bluuebunny/crossref_metadata_2025_split huggingface dataset:
Evolutionary consequences of habitat fragmentation in a localized butterfly
Authors: THOMAS CHRIS D. , HILL JANE K. , LEWIS OWEN T. 
Published: March 2003

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.
Experimental evidence for density dependence of reproduction in great tits
Authors: BOTH CHRISTIAAN 
Published: March 2003

1.  Density dependence of avian reproduction has often been analysed using correlations between annual mean reproductive output and population density. Experiments are necessary to prove that density is the cause of the observed patterns, but so far, three out of four experimental studies do not support a direct causal effect of density on reproduction. 2.  This paper presents experimental evidence that reproductive decisions in great tits, Parus major L., are causally affected by breeding density. The breeding density of great tits was manipulated by providing nest‐boxes at different densities in an ecologically homogeneous area. 3.  Within years the densities in the high and low density plots differed approximately 8‐fold. During the 11 years of the experiment, clutch size, nestling mass and the proportion of birds starting a second brood were all lower in the high density plot. In 5 years with equal breeding densities in both parts, clutch size did not differ between the plots. The patterns found were consistent with the density effects as predicted from the non‐experimental data.
Examination of the nitrogen limitation hypothesis in non‐cyclic populations of cotton rats (Sigmodon hispidus)
Authors: Schetter Timothy A. , Lochmiller Robert L. , Leslie David M. , Engle David M. , Payton Mark E. 
Published: March 2003

1. Nitrogen‐containing nutrients have long been considered a frequently limiting resource to the growth of herbivore populations (nitrogen limitation hypothesis). To explore this hypothesis, we examined the relationships between availability of essential amino acids and concentrations of phenolics in the diets of hispid cotton rats (Sigmodon hispidus) in central Oklahoma and the intrinsic characteristics of their non‐cyclic populations. We posited that lower quality proteins (i.e. essential amino acid composition) and elevated phenolic levels (protein digestion inhibitors) in diets of cotton rats from low‐ compared to high‐density populations, especially during the breeding season, would be supportive of the nitrogen limitation hypothesis. Replicated low‐ and high‐density populations were censused by live‐trapping at 3‐month intervals. Samples of stomach digesta were collected from cotton rats in similar habitats adjacent to trapping grids to determine the botanical and nutrient composition of their diets. 2. During the breeding season, concentrations of essential amino acids were as much as 43% greater in diets of cotton rats from high‐density populations. Dicots, typically higher in protein than monocots, were an important component of diets and were preferred forage in all seasonal collections. Seeds and arthropods were frequently utilized by cotton rats as additional high‐quality sources of essential amino acids. Concentrations of total phenolics in the diet (greater in diets from low‐density populations) were consistent with the nitrogen limitation hypothesis. 3. Density was consistently higher in the high‐density populations throughout the study. Other demographic and body condition parameters were similar between low‐ and high‐density populations in the non‐breeding season, but reproductive activity was greater in high‐density populations during the breeding season. Total number of juveniles recruited into the trappable population over the entire study was about five times greater in high‐ compared to low‐density populations. 4. Our data did not refute the nitrogen limitation hypothesis where levels of essential amino acids and phenolic compounds in the diet during the breeding season may determine annual peak densities of cotton rats that can be supported in their habitat. However, we could not rule out the involvement of other environmental variables such as overhead cover (as well as other unmeasured variables) as contributing factors to determining annual peak densities.
The interactive effects of age and food supply on the breeding ecology of great skuas
Authors: RATCLIFFE N. , FURNESS R.W. , HAMER K.C. 
Published: March 2003

Summary

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.
Can grazing sheep compensate for a daily foraging time constraint?
Authors: Iason G. R. , Mantecon A. R. , Sim D. A. , Gonzalez J. , Foreman E. , Bermudez F. F. , Elston D. A. 
Published: March 2003

