Showing papers in "Journal of Animal Ecology in 2002"

Foraging ranges of solitary bees

Abstract: Summary 1 Habitat requirements of solitary bees include nesting sites, food resources and nesting material. We used translocation experiments to establish foraging distances and measured foraging trip duration to analyse how solitary bees cope with the distance between nesting sites and suitable food plants in different habitat types. 2 Maximum foraging distance between nesting site and food patch was 150–600 m for the 16 bee species examined. Foraging distance was correlated positively with body length. Mean foraging trip duration, measured for seven bee species, ranged from 6 to 28 min and was also correlated with body length. In a study of the polylectic species Osmia rufa , we found a significant decrease in foraging trip duration with increasing number of plant species. Logistic regressions showed that the oligolectic Megachile lapponica nested in trap nests with a probability of 50% if the distance between trap nest and food patch was less than 250 m. The oligolectic Chelostoma rapunculi utilized trap nests when the distance to the nearest food patch was less than 300 m. 3 These experiments showed that solitary bees have a rather small foraging range so local habitat structure appears to be of more importance than large-scale landscape structure. All requirements for sustaining viable populations must be within this range. Therefore, it is necessary to maintain and restore a dense network of habitat patches in landscapes to ensure long-term sustainability of wild bee diversity and their ecological function as pollinators.

Variation in survivorship of a migratory songbird throughout its annual cycle

Abstract: Summary 1 Demographic data from both breeding and non-breeding periods are needed to manage populations of migratory birds, many of which are declining in abundance and are of conservation concern. Although habitat associations, and to a lesser extent, reproductive biology, are known for many migratory species, few studies have measured survival rates of these birds at different parts of their annual cycle. 2 Cormack–Jolly–Seber models and Akaike’s information criterion model selection were used to investigate seasonal variation in survival of a Nearctic – Neotropical migrant songbird, the black-throated blue warbler, Dendroica caerulescens. Seasonal and annual survival were estimated from resightings of colour-ringed individuals on breeding grounds in New Hampshire, USA from 1986 to 2000 and on winter quarters in Jamaica, West Indies from 1986 to 1999. Warblers were studied each year during the May–August breeding period in New Hampshire and during the October–March overwinter period in Jamaica. 3 In New Hampshire, males had higher annual survival (0·51 ± 0·03) and recapture probabilities (0·93 ± 0·03) than did females (survival: 0·40 ± 0·04; recapture: 0·87 ± 0·06). In Jamaica, annual survival (0·43 ± 0·03) and recapture (0·95 ± 0·04) probabilities did not differ between sexes. Annual survival and recapture probabilities of young birds (i.e. yearlings in New Hampshire and hatch-year birds in Jamaica) did not differ from adults, indicating that from the time hatch-year individuals acquire territories on winter quarters in mid-October, they survive as well as adults within the same habitat. 4 Monthly survival probabilities during the summer (May–August) and winter (October–March) stationary periods were high: 1·0 for males in New Hampshire, and 0·99 ± 0·01 for males in Jamaica and for females in both locations. 5 These annual and seasonal survival estimates were used to calculate warbler survival for the migratory periods. Monthly survival probability during migration ranged from 0·77 to 0·81 ± 0·02. Thus, apparent mortality rates were at least 15 times higher during migration compared to that in the stationary periods, and more than 85% of apparent annual mortality of D. caerulescens occurred during migration. 6 Additional data from multiple species, especially measures of habitat-specific demography and dispersal, will improve our understanding of the relative impacts of the breeding, migratory, and winter periods on population dynamics of migratory birds and thus enhance future conservation efforts.

On the misuse of residuals in ecology: regression of residuals vs. multiple regression

Abstract: Summary 1. Residuals from linear regressions are used frequently in statistical analysis, often for the purpose of controlling for unwanted effects in multivariable datasets. This paper criticizes the practice, building upon recent critiques. 2. Regression of residuals is often used as an alternative to multiple regression, often with the aim of controlling for confounding variables. When correlations exist between independent variables, as is generally the case with ecological datasets, this procedure leads to biased parameter estimates. Standard multiple regression, by contrast, yields unbiased parameter estimates. 3. In multiple regression parameters are estimated controlling for the effects of the other variables in the model, and thus multiple regression achieves what residual regression claims to do. 4. Several measures of correlation exist that differ in the way that variance is partitioned among independent variables. These can be estimated multiply, or sequentially if reasons exist for estimating effects of variables in a hierarchical manner.

Body‐size determinants of niche overlap and intraguild predation within a complex food web

Abstract: Summary 1. Body-size may be an important feature of the structure of food webs. Detailed food web data are however scarce, particularly those including ontogenetic dietary shifts within species. We examined the predator guild in a well characterized food web, that of Broadstone Stream (UK), to assess the importance of body-size within and among species in relation to intraguild predation and niche overlap. 2. In agreement with recent food web theory, mutual predation and cannibalism were frequent and occurred in many pairwise permutations. This intraguild predation was strongly asymmetric, being determined by relative body-size within and among species, and seasonal ‘ontogenetic reversals’ in trophic status arose when generations overlapped. 3. Predator size determined dietary overlap, with ontogenetic shifts often outweighing taxonomic differences. Small predators had the narrowest diets, regardless of species, and were limited to feeding on a restricted subset of the total prey size-spectrum. Niche overlap decreased as pairwise differences in body-size increased among and within species. Overlap in diet also tracked seasonal changes in resource availability, being highest in summer, when prey were abundant and small, and declining progressively over time, as prey became scarcer and/or larger. The small predators also became more detritivorous as prey abundance declined and the larger prey species attained size-refugia. 4. The body-size constraints driving feeding relationships within the predator guild, in terms of both resource partitioning and intraguild predation, lend support to recent niche models of food web structure (Warren 1996; Williams & Martinez 2000). The highly interconnected food web of Broadstone Stream appeared to be structured by relatively simple rules, with seasonal and ontogenetic shifts in the size-spectrum accounting for most of the changes in predator diet and trophic position. Encounter rate in time (prey and predator mobility) and space (microhabitat use) and foraging mode also influenced prey vulnerability and niche overlap, but were secondary to the effect of body-size.

