Implications For Population Dynamics Research Articles

Constant breeding and low survival rates in the subtropical Striped Frog in southern Brazil

AbstractSurvival and consequent implications for population dynamics in the subtropical Striped Frog Hypsiboas leptolineatus was studied for 1 year in the southern Brazilian state of Paraná. By means of capture‐marking‐recapture, we estimated survival and capture probabilities in an open population. A total of 583 captures of 374 individuals, comprising 96% male (n = 353) and 4% females (n = 21), resulted in daily survival estimates ranging from 0.808 to 0.998 day−1. The number of individuals captured per capture session varied from 8 to 37 (mean = 21) and was correlated with night length, after a two session lag time (r = 0.78). Males were found vocalizing throughout the year, while females were less common and gravid females were only found in November, March and June. Recapture probability was relatively constant at 25% for males, 24% for females and, overall, 29% of males were recaptured at least once. Young individuals were encountered in all months but one and, being extremely small, were impossible to quantify. Thus, the Striped Frog is active and breeding year‐round as indicated by the constant singing of adult males, the few gravid females found at different times and the frequent encounters of young frogs at all times of year. While survival and captures varied throughout the year, the only seasonality was in the number of captures that increased during longer nights. Nonetheless, recapture probability was constant. These dynamics contrast strongly with most anuran species and especially subtropical and temperate species in other places. This first detailed study of population parameters of a subtropical species with its unusual dynamics may suggest that once studied, other species of anurans may also have surprising population dynamics.

High gene flow and metapopulation dynamics detected for three species in a dryland river system

Summary 1. The highly variable hydrology of dryland rivers has important implications for population dynamics in these systems. In western Queensland, fluctuations in sub-population size are likely to lead to local bottlenecks and extinctions, increasing the need for connectivity and gene flow to maintain population viability. 2. Using microsatellite markers, we explored evidence for this metapopulation structure in two species of freshwater fish (Maquaria ambigua and Tandanus tandanus) and one crustacean (Macrobrachium australiense) in a sub-catchment of the upper Murray–Darling Basin, Australia. 3. Overall, we found very weak genetic structure for all three species. Two species (M. ambigua and M. australiense) showed some significant genetic structure that did not correlate with geographic distance. However, decomposed pairwise regression analysis revealed evidence for intense genetic drift at the waterhole scale, suggesting that local bottlenecks are driving what little genetic structure does exist for these species. 4. The results identify the local impact of bottlenecks on genetic diversity, but highlight the importance of gene flow in maintaining population viability in these highly variable systems. As the impacts of bottlenecks are likely to be tempered by gene flow, it is suggested that the maintenance of connectivity is of paramount importance in this dryland system.

Characteristics of spawning ground fidelity by a diadromous fish: a multi-year perspective

Spawning site fidelity has long been documented for anadromous salmonids, and more recently in estuarine and marine fish. The level of fidelity has implications for population dynamics, conservation, and management. This study extended a previous examination of spawning ground fidelity for common snook, Centropomus undecimalis. We used seines to sample 28 km of Gulf of Mexico beaches, spawning grounds for snook, during spawning season (May through September) from 2007 through 2010. Of 3,304 snook tagged, 171 unique fish were recaptured a total of 186 times and, of those, 166 fish (97.08%) (181 recaptures = 97.31%) were recaptured on the same island where they were tagged. One hundred seven of the recaptures occurred in the same year they were tagged, and 79 occurred one or more years after they were tagged, indicating fidelity was expressed within and across years. Distance between tag and recapture locations within and among years demonstrated fidelity at a spatial scale much smaller than barrier island (mean distance between tag and recapture =1.59 ± 0.12 km; island lengths 6.8, 8.9 and 12.4 km). Furthermore, mapping of capture locations revealed clumped distribution of snook on spawning grounds, further suggesting snook propensity for specific locations. Fish size had no effect on fidelity. We conclude that common snook show a high level of spawning site fidelity in southwest Florida, which may result in spawning group segregation, may influence the response of snook to disturbances, and may have implications for adult-to-nursery ontogenetic connections.

