bluuebunny/crossref_metadata_2025 huggingface dataset:
Linking species diversity to the functioning of ecosystems: on the importance of environmental context
Authors: Cardinale Brad J. , Nelson Karen , Palmer Margaret A.
Published: December 2004

There is currently much interest in understanding how loss of biodiversity might alter ecological processes vital to the functioning of ecosystems. Unfortunately, ecologists have reached little consensus regarding the importance of species diversity to ecosystem functioning because empirical studies have not demonstrated any consistent relationship between the number of species in a system and the rates of ecological processes. We present the results of a simple model that suggests there may be no single, generalizable relationship between species diversity and the productivity of an ecosystem because the relative contributions of species to productivity change with environmental context. The model determined productivity for landscapes varying in species diversity (the number of species in the colonist pool), spatial heterogeneity (the number of habitat types composing the landscape), and disturbance regimes (+/− a non‐selective mortality). Linear regressions were used to relate species diversity and productivity for each of the environmental contexts. Disturbance changed the form of the diversity/productivity relationship by reducing the slope (i.e. the change in productivity per species added to the colonist pool), but spatial heterogeneity increased or decreased this slope depending on the particular habitat types composing the landscape. The cause of the diversity/productivity relationship also changed with environmental context. The amount of variation in productivity explained by species diversity always increased with spatial heterogeneity, while the amount of variation explained by species composition (i.e. the particular species composing the colonist pool) tended to increase with disturbance. These results lead us to conclude that the form and cause of the relationship between species diversity and productivity may be highly dynamic‐changing over both time and space. Because the trends resulted from well‐known mechanisms by which environmental variation alters the absolute and relative abundances of taxa, we suspect this conclusion may be applicable to many different systems.
Estimating effects of constraints on plant performance with regression quantiles
Authors: Cade Brian S. , Guo Qinfeng
Published: December 2004

Rates of change in final summer densities of two desert annuals, Eriogonum abertianum and Haplopappus gracilis, as constrained by their initial winter germination densities were estimated with regression quantiles and compared with mechanistic fits based on a self‐thinning rule proposed by Guo et al. (1998); Oikos 83: 237–245). The allometric relation used was equivalent to S=Nf (Ni)−1=cf (Ni)−1, where S is the ratio of final to initial densities (survivorship), cf is a constant that is a final density specific to the species and environment, Ni is the initial plant density, and Nf is final plant density. We used regression quantiles to estimate cf assuming the exponent of −1 was fixed (model 1, Nf (Ni)−1=cf (Ni)−1) and also obtained estimates by treating the exponent as a parameter to estimate (model 2, Nf (Ni)−1=cf (Ni)λ). Regression quantiles allow rates of change to be estimated through any part of a data distribution conditional on some linear function of covariates. We focused on estimates for upper (90–99th) quantiles near the boundary of the summer density distributions where we expected effects of self‐thinning to operate as the primary constraint on plant performance. Allometric functions estimated with regression quantiles were similar to functions fit by Guo et al. (1998) when the exponent was constrained to −1. However, the data were more consistent with estimates for model (2), where exponents were closer to −0.4 than −1, although model fit was not as good at higher initial plant densities as when the exponent was fixed at −1. An exponential form (model 3, Nf (Ni)−1=cf (Ni)λ

eγNi) that is a generalization of the discrete logistic growth function, where estimates of λ were −0.23 to −0.28 and estimates of γ were −0.003 to −0.006, provided better fit from low to high initial germination densities. Model 3 predictions were consistent with an interpretation that final summer densities were constrained by initial germination densities when these were low (<40 per 0.25 m2 for Eriogonum and <100 per 0.25 m2 for Haplopappus) and were constrained by the self‐thinning process at higher germination densities. Our exponential model (3) estimated with regression quantiles had similar form to the mechanistic relation of Guo et al. (1998) when plotted as a survivorship function, but avoided the unrealistic assumption that all populations attained a similar final density, and was based on a statistical model that has formal rules for estimation and inference.
Comparative analysis of phylogenetic and fishing effects in life history patterns of teleost fishes
Authors: Rochet Marie‐Joëlle , Cornillon Pierre‐André , Sabatier Robert , Pontier Dominique
Published: December 2004

