Showing papers in "Journal of Animal Ecology in 2001"

The geographical structure of British bird distributions: diversity, spatial turnover and scale

Abstract: Summary 1 Using data on the spatial distribution of the British avifauna, we address three basic questions about the spatial structure of assemblages: (i) Is there a relationship between species richness (alpha diversity) and spatial turnover of species (beta diversity)? (ii) Do high richness locations have fewer species in common with neighbouring areas than low richness locations?, and (iii) Are any such relationships contingent on spatial scale (resolution or quadrat area), and do they reflect the operation of a particular kind of species–area relationship (SAR)? 2 For all measures of spatial turnover, we found a negative relationship with species richness. This held across all scales, with the exception of turnover measured as βsim. 3 Higher richness areas were found to have more species in common with neighbouring areas. 4 The logarithmic SAR fitted better than the power SAR overall, and fitted significantly better in areas with low richness and high turnover. 5 Spatial patterns of both turnover and richness vary with scale. The finest scale richness pattern (10 km) and the coarse scale richness pattern (90 km) are statistically unrelated. The same is true of the turnover patterns. 6 With coarsening scale, locations of the most species-rich quadrats move north. This observed sensitivity of richness ‘hotspot’ location to spatial scale has implications for conservation biology, e.g. the location of a reserve selected on the basis of maximum richness may change considerably with reserve size or scale of analysis. 7 Average turnover measured using indices declined with coarsening scale, but the average number of species gained or lost between neighbouring quadrats was essentially scale invariant at 10–13 species, despite mean richness rising from 80 to 146 species (across an 81-fold area increase). We show that this kind of scale invariance is consistent with the logarithmic SAR.

Differential post-fledging survival of great and coal tits in relation to their condition and fledging date

Abstract: Summary 1 We present a multivariate model of the post-fledging survival of juvenile great and coal tits (Parus major L., P. ater L.) in relation to chick body condition and timing of breeding. Radio-telemetry and colour marks were used to track tit families during 20 days from fledging, that is, the period of post-fledging dependence. Data on 342 chicks of 68 broods were obtained. 2 Forty-seven per cent of juveniles died during the observation period, predation being the main cause of mortality. In the first 4 days after fledging the mortality rate was 5–10% per day. 3 Survival of juveniles was positively correlated with fledging mass. Furthermore, survival strongly decreased during the season. In the second half of June, mortality was five times the rate of mid-May. The differential survival resulted in selection for both early fledging and high fledging mass. Juvenile condition was less important for survival in birds that had fledged early in the season. Their survival rates exceeded 70% in all weight classes, whereas in late broods only the heaviest individuals survived equally well. The survival of birds fledging both late and in poor condition was below 20%. Thus, selection for high fledging mass was much stronger in the late season than in early broods. 4 We conclude that the impact of predation after leaving the nest results in selection for early breeding and, particularly in the late season, for high fledging mass. This may explain why the earliest broods have been found to produce most recruits into the breeding population even if they did not profit from maximum food availability during the nestling period. On the other hand, energetic limitations may constrain the begin of egg laying in adult birds. Thus, counteracting evolutionary responses to the seasonal development of food availability (the caterpillar peak) and to the risk of post-fledging mortality (the peak in post-fledging mortality) may have focused the period of optimal reproduction to a narrow time-window.

Life-history strategies in parasitoid wasps: a comparative analysis of ‘ovigeny’

Abstract: 1. Ecologists concerned with life-history strategies of parasitoid wasps have recently focused on interspecific variation in the fraction of the maximum potential lifetime egg complement that is mature when the female emerges into the environment. Species that have all of this complement mature upon emergence are termed 'pro-ovigenic', while those that do not are termed 'synovigenic'. We document and quantify the diversity of egg maturation patterns among 638 species of parasitoid wasps from 28 families. 2. We test a series of hypotheses concerning variation in 'ovigeny' and likely life- history correlates by devising a quantitative index - the proportion of the maximum potential lifetime complement that is mature upon female emergence. 3. Synovigeny, which we define as emerging with at least some immature eggs, was found to be by far the predominant egg maturation pattern (98.12% of species). Even allowing for some taxonomic bias in our sample of species, pro-ovigeny is rare among parasitoid wasps. 4. There is strong evidence for a predicted continuum in ovigeny index among parasitoid wasps, from pro-ovigenic (ovigeny index = 1) to extremely synovigenic species (ovigeny index = 0). 5. As predicted, synovigenic species are longer-lived than pro- ovigenic ones, and ovigeny index and life span are negatively correlated across parasitoid taxa, suggesting a life span cost of concentrating reproductive effort early in adult life. 6. There is equivocal evidence that host feeding (i.e, consumption of host haemolymph and/or tissues by adult wasps) is confined to synovigenic parasitoid wasps. It is also not certain from our analyses whether host feeding is associated with a relatively low ovigeny index. 7. As predicted, egg resorption capability is concentrated among producers of yolk-rich eggs. Also, the hypothesis that it is associated with a tendency towards a low ovigeny index is supported. Parasitoid species that produce yolk-rich eggs also exhibit a lower ovigeny index than species that produce yolk-deficient eggs. 8. Ovigeny index appears to be linked to parasitoid development mode (koinobiosis-idiobiosis). 9. We conclude that 'ovigeny' is a concept applicable to insects generally. [KEYWORDS: feeding strategies; pro-ovigeny; synovigeny; timing of reproduction Leptopilina-boulardi hymenoptera; host-feeding strategies; egg limitation; reproductive-biology; clutch size; independent contrasts; functional-response; asobara-tabida; drosophila-melanogaster; evolutionary argument]

