Showing papers in "Journal of Animal Ecology in 1999"

Scaling of metabolic rate with body mass and temperature in teleost fish

Abstract: Summary 1. We examined published studies relating resting oxygen consumption to body mass and temperature in post-larval teleost fish. The resulting database comprised 138 studies of 69 species (representing 28 families and 12 orders) living over a temperature range of c. 40 °C. 2. Resting metabolic rate (Rb; mmol oxygen gas h–1) was related to body mass (M; wet mass, g) by Rb = aMb, where a is a constant and b the scaling exponent. The model was fitted by least squares linear regression after logarithmic transformation of both variables. The mean value of scaling exponent, b, for the 69 individual species was 0·79 (SE 0·11). The general equation for all teleost fish was 1nRb = 0·80(1nM) – 5·43. 3. The relationship between resting oxygen consumption and environmental temperature for a 50-g fish was curvilinear. A typical tropical fish at 30°C requires approximately six times as much oxygen for resting metabolism as does a polar fish at 0°C. This relationship could be fitted by several statistical models, of which the Arrhenius model is probably the most appropriate. The Arrhenius model for the resting metabolism of 69 species of teleost fish, corrected to a standard body mass of 50 g, was 1nRb = 15·7 – 5·02.T–1, where T is absolute temperature (103 × K). 4. The Arrhenius model fitted to all 69 species exhibited a lower thermal sensitivity of resting metabolism (mean Q10 = 1·83 over the range 0–30 °C) than typical within-species acclimation studies (median Q10 = 2·40, n = 14). This suggests that evolutionary adaptation has reduced the overall thermal sensitivity of resting metabolism across species. Analysis of covariance indicated that the relationships between resting metabolic rate and temperature for various taxa (orders) showed similar slopes but significantly different mean rates. 5. Analysis of the data for perciform fish provided no support for metabolic cold adaptation (the hypothesis that polar fish show a resting metabolic rate higher than predicted from the overall rate/temperature relationship established for temperate and tropical species). 6. Taxonomic variation in mean resting metabolic rate showed no relationship to phylogeny, although the robustness of this conclusion is constrained by our limited knowledge of fish evolutionary history.

Competition for early arrival in migratory birds

Abstract: Summary 1. It is widely accepted that the arrival order of migratory birds is correlated with the condition of the birds, which leads to high quality individuals occupying prime sites. However, the theoretical backgrounds for this argument have been lacking. A simple game-theoretic model of arrival timing is provided which investigates the evolutionary stability of condition-dependent arrival order in territorial migrant birds. 2. Competition for territories or other priority-dependent benefits can lead to arrival dates far preceding the cost-minimizing date (the optimum date in the absence of competition) for all but the weakest individuals. Increasing the number of competitors can generate a ‘cascading’ competition for early arrival, which advances arrival dates further apart from the individual optimum dates for the onset of breeding. 3. At equilibrium, arrival order corresponds strictly to condition order only if marginal costs of advancing arrival are always larger for individuals in lower condition. If spring mortality vacates territories for later-arriving birds, the criterion for ‘honest’ arrival order becomes still stricter: differential survival costs should exist, but survival differences among individuals (or, alternatively, territory quality differences) should not be very large. 4. If the habitat is saturated so that there is a risk of not obtaining a territory at all, or if worst territories are of much lower value than the rest, competition may lead to the majority of the population arriving within a fairly short interval, followed by a much later floating fraction. This synchrony in the arrival of breeders imposes an increasing cost for the lesser fit breeding birds. Thus, arrival costs paid are not necessarily highest for earliest arriving individuals, but for those who have the most to lose if they drop a few steps in the arrival order. 5. Competition for high quality territories can also lead to partial migration, in which case birds in good condition are expected to be most likely to remain resident.

Eliminating autocorrelation reduces biological relevance of home range estimates

Abstract: 1. Destructive subsampling or restrictive sampling are often standard procedures to obtain independence of spatial observations in home range analyses. We examined whether home range estimators based upon kernel densities require serial independence of observations, by using a Monte Carlo simulation, antler flies and snapping turtles as models. 2. Home range size, time partitioning and total straight line distances travelled were tested to determine if subsampling improved kernel performance and estimation of home range parameters. 3. The accuracy and precision of home range estimates from the simulated data set improved at shorter time intervals despite the increase in autocorrelation among the observations. 4. Subsampling did not reduce autocorrelation among locational observations of snapping turtles or antler flies, and home range size, time partitioning and total distance travelled were better represented by autocorrelated observations. 5. We found that kernel densities do not require serial independence of observations when estimating home range, and we recommend that researchers maximize the number of observations using constant time intervals to increase the accuracy and precision of their estimates.

The roles of island area per se and habitat diversity in the species-area relationships of four Lesser Antillean faunal groups