The effects of fishing on life history traits and life history strategies of teleost fishes are analysed by a new comparative method that splits traits into an allometric part (size effect), an autoregressive phylogenetic component, and an environmental component (fishing effect). Both intra‐ and inter‐specific variation of age and size at maturity, fecundity, adult size and egg size are analysed by comparing 84 populations of 49 species submitted to various fishing pressures. Two axes of life history diversification are found among teleosts. One is the well‐known slow‐fast continuum separating short‐lived and early maturing species (like Clupeiformes) from longer‐lived species that mature late relative to their size and spawn larger eggs (like salmonids or Scorpaeniformes). An additional strategy involves the schedule of resource allocation to growth and reproduction. Indeterminate growth allows higher teleosts (e.g. Gadiformes) to reach a large size while maturing early and laying small eggs. Increasing fishing pressure decreases age at maturity and egg size, and increases fecundity at maturity, the slope of the fecundity‐length relationship and relative size at maturity. These compensations for higher adult mortality differ among life history strategies. Indeterminate growth is associated with a greater flexibility in resource allocation to growth and reproduction that facilitates greater resilience to fishing mortality.
Phenolic contents of submerged, emergent and floating leaves of aquatic and semi‐aquatic macrophyte species: why do they differ?
Authors: Smolders A. J. P. , Vergeer L. H. T. , Van Der Velde G. , Roelofs J. G. M.
Published: December 2004

The phenolic content of leaves of 40 aquatic and semi‐aquatic plant species, collected from stands distributed over the Netherlands was investigated. Species with submerged leaves showed a significantly lower phenolic content than species with emergent or floating leaves; the latter two showing no significant differences. Possible explanations for the observed differences are discussed in relation to defence strategies.
Understanding avian responses to forest boundaries: a case study with chickadee winter flocks
Authors: Desrochers André , Fortin Marie‐Josée
Published: December 2004

Avian species diversity and territory location are often associated with sharp forest edges, or boundaries. However, our understanding of behaviours underlying avian distribution near forest boundaries, especially species with large home ranges, remains poor. In a two‐year study, we measured chickadee flock movements in forests at 0–300 m from boundaries in an agricultural landscape near Edmonton, Alberta, Canada. Even though flocks foraged in all forested parts of the study area, only six of the 53 flocks studied foraged further from forest boundaries than expected if they had used their home range irrespectively of the distance to boundaries. We found no evidence that the association of chickadee flocks to forest boundaries resulted from vegetation changes near boundaries, or to changes in their foraging sites and foraging success near boundaries. However, chickadee flocks moved parallel to boundaries (<75 m away) twice as frequently as expected from random movement, thus suggesting that forest boundaries act as movement conduits. Even when birds do not apparently seek special features near forest boundaries, they may be closely associated with them, simply because their passive movements are constrained by habitat barriers defined by boundaries.
The effects of genetic and environmental factors on disease expression (stroma formation) and plant growth in Brachypodium sylvaticum infected by Epichloë sylvatica
Authors: Meijer Gerrit , Leuchtmann Adrian
Published: December 2004

Fungi in the genus Epichloë (Clavicipitaceae, Ascomycota) are endophytic and often mutualistic symbionts of many grasses in temperate areas. Species with a sexual cycle suppress host flowering and seed formation, whereas asexual fungi remain asymptomatic and transmit vertically by seed. Thus, the mode of reproduction may determine whether the symbiosis is mutualistic or parasitic. The level of sexual reproduction (disease expression) varies among different endophytes and on different grass hosts, but factors responsible for this variation, and evolutionary mechanisms leading to one or the other life strategy are not understood. As experimental system, we chose Brachypodium sylvaticum in which the endophyte E. sylvatica can express both reproduction modes. A field experiment was done in plots of a free air carbon dioxide enrichment (FACE) facility. We investigated the effects of three environmental factors (elevated CO2 concentration, shading and fertilisation) and one genetic factor (plant and fungal genotype combination), on plant growth and disease expression. Variation in plant growth was mainly dependant on the genotype and was increased by fertilisation. Elevated CO2 and shading slightly stimulated plant growth, but only in fertilised plants. Disease expression was overwhelmingly dependent on the genotype, hence genetic factors. Fertilisation slightly stimulated disease expression in some genotypes, while the effect of elevated CO2 was negligible, and by interaction with fertilisation inconsistent in the two years. Horizontal transmissions during the experiment, presumably mediated by ascospores, confounded the original infection status of the plants. Contagious infections occurred more frequently in the shade, and in endophyte‐free host plants. The latter suggests that pre‐existing infections render host plants less susceptible to superinfection by choke forming strains. Although our results clearly indicate that disease expression of E. sylvatica has a genetic basis, it is still unclear whether selection on the plants or the fungi is driving the evolution of this association.
Operational sex ratio in terrestrial isopods: interaction between potential rate of reproduction and Wolbachia‐induced sex ratio distortion
Authors: Moreau Jérôme , Rigaud Thierry
Published: December 2004