Demographic processes of upward range contraction in a long-lived Mediterranean high mountain plant

We analyzed demographic data of a long-lived high mountain Mediterranean plant, Silene ciliata Poirret, over a 4-yr period. Selected populations were located at contrasting altitudes at the southernmost margin of the species (Sierra de Guadarrama, central Spain), representing a local altitudinal range at the rear edge of its overall distribution. Previous studies have suggested that differences in the reproduction and performance of individuals at upper and lower populations may have implications for population dynamics. We used matrix analysis to assess their demographic behaviour. Life Table Response Experiments were used to identify the life history stages most relevant to observed differences in population growth rates between populations. Transition matrices revealed great spatio-temporal variability in demographic traits. Seedling recruitment was very low each year in all populations. Maximum longevity of S. ciliata individuals in the lower peripheral population was much lower compared to the central population, probably due to higher adult mortality. Population growth rate (λ) showed a declining trend at the lowest altitude and a relatively stable trend at the central population. Long-term simulations also indicated a great risk of quasi-extinction at the lowest population. Our results suggest that rear edge populations of S. ciliata at Sierra de Guadarrama are suffering demographic processes that may be leading to the latitudinal displacement of the species' range.

The unpredictable effects of mixtures of androgenic and estrogenic chemicals on fish early life

Estuarine shallow areas and coastal lagoons are known to receive and concentrate multiple inputs, either from land, rivers or coastal areas, being intensively impacted by chemical contamination, namely endocrine disrupting chemicals (EDCs). Despite the ubiquitous coexistence of several classes of EDCs in most of these aquatic ecosystems, there is still limited information regarding their combined effects. Furthermore, given the immediate implications for population dynamics, the available laboratory studies almost invariably focus on very specific life history stages, such as embryonic development or reproduction, thus creating a gap on our knowledge of what happens in between. During this ‘intermediate phase’, the newborn larvae and juveniles face numerous challenges whose outcome may impair reproduction or even survival. The black-striped pipefish, Syngnathus abaster, member of the Syngnathidae family (comprising pipefish, seahorses and seadragons), usually breeds in coastal areas such as estuaries, where its newborns are immediately exposed to EDCs. Given the ongoing decline of pipefish populations, together with the observed shrinkage and fragmentation of seagrass meadows, known to be impacted by EDCs, a first reasonable question to address is if pipefish newborns respond to environmentally relevant concentrations of ubiquitous EDCs, either single or in combination. Hence, a seven days exposure experiment to the estrogenic chemical ethinylestradiol (EE 2) and the androgenic chemical tributyltin (TBT), single and in binary mixtures, was conducted. Selected behavioural (e.g. predator avoidance) and developmental variables (e.g. growth) were monitored in pipefish juveniles after EDCs insult. The obtained results indicate that EE 2, TBT, or their combined exposure, do impact pipefish early life. However, the pattern of results emerging from the measured variables clearly indicates that mixtures significantly modulate newborn responses in distinct ways when compared to individual chemical's exposure. These findings further demonstrate the importance of addressing the issue of chemical mixtures of pollutants acting through dissimilar mode of action. Independently of all the observed response variations, an ultimate conclusion seems certain: EE 2 and TBT, single or in combination, induce disruption patterns able to imbalance pipefish survival. Since these (as well as other) contaminants are present in estuarine areas, profound implications in population structure could be expected, ranging from a decrease in recruitment to a disruption of sexual selection. Inexorably, these stressors simultaneously operate in already declining populations.