Movement parameters of ungulates and scale‐specific responses to the environment

Abstract: Summary 1. Most studies of animal movements and habitat selection do not recognize empirically that different components of the environment are important to animals at different scales. Often, availability of habitats is defined at one or more arbitrary spatio-temporal scales, but use of those habitats is constrained to one scale. Identification of scalar movement is the first step in developing models to explain why animals select or move to certain parts of their range. We used a non-linear curve-fitting model of movement rates to identify discontinuities in the scales of movement by woodland caribou Rangifer tarandus caribou collared with global positioning system (GPS) collars. 2. We differentiated intrapatch from interpatch movements, but were unable to distinguish interpatch from migratory-type movements for most combinations of individual caribou by season. Model fit was stronger for winter than summer movements. We suggest that increased patch heterogeneity during the winter resulted in interseason variation in movements and corresponding model fit. 3. Responses by caribou to the environment were scale-dependent. When we applied logistic regressions, land-cover type, energetic costs of movement, and predation risk differentiated the two scales of movement. Intrapatch movements had a lower cost of movement, were associated with cover types where foraging behaviours probably occurred, and were closer to areas of higher predator risk than interpatch movements. 4. Application of the non-linear model will aid in developing mechanism-based approaches to studying resource selection and animal behaviour.

Compartmentalization in plant–insect flower visitor webs

Abstract: 1. Interactions between entomophilous flowering plants and their insect visitors were recorded at two mesotrophic grassland Communities in Norfolk, and a diagrammatic quantitative web produced for each community. 2. The systems were analysed for compartmentalization using the method of Raffaelli & Hall (1992). based on trophic similarity between pairs of species. Good evidence was found for compartmentalization at both sites. 3. Ordination of the data was used to suggest how the species fall into compartments. The likelihood that groups of plants and insects implied by this method represent real compartments in the web was assessed quantitatively, using trophic similarity indices, and qualitatively. by consideration of the species involved. 4. The compartments reflected classic pollination syndromes to some extent, dividing the insect fauna into a group of butterflies and bees, and a group of flies, at both sites. The compartmentalization was also affected by phenology. 5. Dominant interactions fell within compartments in the web, as might be expected in mutualistic systems.

A testable linear model for diversity trends in estuaries

Abstract: Summary 1. In 1934, Adolf Remane constructed a diagram to describe changes in the number of species along a full salinity gradient within the Baltic. Despite fundamental differences in tidal regimes, the Baltic model has been applied directly to estuaries, becoming subsequently the textbook model for estuarine diversity trends. 2. Despite its ubiquity, the Remane diagram has many inconsistent features, making it unsuitable as a quantitative tool for comparing diversity trends between estuaries, including poor definition of x -axis (salinity), y -axis (number of species) and variations in sample location (subtidal/intertidal) than can greatly influence the resulting diversity/ salinity relationship. Consequently, diversity trends within and between estuaries remain to be tested robustly. 3. The major environmental factor influencing the distribution of organisms in estuaries is salinity variation, rather than absolute salinity tolerance as in the Baltic. As salinity range demonstrates a quadratic relationship with mean salinity, an alternative linear model is therefore suggested utilizing mean salinity range at any one point in the estuary ( x -axis) and mean α -diversity of macroinvertebrates ( y- axis) obtained from subtidal samples to allow comparison with river systems and to minimize salinity variability between interstitial and overlying water. 4. The model was tested on an extensive subtidal data set from the Thames estuary (salinity 0‐35), significant negative linear relationships between salinity range and α diversity being apparent for annual and seasonal data sets. Significant models were also possible for both ‘marine’ and ‘freshwater’ halves of the estuary and for meiofauna α diversity. 5. The linear model allows formal, statistical investigation of the differences in diversity trends between estuaries and the development of testable hypotheses on aspects of estuarine diversity, including the causes of the species minimum in estuaries. It also has potential as a management tool enabling the definition of sites of concern that fall below their predicted diversity. 6. The new model of diversity now requires testing in systems additional to the Thames in order to determine whether this relationship is a macroecological phenomenon or restricted to the test system.

Interactive effects of concealment, parental behaviour and predators on the survival of open passerine nests

