Climatic seasonality, resource bottlenecks, and abundance of rainforest birds: implications for global climate change

We demonstrate that within-year climatic variability, particularly rainfall seasonality, is the most significant variable explaining spatial patterns of bird abundance in Australian tropical rainforest. The likely mechanism causing this pattern is a resource bottleneck (insects, nectar, and fruit) during the dry season that limits the population size of many species. The patterns support both the diversity–climatic–stability hypothesis and the species–energy hypothesis but clearly show that seasonality in energy availability may be a more significant factor than annual totals or means. An index of dry season severity is proposed that quantifies the combined effect of the degree of dryness and the duration of the dry season. We suggest that the predicted increases in seasonality due to global climate change could produce significant declines in bird abundance, further exacerbating the impacts of decreased range size, increased fragmentation, and decreased population size likely to occur as a result of increasing temperature. We suggest that increasing climatic seasonality due to global climate change has the potential to have significant negative impacts on tropical biodiversity.     

Species richness and abundance of forest birds in relation to radiation at Chernobyl

The effects of low-level radiation on the abundance of animals are poorly known, as are the effects on ecosystems and their functioning. Recent conclusions from the UN Chernobyl forum and reports in the popular media concerning the effects of radiation from Chernobyl on animals have left the impression that the Chernobyl exclusion zone is a thriving ecosystem, filled with an increasing number of rare species. Surprisingly, there are no standardized censuses of common animals in relation to radiation, leaving the question about the ecological effects of radiation unresolved. We conducted standardized point counts of breeding birds at forest sites around Chernobyl differing in level of background radiation by over three orders of magnitude. Species richness, abundance and population density of breeding birds decreased with increasing level of radiation, even after controlling statistically for the effects of potentially confounding factors such as soil type, habitat and height of the vegetation. This effect was differential for birds eating soil invertebrates living in the most contaminated top soil layer. These results imply that the ecological effects of Chernobyl on animals are considerably greater than previously assumed.     

Predicting changes in the distribution and abundance of species under environmental change

Environmental changes are expected to alter both the distribution and the abundance of organisms. A disproportionate amount of past work has focused on distribution only, either documenting historical range shifts or predicting future occurrence patterns. However, simultaneous predictions of abundance and distribution across landscapes would be far more useful. To critically assess which approaches represent advances towards the goal of joint predictions of abundance and distribution, we review recent work on changing distributions and on effects of environmental drivers on single populations. Several methods have been used to predict changing distributions. Some of these can be easily modified to also predict abundance, but others cannot. In parallel, demographers have developed a much better understanding of how changing abiotic and biotic drivers will influence growth rate and abundance in single populations. However, this demographic work has rarely taken a landscape perspective and has largely ignored the effects of intraspecific density. We advocate a synthetic approach in which population models accounting for both density dependence and effects of environmental drivers are used to make integrated predictions of equilibrium abundance and distribution across entire landscapes. Such predictions would constitute an important step forward in assessing the ecological consequences of environmental changes.     

Mixed responses of farmland birds to the Conservation Reserve Enhancement Program in Pennsylvania

The Pennsylvania Conservation Reserve Enhancement Program (CREP) was initiated in 2000 and within 4 years, 40,000 ha of conservation grasslands were established in southern Pennsylvania. We determined whether CREP habitat has benefitted farmland and grassland bird populations during the 10 years since the program began. From 2001 to 2010, bird surveyors conducted road-side point counts in a 20-county area in south-central Pennsylvania. We observed positive CREP effects on the abundances (in 2009–2010) and changes in abundance (from 2001–2002 to 2009–2010) of 5 species, including eastern meadowlark (Sturnella magna); negative CREP effects for 3 species, including vesper sparrow (Pooecetes gramineus); and no CREP effects for 2 species, including grasshopper sparrows (Ammodramus savannarum). We additionally observed changes in the size and direction of the local CREP effects (within 250 m of count locations) depending on the amount of CREP grassland or field cover in the surrounding landscape (within 5,000 m of survey routes). For example, the local CREP effect on the change in abundance of eastern meadowlarks was 15 times greater at points nested within landscapes with 9% CREP cover compared to landscapes with 1% CREP cover, indicating the potential for greater benefits of adding new CREP grasslands to areas with more CREP habitat already in the surrounding area. We conclude that more careful spatial targeting of CREP enrollment could improve the benefits of the program for farmland and grassland bird populations. © The Wildlife Society, 2013     

Long-term trends in the abundance of breeding Lapwing Vanellus vanellus in relation to land-use change on upland farmland in southern Scotland

Capsule A long-term decline on this habitat is shown to be associated with the intensification of agricultural management, particularly the occurrence of field drainage.

