Showing papers in "Journal of Animal Ecology in 2005"

Planning for success: Serengeti lions seek prey accessibility rather than abundance

Abstract: Summary 1. We used long-term radio-telemetry data to investigate how Serengeti lions ( Panthera leo ) distribute themselves with respect to hunting opportunities. Specifically, we investigate whether lions hunt in areas where prey are easy to capture or where prey are locally abundant. 2. We used resource-selection functions (logistic regressions) to measure the location of kills/carcasses with respect to five different habitats: the view-sheds from large rocky outcrops, river confluences, woodland vegetation, erosion embankments and water sources. 3. As expected for a sit-and-wait predator, resting lions spent more time in areas with good cover. On a broad-scale, lions shifted their ranges according to the seasonal movement of prey, but at a finer scale (< 100 m) lions fed in areas with high prey ‘catchability’ rather than high prey density. Plains lions selected erosion embankments, view-sheds from rocky outcrops, and access to free water. Woodland lions tended to use erosion embankments, and woody vegetation. 4. The results emphasize the importance of fine-scale landscape and habitat features when assessing predator‐prey theory and conservation.

Pairs of extreme avian personalities have highest reproductive success

Abstract: 1. Intraspecific variation in reproductive decisions is generally considered as a reaction to environmental circumstances. We show that variation in reproductive parameters also covaries with intraspecific variation in personality. 2. During 4 years, we studied reproductive parameters in a natural population of great tits in association with a personality trait: exploratory behaviour as measured in a novel environment. 3. Nest success, fledgling size and condition were all correlated with this personality trait. Slow-exploring females had a higher nest success and largest fledglings. Fledgling condition was affected by the interaction between male and female exploratory behaviour, with assortative pairs at both ends of the behavioural spectrum producing fledglings in best condition. Fast-exploring males bred in nestboxes that produced heavy fledglings in other years. 4. We hypothesize that fast-exploring individuals are better able in defending or obtaining a high quality territory, while slow-exploring individuals are either better parents or have better chicks which may, in part, explain the patterns in reproductive success. We discuss how these patterns in reproduction can explain earlier reported relationships between offspring recruitment and avian personality and may result in the maintenance of intraspecific genetic variation in personality.

Are kernels the mustard? Data from global positioning system (GPS) collars suggests problems for kernel home- range analyses with least-squares cross-validation

Abstract: 1. Kernel-density estimation (KDE) is one of the most widely used home-range estimators in ecology. The recommended implementation uses least squares cross-validation (LSCV) to calculate the smoothing factor (h) which has a considerable influence on the home-range estimate. 2. We tested the performance of least squares cross-validated kernel-density estimation (LSCV KDE) using data from global positioning system (GPS)-collared lions subsampled to simulate the effects of hypothetical radio-tracking strategies. 3. LSCV produced variable results and a 7% failure rate for fewer than 100 locations (H = 2069) and a 61% failure rate above 100 points (n = 1220). Patterns of failure and variation were not consistent among lions, reflecting different individual space use patterns. 4. Intensive use of core areas and site fidelity by animals caused LSCV to fail more often than anticipated from studies that used computer-simulated data. 5. LSCV failures at large sample sizes and variation at small sample sizes, limits the applicability of LSCV KDE to fewer situations than the literature suggests, and casts doubts over the method's reliability and comparability as a home-range estimator. © 2005 British Ecological Society.

Root herbivore effects on above-ground herbivore, parasitoid and hyperparasitoid performance via changes in plant quality

Abstract: 1. Plants and insects are part of a complex multitrophic environment, in which they closely interact. However, most of the studies have been focused mainly on bi-tritrophic above-ground subsystems, hindering our understanding of the processes that affect multitrophic interactions in a more realistic framework. 2. We studied whether root herbivory by the fly Delia radicum can influence the development of the leaf feeder Pieris brassicae, its parasitoid Cotesia glomerata and its hyperparasitoid Lysibia nana, through changes in primary and secondary plant compounds. 3. In the presence of root herbivory, the development time of the leaf herbivore and the parasitoid significantly increased, and the adult size of the parasitoid and the hyperparasitoid were significantly reduced. The effects were stronger at low root fly densities than at high densities. 4. Higher glucosinolate (sinigrin) levels were recorded in plants exposed to below-ground herbivory, suggesting that the reduced performance of the above-ground insects was via reduced plant quality. Sinigrin contents were highest in plants exposed to low root fly densities, intermediate in plants exposed to high root fly densities and lowest in plants that were not exposed to root herbivory. 5. Our results show, for the first time, that root herbivory via changes in plant quality can reduce the performance of an above-ground multitrophic level food chain. This underlines the importance of integrating a broader range of above- and below-ground organisms to facilitate a better understanding of complex multitrophic interactions and interrelationships. [KEYWORDS: above–below-ground interactions ; Cotesia glomerata ; Lysibia nana ; Pieris brassicae ; plant–insect interactions]

The effect of climate on adult survival in five species of North Atlantic seabirds

