Declining body size in an endangered large mammal

Body size affects almost every aspect of the biology of a species. According to the ‘resource rule’, decreasing resource availability (e.g. prey density) will lead to a reduction in body size or, alternatively, a decline in mass‐independent energy expenditure. In the present study, we provide a test of this hypothesis, assessing the effect of significantly decreasing prey density on endangered African wild dog (Lycaon pictus) body size and energy expenditure over a 20‐year period. As predicted from the ‘resource rule’, decreasing resource availability resulted in energetic re‐allocation: wild dogs' body size decreased significantly (both shorter and slimmer), whereas our fitness‐related measure of energy expenditure (i.e. litter size) remained constant over time. A phenotypic change of up to 17% within 20 years, as found in the present study, appears to be unprecedented in a nonharvested large mammal, thus advancing the emerging field of eco‐evolutionary dynamics. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 8–12.     

Energetic costs, underlying resource allocation patterns, and adaptive value of predator-induced life-history shifts

We studied costs and benefits of life history shifts of water fleas (genus Daphnia ) in response to infochemicals from planktivorous fish. We applied a dynamic energy budget model to investigate the resource allocation patterns underlying the observed life history shifts and their adaptive value under size selective predation in one coherent analysis. Using a published data set of life history shifts in response to fish infochemicals we show that Daphnia invests less energy in somatic growth in the fish treatment. This observation complies with theoretical predictions on optimal resource allocation. However, the observed patterns of phenotypic plasticity cannot be explained by changes in resource allocation patterns alone because our model-based analysis of the empirical data clearly identified additional bioenergetic costs in the fish treatments. Consequently, the response to fish kairomone only becomes adaptive if the intensity of size selective predation surpasses a certain critical level. We believe that this is the first study that puts resource allocation, energetic costs, and adaptive value of predator induced life-history shifts – using empirical data – into one theoretical framework.     

Size-dependent sex allocation in a monocarpic perennial herb, <Emphasis Type="Italic">Cardiocrinum cordatum</Emphasis> (Liliaceae)

The size-dependent sex allocation model predicts that the relative resource allocation to female function often increases with plant size in animal-pollinated plants. If size effects on reproductive success vary depending on the environmental conditions, however, the size dependency may differ among populations. We tried to detect site-specific variation in size-dependent sex allocation of a monocarpic hermaphrodite with reference to light availability. Multiple flowers and fruits were sampled from the individuals of Cardiocrinum cordatum, a monocarpic understory herb, and pollen, ovule and seed production were measured with reference to the plant size in two populations. Furthermore, frequency and foraging behavior of pollinator visitation was observed. Ovule production per flower increased with plant size in both populations, while pollen production per flower increased with size only in the population under sparse canopy. Therefore, proportional allocation to male function decreased with plant size in the population under closed canopy, but did not change in the population under sparse canopy. Pollinators usually visited only one flower per plant, indicating the negligible geitonogamous pollination in this species. Although seed production under closed canopy was lower than that under sparse canopy, seed-set rate per flower and seed mass per fruit were independent of plant size in either of the populations. Size-dependent sex allocation in this species was site-specific, suggesting that not only resource storage before reproduction (i.e., plant size) but also resource availability of environment throughout the reproductive process (i.e., light availability) affect reproductive performance in this species.     

Resource availability modulates the effect of body size on reproductive development

Within-species variation in animal body size predicts major differences in life history, for example, in reproductive development, fecundity, and even longevity. Purely from an energetic perspective, large size could entail larger energy reserves, fuelling different life functions, such as reproduction and survival (the “energy reserve” hypothesis). Conversely, larger body size could demand more energy for maintenance, and larger individuals might do worse in reproduction and survival under resource shortage (the “energy demand” hypothesis). Disentangling these alternative hypotheses is difficult because large size often correlates with better resource availability during growth, which could mask direct effects of body size on fitness traits. Here, we used experimental body size manipulation in the freshwater cnidarian Hydra oligactis, coupled with manipulation of resource (food) availability to separate direct effects of body size from resource availability on fitness traits (sexual development time, fecundity, and survival). We found significant interaction between body size and food availability in sexual development time in both males and females, such that large individuals responded less strongly to variation in resource availability. These results are consistent with an energy reserve effect of large size in Hydra. Surprisingly, the response was different in males and females: small and starved females delayed their reproduction, while small and starved males developed reproductive organs faster. In case of fecundity and survival, both size and food availability had significant effects, but we detected no interaction between them. Our observations suggest that in Hydra, small individuals are sensitive to fluctuations in resource availability, but these small individuals are able to adjust their reproductive development to maintain fitness.     

