Showing papers in "Journal of Animal Ecology in 1998"

Patterns of natal and breeding dispersal in birds

Abstract: 1. Dispersal is of critical ecological and evolutionary importance for several issues of population biology, particularly population synchrony, colonization and range expansion, metapopulation and source–sink dynamics, and population genetic structure, but it has not previously been possible to compare dispersal patterns across a wide range of species or to study movement outside the confines of local study areas. 2. Using resampling methods, we verified that statistically unbiased estimates of average dispersal distance and of intraspecific variance in dispersal distance could be extracted from the bird ringing data of the British Trust for Ornithology. 3. Using data on 75 terrestrial bird species, we tested whether natal and breeding dispersal were influenced by a species’ habitat requirements, diet, geographical range, abundance, morphology, social system, life history or migratory status. We used allometric techniques to ascertain whether these relationships were independent of body size, and used the method of phylogenetically independent contrasts to ascertain whether they were independent of phylogeny. 4. Both natal and breeding dispersal distances were lower among abundant species and among species with large geographical ranges. Dispersal distances and life-history variables were correlated independent of phylogeny, but these relationships did not persist after controlling for body size. All morphometrical variables (wing length, tarsus length and bill length) were not significantly correlated with dispersal distances after correcting for body size or phylogenetic relatedness. 5. Migrant species disperse further than resident ones, this relation was independent of body size but not of phylogeny. A significant positive relation was observed between breeding dispersal distance and long-term population decline among migrants, but not among residents. 6. The species living in wet habitats disperse further than those living in dry habitats, which could be explained by the greater patchiness of wet habitats in space and/or time. This relationship was observed only for breeding dispersal, suggesting that this habitat variable does not impose the same constraint on natal dispersal.

Impending extinctions of North American freshwater mussels (Unionoida) following the zebra mussel (Dreissena polymorpha) invasion

Abstract: 1. Freshwater mussels (Order Unionoida) are the most imperiled faunal group in North America; 60% of described species are considered endangered or threatened, and 12% are presumed extinct. Widespread habitat degradation (including pollution, siltation, river channelization and impoundment) has been the primary cause of extinction during this century, but a new stress was added in the last decade by the introduction of the Eurasian zebra mussel, Dreissena polymorpha, a biofouling organism that smothers the shells of other molluscs and competes with other suspension feeders for food. Since the early 1990s, it has been spreading throughout the Mississippi River basin, which contains the largest number of endemic freshwater mussels in the world. In this report, we use an exponential decay model based on data from other invaded habitats to predict the long-term impact of D. polymorpha on mussel species richness in the basin. 2. In North American lakes and rivers that support high densities (>3000 m−2) of D. polymorpha, native mussel populations are extirpated within 4–8 years following invasion. Significant local declines in native mussel populations in the Illinois and Ohio rivers, concomitant with the establishment of dense populations of D. polymorpha, suggest that induced mortality is occurring in the Mississippi River basin. 3. A comparison of species loss at various sites before and after invasion indicates that D. polymorpha has accelerated regional extinction rates of North American freshwater mussels by 10-fold. If this trend persists, the regional extinction rate for Mississippi basin species will be 12% per decade. Over 60 endemic mussels in the Mississippi River basin are threatened with global extinction by the combined impacts of the D. polymorpha invasion and environmental degradation.

Competition refuges and coexistence : an example from Serengeti carnivores

Abstract: 1. In the last two decades predator–prey models have shown that ‘refuges’, in which prey can seek respite from predation, are crucial for the persistence of prey and predator. This concept is equally applicable to interspecific competition and, in a heterogeneous environment, species with low competitive ability should seek out ‘competition refuges’ where competition is reduced. 2. Cheetahs have low competitive ability compared with their principal competitors, hyenas and lions, which are directly responsible for their low density. This study uses distribution data collected in the Serengeti National Park in Tanzania over a 4-year period to show that cheetahs are more strongly associated with each other than with their competitors and utilize areas with low-density prey. 3. Cheetahs exhibit local avoidance behaviour in both space and time with respect to lions and hyenas. This behaviour is facultative and is strongest when cheetahs are engaged in activities that might expose them to food loss or increase the risk of close interactions, such as when they are hunting or eating. 4. Lactating cheetahs, whose range is restricted, are more likely to have difficulties finding prey and come into more frequent contact with lions than free-ranging animals. 5. It is argued that although cheetahs always lose in direct competition, they persist in the ecosystem by seeking out ‘competition refuges’ with low densities of lions and hyenas and that their mobility is the key to their continued coexistence with these predators. This pattern of distribution may be generally applicable to other species which, although widely distributed, always occur at low densities.

Individualistic species responses invalidate simple physiological models of community dynamics under global environmental change

Abstract: 1. Most predictions of species distribution and abundance changes in response to global warming relate the individual requirements of a single isolated species to climate variables through some form of climate mapping. This method fails to account for the effects of species dispersal and species interactions, both of which may strongly affect distribution and abundance. 2. We therefore examined the effects of dispersal and species interactions on the distribution and abundance of three Drosophila species in a laboratory system that mimicked a latitudinal cline of 15 °C. We then investigated how species distribution and abundance in this system responded to simulated global warming. 3. Dispersal allowed populations to persist at non-optimum temperatures, overriding physiologically imposed range limits. 4. Temperature determined the outcome of competition. In pairwise interactions, Drosophila subobscura eliminated D. melanogaster or D. simulans at low temperatures but was itself eliminated at high temperatures. 5. Competitive interactions changed abundance and range sizes thus shifting the position of species optima. These changes depended on both the number and the identity of the competing species. 6. Enemy–victim interactions altered range and abundance. Adding the parasitoid Leptopilina boulardi affected the host assemblage directly at high temperatures where the parasitoid was present, and indirectly (mediated by dispersal) at low temperatures where it was scarce or absent. Host species coexisted for longer at low temperatures in clines when parasitoids were present than when they were absent. 7. Simulated global warming produced complex, counter-intuitive effects on distribution and abundance, including the reversal of species’ relative abundance at some temperatures. 8. Because dispersal and species interactions strongly influenced both range and abundance (sometimes in unexpected ways) current species distributions are no guide to what they might be under global climate change. Furthermore, since both these factors are missing from climate envelope models of range and abundance change, their predictions are, at best, incomplete.