Abstract: Summary 1 In long-lived species, adult survival is an important life-history trait. Better knowledge of the effects of non-catastrophic climate variation on the adult survival of long-lived seabirds is therefore needed. However, documentation of such effects is still rare. 2 Using capture–mark–resighting data, we modelled the annual survival rates of five species of seabirds, the common guillemot (Uria aalge), Brunnich's guillemot (Uria lomvia), razorbill (Alca torda), Atlantic puffin (Fratercula arctica) and black-legged kittiwake (Rissa tridactyla). The data spanned 14 years of observation at a colony on Hornoya, off Northern Norway in the western Barents Sea. 3 A method of taking into account ring-loss of marked individuals is proposed. This approach made it possible to keep many observations which otherwise would have to be discarded. 4 The estimated survival rates were higher and less variable than most estimates for the same species from other colonies. There was extensive resighting heterogeneity. Ignoring this effect would have underestimated breeding life span by more than 50% in some species. 5 The analysis presented is the first study to document a correlation between the North Atlantic Oscillation (NAO) index and adult survival in North Atlantic seabirds. Other environmental variables considered were sea surface temperatures (SST) and prey stocks. In most of the species, models incorporating climatological variables were considerably better supported than models with constant survival, time-dependent survival or prey effects. SSTs tended to explain more of the variability in seabird survival than did the NAO. 6 The importance of prey availability was evident in some of the species, but climate was generally a better predictor of survival. Nevertheless, the evidence suggests that meteorological parameters affect seabird mortality only indirectly, possibly through the food chain. This conclusion rests on the observations that most NAO effects are lagged, and that survival rates decreased with increasing SSTs.

Resource dilution effects on specialist insect herbivores in a grassland biodiversity experiment

Abstract: 1. The resource concentration hypothesis predicts that specialist insect herbivores attain higher loads (density per unit mass of the host-plant species) when their food plants grow in high-density patches in pure stands. 2. We tested the resource concentration hypothesis for nine specialist insect herbivore species sampled from a field experiment where plant diversity had been manipulated experimentally, generating gradients of host-plant abundance. 3. The specialist insects responded to varying host-plant abundance in two contrasting ways: as expected, specialist herbivore species were more likely to be present when their host-plant species were abundant; however, counter to predictions, in plots where specialists were present we found strong negative linear relationships between herbivore loads and host-plant abundances - a 'resource dilution' rather than concentration effect. 4. Increased plant species-richness had an additional, but weak, negative influence on loads beyond that due to host-plant abundance. 5. We discuss the implications of resource dilution effects for biodiversity manipulation experiments and for the study of plant–herbivore interactions more generally.

Predation cost of rapid growth: behavioural coupling and physiological decoupling

Abstract: Summary 1. Despite its prominent role in life-history theory, there is no direct empirical evidence for a behaviourally mediated predation cost of rapid growth. Moreover, we know little about how digestive physiology may also influence the shape of the growth/predation risk trade-off function. 2. We determined the role of behaviour and digestive physiology in experiments in which damselfly larvae were induced to grow slowly or rapidly by manipulating photoperiod (time stress), and exposure to a fish predator. 3. We showed that larvae under time stress grew more rapidly. Rapid-growing larvae had a higher foraging activity and a higher growth efficiency. 4. Under predation risk, larvae not only had a lower foraging activity but also a lower growth efficiency. 5. Rapid-growing larvae (i.e. those under time stress) balanced the growth/predation risk trade-off differently and took more risk in the presence of a predator, which resulted in a behaviourally mediated higher predation cost compared to slow-growing larvae. Their higher growth efficiency, however, made this cost smaller compared to a completely behaviourally mediated rapid-growth strategy. 6. Our results provide the first explicit experimental proof of a behaviourally mediated predation cost of rapid growth. Besides a behavioural coupling of growth and predation risk, resulting in the well-known trade-off, we also found a partial decoupling of these two processes by digestive physiology.

Species abundance and the distribution of specialization in host–parasite interaction networks

Abstract: Summary 1. Recent studies have evaluated the distribution of specialization in species interaction networks. Species abundance patterns have been hypothesized to determine observed topological patterns. We evaluate this hypothesis in the context of host‐parasite interaction networks. 2. We used two independent series of data sets, one consisting of data for seven sites describing interactions between freshwater fish and their metazoan parasites and another consisting of data for 25 localities describing interactions between fleas and their mammalian hosts. We evaluated the influence of species abundance patterns on the distribution of specialization in these host‐parasite interaction networks with the aid of null models. 3. In parallel with recent studies of plant‐animal mutualistic networks, our analyses suggest that host‐parasite interactions in these systems are highly asymmetric: specialist parasites tend to interact with hosts with high parasite richness, whereas hosts with low parasite richness tend to interact mainly with generalist parasites. 4. The observed distribution of specialization was predicted by a null model that assumed that species-specific probabilities of being assigned a link during the randomization process were roughly proportional to their relative abundance. Thus, abundant hosts tend to harbour richer parasite faunas, with a high proportion of rare specialists.

