George R. Hess

Disease in metapopulation models : Implications for conservation

Several conservation measures are intended to enhance the movement of individuals among populations. These include the establishment of wildlife corridors, cap- tive breeding and release programs, and translocation of individuals among populations. Many metapopulation models show that increasing movement among populations reduces the chance of metapopulation extinction. However, epidemiological models indicate that increased contact among populations enhances the spread of disease and can trigger epi- demics. I have synthesized elements of mathematical epidemiology with metapopulation models. An analytic model showed that highly contagious diseases of moderate severity spread widely, increasing the probability of metapopulation extinction. I also used a sim- ulation model to examine four spatial arrangements of populations: island, necklace, loop, and spider. When infected individuals were allowed to move freely among populations, all of the arrangements exhibited qualitative behavior similar to that exhibited by the analytic models. The most dangerous diseases were those for which infected populations grew large enough to produce dispersers that infected other populations, but which also reduced the geometric rate of increase for infected populations to near unity. Under those conditions, random demographic and environmental events caused metapopulation extinction. Major differences among the spatial arrangements emerged when a quarantine population was established. A centralized quarantine in the spider and necklace arrangement yielded the most dramatic reductions in metapopulation extinction probability. A single quarantine patch was of little value in an island arrangement. These results have several implications for managing metapopulations. Most notably, some spatial arrangements of populations are more amenable to disease control than others, and establishing a quarantine population can increase the probability of detecting new diseases and reduce the impact of diseases that do appear.

Recommendations for assessing the effectiveness of surrogate species approaches

Surrogate species approaches, including flagship, focal, keystone, indicator, and umbrella, are considered an effective means of conservation planning. For conservation biologists to apply surrogates with confidence, they must have some idea of the effectiveness of surrogates for the circumstances in which they will be applied. We reviewed tests of the effectiveness of surrogate species planning to see if research supports the development of generalized rules for (1) determining when and where surrogate species are an effective conservation tool and (2) how surrogate species should be selected such that the resulting conservation plan will effectively protect biodiversity or achieve other conservation goals. The context and methods of published studies were so diverse that we could not draw general conclusions about the spatial or temporal scales, or ecosystems or taxonomic groups for which surrogate species approaches will succeed. The science of surrogate species can progress by (1) establishing methods to compare diverse measures of effectiveness; (2) taking advantage of data-rich regions to examine the potential effectiveness of surrogate approaches; (3) incorporating spatial scale as an explanatory variable; (4) evaluating surrogate species approaches at broader temporal scales; (5) seeking patterns that will lead to hypothesis driven research; and (6) monitoring surrogate species and their target species.

Communicating clearly about conservation corridors

Conflicting definitions lead to confusion when people communicate about ‘corridors’, particularly when they come from different disciplinary backgrounds. Usage of ‘corridor’ in game management, island biogeography, and metapopulation literature focused on function, namely, the movement of flora and fauna from one area to another. A structural usage of the term arose in the field of landscape ecology as it developed in North America with the matrix–patch–corridor paradigm of landscape structure. ‘Corridor’ is now used to describe both the structural and functional aspects of linear landscape features, often implicitly, in a wide range of disciplinary literature. Lack of a clear and consistent terminology leads to confusion about the goals or implied functions of corridors. Consequently, the manner in which corridors should be designed, managed, and evaluated is also unclear. Proper design and management of a corridor depends critically on a clear and explicit statement of its intended functions. If corridors are not designed to perform well-defined functions, the outcome may be disappointing, or even deleterious. The roles corridors play derive from six ecological functions: habitat, conduit, filter, barrier, source, and sink. These ecological functions have been recognized widely and adopted by a number of disciplines, including conservation biology, wildlife management, landscape ecology, and landscape planning. We review briefly the history of the term ‘corridor’ in the context of conservation, catalog some of its definitions and uses, review the functions of corridors, and differentiate between the structural and functional aspects of corridors. We reject the notion of defining ‘corridor’ succinctly, because of the complex and multiple functions a corridor may serve. Instead, we suggest that conservationists and planners consider and document explicitly all of the possible functions of corridors when designing them. Addressing explicitly these functions when designing a corridor should eliminate much of the confusion surrounding their roles, and focus attention on establishing design criteria for corridors that function as intended.