Effects of habitat fragmentation on carabid beetles: experimental evidence

Abstract: 1. We tested for effects of habitat fragmentation in a controlled, replicated, field experiment, in south-eastern Australia. Our experimental subjects were eight carabid beetle species, and the carabid assemblage (45 species). Monitoring was by pitfall trapping in forest remnants and in adjacent continuous-forest controls. Remnants were of three sizes (0·25, 0·875, 3·062 ha.). Monitoring commenced 2 years prior to habitat fragmentation. Here we present results for 6 years after habitat fragmentation (4 years for species richness). 2. We tested four hypotheses. Hypothesis one: habitat fragmentation reduces species richness in the remaining remnants. Carabid species richness was not different in habitat remnants compared to continuous forest, neither was carabid richness different for remnants of different sizes, or at monitoring sites close to remnant edges compared to sites in remnant interiors. 3. Hypothesis two: populations decline as a result of habitat fragmentation. Two species of eight were completely isolated on remnants and both declined in abundance on remnants compared to control plots in continuous forest. The other six species responded in various ways, which included relative increases and decreases in abundance and no change, but as they were not completely isolated on remnants, their responses could not be explained by isolation. 4. Hypothesis three: remaining subpopulations decline further on smaller habitat remnants than on larger remnants. Three species responded to remnant size; one was most abundant in small remnants, the second was most abundant in large remnants, and the third was equally abundant in small and large remnants and less abundant in medium-sized remnants. 5. Hypothesis four: populations near to remnant edges decline further than populations in remnant interiors. Two species were more likely to occur in remnant interiors than at edges, whereas three species were equally likely to occur in remnant interiors and at edges. 6. The effects of habitat fragmentation may be the consequence of: (i) isolation; and (ii) habitat modification. In this study, we were able to consider effects of isolation for two carabid species. We tested only one aspect of the habitat modification hypothesis, edge effects. We discuss the possibility that other aspects of habitat modification in remnants may explain some of our results and suggest that this is where future research be directed.

A field study of size–fitness relationships in the parasitoid Asobara tabida

Abstract: 1. Many evolutionary models of parasitoid behaviour assume a positive correlation between size and fitness. In this paper we study the size-fitness relationship in the laboratory and in the field using females of the solitary parasitoid Asobara tabida (Hymenoptera: Braconidae). 2. In the laboratory, fecundity, fat reserves and longevity without food were positively correlated with size. 3. Release-recapture experiments in the field showed that dispersal diminishes fat reserves. Dispersal ability is size-dependent: larger females, with larger fat reserves, disperse over larger distances than smaller females. 4. The form of the relationship between size and fitness in the field was estimated in two ways: one based on a comparison of the size distribution of released and recaptured females; the other based on the egg load and fat reserves of wild-caught females. Both showed an accelerating increase of fitness with size. 5. The majority of females appeared to be time-limited. Therefore, the increase in fitness with size is predominantly due to a larger dispersal ability and not to a higher egg load.

Local trophodynamics and the interaction of marine mammals and fisheries in the Benguela ecosystem

Abstract: 1. A method for finding the consequences of long-term generalized press perturbations in multispecies ecological communities, with relatively modest requirements for data, is explicated. The approach uses energetic and allometric reasoning to set some parameter values for which data are not available. The remaining unknown parameters are treated as random variables, enabling the calculation of probability distributions for the outcomes that are of interest. 2. The method is used to investigate the effect of a cull of fur seals on fisheries in the Benguela ecosystem, using a 29-species foodweb for that system. In the case of Cape fur seals treated here, it is found that a cull of seals is more likely to be detrimental to total yields from all exploited species than it is to be beneficial. 3. The influence of weak links on the effects of a cull is investigated. Using both consumption by each species and consumption of each species to define link strength, a clear threshold in link strength is found, indicating that 44% of all links could be deleted from the foodweb without affecting the predictions significantly. Even using a criterion based on consumption by each species alone (conventional dietary proportion data), about the same number of links can be deleted without seriously affecting the predictions of the model. This is a very helpful (and encouraging) result for the design of an observational protocol for systematic efforts to gather data for multispecies modelling.

Geographic expansion of an increasing brown bear population: evidence for presaturation dispersal

Abstract: 1. The distribution of brown bears (Ursus arctos L.) expanding into suitable habitat in Sweden following near extermination was estimated using harvest data from the period 1981–93. Core areas were defined as female concentration areas, where 90% of the hunter-killed females were taken. 2. Three predictions were tested, based on results of earlier bear dispersal studies, which show that females are extremely philopatric. Prediction 1: the relative density of females declines more rapidly from the centre of a core area towards the edge than for males. Prediction 2: males dominate in the peripheral areas, especially males in the age of most active dispersal (2–4 years of age). Prediction 3: females in the periphery are found closer to the edge of the core area than males. 3. The results of the present study supported Predictions 1 and 2, but not Prediction 3. This indicates that males were more prone to disperse from the core areas than females. However, females that did disperse did not differ from males in distance from the core areas, and females were found up to 80–90 km from them. Such long-distance female dispersal has apparently not been previously documented in other bear populations that are stable or declining. The results strongly suggest that presaturation dispersal, i.e. dispersal occurring before the carrying capacity of the habitat has been reached, is occurring in this increasing population. This phenomenon might be more common in increasing populations of large mammals than was previously thought. Regarding conservation of bears, this result is positive for gene flow and metapopulation dynamics and negative for livestock losses in formerly bear-free areas. 4. Core and peripheral areas can be identified based on the age and sex of shot bears. This allowed us to classify Norway as a peripheral area. Bear density appears difficult to estimate near an expansion front because of large differences in densities over short distances.