Abstract: Summary 1. A simple model of relative effects of parental behaviour (parents present vs. absent) and nest concealment on probability of nest predation was evaluated by measuring survival of paired natural/artificial nests of four open nesting passerines over 3 years. 2. The ratio of rodent to corvid predation (i) decreased from yellowhammer (small eggs, ground/near ground nests) through blackcap (small, shrub) to song thrush and blackbird (medium sized, shrub/subcanopy); (ii) was highest in years when rodent abundance peaked ‐ this effect was clear in yellowhammer, detectable in blackcap, but not found in thrushes. An inverse relationship was found between mean annual nest survival and abundance of the major nest predator for each species. Predators differed between poorly (corvids) and well-concealed (rodents) nests in blackcap. 3. The effects on nest survival differed among species, including: positive effect of parental behaviour combined with neutral effect of concealment (thrushes); independent positive effects of behaviour and concealment (yellowhammer); neutral effect of behaviour combined with positive effect of concealment (blackcap). These patterns are consistent with hypothesis that relatively larger species with conspicuous nests (thrushes) should either engage in more vigorous nest defence or their defence is more efficient, compared with smaller species with less conspicuous nests (blackcap). 4. The positive relationship between nest concealment and survival resulted either from an effect of nest site per se (yellowhammer) or from an effect of parental behaviour (blackcap) that changed from negative (nest disclosure) to positive (nest defence) between poorly and well-concealed nests, respectively. The view that lack of a withinspecies relationship between nest concealment and survival (thrushes) results from parental behaviour compensating for predation risk associated with poor nest concealment was not supported. 5. This study demonstrated (i) multiple interactions among factors influencing the rate of nest predation, both between and within species; (ii) potential bias associated with drawing general conclusions from small-scale experiments.

Spatial patterns of benthic diversity: is there a latitudinal gradient along the Norwegian continental shelf?

Abstract: Summary 1 We examined data on soft-sediment macrobenthos (organisms retained on a 1-mm sieve) from a transect of c. 1960 km along the Norwegian continental shelf (56–71°N), covering a range of water depths (65–434 m) and varying sediment properties. 2 A total of 809 species was recorded from 101 sites. Of these, 36% were restricted to one or two sites, and 29% were represented by one or two individuals. No species spanned the entire transect. Polychaetes were the dominant taxonomic group, followed by crustaceans, molluscs and echinoderms. 3 Alpha diversity (sample species richness) was highly variable (35–148 species) but showed no evidence of a relationship to latitude or other environmental variables. 4 Beta diversity was measured as Whittaker’s βW, the number of shared species, complementarity (biotic distinctness) and Bray–Curtis similarity, and there was no evidence of a latitudinal trend on the shelf. Beta diversity increased with the level of environmental variability, and was highest in the southern-central area, followed by the most northern area. Change in environmental variables had a stronger effect on beta diversity than spatial distance between sites. 5 Gamma diversity was computed by pooling samples over large areas. There was no convincing evidence of a latitudinal cline in gamma diversity, but gamma diversity increased with the level of environmental heterogeneity. Mean alpha diversity and gamma diversity were not significantly correlated. Whereas mean complementarity and mean Bray–Curtis similarity were related to gamma diversity, βW was not.

Review of methodology of quantitative reviews using meta‐analysis in ecology

Abstract: Summary 1 Statistical methods for combination of independent results have been well publicized in the ecological literature, and have begun to be used for reviewing research. They provide a considerable advance in scientific rigour over traditional narrative or ‘vote-counting’ reviews. 2 However, other methodological developments for research synthesis have not yet been widely adopted. 3 This review briefly summarizes some of the techniques used for carrying out rigorous reviewing and synthesis of results in medical science, and surveys techniques used by ecological meta-analyses. 4 Many of the methods used to reduce bias and enhance the accuracy, reliability and usefulness of reviews in medical science have not yet been widely used by ecologists. 5 The quality of ecological reviews could be improved by adoption of some of these methods, such as specifying the methods used for literature searching, stating the types of study combined in the review and the strength of evidence they provide, presenting results as a point estimate with a confidence interval, investigating bias in selection of studies using funnel plots, making a clear distinction between the main analysis and subsidiary analyses and interpreting the latter with caution, and performing sensitivity analyses.

The impact of gastrointestinal nematodes on wild reindeer: experimental and cross‐sectional studies

Abstract: Summary 1 It is well known that gastrointestinal nematodes can have a significant impact on the growth of farmed ruminants. The clinical signs of these parasites are often subtle, with production losses mainly due to reductions in host appetite and gut function. However, little is known about the impact of this widespread group of parasites on wild ruminants. We use experiments and cross-sectional data to investigate the effects of gastrointestinal nematodes on a wild host, the Svalbard reindeer. 2 Individually marked reindeer were treated for parasites. Their body mass, back fat depth and fecundity were compared with the control group one year later. The effect of treatment on worm burdens was investigated in a subsample of animals that were culled 3 and 6 months after treatment. Also, the relationship between the intensity of infection and body mass, back fat depth and host pregnancy rates was investigated in cross-sectional data from culled reindeer. 3 The anthelmintic treatment caused an increase in the body mass, back fat depth and fecundity of the reindeer. Treatment depressed the abundance of adult parasites of Ostertagia gruehneri for at least 6 months, but had no significant effect on the abundance of adults of the other dominant parasite species, Marshallagi marshalli, or the abundance of larval stages of either species. 4 In the cross-sectional study, the probability of a reindeer being pregnant in late winter was negatively related to the abundance of adult O. gruehneri when controlling for host body mass. However, no clear evidence were found for an effect of parasitism on host condition in the cross-sectional study. 5 Our experimental results show for the first time in a natural ruminant host population that gastrointestinal nematodes can have a significant effect on host condition and fecundity. However, effects of infection on host condition was not detectable in the cross-sectional study. Also, we show that the experimental effects on host condition and fecundity is most likely to be due to a negative effect of O. gruehneri, while the experimental design did not allow detection of potential negative effects of M. marshalli.

