scispace - formally typeset
Search or ask a question

Showing papers in "Journal of Animal Ecology in 1996"



Journal ArticleDOI
TL;DR: The analyses of habitat selection inside each home range showed that the forest types, characterized by high densities of food and low visibility, were preferred, suggesting that habitat use is allocated in proportion to either food or cover or both.
Abstract: 1. Summer home range size variation and habitat selection of 35 radio-collared adult female roe deer was studied, using kernel home range estimation and compositional analysis of habitat use. 2. Female roe deer adjust the size of their home range in response to decreasing food supply, and the hypothesis that female roe deer utilize the minimum area that sustain their energy requirement cannot be rejected. 3. Home range size increased with the visibility in the home range (the average distance at which sight is blocked by intervening vegetation). This supports the hypothesis that cover is important in reducing the risk of predation and thereby increasing adult survival. 4. Female roe deer spend more time near habitat edges, supporting the hypothesis that different habitat types contain complementary resources, e.g. food and cover or different nutrients. Simultaneous access to several habitat types did not have any effect on home range size, possibly because variation in heterogeneity between different home ranges was too low. 5. Females without fawns had smaller home ranges, possibly because they only need to sustain their own energetic requirements. 6. The analyses of habitat selection inside each home range showed that the forest types, characterized by high densities of food and low visibility, were preferred, suggesting that habitat use is allocated in proportion to either food or cover or both.

426 citations



Journal ArticleDOI
TL;DR: The dynamics of a metapopulation balanced by local colonization and extinction by means of the cellular automaton formalism is introduced and the existence of extinction thresholds when a given fraction of habitat is destroyed is reported.
Abstract: The incidence of habitat destruction on the survivorship of a single metapopulation is studied by means of a spatially explicit model. As the proportion of destroyed sites increases, the structural properties of the resulting landscape change in a non-linear way, showing the existence of critical thresholds and phase transitions. Such critical thresholds are identified by means of an order parameter, which discriminates a quantitative process, i.e. habitat loss, from a qualitative one, i.e. habitat fragmentation. This difference is only well understood using a spatially explicit framework. We introduce on such a fragmented landscape the dynamics of a metapopulation balanced by local colonization and extinction by means of the cellular automaton formalism. The existence of extinction thresholds when a given fraction of habitat is destroyed is reported. These thresholds are determined both by the critical behaviour of the landscape structural properties, and by the demographic properties of the metapopulation. Some differences between these results and those derived from the study of spatially implicit models are described and explained. In particular, the percentage of patch occupancy is lower for a given value of habitat destruction in the spatially explicit formulation. Extinction threshold also take place for a lower destruction value. Some implications for the management of natural landscapes are discussed.

400 citations


Journal ArticleDOI
TL;DR: The under-estimation of long-distance migration by negative exponentials, compared with inverse power functions, may explain why theoretical models haveUnder-estimated the number of occupied patches in metapopulations of H. comma following natural colonization.
Abstract: 1. Metapopulation dynamics of the silver-spotted skipper butterfly Hesperia comma were studied between 1982 and 1991 along a 25-km stretch of chalk hills on the North Downs, Surrey, UK. Sixty-nine patches of suitable habitat were identified, of which 48 were occupied in 1982. Over the 9-year period, 12 patches were colonized, seven went extinct and nine patches remained vacant. Patches were more likely to be colonized if they were relatively large and close to other large, occupied patches. Local populations in small, isolated patches were more likely to go extinct. 2. Within a 70-ha section of a metapopulation, mark-release-recapture techniques were used in 1994 to investigate the effects of local patch area and isolation on movement of individuals among local patches. 988 butterflies were marked of which 133 moved between patches. 67% of between-patch movements were less than 50 m, although the longest recorded distance moved was 1070 m. Butterflies were most likely to move between large patches that were close together. 3. Metapopulation models have assumed that the distribution of distances moved by migrants follows a negative-exponential function. In our study, this distribution fitted an inverse-power function better than a negative-exponential. The under-estimation of long-distance migration by negative exponentials, compared with inverse power functions, may explain why theoretical models have under-estimated the number of occupied patches in metapopulations of H. comma following natural colonization. 4. Per capita emigration and immigration rates were significantly higher in small patches (area < 0.07 ha) compared with medium-sized (0.33-0.78 ha) or large patches (5.66 ha). However, in absolute numbers, more emigrants came from the largest patch where the source population was the largest. 5. The population system studied here shares attributes of several theoretical types of spatially structured population (patchy population, metapopulation, mainland-island system, non-equilibrium system) depending on the temporal and spatial scale examined. The distribution and dynamics of metapopulations should be regarded as being affected by a variety of behavioural and population processes, and real metapopulations can rarely be characterized as a single theoretical type.