Planning open spaces for wildlife: I. Selecting focal species using a Delphi survey approach

Abstract In a world being transformed by human population growth, conservation biology has emerged as one discipline focused on preventing, mitigating, and reversing the loss of species, ecosystems, and landscapes. Because of the need to act quickly with incomplete information, conservation biologists have developed shortcuts that rely on identifying key species to be focused on during planning efforts. We describe a process that can be used to select those species, using a suburbanizing region in the United States as an example. The Triangle region of North Carolina, USA—Raleigh–Durham–Chapel Hill and surroundings—is undergoing rapid suburbanization, resulting in land-use changes that will alter wildlife communities and might result in the loss of some species. We are developing a wildlife conservation plan for the region based on a combination of landscape and focal species approaches. The objective of the research described in this paper was to identify focal species to be used for conservation planning in the region; our effort focused on amphibians, birds, mammals, and reptiles. In theory, habitat conserved by planning for a few carefully chosen focal species is expected to encompass habitat for many other species with similar requirements. To identify focal species, we used a three-part Delphi survey, administered to a panel of experts. The panel identified six landscape types and nine associated focal species: extensive undisturbed habitat (bobcat, eastern box turtle); riparian and bottomland forest (barred owl, beaver); upland forest (ovenbird, broad-winged hawk); mature forest (pileated woodpecker); pastures and grassy fields (loggerhead shrike); and open and early successional forest (northern bobwhite). The panelists generally agreed that a combined landscape—focal species approach was reasonable, but noted a number of problems to be expected during the planning phase. The most critical of these problems are that the approach has not been well tested, required data are often unavailable, and implementation will be difficult in the face of extreme economic pressures to develop land. Administering the Delphi survey was more labor-intensive, and took longer, than we anticipated; it might have been more effective had it been completed more quickly. Nevertheless, we believe this process can be applied to a broad range of conservation problems, which are often characterized by a high degree of uncertainty and the need to act quickly.

Linking Extinction to Connectivity and Habitat Destruction in Metapopulation Models

A metapopulation is a set of populations distributed over a number of patches that are connected, to varying degrees, by dispersal movements (e.g., Levins 1969, 1970; Hanski 1989; Hanski and Gilpin 1991). Single-species metapopulation models focus on the balance between recolonization and local extinction, with metapopulation persistence possible only if the rate at which empty patches are recolonized exceeds the rate at which populations go extinct (Hanski 1991). Conservation biologists have used metapopulation theory to support conservation corridors and other management techniques that create metapopulations. By enhancing the ability of individuals to move among populations, corridors are intended to increase the rate at which locally extinct patches are recolonized (e.g., Mackintosh 1989; Hudson 1991; Saunders and Hobbs 1991; Noss 1992). A metapopulation is also created when people move individuals in captive and managed wild populations among populations to maintain genetic diversity, replenish depleted populations, and establish populations in new areas (e.g., Cade 1988; Griffith et al. 1989; Rahbek 1993). In the absence of empirical data demonstrating that increasing migration rates increases the chance of metapopulation persistence-indeed, examples of metapopulations in nature have been hard to find (Taylor 1991; Harrison 1994)models have played a pivotal role in supporting initiatives that would create metapopulations. The single-species metapopulation model introduced by Levins (1969) has become the centerpiece of the metapopulation paradigm in conservation biology. This in itself is somewhat ironic because Levins was investigating methods to increase the regional effectiveness of crop pest control. In other words, he was trying to figure out how to increase extinctions. His model, expressed in its familiar form, is

Biodiversity Conservation in Local Planning

Local land-use policy is increasingly being recognized as fundamental to biodiversity conservation in the United States. Many planners and conservation scientists have called for broader use of planning and regulatory tools to support the conservation of biodiversity at local scales. Yet little is known about the pervasiveness of these practices. We conducted an on-line survey of county, municipal, and tribal planning directors (n =116) in 3 geographic regions of the United States: metropolitan Seattle, Washington; metropolitan Des Moines, Iowa; and the Research Triangle, North Carolina. Our objectives were to gauge the extent to which local planning departments address biodiversity conservation and to identify factors that facilitate or hinder conservation actions in local planning. We found that biodiversity conservation was seldom a major consideration in these departments. Staff time was mainly devoted to development mandates and little time was spent on biodiversity conservation. Regulations requiring conservation actions that might benefit biodiversity were uncommon, with the exception of rules governing water quality in all 3 regions and the protection of threatened and endangered species in the Seattle region. Planning tools that could enhance habitat conservation were used infrequently. Collaboration across jurisdictions was widespread, but rarely focused on conservation. Departments with a conservation specialist on staff tended to be associated with higher levels of conservation actions. Jurisdictions in the Seattle region also reported higher levels of conservation action, largely driven by state and federal mandates. Increased funding was most frequently cited as a factor that would facilitate greater consideration of biodiversity in local planning. There are numerous opportunities for conservation biologists to play a role in improving conservation planning at local scales.