The role of males in the dynamics of ungulate populations

Abstract: Summary 1 In this review, we focus on how males can affect the population dynamics of ungulates (i) by being a component of population density (and thereby affecting interpretation of log-linear models), and (ii) by considering the mechanisms by which males can actively affect the demographic rates of females. 2 We argue that the choice of measure of density is important, and that the inclusion or exclusion of males into models can influence results. For example, we demonstrate that if the dynamics of a population can be described with a first-order auto-regressive process in a log-linear framework, the asymmetry between the effects of females on the male dynamics and vice versa can introduce a second order process, much in the same way that the interaction between disease and host or predator and prey can. It would be useful for researchers with sufficient data to explore the affects of using different density measures. 3 In general, even in harvested populations with highly skewed sex ratios, males are usually able to fertilize all females, though detailed studies document a lower proportion of younger females breeding when sex ratios are heavily female biased. It is well documented that the presence of males can induce oestrus in females, and that male age may also be a factor. In populations with both a skewed sex ratio and a young male age structure, calving is delayed and less synchronous. We identify several mechanisms that may be responsible for this. 4 Delayed calving may lower summer survival and autumn masses, which may lead to higher winter mortality. If females are born light, they may require another year of growth before they start reproducing. Delayed calving can reduce future fertility of the mother. As the proportion of calves predated during the first few weeks of life is often very high, calving synchrony may also be an important strategy to lower predation rates. 5 We argue that the effects of males on population dynamics of ungulates are likely to be non-trivial, and that their potential effects should not be ignored. The mechanisms we discuss may be important – though much more research is required before we can demonstrate they are.

North Atlantic Oscillation timing of long‐ and short‐distance migration

Abstract: Summary 1 The timing of migration is associated with survival and reproductive risks of migrating species. Hence, variation in factors influencing this timing, such as climate, may have significant life history consequences for migrating species. 2 Using an autoregressive phenological model, we analysed and contrasted the effects of climate (the North Atlantic Oscillation, NAO) and temporal dependence on the long-term (1928–77) dynamics of springtime arrival in three long-distance (83 populations) and three short-distance (52 populations) migratory bird species breeding throughout Norway. 3 Following high NAO winters both long- and short-distance migrants arrived earlier than after low NAO winters. For long-distance migrants, the effect of high NAO winters was probably indirect through improved forage conditions in winter quarters, whereas the effect on short-distance migrants may be related both to improved forage and weather conditions during their northward spring migration. The NAO explained on average 13% (0–46%) and 18% (0–43%) of the interannual variation in arrival dates of long- and short-distance migrants, respectively. 4 For both migrant types, long-term variability in springtime arrival increased with increasing strength of the influence of the NAO on timing of migration. In contrast, the strength of temporal dependence was unrelated to variability in long-term springtime arrival.

Factors affecting pup growth and survival in co-operatively breeding meerkats Suricata suricatta

Abstract: 1. We examined the relative importance of maternal, environmental and social factors for post-weaning pup growth and survival in a co-operatively breeding mammal, the meerkat Suricata suricatta . 2. Pup daily weight gain was primarily influenced by the number of carers per pup and the daily weight gain of those carers. Rainfall and daily temperatures had additional positive and negative effects, respectively, on weight gain of pups born to subordinates. 3. Pup overnight weight loss was primarily influenced by the amount of weight pups gained during the day, and their age. However, pups also lost considerably more weight overnight when temperatures were cold, although such effects were less in large groups. 4. Pup growth rates were positively influenced by the number of carers per pup and carer condition, and negatively influenced by high daytime temperatures. 5. Pup weight at independence was positively associated with weight at emergence and pup weight gain during provisioning, but negatively associated with the extent of over- night weight loss. 6. Pup survival between emergence and independence was related to maternal status, pup sex and overnight weight loss, as well as to group size, daytime temperature and monthly rainfall. 7. Thus, in meerkats, social factors largely, but not wholly, replace the importance of maternal factors that are commonly found to govern reproductive success in non- co-operatively breeding social vertebrates.

A 20‐year study on life‐history traits of a harvested long‐lived temperate coral in the NW Mediterranean: insights into conservation and management needs

Abstract: Summary 1 Long-lived species play a paramount role in the structure and functioning of terrestrial and marine ecosystems. The dramatic increase of human-induced disturbances may be irremediably affecting populations of long-lived species. Data on life-history traits on these species taking into account relevant temporal and spatial scales are very scarce for marine ecosystems, though these data are key to guarantee the conservation of these species and the communities that they shape. 2 This study examined long-term data on life-history traits of the precious red coral (Corallium rubrum L., 1758) in the NW Mediterranean. This was performed by surveying the fate of 67 red coral colonies during 21 years in a shallow-water habitat (27 m depth). 3 Photographic monitoring provided data on survivorship, whole and partial mortality, recruitment and growth of colonies allowing about 1000 observations on life-history traits. 4 Red coral showed a high survivorship, 60% of colonies reaching 22 years of age. Whole mortality was significantly higher in juvenile colonies while partial mortality displayed the inverse pattern. Recruitment showed a pulse mode with a single peak at the beginning of the study and low rates thereafter. Average growth rates over the study period were extremely low: 0·24 ± 0·05 mm year−1 for the basal diameter, 1·78 ± 0·7 mm year−1 for colony height and 3·40 ± 0·31 branches in 22 years. 5 This study also examined size-related effects of harvesting on red coral populations since shallow-water populations (between 10 and 40 m depth) have long been submitted to intensive harvesting. 6 Measurements (basal diameter and height) of 400 colonies corresponding to the largest ones dwelling in 10 currently harvested and two non-harvested populations were compared to analyse the effects on the population's recovery and potential colonies’ maximum sizes. Harvested populations showed values that were about twice as low on average and up to four times lower for maximum values in colony size than in the non-harvested populations. 7 Our study provides indisputable data on the longevity of colonies and the parsimonious population dynamics of C. rubrum. Current populations have shown a dramatic shift in their size structures characterized by the absence of large colonies. Taking into account results obtained in this study full recovery time of shallow-water harvested populations may be measured by several decades or even centuries.