372 citations


Journal ArticleDOI
TL;DR: Results on emigration suggest that conserving an isolated butterfly population is more successful in an area with physical barriers to migration than in an open landscape, and the significance of nectar sources and patch size in successful colonization of empty habitat patches.
Abstract: 1. We studied factors affecting emigration and immigration behaviour in the butterfly Melitaea cinxia by releasing 882 newly emerged marked butterflies into 16 habitat patches in a network of 64 empty patches on an isolated island (area 1.6 km 2 ). 2. Of the 363 butterflies that were recaptured at least once, 40% were recorded in a new patch during their lifetime. Females emigrated earlier and moved further away than males. One-third of males appeared to remain permanently in, whereas females gradually drifted away from, the release patch. 3. High density of butterflies, great abundance of flowers, and large patch area decreased emigration, whereas open landscape around the patch increased emigration. Females that emigrated were on average larger than females that stayed in the patch of release. 4. In total, 152 immigrants were recorded in 32 patches. Numbers of immigrants increased with patch area and abundance of flowering plants. 5. Results on emigration suggest that conserving an isolated butterfly population is more successful in an area with physical barriers to migration than in an open landscape. The possible tendency of butterflies to leave a patch with low density should be taken into consideration in introductions of butterflies to empty habitat patches. 6. Results on immigration indicate the significance of nectar sources and patch size in successful colonization of empty habitat patches.

324 citations


Journal ArticleDOI
TL;DR: Differences between habitats in density, reproductive performance and local survival are consistent with an ideal-despotic/preemptive distribution of individuals, and suggest that this population could be regulated by the availability, distribution, and extent of high and low quality breeding habitats.
Abstract: The distribution of individuals among habitats and their relative success in those habitats can have important consequences for population dynamics. To examine these processes for a long-distance migratory bird species, we studied the population structure, age-specific reproductive output, and local survival of black-throated blue warblers (Dendroica caerulescens, Gmelin) in two breeding habitats differing in shrub density within northern hardwoods forests in New Hampshire, USA. On forest plots with dense shrubs, warblers occurred at higher densities, and fledged significantly more young per capita per season than those occupying areas with lower shrub density. This differential productivity was due to higher reproductive output, mainly through double-brooding, of older (≥ 2 years of age) individuals, which were disproportionately more abundant in high shrub density sites. Clutch initiation dates, clutch sizes, and predation rates at individual nests did not differ significantly between habitats. Mean body mass of nestlings on day 6 following hatching were higher on average on plots with high shrub density, but differences were not significant. Annual return rates, as indices of local survival, did not differ between habitats for older males or for females. Yearling males, however, returned in subsequent years at a significantly lower rate to low shrub density plots, a result of either lower survival or, more likely, dispersal to more suitable habitat in their second year of breeding. Parental age and habitat suitability interact in that older individuals, through their experience and/or dominance, acquire sites of higher quality, which results in higher reproductive output and probably higher survival. These differences between habitats in density, reproductive performance and local survival are consistent with an ideal-despotic/preemptive distribution of individuals, and suggest that this population could be regulated by the availability, distribution, and extent of high and low quality breeding habitats.

321 citations


Journal ArticleDOI
TL;DR: This study investigated its local impact on an infaunal community in the north-eastern Irish Sea and found no difference between fished and unfished areas, which suggests that the effects of fishing disturbance are consistent between replicate samples.
Abstract: Beam-trawling is a source of physical disturbance to marine sedimentary communities in areas less than 50m deep, on the western European continental shelf. Chains attached between the beam-trawl shoes are designed to penetrate the upper few cm of the sediment, which leads to the damage or removal of some infaunal and epifaunal species. In some areas, beam-trawling may be frequent and intense, leading to speculation that it may generate long-term changes in the local benthic fauna. As part of a larger MAFF study examining the ecological effects of beam-trawling, we investigated its local impact on an infaunal community in the north-eastern Irish Sea. Studies of this type are complicated by the heterogeneity of the environment, hence we adopted a replicated, paired control and treatment design to maximize the chances of detecting any effects due to trawling. A side-scan sonar survey revealed that the experimental area was characterized by mobile megaripples in the south-eastern sector of the experimental area and stable sediments with uniform topography in the north-western sector. Multivariate analysis of the species abundances from the control areas separated the fauna into two distinct communities which corresponded to the different substratum characteristics. Data from the two regions were therefore treated separately when testing for the effects of trawling. In the north-western sector, trawling led to 58% decrease in the mean abundance of some taxa and a 50% reduction in the mean number of species per sample. Multivariate analysis revealed that differences between control and fished sites were largely due to the reduction or removal of less common species. These effects were less apparent in the mobile sediments of the south-eastern sector, which had a naturally impoverished fauna and high level of heterogeneity. Univariate variables, such as abundance and the total number of species per sample, indicated that the variation between replicate samples increased as a result of trawling disturbance. However, examination of the community data using an index of multivariate dispersion revealed no difference between fished and unfished areas. This suggests that the effects of fishing disturbance are consistent between replicate samples. Fishing with demersal gears modifies communities in relatively stable sediments. Frequent and repeated physical disturbance by fishing gears may lead to long-term changes in the benthic community structure of these habitats.