Persistence thresholds for phocine distemper virus infection in harbour seal Phoca vitulina metapopulations

Abstract: 1. This paper explores the concept of the critical community size for persistence of infection in wildlife populations. We use as a case study the 1988 epidemic of phocine distemper virus in the North Sea population of harbour seals, Phoca vitulina. 2. We summarize the available data on this epidemic and use it to parameterize a stochastic compartmental model for an infection spreading through a spatial array of patches coupled by nearest-neighbour mixing, with replacement of susceptibles occurring as a discrete annual event. 3. A combination of analytical and simulation techniques is used to show that the high levels of transmission between different seal subpopulations, combined with the small annual birth cohort, act to make persistence of infection impossible in this harbour seal population at realistic population levels. The well known mechanisms by which metapopulation structures may act to promote persistence can be seen to have an effect only at weaker levels of spatial coupling, and higher levels of host recruitment, than those empirically observed.

How can a pelagic seabird provision its chick when relying on a distant food resource? Cyclic attendance at the colony, foraging decision and body condition in sooty shearwaters

Abstract: 1. Sooty shearwaters are abundant in waters too distant from their colonies to account for the observed average frequency with which the chicks are fed. This, and the unexplained cyclic pattern of attendance at their colonies, have led to contradictory interpretations related to evolution of the extreme life histories of pelagic seabirds and their relationship with the marine environment. 2. Study of the provisioning behaviour of individual parent sooty shearwaters indicates that they can rely on productive distant waters, probably 1550. km away, to build up their body reserves and to provision their chick. These long foraging trips on average last 11. days and account for 84% of the foraging time. 3. Birds use this stored energy to cover the costs of performing several successive foraging trips of short duration in nearby less productive waters. These brief trips double the energy flow to the chick and take only 16% of the foraging time but are at the expense of adult body condition, i.e. there is a net energy loss. Foraging successively with short and long trips allows parents to increase by 20% the energy flow to the chick over what it would get if foraging was exclusively in distant waters. 4. The results suggest that maximal fitness in this pelagic seabird is not achieved by maximization of foraging efficiency but is the result of a trade-off system whereby at some stages birds forage with net energy losses. The decision to feed close to or far from the colonies, i.e. to allocate to the chick or to store body reserves, is not related to any endogenous rhythm, nor to the nutritional status of the chick, nor to the length of the previous foraging trip. Rather, it is under the sole control of adult body condition, with the possible existence of a threshold body mass around 750 g. Thus, adult body mass plays a central role in foraging decisions, linking foraging and allocation. 5. The synchronized return of foraging birds every 2. weeks is not due to environmental factors. By modelling attendance patterns with an increasing variance in the foraging routine, it is shown that the periodicity of feeding times is retained throughout the chick rearing period, with a cyclicity of 14. days. This result indicates that cyclic attendance at the level of the whole colony is an emergent property of the two-fold foraging strategy of individual adults. 6. One main consequence of this system appears to be a reduction in competition close to the breeding grounds and this may help explain the existence of huge populations of sooty and short-tailed shearwaters that rely on distant food resources.

Quality vs. quantity: energetic and nutritional trade-offs in parental provisioning strategies

Abstract: 1. When faced with increased brood demand, parent birds provisioning young in the nest can make a variety of adjustments to their foraging and food allocation strategies. Logical extensions of classic optimal foraging theory predict increased provisioning effort to larger broods to be accompanied by changes in load size, foraging distance from the nest, as well as possible changes in the type and size of prey delivered. 2. We assessed such behavioural adjustments and their consequences in pairs of European starlings (Sturnus vulgaris) responding to a range of experimental brood sizes. Parents feeding larger broods increased their visit rates by spending less time in the nestbox and less time around the nestbox colony. High visit rates to larger broods were also associated with larger loads per visit and changes in the type of prey delivered to the nest. As a consequence, chicks in large and small broods received similar rates of food intake, but experienced differences in the nutritional quality of their food. Parents feeding larger brood sizes were able to increase their provisioning effort despite feeding in the same foraging sites, travelling at comparable flight speeds and maintaining similar body masses to parents feeding smaller broods. 3. Parental energetic expenditure, measured through doubly labelled water analyses, showed no effect of the brood size treatment. The greater proportion of indigestible material per gram of food delivered to the larger experimental brood sizes (i.e. soil from the guts of earthworms) was probably responsible for the fact that these chicks grew at slower rates and fledged at lower body masses, although we cannot rule out the possibility of lower growth rates due to higher energetic costs of sibling competition within larger broods. Lighter fledglings from large broods disappeared from the local area earlier in the summer, probably as a result of differential mortality rather than premature natal dispersal. 4. We discuss the adaptive significance of the provisioning trade-off between quantity and quality of food items delivered by parents to the nest, with reference to natural variation in foraging conditions and brood demand.

Abundances of red fox and pine marten in relation to the composition of boreal forest landscapes.

