Summary 1.Positive relationships between the density and distribution of species in taxonomic assemblages are well documented, but the underlying mechanisms remain poorly understood. Two factors that are expected to be important in explaining variation in these relationships are the spatial scale of analysis and the relative mobility of the study species. 2. We examined density-distribution relationships in British butterflies at a variety of spatial scales. Distributions were proportions of grid squares occupied: 50 m grid within 0·25 km 2 areas (local), 500 m grid in 35 km 2 (regional), 10 km grid across England, Wales and Scotland (national), 153 000 km 2 grid squares across Europe (European), and also seven categories of international distribution (Global; 1 = European endemic to 7 = in 5 + continents). Densities were measured using transect counts at local, regional and national scales. 3. Different relationships between density and distribution occurred at different scales of analysis. When we controlled for the effects of mobility and /or phylogenetic association, a positive relationship between density and distribution was apparent at local, regional and national scales. Species' national densities in Britain were positively correlated with their European distribution sizes, but significantly negatively correlated with their global range sizes. 4. Butterfly mobility had a positive effect on distribution and a negative effect on density at all spatial scales. For a given total abundance, more mobile species had lower densities but wider distributions, i.e. they were less aggregated than more sedentary species. 5. The decreasing strength of the density-distribution correlation, and the eventual reversal of the pattern, with the increasing magnitude of difference between the scale at which density was measured relative to distribution, suggests that some element of niche may be important in determining densities and distributions. However, the measure of niche breadth analysed did not explain significant variation in density, distribution, or in the density-distribution relationship.

Summary1. Current national and international frameworks for assessing threats to species have not been developed in the context of climate change, and are not framed in a way that recognises new opportunities that arise from climate change. 2. The framework presented here separates the threats and benefits of climate change for individual species. Threat is assessed by the level of climate-related decline within a species' recently occupied (e.g. pre-1970s) historical distribution, based on observed (e.g. repeat census) and ⁄ or projected changes (e.g. modelled bioclimate space). Benefits are assessed in terms of observed and ⁄ or projected increases outside the recently occupied historical range. 3. Exacerbating factors (e.g. small population size, low dispersal capacity) that might increase levels of threat or limit expansion in response to climate change are taken into consideration within the framework. Protocols are also used to identify levels of confidence (and hence research and ⁄ or monitoring needs) in each species' assessment. 4. Observed and projected changes are combined into single measures of expected decline and increase, together with associated measures of confidence. We weight risk classifications towards information that is most certain. Each species is then placed in one of six categories (high risk, medium risk, limited impact, equivalent risks & benefits, medium benefit, high benefit) reflecting whether climate change is expected (or has been observed) to cause net declines or increases in the region considered, based on the balance of benefits and threats. 5. We illustrate the feasibility of using the framework by applying it to (i) all British butterflies (N = 58 species) and (ii) an additional sample of British species: 18 species of plants, bats, birds and beetles. 6. Synthesis. Our framework assesses net declines and increases associated with climate change, for individual species. It could be applied at any scale (regional, continental or global distributions of species), and complements existing conservation assessment protocols such as red-listing. Using observed and projected population and ⁄ or range data, it is feasible to carry out systematic conservation status assessments that inform the development of monitoring, adaptation measures and conservation management planning for species that are responding to climate change.