Selfish genetic elements distorting sex ratio are known in several arthropods. By inducing a deficit of males, these sex ratio distorters may modify sexual selection by reversing the sex that competes for a mate. They also have potential to reduce the male proportion to values limiting mating possibilities and therefore limiting population size. Wolbachia endosymbionts are intracytoplasmic, vertically transmitted bacteria that convert genotypic males of terrestrial isopods (woodlice) into functional females. We have tested the impact of these feminizing symbionts on the operational sex ratio (OSR) in three woodlice species. Preliminary experiments consisted in estimating the potential rate of reproduction in males and females, and measuring the dynamics of the onset of reproduction in the wild. These parameters were then combined with population sex ratio to discriminate key factors influencing the OSR. The results suggest that the high potential rate of reproduction of males and the asynchrony in female receptivity both counterbalance female‐biased sex ratios. The result is an overall balanced or slightly female‐biased OSR. Male deficit can therefore not be considered as a factor strongly limiting reproduction in woodlice. Some females were nevertheless found not mated in the wild at the beginning of the reproductive season, most of them being infected by Wolbachia. This suggests that uninfected females may have an advantage as the first mate. Consequences of these findings on woodlice population dynamics are discussed.
Response of galling invertebrates on Salix lanata to reindeer herbivory
Authors: Olofsson Johan , Strengbom Joachim
Published: December 2004

Browsing and defoliation often increase the densities of insect herbivores on woody plants. Densities of herbivorous invertebrates were estimated in a long‐term grazing manipulation experiment. More then 30‐yr‐old fences allow us to compare densities of invertebrate herbivores on Salix lanata in areas heavily grazed and areas lightly grazed by reindeer. The number of gall‐forming insects (Pontania glabrifons) and gall‐forming mites were higher on the heavily grazed shrubs than on lightly grazed shrubs. In contrast to most short‐term studies, the heavily grazed S. lanata had shorter current annual shoots. No difference in leaf size, leaf nitrogen content, or C:N ratio between grazing intensities were detected. However, the enhanced natural δ15N value indicates that heavily grazed shrubs get a higher proportion of their N directly from reindeer faeces. Leaf weight per unit area and relative fluctuating asymmetry of leaf shape increased in heavily grazed S. lanata. Enhanced relative fluctuating asymmetry might indicate higher susceptibility to herbivores. Long‐term grazing seems to increase the density of invertebrate herbivory in the same way as short‐term grazing, even if the plant responses differ substantially.
Hummingbird avoidance of nectar‐robbed plants: spatial location or visual cues
Authors: Irwin Rebecca E.
Published: December 2004

Broad‐tailed and rufous hummingbirds avoid plants and flowers that have recently been visited by nectar‐robbing bees. However, the cues the hummingbirds use to make such choices are not known. To determine the proximate cues hummingbirds use to avoid visiting nectar‐robbed plants, I conducted multiple field experiments and one aviary study using the nectar‐robbed, hummingbird‐pollinated plant Ipomopsis aggregata. In the first field experiment, free‐flying hummingbirds were presented with plants in which I manipulated nectar volume and the presence of nectar‐robber holes. Hummingbirds visited significantly more plants with nectar and probed more available flowers on those plants, regardless of the presence of nectar‐robber holes. Thus, I hypothesized that hummingbirds may avoid robbed plants based on their spatial memory of unrewarding plants and/or visual cues that nectar absence provides. In an aviary study, I removed spatial cues by re‐randomizing the position of plants after each hummingbird‐foraging bout, but hummingbirds still selected plants with nectar. Nectar may provide a visual cue in I. aggregata flowers because corollas are translucent, and nectar is visible through the side of the corolla. To determine if hummingbirds use this visual cue to avoid plants with no nectar, I masked corolla translucence in a field study by painting flowers with acrylic paint. Hummingbirds still visited significantly more plants with nectar and probed more flowers on those plants, whether or not the corollas were painted. These results suggest that hummingbirds use nectar as a proximate cue to locate and avoid non‐rewarding, nectar‐robbed plants, even in the absence of spatial cues and simple visual cues.
Shading enhances the quality of willow leaves to leaf beetles – but does it matter?
Authors: Sipura Mika , Tahvanainen Jorma
Published: December 2004

Plant shading is commonly recognised as a factor, which increases susceptibility of plants to attack by herbivorous insects. In this study we experimentally investigated the effect of host plant shading on two willow‐feeding leaf beetles, Galerucella lineola feeding upon Salix phylicifolia and Phratora vitellinae feeding upon Salix myrsinifolia. Both beetle species were more abundant on potted willows growing in open habitats than on the same clones placed under the shade of trees. However, in the laboratory the food preference by adults and larval performance showed that the shaded willows are actually better food for both beetle species. On the contrary, when larvae were reared in the field under natural abiotic conditions, we found no difference in larval performance, or if any, even better performance in open habitats. Apparently, higher and more variable daily temperatures in open habitats accelerated the growth of the larvae. When adults were let to emigrate from or immigrate to potted willows, which had been grown in the same conditions but placed either in the open or shady habitats, adults preferred exposed willows. Invertebrate predators were more abundant in open habitats, but we found no differences in leaf beetle mortality by natural enemies between the habitats. Although the larval performance appeared to be approximately equal in the two habitats during the unusually warm study period, we suggest that under suboptimal temperatures the better abiotic conditions of open sites can easily override the better food provided by shaded habitats. The selection of abiotic habitat thus plays a significant role in the adaptive habitat and host plant selection of these beetles within the gradient of shadiness.
Suspension feeders transform massive amounts of seston in large northern rivers
Authors: Malmqvist Björn , Wotton Roger S. , Zhang Yixin
Published: December 2004