301 citations


Journal ArticleDOI
TL;DR: An analysis of the time of death in 63 cases where kestrels, Falco tinnunculus L. had raised broods of manipulated size and were subsequently reported freshly dead confirms the interpretation of the manipulation effects on local survival as due to mortality rather than emigration.
Abstract: 1. Costs of reproduction have been assessed experimentally by measuring subsequent survival and reproduction of parent animals raising enlarged and reduced numbers of offspring. Reported effects on survival have so far always referred to local survival of marked individuals in the study population. They do not provide definitive proof of a cost of reproduction, since reduced local survival may be due either to reduced survival or to an increased tendency to emigrate from the study area. Therefore, it is important to assess mortality rates in connection with brood size experiments. 2. We report an analysis of the time of death in 63 cases where kestrels, Falco tinnunculus L. had raised broods of manipulated size and were subsequently reported freshly dead. 60% of the parents raising two extra nestlings were reported dead before the end of the first winter, compared to 29% of those raising control or reduced broods. This result confirms our interpretation of the manipulation effects on local survival as due to mortality rather than emigration. The extra mortality occurred in the winter following the brood enlargement. 3. Kestrel parents in these experiments have been shown to adjust their daily energy expenditure to the modified brood size. Increased parental effort in this species thus entails an increased risk of death half a year later.

295 citations


Journal ArticleDOI
TL;DR: Observations of fat reserves of live great tits within a marked, free-living population in Wytham Woods, Oxford, UK agree with the predictions, supporting the arguments for a cost of fattening, that fattens follows a strategic response to resource predictability; and that social status is a component in that resourceredictability.
Abstract: 1. It has generally been assumed that reduced winter fat reserves in small birds are symptomatic of environmental constraints on their ability to fatten further. Contrary to this, recent theoretical work on foraging and fat storage suggests that such observations result as a strategic response to environmental stress, determined by a balance of fattening costs and benefits that differ between individuals. Fat reserves should therefore be adjusted to the minimum required as insurance against some period of stress, but birds should respond to the resource variance (predictability) as well as the mean abundance. Hence, birds facing greater starvation risk should carry more fat than those facing less risk. 2. These hypotheses predict that, in a winter-flocking species with a clear dominance hierarchy, and in which social status may determine a bird's access to food, individuals of different social status should differ in the fat reserves that they carry as follows: (i) fat should increase with declining temperature; (ii) the rate of diurnal fattening should be reduced when food is abundant; (iii) fat reserves should be increased when food is scarce; and (iv) dominant individuals should carry less fat than subdominants. This paper tests these predictions by observations of fat reserves of live great tits within a marked, free-living population in Wytham Woods, Oxford, UK. It then examines the relationship between the winter fat and over-winter survival of known individuals. 3. Fat increased with declining temperature on the day of capture rather than at any previous time, suggesting that it acted as a proximate factor in fat regulation. The rate of fattening was lower when food was more predictable. Reserves increased when food was scarce but this depended on the bird's status, dominants carrying less fat than subdominants. Evidence that the ability to fatten might be subject to external constraint was found only in the least dominant individuals. 4. When food was abundant, fat reserves failed to predict over-winter survival, suggesting that the birds were able to maintain their individual optimum reserves. When food was scarce, low ranking birds were more likely to survive if fat, while the survival of dominants was independent of fatness, suggesting that they were still able to achieve their individual optimum reserves. 5. These results agreed with the predictions, so supporting the arguments (i) for a cost of fattening, (ii) that fattening follows a strategic response to resource predictability; and (iii) that social status is a component in that resource predictability.

290 citations


Journal ArticleDOI
TL;DR: This work describes the spatial organization, holts, and habitat of otters in the sea and freshwater, and investigates their social behaviour, hunting behaviour and strategies, and populations, survival, and mortality.