Aims To assess long-term trends in the number of breeding Lapwing and determine the relationship between these trends and changes in agricultural management on an upland study area.

Methods Breeding Lapwing were counted along two road transects in nine years between 1980 and 2002, and on one extensive plot in 1980, 1990 and 2000. Counts along the road transects were made from a vehicle and the fields used for nesting were recorded. Changes in field management along the transects were monitored annually between 1980 and 1990, and habitat composition assessed in 1980, 1985, 1990 and 2000.

Results During the first 20 years of study the number of breeding Lapwing declined substantially on all three count areas and by 77% overall, with further declines on both transects in 2002. The area of unimproved grassland and arable on these transects also declined substantially due to conversion to improved grass. Fields that comprised either unimproved grassland or arable were most likely to hold nesting Lapwing, while the chance of a field losing its nesting Lapwing was positively associated with the occurrence of drainage. Drainage and conversion to improved grass were closely linked.

Conclusions Agricultural intensification is a probable cause of decline in the number of breeding Lapwing in upland areas. Such declines may have been widespread in upland areas following increased agricultural intensification in recent decades.     

Dispersal and the interspecific abundance-occupancy relationship in British birds

Aim To test the prediction that deviations of species from the positive interspecific relationship between abundance and occupancy (a measure of geographical range size) are related to differences in dispersal. Location Great Britain. Methods Quantitative data on the abundances, occupancy and dispersal distances of British birds are compared using phylogenetic comparative methods. Results Measures of natal and adult dispersal distance, and the intraspecific variance in these parameters, explain little variation in occupancy in addition to that accounted for by population size. Individual dispersal variables failed to explain significant variance when added individually to a model with population size as a predictor. Migrants and species using wet habitats tend to disperse further than residents and dry habitat species. Analysing these four groups separately revealed effects of dispersal only on the occupancy attained by dry habitat species. Conclusions The only consistent predictor of occupancy in these analyses was population size.     

Local extinction of British farmland birds and the prediction of further loss

1.  The populations of many UK farmland birds declined between 1970 and 1990, frequently accompanied by contractions in breeding ranges. Ornithological atlas data, land use data and environmental data at the scale of 10-km squares were used to investigate the relationship between local extinctions and habitat suitability for six species, and to predict where future losses are most likely.
2.  For each species we tested the hypothesis that local extinctions were concentrated in environments that were inherently less suitable. We also tested the hypothesis that spatial patterns of loss were not independent between species due to their concurrence in the same habitats.
3.  Multivariate analyses (PCA) showed that areas where each species became extinct between 1970 and 1990 were more similar in land use type, climate and topography to areas where a species was never present than those where it was retained; local extinction was more likely in less suitable environments. Multiple logistic regression showed that for five of the six species the environmental gradient best predicting presence or absence in 1970 was also that best predicting loss between 1970 and 1990. For the six species studied, local extinctions were least likely in lowland arable areas.
4.  For any pair of species, local extinctions were more frequent outside the area of overlap of the two species' ranges than inside. Within the area of overlap, species tended to be lost from the same squares. For each species, likelihood of local extinction declined with increasing number of the other five species present.
5.  We used model parameters to map the probability of future local extinctions of the six species considered, allowing the identification of key areas for conservation management at a spatial scale appropriate to agri-economic incentives.     

Assessing the impact of Entry Level Stewardship on lowland farmland birds in England

Agri‐environment schemes (AES) are central to the conservation of Europe’s farmland biodiversity. The UK Government’s Public Service Agreement target seeks to reverse the decline of farmland birds in England by 2020 through the use of AES. The Entry Level Stewardship (ELS) scheme, introduced in 2005, is the first non‐competitive, broad‐uptake stewardship scheme designed to deliver simple but effective environmental management on farms throughout England. Approximately 5 million hectares of farmland are currently entered into Environmental Stewardship (ES) agreements, and given the scale of this investment it is important to verify the efficacy of ELS as a mechanism for delivering biodiversity benefits. We used spatially referenced ES agreement data to assess the influence of specific management options and combinations of options on farmland bird population changes between 2005 and 2008 derived from standardized surveys carried out on over 2000 lowland farmland 1‐km squares in England. The results showed only limited evidence for short‐term effects of ES. Only Corn Bunting Emberiza calandra and Common Starling Sturnus vulgaris showed some landscape‐specific positive associations with area under ELS management. There was also limited evidence for positive associations between specific management options and the population changes of the species at which they were aimed. The current pattern of option uptake may be limiting benefits of ELS, with boundary management accounting for over 50% of scheme compensation value. However, the time required for option maturation and the time lags in bird population responses to environmental change mean that it is too early to reach definitive conclusions about the success or failure of the scheme. The findings of this study will assist in the development of ELS by feeding into the 5‐year review scheduled for 2010 and have implications for the development of similar schemes elsewhere in Europe.     