The concentration and transport of faecal pellets (FPs) produced by blackfly (Diptera: Simuliidae) larvae were estimated in a large, free‐flowing river in the north of Sweden during 1997–1999. FPs were abundant from May through August and FP loads in transport peaked at 429 t dry mass d−1 at a site in the lower part of the river in 1997. Daily transport at the same site, averaged over each study period (16 Jun.–18 Sep. 1997, 6 Apr.–10 Sep. 1998, and 21 Apr.–5 Aug. 1999), was estimated to be 93.7 t dry mass (3.7 t carbon), 47.5 t dry mass (1.9 t carbon and) and 69.2 t dry mass (2.7 t carbon), respectively. On a large scale, there was a downstream trend of increasing FP concentration and, during periods with greater discharge, sedimentation was reduced so that more material was exported from the river. Samples from six sites in a regulated river (into which our focal river flows) parallel to six sites in the unregulated tributary showed considerably lower FP concentrations in the regulated river, presumably because of much smaller blackfly populations as a consequence of habitat loss through damming. A survey of two other large, unregulated rivers in northern Sweden confirmed that these also carry large amounts of FPs. We conclude that the transformation of small suspended food particles into considerably larger FPs by huge populations of filter‐feeding blackfly larvae is a major process in large northern rivers.
Evolution of pistil length as a choice mechanism for pollen quality
Authors: Lankinen Åsa , Skogsmyr Io
Published: December 2004

During the fertilisation process in plants, pollen tube growth rate may be selected as a trait important in male to male competition. Since female morphology provides the necessary selective arena for such competition, we investigate if sexual selection theory can be used to explain the evolution of pistil length as a female choice mechanism. This choice is performed by direct interference with male to male competition. Furthermore, the sessile nature of plants limits the number of mates a female can choose between, which could limit the benefit a female can gain from distinguishing between donors. To mirror these circumstances, we model a situation when there are only two competitors at a time. Using a game theoretical approach we show that if pollen tube growth rate can be used as an indication of heritable quality, pistil length can be selected in response to variation of this trait. We further find that length of the pistil affects selection of pollen tube growth rate. Thus female preference and male competitive ability co‐evolve, but this does not necessarily lead to a positive relationship between the two. Under certain circumstances we find a negative relation instead. Given realistic differences in male quality, the model indicates that there is a potential for evolution of female morphology as a choice mechanism for pollen quality.
Reproductive biology in diploid Anthericum ramosum and tetraploid A. liliago (Anthericaceae)
Authors: Rosquist Gabrielle
Published: December 2004

Polyploidy is often thought to be associated with asexual reproduction, self‐compatibility and self‐pollination. Polyploids are also often regarded as being less sensitive than diploids to the negative effects of inbreeding. The reproductive biology of two lilioid species, the diploid Anthericum ramosum and the tetraploid A. liliago, was compared in a series of greenhouse and field experiments. Pollination experiments were carried out to study agamospermy (seed set without fertilization), self‐compatibility and early inbreeding depression for both species. Tetraploid A. liliago showed no greater ability to self‐pollinate than diploid A. ramosum and neither of the species showed agamospermy. Both species were self‐compatible, but levels of fruit and seed set were lower after self‐fertilization than after cross‐fertilization in A. ramosum (P<0.001). No reduction in fecundity after self‐fertilization was observed in A. liliago. Fruit set in natural populations of A. ramosum increased significantly (P<0.001) after emasculation (which excluded the possibility of autodeposition of pollen within flowers). In contrast, emasculation of flowers reduced the fruit set within natural populations of A. liliago (P<0.05). The reproductive biology of diploid A. ramosum and tetraploid A. liliago is discussed in the context of polyploidization and the reduced fruit and seed set after self‐fertilization in A. ramosum is discussed in terms of early inbreeding depression.
Interspecific variation in sapling mortality in relation to growth and soil moisture
Authors: Caspersen John P. , Kobe Richard K.
Published: December 2004

