fragmentation can disrupt dispersal and migration in several ways. First, edges of the remnant habitat patches may act as filters or barriers that discourage or impede movement. Second, increased distances between suitable habitat patches may influence the likelihood of successful movement. Last, the composition and structure of the intervening landscape mosaic may influence the permeability of the landscape to movements by different organisms. If fragmentation impedes seasonal migration, wildlife may be cut off from seasonal resources. If dispersal routes are blocked or altered, organisms may experience higher rates of mortality when trying to disperse, or they may be stopped completely, leading to unsustainably high densities of organisms in remnant patches, resulting in increases in mortality. Habitat fragmentation may impede gene flow and lead to genetic isolation. Gene flow is critical to population viability, as it helps maintain local genetic variation and spreads potentially adaptive genes. Genetic isolation can be a mechanism for the creation of new populations and even species; however, small remnant populations inhabiting fragmented landscapes are more likely to suffer from inbreeding and low genetic variation, which can increase vulnerability to other stressors and lead to local extinctions. Retaining or restoring connectivity counteracts these negative effects of genetic isolation. Landscape connectivity is essential across large areas (connectivity across ecoregions or continents is critical for some species) and over long timeframes (connectivity over many years or generations) to allow species’ range shifts in response to long-term ecological change. Projected climate change over the next few decades will change ecosystem structure, species composition, and diversity. Changes in biophysical conditions will likely lead to species replacement in communities (community turnover) and latitudinal and elevational shifts in geographic ranges. During episodes of climate change since the Pleistocene, vegetation zones or communities did not move as a whole in response to climate shifts; rather, species responded individually to climate change, according to their own individual and largely independent environmental tolerances, dispersal abilities, and responses to biotic interactions. Current climate change appears to be occurring substantially faster than in the pre-historical record, meaning that the ecological conditions required by many species (their niches) may be shifting faster than species can adapt. These pressures, caused by changes in climatic conditions encountered by species in their current distributions, are compounded by habitat loss and fragmentation. The resulting obstacles to migration may impede species’ abilities to adapt to climate change, to such an extent that many species could be driven to extinction. Effects of Connectivity on Disease and Biotic Invasions The extent to which landscapes are connected or fragmented may also affect the rate and pattern of disease spread and invasion by nonnative species. Species introductions, which can radically alter ecosystems, include plant and animal diseases as well as competitors and predators against which native communities may not have evolved defenses. It is important for managers to consider how changes in connectivity and fragmentation may influence the spread of diseases and invasive species, and to recognize that these influences are not necessarily unidirectional, but rather depend on the characteristics of the particular species and landscape in question. Intact, well-connected landscapes can serve as conduits for many invasive species if they disperse in similar ways to native species. In other cases, processes that fragment habitats for native species may simultaneously provide connections that can facilitate biotic invasions. For example, the recent Asian carp invasion (including grass carp (Ctenopharyngodon idella), silver carp (Hypophthalmichthys 4 esa © The Ecological Society of America • esahq@esa.org

© The Ecological Society of America • esahq@esa.org esa 5 ISSUES IN ECOLOGY NUMBER SIXTEEN FALL 2012 molitrix), and bighead carp (H. nobilis)) in the Mississippi River watershed, and their potential spread into the Great Lakes, illustrate how human-constructed connections (canals) can both fragment the terrestrial environment and provide new corridors between aquatic systems. Similarly, while roads can fragment vegetated habitats, they can simultaneously serve as conduits for some invasive species, such as cheatgrass (Bromus tectorum), yellow star thistle (Centaurea solstitialis), and other invasive species that benefit from the openings created by roads. Connectivity for pathogens and parasites is largely a function of host distribution and abundance. While disease persistence benefits from increased host connectivity, it does not necessarily follow that these conditions are optimized only in well-connected landscapes. Landscape disturbance and fragmentation can increase host abundance and alter host distribution, and these changes can increase connectivity for pathogens and parasites for which the hosts constitute the true “landscape” across which movement occurs (Box 1). However, fragmentation may also lead to the isolation of smaller host subpopulations, which then may become more susceptible to disease or invasions. In other situations, isolation resulting from landscape fragmentation may protect a population from disease. For example, plague (Yersinia pestis) in Colorado prairie dog (Cynomys ludovicianus) populations was shown to be less prevalent in more remote, isolated populations than in those more closely grouped together. Measuring, Analyzing and Designing Landscape Connectivity Measuring structural connectivity has increasingly become a routine objective