On the misuse of residuals in ecology: testing regression residuals vs. the analysis of covariance

Abstract: Summary 1 An analysis of variance (anova) or other linear models of the residuals of a simple linear regression is being increasingly used in ecology to compare two or more groups. Such a procedure (hereafter, ‘residual index’) was used in 8% and 2% of the papers published during 1999 in the Journal of Animal Ecology and in Ecology, respectively, and has been recently recommended for studying condition. 2 Although the residual index is similar to an analysis of covariance (ancova), it is not identical and is incorrect for at least four reasons: (i) the regression coefficient used by the residual index differs from the one used in ancova and is not the least-squares estimator of the model. (ii) in contrast to the ancova, the error d.f. in the residual index are overestimated because of the estimation of the regression coefficient. (iii) the residual index also assumes the homogeneity of regression coefficients (parallelism assumption), which should be tested with a special ancova design. (iv) even if the assumptions of the linear model hold for the original variables, they will not hold for the residuals. 3 More importantly, the residual index is an ad hoc sequential procedure with no statistical justification, unlike the well-known ancova. For these reasons, I suggest that a t-test or an anova of the residuals should never be used in place of an ancova to study condition or any other variable.

Weak cross-species relationships between body size and trophic level belie powerful size-based trophic structuring in fish communities

Abstract: 1. Body size determines rates of respiration and production, energy requirements, mortality rates, patterns of predation and vulnerability to mortality. Body size distributions are often used to describe structure and energy flux in communities and ecosystems. 2. If clear relationships can be established between body size and trophic level in fishes, they may provide a basis for integrating community and ecosystem analyses based on size spectra, food webs and life histories. 3. We investigated relationships between the body sizes (weight and length) of northeast Atlantic fishes and their trophic level. The abundance of 15N, as determined by stable isotope analysis, was used as an index of trophic level. 4. Cross-species and comparative analyses demonstrated that body size was unrelated or weakly related to trophic level. Thus allometric relationships between body size and trophic level could not be used to predict the trophic structure of fish communities. 5. The results of the cross-species analyses contrasted with patterns in the size and trophic structure of entire fish communities. When fish communities were divided into size classes, there were strong positive relationships between size class and trophic level. The slope suggested a mean predator: prey body mass ratio of 80:1. 6. Our results suggest that body size does not provide a useful surrogate of trophic level for individual species, but that body size is an excellent predictor of trophic level within the community, providing an empirical basis for integrating community analyses based on models of trophic structure and body size distributions.

Climate and population density induce long‐term cohort variation in a northern ungulate

Abstract: Summary 1. Density-dependent and climatic conditions experienced by individuals before and after birth differ considerably between cohorts. Such early environmental variability has the potential to create persistent fitness differences among cohorts. Here we test the hypothesis that conditions experienced by individuals in their early development will have long-term effects on their life history traits. 2. We approached this by analysing and contrasting the effects of climate (the North Atlantic Oscillation, NAO) and population density at year of birth on cohort birth weight, birth date, litter size, age of maturity, survival and fecundity of Soay sheep, Ovies aries L., ewes in the population on the island of Hirta, St Kilda, Scotland. 3. Significant intercohort variations were found in life history traits. Cohorts born after warm, wet and windy (high NAO) winters were lighter at birth, born earlier, less likely to have a twin and matured later than cohorts born following cold and dry (low NAO) winters. High population densities in the winter preceding birth also had a negative effect on birth weight, birth date and litter size, whereas high postnatal densities delayed age of first reproduction. 4. High NAO winters preceding birth depressed juvenile survival but increased adult survival and fecundity. The negative influence of high NAO winters on juvenile survival is likely to be related to mothers’ compromised physical condition while the cohort is in utero , whereas the positive influence on adult survival and fecundity may relate to the improved postnatal forage conditions following high NAO winters. High preand postnatal population densities decreased juvenile (neonatal, yearling) and adult (2‐4 years) survivorship but had no significant effect fecundity.

Butterfly numbers and weather: predicting historical trends in abundance and the future effects of climate change

Abstract: Summary 1 The effect of weather on the size of British butterfly populations was studied using national weather records and the Butterfly Monitoring Scheme (BMS), a national database that has measured butterfly abundance since 1976. 2 Strong associations between weather and population fluctuations and trends were found in 28 of 31 species studied. The main positive associations were with warm summer (especially June) temperature during the current and previous year, low rainfall in the current year and high rainfall in the previous year. Most bivoltine species benefited from warm June weather in the current year, three spring species and two that overwinter as adults benefited from warm weather in the previous summer, and most species with moist or semi-shaded habitats increased following high rainfall and cooler weather in the previous year. 3 Simple models incorporating weather variables and density effects were constructed for each species using the first 15 years’ population data (1976–90). These fitted the observed data for that period well (median R2 = 70%). Models were less good at predicting changes in abundance over the next 7 years (1991–97), although significant predictive success was obtained. 4 Parameter values of models were then adjusted to incorporate the full 22-year data-run. For the eight species whose models had best predicted population changes or fitted the data well (R2 > 85%), models were run from 1767 to 1997, using historical weather records, to ‘predict’ trends in abundance over the past two centuries. For three species it was possible to compare predicted past trends with contemporary accounts of abundance since 1800. In each case, the match between predictions and these qualitative assessments was good. 5 Models were also used to predict future changes in abundance, using three published scenarios for climate change. Most, but not all, species are predicted to increase in the UK under warmer climates, a few species stayed stable, and only one species – the agricultural pest Pieris brassicae (Cabbage White) – is predicted to decline.