Abstract: The effects of human-caused fragmentation of boreal forest on the abundance of red fox Vulpes vulpes L. and pine marten Martes martes L. were studied by combining the Finnish wildlife-triangle snow-track data (1990-94) with land-use and forest resources data employing the GIS. Two study areas (each 45 000 km(2) ) located in northern and southern Finland were selected for the investigation. The extent of landscape that best explained predator abundance (tracks per 10 km 24 h(-1) ) was the same (about 100 km(2) ) in both species and study areas. The decreasing proportion of older forest and the increasing proportions of young forest and agricultural land in the landscape positively affected track density of red fox. The relationship between agricultural land and fox abundance, however, was characterized by a convex curve peaking at 20-30% of agricultural land. With the habitat classification used, landscape composition explained 26% and 11% of the spatial variation in fox abundance in the northern and southern study area, respectively. The relationship between landscape composition and pine marten abundance was not as clear as in that of red fox. Landscape composition explained 10% and 6% of spatial variation in pine marten abundance in the northern and southern study area, respectively. In both areas a positive impact occurred with the increasing proposition of young forest in the landscape, but in the northern area the negative effect of increasing proportion of agricultural land was dominant. The abundances of red fox and pine marten were not negatively correlated, indicating that competition or intraguild predation by red fox do not determine abundance of pine marten on a landscape scale. A general increase in predation pressure by generalist predators in fragmented forest landscapes has been an intensively discussed conservation problem during recent years. We conclude that the red fox is a species potentially able to cause elevated predation pressure in boreal landscapes fragmented by human activities, but that the evidence against the pine marten is weaker.

Large-scale climatic fluctuation and population dynamics of moose and white-tailed deer

Abstract: 1. Mech et al. (1987) documented cumulative, negative effects of previous winters’ snow on rates of population increase in moose (Alces alces) and white-tailed deer (Odocoileus virginianus), but noted no effect of predation by wolves (Canis lupus). Those results were contested by Messier (1991), who analysed smoothed versions of the original abundance data and reported no effect of snow accumulation on population dynamics of either species, but strong effects of wolf predation and food competition. 2. McRoberts, Mech & Peterson (1995) contended that the conclusions reached by Messier (1991) were an artefact of the use of smoothed data. In a subsequent re-analysis of the smoothed data, Messier (1995) argued that the lack of an effect of snow after one year precluded the potential for a cumulative effect beyond one year. 3. We re-analysed original and smoothed data on dynamics of moose and white-tailed deer densities using the same methods as Mech et al. (1987) and Messier (1991), but we used a measure of global climatic fluctuation, the North Atlantic Oscillation (NAO) index. The NAO is the atmospheric process determining most interannual variation in snowfall and winter temperatures in northern latitudes, and its phases drive decadal trends in wintertime precipitation. 4. We observed that rates of increase of moose and white-tailed deer in both the original and smoothed data were influenced by global climatic fluctuation at 2- and 3-year lags, as well as by delayed density-dependent feedback and wolf predation.

The relative role of density‐dependent and density‐independent survival in the life cycle of Atlantic salmon Salmo salar

Abstract: 1. Density-dependent factors appeared important for the survival of juvenile Atlantic salmon in the River Imsa whilst density-independent factors were more important for the older fish at sea. In fresh water, density dependence was indicated by a stock−recruitment relationship with increasing loss-rates from eggs to smolts and from eggs to adults as egg density increased. 73% of the loss-rates were explained by variation in egg density. At sea, density independence was indicated by the lack of a significant relationship between loss-rates and smolt densities. 2. The relationship between smolt density and initial egg density was best described by an asymptotic ‘Cushing’ type relationship with a plateau at densities higher than approximately 60 000 eggs for the total river areas of 10 000 m2. The number of smolts developed from the eggs spawned varied between 350 and 2400. 3. The relationship between smolt biomass in wet mass (kg 10 000 m−2) or energy (kJ 10 000 m−2) and the amount of salmon eggs in the River Imsa increased asymptotically. Annual smolt biomass ranged from 13 to 88 kg 10 000 m−2, or 66 000 and 431 000 kJ 10 000 m−2. Variation in egg density accounted for approximately 45% of the variation in smolt biomass (mass or energy). 4. Total wet mass and energy of adults (kg 10 000 m−2 and kJ 10 000 m−2) produced in relation to the amount of eggs at the start of the year-class, were not significantly correlated, due to a high variation among years. The biomass of adults ranged from 73 kg 10 000 m−2 to 655 kg 10 000 m−2 and in energy from 370 000 kJ 10 000 m−2 to 3 270 000 kJ 10 000 m−2. 5. Total adult biomass (adults caught at sea and in rivers) and the returning adults to the River Imsa in mass or energy were correlated with the size of the smolt cohort from which they originated. Yearly total adult biomass ranged between 240 and 3711 kg 10 000 m−2, when the number of smolts ranged from 397 to 2751, respectively. The biomass of adults returning to the River Imsa was between 59 and 614 kg, produced from between 672 and 1621 smolts.