How species richness and total abundance constrain the distribution of abundance

The species abundance distribution (SAD) is one of the most intensively studied distributions in ecology and its hollow‐curve shape is one of ecology's most general patterns. We examine the SAD in the context of all possible forms having the same richness (S) and total abundance (N), i.e. the feasible set. We find that feasible sets are dominated by similarly shaped hollow curves, most of which are highly correlated with empirical SADs (most R² values > 75%), revealing a strong influence of N and S on the form of the SAD and an a priori explanation for the ubiquitous hollow curve. Empirical SADs are often more hollow and less variable than the majority of the feasible set, revealing exceptional unevenness and relatively low natural variability among ecological communities. We discuss the importance of the feasible set in understanding how general constraints determine observable variation and influence the forms of predicted and empirical patterns.     

Body size and latitudinal gradients in regional diversity of New World birds

Are latitudinal gradients in regional diversity random or biased with respect to body size? Using data for the New World avifauna, I show that the slope of the increase in regional species richness from the Arctic to the equator is not independent of body size. The increase is steepest among small and medium‐sized species, and shallowest among the largest species. This is reflected in latitudinal variation in the shape of frequency distributions of body sizes in regional subsets of the New World avifauna. Because species are added disproportionately in small and medium size classes towards low latitudes, distributions become less widely spread along the body size axis than expected from the number of species. These patterns suggest an interaction between the effects of latitude and body size on species richness, implying that mechanisms which vary with both latitude and body size may be important determinants of high tropical diversity in New World birds.     

Using chorotypes to deconstruct biogeographical and biodiversity patterns: the case of breeding waterbirds in Europe

Aim To deconstruct the biodiversity pattern of the 152 waterbird species breeding in Europe to better understand its multiple causal processes. Location Continental Europe, Iceland and the British Isles. Methods We considered the orders that are typically comprised by swimming, diving or wading birds, which inhabit marshes, fens, peatlands and fresh, brackish or salt waters, including coastal waters. We used the 55 main river basins of Europe as geographical units, and searched for either chorotypes (groups of similar species distributions) or gradual replacement of species throughout the river basins. Chorotypes were recognized by applying a probabilistic classification method to the distributions. Then we used GLM to characterize the extent and the species richness of each chorotype according to energy availability (higher levels of environmental energy favouring the presence of species), climatic stress due to an excess of energy, availability of water, productivity, seasonality and surface area. Results One hundred and forty species significantly aggregate into nine chorotypes. The other 12 species, most of them marine, are centred on Great Britain, dropping away progressively on coasts further away from there. Differences in either the availability of energy or climatic stress significantly characterized the distribution of seven chorotypes comprising 90.8% of the species. Main conclusions Chorotypes are meaningful and useful to deconstruct biodiversity patterns. Our results suggest that energy is the main factor related to the biogeographical patterns of breeding waterbirds in Europe, and provide an insight into regional trends of species richness previously analysed with a habitat‐scale perspective.     

Contrasting patterns of elevational zonation for birds and mammals in the Andes of southeastern Peru