Shared predators and indirect trophic interactions: lemming cycles and arctic-nesting geese

Abstract: Summary 1 We investigated the hypothesis that cyclic lemming populations indirectly affect arctic-nesting greater snow geese (Anser caerulescens atlanticus L.) through the behavioural and numerical responses of shared predators. 2 The study took place on Bylot Island in the Canadian High Arctic during two lemming cycles. We recorded changes in foraging behaviour and activity rate of arctic foxes, parasitic jaegers, glaucous gulls and common ravens in a goose colony during one lemming cycle and we monitored denning activity of foxes for 7 years. We also evaluated the total response of predators (i.e. number of eggs depredated). 3 Arctic foxes were more successful in attacking lemmings than goose nests because predators were constrained by goose nest defence. Predators increased their foraging effort on goose eggs following a lemming decline. 4 Activity rates in the goose colony varied 3·5-fold in arctic foxes and 4·8-fold in parasitic jaegers, and were highest 2 and 3 years after the lemming peak, respectively. The breeding output of arctic foxes appeared to be driven primarily by lemming numbers. 5 Predators consumed 19–88% of the annual goose nesting production and egg predation intensity varied 2·7-fold, being lowest during peak lemming years. Arctic foxes and parasitic jaegers were the key predators generating marked annual variation in egg predation. 6 Our study provides strong support for short-term, positive indirect effects and long-term, negative indirect effects of lemming populations on arctic-nesting geese. The outcome between these opposing indirect effects is probably an apparent competition between rodents and many terrestrial arctic-nesting birds.

Density‐dependent growth in brown trout: effects of introducing wild and hatchery fish

Abstract: Summary 1 Although it is not clear to what extent density dependence acts on the survival, emigration or growth of organisms, experiments testing alternative explanations are rare. A field experiment on 1-year-old brown trout ( Salmo trutta L.) was undertaken to address the following questions: are the mortality, movement and growth of wild stream-living trout affected by population density? If so, are the density-dependent effects of released hatchery trout different from the effects of wild fish? 2 In each of two small streams, two replicate treatment blocks were used, each with four treatments assigned to stream sections 50–70 m in length: (1) control, no fish was introduced and population density was kept at its original level. (2) Trout biomass was doubled by introducing additional wild fish. (3) Trout biomass was doubled by introducing additional hatchery fish. (4) Hatchery trout were introduced, but biomass was kept at its original level by the removal of some resident wild fish. 3 We found no treatment effects on the recapture rates of resident trout, which suggests that survival was not strongly affected by competition. They were also remarkably stationary, regardless of treatment. However, trout growth rate was reduced to the same extent in both treatments with increased density, suggesting that growth was negatively density-dependent, and that the density-dependent effects of hatchery trout and introduced wild fish were similar. 4 Wild resident fish grew faster than introduced wild trout, which in turn grew faster than hatchery trout. Hatchery fish and introduced wild fish moved more than wild resident fish. 5 The results show that population density affected growth in trout parr. We conclude that competition is not limited to the underyearlings, as has previously been suggested, and that density-dependent growth is the main density-dependent response in yearling trout. Furthermore, this effect was the same for wild and hatchery-reared competitors, suggesting that stocking of hatchery fish may affect natural populations negatively through density dependence.

Heterotroph species extinction, abundance and biomass dynamics in an experimentally fragmented microecosystem

Abstract: Summary 1. Species extinction in fragmented habitats is a non-random process described by transient, rather than equilibrium dynamics. Therefore, ‘static’ approaches focusing on experimentally established spatial gradients of diversity may fail to capture essential aspects of ecosystem responses to species loss. 2. Here we document temporal changes in microarthropod species abundance, biomass and richness during a community disassembly trajectory following experimental habitat fragmentation of a moss-based microecosystem. 3. Habitat fragmentation reduced heterotrophic species richness and community biomass in remnant moss fragments. Extinction was biased towards rare species, and thus occurred initially without significant changes in total community abundance and biomass. Eventual reductions in abundance and biomass were found to lag behind observed declines in species richness. 4. The presence of moss-habitat corridors connecting fragments to a large ‘mainland’ area coupled with an immigration rescue effect maintained microarthropod richness, abundance and biomass within remnant fragments. 5. Our results indicate that both the order of species loss and the dynamics of remnant populations influence the magnitude and timing of ecosystem-level responses to habitat destruction and isolation.

Condition-dependent dispersal and ontogeny of the dispersal behaviour: an experimental approach

Abstract: Summary 1 Although little investigated, developmental processes that generate dispersal condition evolution of this behaviour. We have shown previously prenatal and postnatal influences on dispersal in the common lizard (Lacerta vivipara). The observation of these developmental processes was based on independent experiments; our primary goal in this paper is to test their interactions. Interactions could indeed be a source of inconsistencies in studies because they can mask, or even reverse, effects of factors treated additively. 2 We studied dispersal of juveniles released in natura from 416 pregnant females captured in the field. We used a factorial design to test interactions among the maternal habitat (dry vs. humid), prenatal conditions (temperature, humidity), and postnatal environments (dry vs. humid). 3 We found that juvenile dispersal was dependent on the humidity level at different developmental stages, but with varying and sometimes opposite effects. Dispersal was also influenced by the temperature during gestation and by populational differences not related to humidity (differences between replicated populations for the maternal and postnatal habitats). 4 These results confirm our previous findings that dispersal of the common lizard is condition-dependent and has multiple causation. In addition, most of the effects exhibited interactions, and the ontogeny of dispersal appeared as a sequential process where the maternal habitat conditioned prenatal influences, and the prenatal environment modulated postnatal influences. 5 The robustness of our results is supported by the finding of the same interactions in independent tests on both juvenile males and females. This militates in favour of future studies on the multiple causation of dispersal because the same dispersal status might originate from different causes, and different dispersal outcomes might be due to the same factor.