Transcriptomic response of red grouse to gastro‐intestinal nematode parasites and testosterone: implications for population dynamics

A central issue in ecology is in understanding the relative influences of intrinsic and extrinsic effects on population regulation. Previous studies on the cyclic population dynamics of red grouse (Lagopus lagopus scoticus) have emphasized the destabilizing effects of either nematode parasites or territorial behaviour and aggression. The potential interacting effects of these processes, mediated through density-dependent, environmentally induced alterations of host immunocompetence influencing susceptibility to parasites have not been considered. Male red grouse at high density are more aggressive, associated with increased testosterone, which potentially could lead to reduced immunocompetence at a stage when parasites are most prevalent. This could depress individual condition, breeding performance and survival and thus drive or contribute to overall reductions in population size. Here, we characterize the transcriptomic response of grouse to nematode parasite infection and investigate how this is subsequently affected by testosterone, using a microarray approach contrasting red grouse with high and low parasite load at both high and low testosterone titre. A suite of 52 transcripts showed a significant level of up-regulation to either chronic parasite load or experimental parasite infection. Of these, 51 (98%) showed a reduced level of expression under conditions of high parasite load and high testosterone. The genes up-regulated by parasites and then down-regulated at high testosterone titre were not necessarily associated with immune response, as might be intuitively expected. The results are discussed in relation to the fitness and condition of individual red grouse and factors influencing the regulation of abundance in natural populations.

Metabolic responses of Glossina pallidipes (Diptera: Glossinidae) puparia exposed to oxygen and temperature variation: Implications for population dynamics and subterranean life

Understanding the factors affecting insect gas exchange in subterranean environments is critical to understanding energy budgets and predicting mortality under field conditions. Here, we examine the metabolic rate (MR) responses of tsetse puparia, which remain underground for ca. 1 month in this life-stage, to varying oxygen and temperature. First, the effects of temperature and oxygen on puparial MR were investigated by ramping temperature from 15 to 35°C under 10, 21 or 40% O2. Overall, temperature was the dominant effect on puparial MR although O2 had small but significant impacts. Second, critical O2 concentration (PCRIT) for MR of puparia was examined across a range of oxygen concentrations (0–40%). PCRIT was 6% O2 which is similar to PCRIT in other basal arthropods but relatively high for inactive or subterranean insects. Third, we asked if puparia exposed to anoxia might experience oxygen debt, potentially indicative of anaerobic metabolism or cellular repair. Metabolic responses to anoxia were limited or insignificant, but MR was marginally elevated (∼15%) in anoxia-exposed (4h) puparia by 12h post-anoxia. Finally, we examined the ability of puparia to withstand water submersion, thus simulating flooding conditions frequently experienced in tropical soil habitats. Puparia were unable to survive submersion for >24h suggesting limited flooding tolerance. These novel results suggest that soil conditions experienced by puparia should not be limiting for MR, except possibly under high temperature-low O2 conditions. Due to a large safety margin between PCRIT and soil oxygen levels and limited effects of oxygen on metabolism during temperature ramping experiments, we suggest that Glossina pallidipes puparia are not particularly susceptible to oxygen availability in their natural environment. However, soil flooding associated with tropical rainfall likely imposes strong selection on tsetse populations and may have had important effects for tsetse energy budgets and evolution.

Complete compensation in Daphnia fecundity and stage‐specific biomass in response to size‐independent mortality

1. Recent theory suggests that compensation or even overcompensation in stage-specific biomass can arise in response to increased mortality. Which stage that will show compensation depends on whether maturation or reproduction is the more limiting process in the population. Size-structured theory also provides a strong link between the type of regulation and the expected population dynamics as both depend on size/stage-specific competitive ability. 2. We imposed a size-independent mortality on a consumer-resource system with Daphnia pulex feeding on Scenedesmus obtusiusculus to asses the compensatory responses in Daphnia populations. We also extended an existing stage-structured biomass model by including several juvenile stages to test whether this extension affected the qualitative results of the existing model. 3. We found complete compensation in juvenile biomass and total population fecundity in response to harvesting. The compensation in fecundity was caused by both a higher proportion of fecund females and a larger clutch size under increased mortality. We did not detect any difference in resource levels between treatments. 4. The model results showed that both stages of juveniles have to be superior to adults in terms of resource competition for the compensatory response to take place in juvenile biomass. 5. The results are all in correspondence with that the regulating process within the population was reproduction. From this, we also conclude that juveniles were superior competitors to adults, which has implications for population dynamics and the kind of cohort cycles seen in Daphnia populations. 6. The compensatory responses demonstrated in this experiment have major implications for community dynamics and are potentially present in any organisms with food-dependent growth or development.