Habitat-specific normal and reverse diel vertical migration in the plankton-feeding basking shark

Abstract: Summary 1. Megaplanktivores such as filter-feeding sharks and baleen whales are at the apex of a short food chain (phytoplankton‐zooplankton‐vertebrate) and are sensitive indicators of sea-surface plankton availability. Even though they spend the majority of their time below the surface it is still not known how most of these species utilize vertical habitat and adapt to short-term changes in food availability. 2. A key factor likely to control vertical habitat selection by planktivorous sharks is the diel vertical migration (DVM) of zooplankton; however, no study has determined whether specific ocean-habitat type influences their behavioural strategy. Based on the first high-resolution dive data collected for a plankton-feeding fish species we show that DVM patterns of the basking shark Cetorhinus maximus reflect habitat type and zooplankton behaviour. 3. In deep, well-stratified waters sharks exhibited normal DVM (dusk ascent‐dawn descent) by tracking migrating sound-scattering layers characterized by Calanus and euphausiids. Sharks occupying shallow, inner-shelf areas near thermal fronts conducted reverse DVM (dusk descent‐dawn ascent) possibly due to zooplankton predator‐prey interactions that resulted in reverse DVM of Calanus . 4. These opposite DVM patterns resulted in the probability of daytime-surface sighting differing between these habitats by as much as two orders of magnitude. Ship-borne surveys undertaken at the same time as trackings reflected these behavioural differences. 5. The tendency of basking sharks to feed or rest for long periods at the surface has made them vulnerable to harpoon fisheries. Ship-borne and aerial surveys also use surface occurrence to assess distribution and abundance for conservation purposes. Our study indicates that without bias reduction for habitat-specific DVM patterns, current surveys could under- or overestimate shark abundance by at least 10-fold.

Scale‐dependent habitat use in a long‐ranging central place predator

Abstract: Summary 1. It is predicted that the movements of foraging animals are adjusted to the hierarchical spatial distribution of resources in the environment, and that decisions to modify movement in response to heterogeneous resource distribution are scale-dependent. Thus, controlling for spatial scales of interaction with environment is critical for a better understanding of habitat selection, which is likely to follow scale-dependent processes. 2. Here we study the scales of interactions and habitat selection in a long-ranging marine predator foraging from a central place, the yellow-nosed albatross. We use firstpassage time analysis to identify the scales of interaction with environmental variables and compositional analysis to study habitat selection. 3. Of 26 birds, 22 adopted an area restricted search (ARS) at a scale of 130 ± 85 km, and 11 of these 22 birds adopted a second, nested ARS scale at 34 ± 20 km. Habitat use differed according to the spatial scale considered. At the oceanic basin macro-scale, birds foraged in pelagic, subtropical waters. Birds commuted to the ARS zones after a c. 1500-km trip to reach predictable turbulence zones from Agulhas return current, where primary productivity was enhanced at large scale. At a smaller, meso-scale, birds increased their search effort according to sea surface height anomalies (SSHa) and chlorophyll- a concentrations (Chl- a ), indicating association with productive cyclonic eddies. 4. Among birds, differences in search pattern were noted: 11 birds concentrated their search effort directly at a small scale of 77 ± 22 km, avoiding anticyclonic eddies. The 11 other birds showed two scales of ARS pattern: (i) first at 180 ± 90 km with a preference for high Chl- a concentrations but unrelated to SSHa; and (ii) secondly at a nested scale at 34 ± 20 km related exclusively to SSHa where prey patches were expected to be distributed at this scale. This second group of birds appeared to be less efficient, spending more time at sea for the same mass gain than the first group. 5. Our study is the first to demonstrate scale-dependent adjustments, with interindividual variability, in relation to environmental features for predators with a central-place constraint.

Importance of parasites and their life cycle characteristics in determining the structure of a large marine food web

Abstract: Summary 1Despite their documented effects on trophic interactions and community structure, parasites are rarely included in food web analyses. The transmission routes of most parasitic helminths follow closely the trophic relationships among their successive hosts and are thus embedded in food webs, in a way that may influence energy flow and the structure of the web. 2We investigated the impact of parasitism on the food web structure of a New Zealand intertidal mudflat community. Different versions of the food web were analysed, one with no parasites, one with all parasite species and several other versions, each including a single parasite species. We measured key food web metrics such as food chain length, linkage density and proportions of top, intermediate and basal species. 3The inclusion of all parasite species in the food web resulted in greatly increased mean and maximum food chain length, but had little impact on linkage density and realized connectance. The main change caused by introduction of parasites was the relegation of a number of species from top predators to intermediate status, although the addition of parasites as top predators left the actual ratio of predators to prey relatively unchanged. 4When individual parasites were added to the food web, their effect on food web properties was generally minimal. However, one trematode species that affected several host species, because of its complex life cycle and low host specificity, produced food web properties similar to those in the web version including all parasite species. 5The respective effect of individual parasite species was roughly proportional to the number of host species they affected, and thus the life cycle characteristics of parasites determine to a large extent their impact on food web structure. The next step would be to quantify how they affect energy flow through the web.

Climate change, ice conditions and reproduction in an Arctic nesting marine bird: Brunnich's guillemot (Uria lomvia L.)

Abstract: Summary 1 We compared the reproduction of a marine diving bird, Brunnich's guillemot (Uria lomvia), breeding at two Arctic colonies close to the northern and southern limits of the species’ range in the Canadian Arctic. 2 At both colonies, timing of breeding for Brunnich's guillemots was positively correlated with summer ice cover, which was determined by winter and spring temperatures. Spring temperatures also modified the effects of ice conditions on timing of breeding. 3 At Coats Island, northern Hudson Bay, in low Arctic waters, the date of egg-laying has advanced since 1981, simultaneous with a decrease in summer ice cover in surrounding waters. Lower ice cover in this region is correlated with lower chick growth rates and lower adult body mass, suggesting that reduction in summer ice extent is having a negative effect on reproduction. 4 Conversely, at Prince Leopold Island, in the High Arctic, there has been no trend in summer ice cover and no detectable change in timing of breeding. Reproduction there is less successful in years of late ice than in years of early ice break-up. 5 Current trends suggest that continued warming should benefit birds breeding on the northern limit of the species range, while adversely affecting reproduction for those on the southern margin. The probable result will be an eventual northward displacement of the population. Although this type of effect has been widely predicted, this study is among the first to demonstrate a potential causal mechanism.