of researchers and policy makers, as Geographic Information Box 1. Habitat Fragmentation and Increased Disease Transmissivity An important consequence of fragmentation in forested habitats is the loss of species diversity. Those species that thrive in fragmented habitats tend to be more generalist or opportunistic, or have traits such as smaller home range requirements and tolerance for higher densities. Fragmentation can actually increase connectivity from the perspective of a disease-causing pathogen. Higher densities of hosts increase opportunities for transmissivity, and the host population is the true “landscape” across which pathogen movement occurs. This is the case for the tick-transmitted bacterium (Borrelia burgdorferi) that causes Lyme disease. Its host, the white-footed mouse (Peromyscus leucopus), has become increasingly common in small forest fragments (<2 ha) in New England, likely resulting from its small home range requirements combined with release from competitors and predators in smaller forest patches. P. leucopus is the principal natural reservoir for Lyme disease. Higher densities of ticks infested with B. burgdorferi are found in smaller forest fragments (Figure 1), which may result from higher densities of white-footed mouse in these smaller fragments, presenting more opportunities for ticks to feed on the mice. Consequently, humans living near these small forest fragments may have a higher risk of exposure to Lyme disease relative to those near larger forest fragments. Figure 1. Relationship between measures of Lyme disease risk and forest patch area in a fragmented landscape in New York state. a) Density of nymphal ticks is higher in smaller forest fragments. b) Percentage of nymphal ticks infected with the bacterium Borrelia burgdorferi is higher in smaller forest fragments. c) Density of nymphal ticks infected with the B. burgdorferi is higher in smaller forest fragments. (Source: Allan, B.F., F. Keesing, and R.S. Ostfeld. 2003. Effect of forest fragmentation on Lyme disease risk. Conservation Biology. 17: 267–272). Image used with permission of John Wiley and Sons. (a) (b) (c)

ISSUES IN ECOLOGY NUMBER SIXTEEN FALL 2012 6 esa © The Ecological Society of America • esahq@esa.org System (GIS) and remote sensing tools become more widely available, affordable, and scalable. However, measuring functional connectivity using the movements of individual organisms can be logistically complicated. Even the largest studies using the most appropriate technologies can track only relatively few individuals over modest time periods, and controlled experiments addressing movements and dispersal at relevant scales are extremely difficult to implement. One way to address this difficulty is to measure gene flow, which may more accurately and efficiently reflect functional connectivity across large landscapes. Genetic studies avoid the logistic and financial costs of tracking individual animals and integrate only those movements that produce meaningful population impacts – dispersals that result in breeding or emigration. A shortcoming of this approach is that current genetic patterns may not reflect the impact of current landscape features, especially for species with large population sizes or long generation times, or species affected by unobserved events, such as genetic bottlenecks caused by past epidemics or human persecution. In addition, genetic connectivity may be masked in some instances by local adaptation, which can drive genetic distinctiveness even in a well-connected landscape, by selecting for particular characteristics of the local environment. A common product of connectivity analysis is a map of predicted core areas, linkage zones, or barriers. Such maps often become the basis for management actions. Several tools can be used to map these features, and each has unique strengths and weaknesses. All of the approaches described in the next section depend on accurately defining landscape resistance (an indication of how well a landscape can be traversed by a given species), a challenging task when only a limited amount of information about species habitat preferences is available. Furthermore, connectivity models can be difficult to validate. Several research teams are working to develop methods to rigorously estimate species-specific resistance from data on gene flow, genetic distances, habitat use, and movement paths. Simple estimates of resistance, based on the extent to which landscapes are impacted by roads, loss of natural land cover, increased edge effects, spread of invasive species, and other direct human impacts measures may be useful for some generalist species, but are insufficient for addressing species-specific movements and habitat needs. Thus, recent developments in connectivity modeling combine a structural landscape approach, identifying both the potential for and obstacles to long-term habitat shifts, with a functional approach that highlights the specific connectivity needs of species with restricted habitat requirements. Modeling Approaches for Identifying and Quantifying Landscape Connectivity We describe five widely-used analytical approaches, all implemented in a GIS environment, to assist planners in mapping and prioritizing landscape connections. Each approach has specific data requirements that often require input from biologists to help define model parameters. In addition, each approach is designed to meet different objectives and will, therefore, produce different outcomes. Least-cost analysis identifies the least costly route that an animal can take from one area to another. The method assumes that the animal incurs a cost as it moves over an area, where “cost” may reflect the actual energy expended to move over the area, mortality risk, or impact on future reproductive potential. In practice, cost