Survival costs of chick rearing in black-legged kittiwakes

Abstract: 1. We tested for costs of chick rearing in the black-legged kittiwake Rissa tridactyla (Linnaeus) by removing entire clutches from 149 of 405 randomly selected nests, in which one or both mates was colour-banded. After the manipulation, we monitored adult nest attendance and body condition at unmanipulated and manipulated nests, and measured the survival and fecundity of these adults the following year. 2. Late in the chick-rearing period, adults from unmanipulated nests (i.e. with chicks) went on significantly longer foraging trips, and were significantly lighter for their size, than adults from manipulated nests (i.e. without chicks). 3. Adults from unmanipulated nests also survived to the following nesting season at a significantly lower rate than those from the manipulated nests (0·898 vs. 0·953), suggesting that attempting to raise chicks can reduce life expectancy by 55%. 4. There was a tendency for adults from nests that were unmanipulated in year one to have lower reproductive success in year two, primarily because of reduced fledging success, and a higher incidence of non-breeding. 5. These findings suggest that mass loss in kittiwakes during chick rearing may not be adaptive. Raising chicks can lead to reproductive costs, and the causal mechanism appears to be a reduction in body condition. 6. We compare our results with previous brood (or clutch) size manipulation experiments that have measured adult body condition, survival and/or future fecundity. Although the empirical evidence suggests that long-lived species are more likely to experience survival costs than short-lived species, we believe the opposite may be true. We suggest that shifting the experimental protocol of cost of reproduction studies from brood enlargements (an approach taken in most prior studies) to brood reductions will provide more accurate quantifications of naturally occurring costs. 7. The cost of reproduction is one mechanism proposed to explain the reduced survival rates reported for kittiwake populations in the North Atlantic relative to those in the North Pacific ocean. Oceanographic data, however, suggest that lower food availability may limit survival of kittiwakes in the North Atlantic where a deeper mixed layer and reduced primary production combine to make conditions less favourable for this seabird during the winter months.

Mortality during dispersal and the cost of host‐specificity in parasites: how many aphids find hosts?

Abstract: 1. For a full assessment of explanations for the evolution of host-specificity it is necessary to estimate the probability that a dispersing parasite finds a host. We develop a method of estimating this success rate from samples of dispersing parasites and populations resident on hosts. 2. Applying this method to data on the bird cherry-oat aphid, Rhopalosiphum padi (L.), from southern Scotland in 1984-92, we estimate that 0.6% of the autumn migrants find hosts. 3. With such a low success rate, there should be selection for a broadening of host range, to include any host on which the colonist's fitness is more than about 0.6% of that on the normal hosts. We argue that neither nutrition nor the need for 'enemy-free space' are sufficient explanations of the host-specificity of this animal, and propose instead that it is the host's role as a rendezvous for mating that constrains the migrants to their costly host-specificity. 4. We also discuss the implications of this low success rate for the hypothesis that aphids speciate sympatrically through the formation of host races.

Multi-species territoriality and dynamic of neotropical forest understorey bird flocks

Abstract: 1. Eleven contiguous mixed-species bird flocks, with colour-banded individuals, were monitored continuously during 3 years in a 132-ha study area of primary rainforest in French Guiana. 2. Flock members were divided into six categories according to their flocking propensity and occurrence: 10 core or permanent species and 56 regular, occasional or incidental species. Each core species was represented by a single breeding pair with their fledglings and extra ‘floaters’ (unmated subadults and adults). 3. Flock home ranges overlapped slightly, but were communally defended by all core species in areas of overlap. Their size varied from 3·2 to 14·3 ha and was inversely correlated with vegetation density, but not flock size or species composition. 4. Flock number, size and composition, as well as boundaries were highly stable between seasons and years. Each flock had a single permanent gathering site and bathing site in late afternoon, the latter sometimes shared by 2–3 flocks. 5. Core species produced 0·18–0·73 fledglings per pair per year, which stayed in their natal flock for 200 to over 421 days. Then, these individuals usually moved between two and six different flocks, sometimes for up to 3 years, before finding a mate and a flock where they could settle and breed. Once breeding, they probably remained for life in the same flock. The mean annual survival rate was at least 0·75. 6. This highly evolved and stable organization, associated with a low breeding success and high survival rate was a critical factor maintaining low species density, delayed reproduction and a proportion of floating individuals buffering population fluctuations. 7. These social groups with their multi-species territoriality and co-evolved roles of flock members were similar to those described elsewhere in South America. They seem to be a general phenomenon in neotropical lowland rainforests.

Diet overlap and relative abundance of sympatric dasyurid carnivores: a hypothesis of competition

Abstract: 1. Diet overlap among age and sex classes of sympatric dasyurid carnivores (Marsupialia: Dasyuridae) at Cradle Mountain National Park in Tasmania, Australia, was determined to assess the likelihood of current interspecific competition, which could influence and explain the disparate population densities of the three species. 2. The carnivore guild divided into two groups based on body size and prey size, within which diet overlapped: Tasmanian devils (Sarcophilus laniarius) and male spotted-tailed quolls (Dasyurus maculatus), which consumed larger prey species, and female spotted-tailed quolls and eastern quolls (D. viverrinus), which consumed smaller prey species. Male spotted-tailed quolls overlapped in diet with adult devils in winter, but not in summer. However, in summer the small number of male spotted-tailed quolls overlapped in both body weight and diet with a large cohort of young devils. Too few data were obtained to repeat these analyses with female and young spotted-tailed quolls and eastern quolls, but results indicated that a similar pattern of overlap may occur. 3. Spotted-tailed quolls would experience the highest degree of dietary overlap with another species of carnivore, with all age and sex classes experiencing overlap for much of the year. Adult devils and young eastern quolls would both be free of overlap for more than half the year. 4. No indications of seasonal food limitation, when competition is most likely to occur, were found during this study, but this may occur over a longer time scale. 5 If the high degree of diet overlap experienced by spotted-tailed quolls means higher competitive pressure, this may explain the low density of this species at Cradle Mountain. 6. These results, high levels of interference experienced by spotted-tailed quolls, and the behavioural and numerical dominance of devils, the largest species in the guild, support Brown & Maurer’s (1986) and Cotgreave’s (1993) ideas that competitive dominance may be more important than energetic equivalence in determining relationships between body size and abundance in local assemblages of animals.