Abstract: Summary 1. We analysed the relationships between species richness, island area, and habitat diversity for birds, bats, butterflies, and reptiles and amphibians on 19 islands in the Lesser Antilles. Habitat diversity was quantified by Simpson’s index based on the total areas of five vegetation types on each island. Island area varied over two orders of magnitude (13‐1510km 2 ) and habitat diversity varied between 1 and 3·7 equivalents of equally abundant habitat types. 2. Because the Lesser Antilles consist of an inner arc of high, volcanic islands and an outer arc of low-lying islands formed of uplifted marine sediments, correlations between area and elevation (r 2 =0·32) and between area and habitat diversity (r 2 =0·40) were weak. Habitat diversity was, however, strongly correlated with maximum island elevation (r 2 =0·85). 3. Simple correlations of species richness with island area were significant for all four faunal groups, and simple correlations of species richness with elevation and habitat were significant for all groups except bats. In multiple regressions of species richness on area and habitat diversity together, area was a significant eAect for birds and bats, and habitat diversity was a significant eAect for birds, butterflies, and reptiles and amphibians. 4. These results suggest that the four Lesser Antillean taxonomic groups diAer in their responses to area and habitat diversity. For butterflies and for reptiles and amphibians, the relationship of species richness to area is probably a fortuitous consequence of a relationship between habitat diversity and area. Bird species richness responds independently to both habitat diversity and area, and bat species richness is influenced by area but not by habitat diversity. 5. We suggest that this variation is related to diAerences in several biological traits of the diAerent faunal groups. Strong habitat-diversity eAects are likely in taxa with high degrees of habitat specialization, populations large enough to have a low probability of stochastic extinction, life-cycles that include a resistant resting stage that reduces vulnerability to catastrophic extinction, or a combination of these traits. In contrast, strong area eAects are likely in taxa with weak habitat specialization, low population density, or both. 6. At least in Lesser Antillean birds, it is unlikely that immigration depends on island size. Therefore, the species‐area relationship for birds is probably generated by island-size-dependent extinction. Among the four taxonomic groups we studied, only butterflies are likely to show a ‘rescue eAect’ stemming from frequent between-island movement of individuals, as only butterflies exhibited low levels of endemism and lacked a unique area eAect for species richness.

Structural change in an exploited fish community: a consequence of differential fishing effects on species with contrasting life histories

Abstract: 1. An understanding of the links between life histories and responses to exploitation could provide the basis for predicting shifts in community structure by identifying susceptible species and linking life-history tactics with population dynamics. 2. We examined long-term trends in the abundance of species in the North Sea bottom-dwelling (demersal) fish community. Between 1925 & 1996 changes in species composition led to an increase in mean growth rate, while mean maximum size, age at maturity and size at maturity decreased. The demersal fish community was increasingly heavily fished during this period. 3. Trends in mean life-history characteristics of the community were linked to trends in abundance of component species. An approach based on phylogenetic comparisons was used to examine the differential effects of fishing on individual species with contrasting life histories. 4. Those species that decreased in abundance relative to their nearest relative, matured later at a greater size, grew more slowly towards a greater maximum size and had lower rates of potential population increase. The phylogenetically based analyses demonstrated that trends in community structure could be predicted from the differential responses of related species to fishing. 5. This is the first study to link exploitation responses of an entire community to the life histories of individual species. The results demonstrate that fishing has greater effects on slower growing, larger species with later maturity and lower rates of potential population increase. The comparative approach provides a basis for predicting structural change in other exploited communities.

Food regulates the Serengeti wildebeest: a 40-year record

Abstract: Summary 1. The migratory wildebeest (Connochaetes taurinus (Burchell)) population of Serengeti has been monitored for 40 years (1958–1998). The population increased from 1963 to 1977, stabilized from 1977 to 1993 and declined during drought in 1993–94. These changes provided a significant contrast in densities and the opportunity to determine the life history stages where survival was density dependent and their causes. 2. There was a delay in age of maturity and a decline of both yearling and adult pregnancy rates at higher densities. Changes in yearling pregnancy had minimal effect on number of calves born. Adult pregnancy rate dropped from 95% to 88% 3. The greatest absolute mortality occurred in newborn calves (0–4 months old). The most variable survival was in dry-season calf mortality (5–11 months old). 4. Density dependence was clearly present in adult mortality and this appeared to be the regulating life stage. Density dependence was also found in dry-season calf mortality. The drop in pregnancy rate was only weakly density dependent. 5. The main cause of mortality (75% of cases) was undernutrition. Bone marrow condition showed that more animals died in extremely poor condition as the population increased and predators did not cause these deaths. 6. Predation played only a minor role in limiting the wildebeest population. Predators caught animals in moderate condition during the increase phase and increasingly healthy condition during years of high population density. The main predators were lion [Panthera leo (Linnaeus)] and hyaena (Crocuta crocuta Erxleben). 7. Adult mortality was significantly negatively related to food per capita. This could be viewed either as curvilinear density dependence, or as two phases of mortality. The first phase was during population increase when mortality was inversely density dependent and may have been caused by a constant predation rate. The second was during stability and decline and comprised the additive effects of undernutrition and predation. 8. Rainfall in the dry season was the most important extrinsic determinant of food supply for the wildebeest population but food was related to mortality only when population density was incorporated. 9. At present the wildebeest population appears regulated by natural causes through food supply. Human-caused mortality appears to be a minor factor. However, if the number of wildebeest killed by humans increases as human population increases, then there could be a permanent decline in wildebeest, and a major change in the whole ecosystem, because wildebeest are the keystone species. Continued monitoring is essential for conservation of the Serengeti ecosystem.

Using local–regional richness plots to test for species saturation: pitfalls and potentials

Abstract: 1. Plots of local versus regional species richness are an exciting new tool for testing for species saturation in ecological communities. In this method, the local richness of a community is plotted as a function of its regional richness for different biogeographical regions. A proportional relationship between local and regional richness is interpreted as evidence for an unsaturated community, that is, a community with strong evolutionary limits to local richness. There will be no correlation between local and regional richness in a saturated community, that is, a community whose local species richness is limited largely by ecological processes. 2. Although at least 36 data sets have now been analysed using local–regional richness plots, there has not been much critical evaluation of the method. This paper provides such a critique, focusing on the selection of communities for comparison, the prevalence of pseudoreplication and multiple null models, and the effects of differing region size. 3. Local–regional richness plots are best suited for comparing similar habitats between different regions, not different habitats in a single region. In the latter, taxa effects and species pool effects are confounded. 4. Four very different types of local–regional richness plots have been published. Each type of plot has important underlying assumptions which are often not addressed by ecologists. 5. Of the 36 data sets reviewed in this paper, 13 were spatially pseudoreplicated, and 2 were temporally pseudoreplicated. Furthermore, ecologists differ in their choice of null model, with the result that the same local–regional plot could be interpreted as evidence for saturation by one ecologist, and for lack of saturation by another. 6. Differences in region size can result in pseudosaturation, the appearance of saturation by an unsaturated community. A simple model demonstrates this phenomenon. Other sources of error in estimating the regional species pool are also of concern. 7. In conclusion, local–regional richness plots are a potentially useful tool for distinguishing saturated from unsaturated communities, but should be used cautiously, and in conjunction with other supporting evidence (such as the presence or absence of competitive exclusion, resource limitation, density compensation and the effects of species invasions).