To examine the causes of landscape variation in forest community composition, we have quantified sapling mortality as a function of growth and soil moisture for seven dominant species in transition oak‐northern hardwood forests of the northeastern USA. We located saplings in sites that encompassed a wide range of variation in soil moisture and light availability. In mesic conditions, the probability of mortality decays rapidly with increasing growth among shade tolerant species and more gradually among shade intolerant species: the rank order of survivorship at low growth rates is Tsuga canadensis>Fagus grandifolia>Acer saccharum>Fraxinus americana>Acer rubrum>Quercus rubra>Pinus strobus. The relationship between probability of mortality and growth does not vary with soil moisture among species insensitive to drought: Tsuga canadensis, Quercus rubra, and Pinus strobus. However, probability of mortality increases substantially with decreasing soil water availability for the other four species. Acer saccharum and Fagus grandifolia have high mortality rates under xeric conditions even when their growth is not suppressed. Acer rubrum and Fraxinus americana exhibited a steady but more gradual increase in the probability of mortality with decreasing soil moisture. Among the five deciduous hardwood species we examined there is a weak inverse relationship between the ability to survive growth suppression, a measure of shade tolerance, and the ability to survive in xeric conditions, a measure of drought tolerance. Tsuga canadensis, however, is tolerant of growth suppression and exhibits high survivorship in xeric conditions, while Pinus strobus is intolerant of growth suppression but insensitive to soil moisture. Species differences in water‐dependent mortality are consistent with the species distributions across landscape gradients of soil water availability.
Diversity loss, recruitment limitation, and ecosystem functioning: lessons learned from a removal experiment
Authors: Symstad Amy J. , Tilman David
Published: December 2004

A five‐year removal experiment in which plant functional group diversity was manipulated found strong limitation of ecosystem functioning caused by the differing abilities of remaining functional groups to recruit into space left unoccupied by the plants removed. We manipulated functional group diversity and composition by removing all possible combinations of zero, one, or two plant functional groups (forbs, C3 graminoids, and C4 graminoids), as well as randomly chosen biomass at levels corresponding to the functional group removals, from a prairie grassland community. Although random biomass removal treatments showed no significant effect of removing biomass in general on ecosystem functions measured (P>0.05), the loss of particular functional groups led to significant differences in above‐ (P<0.001) and belowground (P<0.001) biomass, rooting‐zone (P=0.001) and leached (P=0.01) nitrogen, nitrogen mineralization (P<0.001), and community drought resistance (P=0.002). Many of these differences stemmed from the marked difference in the ways remaining functional groups responded to the experimental removals. Strong recruitment limitation of C4 graminoids resulted in large areas of open ground, high nutrient leaching, and high community drought resistance in plots containing just this functional group. In contrast, rhizomatous C3 graminoids quickly colonized space and used soil resources made available by the removal of other groups, leading to lower soil nitrate in plots containing C3 graminoids. These effects of recruitment limitation on ecosystem functioning illustrate possible effects of diversity loss not captured by synthetic experiments in which diversity gradients are created by adding high densities of seeds to bare soil.
Seed mass and seed nutrient content as predictors of seed output variation between species
Authors: Henery Martin L. , Westoby Mark
Published: December 2004

In patch‐occupancy models for vegetation, propagule output per area occupied is a key species trait, influencing the potential to colonize vacant patches, and hence species dynamics and coexistence. We estimated seed output across a range of species and quantified its relationship to seed dry mass, seed N and P content, and accessory costs in fruiting structures. Fruiting and seed production data were obtained for 47 woody perennial species, spanning an almost 3000‐fold range of seed mass, over a period of one year in Ku‐ring‐gai Chase National Park, New South Wales, Australia. Seed output was measured as numbers per m2 canopy outline and per m2 leaf area. 
Of cross‐species variation in seed output per m2 canopy outline per year, 72% could be predicted from seed mass alone, with a directly inverse relationship (log‐log slope not significantly different from −1). Seed output per m2 leaf area could be predicted somewhat more tightly (75%), indicating leaf area per canopy outline area accounted for some cross‐species variation. Reproductive production per m2 occupied per year varied much less than seed mass and accounted for the remaining variation in seed output. Although accessory costs were about equal in magnitude to seed mass as a component of aggregate investment per seed, they were strongly correlated with seed mass, and consequently did not add substantially to the predictive power. 
Total mass of N or P per seed were found to be slightly but significantly better predictors of seed output variation than dry seed mass (83% and 78%, respectively). This supports the idea that mineral nutrients are a more fundamental currency for seed production than dry mass. Seed mass, whether measured as dry mass or as N or P, appears to be the principal driver of variation in seed output per m2 occupied, and consequently is among the most important dimensions of ecological variation across coexisting species.
Life on the edge: the demography of short‐season populations of deer mice
Authors: Millar John S. , McAdam Andrew G.
Published: December 2004