Evolutionary consequences of habitat fragmentation in a localized butterfly

Abstract: 1 Theoretical studies of the costs and benefits of migration predict that evolutionary changes in dispersal traits may take place in response to habitat fragmentation 2 For the butterfly Plebejus argus, we investigated five morphological characters potentially associated with flight ability, in locations that varied in level of habitat fragmentation The traits were: total mass, relative thorax mass (containing flight muscle), relative abdomen mass (containing reproductive organs), relative wing area and wing aspect ratio All characters were measured on individuals reared in a common environment 3 Morphology was related to the level of habitat fragmentation in both limestone and heathland habitats Total mass increased with decreasing heathland habitat area (over the range 13 000–10 ha) Relative allocation to the thorax increased, while allocation to the abdomen decreased, with declining limestone habitat area (over the range 3·5–0·2 ha) Morphological characters were not significantly correlated with habitat isolation 4 Significant family effects for total mass, relative thorax mass, relative abdomen mass and (for females only) relative wing area indicate that these traits may have a heritable component, and therefore have the potential to respond to selection acting on flight ability 5 We suggest that evolutionary changes in life history traits are taking place in response to changes in landscape structure: in P argus, these traits may be influenced by the effects of mate-location strategy on emigration rates Specific changes in traits can be complex, and may vary among species and populations 6 P argus had a 50% chance of occurring in heathland fragments of 33 ha, and was present on all heathlands above 50 ha, but population systems may not yet have achieved equilibrium P argus is unlikely to undertake major evolutionary changes in response to reduced habitat area in heathlands > 50 ha Extinctions from smaller heathlands can be explained most plausibly by population/vegetation dynamics Therefore, evolutionary changes in morphology may be more likely to be symptomatic of populations with altered costs and benefits of migration, rather than to be a direct cause of extinction

Male mating success and risk of predation in a wolf spider: a balance between sexual and natural selection?

Abstract: 1 Traits that benefit males through sexual selection are simultaneously expected to impair males by provoking costs through natural selection If we consider the two male fitness components, mating success and viability, then we may expect that the increase in male mating success resulting from a larger trait size will be counterbalanced by an increase in viability costs 2 We studied the benefits and costs of male mate searching and sexual signalling activity in the wolf spider Hygrolycosa rubrofasciata In the field, males search females actively and court them by drumming dry leaves with their abdomen Females have been shown to prefer males with high drumming rate Male moving and especially drumming is energetically highly demanding and drumming results in significant mortality costs 3 Our objective in this study was to determine whether male mate-searching activity or drumming activity affect male mating success and the risk of males being predated 4 It was evident that both higher mate-searching activity and higher drumming activity benefited males by increasing their mating success Higher mate-searching activity clearly impaired males by causing direct increase in predation risk There was also a slight tendency that more actively drumming males had higher risk of predation and from all of the predated males 133% were caught directly after they had drummed Furthermore, male drumming activity decreased drastically in the presence of the predator 5 We conclude that in H rubrofasciata both increased mate-searching activity and drumming activity benefit males through sexual selection, but at the same time natural selection provokes direct balancing costs on the same traits

Cohort variation in male survival and lifetime breeding success in red deer

Abstract: We investigated cohort differences in the lifetime breeding success and survival of male red deer Cervus elaphus L. in an increasing population on the Isle of Rum, Scotland. There were significant differences in survival through different stages of the life span between 15 cohorts of males, ranging between: 0.26-1.00, calf survival through first winter; 0.56-1.00, yearling survival; 0.44-0.94, adult survival. This variation in survival was related to environmental conditions in the cohorts' year of birth, whilst controlling for annual effects. For 10 cohorts of males with complete lifetime data, mean breeding success also varied significantly, between 0.83 and 3.86. This variation, too, was associated with environmental conditions in the cohorts' year of birth. Since in many sexually dimorphic mammals, the growth and survival of males is more strongly affected by adverse environmental conditions than that of females, we expected that cohort variation would be more extreme amongst males than females. This was true, for both cohort survival and reproduction. We expected to observe selection on cohort adult body size, related to survival or reproduction. However, there was no evidence of variation in adult body size between cohorts, nor for relationships between differences in body size between cohorts and measures of survival or breeding success. Cohorts which underwent high intial mortality subsequently experienced higher adult survival than cohorts not subjected to high density-related selection early in life.

Plumage dichromatism of birds predicts introduction success in New Zealand

Abstract: 1. Reintroduction of plants and animals has become a common practice in the attempt to establish, re-establish or augment endangered populations. The fate of these translocation programmes is likely to depend, on the one hand, on several ecological attributes of the introduced species, on the other hand on introduction effort. 2. In a recent paper, Veltman, Nee & Crawley (1996) reported the correlates of introduction success of exotic birds released by humans in New Zealand before 1907: introduction effort (minimum number of release events and minimum number of released propagules) accounted for most of the variance in introduction success. 3. One of the factors potentially affecting the establishment of new populations concerns the intensity of sexual selection. Sexually selected species may be more vulnerable to extinction risks for several reasons (they may be more sensitive to both environmental and demographic stochasticity) and therefore have lower introduction success when compared to nonsexually selected species. 4. In the present study the data set reported by Veltman et al. (1996) was used to address the particular issue concerning sexual selection and introduction success. Each bird species was scored as monochromatic or dichromatic; plumage dichromatism is supposed to have evolved under sexual selection pressures. These scores were then correlated to the fate of introduction. 5. Plumage dichromatism was found to be, as expected, a significant predictor of introduction success, after removing the potential confounding effect of introduction effort. Dichromatic species had lower introduction success than monochromatic species. This pattern holds true for nonpasserines and passerines, although the correlation between plumage dichromatism and introduction success was not significant for the latter. 6. These results indicate that dichromatic species may experience reduced chances of establishing new populations when compared to monochromatic species. The presumed mechanisms involved in this phenomenon range from a reduced ability to adapt to a novel environment, to an increase in the risks of extinction through environmental and demographic stochasticity.