The structure of an aphid–parasitoid community

Abstract: 1 A quantitative parasitoid web was constructed describing the trophic relationships between the community of aphids, parasitoids and secondary parasitoids in an abandoned field in southern England Root aphids were omitted and secondary parasitoids were associated with aphids rather than primary parasitoids All aphids, parasitoids and trophic links were expressed in the same units (m−2) Over a 2-year period, separate webs were constructed for every month that aphids and parasitoids were active in the field 2 Twenty-six species of plants were attacked by 25 species of aphid which were parasitized by 18 species of primary parasitoids The primary parasitoids were attacked by 28 species of secondary parasitoids, of which 18 directly attacked the still living aphid (hyperparasitoids) and 10 attacked the aphid after mummification (mummy parasitoids) The aphids were attended by three species of ants 3 Eleven webs were constructed for the months May to September 1994 and May to October 1995 Aphids and primary parasitoids were most abundant and diverse in June, and secondary parasitoids one month later 4 The ratios of the number of aphid species to the number of species of primary parasitoid and hyperparasitoid were relatively constant across webs, as was the ratio of the number of links involving hyperparasitoids and mummy parasitoids The ratio of the total number of links to the total number of species increased with web size 5 The relative abundance of the species in the different webs was well described by a Dirichlet distribution with a common parameter This implies a gamma distribution of aphid abundances in the field with few common and many rare species 6 Predator (ie parasitoid) overlap graphs were constructed to assess the potential for indirect interactions between aphids Mummy parasitoids were the most important group linking different aphids A quantitative overlap diagram was developed to illustrate the potential strengths of indirect linkages Common aphid species shared few strong, indirect links via primary parasitoids or hyperparasitoids, but could be strongly linked by mummy parasitoids Symmetrical links were uncommon, and rare species were potentially strongly influenced by the presence of common aphids with which they shared parasitoids 7 Ant-attended aphids were attacked by fewer species of primary parasitoids and hyperparasitoids than those species not attended by ants Species of mummy parasitoids attack, on average, approximately twice the number of host species than species of either primary parasitoids or hyperparasitoids

The spatial structure of populations

Abstract: 1. Studies of the spatio-temporal dynamics and structure of populations have identified many categories of population type. However, recognized categories intergrade, making it difficult to assign empirical population systems to single categories. 2. We suggest that most population categories can be arranged along two axes that combine per capita birth (B), death (D), emigration (E) and immigration (I) rates. The ‘Compensation Axis’ describes the source-sink component of population structure, with source populations exporting individuals (B > D, E > I) and sinks and pseudosinks consuming individuals (B < D, E < I). The ‘Mobility Axis’ describes the involvement of a local population in regional (I + E) rather than local (B + D) processes, running from separate populations, through metapopulations, to patchy populations. 3. Each sample area within a spatially structured population system can potentially be assigned to a position along each of these axes, with individual sample areas weighted by local population size. The positions of these sample areas and their relative weightings allow the relative importance of different types of process to be judged. A worked exampled is provided, using the butterfly Hesperia comma. This approach shifts the emphasis from pattern (categories that real population systems do not fit) onto process. 4. In many systems, continuous variation in habitat quality and demographic parameters make clear distinctions between ‘habitat’ and ‘non-habitat’ difficult to sustain. In such cases, we advocate the use of a spatial grid system, with effects of patch size and isolation combined into a single, weighted distance function (neighbourhood). 5. The relative importance of different processes depends on the spatial scale at which the system is observed. This again emphasizes the value of a process-based approach.

Predation, group size and mortality in a cooperative mongoose, Suricata suricatta.

Abstract: 1. In social mammals where group members cooperate to detect predators and raise young, members of small groups commonly show higher mortality or lower breeding success than members of large ones. It is generally assumed that this is because large group size allows individuals to detect or repel predators more effectively but other benefits of group size may also be involved, including reduced costs of raising young and more effective competition for resources with neighbouring groups. 2. To investigate the extent to which predation rate affects survival, we compared mortality rates in two populations of suricates (Suricata suricatta), one living in an area of high predator density (Kalahari Gemsbok Park) and one living in an area of relatively low predator density (neighbouring ranchland). Most aspects of feeding ecology and growth (including time spent feeding, daily weight gain, growth, adult body weight, breeding frequency and neonatal mortality) were similar in the two populations. In contrast, mortality of animals over 3 months old was 1·7 times higher in the Park than on ranchland. 3. Mortality of juveniles between emergence from the natal burrow and 6 months of age was higher in small groups than large ones in the Park but significantly lower in small groups than large ones on ranchland. Adult mortality declined in larger groups in both areas. 4. The tendency for survival to be low in small groups had far-reaching consequences for the risk of group extinction. During a year of low rainfall in the Park, all groups of less than nine animals became extinct and population density declined to around a third of its initial level. We argue that high group extinction rates are to be expected in species where survival declines in small groups and mortality rates are high.