Plant phenology, migration and geographical variation in body weight of a large herbivore: the effect of a variable topography

Abstract: 1. A central concept in ruminant foraging ecology is that even slight changes in plant quality affect body growth substantially, because ruminants not only gain more protein and energy but also use less time for rumination when eating high-quality forage. Increased access to highly nutritious forage is thus regarded as an important driving force in the evolution of migration in large herbivores, because the temporal and spatial variation in plant quality is huge. Body weight is in turn a major determinant of reproductive performance and survival in ungulates, and therefore important for population dynamics. 2. In the temperate region, the vertical movement of cervids from a low-elevation winter range to a high-elevation summer range is the most common pattern of migration. Migration to high elevations during summer is regarded as a strategy to increase energy intake among northern temperate ungulates. 3. It has been suggested that there is increased forage quality at high elevation. This leads to the prediction that body weights are positively correlated with the proportion of high-altitude habitat. Alternatively, cervids may benefit due to prolonged access to newly emerged forage as they migrate along an altitudinal gradient. If so, it can be predicted that body weights are highest in regions with the highest diversity of different altitudes. 4. The facing of slopes, i.e. the aspect, is also regarded as an important part of the habitat for deer. Access to north-facing slopes is regarded as favourable because plant quality may be higher on north-facing slopes; access to a variety of different aspects is also favourable because this may lead to a variable snow melt and thus a longer time period with access to high-quality forage. 5. In order to test the above predictions (under points 3 and 4), we analysed the spatial variability of body weight of 8452 female and 12 474 male red deer harvested during 1975-98 from 105 municipalities in Norway. 6. Body weight of red deer was positively related to the diversity of altitudes, and negatively related to the proportion of high-altitude habitat per se. This demonstrated that the altitudinal migration of cervids is not necessarily due to increased forage quality at high elevation, but rather that migrating cervids gain from prolonged access to newly emerging forage along an altitudinal gradient as they migrate to high elevation during early summer. 7. There was also no support for the hypothesis that access to a high proportion of north-facing slopes was favourable; rather, body weight of red deer was correlated with access to a diversity of aspects. There was thus clear evidence that a variable topography, measured as different altitude levels and aspects, was positively related to body weight of red deer. 8. We discuss the possible causes of why an ideal free distribution is not reached, and conclude that phenological differences in plants related to topography can have a large impact on body weights of cervids, and therefore induce considerable spatial heterogeneity in population dynamics.

Trawling disturbance can modify benthic production processes

Abstract: 1. Trawling disturbance has wide-ranging impacts on the marine environment and is well known to modify benthic habitat and community structure. This has led to speculation about the positive and negative impacts of trawling on ecosystem processes such as production. 2. Existing theory suggests that frequent trawling disturbance may lead to the proliferation of smaller benthic species, with faster life histories, because they can withstand the mortality imposed by trawling and benefit from reduced competition or predation as populations of larger species are depleted. Since smaller species are more productive, trawling disturbance may 'farm the sea', with knock-on benefits for consumers, including fish populations. 3. We conducted the first large-scale studies of trawling effects on benthic production across quantified gradients of trawling disturbance on real fishing grounds in two regions (Silver Pit and Hills) of the North Sea. There were 27- and 10-fold differences in levels of beam trawl disturbance among the Silver Pit and Hills sites, respectively. 4. Size structure was described using normalized size-spectra, and the slopes and intercepts of these spectra were related to levels of trawling disturbance. Production was estimated from the size spectra, using a new allometric relationship between body mass and the production to biomass (P:B) ratio of marine invertebrates. The general validity of the relationship was confirmed using a phylogenetic comparative approach. 5. In the Silver Pit region, trawling led to significant decreases in infaunal biomass and production. The abundance of larger individuals was depleted more than smaller ones, as reflected by the positive relationship between the slope of the normalized size spectra and trawling disturbance. The effects of trawling disturbance were not significant in the epifaunal community. In the Hills region, where the range of trawling disturbance was lower, trawling disturbance did not have significant effects on biomass or production. 6. In the Silver Pit, relative infaunal production (production per unit biomass) rose with increased trawling disturbance. This was attributable largely to the dominance of smaller animals in the disturbed communities. The increase in relative production did not compensate for the loss of total production that resulted from the depletion of large individuals. There was some evidence for the proliferation of small polychaetes at moderate levels of disturbance, but at higher levels of disturbance their biomass and production fell. 7. We conclude that reported increases in the biomass and production of small infaunal invertebrates in the North Sea are attributable largely to recent increases in primary production that were driven by climate change, and not to the effects of trawling disturbance.