Structure of a diverse tropical forest insect–parasitoid community

Abstract: Summary 1 Quantitative host–parasitoid food webs are descriptions of community structure that include data on the abundance of hosts and parasitoids, and the frequency of links between them, all expressed in the same units. 2 Quantitative host–parasitoid food webs were constructed describing the community of leaf-mining insects (Diptera, Coleoptera and Lepidoptera) and their parasitoids (Hymenoptera) in an 8500-m 2 area of moist tropical forest in Belize, Central America, over a 1-year period. 3 The summary food web, containing data for the whole year, is we believe the most diverse quantitative host–parasitoid web yet described. It contains 93 species of leaf-miner, 84 species of parasitoid and 196 links between hosts and parasitoids. 4 Most parasitoids were generalists, with individual parasitoid species recorded as parasitizing up to 21 host species. In contrast, most leaf-miners were specialists, with 114 links documented between leaf-miners and their host plants. 5 A robustness analysis was used to reveal the effects of different sampling intensities on food web statistics. The results suggest that the sampling had revealed most of the species of host and parasitoid in the community, but further interactions among species would continue to be detected with additional sampling. Measures of the ratio of hosts to parasitoids and of realized connectance were relatively insensitive to sampling intensity. 6 Three seasonal webs were constructed, revealing temporal changes in the structure of the community. There was greater turnover in host species composition than parasitoid species composition among seasons, but most web statistics remained relatively constant across seasons. 7 Both the summary web and the seasonal webs show low levels of compartmentalization, suggesting that the host–parasitoid community is not divided into relatively discrete subwebs with largely independent dynamics. 8 The extent of potential indirect interactions between pairs of hosts was assessed by constructing quantitative parasitoid overlap graphs. These suggest that abundant species are likely to have greater indirect effects on less abundant species than vice versa , and that species in the same taxonomic order are more likely to interact indirectly. The graphs do not support the hypothesis that species sharing the same host plant are more likely to interact via shared parasitoids.

Spatial organization, group living and ecological correlates in low‐density populations of Eurasian badgers, Meles meles

Abstract: Summary 1 Territoriality and group living are described in a low-density population of Eurasian badgers, Meles meles L., by studying the patterns of spatial grouping and territory marking, as well as the differences between individuals in some of their characteristics (body condition and dispersal) and in their space use (seasonally, periods of activity and interaction between pairs of individuals) under strong seasonal fluctuations in the availability of the key resource (young rabbits, Oryctolagus cuniculus L.). Finally, the role of the spatial distribution of the main prey (young rabbits) in the development of sociality was also studied in order to test some of the assumptions and predictions of the resource dispersion hypothesis (RDH). 2 Badgers were territorial, showing a flexible system of territory marking, which includes the marking of the most used areas (sett-latrines at the centres of activity) and additionally, at the smaller territories, a system of border-latrines in the areas of contact between territories. The maximum use of border-latrines was associated with the reproductive season, and that of sett-latrines with the season of food scarcity. 3 In the study area where badgers had rabbits as main prey, territories were occupied by small groups of animals, formed by one adult female who reproduced, one adult male who also showed signs of reproductive activity, the cubs of the year (if there was reproduction) and some animals born during previous years, which remained in their natal territory until their dispersal (normally during the mating season of their third or fourth year of life). This system was not strictly fixed as males, given the opportunity, expanded their territories to encompass additional females. Territories in another study site were occupied by one adult female (marked), plus the cubs of the year and another adult individual (unmarked). 4 In winter and spring dominant females and subordinates used only a small fraction of their territories, moved short distances, at a low speed and covering small areas per night. These seasons corresponded with the reproduction of rabbits (highest food availability). Dominant females were the only individuals using all the territory available in the summer (lowest food availability), when badgers had the worst body condition. Food availability increased again in autumn, as did body condition, while range sizes were again reduced. Dominant males used the same proportion of their territories over all seasons. However, in winter (reproductive season) they moved faster, over longer distances, and covered larger areas per period of activity. These results indicate that use of space by dominant males was affected by different factors from that of dominant females and subordinates. 5 RDH does not seem to explain group living in our populations because: (a) territoriality in each pair of primary animals was driven by different factors (trophic resources for females and females for males); (b) dominant males acted as expansionists; and (c) territory size was related to its richness and not to patch dispersion. 6 We propose an integrative hypothesis to explain not only group formation but also interpopulation variability in the social organization of badgers within ecological, demographic and behavioural constraints and in the light of current theory on delayed dispersal.

Reindeer summer activity pattern in relation to weather and insect harassment

Abstract: Summary 1 During the brief growing season in Arctic and high mountain ecosystems, undisturbed grazing is crucial in order to maximize growth and fattening. During summer 1997 we investigated the influence of weather and insect harassment on the behaviour and group dynamics of reindeer ( Rangifer tarandus tarandus L.). 2 Climatic data, activity of parasitic flies, and female reindeer behaviour were recorded from two wild reindeer populations in Southern Norway. Temperature and solar irradiation were good predictors of oestrid fly activity. Throughout the warm summer, reindeer were exposed to vigorous oestrid fly harassment, which caused dramatic decrease in feeding and lying, and increase in walking, running and standing. This behavioural change may compromise the physical condition of individuals entering winter. 3 Mosquitoes had little influence on reindeer activity patterns. In the absence of oestrid flies, weather parameters had no influence on reindeer activity pattern. Even during the warmest days, no signs of heat stress were recorded. Accordingly, snow patches, marshes and windy mountaintops were used primarily to avoid oestrid fly harassment. Thus, most disruptions of feeding that are often reported on warm days are responses to oestrids, not thermal stress.