Influences of Temperature onHomalodisca vitripennis(Hemiptera: Cicadellidae) Survival Under Various Feeding Conditions

Survival of the glassy-winged sharpshooter, Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae), was studied under various constant temperatures and feeding conditions. When provided a host plant (Citrus limon L. Burm. f.) to feed on during a 21-d trial, 100% mortality occurred at 0.1, 3.2, and 40.1 degrees C, whereas an average of 74-76% of adults survived in the 13.2-24.5 degrees C range. When individually confined with moist cotton, adult longevity was greatest (16.3 d) at 13.3 degrees C, but it was <3 d at -2.4 and 36.2 degrees C. In a companion study comparing the presence versus absence of a host plant, the presence of a host plant was not a significant factor influencing survival at temperatures < or =7.8 degrees C but was at temperatures > or =18.9 degrees C. The relationship between temperature and survival was described by a nonlinear function that estimated the optimum temperature in each feeding regimen: no host plant or moist cotton (5.5 degrees C), moist cotton (9.9 degrees C), and accessible host plant (25.1 degrees C). The model quantitatively predicted that H. vitripennis would survive longer periods at a wider temperature regimen when provided with a host plant than when provided with water alone (moist cotton) or when provided with neither plant host nor water. Our results suggest that continuous exposure to either low (<5 degrees C) or high (>30 degrees C) temperatures are detrimental for adult survival. Specifically, low temperatures caused early mortality because of inhibition of feeding activity and presumably this threshold lies between 7.8 and 13.2 degrees C. Furthermore, this study clearly shows that temperature may influence the survival of H. vitripennis adults regardless of feeding regimens, and its implications for population dynamics are discussed.

A diffusion‐based approach to stochastic individual growth and energy budget, with consequences to life‐history optimization and population dynamics

Using diffusion processes, I model stochastic individual growth, given exogenous hazards and starvation risk. By maximizing survival to final size, optimal life histories (e.g. switching size for habitat/dietary shift) are determined by two ratios: mean growth rate over growth variance (diffusion coefficient) and mortality rate over mean growth rate; all are size dependent. For example, switching size decreases with either ratio, if both are positive. I provide examples and compare with previous work on risk-sensitive foraging and the energy-predation trade-off. I then decompose individual size into reversibly and irreversibly growing components, e.g. reserves and structure. I provide a general expression for optimal structural growth, when reserves grow stochastically. I conclude that increased growth variance of reserves delays structural growth (raises threshold size for its commencement) but may eventually lead to larger structures. The effect depends on whether the structural trait is related to foraging or defence. Implications for population dynamics are discussed.

Cumulative effects of low salinity on larval growth and biochemical composition in an estuarine crab, Neohelice granulata

Throughout the larval development of a highly euryhaline crab, Neohelice (= Chasma- gnathus) granulata, we studied effects of reduced salinities (15 and 25‰ vs. seawater control at 32‰) on larval dry mass (W ) and biochemical composition (total lipid and protein). In the early zoeal stages (I, II), reduced salinity caused delayed moulting, while W and biochemical composition at ecdysis remained unaffected. This suggests that the capability of the Zoea I for hyper-osmoregulation (demonstrated in a previous study) mitigated potential effects of hypo-osmotic stress on biomass accumulation and biochemical composition, but did not prevent a developmental delay. After contin- ued rearing at low salinities, later larval stages showed increasingly negative effects on growth. The Zoea IV, for instance, revealed at 15‰ significantly reduced W, lipid and protein contents compared to higher salinities, probably as a consequence of weak osmoregulatory capabilities in the zoeal stages II and III. Likewise, average daily rates of biomass accumulation (taking into account salinity- induced variations in development duration) were lower at reduced salinities. These negative effects on larval growth persisted throughout the Megalopa, in spite of a strong capability in this stage to hyper-osmoregulate. This indicates that the mitigating force of osmoregulation in the final larval stage was not strong enough to offset cumulative effects of a continuous exposure to osmotic stress lasting from hatching throughout the more stenohaline zoeal stages. Osmoregulation in N. granulata has implications for population dynamics in the field, where salinity conditions prevailing during the development in the plankton may influence larval condition and survival at metamorphosis, eventually affecting recruitment.