Butterfly responses to habitat edges in the highly fragmented prairies of Central Iowa

Abstract: Summary 1 The behaviour of two butterfly species, a habitat specialist (Speyeria idalia) and a habitat generalist (Danaus plexippus), was tracked at four prairie edges to determine the extent to which edges act as a barrier to emigration. The four edge types studied were crop, road, field and treeline. The edges differed in structure ranging from high-contrast (treeline) to low-contrast (field). 2 S. idalia, the habitat specialist, responded strongly to all edges, even those with low structural contrast. However, S. idalia’s response was strongly affected by conspecific density at crop and field edges; individuals were less likely to exit from high density plots. S. idalia responded to edges both by turning to avoid crossing them, and returning to the plot if they had crossed. 3 D. plexippus responded strongly only to treeline edges. Wind direction and time of year were important factors influencing behaviour at edges for this species. Conspecific density was not a significant factor affecting their behaviour. D. plexippus responded to edges by not crossing them, but rarely returned once they had crossed. 4 In highly fragmented landscapes, such as the one in which this study occurred, butterflies which show little or no response to edges may exhibit high emigration rates because of the high probability of encountering an edge in small habitat patches. Butterflies may respond strongly to even subtle habitat boundaries, but those responses may be modified by the edge structure, local environment or other conditions. Therefore, modifying edge structure may be a way to influence emigration rates, making it a useful tool for conservation.

Factors affecting first‐year survival in grey seals and their implications for life history strategy

Abstract: 1In order to estimate the effect of weaning mass and body condition on the post-weaning survival of grey seal pups from the Isle of May, Scotland in 1998 during their first year of life, a simultaneous analysis of live resighting and dead recovery mark–recapture data was used. A new type of tag was employed which allowed individuals to be identified when resighted alive (Hall, Moss & McConnell 2000) as well as when found dead. 2The probability of post-weaning survival to age 1 increased with body condition at weaning and differed between the sexes. Regardless of pup condition and time of year, the odds of survival for female pups over a 2-month interval was estimated to be 3·37 (SE = 1·30) times higher than for males. Regardless of sex, a 1 standard deviation increase in pup condition was estimated to increase the odds of survival by a factor of 1·422 (SE = 0·226). For a male pup in average condition (0·41 kg cm−1) the estimated annual survival after adjusting for tag-loss was 0·193 (SE = 0·084); for a female pup in average condition (0·39 kg cm−1) it was 0·617 (SE = 0·155). 3The effect of condition at weaning on survival was significantly greater for male pups than for females. This implies that high quality females should invest more heavily in their male pups because the marginal return, in terms of increased reproductive value, from any additional expenditure is twice that for females. Male pups in our sample were significantly heavier at weaning and in better condition than female pups. However, this does not provide conclusive support for our predictions, because we could not control for the effects of maternal size on weaned mass.

Effects of food availability on the distribution of migratory warblers among habitats in Jamaica

Abstract: Summary 1. Theoretical arguments suggest that distributions of migratory birds in winter should match patterns of food availability, but in reality the match between migrants and their food may be imperfect because, for various reasons, birds may be unable to ‘track’ food resources. We tested the hypothesis that food availability influences the distribution of migratory canopy-foraging insectivorous warblers wintering in Jamaica. 2. Over a wide spatial scale (24 sites on the island), warbler abundance varied signi ficantly among sites and habitats and was significantly dependent on measures of arthro pod biomass. Alternative factors (vegetation characteristics, resident bird competitor abundance, predator abundance) were not correlated with warbler abundance. 3. Over a short temporal scale (about 2 weeks) at a single site, warbler abundance increased as predicted quantitatively after a natural, rapid increase in arthropod biomass. 4. Over a longer temporal scale (the duration of a winter), changes in density and persistence of individually marked American redstarts ( Setophaga ruticilla , L.) on six sites corresponded with concurrent fluctuations in arthropod biomass. 5. These results document a strong association between arthropod biomass and warbler abundance in time and space, suggesting that warblers wintering in Jamaica distribute themselves in response to food resources. 6. We hypothesize that dominance hierarchies and variable movement strategies operate in concert with birds’ responses to food to influence the distribution of wintering warblers at different spatial scales. Whether food availability determines habitat quality remains to be investigated.

Ungulate foraging strategies: energy maximizing or time minimizing?

Abstract: Summary 1. Many classical models of ungulate foraging are premised on energy maximization, yet limited empirical evidence and untested currency assumptions make the choice of currency a non-trivial issue. The primary constraints on forage intake of ungulates are forage quality and availability. Using a model that incorporates these two constraints, we predicted the optimal biomass of forage patches for ungulate grazers using an energy maximizing vs. a time minimizing strategy. 2. We tested these predictions on wood bison ( Bison bison athabascae Rhoads) grazing naturally occurring sedge ( Carex atherodes Spreng). The digestive constraint was determined by a series of ad libitum feeding trials using sedge at different stages of growth. Sedge digestibility declined with biomass. Ad libitum intake of sedge by bison declined with sedge digestibility and thus decreased with sedge biomass. On the other hand, short-term sedge intake rates of wood bison increased with biomass. 3. Incorporation of these constraints resulted in the prediction that daily energy gain of bison should be maximized by grazing patches with a biomass of 10 g m - 2 , whereas a bison could minimize daily foraging time needed to fulfil its energy requirement by cropping patches with a biomass of 279 g m - 2 . 4. To test these quantitative predictions, we used a staggered mowing regime to convert even-aged stands of sedge to a mosaic of patches varying in quality and quantity. Observations of bison grazing these mosaics indicated that patches of biomass below 120 g m - 2 were avoided, while patches of biomass 156 and 219 g m - 2 were highly preferred, with the greatest preference for the latter. 5. These results indicate that bison were behaving as time minimizers rather than energy maximizers. Daily cropping times of free-ranging bison from the literature corroborate our results.