Abstract. To determine the generality of avian diversity patterns, we investigated patterns of elevational zonation shown by birds and mammals along the eastern slope of the Andes Mountains in southeastern Peru. The strong environmental gradient sampled, entirely within Peru's Manu National Park and Biosphere Reserve, supports highly diverse faunas. Elevational distributions of 901 bird species, 129 bat species, and twenty-eight species of native mice exhibit contrasting patterns in species richness, species composition, and species turnover. Birds and bats showed smooth declines of species richness with elevation, whereas the richness of mouse assemblages was unrelated to elevation. For all three groups, the greatest differences were between lowland and highland faunas, although cutoff points for this contrast varied among groups (≈ 500 m for birds, 750 m for bats, and 1000 m for mice). Differences in composition also separated bird and bat faunas on either side of c. 1400 m (the boundary between montance forest and cloud forest); for mice, this faunal transition may take place nearer to 2000 m. Bird and bat faunas lacked the more discrete zonations suggested for mouse assemblages, as indicated by elevational range profiles and nested subset analyses. Distinct highland assemblages are apparent in two-dimensional histograms of range limits of birds and mice, but not for bats. Highland bat species occupy broader elevational ranges than lowland bat species, but for both birds and mice, species at intermediate elevations had the broadest amplitudes. Finally, clumping of range maxima and minima along the gradient identified zones of pronounced species turnover in each group, but these were generally not strongly associated with the locations of ecotones. Differences in zonation of these groups appear to reflect their different biological attributes and phylogenetic histories. Such differences obviously complicate discussions of ‘general’ diversity patterns, and limit the usefulness of birds to forecast or predict diversity patterns in other more poorly known groups—other groups may show elevated diversity and endemism in areas where avian diversity patterns appear unremarkable. The pronounced contrasts between bats and mice, and the generally intermediate character of avian patterns, suggest that future analyses might profitably partition birds into finer, more homogeneous groups of historically and/or ecologically similar species. Group differences in zonation may ultimately prove explicable with information on both species-abundance patterns and resource distributions.     

Changes in bird populations on sample lowland English farms in relation to loss of hedgerows and other non-crop habitats

Farmland bird population trends were examined on a sample of lowland English farms to assess the relative importance of habitat loss and habitat degradation. Data were extracted from 11 farms surveyed by territory mapping between 1966 and 1986 as part of the British Trust for Ornithology's Common Birds Census. The population size of 38 bird species was quantified for each farm in each year. The extents of five non-crop habitats were measured at 4-yearly intervals on each farm. The farms were selected because some had undergone extensive removal of non-crop habitats while others had undergone little or none. Although declines were commonest on farms where the severest habitat loss had taken place, we found no evidence that habitat loss was the main factor causing population declines: all 11 farms had significant numbers of declining species, even where habitat loss was minimal. Furthermore, general linear modelling found no significant effects of habitat loss on population trends and principal-components analysis found limited effects of habitat extent on community composition. These results suggest that habitat loss is of secondary importance in causing farmland bird population declines. We suggest that other processes, such as habitat degradation, may have caused a baseline population decline in at least 10 farmland bird species and that declines may have been exacerbated by localised habitat loss. Received: 4 February 1998 / Accepted: 1 April 1998     

Pattern in the distribution of Britain's upland breeding birds

We use a quantitative approach to identify fifty-eight species of birds which breed in association with the British uplands. Similarities and differences between this list of 'upland birds' and previous more subjective lists are discussed. We then study pattern in the distribution of these species throughout the uplands. A high degree of regionalization is found, and interpreted in terms of the habitat composition of different regions, and known bird–habitat associations. Different regions differ widely, not only with respect to their bird species composition, but also in the number and conservation importance of their upland bird assemblages. In particular, we contrast the uplands of Wales and England with those of Scotland. The Welsh and English uplands contain a relatively low number of upland bird assemblages and are divided into a few large regions, each dominated by a single assemblage type. In comparison, the Scottish uplands are more varied, both in terms of the total number of assemblages, and the range of assemblages found at a small scale. The study provides a means of viewing any upland region within the national context.     

The positive correlation between avian species richness and human population density in Britain is not attributable to sampling bias

Aim  To assess whether spatial variation in sampling effort drives positive correlations between human population density and species richness.
Location  British 10 × 10 km squares.
Methods  We calculated three measures of species richness from atlas data of breeding birds in Britain: total species richness, species richness standardised for sampling effort, and the number of species only recorded in supplementary casual records in a manner not standardised for survey effort. We then assessed the form of the relationship between these richness estimates and human population density, both with and without taking spatial autocorrelation into account.
Results  Both total and standardised species richness exhibit similar species richness–human population density relationships; species richness generally increases with human population density, but decreases at the very highest densities. Supplementary species richness is very weakly correlated with human population density.
Main conclusions  In this example, sampling effort only slightly influences the form of species richness–human population density relationships. The positive correlation between species richness and human population density and any resultant conservation conflicts are thus not artefactual patterns generated by confounding human density and sampling effort.