Continental-aquatic mollusk hydrochemical occurrence patterns: Implications for population dynamics and paleoenvironmental reconstruction

Paired water and mollusk samples collected from 90 lakes, springs, and wetlands throughout the northwestern quarter of the United States suggest that, rather than just occupying discrete total dissolved solids (TDS) ranges as is often described, molluscan distribution is also related to solute types produced by solute evolution. This may explain why water bodies having similar environmental characteristics and TDS values have different molluscan assemblages. Therefore, continental-aquatic mollusk occurrences define solute and TDS fields that offer a means to assess past hydrology and climate. These relations also provide new insights into mollusk population dynamics. For example, molluscan distribution on the aquatic landscape is not a uniform continuum, but rather is a solute compositional mosaic, where taxa in adjoining lakes may be as isolated from each other as taxa on opposite sides of the continent. Knowledge regarding the relation of mollusk occurrence and solute composition can assist management decisions in sustaining threatened, sensitive, or endemic taxa, designing conservation programs, resolving wetland and spring mitigation issues, and determining the potential biological impacts from interbasin water transfers or ground water pumping.

The Role of a Seed Bank in Establishment and Persistence of Aeschynomene virginica, a Rare Wetland Annual

Abstract Seed banks of Aeschynomene virginica may have significant implications for population dynamics and conservation of this rare, freshwater wetland, annual plant. Little work, however, has been done on the size and extent of the seed bank of this threatened plant. Experimental seed banks were placed at three sites to estimate overwinter seed survivorship and the potential for long-term seed dormancy. The soil beneath 5 extant populations was sampled after current-year seed germination and before seed fall to determine the presence and spatial distribution of seeds surviving more than one season. An average of 47.4 ± 2.8 (± 1 SE) of the 125 seeds placed in the field survived over winter. Of those seeds that were recovered from experimental seed banks, an average of 37.3 ± 1.7 germinated under greenhouse conditions. When the seed coats of ungerminated seeds recovered from experimental seed banks (10.3 ± 0.3) were scored, 4.9 ± 0.8 seeds germinated. Analysis of soil cores from the field showed high var...

Experimental evidence for density-dependent survival in mallard (Anas platyrhynchos) ducklings

It is unresolved to what extent waterfowl populations are regulated by density-dependent processes. By doing a 2-year crossover perturbation experiment on ten oligotrophic boreal lakes we addressed the hypothesis that breeding output is density dependent. Wing-clipped mallard (Anas platyrhynchos) hens were introduced with their own brood and then monitored for 24 days. Predicted responses were that per capita duckling and hen survival would be lower in high-density than in low-density treatments. Survival was evaluated by model fitting in program MARK. Density, year, and lake were used as main effects, while day after introduction, a weather harshness index, and presence of hens were covariates. Daily survival in ducklings was lower in the high-density treatment, but this effect was year dependent. The highest-ranking model for duckling survival also included a positive effect of duckling age and presence of hens, and a negative effect of harsh weather. Density did not affect female survival although there was a prominent year effect. The highest-ranking model for female survival also included negative effects of day after introduction and harsh weather. This is the first study to report density-dependent survival in experimentally introduced ducklings in a natural setting. Implications for population dynamics and management of harvested populations are far-reaching if such regulation occurs in some years, but not in others.