1. Theoretical studies of large herbivore foraging assume that total daily grazing time is a key constraint on daily intake and diet choice. We experimentally tested this assumption and investigated the effects of food availability on the ability of grazing sheep to compensate for restriction of available daily grazing time. 2. Foraging behaviour, intake and diet digestibility by sheep, were measured on grass pastures in a replicated 2 × 2 factorial experiment, in which overnight access to pasture was varied (restricted overnight and continuous access) on two sward heights (5·5 and 3·0 cm), representing high and low food availability. 3. Regardless of food availability, the overnight‐restricted sheep fed for almost all of the available grazing time by grazing for fewer, longer foraging bouts, but still had much shorter total daily grazing time than the continuous access sheep. 4. In response to overnight penning, the sheep had a significantly higher instantaneous rate of intake achieved mainly via larger bites. The continuous access sheep were hence not maximizing their short‐term rate of intake, whilst grazing according to the daily schedule considered normal for sheep. 5. The behavioural responses to overnight food restriction were able to counteract the reduction in daily grazing time only where food availability was high. In contrast on short swards overnight grazing restriction led to a reduction in total daily intake. We suggest that the interactions between the factors considered as constraints on foraging behaviour of herbivores are, as yet, only poorly quantified.
Eliminating autocorrelation reduces biological relevance of home range estimates
Authors: DE Solla Shane R. , Bonduriansky Russell , Brooks Ronald J. 
Published: March 2003

1. Destructive subsampling or restrictive sampling are often standard procedures to obtain independence of spatial observations in home range analyses. We examined whether home range estimators based upon kernel densities require serial independence of observations, by using a Monte Carlo simulation, antler flies and snapping turtles as models. 2. Home range size, time partitioning and total straight line distances travelled were tested to determine if subsampling improved kernel performance and estimation of home range parameters. 3. The accuracy and precision of home range estimates from the simulated data set improved at shorter time intervals despite the increase in autocorrelation among the observations. 4. Subsampling did not reduce autocorrelation among locational observations of snapping turtles or antler flies, and home range size, time partitioning and total distance travelled were better represented by autocorrelated observations. 5. We found that kernel densities do not require serial independence of observations when estimating home range, and we recommend that researchers maximize the number of observations using constant time intervals to increase the accuracy and precision of their estimates.
Different impact of pupal predation on populations of Epirrita autumnata (Lepidoptera; Geometridae) within and outside the outbreak range
Authors: Tanhuanpää Miia , Ruohomäki Kai , Kaitaniemi Pekka , Klemola Tero 
Published: March 2003

1. We studied the effect of pupal predation on Epirrita autumnata populations within and outside the outbreak range, in northern and southern Finland, respectively. The purpose of the study was to evaluate the potential of pupal predation to contribute to the contrasting dynamics of northern and southern populations. In addition to overall pupal survival, we separately estimated the influence of small mammal and invertebrate predation. 2. Each summer for a period of 5 years, pupae inside their cocoons were buried in the ground. The pupae were re‐collected before the flight season and checked for damage. Direct and delayed density dependence of pupal survival was tested using the current and the previous year larval density indices, respectively. 3. Annual survival of pupae from both small mammal and invertebrate predation was lower in the south. Daily survival rates did not differ between the south and the north indicating that the difference in annual pupal survival is most likely due to the longer pupal period in the south. 4. In the south, pupal survival was negatively density dependent and resulted from small mammal predation. The results indicate that small mammals may prevent E. autumnata populations from reaching high densities in the south. 5. In the northern study area with a current outbreak, total pupal survival was density independent. Also small mammal predation was density independent, most likely due to predator satiation. Moreover, there was no difference in pupal survival between northern areas with and without a current outbreak. The results indicate that pupal predation was not a significant factor in termination of the outbreak, nor is it likely to contribute to the maintenance of the cyclic density fluctuations. 6. We conclude that low pupal survival and its density dependence are likely to contribute to stability of E. autumnata populations in southern Fennoscandia. On the contrary, shorter pupal period and the lack of density dependent pupal predation may make northern populations more prone to outbreaks. The potential regulating effect of pupal predation in southern populations is due to small mammals being able to respond to changes in E. autumnata densities.
Dietary protein constraint on age at maturity: an experimental test with wild deer mice
Authors: McAdam Andrew G. , Millar JohN. S. 
Published: March 2003