Journal ArticleDOI
TL;DR: It is argued that retarded phenological development, during periods of cooler weather, enhances diet quality because leaf :stem ratios and digestibility of plant parts decline more slowly, and weight gain during the early summer growth spurt should be rapid during cool May-June weather, increasing the probability of conception in the autumn.
Abstract: Density-independent weather effects can have important consequences for the demography of terrestrial herbivores because precipitation, temperature and insolation influence plant phenology, forage quality and biomass production, which in turn affects the habitat carrying capacity. Since forage digestibility influences intake and weight gain, life-history traits of young, growing animals are likely to reflect variation in the prevailing weather. This paper specifically investigates spatial and temporal variation in age at maturation in female red deer (Cervus elaphus) in Norway in relation to climate variables known to influence primary production. Our findings are corroborated by analysing differences in age at maturation in 21 cohorts of red deer on the Isle of Rum, Scotland. In Norway the majority of females ovulated as yearlings and calved for the first time as 2-year-olds. The proportion calving for the first time at two years varied from 0.23 to 0.67 between regions and fluctuated from 0.46 to 0.76 between cohorts. On Rum, where age of maturation was delayed at least a year, the proportion calving for the first time as 3-year-olds varied between cohorts from 0.0 to 0.89. In a subset of yearlings culled in Norway at the time of conception, the spatial and temporal differences in ovulation rates were related to the geographical and annual variation in body weight. 5. Both the spatial and temporal variation in the proportion of 2-year-olds calving in Norway, and cohort differences in the proportion calving as 3-year-olds on Rum, were negatively related to variation in May-June degree days 12 months earlier. Although primary production on the preferred herb-rich Agrostis-Festuca grasslands was positively correlated with temperature in May and June on Rum, the proportion of females calving as three years old, was negatively correlated with annual differences in May June primary production. We argue that retarded phenological development, during periods of cooler weather, enhances diet quality because leaf :stem ratios and digestibility of plant parts decline more slowly. Thus, weight gain during the early summer growth spurt should be rapid during cool May-June weather, increasing the probability of conception in the autumn. Since density-independent variation in food availability also influences fitness components which commonly have a more pronounced influence on population demography, for example offspring survival, we argue that our results highlight the potential importance of variation in weather on herbivore abundance.

Journal ArticleDOI
TL;DR: Data from Butterfly Monitoring Scheme transect counts were used to investigate the decline in synchrony of population fluctuations with increasing distance between sample units, and to evaluate the relative effects of local and regional scale processes on population fluctuations.
Abstract: 1. Data from Butterfly Monitoring Scheme (BMS) transect counts were used to investigate the decline in synchrony of population fluctuations with increasing distance between sample units, and to evaluate the relative effects of local and regional scale processes on population fluctuations. 2. Each BMS transect is divided into sections and numbers (densities) of butterflies are recorded separately in each section. Local scale analyses examined population fluctuations on different sections within a single transect. Regional scale analyses examined fluctuations on different transects. 3. At the local scale population dynamics were found to be more closely synchronized between very close population units (sections) than more distant ones. There was considerable variation between data sets but on average this correlation declined relatively quickly over 1-2 km. 4. At the regional scale, where local environmental heterogeneity was averaged out (sections were lumped together within transect sites), the decline in synchrony with increasing distance (up to 200 km) was very small compared with local scale decrease, and populations remain partially synchronized throughout the range studied due to regionally correlated weather patterns. 5. Butterfly dispersal had a significant effect on synchrony at a local scale (several km): the dynamics of populations of relatively mobile species remained correlated over relatively long distances. At the regional level, however, mobility was not a significant factor, implying that widespread environmental stochasticity is of overriding importance at this scale. 6. Although mobility and distance are significant factors in determining population synchrony/asynchrony, the low r 2 values attributable to them indicate that these factors are actually contributing relatively little to the overall dynamics. Butterfly species show local variation in population dynamics, nested within broad scale synchrony which is presumably generated by the climate. Heterogeneity in population dynamics, in local environments, potentially aids metapopulation persistence by buffering the effects of high levels of temporal environmental stochasticity.

Journal ArticleDOI
TL;DR: In this paper, the authors examined long-term changes in the structure and composition of the groundfish species assemblage in three regions of the north-western North Sea using data collected during the months July-September over the period 1929-53 are compared with more recent August groundfish survey data covering the period 1980-93.
Abstract: This paper examines long-term changes in the structure and composition of the groundfish species assemblage in three regions of the north-western North Sea. Scottish fisheries research vessel data collected during the months July-September over the period 1929-53 are compared with more recent August groundfish survey data covering the period 1980-93. Trends in the whole groundfish assemblage, and in a subset of the assemblage that is not targeted by commercial fisheries, are described. Long-term differences in species assemblage were subtle, and were most apparent in the dominance structure. Species diversity in the whole groundfish assemblage was marginally greater in the period 1929-53, but no difference was apparent within the non-target species assemblage. For the whole groundfish assemblage, diversity was greatest in the inshore region and least in the offshore area, but there was no obvious spatial gradient for the non-target species assemblage. Multivariate analyses indicated long-term changes and between-area differences in the species composition for both the whole groundfish assemblage and the non-target species subset. More detailed examination of the data revealed that the long-term changes resulted from relatively small and subtle differences in the relative abundance of rarer species, such as grey gurnard Eutrigla gurnardus (L.) and spur-dog Squalus acanthius L.. In contrast, changes in the relative abundance of the more common species, such as Norway pout Trisopterus esmarkii (Nilsson) and whiting Merlangius merlangus (L.), explained much of the between-area variation. Examination of species aggregated length-frequency distributions suggested that by the 1980s there had been a shift towards assemblages in which smaller fish were more highly represented. This was only apparent, however, in the whole groundfish species assemblage ; the length-frequency distributions of non-targeted species were almost identical in the two time periods. Overall, the results suggest that, although differences in the structure of the whole fish assemblage can be detected, the non-target groundfish assemblage appears to have remained relatively unchanged, despite a century of intensive fishing activity.