Generating confidence intervals for composition-based landscape indexes

Many landscape indexes with ecological relevance have been proposed, including diversity indexes, dominance, fractal dimension, and patch size distribution. Classified land cover data in a geographic information system (GIS) are frequently used to calculate these indexes. However, a lack of methods for quantifying uncertainty in these measures makes it difficult to test hypothesized relations among landscape indexes and ecological processes. One source of uncertainty in landscape indexes is classification error in land cover data, which can be reported in the form of an error matrix. Some researchers have used error matrices to adjust extent estimates derived from classified land cover data. Because landscape diversity indexes depend only on landscape composition – the extent of each cover in a landscape – adjusted extent estimates may be used to calculate diversity indexes. We used a bootstrap procedure to extend this approach and generate confidence intervals for diversity indexes. Bootstrapping is a technique that allows one to estimate sample variability by resampling from the empirical probability distribution defined by a single sample. Using the empirical distribution defined by an error matrix, we generated a bootstrap sample of error matrixes. The sample of error matrixes was used to generate a sample of adjusted diversity indexes from which estimated confidence intervals for the diversity indexes were calculated. We also note that present methods for accuracy assessment are not sufficient for quantifying the uncertainty in landscape indexes that are sensitive to the size, shape, and spatial arrangement of patches. More information about the spatial structure of error is needed to calculate uncertainty for these indexes. Alternative approaches should be considered, including combining traditional accuracy assessments with other probability data generated during the classification procedure.

Creating effective poster presentations: AMEE Guide no. 40

Poster presentations have become an important part of professional meetings and are recognized as valuable tools for teaching and assessment. An effective poster is a visual communication tool that will help you engage colleagues in conversation, convey your main point to large numbers of people, and advertise your work. An effective poster is a highly condensed version of a research paper constructed primarily of visual displays of data with just enough supporting text to provide context, interpretation, and conclusions. A new AMEE Guide, ‘Creating Effective Poster Presentations’, provides guidance and is illustrated with annotated examples.

Southern two-lined salamanders in urbanizing watersheds

Forested riparian buffers are an increasingly common method of mitigating the negative effects of impervious surface cover on water quality and wildlife habitat. We sampled larval southern two-lined salamanders (Eurycea cirrigera) in 43 streams, representing the range of impervious surface cover and forested riparian buffer width across Wake County, NC, USA. Larval abundance decreased with increasing impervious surface cover in the upstream catchment, but was not affected by buffer width. This is likely a result of an incomplete buffer system and culverts or other breaches along streams. Larval abundance increased with detritus cover in the stream to a threshold and then decreased as detritus continued to increase. As percent pebble substrate in the stream increased, especially in perennial streams, larval salamander abundance also increased. We suspect salamanders were unable to migrate with the water column during dry periods in intermittent streams with sedimented interstices below the surface, resulting in low abundances. A combination of increased peak flows and sedimentation, reduced base flow, and chemical changes likely reduces the abundance of salamanders in urban and suburban streams. We suggest creation of catchment-wide, unbreached buffers to maintain the integrity of stream habitats in urbanizing watersheds.

Urban Morphology Drives the Homogenization of Tree Cover in Baltimore, MD, and Raleigh, NC

Heterogeneous land cover patterns contribute to unique ecological conditions in cities and little is known about the drivers of these patterns among cities. We studied tree cover patterns in relationship to urban morphology (for example, housing density, parcel size), socioeconomic factors (for example, education, income, lifestyle characteristics), and historical legacies in Baltimore, Maryland, and Raleigh, North Carolina. Utilizing a multimodel inference approach and bivariate analyses, we analyzed two primary datasets employed in previous research predicting urban tree cover—one comprising continuous data (US Census), and the other consisting of categorical variables (Claritas PRIZM) that incorporate consumer purchasing data. Continuous data revealed that urban morphological characteristics were better predictors of tree cover patterns than socioeconomic factors in Raleigh and Baltimore at the parcel and neighborhood scales. Although the categorical dataset provided some evidence for the importance of socioeconomic and lifestyle characteristics in predicting tree cover patterns, the hierarchical nature of these data preclude separating the impacts of these factors from levels of urbanization. Bivariate analyses of continuous and categorical variables revealed that the highest correlation coefficients were associated with variables describing urban morphology—parcel size, percent pervious area, and house age. In Baltimore, historical census data were better predictors of present-day tree cover than census data from recent years. Most notably, parcel size, a key predictor of tree cover, has decreased with time in Raleigh to sizes consistently seen in Baltimore. Our findings demonstrate that urban morphology, the main driver of tree cover patterns in these cities, may lead to the homogenization of tree canopy in Raleigh and Baltimore in the future.