Describing breeding territories of migratory passerines: suggestions for sampling, choice of estimator, and delineation of core areas

Abstract: Summary 1. The goals of this study were to investigate the possibility of using kernel techniques to estimate male breeding territory size and delineate core areas, focusing on a small nontransmitter bearing bird, the cerulean warbler. We then compared the performance of kernel estimators with traditionally used minimum convex polygons (MCP). 2. Given the lack of a consistent across-male sample size‐area relationship, we opted to use each male’s full set of locations in the kernel calculation rather than standardizing sample size across males. 3. All locations collected for each male were biologically independent though statistically autocorrelated. Subsampling locations did not achieve independence even at time intervals far exceeding biological independence. 4. The physical space bounded by kernel and MCP methods differed drastically in certain cases, especially in situations where there were large areas within a territory that were never visited during our data collection sessions. 5. Kernel methods of territory estimation were far more accurate and informative than MCP for cerulean warblers. We suggest that evenly sampling individuals in a biologically relevant manner during a strictly defined study period is more important than standardizing sample size across individuals. Furthermore, sampling regimes can safely be guided by biological vs. statistical independence timelines. 6. Avian biologists should consider kernel estimators as an option especially for habitat selection studies where accurate territory boundary and size estimation is crucial.

Density-dependent growth of young-of-the-year Atlantic salmon Salmo salar in Catamaran Brook, New Brunswick

Abstract: Summary 1 While density-dependent mortality and emigration have been widely reported in stream salmonid populations, density-dependent growth is less frequently detected. A recent study suggests that density-dependent growth in stream salmonids occurs at low densities, whereas density-dependent mortality and emigration occur at high densities. 2 To test the hypothesis that density-dependent growth occurs primarily at low rather than at high densities, we examined the relationship between average fork length and population density of young-of-the-year (YOY) Atlantic salmon at the end of the growing season using a 10-year data set collected on Catamaran Brook, New Brunswick. We tested whether (1) average body size decreases with increasing density; (2) the effect of density on average body size is greatest at low densities; (3) growth rate will decrease most rapidly at low effective densities [Σ(fork length)2]; (4) density-dependent growth is weaker over space than over time; and (5) the strength of density-dependent growth increases with the size of the habitat unit (i.e. spatial scale) when compared within years, but not between years. 3 There was a strong negative relationship between the average body size and population density of YOY Atlantic salmon in the autumn, which was best described by a negative power curve. Similarly, a negative power curve provided the best fit to the relationship between average body size and effective density. Most of the variation in average body size was explained by YOY density, with year, location and the density of 1+ and 2+ salmon accounting for a minor proportion of the variation. 4 The strength of density-dependent growth did not differ significantly between comparisons over space vs. time. Consistent with the last prediction, the strength of density-dependent growth increased with increasing spatial scale in the within-year, but not in the between-year comparisons. 5 The effect of density on growth was strongest at low population densities, too low to expect interference competition. Stream salmonid populations may be regulated by two mechanisms: density-dependent growth via exploitative competition at low densities, perhaps mediated by predator-induced reductions in drift rate, and density-dependent mortality and emigration via interference competition at high densities.

Species richness estimators: how many species can dance on the head of a pin?

Abstract: Summary 1Several species richness estimators (two non-parametric, four based on rarefaction curves and two from fitting of abundance distributions) were compared by examining their performance in estimating the species richness for two moth data sets from the United Kingdom. Comparisons were also made using data simulated from the fitted abundance distributions. 2The different species richness estimators gave different estimates. The non-parametric estimates and the rarefaction estimates were similar, but were smaller than the parametric estimates. When the simulated data were used, the only methods to give estimates near the true value was the parametric method using the distribution from which the data were simulated. 3At present it is impossible to decide whether any of the estimation methods will give a realistic estimate, as not enough is known about the true numbers of species in communities. Until this is rectified, the most that can be hoped for is to obtain upper and lower bounds on species richness.

Relationships between taxonomic resolution and spatial scales of multivariate variation

Abstract: Summary 1. We investigated the effects of changes in taxonomic resolution on analyses of patterns of multivariate variation at different spatial scales for the highly diverse fauna inhabiting holdfasts of the kelp Ecklonia radiata . 2. Multivariate analyses were performed using several transformations to examine differences in spatial patterns of variation from metres up to hundreds of kilometres for composition vs. relative abundance in assemblages. 3. The greatest variability in assemblages occurred at the smallest spatial scale, from plant to plant, pointing to the existence of important small-scale processes. The proportional amount of variation at the smallest spatial scale decreased with decreasing taxonomic resolution (i.e. from species through to phyla). For composition, the next-greatest source of variation was at the largest spatial scale (hundreds of kilometres), while for relative abundance, the next-greatest source of variation was at the level of sites (hundreds of metres to kilometres). 4. For abundance data, location-level variation became less important and site-level variation became more important with decreasing taxonomic resolution, while for compositional data, the opposite pattern occurred. This suggests that variation in the presence of species or taxa at a particular location along the coast is driven by large-scale processes, while variation in relative abundances within locations is driven by mediumscale processes. 5. The lack of significant variation in the proportional abundances of phyla at large spatial scales suggests that some consistency of pattern may emerge at larger scales (spatial and/or taxonomic), even in the presence of high small-scale variability. These findings strengthen the idea that substantial local variation need not preclude the existence of broad-scale organization in ecological patterns and biodiversity.