Journal ArticleDOI
TL;DR: For tropical fishes on small, artificial reefs, patterns of species richness and abundance established at settlement can be modified considerably over small spatial and temporal scales by differences in refuge availability, and habitat complexity need not provide permanently safe sites to affect patterns of coexistence.
Abstract: We examined assemblage structure of tropical fishes on small, artificial reefs to determine if differences in refuge availability could modify patterns of species richness and abundance of fishes established at recruitment. Our artificial reefs were designed to provide prey refuge of two types (permanent and transient). Permanent refugia are those that physically exclude predators, i.e. small holes provide permanent refuge from large-bodied predators. Transient refugia result when habitat complexity increases the probability that prey will elude predators. These refugia do not physically exclude predators and thus provide no permanently safe sites. We conducted this experiment at two widely separated locations on Australia's Great Barrier Reef. The species pools of reef fishes, recruitment rates and predator densities all differ between these locations. At neither location were patterns of recruitment influenced by the presence of either type of refuge. By the end of the experiment, however, there were more resident fishes on reefs with additional refugia. Species richness of residents was positively related to total abundance of residents. Therefore, refuge availability indirectly affected species richness through its effect on abundance. There was no indication, however, that permanent refugia provided any greater protection to prey species than did transient refugia. Our results therefore indicate that for these communities, patterns of species richness and abundance established at settlement can be modified considerably over small spatial and temporal scales by differences in refuge availability. Furthermore, habitat complexity need not provide permanently safe sites to affect patterns of coexistence.

Journal ArticleDOI
TL;DR: Using long-term records of individual reproduction and survival in the Soay sheep of St Kilda, it is shown that the costs and benefits of breeding to animals of different weight categories vary with population density and stochastic dynamic programming is used to predict the optimal fecundity of animals belonging to each category at high and low population density.
Abstract: 1. Though models of life-history decisions are traditionally based on age-related changes in the costs and benefits of reproduction, in nature both costs and benefits vary with individual differences in phenotype as well as with environmental changes. 2. Using long-term records of individual reproduction and survival in the Soay sheep of St Kilda, we show that the costs and benefits of breeding to animals of different weight categories vary with population density. 3. Subsequently, we use stochastic dynamic programming to predict the optimal fecundity of animals belonging to each category at high and low population density. Optimal strategies of fecundity vary with population density as well as between different weight categories of sheep. However, there is no evidence that the sheep track density-related changes in optimal fecundity. Instead, their behaviour approximates to an average, weight-related optimum that is well adapted to the range of conditions that they encounter.

Journal ArticleDOI
TL;DR: This book will not become a unity of the way for you to get amazing benefits at all, but, it will serve something that will let you get the best time and moment to spend for reading the book.
Abstract: It sounds good when knowing the science and the endangered species act in this website. This is one of the books that many people looking for. In the past, many people ask about this book as their favourite book to read and collect. And now, we present hat you need quickly. It seems to be so happy to offer you this famous book. It will not become a unity of the way for you to get amazing benefits at all. But, it will serve something that will let you get the best time and moment to spend for reading the book.

Journal ArticleDOI
TL;DR: An analysis of patterns in the abundance, geographic range size and body size of a monophyletic animal group, the Anseriformes (wildfowl), using data on global abundance and range size for almost all (probably) extant species and comparative methods to control for phylogenetic effects is presented.
Abstract: 1. A large body of literature has addressed macroecological interactions between abundance, geographic range size and body size across species in animal assemblages. Studies have examined a variety of taxa, at a variety of spatial scales, using a variety of measures of the relevant variables, and with assemblages defined in different ways. Given this heterogeneity, it has been difficult to draw firm conclusions about the nature of these relationships. 2. Here we present an analysis of patterns in the abundance, geographic range size and body size of a monophyletic animal group, the Anseriformes (wildfowl), using data on global abundance and range size for almost all (probably) extant species and comparative methods to control for phylogenetic effects. As far as we are aware, this is the first study to examine these patterns for an entire group at this scale. 3. Population and range sizes are log-normally distributed in wildfowl, whereas the body mass distribution is positively log-skewed. There is a consistent relationship only between abundance and measures of geographic range size. Body size explains little variance in either of these two variables, but some variance in abundance unexplained by geographic range size can be accounted for by life-history traits relating to the rate at which offspring are raised. 4. Abundance, geographic range size and body size all show latitudinal variation, but this is not consistent between variables, and does not appear to affect the relationships observed between them. Wildfowl listed as threatened with extinction have smaller global population sizes and geographic ranges than species considered non-threatened, but it is population size, rather than range size, that seems to contribute most strongly to categorization as threatened or otherwise. 5. Phylogenetic and across-species analyses generally reveal the same patterns. 6. The results are discussed in relation to previous studies and to hypotheses concerning patterns in, and mechanisms structuring, animal assemblages.