Effects of an organophosphate on Daphnia magna at suborganismal and organismal levels: Implications for population dynamics

The effects and recovery of 24 h pulse exposure to paraoxon-methyl on Daphnia magna were recorded for various endpoints to study the propagation of effects through various biological levels of organization. Above a threshold concentration (2.2 μg L −1), all selected endpoints were affected. Thus, effects at the suborganismal level (e.g., the biomarker response: transient inhibition of cholinesterase (ChE) activity) were accompanied by effects at the organismal (survival, reduction in reproductive performance, decrease in body size) and population (reduced population growth rate) levels. At intermediate and sublethal concentrations, exposure induced a transient ChE inhibition that was also accompanied by effects at the organismal level and that exerted long-term effects on population dynamics. At lower concentrations, although ChE activity was affected, there was no propagation of effects to higher biological levels. This study shows that effects of pulse exposure to organophosphates propagate from the suborganismal level toward the population level and demonstrates the significance of transient ChE inhibition on population dynamics.

Historical changes in the abundance and distribution of ovigerous red king crabs (Paralithodes camtschaticus) in Bristol Bay (Alaska), and potential relationship with bottom temperature

AbstractDistribution and abundance of ovigerous female red king crabs (Paralithodes camtschaticus) in the southeast Bering Sea from 1975 to 2001 were investigated using data collected during National Marine Fisheries Service annual trawl surveys. Peak abundance of ∼140 million crabs was observed in 1978, and declined rapidly to a low of just over 6 million in 1986. Abundance fluctuated from ∼6 to 22 million from the late 1980s through 2001, with a single strong recruitment event that resulted in ∼35 million ovigerous females observed in 1998. Changes in abundance were accompanied by changes in distribution. During the late 1970s the population was typified by high abundance to the southwest, along the northern shore of Unimak Island and the Alaska Peninsula. By the mid‐1980s the population's average center of abundance shifted substantially to the northeast and was found in central Bristol Bay. The distribution remained similar throughout the 1990s. Changes in distribution during the late 1970s and early 1980s coincided with changes in early summer near‐bottom temperature. The 1970s were typified by a pool of very cold water (&lt;1°C) within central Bristol Bay. This retreated in ∼1978, and was not observed in consecutive summers during the remainder of the time series. The northeastward shift in the population, measured as the distance between Unimak Pass and the average center of abundance, showed a negative correlation with the geographic extent the cold‐pool. Abundance calculated for smaller spatial strata indicate that changes in distribution were not simply the result of relative abundance phenomena or solely generated by mortality in southwestern Bristol Bay, but also reflected regional increases in absolute abundance. Total broodstock abundance declined after 1978, but abundance in the western and northern areas of the region increased until at least 1982. The fact that distribution patterns change over time may have implications for population dynamics and fishery management. Changes in spatial population structure may affect recruitment patterns via changes in larval dynamics, and management might benefit if the causes of geographic displacement can be identified and predicted.

Effects of age, density and sex ratio on reproductive effort in male reindeer (Rangifer tarandus)

AbstractIn sexually dimorphic ungulates, male reproductive success depends on fighting with other males for access to females during a brief rutting season. Large body size is necessary for success in intrasexual competition, and a few large‐sized males are often able to monopolize access to female groups. Earlier studies have reported that reproductive effort increases with age until prime‐age is reached, and one study that population density lowered effort in (older) males. No study has directly assessed whether there is within‐age‐class variation in effort resulting from varying levels of intra‐male competition. It is reported here the weight loss during the rutting season of 54 individual male reindeer Rangifer tarandus coming from eight herds with varying density (3.3–6.0 deer/km2) and sex ratio (4–28% males). In agreement with earlier studies, reproductive effort was lower for young (1‐ to 2‐year‐old) than for prime‐aged (3‐ to 5‐year‐old) males both on an absolute and relative scale. Among 1‐year‐old males (n=33), effort was lower as sex ratio became closer to even, but density during the rutting season had no effect. This suggests that yearling males take a more active role when prime‐aged males are absent. In addition to the insight into male ungulate life history, understanding male rutting behaviour may also have implications for population dynamics.