Predictors of reproductive cost in female Soay sheep

Abstract: 1. We investigate factors influencing the trade-off between survival and reproduction in female Soay sheep (Ovis aries). Multistate capture-recapture models are used to incorporate the state-specific recapture probability and to investigate the influence of age and ecological conditions on the cost of reproduction, defined as the difference between survival of breeder and non-breeder ewes on a logistic scale. 2. The cost is identified as a quadratic function of age, being greatest for females breeding at 1 year of age and when more than 7 years old. Costs, however, were only present during severe environmental conditions (wet and stormy winters occurring when population density was high). 3. Winter severity and population size explain most of the variation in the probability of breeding for the first time at 1 year of life, but did not affect the subsequent breeding probability. 4. The presence of a cost of reproduction was confirmed by an experiment where a subset of females was prevented from breeding in their first year of life. 5. Our results suggest that breeding decisions are quality or condition dependent. We show that the interaction between age and time has a significant effect on variation around the phenotypic trade-off function: selection against weaker individuals born into cohorts that experience severe environmental conditions early in life can progressively eliminate low-quality phenotypes from these cohorts, generating population-level effects.

Weather packages: finding the right scale and composition of climate in ecology

Abstract: Summary 1. Animals are affected by local weather variables such as temperature, rainfall and snow. However, large-scale climate indices such as the North Atlantic Oscillation (NAO) often outperform local weather variables when it comes to explain climate-related variation in life history traits or animal numbers. 2. In a recent paper, Hallet et al . (2004, Nature , 430 , 71‐75) document convincingly why this may happen. In this perspective, we identify from the literature three mechanisms why this is so: (1) the time window; (2) the spatial window; and (3) the weather composition component of climate. 3. Such an understanding may be used to derive even better ‘weather packages’ than the NAO.

Decomposing the variation in population growth into contributions from multiple demographic rates

Abstract: Summary 1 The decomposition of variation in population growth into the relative contributions from different demographic rates has multiple uses in population, conservation and evolutionary biology Recent research has favoured methods based on matrix models termed ‘life-table-response experiments’ or more generally ‘the retrospective matrix method’, which provide an approximation of a complete demographic decomposition The performance of the approximation has not been assessed 2 We compare the performance of the retrospective matrix method to a complete decomposition for two bighorn sheep populations and one red deer population 3 Different demographic rates make markedly different contributions to variation in growth rate between populations, because each population is subject to different types of environmental variation 4 The most influential demographic rates identified from decomposing observed variation in population growth are often not those showing the highest elasticity Consequently, those demographic rates most strongly associated with deterministic population growth are not necessarily strongly associated with temporal variation in population growth 5 The retrospective matrix method provides a good approximation of the demographic rate associated most strongly with variation in population growth However, failure to incorporate the contribution of covariation between demographic rates when decomposing variation in population growth can lead to spurious conclusions

Variation of primary production during winter induces synchrony in survival rates in migratory white storks Ciconia ciconia

Abstract: Summary 1. The environmental conditions that migratory birds experience during their stay at different locations throughout the year can have significant impact on the variation of annual survival rates. Because migrants often crowd during non-breeding, environmental conditions can affect survival rates of individuals originating from different breeding populations, thus also provoking spatial synchronisation of annual survival rates. The identification of critical environmental factors affecting survival is therefore crucial for understanding large-scale population dynamics of migrants. 2. We studied temporal and spatial variation of survival rates of migratory white stork Ciconia ciconia from eastern Germany and Poland and examine factors associated with this variation. We used resighting and dead-recovery data from more than 30 000 individuals sampled over 19 years to estimate survival. 3. Survival rates of juvenile and adult white storks originating from the two countries varied in parallel over time. Thus, variation in survival was caused by environmental variation to which individuals of both age classes from both countries were similarly sensitive. Variation in the primary production (as reflected by the normalized difference vegetation index) at one staging area in the eastern Sahel that is visited from October to November contributed up to 88% to the temporal variation in survival. Annual survival was reduced when primary production in the Sahel was low. 4. As the population growth rate of white storks is very sensitive to changes in survival, variation in primary production in the Sahelian staging area contributed significantly to population changes and is likely to be an important reason for population synchrony. 5. This study demonstrates that dynamics of migratory bird populations inhabiting large breeding areas can result from environmental variation encountered at a staging area that is visited for only a short period of time.