Species richness of insects and mites on trees: expanding Southwood

Abstract: Summary 1 Data on the species richness of phytophagous insects and mites associated with 25 tree genera occurring as natives in Germany were compiled and compared to data for British trees published by Kennedy & Southwood (1984). For tree genera occurring in Germany and Britain patterns of species richness and composition of phytophage faunas were similar. 2 Present abundance of trees, their distributional history during the Holocene, morphological traits and taxonomic isolation were used to explain the variance of species richness and proportion of specialists across tree genera occurring as natives in Germany. Tree genera were either used as independent data points or to calculate phylogenetically independent contrasts. For the latter approach, a phylogeny for the tree genera was generated from published rbcL gene sequences. In general, the conclusions from the two types of analyses were similar. 3 The species richness of phytophages on German tree genera were positively related to present tree abundance, tree height and tree abundance derived from pollen samples. For phylogenetically independent contrasts the length of time a genus was present since the end of the last glaciation also became significant. 4 The proportions of specialists showed a negative relationship with present abundance of trees, a positive relationship with taxonomic isolation as well as the length of time of genus was present since the end of the last glaciation. For phylogenetically independent contrasts only the latter two variables remained significant. 6 Overall the results support the species–area and the coevolutionary hypotheses.

High predictability in introduction outcomes and the geographical range size of introduced Australian birds: a role for climate

Abstract: Summary 1 We investigated factors hypothesized to influence introduction success and subsequent geographical range size in 52 species of bird that have been introduced to mainland Australia. 2 The 19 successful species had been introduced more times, at more sites and in greater overall numbers. Relative to failed species, successfully introduced species also had a greater area of climatically suitable habitat available in Australia, a larger overseas range size and were more likely to have been introduced successfully outside Australia. After controlling for phylogeny these relationships held, except that with overseas range size and, in addition, larger-bodied species had a higher probability of introduction success. There was also a marked taxonomic bias: gamebirds had a much lower probability of success than other species. A model including five of these variables explained perfectly the patterns in introduction success across-species. 3 Of the successful species, those with larger geographical ranges in Australia had a greater area of climatically suitable habitat, traits associated with a faster population growth rate (small body size, short incubation period and more broods per season) and a larger overseas range size. The relationships between range size in Australia, the extent of climatically suitable habitat and overseas range size held after controlling for phylogeny. 4 We discuss the probable causes underlying these relationships and why, in retrospect, the outcome of bird introductions to Australia are highly predictable.

Intraspecific resource partitioning in brown trout: the temporal distribution of foraging is determined by social rank

Abstract: Summary 1. Animals can reduce the competition for a limiting resource by temporal segregation, whereby individuals exploit the resource at different times. However, the pay-offs may vary predictably over time, and it can be predicted that (a) more dominant competitors should gain access to resources at the preferred times and (b) the degree of temporal segregation will vary with the intensity of competition. 2. Here we show experimentally that individual brown trout Salmo trutta (L.) made sequential use of foraging areas, with dominant individuals feeding mainly at the most beneficial times of dusk and the early part of the night while more subordinate fish fed at other times. 3. However, the degree of overlap in foraging times between high-ranking fish was dependent on energetic demands. At low temperatures (when requirements were low) the temporal activity patterns of top-ranking fish were synchronized, with foraging concentrated at the preferred times. In contrast, when temperature was raised to increase energetic requirements, activity patterns showed strong temporal segregation: the most dominant fish remained predominantly nocturnal, whereas second-ranking fish became increasingly diurnal. 4. This is the first experimental demonstration of shifts in the daily pattern of activity caused by varying intensity of intraspecific competition.

Density–distribution relationships in British butterflies. I. The effect of mobility and spatial scale

Abstract: 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 km2 areas (local), 500 m grid in 35 km2 (regional), 10 km grid across England, Wales and Scotland (national), 153 000 km2 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.

An experimental test of the link between foraging, habitat selection and thermoregulation in black rat snakes Elaphe obsoleta obsoleta

Abstract: Summary 1 Most physiological processes are temperature-dependent. Thus, for ectotherms, behavioural control of body temperatures directly affects their physiology. Ectotherms thermoregulate by adjusting habitat use and therefore thermoregulation is probably the single most important proximate factor influencing habitat use of terrestrial reptiles, at least in temperate climates. 2 Snakes have been shown to raise their body temperature following feeding in a laboratory thermal gradient, presumably to enhance digestion. This experiment was exported to the field to explore the link between feeding, habitat selection and thermoregulation in free-ranging snakes. 3 Experimental feeding was conducted in the laboratory and in the field on black rat snakes (Elaphe obsoleta obsoleta) that had temperature-sensitive radio-transmitters surgically implanted. 4 Snakes had higher mean body temperatures following feeding than prior to feeding in a laboratory thermal gradient. 5 Some, but not all evidence, indicated that black rat snakes increased their mean body temperature following feeding in the field. Indices of thermoregulation indicated that the snakes thermoregulated more carefully and more effectively after they had eaten. 6 Forest edges provided the best opportunities for thermoregulation in the study area. Black rat snakes were less likely to move following feeding when fed in edges than when fed in the forest and were more likely to be found in edges following feeding, whether they had been fed in the forest or in an edge. 7 Results of this study and one previous study suggest that thermoregulatory behaviour of snakes following feeding in the laboratory is a reliable predictor of their behaviour in the field. A review of 13 studies of the thermoregulatory behaviour of snakes following feeding in the laboratory revealed that not all species behave similarly. However, the quality and number of studies currently available is not adequate for testing hypotheses about which species should change thermoregulatory behaviour in response to eating and which should not.