Fuelling flight in a parasitic wasp: which energetic substrate to use?

1. Flight is an energy‐demanding behaviour in insects. In parasitic wasps, strategies of nutrient acquisition and allocation, resulting life‐history trade‐offs and relationships with foraging strategies and resource availability have received much attention. However, despite the ecological importance of dispersal between host and food patches, and the great impact energy diverted to flight should have on lifetime reproductive success, the eco‐physiology of flight in parasitoids is poorly understood. 2. The objective of this study is to (i) identify the energetic resources used to fuel flight, and (ii) relate nutrient type and rate of utilisation to selective pressures in terms of resource availability posed by the environment. 3. Using a flight mill and biochemical assays, we compared flight performance and nutrient dynamics during flight between two reproductive modes of the parasitoid Venturia canescens Gravenhorst, which is known to thrive preferentially in contrasted environments (i.e. natural vs. anthropogenic habitat), differing notably in host and food distribution. 4. Biochemical analyses of different nutrient types showed that glycogen is the flight fuel used by this species, yet no significant differences in its dynamics in flight were found between the two reproductive modes. 5. Results suggest that both glycogen quantity and flight performance are related to the diverging ecological conditions experienced by thelytokous and arrhenotokous strains.     

Plasticity and ontogenetic drift of biomass allocation in response to above- and below-ground resource availabilities in perennial herbs: a case study of <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis>

Changes in plant biomass allocation in response to varying resource availabilities may result from ontogenetic drift caused by allometric growth (i.e., apparent plasticity), a true adjustment of ontogenetic trajectories (true plasticity) or both (complex plasticity). Given that the root allocation of annual species usually decreases during the growth, the developmentally explicit model predicts that annual herbs will exhibit true plasticity in root allocation under above-ground resource limitation and apparent plasticity for moderate stress of below-ground resource. For perennial species, the root allocation of which increases during growth, the reverse patterns would be expected. In this study, we tested the developmentally explicit model with a perennial weed, Alternanthera philoxeroides (Mart.) Griseb. We report its adaptive changes and ontogenetic drift of root allocation in response to different resource levels (i.e., light, water and nutrient availability) by comparing root allocation on both an age and a size basis. The root allocation of A. philoxeroides increased with the size (i.e., ontogenetic drift) during the growth, and exhibited significant changes in response to different resource availabilities. Furthermore, the root allocation in response to water or nutrient availability exhibited typical complex plasticity, while the light stress only slowed down the growth, with the ontogenetic trajectory unchanged (apparent plasticity). The contrasting responses to above-ground and below-ground stresses were consistent with the prediction of the developmentally explicit model.     

Rainbow trout in seasonal environments: phenotypic trade‐offs across a gradient in winter duration

Survival through periods of resource scarcity depends on the balance between metabolic demands and energy storage. The opposing effects of predation and starvation mortality are predicted to result in trade‐offs between traits that optimize fitness during periods of resource plenty (e.g., during the growing season) and those that optimize fitness during periods of resource scarcity (e.g., during the winter). We conducted a common environment experiment with two genetically distinct strains of rainbow trout to investigate trade‐offs due to (1) the balance of growth and predation risk related to foraging rate during the growing season and (2) the allocation of energy to body size prior to the winter. Fry (age 0) from both strains were stocked into replicate natural lakes at low and high elevation that differed in winter duration (i.e., ice cover) by 59 days. Overwinter survival was lowest in the high‐elevation lakes for both strains. Activity rate and growth rate were highest at high elevation, but growing season survival did not differ between strains or between environments. Hence, we did not observe a trade‐off between growth and predation risk related to foraging rate. Growth rate also differed significantly between the strains across both environments, which suggests that growth rate is involved in local adaptation. There was not, however, a difference between strains or between environments in energy storage. Hence, we did not observe a trade‐off between growth and storage. Our findings suggest that intrinsic metabolic rate, which affects a trade‐off between growth rate and overwinter survival, may influence local adaptation in organisms that experience particularly harsh winter conditions (e.g., extended periods trapped beneath the ice in high‐elevation lakes) in some parts of their range.     