Digestive bottleneck affects foraging decisions in red knots Calidris canutus. I. Prey choice

Abstract: Summary 1. Rate-maximizing foragers that only divide their time between searching and handling prey should, according to the classical contingency model (CM), only select those prey whose energy content per unit handling time (i.e. profitability) exceeds or equals long-term average energy intake rate. 2. However, if digestively constrained foragers were to follow this so-called ‘zero-one rule’, they would need to take digestive breaks and their energy intake over total time would not be maximized. They should, according to the digestive rate model (DRM), also consider the rate at which a prey type is digested (i.e. digestive quality), such that time lost to digestive breaks is minimized. 3. In three different contexts, we tested these competing models in a mollusc-eating shorebird, the red knot ( Calidris canutus ), that is often digestively constrained due to its habit of ingesting its bulky prey whole. Measurements on gizzard size (using ultrasonography) and prey-characteristics confirmed that in each test the birds were digestively bottlenecked and should thus follow the DRM in order to maximize long-term energy intake. 4. In the first experiment, knots were offered a choice between two fully exposed prey, and tended to select prey by the criterion of digestive quality rather than profitability. 5. In the second experiment, knots were offered two buried prey types and preferred the highest quality prey to the most profitable prey. 6. In the wild, knots mainly fed on high quality Mya and largely ignored poor quality, but equally profitable, Cerastoderma . 7. Thus, each test verified the predictions of the DRM and rejected those of the CM. Given that many species face digestion constraints, we expect that the DRM is likely to explain diet composition in many more studies.

Correlates of survival rates for 10 African ungulate populations: density, rainfall and predation

Abstract: Summary 1 Through reconciling census totals with population structure, annual survival rates were estimated for the juvenile, yearling and adult stages of 10 ungulate species over 14 years or longer in South Africa's Kruger National Park. During this period four species maintained high abundance levels, while six species declined progressively in abundance. 2 Multiple regression models fitted to these estimates indicated that juvenile survival was sensitive to annual variability in rainfall for most of these species, especially in the dry season component, but with no density feedback apparent. Rainfall components affected adult survival in several of the declining species, while negative density dependence in adult survival was evident for three of the four species that maintained high abundance. A negative effect of past prey availability, indexing putative changes in predator abundance, on adult survival was more strongly supported statistically among the declining species than the lagged effect of prior rainfall, potentially affecting herbaceous vegetation cover and composition. 3 The high sensitivity of juvenile survival to environmental variability among these ungulate species was consistent with the general pattern identified for large mammalian herbivores, although the absence of any survival response counteracting the density declines was surprising. The susceptibility of adult survival to environmental influences for the declining species appeared unusual and probably reflected an interaction between nutritional shortfalls and a numerical increase in lions, preying largely upon the adult segment of these species. The ungulate species that persisted at high abundance seemed resistant to effects of rainfall on food resources and evidently drove the changes in predator abundance. The sharp density effect on adult survival among these species could indicate prey switching by lions following changes in their relative availability. 4 Findings extend past generalizations about the demographic processes underlying the population dynamics of large mammalian herbivores and reveal how the survival rates of particular population segments respond differently to environmental influences. Demographic patterns help reveal the interplay of changing resource supplies, predation pressure and population abundance on population changes.

The lognormal distribution is not an appropriate null hypothesis for the species–abundance distribution

Abstract: Summary 1. Of the many models for species‐abundance distributions (SADs), the lognormal has been the most popular and has been put forward as an appropriate null model for testing against theoretical SADs. In this paper we explore a number of reasons why the lognormal is not an appropriate null model, or indeed an appropriate model of any sort, for a SAD. 2. We use three empirical examples, based on published data sets, to illustrate features of SADs in general and of the lognormal in particular: the abundance of British breeding birds, the number of trees > 1 cm diameter at breast height (d.b.h.) on a 50 ha Panamanian plot, and the abundance of certain butterflies trapped at Jatun Sacha, Ecuador. The first two are complete enumerations and show left skew under logarithmic transformation, the third is an incomplete enumeration and shows right skew. 3. Fitting SADs by χ 2 test is less efficient and less informative than fitting probability plots. The left skewness of complete enumerations seems to arise from a lack of extremely abundant species rather than from a surplus of rare ones. One consequence is that the logit-normal, which stretches the right-hand end of the distribution, consistently gives a slightly better fit. 4. The central limit theorem predicts lognormality of abundances within species but not between them, and so is not a basis for the lognormal SAD. Niche breakage and population dynamical models can predict a lognormal SAD but equally can predict many other SADs. 5. The lognormal sits uncomfortably between distributions with infinite variance and the log-binomial. The latter removes the absurdity of the invisible highly abundant half of the individuals abundance curve predicted by the lognormal SAD. The veil line is a misunderstanding of the sampling properties of the SAD and fitting the Poisson lognormal is not satisfactory. A satisfactory SAD should have a thinner right-hand tail than the lognormal, as is observed empirically. 6. The SAD for logarithmic abundance cannot be Gaussian.