Journal ArticleDOI
TL;DR: A predator-prey model previously parameterized for voles and weasels is extended to two prey species, a 'Microtus' (field vole) type which is competitively superior to but more vulnerable to predation than a 'Clethrionomys' (bank vole" type, which explains four patterns in the dynamics of multispecies rodent assemblages in Fennoscandia.
Abstract: We extend a predator-prey model previously parameterized for voles and weasels to two prey species, a 'Microtus' (field vole) type which is competitively superior to but more vulnerable to predation than a 'Clethrionomys' (bank vole) type. The model explains four patterns in the dynamics of multispecies rodent assemblages in Fennoscandia : a predictable shift in the relative abundances of different prey species during one multiannual population cycle ; long-term (supracyclic) variation in relative prey abundances ; an association between the amplitude of population oscillations and the type of the numerically dominant prey species ; and increasing rodent species number with increasing latitude. The model results illustrate the complex and often unexpected behaviour of strongly connected multispecies assemblages, of which the Fennoscandian rodent-predator community is a prime example. Since the mid 1980s, rodent oscillations in many, though not all, parts of northern Fennoscandia have become distinctly less regular (non-cyclic), a change which is reflected in the entire animal community linked to the keystone species, the arvicoline rodents. We demonstrate that such long-term changes in the amplitude and regularity of rodent oscillations are not unexpected in multispecies prey-predator assemblages.

Journal ArticleDOI
TL;DR: It is shown that the detection of density-dependent processes is a function of study spatial scale, so that appropriately scaled studies commonly reveal density dependence, while inappropriately scaled studies do not.
Abstract: Regulation of populations by density-dependent mechanisms is one of the basic tenets of theory in population biology. Yet, it has been argued that population studies rarely reveal density dependence. We show that the detection of density-dependent processes is a function of study spatial scale, so that appropriately scaled studies commonly reveal density dependence, while inappropriately scaled studies do not. The detection of density dependence and the potential for spatial structure in population density were examined in 79 insect population studies. The potential for spatial structure within the study area was estimated from population movement relative to the area over which density was measured. Relative movement was classified as 'high' (and potential for spatial population structure 'low') if more than 10% of the population moved across the boundary of this area each generation. Analyses were stage-based, addressing the prevalence of density-dependent processes, rather than population regulation. In mobile stages (small and large instars and adults), density dependence was found in 69-73% of studies with low potential spatial structure ('high' relative population movement), and in 23-35% of studies with high potential structure ('low' movement). In immobile stages (eggs and pupae), density dependence was rarely detected in studies larger than 0.1 hectare (7-16%), but often detected in smaller-scale studies (65-69%). One-third of studies reporting both density relationships and 'key factors' (sources of mortality that are most correlated with population fluctuation) found the key factor to be directly density-dependent. Density-dependent growth was detected in at least one stage in 74-76% of studies that tested for density relationships. Study length (in generations) was not related to the frequency of detection of density dependence in studies with low potential spatial structure in population density. This result calls into question the need for long-term studies to detect density relationships. The detection of population regulation may be hampered by scale effects similar to those demonstrated here.

Journal ArticleDOI
TL;DR: Whenever ecologists attempt to understand which traits or characteristics allow different taxa to be successful in different ecological circumstances, phylogenetic information should be incorporated into their analyses whether the data are experimental or observational.
Abstract: Molecular genetic information is becoming widely available and can reveal when (in relative or absolute time) pairs of individuals within a species or pairs of higher-level taxa last shared a common ancestor. Such genealogical or phylogenetic information is usually ignored by ecologists when performing cross-taxonomic analyses, probably because ecological and evolutionary time-scales are considered to be very different. Whenever ecologists attempt to understand which traits or characteristics allow different taxa to be successful in different ecological circumstances, phylogenetic information should be incorporated into their analyses whether the data are experimental or observational. Closely related species share many traits in addition to those responsible for ecological success in particular circumstances, and phylogenetically controlled analyses help to eliminate the effects of such confounding variables. When genealogical information is available showing when, in relative time, a sample of individuals from a population or species last shared a common ancestor, it is often possible to infer the population dynamic history. In particular, some modes of population change (e.g. linear or exponential growth, population stasis, population decline) can be shown to provide a much better fit the data than others. Phylogenetic analysis can show the relative extent to which new species evolve into vacant niches rather than partition old niches.