An energetic correlate between colony size and foraging effort in seabirds, an example of the Adélie penguin Pygoscelis adeliae

Central-place foraging seabirds alter the availability of their prey around colonies, forming a "halo" of reduced prey access that ultimately constrains population size. This has been indicated indirectly by an inverse correlation between colony size and reproductive success, numbers of conspecifics at other colonies within foraging range, foraging effort (i.e. trip duration), diet quality and colony growth rate. Although ultimately mediated by density dependence relative to food through intraspecific exploitative or interference competition, the proximate mechanism involved has yet to be elucidated. Herein, we show that Adélie penguin Pygoscelis adeliae colony size positively correlates to foraging trip duration and metabolic rate, that the metabolic rate while foraging may be approaching an energetic ceiling for birds at the largest colonies, and that total energy expended increases with trip duration although uncompensated by increased mass gain. We propose that a competition-induced reduction in prey availability results in higher energy expenditure for birds foraging in the halo around large colonies, and that to escape the halo a bird must increase its foraging distance. Ultimately, the total energetic cost of a trip determines the maximum successful trip distance, as on longer trips food acquired is used more for self maintenance than for chick provisioning. When the net cost of foraging trips becomes too high, with chicks receiving insufficient food, chick survival suffers and subsequent colony growth is limited. Though the existence of energetic studies of the same species at multiple colonies is rare, because foraging metabolic rate increases with colony size in at least two other seabird species, we suggest that an energetic constraint to colony size may generally apply to other seabirds.     

林木细根寿命及其影响因子研究进展

 细根周转要消耗大量的C,它影响森林生态系统C分配格局与过程和养分循环,对生态系统生产力具有重要意义。细根的周转取决于细根的寿命,细根寿命越短,周转越快,根系对C的消耗也越多。大量研究表明,细根的寿命与地上部分C向根系供应的多少有密切关系,同时也与细根直径大小、土壤中N和水分的有效性、土壤温度以及根际周围的土壤动物和微生物的活动有关。本文综述了国外近年来在该领域里的研究进展,特别是对控制细根寿命的机理和主要影响因子进行了评述,目的是引起国内研究者的关注,促进我国根系生态学的研究与发展。     

Predicting abundance–body size relationships in functional and taxonomic subsets of food webs

Abundance–body size relationships are widely observed macroecological patterns in complete food webs and in taxonomically or functionally defined subsets of those webs. Observed abundance–body size relationships have frequently been compared with predictions based on the energetic equivalence hypothesis and, more recently, with predictions based on energy availability to different body size classes. Here, we consider the ways in which working with taxonomically or functionally defined subsets of food webs affected the relationship between the predicted and observed scaling of biomass and body mass in sediment dwelling benthic invertebrate communities at three sites in the North Sea. At each site, the energy available to body size classes in the “whole” community (community defined as all animals of 0.03125–32.0 g shell-free wet weight) and in three subsets was predicted from estimates of trophic level based on nitrogen stable isotope analysis. The observed and predicted scalings of biomass and body size were not significantly different for the whole community, and reflected an increase in energy availability with body size. However, the results for subsets showed that energy availability could increase or decrease with body size, and that individuals in the subsets were likely to be competing with individuals outside the subsets for energy. We conclude that the study of abundance–body mass relationships in functionally or taxonomically defined subsets of food webs is unlikely to provide an adequate test of the energetic equivalence hypothesis or other relationships between energy availability and scaling. To consistently and reliably interpret the results of these tests, it is necessary to know about energy availability as a function of body size both within and outside the subset considered.     

Energy expenditure in Crocidurinae shrews (Insectivora): is metabolism a key component of the insular syndrome?