We documented populations of deer mice in the Kananaskis Valley, southwestern Alberta, Canada from 1979 to 1997 to determine whether these short‐season populations were more, or less, variable than populations in more temperate environments. We then examined patterns of reproduction, age‐specific survival, and immigration to explain variation in summer population growth. Population densities showed no multi‐annual periodicity and were generally low. At maximum, numbers doubled over the breeding season, but declined over the breeding season in 4 of 16 yr. Variability in population density was low, and similar to that of Peromyscus populations in more temperate environments. No demographic parameters were related to spring population densities, and immigration rates were low when conditions for survival of nestlings and adults were favorable. Variation in summer population growth was attributed primarily to variation in nestling survival among years.
Why does herbivore sex matter? Sexual differences in utilization of Fucus vesiculosus by the isopod Idotea baltica
Authors: Jormalainen Veijo , Honkanen Tuija , Mäkinen Anita , Hemmi Anne , Vesakoski Outi
Published: December 2004

Optimal feeding is often constrained by predation avoidance, which may direct feeding to take place on more sheltered, but less profitable, parts of the habitat. Though constrained, feeding preferences and/or digestive capabilities should co‐evolve with the energetic and nutritional needs of the animal. These needs, as well as risk taking for these goals, may differ between the sexes. We studied sexual differences in the utilization of the brown alga Fucus vesiculosus by the marine isopod Idotea baltica. In a feeding preference test with small pieces of alga both sexes prefer the apical, newly grown parts to the basal parts. However, males show stronger preference than females. When isopods are reared on the apical or basal part of the alga alone, sexes grow differently on these parts. Females gain weight equally well and their intermolt duration is the same on both parts of the alga, while males perform clearly worse on the basal than on the apical part. Despite the better quality of the apical parts of the alga, in a grazing experiment in a “semi‐natural” environment with whole bushes of F. vesiculosus also the basal parts are consumed, and even preferred over apical parts by females. We suggest that feeding preferences are subordinate to microhabitat choice, and therefore predation avoidance constrains optimal feeding. We hypothesize that the better ability of females than that of males to utilize the basal parts of the alga for growth is a co‐adaptation to their risk‐averse general reproductive strategy. We discuss the sexual differences in host plant utilization in the context of reproductive strategies of the sexes, especially different risk sensitivity of foraging, and its potential consequences on plant‐herbivore interaction.
The importance of sand albedo for the thermal conditions on sea turtle nesting beaches
Authors: Hays G. C. , Ashworth J. S. , Barnsley M. J. , Broderick A. C. , Emery D. R. , Godley B. J. , Henwood A. , Jones E. L.
Published: December 2004

At Ascension Island and Cyprus, major nesting areas for green turtles (Chelonia mydas) in the Atlantic and Mediterranean, respectively, visual inspection shows some beaches are light in colour while others are darker. We objectively measured the albedo of the sand on different beaches, i.e. the percentage of the incident solar radiation that was reflected from the sand surface. At sites where albedo was recorded, we also measured the temperature of the sand at nest depths. At both rookeries, the sand temperature was markedly higher on darker beaches due to greater absorption of the incident solar radiation over the diurnal cycle. Temperature loggers buried at nest depths revealed seasonal changes in temperature on both islands, but showed that the lowest temperatures found on the darker beaches rarely dropped below the highest temperatures on the lighter beaches. Sea turtles exhibit temperature‐dependent sex determination. Since sand albedo is a major avenue for the production of a range of incubation temperatures on both islands, it will also have profound implications for hatchling sex ratios. In comparison with both Ascension Island and Cyprus, for samples collected from sea turtle rookeries around the world there was an even greater range in sand albedo values. This suggests that sand albedo, a factor that has previously received little consideration, will have profound implications for nest temperatures, and hence hatchling sex ratios, for other populations and species.
Phenotypic plasticity for dispersal ability in the seed heteromorphic Crepissancta (Asteraceae)
Authors: Imbert Eric , Ronce Ophélie
Published: December 2004

In several species of Asteraceae with seed heteromorphism, the proportion of seeds equipped with wind‐dispersal structure decreases plastically under stressful conditions. This contradicts the adaptive expectation that the dispersal rate should increase with stress. Alternatively, developmental constraints related to the ontogeny of the capitulum may explain these observations, as they may explain the effect of senescence on seed morph proportions. In the present paper, we report on an experiment investigating the effect of stress and senescence on the seed heteromorphic Crepissancta (Asteraceae). Plants were subjected to one of four different treatments: control, simulated herbivory, nutrient depletion, and simulated herbivory plus nutrient depletion. Plants experiencing herbivory produced more fecund seed heads immediately after the application of stress, which was later compensated by a decrease in fecundity per seed head. Environmental stress increased the proportion of seeds with dispersal structures. Though senescence had large effects on fecundity, it had no effect on seed morph proportions in nonstressed plants. These findings agree with adaptive expectations about the effect of stress on dispersal ability, and contrast with results obtained in other seed heteromorphic Asteraceae. Our results cast doubts on the existence of strong developmental constraints acting on the seed morph proportions in Crepissancta.
On the measurement of species diversity incorporating species differences
Authors: Shimatani Kenichiro
Published: December 2004