Costs of ant attendance for aphids

Abstract: 1. Interactions between aphids and ants are considered to be mutualistic, with both partners benefiting. Costs associated with such interactions are likely to be less obvious, although they can be expected, especially if these associations are facultative. 2. It is demonstrated here that there are costs in several life-history parameters to individual aphids resulting from ant attendance. Over several generations Aphis fabae cirsiiacanthoides feeding on Cirsium arvense, at a range of developmental stages, suffered significant costs when tended by Lasius niger, e.g. in terms of a prolonged developmental time, delayed offspring production, proportionally smaller gonads, fewer well developed embryos and a reduced mean relative growth rate. These effects are similar to those observed when aphids feed on poor quality plants. 3. This is the first indication that there is a cost for aphids associated with ant attendance. The significance of this for the evolution of ant attendance in aphids is discussed.

Host-specificity of folivorous insects in a moist tropical forest

Abstract: 1. To assess the degree of herbivore host-specificity in the moist tropical forest on Barro Colourado Island, Panama, I conducted an extensive series of feeding trials on the common insect herbivores from 10 tree species. 2. The herbivores were offered leaves from both congeneric and confamilial plant species to their known host species, as well as leaves from the most abundant tree species in the forest. 3. The amount of damage caused by these herbivores to young, expanding leaves was also measured on nine of the tree species. 4. Of 46 herbivores species (seven Coleoptera, one Orthoptera, 38 Lepidoptera), 26% were specialized to a single plant species, 22% were limited to feeding on a single genus and 37% were able to feed on several genera within a single family. The remaining 15% were generalists, able to feed from several different plant families. 5. The causes of leaf damage varied extensively across the tree species. On average, specialist herbivores caused 58% of the damage to young leaves, generalists herbivores 8% and fungal pathogens 34%. For four of the tree species, pathogens were the most important cause of leaf damage. 6. In this forest, most chewing herbivores appear to have fairly narrow diets, and these specialists are responsible for most of the insect herbivory.

Diurnal and seasonal mass variation in blackbirds Turdus merula: consequences for mass‐dependent predation risk

Abstract: 1. A bird’s mass in winter should reflect the trade-off between the benefits of an increased energy store to reduce starvation risk, and its costs in terms of increasing mass-dependent predation risk. State-dependent models of this trade-off predict that as starvation risk increases then energy reserves should (i), increase, and (ii) be acquired earlier in the day. 2. Blackbirds increased their minimum weight by 25. ±. 1·5% in midwinter; adults weighed significantly more than juveniles in midwinter, but weighed less in late winter. Weight for both sexes, and particularly adults, decreased with increasing daylength, but weight increases with respect to temperature decreases were most pronounced in males. Female blackbirds showed relatively small seasonal weight changes so that their midwinter mass-dependent predation risk relative to males was independent of diurnal weight changes. 3. Blackbirds lost at least 1–9% of their body weight overnight depending on temperature. Over 60% of the weight lost was then regained in the first 3 hours after dawn, with only 5% being regained after 12.00 h. The pattern of early morning weight gain was maintained throughout the winter, with the ratio of weight gain rates in the first 4. h of dawn compared to the rest of the day being c.. 3:1 in midwinter (when individuals maintained high weights) and in early spring (when individuals were losing weight seasonally). 4. The results agree with the theoretical prediction that energy reserves should increase as risk of starvation increased (as measured by seasonal factors such as cold, short midwinter days). There was, however, little change in the seasonal pattern of diurnal weight gain, with weight always being gained early in the day. In the likely absence of foraging constraints within the study system, this result fits the predictions of state-dependent foraging models where animals can escape from predation after achieving weight gains, by either using refuges or adopting low-risk foraging options.

Population dynamics of the Indian meal moth:Demographic stochasticity and delayed regulatory mechanisms.

Abstract: 1. Laboratory populations of the Indian meal moth [Plodia interpunctella (Hubner) (Lepidoptera: Pyralidae)], undergo sustained periodic fluctuations in abundance. The period is just longer than the generation time. The fluctuations are accentuated in the presence of the P. interpunctella granulosis virus (PiGV). 2. Time series spanning 8–10 generations from three replicate populations of the virus-free (VF) system and three from the virus-infected (VI) system are investigated using nonparametric autoregressive time series models. 3. The dynamics are concluded to correspond to a third order process consistent with interactions in a three-dimensional stage-structured model for both systems. The functionally different interactive stages are believed to be the egg stage (preyed upon by larvae), small larvae (competing for resources and cannibalized by large larvae) and large larvae (competing for resources). 4. The virus is seen as a modulator of the host vital rates more than an independent agent in a trophic host–pathogen interaction. The virus increases developmental time and decreases fecundity of the moths. 5. A significantly nonlinear additive autoregressive model of order 3 appears to give a parsimonious description of the series. 6. The demographic (birth and death) nature of the stochasticity inherent in the system is explicitly incorporated in the statistical model for the time series by assuming an overdispersed Poisson process. The variability around the skeleton is found to conform closely to this assumption. The demographic nature of the stochasticity cannot be fully understood on the basis of Gaussian (least-squares) models on transformed (variance-stabilized) data. 7. Significant density dependencies are found at a 1-week lag, a 2- to 3-week lag and at a 6- to 7-week lag. These are argued to be the signatures of within-stage competition, between-stage interactions and reproduction, respectively. Negative and statistically significant density dependence is apparent for the first two of these. No significant negative density dependence is apparent in the lag corresponding to reproduction. 8. The fluctuations in both the VF and VI system appear to represent limit cycles or weakly dampened cycles clothed by Poisson demographic stochasticity. 9. The enhanced cycles of the VI system are demonstrated to be consistent with a situation where the functional forms for the interactions are nearly the same as for the VF, but with delay structure shifted by just less than a week.