is usually estimated simply as the inverse of habitat suitability. Habitats that the animal favors are assigned low cost while unsuitable habitats are assigned high cost. The least-cost path is the contiguous collection of cells that has the lowest cumulative cost as the path crosses from one endpoint (such as a park, natural area, or known population; sometimes referred to as a node or patch) to the other endpoint. Computers using GIS software can easily identify this path. Because the least-cost path is only one cell wide (for example, the center panel in Figure 2), it is often not a realistic area to propose for conservation. Therefore, analysts usually identify the least-cost corridor (shown in red in the panel on the right in Figure 2), which is a swath of cells expected to provide a low-cost route for movement. Increased distance between two nodes or patches also results in higher costs. This latter assumption is important, in that some species may be able to identify and take advantage of shorter linkages, while others operate at a finer scale of perception and therefore may not be able to consider total corridor length. Correctly assigning these cost values (also referred to as

© The Ecological Society of America • esahq@esa.org esa 7 ISSUES IN ECOLOGY NUMBER SIXTEEN FALL 2012 resistance or permeability values) is the most problematic aspect of least-cost analysis and the other approaches described here. Factorial least-cost paths address one major limitation of traditional least-cost path and least-cost corridor analyses in that they are limited to predictions of connectivity between single sources and single destinations. While this may be ideal in the case where one is interested in the lowest cost routes between two focal conservation areas, many situations require a more comprehensive analysis of connectivity. For example, corridor connectivity may need to be calculated between thousands of sources and a single destination, or between hundreds of sources and hundreds of destinations distributed across a complex landscape. A factorial implementation of least-cost paths integrates a vast number of paths to show a network of connectivity across large and complex landscapes, such as a factorial least-cost path analysis among hundreds of points across a resistance surface (Figure 3). Densities of paths are shown in a gradient from yellow to red, with red paths representing routes that are predicted to contain the least-cost paths between many pairs of source and destination points. Additionally, while factorial approaches are most common among leastcost approaches, they can also be integrated into graph and circuit analysis as well. Circuit theory treats the landscape as if it were a large electrical circuit, in which all cells in the landscape can support movement. An important distinction between this and other methods is that while circuit approaches can be used to delineate corridors (with additional processing), they are most useful in analyzing and describing how well connected source and destination habitat patches may be, given multiple movement pathways. Wellconnected habitat patches have wide, continuous habitat between them, while paths between poorly-connected habitat patches might have constrictions and bottlenecks, each of which can be identified using a circuit based approach. Current maps (Figure 4) can be a useful way to visualize a circuit-theoretic analysis. Current strength reflects the predicted probability of movement between the two points or habitat patches. Current maps can be difficult to interpret: higher current (usually depicted in yellow) may occur in a cell because resistance is low, because most paths are forced through that area because of high resistance elsewhere, or because the analysis is spatially constrained (Figure 4a). Lower current (usually depicted in blue), in turn, may imply either high resistance in the underlying layer, or simply that there are many equally good alternate paths for movement. In this way, circuit models may more accurately approximate how organisms move through real landscapes. Despite their relative complexity, these maps are useful for evaluating connectivity and identifying constrained areas (bottlenecks) for possible conservation action. In Figure 4a, where the Figure 2. A least cost path analysis. Figure 3. A factorial least-cost path analysis, evaluating leastcost paths (lines in blue to red, with red showing paths with the lowest costs) among many source areas (green points)

ISSUES IN ECOLOGY NUMBER SIXTEEN FALL 2012 8 esa © The Ecological Society of America • esahq@esa.org model was confined to a particular habitat corridor, high current values clearly indicate a bottleneck where movement is funneled into a narrow space. In Figure 4b, where the analysis was conducted in a different landscape and not constrained to a corridor, few major bottlenecks are apparent but some areas have higher current due to lower resistance or proximity to the nearest edges of the habitat patches, which are shown in beige. Graph theory combined with least-cost modeling or circuit theory provides several useful enhancements to landscape connectivity assessment and modeling. The landscape itself can be likened to an interlaced web or network that is composed of habitat patches (graph theory modeling uses centers, or “nodes,” of these patches as points of connection) and the connections between these patches (the linear representations of which are described in graph theory language as “edges”) (Figure 5). Once identified, nodes and edges can be prioritized based on their overall contribution to the landscape network, for instance by evaluating how many potential connections rely on each node or edge. This approach allows for multiple least-cost pathways to be evaluated for their contribution