Journal ArticleDOI
TL;DR: This work estimated the relationship between body size and fitness for Achrysocharoides zwoelferi, a parasitoid of leaf mining moths on willow, and found that fitness increased rapidly with size over the first two-thirds of the observed size range, but plateaued in the last third of the range.
Abstract: A knowledge of the relationship between body size and fitness is very important in many models of parasitoid behaviourial ecology. We estimated this relationship for Achrysocharoides zwoelferi (Hymenoptera, Eulophidae), a parasitoid of leaf mining moths (Gracillariidae) on willow (Salix). The size distribution of female wasps emerging from their pupae was compared with the size distribution of wasps caught while searching for hosts. A parametric statistical approach was used to obtain a function relating size to fitness. Laboratory estimates of the influence of size on a component of fitness, longetivity, were obtained under two different sets of experimental conditions. In the field, fitness increased rapidly with size over the first two-thirds of the observed size range, but plateaued (or possibly declined) in the last third of the range. One laboratory experiment showed no effect of size on fitness while the other did show an effect although much weaker than that observed in the field. This suggests caution in using results from the laboratory in parameterising behaviourial ecological models. The results are compared with two other field estimates of the influence of size on fitness which show broadly similar patterns.

Journal ArticleDOI
TL;DR: Female agaonine fig wasps enter Ficus fruits (figs), where they pollinate the flowers and oviposit into the ovaries of a proportion of the flowers via their styles, to help maintain the evolutionary stability of the fig-fig wasp mutualism.
Abstract: Female agaonine fig wasps enter Ficus fruits (figs), where they pollinate the flowers and oviposit into the ovaries of a proportion of the flowers via their styles. The fig trees are totally dependent on these fig wasps for sexual reproduction, as the wasps fertilize the figs' seeds and transfer pollen between trees, while the wasps need the fig trees' ovaries as feeding sites (galled seeds) for their progeny. Since the wasp progeny destroy seeds, the question arises as to why selection has not led to increasingly higher fig wasp fecundities and the eventual collapse of the mutualism. The stability of the mutualism could be maintained by pollinators having short ovipositors, restricting oviposition to short-styled flowers so that long-styled flowers produce seeds. However, in most of the monoecious Ficus species where measurements were taken, pollinators generally possessed ovipositors of sufficient length to reach the ovaries of a large majority of the flowers. This was confirmed by the presence of pollinator progeny in flowers with long styles. Complete seed destruction may also be avoided by a proportion of fig ovaries being inviolate to oviposition by fig wasps, independent of their style lengths. However, experimental increases in pollinator foundresses per fig in F. burtt-davyi showed that all the ovaries that could be reached by the wasps' ovipositors were potentially exploitable by agaonine fig wasps. Relative production of wasps and seeds was found to be largely dependent on the interplay between the number of eggs inside each wasp, the number of accessible ovaries in individual figs, and the average number of foundresses entering each fig. On average, there were not enough foundresses entering each fig to utilize all accessible ovaries. Possible factors affecting the total entry numbers of wasps into figs include ostiolar closure rates and wasp population densities in the areas surrounding fig trees. As many ovaries remain unused because of a frequent shortage of wasp eggs per fig, fig trees can continue to produce enough seeds for the continued evolutionary stability of the fig-fig wasp mutualism.

Journal ArticleDOI
TL;DR: The low number of ant and plant partners per compartment, coupled with an apparently high temporal and spatial stability of ant-myrmecophyte interactions, suggests that compartments are the appropriate scale at which to investigate coevolution in ant-related systems.
Abstract: In the tropics, many plants offer housing and food for their specialized ant partners which, in return, offer benefit in the form of defence and/or nutrients, thus forming mutualistic bonds. Such ant-plants, also called myrmecophytes, occur together at a local scale, generating community patterns of mutualistic ant-plant associations. Here, we present the first fully quantitative description of an ant-myrmecophyte community. The study site in Central Amazonian tropical rainforest had a high myrmecophyte density of about 380 ind. ha -1 . Sixteen myrmecophyte and 25 ant species were recorded, the species abundance rank curves being highly uneven. The ant-myrmecophyte matrix was highly compartmentalized, and a Monte Carlo simulation showed that the observed pattern was not a product of chance and sample size (P < 0.0001). Cluster analyses indicated that compartments were partially explained by occurrence of the ants in phylogenetically related host plants, but not by habitat specificity. The connectance of the ant-plant community was 12%. This value seems quite low when compared with published results from other mutualistic systems (pollinator and seed-dispersor), after controlling for the total number of interacting species. The high frequency of null interactions in the ant-myrmecophyte system could not be explained by the 'phenological non-coincidence hypothesis', since both ant and plant partners occur together throughout the year. Ant-plant interactions were highly asymmetrical: ant species had fewer partners than plant species and ants were more dependent on the plants than the reverse. These asymmetries are in the opposite direction to those recorded for plant-pollinators and plant-dispersors; however, they seem to be the product of the same underlying process: differential fitness benefits between mutualistic partners. The low number of ant and plant partners per compartment, coupled with an apparently high temporal and spatial stability of ant-myrmecophyte interactions, suggests that compartments are the appropriate scale at which to investigate coevolution in ant-myrmecophyte systems.