Cats protecting birds: modelling the mesopredator release effect

Abstract: 1. Introduced predators account for a large part of the extinction of endemic insular species, which constitutes a major component of the loss of biodiversity among vertebrates. Eradication of alien predators from these ecosystems is often considered the best solution. 2. In some ecosystems, however, it can generate a greater threat for endemic prey through what is called the ‘mesopredator release’. This process predicts that, once superpredators are suppressed, a burst of mesopredators may follow which leads their shared prey to extinction. 3. This process is studied through a mathematical model describing a three species system (prey–mesopredator–superpredator). Analysis of the model, with and without control of meso- and superpredators, shows that this process does indeed exist and can drive shared prey to rapid extinction. 4. This work emphasizes that, although counter-intuitive, eradication of introduced superpredators, such as feral domestic cats, is not always the best solution to protect endemic prey when introduced mesopredators, such as rats, are also present.

The foraging performance of great and blue tits (Parus major and P. caeruleus) in relation to caterpillar development, and its consequences for nestling growth and fledging weight

Abstract: 1. We analysed the effect of prey density and size on the foraging performance of great and blue tit (Parus major L., P. caeruleus L.) parents, and its consequences for the growth and fledging weight of nestlings. Because fledging weight is a determinant of subsequent survival and therefore fitness, foraging decisions of the parents play a key role in the reproductive system of tits. The analysis quantifies (i) the rate at which energy is delivered to the nestlings in relation to prey size and abundance, and (ii) the growth rates of nestlings and the resulting fledging weight in relation to the rate of food delivery by the parents. 2. The searching time per prey item increased exponentially with decreasing prey biomass. During the peak abundance of caterpillars, the average searching time per item was 2·5–3 min instead of 5–6 min before and after the peak. Searching time was significantly reduced when the birds returned to the foraging site where the preceding prey was found. This accords with the clumped distribution of caterpillars within the canopy. 3. The foraging performance (in mg caterpillars per min) was maximal when caterpillars were both abundant and large, i.e. shortly before they left the trees for pupation. The high feeding frequency and the large prey then caused a peak energy flow rate to the nestlings of 4–5 times the rate before or after the caterpillar peak. This suggests that the foraging success and rate of food delivery by tit parents was primarily determined by the abundance and size of prey. 4. The growth rate of nestlings, as well as their fledging weight was correlated with the rate of food delivery. Low feeding performance of the parents resulted therefore in poor relative growth rates of only 0·3–0·6 of the rate achieved under optimal conditions and, as a consequence, in a low fledging weight. This indicates that tit parents have restricted options to adjust prey delivery rates according to the requirements of the brood. 5. The results give insight into the chain of causal mechanisms through which an environmental factor (availability of food) has a strong and immediate effect on fitness (growth, fledging weight and, thus, survival of the nestlings). The importance of caterpillar size for foraging success and prey delivery rates of parent tits makes clear, why the phase of best foraging conditions is shorter than the period during which caterpillars are available. The relationships we quantified give a proximate explanation for the great effects that temperature and caterpillar growth have on the between-year variation in selection intensity for laying date observed in other studies.

Food availability and the nocturnal vs. diurnal foraging trade‐off in juvenile salmon

Abstract: 1. Much attention has been devoted to explaining the spatial distribution of foraging animals, but rather little to their temporal distribution (i.e. whether they are diurnal, nocturnal or crepuscular). Many animals face predictable diel cycles of food availability or predation risk, and so the approach of measuring the relative ratio of mortality risk to food gained (the ‘minimize μ/f’ rule) can be applied equally as well to different time periods of the day as to alternative food patches or habitats. 2. This method is used here to investigate the diel activity patterns of juvenile Atlantic salmon, which have previously been shown to become increasingly biased towards nocturnal activity in winter, hiding for much of the day in streambed refuges. Calculations based on published data show that nocturnal foraging in winter is far safer per unit of food obtained than is diurnal, despite greatly reduced food capture efficiency at night-time light levels. 3. Using an automated activity monitoring system based on passive integrated transponder (PIT) tags, this study shows that winter diel activity patterns in salmon are dependent on food availability. A change in food density led to a parallel change in time spent in the refuge, but (as predicted by the μ/f rule) the effect was greatest at the time of day with the least favourable ratio of predation cost to feeding benefit. Thus an experimental increase in food availability led to a 16% reduction in time spent in nocturnal foraging but a 98% reduction in time spent foraging by day, with fish spending only 0·6% of the daylight hours out of the refuge at the highest food density. 4. However, brief daytime foraging bouts had a major impact on growth rates (presumably because feeding efficiency was much greater in daylight), especially when food was scarce. Daytime feeding was thus profitable in terms of rapid food acquisition but normally suboptimal in terms of risk of predation. 5. Daily activity patterns are therefore suggested to be the result of a complex trade-off between growth and survival, which takes account of diel fluctuations in food availability, food capture efficiency and predation risk; individual variation in the extent of diurnal feeding in salmon may result from state-dependent differences in the benefits of rapid feeding and growth.

Dynamics of a harvested moose population in a variable environment

Abstract: 1 Population size, calves per female, female mean age and adult sex ratio of a moose (Alces alces) population in Vefsn, northern Norway were reconstructed from 1967 to 1993 using cohort analysis and catch-at-age data from 96% (6752) of all individuals harvested 2 The dynamics of the population were influenced mainly by density-dependent harvesting, stochastic variation in climate and intrinsic variation in the age-structure of the female segment of the population 3 A time delay in the assignment of hunting permits in relation to population size increased fluctuations in population size 4 Selective harvesting of calves and yearlings increased the mean age of adult females in the population, and, because fecundity in moose is strongly age-specific, the number of calves per female concordantly increased However, after years with high recruitment, the adult mean age decreased as large cohorts entered the adult age-groups This age-structure effect generated cycles in the rate of recruitment to the population and fluctuations introduced time-lags in the population dynamics 5 An inverse relationship between recruitment rate and population density, mediated by a density-dependent decrease in female body condition, could potentially have constituted a regulatory mechanism in the dynamics of the population, but this effect was counteracted by a density-dependent increase in the mean age of adult females 6 Stochastic variation in winter snow depth and summer temperature had delayed effects on recruitment rate and in turn population growth rate, apparently through effects on female body condition before conception