When pairwise differences (relatedness) between species are numerically given, the average of the species differences weighted by relative frequencies can be used as a species diversity index. This paper first theoretically develops the indices of this type, then applies them to forestry data. As examples of diversity indices, this paper explores the taxonomic diversity and the newly introduced amino acid diversity, which is a modification of the nucleotide diversity in genetics. The first, mathematical part shows that both indices can be decomposed into three inner factors; evenness of relative frequencies (=the Simpson index), the simple average over species differences regardless of relative frequencies, and the taxonomic or genetic balance in relative frequencies. The taxonomic diversity has another decomposition: the sum over the Simpson indices at all the taxonomic levels. The second part examines the effects of different forest management techniques on diversity. It is shown that a thinning operation for promoting survival of specific desirable species also contributed to increasing the taxonomic diversity. If we calculated only conventional indices that do not incorporate species relatedness, we would simply conclude that the thinning did not significantly affect the diversity. The theoretical developments of the first part complement the result, leading us to a better interpretation about contrasting vegetation structures. The mathematical results also reveal that the amino acid diversity involves redundant species, which is undesirable when measuring diversity; hence, this index is used to demonstrates crucial points when we introduce species relatedness. The results suggest further possibilities of applying diversity indices incorporating species differences to a variety of ecological studies.
Do plant toxins impose constraints on herbivores? An investigation using compartmental analysis
Authors: Lambdon Philip W. , Hassall Mark
Published: December 2004

Plant secondary metabolites are often acutely toxic to animals, but the influence of this property on interactions with herbivores has been debated vigorously. In this paper we apply a compartmental model, developed to investigate the movement of insecticides through the bodies of animals, to quantify the effect of natural plant toxins. A simulation based on the model is also used to examine the degree of tolerance necessary for a herbivore to avoid illness. Comparison with studies from insecticide science suggests that the required level of tolerance is high enough to impose major ecological limitations on herbivory at naturally occurring concentrations. Various adaptive mechanisms could be employed to overcome this inhibitory effect, and the model allows a precise analysis of their relative importance in a given species. More generally, elasticity analysis shows that these mechanisms differ considerably in efficiency, with desensitisation at the molecular site of action cited as particularly cost effective. However, such efficient mechanisms are often toxin‐specific, and more likely to evolve in herbivores which display a high degree of host‐plant constancy. It should therefore be less costly to achieve tolerance in oligophagous herbivores than polyphagous ones. Reduced concentrations of secondary metabolites in cultivated species could curtail their effectiveness dramatically and universally.
Determinants of polyphagy by a woolly bear caterpillar: a test of the physiological efficiency hypothesis
Authors: Singer Michael S.
Published: December 2004

The physiological efficiency hypothesis argues that the physiological efficiency of food utilization determines feeding habits of herbivorous insects. Although relatively unsuccessful at explaining dietary specificity, it may explain the food‐mixing habit of individually polyphagous herbivores because they may opportunistically increase physiological efficiency by optimizing nutrient balance or diluting toxins in the course of feeding on multiple host‐plant species. This study tests two predictions of the physiological efficiency hypothesis with the woolly bear caterpillar, Grammia geneura (Strecker) (Lepidoptera: Arctiidae). Namely, both herbivore performance (survival, developmental rate, pupal mass) and growth efficiency should be better on mixed‐plant diets than on single‐plant diets. In a series of three laboratory experiments, I found that larval survival and developmental rate on mixed‐plant diets were superior to performance on single‐plant diets in only one of four cases. In all other cases, mixed‐plant diets were either inferior (female pupal mass) to single‐plant diets or not detectably different from them. Larval growth efficiency on mixed‐plant diets was never superior to that on single‐plant diets. In mixed‐plant treatments, caterpillars often selected a diet that included plant species of relatively low suitability alone, thereby suffering reduced performance and growth efficiency. These results contradict predictions of the physiological efficiency hypothesis, indicating the limitations of the conventional focus on the physiological constraints on food utilization as an explanation for both individual polyphagy and dietary specificity in herbivorous insects.
Lifetime reproductive success in common buzzard, Buteo buteo: from individual variation to population demography
Authors: Krüger Oliver , Lindström Jan
Published: December 2004