A key question facing conservation biologists is whether declines in species' distributions are keeping pace with landscape change, or whether current distributions overestimate probabilities of future persistence. We use metapopulations of the marsh fritillary butterfly Euphydryas aurinia in the United Kingdom as a model system to test for extinction debt in a declining species. We derive parameters for a metapopulation model (incidence function model, IFM) using information from a 625-km2 landscape where habitat patch occupancy, colonization, and extinction rates for E. aurinia depend on patch connectivity, area, and quality. We then show that habitat networks in six extant metapopulations in 16-km2 squares were larger, had longer modeled persistence times (using IFM), and higher metapopulation capacity (lambdaM) than six extinct metapopulations. However, there was a > 99% chance that one or more of the six extant metapopulations would go extinct in 100 years in the absence of further habitat loss. For 11 out of 12 networks, minimum areas of habitat needed for 95% persistence of metapopulation simulations after 100 years ranged from 80 to 142 ha (approximately 5-9% of land area), depending on the spatial location of habitat. The area of habitat exceeded the estimated minimum viable metapopulation size (MVM) in only two of the six extant metapopulations, and even then by only 20%. The remaining four extant networks were expected to suffer extinction in 15-126 years. MVM was consistently estimated as approximately 5% of land area based on a sensitivity analysis of IFM parameters and was reduced only marginally (to approximately 4%) by modeling the potential impact of long-distance colonization over wider landscapes. The results suggest a widespread extinction debt among extant metapopulations of a declining species, necessitating conservation management or reserve designation even in apparent strongholds. For threatened species, metapopulation modeling is a potential means to identify landscapes near to extinction thresholds, to which conservation measures can be targeted for the best chance of success.

Summary1. Most species' surveys and biodiversity inventories are limited by time and money. Therefore, it would be extremely useful to develop predictive models of animal distributions based on habitat, and to use these models to estimate species' densities and range sizes in poorly sampled regions. 2. In this study, two sets of data were collected. The ®rst set consisted of over 2000 butter¯y transect counts, which were used to determine the relative density of each species in 16 major habitat types in a 35-km 2 area of fragmented landscape in north-west Wales. For the second set of data, the area was divided into 140 cells using a 500-m grid, and the extent of each habitat and the presence or absence of each butter¯y and moth species was determined for each cell. 3. Logistic regression was used to model the relationship between species' distribution and predicted density, based on habitat extent, in each grid square. The resultant models were used to predict butter¯y distributions and occupancy at a range of spatial scales. 4. Using a jack-knife procedure, our models successfully reclassi®ed the presence or absence of species in a high percentage of grid squares (mean 83% agreement). There were highly signi®cant relationships between the modelled probability of species occurring at regional and local scales and the number of grid squares occupied at those scales. 5. We conclude that basic habitat data can be used to predict insect distributions and relative densities reasonably well within a fragmented landscape. It remains to be seen how accurate these predictions will be over a wider area.

Summary 1.The interspecific density-distribution relationship is a general and robust pattern that has been described as a rule in community ecology. Many theoretically plausible causes of the relationship have been described, but it is still disputed which factor(s) are most important. 2. Using data on the densities and distributions of butterflies and their host plants collected in a 35-km 2 area of north Wales, and data on butterfly mobility, niche breadth, habitat breadth and distance from range margins, we examined five of the principal explanatory mechanisms. 3. We found that several variables were significantly correlated with density or distribution. Habitat breadth, mobility and distance from range margin had significant positive effects on butterfly distribution. Host-plant density was significantly positively related to butterfly density; mobility was significantly negatively related to density. 4. Despite these results, we could not unambiguously demonstrate that one hypothesis (or several interacting hypotheses) generated density-distribution correlations. The most conclusive evidence was that statistical patterns of distribution (aggregation models) underpinned the positive density-distribution relationship seen amongst the more mobile butterflies. The results provided evidence against the metapopulation dynamic explanation, and were equivocal with respect to the contributions of range position, niche breadth and resource availability. 5. An alternative approach was to explore deviations from the underlying relationship between density and distribution, rather than concentrating on the correlation itself. This approach was much more successful: we demonstrated that species that occurred at high densities relative to their distributions used aggregated resources and were relatively sedentary; whereas those that occurred at low densities relative to their distributions used less aggregated resources, and were more mobile. Mobile species had less aggregated distributions than did relatively sedentary species. 6. Given that the interspecific density-distribution pattern appears to be almost ubiquitous and that the proposed explanations are not mutually exclusive, faster progress may be made by examining deviations from the pattern than from further analysis of the pattern itself.