Immunocompetence and condition-dependent sexual advertisement in male house sparrows (Passer domesticus)

Abstract: Summary1. We tested the condition-dependent hypothesis of sexual advertisement in housesparrows (Passer domesticus). Male house sparrows have a bib of black featherswhich serves as both a badge of social status and as a cue for female choice. Wemanipulated environmental conditions during the premoult period of juvenilehouse sparrows kept in outdoor aviaries. Birds were assigned to two treatments dif-fering in the amount of dietary proteins, which are known to a•ect the expressionof immune response in birds. We tested whether birds in the protein-rich grouphad better immune responses and developed larger bibs than birds reared on a pro-tein-poor diet. We also checked whether immune response was a predictor of survi-val and parasite resistance.2. Individuals with higher cellular immune response at capture had greater prob-ability to survive during the 3months of the experiment, and they had a higherprobability to recover from infection with Haemoproteus sp. (a blood parasite).Conversely, birds with high immunoglobulin concentrations at capture had ahigher probability of mortality.3. Birds on the protein-rich diet had a higher cellular immune response comparedto birds in the protein-poor treatment. Humoral immune response showed theopposite pattern, being higher for birds in the protein-poor treatment. We did notfind any e•ect of food quality on the development of the badge, assessed as the sizeof the trait and its colour properties.4. In conclusion, our results support the view that immune defences are importantfor survival and parasite resistance in natural populations, and that they might becostly to produce. On the other hand, we did not find support for the condition-dependent hypothesis of sexual advertisement, suggesting that the badge may notbe a costly trait to produce. However, badge size could reflect other aspects of con-dition. The kind of pigments involved in colour signals may be the key factor deter-mining the production costs of such traits.Key-words: immune response, immunoglobulins, parasite resistance, sexual signals,survival, T-cell response.Journal of Animal Ecology (1999) 68, 1225–1234Introduction

Juvenile migration in brown trout: a consequence of energetic state

Abstract: 1. We explored the mechanisms determining age and size at juvenile migration in brown trout Salmo trutta L. A 133Cs tracer methodology was used to estimate food consumption of juvenile brown trout in a Norwegian stream, and the energy budgets of early migrants and stream residents were compared. 2. Fast-growing brown trout migrated to the lake earlier and at a smaller body size than slower-growing individuals. The 2+ migrants were significantly larger than those that remained 1 or more years longer in the stream. The 3+ migrants were significantly larger than the 2+ migrants. Some fast-growing males matured in the stream, whereas all females left the stream before maturing sexually. 3. The food consumption and the energy budgets for 2+ migrants were more than four times higher than those of the resident 2+ fish. Total energy allocated to growth was also higher among migrants, and the total metabolic costs were five times higher among migrants than among resident fish. 4. The proportional energy allocation to growth among the 2+ migrants was much lower (about half) than that of those remaining longer in the stream. The reduction in the proportion of energy available for growth from age 1+ to 2+ was larger among migrants (88%) than among resident fish (68%). Reduction in the proportion of energy available for growth is a probable explanation for why migrations are initiated at age 2. 5. Our study supports the hypothesis that fast-growing individuals shift their niche earlier and at a smaller body size than slower-growing individuals because they maintain higher metabolic rates and are energetically constrained at a younger age by limited food resources than slow growers.

Estimating the contributions of population density and climatic fluctuations to interannual variation in survival of Soay sheep

Abstract: Summary 1. The relative importance of density-dependent and -independent factors on interannual variation in over-winter survival was investigated in the fluctuating population of Soay sheep on St Kilda, Scotland, over the period 1985–96. 2. Population density had a negative effect on survival in lambs and adult males while adult female survival showed no evidence of density dependence over the observed range of population densities. 3. Climatic fluctuations associated with the winter North Atlantic oscillation index (NAO) also affected survival, which decreased in winters that were relatively warm, wet and windy. The effect was most pronounced in lambs. 4. Survival was modelled using logistic regression analysis with and without year fitted as a random effect. The former incorporated stochastic year to year variation in survival. Results from the two modelling approaches were similar in terms of the regression coefficients estimated. However, the standard errors of the year-dependent covariates, population size and NAO, were underestimated when the random year effect was ignored, leading to incorrect inferences about the relative significance of terms being made. 5. Using both modelling approaches, density dependence was found to have a greater influence on survival than the effect of NAO in lambs and adult males, whereas in adult females NAO was the more important. 6. Once random between-year effects were taken into account, the individually varying terms such as body weight and faecal egg count were the most significant factors explaining differences in survival.

Predator–prey relationships: arctic foxes and lemmings

Abstract: 1. The number of breeding dens and litter sizes of arctic foxes Alopex lagopus were recorded and the diet of the foxes was analysed during a ship-based expedition to 17 sites along the Siberian nor ...

Habitat selection as a major resource partitioning mechanism between the two sympatric sibling bat species Myotis myotis and Myotis blythii