A cascade of morphological, ecological, demographical and behavioural changes operates within island communities compared to mainland. We tested whether metabolic rates change on islands. Using a closed circuit respirometer, we investigated resting metabolic rate (RMR) of three species of Crocidurinae shrews: Suncus etruscus, Crocidura russula, and C. suaveolens. For the latter, we compared energy expenditure of mainland and island populations. Our measurements agree with those previously reported for others Crocidurinae: the interspecific comparison (ANCOVA) demonstrated an allometric relation between energy requirements and body mass. Energy expenditure also scaled with temperature. Island populations (Corsica and Porquerolles) of C. suaveolens differed in size from mainland (gigantism). A GLM showed a significant relationship between energy expenditure, temperature, body mass and locality. Mass specific RMR allometrically scales body mass, but total RMR does not significantly differ between mainland and island, although island shrews are giant. Our results are consistent with other studies: that demonstrated that the evolution of mammalian metabolism on islands is partially independent of body mass. In relation to the insular syndrome, we discuss how island selective forces (changes in resource availability, decrease in competition and predation pressures) can operate in size and physiological adjustments.     

Neuropeptide Y influences acute food intake and energy status affects NPY immunoreactivity in the female musk shrew (Suncus murinus)

Neuropeptide Y (NPY) stimulates feeding, depresses sexual behavior, and its expression in the brain is modulated by energetic status. We examined the role of NPY in female musk shrews, a species with high energetic and reproductive demands; they store little fat, and small changes in energy can rapidly diminish or enhance sexual receptivity. Intracerebroventricular infusion of NPY enhanced acute food intake in shrews; however, NPY had little affect on sexual receptivity. The distribution of NPY immunoreactivity in the female musk shrew brain was unremarkable, but energy status differentially affected NPY immunoreactivity in several regions. Similar to what has been noted in other species, NPY immunoreactivity was less dense in brains of ad libitum shrews and greater in shrews subjected to food restriction. In two midbrain regions, both of which contain high levels of gonadotropin releasing hormone II (GnRH II), which has anorexigenic actions in shrews, NPY immunoreactivity was more sensitive to changes in food intake. In these regions, acute re-feeding (90-180 min) after food restriction reduced NPY immunoreactivity to levels noted in ad libitum shrews. We hypothesize that interactions between NPY and GnRH II maintain energy homeostasis and reproduction in the musk shrew.     

Feeding Competition and Group Size in Alouatta pigra

Researchers consider group size in primates to be determined by complex relationships among numerous ecological forces. Antipredator benefits and better resource defense are the primary pressures for large groups. Conversely, intragroup limited food availability, can result in greater intragroup feeding competition and individual energy expenditure in larger groups, creating energetic advantages for individuals in small groups and placing an upper limit group size. However, the extent to which food availability constrains group size remains unclear for many species, including black howlers (Alouatta pigra), which ubiquitously live in small social groups (≤10 individuals). We studied the relationship between group size and 2 key indices of feeding competition—day journey length and activity budgets—in 3 groups of wild Alouatta pigra at a hurricane-damaged site in Belize, Central America. We controlled for differences in food availability between home ranges (food tree density) and compared both indicators of feeding competition directly with temporal variation in food availability for each group. Our results show no consistent association between resource availability, group size, and either index of competition, indicating that feeding competition does not limit group size at the site—i.e., that larger groups can form without increased costs of feeding competition. The results support the search for other explanations, possibly social ones, for small group size in the primates, and we conclude with suggestions and evidence for such alternative explanations.     

When should species richness be energy limited,and how would we know?

Energetic constraints are fundamental to ecology and evolution, and empirical relationships between species richness and estimates of available energy (i.e. resources) have led some to suggest that richness is energetically constrained. However, the mechanism linking energy with richness is rarely specified and predictions of secondary patterns consistent with energy‐constrained richness are lacking. Here, we lay out the necessary and sufficient assumptions of a causal relationship linking energy gradients to richness gradients. We then describe an eco‐evolutionary simulation model that combines spatially explicit diversification with trait evolution, resource availability and assemblage‐level carrying capacities. Our model identified patterns in richness and phylogenetic structure expected when a spatial gradient in energy availability determines the number of individuals supported in a given area. A comparison to patterns under alternative scenarios, in which fundamental assumptions behind energetic explanations were violated, revealed patterns that are useful for evaluating the importance of energetic constraints in empirical systems. We use a data set on rockfish (genus Sebastes) from the northeastern Pacific to show how empirical data can be coupled with model predictions to evaluate the role of energetic constraints in generating observed richness gradients.     