Small mammals cycles in northern Europe: patterns and evidence for a maternal effect hypothesis

Abstract: 1. Voles undergo pronounced oscillations over periods of 3–5 years in northern Europe. A latitudinal gradient of cycle periods and amplitudes has been reported for Fennoscandia, with periods and amplitudes increasing towards northern latitudes. 2. This study formulates a discrete time model based on maternal effects to explain the density fluctuation patterns of microtine rodents. The phenotypic transmission of quality from mothers to offspring generates delayed density dependence, which produces cyclic behaviour in the model. 3. The dynamic patterns predicted by the maternal effect model agree with data. We conclude that the maternal effect hypothesis is a plausible, parsimonious explanation for vole-density cycles in northern Europe.

Spatial dynamics of two competing specialist parasitoids in a host metapopulation

Abstract: 1. We investigated spatial dynamics in two specialist larval parasitoids, Cotesia melitaearum and Hyposoter horticola, attacking the Glanville fritillary butterfly, Melitaea cinxia, in Finland. 2. Presence of C. melitaearum in a host larval group significantly reduced the rate of parasitism by H. horticola (18 vs. 33% in groups with and without C. melitaearum), but there was no significant reverse effect. The parasitism rates at the level of local populations showed a similar trend. 3 Hyposoter horticola females moved frequently among larval groups, whereas C. melitaearum females often spent several days at a single larval group. In agreement with these behavioural observations about movements within populations, H. horticola was found to have a higher colonization rate of host populations than C. melitaearum. 4. At the within-population level, C. melitaearum tended to occupy large larval groups in the centre of the host population, whereas H. horticola parasitized also small and more isolated (peripheral) larval groups, especially in the presence of C. melitaearum. At the metapopulation level, host population size had a significant positive effect on the presence of local populations of both parasitoid species, but isolation had a significant negative effect on the presence of C. melitaearum only. 5. These results suggest that C. melitaearum is a superior competitor, but an inferior disperser to H. horticola, which facilitates the co-existence of the two species both at the level of local populations and at the level of a metapopulation. 6. Data from a 50-patch network showed a decline in the number of host populations from 34 to 13 in 4 years. This decline caused a near-extinction of the superior competitor, but inferior disperser, C. melitaearum, whereas the abundance of the inferior competitor, but superior disperser, H. horticola, remained relatively constant, in agreement with the prediction of a theoretical model.

The interactive effects of age and food supply on the breeding ecology of great skuas

Abstract: Age-specific variation in the reproductive performance of great skuas in Shetland, U.K., was investigated during a period of fluctuating sandeel availability between 1988 and 1993. Increased sandeel abundance was associated with earlier laying dates, increased clutch volumes and improved fledging success. Sandeel abundance had no effects on clutch size and hatching success. Parental age improved haying date and clutch size in a nonlinear manner. Laying date became earlier in the youngest age classes, with the effect being negligible in older birds. Clutch size increased with age up to 18 and then declined in older birds. Clutch volume increased with age and the probability of nest predation declined with age. There were no significant interactive effects of age and year on laying date nor clutch volume, indicating that birds of all ages benefited equally from increases in sandeel abundance. This suggests that lack of breeding experience rather than deficient foraging skills are responsible for young birds laying later. There was a significant interactive effect of year and age on fledging success that was associated with changes in sandeel availability. Success was uniformly low for birds of all ages in 1988-90 when sandeel abundance was low. Success increased with age in 1991 and 1992 during which time food supply improved. During 1993 food supply was abundant and fledging success was high in all age classes. Age-specific improvements in laying date, clutch size and nest predation were best explained by learning of skills associated with experience of breeding. Improvements in fledging success with age were probably best explained by the learning of skills associated with foraging.

The effect of age at first breeding on Ural owl lifetime reproductive success and fitness under cyclic food conditions

Abstract: 1. Individuals are expected to balance the costs and benefits underlying the trade-off between current and future reproduction. If starting to breed does not seriously lower future reproductive output, individuals that start breeding early in their life should have a higher fitness than individuals that postpone their breeding career. We studied how the fluctuations in food supply interacted with Ural owl’s age at first breeding, lifetime reproductive success (LRS) and fitness. 2. During the period 1977–95, 126 Ural owl females started and ended their breeding career in a study area in southern Finland. Voles, the owls’ main food source, showeda 3-year cycle of low, increase and peak population numbers. We recorded when the females started to breed and how many fledglings they produced. For 57 females the age at first breeding was known. 3. LRS of female Ural owls varied from 0 to 33 fledglings (mean 6.7±0.52 SE). The variance in LRS was explained by variation in the components: breeding lifespan (97%); nest success (23%); and average clutch size (15%). 4. Survival of breeding females was low (62%) after a peak year, when the vole population crashed. In other phases the survival was 85–95%. Females that started breeding in a peak year had half the LRS of females that started in an increase year. 5. There was a strong interaction between the vole cycle and age at first breeding. 1-year-olds started in a peak and 2-year-olds in an increase year. 6. There was no effect of age at first breeding on LRS for females that started breeding in the same phase of the vole cycle. 7. Females that started breeding at age 1–3 years had equal fitness, whereas females that started at age ≥4 had a lower fitness. Females that postponed first breeding as a two-year-old in an increase year had a lower fitness than females that did not do so. Females that postponed first breeding as a 1-year-old in a peak year had equal fitness to females that did not do so. 8. Cyclic fluctuation in food supply clearly constrains the option as to during what phase and at what age to start breeding. In terms of fitness, the optimal age to start breeding depends on the phase of the vole cycle at hatching.