Abstract: 1. Ecomorphological studies have described local bat communities as densely-packed species assemblages arising from non-deterministic processes. Together with the observation that insectivorous bats readily exploit patchy, often unlimited trophic resources, this has even led to the claim that partitioning of niche space may be absent. However, the paucity of data on resource exploitation per se among bat guilds rarely allows these assertions to be verified. In particular, the mechanisms allowing the co-existence of similar species have proved difficult to determine. 2. As a subset of an insectivorous guild, the vespertilionid bats Myotis myotis and M. blythii offer an opportunity to examine this question. Genetically closely-related, they are morphologically almost identical. Yet, as established by faecal analysis, they exploit distinct trophic niches, preying upon ground- and grass-dwelling prey taxa, respectively. The distinct habitat requirements of their basic prey suggest that ecological segregation may stem primarily from a differential allocation of foraging space. 3. The present study tests the hypothesis that sympatric M. myotis and M. blythii segregate spatially to an extent which prevents competitive interference. I performed radio-tracking on Swiss sympatric populations. Using multivariate analyses and randomized contingency table procedures, I looked for (i) habitat overlap and overall inter-specific differences in habitat choice, and (ii) the habitat preferences exhibited by individuals when foraging within their own feeding areas. 4. Primary foraging habitats were largely species-specific. Not only were they spatially segregated, but they also differed structurally. All habitats selected by M. myotis offered a high accessibility to ground-dwelling prey (freshly-cut meadows, mown grass in intensively cultivated orchards, forests without undergrowth), whereas grassland predominated in all habitats of M. blythii (steppe, unmown meadows, pastures). This corroborates the predictions drawn from dietary niches. 5. This study shows that refined mechanisms of resource partitioning, not predictable by the study of morphological characters or echolocation alone, may still account for the organization of parts of insectivorous bat guilds. It also supports the view that habitat selection may prove to be a major mode of resource allocation amongst similar insectivorous bats, particularly for species exploiting limited food supplies.

Flexible niche partitioning via a foraging mode shift: a proposed mechanism for coexistence in stream‐dwelling charrs

Abstract: Summary 1. Foraging behaviour, diet and interference competition were examined for two morphologically similar charrs, Salvelinus malma (Dolly Varden) and S. leucomaenis (white-spotted charr), under varying food resource conditions over four summers in a Japanese mountain stream. Data were used to test predictions from a mechanistic model of resource partitioning developed from an earlier field experiment. 2. The charrs adopted one of two distinct foraging modes, ambushing drifting invertebrates from relatively fixed foraging positions or actively searching for benthic prey over large areas. The proportion of benthos foragers increased markedly when drifting prey declined, and was much greater in S. malma than S. leucomaenis when drift was lean, upholding predictions made from our earlier experiment. 3. For drift foragers of both species, frequency of foraging attempts decreased as drift rate declined, and aggressive encounters increased. In contrast, for benthos foragers of both species the frequency of foraging attempts was essentially constant across the range of benthos biomass measured, and aggressive encounters remained low. 4. Salvelinus leucomaenis ate larger drifting prey than S. malma, even though the former charr were smaller. In contrast, S. malma foraged on benthic prey at a higher rate than S. leucomaenis, although there was no difference in prey mass. Thus, the optimal point to shift to benthic foraging is at a higher drift threshold for S. malma than S. leucomaenis, most probably due to differences in jaw morphology. Moreover, because dominance for favourable drift foraging positions was based on size alone, S. malma shifted to benthos foraging at a larger size than S. leucomaenis, as predicted by a simple model. 5. Charr consumed distinct prey types according to their foraging mode. Drift foragers primarily ate terrestrial invertebrates, whereas benthos foragers ate mainly chironomid larvae. Consequently, diet overlap was high when drift was abundant and both species were drift foragers, but declined as drift declined and S. malma shifted to benthos foraging. Therefore, species-specific differences in foraging mode shifts across the resource gradient explained the flexible resource partitioning we observed, and probably account for the coexistence of these congeneric charrs in zones of sympatry in northern Japan.

Viruses of the Serengeti: Patterns of infection and mortality in African lions

Abstract: Summary 1. We present data on the temporal dynamics of six viruses that infect lions (Panthera leo) in the Serengeti National Park and Ngorongoro Crater, Tanzania. These populations have been studied continuously for the past 30 years, and previous research has documented their seroprevalence for feline herpesvirus, feline immunodeficiency virus (FIV), feline calicivirus, feline parvovirus, feline coronavirus and canine distemper virus (CDV). A seventh virus, feline leukaemia virus (FeLV), was absent from these animals. 2. Comprehensive analysis reveals that feline herpesvirus and FIV were consistently prevalent at high levels, indicating that they were endemic in the host populations. Feline calici-, parvo- and coronavirus, and CDV repeatedly showed a pattern of seroprevalence that was indicative of discrete disease epidemics: a brief period of high exposure for each virus was followed by declining seroprevalence. 3. The timing of viral invasion suggests that different epidemic viruses are associated with different minimum threshold densities of susceptible hosts. Furthermore, the proportion of susceptibles that became infected during disease outbreaks was positively correlated with the number of susceptible hosts at the beginning of each outbreak. 4. Examination of the relationship between disease outbreaks and host fitness suggest that these viruses do not affect birth and death rates in lions, with the exception of the 1994 outbreak of canine distemper virus. Although the endemic viruses (FHV and FIV) were too prevalent to measure precise health effects, there was no evidence that FIV infection reduced host longevity.

Habitat duration, predation risk and phenotypic plasticity in common frog (Rana temporaria) tadpoles

Abstract: Summary 1. Common frogs (Rana temporaria) breed readily in small pools and thus expose their offspring to catastrophic mortality by desiccation. Amphibian larvae exhibit considerable phenotypic plasticity in metamorphic traits, and some species respond to environmental uncertainty by metamorphosing earlier and at smaller size. In a factorial laboratory experiment, we studied whether common frog tadpoles possess this ability. 2. We also studied the interaction between pool drying and predation risk, because in a previous study the presence of a predatory dragonfly larva delayed metamorphosis of the tadpoles. 3. We gradually removed water from half the experimental containers, while in the other half the water volume was kept constant. In the laboratory it was possible to remove the effect of increased water temperature in the decreasing volume treatment by using fluorescent lights. Tadpoles responded to decreasing volume by metamorphosing earlier and at smaller size. A greater proportion of the tadpoles metamorphosed in the decreasing volume treatment. 4. Tadpoles were less active at decreasing water level and there were significant positive correlations between activity late in the experiment and metamorphic size. This suggests that the metamorphic response to habitat drying is behaviourally mediated. 6. Early in the experiment, tadpoles developed slower in the presence of predators. At metamorphosis presence of a dragonfly larva had no effect in the whole data set, but when the constant volume treatment was analysed separately, larval period was longer in the presence of a predator. 7. Our results indicate that common frog larvae are able to respond to pond-drying adaptively by speeding up their development, and that temperature advantage is not needed to induce this adaptive plasticity. Furthermore, pond-drying seems to be a more important determinant of development rate than the presence of odonate predators.