1.Five populations ofPeromyscus maniculatus borealis(Mearns) were live‐trapped in the Kananaskis Valley, Alberta, throughout the breeding seasons of 1996 and 1997, to test the hypothesis that growth and maturation of young‐of‐the‐year (YY) females are limited by dietary protein.2.In 1996, two populations received a high protein (30%) food supplement, one received a high energy, but low protein (14%) food supplement and two others were unsupplemented. In 1997, three populations received the protein (30%) supplement and two served as controls.3.In 1996, control populations had high nestling growth rates and many (43%) YY females bred. In 1997, nestling growth rates were lower in control populations and no YY females bred.4.Supplementation of high protein food resulted in consistently high nestling growth rates in both 1996 and 1997, and a significantly higher proportion of YY females bred than controls in both years.5.Supplementation of high energy, but low protein food had no effect on YY growth or maturation.6.Neither protein nor energy supplementation had any effects on the number of litters conceived per season, litter success, litter size or sex ratio of litters born to over‐wintered females.7.We conclude that growth and maturation of YY femalePeromyscus maniculatusin this area are limited by the availability of dietary protein.
Life‐history consequences of predation for a subantarctic beetle: evaluating the contribution of direct and indirect effects
Authors: Ernsting G. , Brandjes G. J. , Block W. , Isaaks J. A. 
Published: March 2003

1.Recently, a small predatory beetle,Trechisibus antarcticus(Carabidae), was accidentally introduced onto the island of South Georgia, sub‐Antarctic.2.From the presumed site of introduction the beetle is invading the coastal lowland area, building up high densities locally in the tussock‐forming grassParodiochloa flabellata.3.In the coastal area the endemic detritivorous/herbivorous beetleHydromedion sparsutum(Perimylopidae) is common, especially in and beneath the tussocks.4.The first three, out of six, larval instars ofH. sparsutumare easily taken prey by the carabid.5.In sites colonized by the carabid, total abundance and the ratio between larval and adult numbers ofH. sparsutumare far lower, and its adult body size clearly larger, than in comparable sites where the carabid is absent.6.Two hypotheses are proposed for explaining the increase in adult body size ofH. sparsutum: (i) the increase is a direct effect of predation: selection by the predator favours large hatchlings and/or larvae with a high growth rate; and (ii) the increase is an indirect effect of predation: by lowering the density ofH. sparsutum, predation has increased its per capita food supply, enabling a higher growth rate and a larger adult body size.7.A food addition experiment in a carabid‐free site showed availability of high quality food to be insufficient for sustaining the initial larval population.8.In the laboratory, females from the predator‐infested sites produced larger eggs and hatchlings than females from the carabid‐free sites, but mass specific growth rates of the larvae were not higher.9.Field and laboratory data give stronger support to the food hypothesis than to the size selectivity hypothesis.
Competition for early arrival in migratory birds
Authors: Kokko Hanna 
Published: March 2003