Fine-scale genetic structure in a free-living ungulate population.

The fine-scale genetic structure of wild animal populations has rarely been analysed, yet is potentially important as a confounding factor in quantitative genetic and allelic association studies, as well as having implications for population dynamics, inbreeding and kin selection. In this study, we examined the extent to which the three spatial subunits, or hefts, of the Village Bay population of Soay sheep (Ovis aries) on St Kilda, Scotland, are genetically structured using data from 20 microsatellite and protein loci. Allele frequencies differed significantly among three hefts in all the study years we considered (1987-2000 inclusive). Small but significantly positive F(ST) and negative F(IS) values were observed in most years, indicating that the hefts are genetically differentiated, and that within each heft there is more observed heterozygosity than would be expected if each were an isolated breeding population. Males showed less fidelity to their natal heft, and as a consequence higher levels of relatedness within hefts were observed among females than among males. There was a significant negative relationship between geographical proximity and relatedness in pairwise comparisons involving females, and on average pairs of females located within 50 m of each other were related at the equivalent level of second cousins. Structure is therefore largely driven by incomplete postnatal dispersal by females. Mating appears to be random with respect to the spatial-genetic substructure of the hefts, and therefore genetic structure does not contribute to the overall rate of inbreeding in the population. However, genetic substructure can lead to allelic associations and generate environmental effects within lineages that have the potential to confound heritability analyses and allelic association studies.

Seasonal variation in the niche, habitat availability and population fluctuations of a bivoltine thermophilous insect near its range margin.

We investigated the niche requirements of the summer and autumn/spring generations of the bivoltine butterfly, Polyommatus bellargusRott., and their implications for population dynamics at sites occurring near its northern range margin. The larvae of this species are sedentary, and the turf height and shelter of Hippocrepis comosa foodplants selected for egg-laying accurately predict larval distributions within United Kingdom (UK) sites. We found a significant shift between the plants used for egg-laying in each generation, with the niche occupied by summer-feeding larvae being broader and different to the autumn one. Measurements of soil temperature confirmed that the short, sheltered foodplants selected by ovipositing females in autumn placed the autumn/spring-feeding generation of larvae in the warmest available microclimates within sites. In late spring, egg-laying females avoided the hottest spots but extended egg-laying into taller, less sheltered (relatively cool) turf where the microclimate was similar to that experienced by autumn/spring-feeding larvae. Using each generations' definition of niche requirement, we analysed surveys of foodplant populations available on 24 UK sites for P. bellargus, and estimated that nearly twice as many plants were available to the summer-feeding larvae compared to those feeding in the autumn. Annual adult population counts match these seasonal differences in site carrying capacity; first generation counts (from autumn-laid eggs) were generally half as abundant as in the second generation, and more variable. These results suggest that the seasonal cycle of niche switches represents an annual (autumn-spring) bottleneck for populations of this butterfly at its northern range margin. Under climate warming we predict that the inter-generational difference in niche availability, carrying capacity and population size will be reduced. We recommend revised management requirements for this threatened species under current and predicted climates in northern Europe.

A Continuum Formulation of the Ideal Free Distribution and Its Implications for Population Dynamics

The ideal free distribution is a description of how organisms would distribute themselves in space if they were free to move so as to maximize fitness. The standard formulation of the ideal free distribution envisions the environment as consisting of finitely many discrete habitats. In this paper, a version of the ideal free distribution is derived for the case where the environment is a continuum. The continuum formulation allows computation of average fitness at the population level by taking account of both local fitness and the spatial distribution of the population. An example shows that the average fitness may have a different form than the local fitness; in particular, if local fitness is described by a logistic equation at each location, the average fitness may obey the θ-logistic equation of F. J. Ayala et al. (1973, Theor. Popul. Biol. 4, 331–356). This gives a mechanistic derivation of the θ-logistic equation.