Journal ArticleDOI
TL;DR: It is concluded that the behavioural biology of both immature and adult Eunica bechina is finely linked with the utilization of a host plant where ant visitation patterns strongly affect larval survival.
Abstract: Larvae of the nymphalid butterfly Eunica bechina feed on young leaves of Caryocar brasiliense, a shrub of the Brazilian savannah that is frequently visited by nectar-gathering ants. Eggs are not removed by ants, but ant occupation on plants affects the oviposition behaviour of the butterflies. Adult females avoid laying eggs on highly visited plant locations and visual cues were demonstrated to mediate oviposition by the butterflies. Plant branches with artificial rubber ants were significantly less infested than control branches with rubber circles. This is the first demonstration that ant presence per se can be enough to produce an avoidance response by ovipositing females in a non-myrmecophilous butterfly. Larval mortality was strongly affected by the level of ant visitation to the host plants, and vulnerability to ant predation decreased with larval size. Stick-like frass chains constructed by the larvae at leaf margins were demonstrated to be a safe refuge against ant attacks on the host plant. Live termites placed on leaves were attacked by foraging ants in significantly greater numbers than those placed on the frass chains. It is concluded that the behavioural biology of both immature and adult Eunica bechina is finely linked with the utilization of a host plant where ant visitation patterns strongly affect larval survival.

Journal ArticleDOI
TL;DR: The model predicted that the level of cellulolytic activity of microbes in the rumen can affect the energy obtained from the diet, and predicted that microbial adaptation would cause browsers to suffer a penalty on switching from browse to grass, thereby reinforcing diet choice.
Abstract: 1. Two contrasting explanations of niche separation in ruminants emphasize the importance of body mass-related trends in energy requirements and food-processing ability (the diet-quality assumption) and of physiological adaptations to diet type (grass or browse; the diet-type assumption). 2. The quality of the diet consumed in the dry season by 21 species of African ruminants was estimated from observed fermentation rates in the rumens of culled animals using a simulation model of digesta kinetics. The model predicts the effect of food composition (including tannins) on a number of variables on which previous arguments about physiological adaptations have been based: cellulolytic activity, digestion kinetics and volatile fatty acid proportions. 3. The predicted diet quality (potential digestibility) varied from 0.90 in small animals to 0.75 in large ones, and was negatively related to body mass with a shallow allometric exponent(- 0.035 +/- 0.0109). 4. The allometric exponent scaling body mass to predicted energy assimilation rate was 0.852+/-0.0474. The abundance and quality of food available were apparently adequate to allow greater net energy intake, relative to requirements, by large animals than small ones. 5. No difference existed in the predicted potential digestibility or net energy yield from the diet of browsing and grazing species after controlling for body mass. Therefore, although the diets differed in their botanical and chemical composition, the end result had little effect on the nutritional ecology of the animals with different diet types. 6. The model predicted that the level of cellulolytic activity of microbes in the rumen can affect the energy obtained from the diet. Microbial adaptation would cause browsers to suffer a penalty on switching from browse to grass, thereby reinforcing diet choice. 7. The presence of tannins in the diets of browsers was predicted to decrease rumen fermentation rate and increase the digestibility of the diet required to match observed fermentation rates.

Journal ArticleDOI
TL;DR: The work here, combined with the previous work, demonstrates the importance of priority effects with respect to the outcome of competition and intraguild predation in these two taxonomically distant species.
Abstract: 1. We tested for the importance of priority effects on the interactions between immatures of the mosquito, Culiseta longiareolata, and immatures of the green toad, Bufo viridis, the two most abundant species of many temporary pools of the Negev Desert, Israel. Previously, we reported that when these two taxonomically distant species were placed together as early stage larvae, they competed strongly and when Culiseta larvae had the size advantage, they preyed on Bufo hatchlings. Here, we tested the interaction between these two species in artificial pools when Bufo enters first and develops prior to Culiseta entering. 2. We also tested for intra-specific competition and for the impact of both mosquito larva and tadpole on invading species. 3. We assessed these interactions by experimentally manipulating various combinations and densities of Bufo and Culiseta. We introduced early stage Bufo larvae 12 days prior to introducing first instar Culiseta larvae. 4. Bufo exhibited a strong intraspecific density-dependent effect: tadpoles were smaller and reached metamorphosis later at the higher density. We did not demonstrate an intraspecific density-dependent effect in Culiseta. 5. Culiseta larvae affected neither survival nor development rates of the larger Bufo. Early stage Culiseta larvae were vulnerable to predation by the larger Bufo but later instar Culiseta larvae were no longer vulnerable. Bufo reduced larval development rate of Culiseta but did not affect pupal size. 6. Bufo tadpoles virtually eliminated, and Culiseta larvae strongly reduced, the number of individuals of an invading ceratopogonid midge species reaching the pupal stage. 7. The work here, combined with our previous work, demonstrates the importance of priority effects with respect to the outcome of competition and intraguild predation.