We studied annual and lifetime reproductive success (LRS) of both sexes of common buzzard Buteo buteo in eastern Westphalia, Germany. We followed a bottom‐up approach starting from individual breeding attempts, over lifetime reproductive success to derive population demography. Annual breeding performance and survival followed a quadratic relationship with breeding experience; individuals starting their breeding career were less likely to survive and breed successfully than birds of intermediate breeding experience. According to an analysis of selection gradients, both the opportunity and intensity of selection peaked in the early stages of the breeding career. The distribution of both LRS and another fitness measure, λ, was highly skewed, with ca 17% of adult birds producing 50% of fledglings in both sexes. Besides breeding life span and number of breeding attempts, habitat quality and plumage morph were significant predictors of LRS. There were strong differences in LRS and λ between the plumage morphs in both sexes: intermediate pigmented buzzards were much more successful than either dark or light ones. There was no significant difference between buzzard cohorts either in LRS or λ, nor did these fitness measures differ between individuals starting their breeding career at different conditions of food availability. Based on individual life histories, we formed a transition matrix and analysed its properties to study the population as a whole. This analysis showed that the population growth rate was close to unity (0.906, bootstrapped 95% confidence limits: 0.834 and 0.962). Analysis of reproductive values and elasticities further emphasised colour morph differences: the contribution of intermediate individuals to population growth greatly exceeded that of dark or light individuals. Thus most phenomena on all levels from individual breeding attempts over lifetime reproductive success to population demography can be explained by the fitness differences between the colour morphs with the intermediate morph maintaining the current population renewal potential.
Field ecology of freeze tolerance: interannual variation in cooling rates, freeze‐thaw and thermal stress in the microhabitat of the alpine cockroach Celatoblatta quinquemaculata
Authors: Sinclair Brent J.
Published: December 2004

Microclimate recordings were made over four years on the Rock and Pillar Range, New Zealand. These are used in conjunction with mortality and freezing data derived in the laboratory to make inferences about the winter thermal ecology of the freeze‐tolerant alpine cockroach Celatoblatta quinquemaculata (Dictyoptera: Blattidae). Threshold temperatures are identified through laboratory experiments at ecologically relevant cooling rates: 0°C; −3.1°C (FP5: the temperature at which 5% of cockroaches are expected to be frozen); and −4.5°C (FP50). The maximum cooling rate in the microhabitat across any of the thresholds was 0.06°C min−1, considerably slower than the 1°C min−1 normally used in laboratory studies. Freeze‐thaw events occurred regularly in the field, and temperature minima occasionally fell to temperatures lethal to C. quinquemaculata. Variability in snow cover contributed to interannual variation in microclimate temperatures. Decreased snow cover is predicted with climate change scenarios, and this will probably result in more freeze‐thaw cycles and more extreme minimum temperatures in this environment. It is concluded that the limited environmental tolerances of the animals living in this habitat make the few degrees of interannual variation ecologically relevant.
Does the seed size/number trade‐off model determine plant community structure? An assessment of the model mechanisms and their generality
Authors: Leishman Michelle R.
Published: December 2004

This paper examines four key mechanisms of the seed size/number trade‐off (SSNT) models to assess their relevance to a general understanding of plant community structure. Mechanism 1 is that large seeds have a greater probability of winning in competition against smaller seeds. I provide interspecific experimental evidence that there is a competitive hierarchy among seedlings based on seed size. Mechanism 2 is that a trade‐off exists between the number and size of seeds produced for a given reproductive allocation. Negative correlations between seed size and number were found consistently across a range of species from a range of habitats, from published literature. Mechanism 3, that seedling‐seedling competition is an important influence on species composition, was found to exist potentially in a range of environments, including annual‐dominated, post‐fire and gap‐dynamic communities. However, there is little quantitative evidence available and this is likely to be a restrictive mechanism. Mechanism 4, that small seeds are superior colonists due to their greater number, was tested in a field experiment in a calcareous grassland community. No supporting evidence was found, suggesting that the SSNT is not an important determinant of structure in this community. Thus two of the four mechanisms can be considered to hold true generally, while the third mechanism may be valid in particular environments. The fourth mechanism did not apply in the community tested, but could be tested in a wider range of communities.
The impact of stochasticity on the behaviour of nonlinear population models: synchrony and the Moran effect
Authors: Greenman J. V. , Benton T. G.
Published: December 2004

Environmental variation is ubiquitous, but its effects on nonlinear population dynamics are poorly understood. Using simple (unstructured) nonlinear models we investigate the effects of correlated noise on the dynamics of two otherwise independent populations (the Moran effect), i.e. we focus on noise rather than dispersion or trophic interaction as the cause of population synchrony. We find that below the bifurcation threshold for periodic behaviour (1) synchrony between populations is strongly dependent on the shape of the noise distribution but largely insensitive to which model is studied, (2) there is, in general, a loss of synchrony as the noise is filtered by the model, (3) for specially structured noise distributions this loss can be effectively eliminated over a restricted range of distribution parameter values even though the model might be nonlinear, (4) for unstructured models there is no evidence of correlation enhancement, a mechanism suggested by Moran, but above the bifurcation threshold enhancement is possible for weak noise through phase‐locking, (5) rapid desynchronisation occurs as the chaotic regime is approached. To carry out the investigation the stochastic models are (a) reformulated in terms of their joint asymptotic probability distributions and (b) simulated to analyse temporal patterns.