Consequences of maternal size for reproductive expenditure and pupping success of grey seals at North Rona, Scotland

Abstract: 1 The reproductive performance of individually marked mothers aged between 4 and 36 years breeding at the established grey seal colony of North Rona, Scotland was studied Natality rate was between 0·805 and 0·975 for these females during 1979–95 and 57% of females produced 74% of the pups born Mothers pupped successfully on N Rona after absences of up to 5 years 2 The average maternal postpartum mass (MPPM) of mothers was 190 ± 23 (SD) kg, larger than had been recorded from this colony previously Although annual mean MPPM increased during the study, there were increases and decreases in individuals’ MPPM between years 3 Pup mass at birth and pup growth rate were related to MPPM and date of parturition No evidence of differential postpartum expenditure in the sexes was found Relative pup birth mass decreased with MPPM but relative pup weaning mass remained constant over the range of MPPM 4 Maternal mass expenditure during lactation averaged 39% of MPPM, and the consequences for MPPM in the year following either high or low relative expenditure were inversely related to relative expenditure in the first year However, mothers increased their mass after skipping breeding in a year 5 General Linear Models and REML analyses indicated that expenditures were significantly different between mothers when other variables and factors had been taken into account In general, maternal expenditure was greater for animals of larger masses and duration of lactation, but corrected maternal expenditure of longer individuals was less than expected Pup mass at weaning was influenced by mother's identity and year and there was evidence that individual mothers which were longer for their weight raised smaller pups 6 The life history consequences of reproductive expenditure in any year appeared in subsequent breeding patterns The cost of breeding for larger animals and larger expenditures was indicated by lower pupping success rates in years following births and skipped breeding years

The taxonomic distinctness of coastal bottom‐dwelling fish communities of the North‐east Atlantic

Abstract: 1. New techniques for identifying the average taxonomic range of species assemblages were applied to an extensive dataset of bottom-dwelling fish in the coastal waters of NW Europe. These taxonomic distinctness indices provided much greater resolution than traditional diversity indices as they incorporated information on taxonomic relationships into an index which measures species dominance. Unlike standard measures of species richness and diversity, the mean value of these statistics is independent of sampling effort, and this allows objective comparisons to be made between samples from studies where sampling effort is not standardized. 2. A reduction in the average taxonomic range between the fauna of western waters of the UK and that of the southern North Sea was consistent with the general decline in species richness observed between these regions, and suggests that these two factors may be spatially positively correlated. Indices calculated for individual samples of fish on a local scale, however, did not all fit this trend. 3. Much of the variability in taxonomic diversity within the coastal waters of NW Europe was caused by the variable geographical distribution of the elasmobranchs. Of all the families which comprise the fish communities, this group has life-history characteristics which make it most susceptible to impact by commercial trawl fisheries. 4. The use of taxonomic distinctness measures provided additional insights, of relevance to biodiversity assessment, suggesting that they might usefully be applied to other aquatic and terrestrial fauna.

The use of conspecific reproductive success for breeding habitat selection in a non-colonial, hole-nesting species, the collared flycatcher

Abstract: Summary 1. Breeding habitat quality strongly affects fitness. Therefore, individuals are likely to select their breeding habitat after gathering information on quality of potential breeding patches. In the study reported in this paper, we investigated whether local reproductive success of conspecifics in a patch (patch reproductive success, PRS) could be used to assess habitat quality and make dispersal decisions in a non colonial, hole-nesting, passerine bird, the collared flycatcher (Ficedula albicollis). 2. Assumptions for such a breeding habitat selection mechanism were met: the habitat patches were of different quality, as measured by PRS, with relative quality varying between years, and PRS was autocorrelated in time. 3. As in many other species, breeding dispersal was related in both sexes to individual reproductive success before dispersal: unsuccessful individuals were more likely to disperse than successful ones. 4. Additively, PRS influenced both breeding and natal dispersal. This effect depended on sex. Dispersal was negatively related to PRS in females, and positively in males with low competitive ability (juvenile and unsuccessful adult males). 5. Breeding dispersal was positively related to flycatcher density in males, but negatively in females. Moreover, when included in the analyses, laying date of the previous breeding attempt (which should be correlated with competitive ability) replaced the effect of PRS in adult males. 6. The observed patterns can be explained by PRS (or another variable correlated with PRS) being used to assess different components of habitat quality in each sex, in relation to intraspecific competition pressure. We suggest that males of low competitive ability may either use PRS to assess the level of expected intraspecific competition the following year, and choose to disperse from high PRS woodlands, or be forced to disperse when PRS is high because of strong competition. Females might use PRS to assess the expected reproductive output if breeding in the patch the following year.