to the configuration of the overall network. This approach is particularly useful when modeling connectivity between large reserve sets (assemblages of patches, parks, or protected areas), identifying isolated reserve sets within the context of the modeled landscape, evaluating the robustness of multiple connections within the landscape network, node/connection prioritization, and evaluating the consequences of losing nodes due to competing factors such as development pressure or fiscal constraints. The resistant kernel approach to connectivity modeling is based on least-cost dispersal from a defined set of sources. The model calculates the expected relative density of dispersing individuals in each cell around the source, given the dispersal ability of the species, the nature of the dispersal function, and the resistance of the landscape. Once the expected density around each source cell (the smallest unit of space that is modeled containing individuals dispersing to other parts, or cells, in the landscape) is calculated, the kernels surrounding all sources are summed to give the total expected density at each cell. The results of the model are surfaces of expected density of dispersing organisms at any location in the landscape, in contrast to the physical delineation of linkages or corridors. The resistant kernel approach has a number of advantages for assessing population connectivity. First, unlike most corridor prediction efforts, but similar to circuit-based approaches, it is spatially comprehensive, provides prediction and mapping of expected migration rates for every cell in the study area extent, and can do so for large geographic extents rather than only for a few selected linkage zones. Second, this approach allows assessment of how species with different movement patterns and disperFigure 4. Current maps, illustrating a) a landscape where analysis was confined to a corridor and b) a different landscape where the analysis was not confined to a corridor. Figure 5. A graph theory model depicting connectivity between habitat patches. (a) (b)

© The Ecological Society of America • esahq@esa.org esa 9 ISSUES IN ECOLOGY NUMBER SIXTEEN FALL 2012 sal abilities are affected by a range of landscape change and fragmentation scenarios. This approach is useful for characterizing connectivity across continuous surfaces but does not identify individual linkages or corridors without additional analyses. Figure 6 shows an individual resistant kernel around a) a single source cell and b) the cumulative resistant kernel surface created from summing all individual kernels for all habitat cells. Red areas are predicted to have high frequency of occupancy by dispersers, while blue areas are predicted to experience low rates of dispersal. Case Study 2 provides an example of the use of the resistant kernel approach in evaluating fragmenting effects of roads on amphibian populations. Considering Resolution and Focus in Connectivity Design The resolution at which connectivity is ecologically meaningful varies enormously, depending on the species in question. For example, consider the scale of connectivity relevant to a beetle versus a bison. In practice, we tend to design and plan for connectivity at a human scale, meaning that we visualize connectivity in terms of landscape management units in a policy framework. Spatially extensive maps (thousands of kilometers), with coarse grained resolution (for example, in the hundreds of meters per pixel or measurement unit) can depict a network of numerous habitat blocks and the connections among them. Such maps may serve as decision-support tools for managers, or provide a high-level vision of landscape connectivity. They may be used to alert decisionmakers to potential threats to large-scale connectivity as well as conservation opportunities. However, these types of maps are often too coarse to inform specific conservation action plans. Examples include the Yellowstone to Yukon initiative, Arizona Wildlife Linkage Assessment, California Essential Habitat Connectivity, Two Countries-One Forest (Case Study 3), Washington Connected Landscapes, and the Bhutan Biological Corridor Complex. The two largest challenges for coarse-grained analysis and mapping are the identification and delineation of core habitat blocks (areas whose conservation value derives from the species and ecological processes within them) and determining which pairs or sets of blocks can feasibly be connected in a way that promotes functional connectivity and meets conservation goals. Once habitat blocks have been identified, various techniques may be used to map the connections (or linkages) among them, including least-cost path analysis, graph theory, or individual-based movement models. Finer-grained linkage designs can guide sitespecific actions to conserve connectivity between specific habitat areas that are relevant to the distances and ways in which species of interest move across the landscape. To develop maps for these plans, landscape connectivity planners typically select a suite of focal species and use the union of their corridors or movement pathways (usually produced by least-cost modeling) to serve as a preliminary linkage design for the entire biota. For instance, each of the 27 linkage plans in California (South Coast Missing Linkages project, available at www.scwildlands.org) and Arizona (the Arizona Missing Linkages project, www.corridordesign.org) was designed to meet the needs of several focal species including mammals, reptiles, amphibians, plants, and invertebrates. Focal species included areasensitive species, species with short or habitatrestricted dispersal movements, and species Figure 6. Resistance kernel modeling: a) single-kernel analysis and b) the cumulative resistance surface of all kernels across the landscape. (a) (b)