Climate change and breeding success: decline of the capercaillie in Scotland

Abstract: Summary 1 The number of capercaillie in Scotland has fallen since the 1970s Previous work showed that low breeding success, exacerbated by deaths of fully grown birds flying into forest fences, was the primary cause of the decline The hypothesis that climate change caused the lower breeding success was investigated in this study 2 Temperature usually rose during April There was no trend in mean April temperature during the study (1975‐99) but there was a progressive cooling in mid-April relative to the rest of the month, such that the normal April warming was increasingly delayed 3 Hens reared more chicks when the temperature rose more in early April It is suggested that this stimulated timely plant growth, so improving the laying hens’ plane of nutrition and the viability of their chicks 4 Hens also reared more chicks when late May was warmer and early June was warmer and had fewer rain days Young chicks may have foraged more successfully in warm dry conditions However, neither temperature nor rain days in late May or early June showed any trend during the study 5 Increasingly protracted spring warming seems to have been a major cause of the decline of the capercaillie in Scotland

Factors affecting breeding dispersal in the facultatively colonial lesser kestrel: individual experience vs. conspecific cues

Abstract: Summary 1 The role of individual experience vs. the use of conspecific cues on breeding dispersal decisions have seldom been determined in colonial birds. We studied causes of breeding dispersal in the lesser kestrel (Falco naumanni), a species that breeds in colonies of variable size as well as solitarily. During a 6-year study in Spain, we gathered information on 486 subsequent breeding attempts and on 26 explanatory variables which evaluated individual experience, conspecific cues in terms of breeding performance and colony size, and different ecological and populational characteristics. 2 Two decisions were separately analysed: whether or not to disperse, and how far to move. Generalized Linear Mixed Models (GLMMs) allowed us to identify the relative contribution of each explanatory variable while controlling for the non-independence of individual dispersal decisions across years. 3 Females seemed to disperse more often than males (34% vs. 19%), and both sexes apparently dispersed less with age. However, a GLMM showed that experience (i.e. the number of years a bird bred in a particular colony) was the only factor influencing breeding dispersal. Birds showed higher site fidelity the greater their experience in a colony, which could be related to benefits derived of increased local familiarity. A second GLMM showed that, before birds acquired experience in a particular colony, individual nest failure due to predation and proximity to other colonies increased the probability of dispersal, dispersal being also higher in colonies with poor conspecific breeding success. Furthermore, solitary nesting birds were more prone to disperse and dispersal probability decreased the larger the colony of origin, according to fitness expectations associated with colony size. 4 A GLMM explaining dispersal distances retained two variables – birds dispersed farther the lower the breeding density in the surroundings, and the larger the distance to the nearest colony. Dispersing birds tended to settle within their previous foraging areas (median dispersal distance = 1·6 km), being constrained by the availability of nearby colonies. 5 Lesser kestrels mainly cue on their own breeding performance and experience in a particular colony at the time of taking a dispersal decision. However, inexperienced birds also partially cue on the size and breeding success of their own colonies (but not on the size or breeding performance of other colonies), and birds moved larger distances when dispersing from areas of low populational density. These results support some degree of conspecific attraction.

Monitoring a marine ecosystem using responses of upper trophic level predators

Abstract: 1 This study examined the changing status of the marine ecosystem at the island of South Georgia (Southern Ocean) using up to 27 variables measured over 22 years from three upper trophic level predators that specialize in foraging upon krill (Eupuasia superba Dana). These variables included population size, breeding performance, offspring growth rate, foraging behaviour and diet. A method was developed for reducing these multivariate time-series to a single vector, called a combined standardized index (CSI). 2 Sensitivity analyses showed that missing values had a large effect upon the accuracy of the CSI but this effect was reduced if the individual variables were highly correlated. The level of correlation and proportion of missing values within the empirical data set were within the acceptable range. Individual variables had widely varying influence upon the CSI but, in general, those with longer time-series had the greatest influence. 3 Principal components analysis showed that variables representing offspring growth tended to explain the greatest proportion of the variability in the CSI and this was followed by variables representing diet. 4 There were 3 years in which the CSI showed extreme and significantly low values. There was a significant non-linear functional response (similar to the Holling Type II functional response) between the overall CSI and krill biomass and a similar relationship existed when the CSI was calculated for each species individually. 5 Separate analysis of variables that were likely to be representative of changing population size showed the presence of a significant decline between 1977 and 1998. There was no trend in the CSI from variables representative of foraging conditions during the summer breeding season. The study has shown that the marine ecosystem at South Georgia shows acute but transient variability that is amplified in the response of upper trophic-level predators. There is less certainty that trends in populations are a consequence of shifts in the degree to which the ecosystem can support krill-feeding seals and penguins.