Summary1. It is widely accepted that the arrival order of migratory birds is correlated with the condition of the birds, which leads to high quality individuals occupying prime sites. However, the theoretical backgrounds for this argument have been lacking. A simple game‐theoretic model of arrival timing is provided which investigates the evolutionary stability of condition‐dependent arrival order in territorial migrant birds.2. Competition for territories or other priority‐dependent benefits can lead to arrival dates far preceding the cost‐minimizing date (the optimum date in the absence of competition) for all but the weakest individuals. Increasing the number of competitors can generate a ‘cascading’ competition for early arrival, which advances arrival dates further apart from the individual optimum dates for the onset of breeding.3. At equilibrium, arrival order corresponds strictly to condition order only if marginal costs of advancing arrival are always larger for individuals in lower condition. If spring mortality vacates territories for later‐arriving birds, the criterion for ‘honest’ arrival order becomes still stricter: differential survival costs should exist, but survival differences among individuals (or, alternatively, territory quality differences) should not be very large.4. If the habitat is saturated so that there is a risk of not obtaining a territory at all, or if worst territories are of much lower value than the rest, competition may lead to the majority of the population arriving within a fairly short interval, followed by a much later floating fraction. This synchrony in the arrival of breeders imposes an increasing cost for the lesser fit breeding birds. Thus, arrival costs paid are not necessarily highest for earliest arriving individuals, but for those who have the most to lose if they drop a few steps in the arrival order.5. Competition for high quality territories can also lead to partial migration, in which case birds in good condition are expected to be most likely to remain resident.
Does large size make daughters of the blue‐footed booby more expensive than sons?
Authors: Torres Roxana , Drummond Hugh 
Published: March 2003

Summary 1. In species where size dimorphism emerges during parental care, individuals of the larger sex are presumably more expensive to produce than individuals of the smaller sex. 2. We evaluated whether female offspring of the blue‐footed booby, because of their larger size, require greater parental feeding expenditure than males. 3. A field experiment compared growth rates of sons and daughters when hand‐fed the same amount of food, and a descriptive field study compared the parental feeding rates to fledgling sons and daughters. 4. Females maintained greater rates of mass and ulna increase than males when fed the same amount of food. 5. First‐hatched chicks had higher probability to be fed than second‐hatched chicks. However, male and female fledglings were fed at similar rates by their parents. 6. No evidence that daughters of the blue‐footed booby receive a greater feeding expenditure than sons was found. Faster motor development of males in this facultatively siblicidal species might explain why they grow more slowly than females with the same food budget.
Comparative patterns of philopatry and dispersal in two common mole‐rat populations: implications for the evolution of mole‐rat sociality
Authors: Spinks Andrew C. , Jarvis Jennifer U. M. , Bennett Nigel C. 
Published: March 2003

Summary 1. The aridity food‐distribution hypothesis (AFDH) maintains that ecological constraints and foraging risks in arid environments curtail dispersal and promote the evolution of cooperatively foraging social groups within African mole‐rat populations. To assess the influence of habitat constraints upon mole‐rat social behaviour and hence the validity of the AFDH, we investigated the differential patterns of dispersal and philopatry in two populations of the common mole‐rat, Cryptomys hottentotus hottentotus Lesson, one from a more ‘optimal’ habitat (mesic) and another from a ‘suboptimal’ habitat (arid). 2. Rates of immigration and emigration were markedly lower in the arid relative to the mesic site, reflecting divergence in the ecological constraints upon dispersal. 3. The rates of dispersal at both sites increased as a function of group size. However, for any given group size, colony attrition was significantly greater at the mesic site than at the arid site, probably reflecting the relaxation of constraints upon dispersal in mesic areas. 4. There was no sex bias in dispersal at either study site. 5. At the mesic sites dispersing and nondispersing individuals were of comparable mass, whilst at the arid site there was a trend for dispersing animals to be heavier than their nondispersing counterparts. 6. This investigation revealed marked differences between the study populations in the nature of dispersal. This reflects adaptive variation in social behaviour between the regions, and the results suggest that delayed dispersal and cooperation may be more crucial to individual survival in arid than in mesic areas. As such, these findings provide support for the underlying contention of the AFDH that ecological constraints on successful dispersal and colony formation in arid areas have promoted a greater degree of social cohesion in mole‐rats occurring in these regions, ultimately advancing the evolution of group‐living and cooperative foraging in the Bathyergidae.