Metapopulation consequences of site fidelity for colonially breeding mammals and birds

Abstract: Summary 1. Many far-ranging bird and mammal species aggregate in colonies to breed, and most individuals remain faithful to one colony. Here, we use modelling to explore the consequences of this site fidelity for the metapopulation dynamics of such species. 2. We develop a spatially explicit model of the annual transfer process between colonies. We apply it to different spatial arrangements of 20 identical colonies and thus demonstrate that connectivity alone can, in the short term, give rise to heterogeneity in colony size. 3. We place the annual transfer model within a state-structured population model and examine the consequences of local and global density dependence for long-term dynamics. For each scenario, we investigate the implications of the strength of site fidelity, the cost of migration and the population’s intrinsic growth rate. 4. Our results suggest that, under local density dependence, site fidelity slows down the colonization process and can temporarily trap the entire population in a subset of the available potential colonies. When site fidelity is strong, the metapopulation follows a step-like trajectory. Population growth occurs only rarely because individuals must overcome their site-fidelity to found new colonies. Even though this effect is temporary, it renders the entire metapopulation vulnerable to rare catastrophic collapses. 5. Under global density dependence, site fidelity imposes competition between colonies for the limiting resource. Stochastic events lead to the dominance of certain colonies and the temporary extinction of others. If site fidelity is strong, it can permanently prevent the metapopulation from occupying all available potential colonies. 6. We conclude that, irrespective of the mechanism of population regulation, colonially breeding species that show strong site fidelity are likely to occupy only a portion of the breeding habitat available to them.

Multiple edge effects and their implications in fragmented landscapes

Abstract: Summary 1. Habitat edges are thought to explain much of the negative effects arising from habitat fragmentation; however, progress has been limited in extrapolating edge effects to different situations because ecologists still do not understand if and how multiple edges interact within fragments. It also remains controversial whether edge effects govern patch-size effects, such as area sensitivity, observed in many migratory songbirds. 2. I examined how multiple edges within fragments may intensify edge responses by investigating spatial distributions of an area-sensitive songbird that breeds in temperate grasslands of North America, the bobolink ( Dolichonyx oryzivorus Linnaeus). I tested whether bobolinks avoid edges and whether avoidance is stronger near two edges (doubleedge plots) than near only one edge (single-edge plots). I subsequently linked bobolink distributions to landscape maps that vary in the amount of habitat and degree of fragmentation to explore some potential implications of multiple edges on patch- and landscapelevel distributions. 3. Multiple edges appeared to influence the magnitude of observed edge effects, in which the probability of bobolink occurrence was four times lower in double-edge plots and two times lower in single-edge plots than in the interior of grasslands. Within singleedge plots, the probability of occurrence increased with increasing distance from edge. Within double-edge plots, the probability of occurrence increased as a function of the nearest and next-nearest distances from edges. Multiple edges also appeared to increase the extent of edge effects, or distance of edge influence, which was estimated to be approximately 11‐33% greater in double-edge plots than in single-edge plots, depending on the next-nearest distance from edge. 4. Extrapolating local bird distributions to landscape models suggests that edge effects can have strong influences on large-scale distributions and that models incorporating multiple edge effects are different to simple nearest-edge models only in highly fragmented landscapes, regardless of landscape composition. Furthermore, edge effects can lead to patch-size effects similar to empirical patterns of area sensitivity observed in this species. I conclude that edge effects can be intensified when multiple edges collide, a feature that permeates many fragmented landscapes.

The effect of propagule size on the invasion of an alien insect

Abstract: 1. The movement of species from their native ranges to alien environments is a serious threat to biological diversity. The number of individuals involved in an invasion provides a strong theoretical basis for determining the likelihood of establishment of an alien species. 2. Here a field experiment was used to manipulate the critical first stages of the invasion of an alien insect, a psyllid weed biocontrol agent, Arytainilla spartiophila Forster, in New Zealand and to observe the progress of the invasion over the following 6 years. 3. Fifty-five releases were made along a linear transect 135 km long: 10 releases of two, four, 10, 30 and 90 psyllids and five releases of 270 psyllids. Six years after their original release, psyllids were present in 22 of the 55 release sites. Analysis by logistic regression showed that the probability of establishment was significantly and positively related to initial release size, but that this effect was important only during the psyllids' first year in the field. 4. Although less likely to establish, some of the releases of two and four psyllids did survive 5 years in the field. Overall, releases that survived their first year had a 96% chance of surviving thereafter, providing the release site remained secure. The probability of colony loss due to site destruction remained the same throughout the experiment, whereas the probability of natural extinction reduced steeply over time. 5. During the first year colonies were undergoing a process of establishment and, in most cases, population size decreased. After this first year, a period of exponential growth ensued. 6. A lag period was observed before the populations increased dramatically in size. This was thought to be due to inherent lags caused by the nature of population growth, which causes the smaller releases to appear to have a longer lag period.

Relating predation mortality to broad-scale habitat selection

Abstract: Summary 1. Few studies have related metrics of fitness to broad-scale, multivariate patterns of resource selection. 2. Our objective was to relate long-term predation-mortality patterns for adult woodland caribou [ Rangifer tarandus caribou (Banfield)] from Alberta, Canada (1991‐2002), with patterns of multivariate habitat selection. 3. We first compared probabilities of radio-tracked caribou dying ( n = 55) from predation in habitats within the home range, controlling for habitat availability, with that expected from habitat selection probabilities for the same animals during life. We then compared survival rates of caribou ( n = 141) possessing dissimilar patterns of habitat selection using a Cox proportional hazards regression model. 4. Patterns in habitat-specific predation mortality differed significantly from expected given probabilities of habitat selection during life ( P < 0·0001). Cox regression indicated that mortality rates of caribou due to predation were affected significantly by and can be predicted from patterns of selection ( P = 0·02). 5. Our results strongly suggest that uplands (primarily mixed deciduous and coniferous forest) present caribou with higher than expected levels of predation risk, and that caribou can avoid predation by maximizing selection of peatlands (open, coniferdominated bogs and fens). 6. Approaches presented in this study may be useful for ecologists interested in assessing the influence of mortality factors on broad-scale, multivariate resource selection. Linking metrics of fitness to multivariate resource selection will enable us to ask questions of evolutionary ecology once restricted to only the finest ecological scales.