The role of plant trichomes and caterpillar group size on growth and defence of the pipevine swallowtail Battus philenor

Abstract: Summary 1. The California population of the pipevine swallowtail Battus philenor is a specialist on the Dutchman’s pipe Aristolochia californica , an endemic vine that is densely covered with trichomes. Populations of B. philenor outside California use other Aristolochia species that are largely glabrous. The average clutch size of the pipevine swallowtail is larger in California compared with populations elsewhere and larvae feed gregariously until late in the third instar. 2. In the field, caterpillars consumed more leaf material and showed preference for portions of leaves with trichomes removed. However, large groups of caterpillars were consistently observed feeding on the apical portion of the plant, where trichome density was highest. Smaller groups of caterpillars were observed feeding more often on mature leaves on the lower portions of the plant, where trichome density was lower. 3. Laboratory experiments showed that the walking speed of a commonly observed predator, larvae of the green lacewing Chrysopa carnea , was reduced as trichome density increased. Furthermore, lacewing search efficiency and capture rate of a model prey item were compromised by high trichome density. 4. In an additional field experiment, no difference was found in the percentage mortality of groups of four and 12 caterpillars. However, growth rate of the larger group was accelerated by 25% compared with smaller groups. In an experiment using a ladybird beetle larva Hippodamia convergens as the predator, no difference was observed in absolute mortality of caterpillars, suggesting that group size does not function directly as a defence against predators. 5. First instar caterpillars are most vulnerable to predators, thus feeding in larger groups may benefit caterpillars by accelerating growth. Feeding in large groups may also be an effective strategy for B. philenor to overcome plant trichomes and feed on portions of the plant conducive to faster development. However, feeding on areas with dense trichomes does not appear to provide larvae with a refuge from predators.

Post‐fledging care, philopatry and recruitment in brown thornbills

Abstract: Summary 1. We describe patterns of post-fledging care, dispersal and recruitment in four cohorts of brown thornbills Acanthiza pusilla . We examine what factors influence post-fledging survival and determine how post-fledging care and the timing of dispersal influence the probability of recruitment in this small, pair breeding, Australian passerine. 2. Fledgling thornbills were dependent on their parents for approximately 6 weeks. Male fledglings were more likely than female fledglings to survive until independence. For both sexes, the probability of reaching independence increased as nestling weight increased and was higher for nestlings that fledged later in the season. 3. The timing of dispersal by juvenile thornbills was bimodal. Juveniles either dispersed by the end of the breeding season or remained on their natal territory into the autumn and winter. Juveniles that delayed dispersal were four times more likely to recruit into the local breeding population than juveniles that dispersed early. 4. Delayed dispersal was advantageous because individuals that remained on their natal territory suffered little mortality and tended to disperse only when a local vacancy was available. Consequently, the risk of mortality associated with obtaining a breeding vacancy using this dispersal strategy was low. 5. Males, the more philopatric sex, were far more likely than females to delay dispersal. Despite the apparent advantages of prolonged natal philopatry, however, only 54% of pairs that raised male fledglings to independence had sons that postponed dispersal, and most of these philopatric sons gained vacancies before their parents bred again. Consequently, few sons have the opportunity to help their parents. Constraints on delayed dispersal therefore appear to play a major role in the evolution of pair-breeding in the brown thornbill.

Invasion of a stream food web by a new top predator

Abstract: Summary 1. A new top predator, the dragonfly Cordulegaster boltonii Donovan, ‘invaded’ a stream with a well-described food web. 2. The pre-invasion web was species-poor but complex, with prevalent intraguild predation, cannibalism and omnivory. Such characteristics differ from expectations based upon the early food web literature, but are consistent with more recent empirical webs and theoretical developments. 3. Exhaustive sampling was necessary to describe web structure, with the gut contents of several hundred individuals being required to reach the asymptote of the total number of links for individual species. There was no single ‘standard’ sample size that was applicable for estimating the number of links: sampling ‘ x ’ guts gave a different fraction of the asymptotic value for different species. Smaller predators were more prone to underestimation of links than larger species higher in the web. 4. The number of feeding links, trophic status and the degree of omnivory increased progressively with predator body size, both within and among species. The diet of each predator species (or instar) was effectively a subset of the diet of the next largest predator. 5. The invader was extremely polyphagous and fed at all trophic levels. Mean chain length increased by half a link following the invasion. Web complexity, and omnivory in particular, also increased. Pre- and post-invasion webs displayed intervality and rigid circuitry. The resident predators were frequently eaten by the invader, but the only significant predators of C. boltonii were larger conspecifics. Although no species have yet been deleted, there has been a 21% increase in links for a 6% increase in species since the invasion, suggesting that the members of the web had become more tightly packed within niche space. Most prey species were eaten by every predator species (including C. boltonii ), indicating the potential for strong apparent competition within the web.