Resource partitioning among British and Irish mustelids

Abstract: Summary 1. Seven species of mustelid carnivore live in the British Isles: weasel, stoat, mink, polecat, pine marten, badger and otter. Recent studies have hypothesized that coexistence of these species is facilitated by partitioning of resources according to prey size, particularly that of mammalian prey. This hypothesis has been supported by evidence of character displacement derived from even size ratios in skull length and canine diameter. 2. To test whether this hypothesis is supported by empirical data, 98 studies of the diet of mustelids living in Great Britain and Ireland were analysed. Two main predictions were tested; that larger males ate larger prey than females and that larger species ate larger prey than smaller species. 3. Male mustelids ate larger prey than females but there was no relationship between predator size and prey size, either for all species or when largely vermivorous badgers and piscivorous otters were excluded. There was no difference in dietary niche breadth between the sexes. Dietary niche breadth increased with body size in the assemblage excluding otters and badgers. The dietary niches of mustelids were partitioned along several axes, none of which was clearly related to prey size. 4. The dietary niches of the five species living in Ireland (stoat, mink, pine marten, otter and badger) were more similar to one another in Ireland than in Great Britain and there was no difference in niche breadth between Irish and British mustelids. There was no evidence of competitive release in the diets of Irish mustelids. 5. Resource partitioning according to prey size is apparent between the sexes within species, and this is probably correlated to sexual selection for size dimorphism. Empirical data do not, however, support a hypothesis of resource partitioning according to prey size between species. Interspecific aggression provides an alternative hypothesis explaining character displacement among mustelids.

The life‐history basis of latitudinal diversity gradients: how do species traits vary from the poles to the equator?

Abstract: 1. Latitudinal variation among species in life-history traits is often suggested to contribute to high tropical species richness. However, traditional methods of analysing such variation rarely control for phylogeny and latitudinal range overlap between species, potentially giving misleading results. 2. Using a method of pairwise independent contrasts which overcomes these problems, I tested for latitudinal variation among bird species in a number of traits which have been linked, theoretically or empirically, with both latitude and species richness. 3. This method indicates strong support for Rapoport's Rule and decreasing clutch size towards the equator in both hemispheres, but only partial support for decreasing body size and ecological generalism towards the equator. 4. Indirect measures of sexual selection (sexual dichromatism and size dimorphism) show no variation with latitude; an apparent increase in dichromatism towards the equator is shown to be an artefact of phylogeny. 5. Many of the associations between life history and latitude were not detected by traditional cross-species analyses, highlighting the importance of incorporating phylogeny and overlap in studies of geographical life-history variation. Establishing associations between life-history traits and latitude does not prove, but is a necessary prerequisite for., a link between these traits and latitudinal diversity gradients.

Functional response of a generalist insect predator to one of its prey species in the field

Abstract: Summary 1. The shield beetle Cassida rubiginosa Muller (Coleoptera, Chrysomelidae) is one of the most prominent herbivores on creeping thistle, Cirsium arvense (L.) Scop. and is considered as a biological control agent against this weed. In order to investigate if augmented densities of this species could be sustained, predation on C. rubiginosa larvae was studied in the field with the aid of continuous video surveillance of individual prey during three seasons at two different study sites. 2. The paper wasp Polistes dominulus Christ (Hymenoptera, Vespidae) was responsible for 99·4% predation on C. rubiginosa larvae at both study sites. Prey densities were always reduced to a few individuals at the sites after a few days only. Thus, P. dominulus may annihilate the effect of augmentative releases of the beetle as a biocontrol agent. 3. P. dominulus showed no aggregation in patches of high host density (numerical response). The observed density dependence was attributed to the functional response of the wasp. 4. P. dominulus showed a Type III functional response. In mark-and-recapture experiments we found no indication that this type of functional response was the result of single wasps specializing on this particular prey type. 5. Type III functional responses in generalists can result by switching to prey species which become temporarily abundant. Since in a natural setting a mixture of prey species coexists in different abundance and which are differentially accepted by generalist predators, it is imperative to study their functional response in the field.

Colonization rates and distances of a host butterfly and two specific parasitoids in a fragmented landscape

Abstract: Summary 1 We describe the pattern of colonization of suitable, but currently empty, habitat by a host butterfly and two specialist parasitoids living in a highly fragmented landscape. 2 Using survey data collected over 8 years, field sampling and small-scale experiments we show that the ability of the Glanville fritillary butterfly ( Melitaea cinxia ) to colonize new habitat is intermediate between that of its two larval primary parasitoids. 3 The butterfly forms a classic metapopulation, which the parasitoid Hyposoter horticola experiences as a single patchily distributed host population because of its high rate of dispersal and long colonization distances. In contrast, most of the local butterfly populations are presently inaccessible to the parasitoid Cotesia melitaearum, which has a limited dispersal range and therefore persists only in tightly clustered networks of host populations. 4 At the regional scale, the butterfly may escape C. melitaearum by colonizing empty habitat, but host dispersal does not limit parasitism by H. horticola, which consequently must be limited by local interaction. 5 The parasitoid H. horticola mostly avoids direct competition with C. melitaearum because the majority of H. horticola populations are outside the range of dispersal by current C. melitaearum populations. In contrast, all C. melitaearum populations experience competition with H. horticola .