Digestive bottleneck affects foraging decisions in red knots Calidris canutus. II. Patch choice and length of working day

Abstract: Summary 1. When prey occur at high densities, energy assimilation rates are generally constrained by rates of digestion rather than by rates of collection (i.e. search and handle). As predators usually select patches containing high prey densities, rates of digestion will play an important role in the foraging ecology of a species. 2. The red knot Calidris canutus shows tremendous inter- and intra-individual variation in maximum rates of digestion due to variation in the size of the processing machinery (gizzard and intestine), which makes it a suitable species to study the effects of digestive processing rate on foraging decisions. 3. Here we report on patch use, prey choice, and daily foraging times as a function of gizzard size in free-ranging, radio-marked, red knots. As knots crush their bulky bivalve prey in their gizzard, the size of this organ, which we measured using ultrasonography, determines digestive processing rate. 4. Using the digestive rate model, we a priori modelled patch use, prey choice, and daily foraging times as a function of gizzard mass. Focusing on two contrasting patches, birds with small gizzards were expected to feed on high-quality (soft-bodied) prey found in low densities in the one patch, while birds with large gizzards were expected to feed on low-quality (hard-shelled) prey found in high densities in the other patch. Assuming that red knots aim to balance their energy budget on a daily basis, we expected daily foraging time to decline with gizzard mass. 5. Observed patch and prey choices were in quantitative agreement with these theoretical predictions. Observed daily foraging times were only in qualitative agreement: they declined with gizzard mass but less steeply than predicted. 6. We discuss that red knots might be aiming for a slightly positive energy budget in order to (i) refuel their stores depleted during migration, and (ii) to insure against unpredictability in supply and demand during winter. Red knots arriving from their breeding grounds with small gizzards are only able to realize this aim when densities of soft-bodied prey are high, which is the case in late July and early August. Rapidly declining soft-bodied prey densities throughout late summer pose a major penalty for individuals arriving late at their wintering grounds. 7. The long daily foraging periods required by knots with small gizzards are only feasible through ‘tide-extension’. In our study area, birds can and do raise the daily low tide period from 12 h to almost 17 h by moving along with the tide in an easterly direction, subsequently flying back to their starting point at the high tide roost.

Hydrogen isotope ratios in animal body protein reflect trophic level

Abstract: 1Carbon and nitrogen isotope ratios in consumer tissues are known to correlate with diet isotope composition, and nitrogen isotope ratios are observed to increase with increasing trophic level2We analysed nitrogen and hydrogen isotope ratios of collagen from 19 species of British fish, birds and mammals to investigate how δD also correlated with trophic level and with feeding environment (terrestrial or aquatic)3A strong relationship between trophic level and δD was discovered for both terrestrial and aquatic consumers4The correlation between trophic level and δ15N was apparent for terrestrial consumers, but less so for aquatic consumers5No differentiation was found between δD of aquatic and terrestrial consumers at the same trophic level6This observation should provide an additional tool in the study of current and ancient animal and human food web ecology

Population dynamics of wetland fishes: spatio-temporal patterns synchronized by hydrological disturbance?

Abstract: Summary 1. Drought is a natural disturbance that can cause widespread mortality of aquatic organisms in wetlands. We hypothesized that seasonal drying of marsh surfaces (i.e. hydrological disturbance) shapes spatio-temporal patterns of fish populations. 2. We tested whether population dynamics of fishes were synchronized by hydrological disturbance (Moran effect) or distance separating study sites (dispersal). Spatio-temporal patterns were examined in local populations of five abundant species at 17 sites (sampled five times per year from 1996 to 2001) in a large oligotrophic wetland. 3. Fish densities differed significantly across spatio-temporal scales for all species. For all species except eastern mosquitofish ( Gambusia holbrooki ), a significant portion of spatio-temporal variation in density was attributed to drying events (used as a covariate). 4. We observed three patterns of response to hydrological disturbance. Densities of bluefin killifish ( Lucania goodei ), least killifish ( Heterandria formosa ), and golden topminnow ( Fundulus chrysotus ) were usually lowest after a dry down and recovered slowly. Eastern mosquitofish showed no distinct response to marsh drying (i.e. they recovered quickly). Flagfish ( Jordanella floridae ) density was often highest after a dry down and then declined. Population growth after a dry down was often asymptotic for bluefin killifish and golden topminnow, with greatest asymptotic density and longest time to recovery at sites that dried infrequently. 5. Fish population dynamics were synchronized by hydrological disturbance (independent of distance) and distance separating study sites (independent of hydrological disturbance). Our ability to separate the relative importance of the Moran effect from dispersal was strengthened by a weak association between hydrological synchrony and distance among study sites. Dispersal was the primary mechanism for synchronous population dynamics of flagfish, whereas hydrological disturbance was the primary mechanism for synchronous population dynamics of the other species examined. 6. Species varied in the relative role of the Moran effect and dispersal in homogenizing their population dynamics, probably as a function of life history and ability to exploit dry-season refugia.