Community persistence among stream invertebrates tracks the North Atlantic Oscillation

Abstract: • 1. The North Atlantic Oscillation (NAO) affects marine ecosystems, lakes and some terrestrial ecosystems around the Atlantic rim, but evidence for effects on rivers is scarce. For 14 years, we sampled riverine macroinvertebrates in eight independent streams from the Llyn Brianne experimental catchments in central Wales. We assessed whether year-to-year persistence in rank abundance and species composition tracked the NAO. • 2. Persistence was quasi-cyclical and varied synchronously across all streams, irrespective of chemistry and catchment land use. Invertebrate communities in all stream types or habitats switched significantly from being highly persistent during negative phases of the NAO (winter index 1 = mild, wet winters). These effects occurred in both rare and common taxa. • 3. Acid episodes could not explain low persistence in positive NAO years because variations in acid-base status were not linked to the NAO. Moreover, fluctuations in persistence were apparent even in well-buffered streams. • 4. Discharge in adjacent gauged catchments increased in positive NAO years by 15–18% but neither flow variability nor flow maxima were higher. Nor were variations in invertebrate persistence at Llyn Brianne directly correlated with discharge pattern. Discharge variations alone were therefore insufficient to explain links between persistence and the NAO, but we cannot exclude subtle effects due either to flow or temperature. • 5. These data illustrate how the persistence of invertebrate communities varies through time in fluctuating environments. Positive phases of the NAO are accompanied by ecological instability in the Llyn Brianne streams, although the exact mechanisms are currently unclear. The effects of the NAO might confound or obscure other long-term change in rivers such as recovery from acidification or the effects of global warming.

A long-term study of reproductive performance in tree swallows: the influence of age and senescence on output

Abstract: Summary 1 We describe age-related reproductive performance and recapture rates of tree swallows (Tachycineta bicolor Vieillot) based on a 25-year study of a nestbox population in south-eastern Ontario, Canada (1975–99). 2 Performance improved from first-time breeders to intermediate-aged birds. Nest initiation advanced, and clutch size increased in both sexes. In females the number of hatchlings and fledglings increased, and the proportion of nests failing completely declined. Performance declined in females after ‘middle-age’, in the number of young fledged, and the proportion of young fledged relative to initial clutch and brood size. Also, the proportion of nests that failed completely increased in the oldest birds. Males showed similar patterns. 3 An index of performance incorporating clutch size, hatching and fledging efficiency, and two measures of total nest failure increased to, then declined after, 4 years of age in females and 3 years in males. The relationship between this index and age was best predicted by quadratic regression. 4 We found no support for three of four hypotheses to explain improvement in performance with age. Recapture rates declined after age 4Y in males, but remained unchanged in females until age 7Y +, while output decreased in both sexes (Residual Reproductive Value). Birds breeding repeatedly did not perform better during their first attempt compared to birds that bred only once (Selection). Birds with varied breeding experience did not differ in their performance within age-groups (Breeding Experience). We did find support for the Breeding Age hypothesis; in females with no breeding experience, there was a successive advance in laying and increase in clutch size from 2 to 4 years of age. 5 Improved performance may be due to skills acquired with age, such as those devoted to feeding and balancing energy demands, which are necessary to prepare and maintain individual condition prior to, and during, breeding. Senescence in performance after ‘middle-age’ may result from accumulated costs of previous breeding effort which have been identified in this species based on research elsewhere.

Habitat heterogeneity affects population growth in goshawk Accipiter gentilis

Abstract: 1. The concept of site-dependent population regulation combines the ideas of Ideal Free Distribution-type of habitat settlement and density dependence in a vital rate mediated by habitat heterogeneity. The latter is also known as habitat heterogeneity hypothesis. Site-dependent population regulation hypothesis predicts that increasing population density should lead to inhabitation of increasingly poor territories and decreasing per capita population growth rate. An alternative mechanism for population regulation in a territorial breeding system is interference competition. However, this would be expected to cause a more even decrease in individual success with increasing density than site-dependent regulation. 2. We tested these ideas using long-term (1975-99) population data from a goshawk Accipiter gentilis population in Eastern Westphalia, Germany. 3, Goshawk territory occupancy patterns and reproduction parameters support predictions of site-dependent population regulation: territories that were occupied more often and earlier had a higher mean brood size. Fecundity did not decrease with increasing density in best territories. 4. Using time-series modelling, we also showed that the most parsimonious model explaining per capita population growth rate included annual mean habitat quality, weather during the chick rearing and autumn period and density as variables. This model explained 63% of the variation in per capita growth rate. The need for including habitat quality in the time-series model provides further support for the idea of site-dependent population regulation in goshawk.

Associations between energetics and over-winter survival in the short-tailed field vole Microtus agrestis

Abstract: Summary 1 Over-winter mortality in small mammals is influenced strongly by low ambient temperatures. Individuals with greater thermogenic capacities might then be expected to survive better than those with lower thermogenic capacities. 2 To test this hypothesis, short-tailed field voles Microtus agrestis (Linnaeus) were captured during the winters of 1995/96 and 1996/97 at two field sites near Aberdeen (57°N). The captured animals were tagged and taken back to the laboratory, where their resting metabolism (RMR), thermogenic capacity (NA induced metabolism excluding RMR) and body mass were measured. 3 Body mass, RMR and thermogenic capacity did not differ significantly between the start and end of winter in voles that were captured at both times. 4 Body mass varied significantly over the winter months, being lowest in January and highest in March. Thermogenic capacity also varied over the winter and the variation was linked significantly with changes in ambient temperature, suggesting that either voles with greater thermogenic capacity were more likely to be active on cold nights, or voles were flexible in their thermogenic capacities. RMR did not vary significantly over the winter. 5 Animals that survived the winter had a significantly higher residual RMR than those that died (or permanently emigrated) but the survivors did not have significantly greater body masses, RMRs, thermogenic capacities or residual thermogenic capacities.