Benefits of early arrival at breeding grounds vary between males

Abstract: Summary 1. It is often assumed that, in migratory birds, males of highest phenotypic quality are the first to arrive at breeding grounds, as only males in good body condition can afford the costs of early arrival. Here it is argued that variation in how much an individual can gain from being early may sometimes override these patterns of condition dependence. Benefits from early arrival will depend on how good the chances of an individual are of successfully occupying and defending a high-quality territory. Other studies have shown that these chances depend on male resource-holding power (RHP) and on value asymmetries (VA), as resulting from prior ownership. 2. Patterns of spring arrival were studied in a polygynous population of the dusky warbler Phylloscopus fuscatus Blyth, a species with high breeding-site tenacity. During the period of postnuptial moult, just before departure to winter quarters, about half of the males preoccupied and vigorously defended high-quality territories. Apparently these males enhanced their success in competition over these territories in the following breeding season by taking advantage of having prior ownership. 3. Males with good chances of obtaining a high-quality territory (i.e. males that had occupied such territories before) were the first to arrive in spring, suggesting that high motivation drives early arrival. In contrast, there was no indication for male quality or body condition to affect arrival patterns. 4. Interestingly, males of small body size, possibly reflecting low RHP, invested more in gaining prior ownership than did large males. 5. The present study suggests that, in some species, individual variation in benefits rather than in costs resulting from early arrival determine which males are the first to arrive.

Variability in interaction strength and implications for biodiversity

Abstract: Summary 1. We examined the species richness of theoretical communities in relation to interaction strength between species. 2. To do so, we used randomly constructed interaction matrices for competitive systems. To determine co-existence, we tested for local stability and equilibrium feasibility of these theoretical assemblages. 3. As expected, we found that a low mean species interaction strength could allow for many species to co-exist. However, variance in the interaction strengths may alter previous results; two systems with the same mean interaction strength show markedly different diversity depending critically on the magnitude of the variance. If species are similar enough then many can co-exist, even if they compete strongly. 4. In addition we found that the species richness of a competitive community can greatly depend on the correlation between interaction strengths, an issue that so far has gone unreported. This correlation, a result of trade-offs between species’ characteristics, may profoundly increase the potential for stable co-existence of a highly species-rich community. 5. Competition may not be an anathema to diversity. Statistical properties of species’ interactions may be critical factors that contribute to the explanation of species diversity in natural communities.

Model complexity and population predictions. The alpine marmot as a case study

Abstract: Summary 1. During the past 15 years, models have been used increasingly in predictive population ecology. Matrix models used for predicting the fates of populations are often extremely basic, ignoring density dependence, spatial scale and behaviour, and often based on one sex only. We tested the importance of some of these omissions for model realism, by comparing the performance of a variety of population models of varying levels of complexity. 2. Detailed data from more than 13 years of behavioural and demographic research on a population of alpine marmots Marmota marmota in Berchtesgaden National Park, southern Germany, were used to parameterize four different population models. The models ranged from a simple population-based matrix model, to a spatially explicit behaviour-based model. 3. The performance of the models was judged by their ability to predict basic population dynamics under equilibrium conditions. Only a spatially explicit individual-based model ignoring optimal behaviour predicted dynamics significantly different to those observed in the field, highlighting the importance of considering realistic patterns of behaviour in spatially explicit models. 4. Using realistic levels of environmental and demographic stochasticity, variance in population growth rates predicted by all models was high, even within the range of population densities experienced in the field. This emphasizes the difficulty of using population-level field data to determine overall patterns of density dependence for use in population models. 5. All models were also used to predict potential density-dependent effects on alpine marmot population growth. In this regard, the models differed greatly. It was concluded that the simplest matrix model was adequate for making predictions regarding population sizes or densities under equilibrium conditions, but that for predictions requiring an understanding of transient dynamics only the behavioural model would be adequate. 6. An emergent feature of this study of alpine marmot population dynamics was the prediction of a demographic Allee effect with a profound influence on population dynamics across a very broad range of population sizes. Three mechanisms were identified as underlying this Allee effect: stochastic skews in sex ratio and demographic composition at low population sizes; less efficient social thermoregulation during hibernation in small groups; and difficulties with mate finding during dispersal, even at relatively high population sizes.

Capital-breeding and reproductive effort in a variable environment: a longitudinal study of a viviparous snake

Abstract: Summary 1 The ways that fluctuations in prey abundance and weather conditions can affect reproductive output in a ‘capital breeding’ ectotherm, the aspic viper (Vipera aspis) were examined 2 Our longitudinal study confirms that female aspic vipers adjust reproductive investment by integrating allocations of energy from stores (‘capital’) and facultative feeding (‘income’) Thus, long-term energy storage enabled females to reproduce successfully even in years when prey were scarce 3 Not surprisingly, temporal changes in body reserves of female vipers preparing for reproduction depended upon current feeding rates However, the mean environmental temperature during the active season also affected mass gain 4 Allometric patterns suggest that reproductive output was limited by energy availability in 8 out of the 9 years of our study In the other year, high prey availability in the preceding season meant that reproductive output was maximized within the constraints set by maternal body size (and thus, abdominal volume) 5 High summer temperatures increased basking opportunities of gravid vipers and thus accelerated gestation However, maternal metabolic costs also increased in such situations, resulting in low postpartum body condition