Effects of parental quality and effort on the reproduction of common terns

Abstract: 1. Age-related patterns of reproduction in birds are often interpreted in terms of changing reproductive value with age: as residual reproductive value decreases with age birds have to increase their effort at younger age. In contrast, the ‘state-based’ approach to life history postulates that not age per se determines reproductive decisions, but the ‘state’, reflecting condition or quality that may differ considerably between individuals of the same age. In this respect, we investigated the stability of body conditon, as well as individual quality differences in common terns Sterna hirundo and outline the role of parental quality and parental effort in reproductive performance. For lifetime identification we marked terns with passive transponders. Quality and effort were derived from body condition, which we monitored during the whole breeding season and every year for a period of up to 5 years. 2. Parameters of body condition and reproduction varied significantly between individuals, whereas intra-individual variation over years was very low. Hence, body condition is very stable and differs considerable between birds. 3. Among the factors number of chicks, year and parental body condition, the latter was the only significant factor influencing chick growth rates. Furthermore, high growth rates of chicks were correlated with good female condition, and breeding success was positively correlated with body condition of both parents during incubation and chick-rearing. 4. Parental effort increased significantly with increasing breeding success. The amount of expended effort depended on condition during incubation: mass loss was correlated with incubation mass, but not with mass during chick-rearing. 5. Age was not an important factor in terms of condition or quality and also in terms of reproduction. The age independent condition reflects bird quality, enabling high quality birds to invest high parental effort resulting in high reproductive success over years, yet without negative consequences on future reproduction. 6. We conclude that in the common tern condition is a stable and age-independent individual feature with substantial effects on reproduction. Hence, our data strongly support the ‘state-based’ approach to life history. The high constancy in condition within and great differences between birds may also result in differences in lifetime reproductive success and fitness.

Numerical and functional responses in generalist predators: hen harriers and peregrines on Scottish grouse moors

Abstract: Summary 1. The functional and numerical responses of two generalist raptors (hen harrier and peregrine) were studied on one moor for 6 years and on five other moors for 5 years. 2. Neither species showed numerical responses to grouse abundance. Harrier densities were highest in areas and years where their small prey (meadow pipits and small mammals) were most abundant. Peregrine densities were highest on southern study moors, probably in association with high abundance of racing pigeons. 3. For harriers preying on grouse chicks, the available data fitted a sigmoidal or type III functional response. Peregrines showed a type II response to adult grouse densities around eyries. 4. The proportion of grouse chicks taken by harriers was estimated to have been highest at densities of 67 chicks km–2 (equivalent to a mean of about 12 broods km–2). The proportion of adult grouse taken by peregrines appeared to be inversely density dependent, such that an increasing proportion of grouse was taken at grouse densities below 20 km–2. 5. In the absence of persecution, the impact of harriers on grouse populations is most likely to be greatest on moors where alternative prey and thus harriers are abundant. The question of whether harriers may dampen grouse population cycles at low grouse density is discussed.

Delayed maturation in female bank voles: optimal decision or social constraint?

Abstract: 1. Density, maturation and survival of female bank vole (Clethrionomys glareolus) in the northern taiga of Finnish Lapland were studied using long-term capture–mark–recapture data from two large grids, one food-addition grid and one control grid, in 1982–94. 2. The density on the food grid was consistently higher than the density on the control grid. 3. Females born early in the breeding season usually matured, except at very high densities. Those born later in the summer season commonly delayed maturation to the following spring. 4. Winter survival of sub-adult (having delayed maturation) females was significantly higher than survival of adult (breeding) females. However, empirical values of sub-adult and adult survival, as well as difference between them, were not consistent with survival values assumed in theoretical models on optimal deferred breeding. 5. There was a density-dependent relationship between the maturation rate of young voles and the density of already established breeding females (both bank voles and all Clethrionomys together; C. rutilus and C. rufocanus occasionally occurred on the study grids). This density dependence was different for the two grids (weaker on the food-addition grid). 6. These findings are discussed within an evolutionary context: we have, on the basis of these findings, no evidence suggesting that the observed delayed maturation represents an evolutionary optimal strategy. Rather, there is evidence suggesting that the delay is due to social constraints.

Senescence in a short-lived migratory bird: age-dependent morphology, migration, reproduction and parasitism

Abstract: 1. Senescence reflects age-dependent changes in residual reproductive value. Annual survival rates of the barn swallow Hirundo rustica L. increased from 1- to 2-year-old individuals, but decreased among 5 years old or older individuals. Estimates of age-dependent reproductive value showed a similar pattern. 2. Longitudinal data from two long-term population studies were used to test whether a number of different measures of performance (condition-dependent morphological traits, migratory performance, reproductive success, intensity of parasitism) changed among individuals when reaching old age. 3. The length of the outermost tail feathers (a secondary sexual character) decreased among old individuals, while two measures of individual developmental instability increased with age. Migratory performance decreased in old barn swallows as reflected by a delay in spring arrival at the breeding grounds. Reproductive performance measured as seasonal reproductive success decreased with age. The intensity of infestations with an haematophagous mite and a mallophagous ectoparasite increased among old barn swallows. 4. These results suggest that the condition-dependent secondary sexual character, developmental stability, and measures of migratory and reproductive performance deteriorated, and the frequency of parasitism increased among old individuals. Ageing was thus associated with a general deterioration of performance.

Repeated selection of morphometric traits in the Soay sheep on St Kilda

Abstract: 1. Long-term studies allow the outcomes of repeated selection events to be monitored. Here, we investigate phenotypic selection in successive winter mortality events in the Soay sheep of St Kilda, Scotland, between 1985 and 1996. Selection of three quantitative morphometric traits, body weight, hindleg length and incisor arcade breadth, was investigated in different sectors of the population. 2. Evidence from fitness differentials of positive directional selection for large size was repeatedly found in lambs and adult females. Selection in the opposing direction was only found in one year in lambs. 3. Selection gradients showed that in most years when significant selection occurred, body weight was the focus of direct selection, whereas selection of hindleg length and incisor breadth was indirect, arising from their correlation with body weight. 4. Selection was strongest in years of low over-winter survival and almost absent in years when survival was high. Intensity of selection was greatest in lambs, emphasizing the differences in selection pressure experienced by different sectors of the population, in addition to the temporal variation in selection pressure due to population density and environmental conditions. 5. Despite repeated positive selection of body weight, no evidence of a change in the population mean was found over the course of the study.