Resource-dependent sex-allocation in a simultaneous hermaphrodite

Most sex allocation theory is based on the relationship between the resource investment into male and female reproduction and the consequent fitness returns (often called fitness-gain curves). Here we investigate the effects of resource availability on the sex allocation of a simultaneously hermaphroditic animal, the free-living flatworm Macrostomum lignano. We kept the worms under different resource levels and determined the size of their testes and ovaries over a period of time. At higher resource levels, worms allocated relatively more into the female function, suggesting a saturating male fitness-gain curve for this species. A large part of the observed effect was due to a correlated increase in body size, showing size-dependent sex allocation in M. lignano. However, a significant part of the overall effect was independent of body size, and therefore likely due to the differences in resource availability. Moreover, in accordance with a saturating male fitness-gain curve, the worms developed the male gonads first. As the group size was kept constant, our results contrast with expectations from sex allocation models that deal with local mate competition alone, and with previous experiments that test these models.     

A model for seed size variation among plants

Summary I developed a model for seed size variation among plants assuming that the pollen captured per flower depends on both the allocation to pollen capture mechanisms per flower and the number of flowers on each plant. I showed that the optimal seed size increases with (1) the total resource allocation to reproduction, (2) decreasing outcross pollen availability, (3) decreasing probability of seedling establishment and (4) decreasing selfing rate. However, optimal seed size does not depend on the total resource allocation if the total number of pollen grains captured by a plant increases linearly with its flower number. In addition, the optimal seed size is not always positively correlated with the optimal resource allocation to pollen capture mechanisms per flower. I discussed implications of the results for seasonal decline in seed size and seed size variations among populations, such as alutitudinal variation.     

Energetic constraints, size gradients, and size limits in benthic marine invertebrates

Populations of marine benthic organisms occupy habitats witha range of physical and biological characteristics. In the intertidalzone, energetic costs increase with temperature and aerial exposure,and prey intake increases with immersion time, generating sizegradients with small individuals often found at upper limitsof distribution. Wave action can have similar effects, limitingfeeding time or success, although certain species benefit fromwave dislodgment of their prey; this also results in gradientsof size and morphology. The difference between energy intakeand metabolic (and/or behavioral) costs can be used to determinean energetic optimal size for individuals in such populations.Comparisons of the energetic optimal size to the maximum predictedsize based on mechanical constraints, and the ensuing mortalityschedule, provides a mechanism to study and explain organismsize gradients in intertidal and subtidal habitats. For specieswhere the energetic optimal size is well below the maximum sizethat could persist under a certain set of wave/flow conditions,it is probable that energetic constraints dominate. When theopposite is true, populations of small individuals can dominatehabitats with strong dislodgment or damage probability. Whenthe maximum size of individuals is far below either energeticoptima or mechanical limits, other sources of mortality (e.g.,predation) may favor energy allocation to early reproductionrather than to continued growth. Predictions based on optimalsize models have been tested for a variety of intertidal andsubtidal invertebrates including sea anemones, corals, and octocorals.This paper provides a review of the optimal size concept, andemploys a combination of the optimal energetic size model andlife history modeling approach to explore energy allocationto growth or reproduction as the optimal size is approached.     

Regulatory Mechanisms of Reproductive Effort in Plants II. Plasticity in Reproductive Energy Allocation and Propagule Output of Glycine max Merr. (Leguminosae) Cultivated at Varying Densities and Nitrogen Levels

Abstract Plasticity in growth, reproductive energy allocation (RA), and reproductive output were studied in Glycine max Merr. Cv. Enrei (Leguminosae) grown under varying densities and soil nitrogen levels.
Marked plastic responses were detected in individual biomass, the patterns of resource allocation to total reproductive structures (RA) and also to propagules, reproductive outputs, and propagule weight under changing densities and soil nitrogen levels. Plants cultivated at higher densities exhibited proportionately lower individual biomass, lower RA, lower seed output, and smaller seed size in response to increasing density and decreasing soil nitrogen levels, although some deviations were observed, especially in the highest density plot with no fertilization. Differences due to different N-levels were not as great as those to changing density, which may in part be due to the fact that soybean has nitrogen-fixing bacteria in root tubercles, just as in any other Leguminosae. Fecundity was also maintained at the similar high rates of 80–97% in all plots examined, although slight but steady decreases were noted with increasing density. This resemblance in fecundity may be due to its strong inbreeding system.
Another important finding was that seed production under limited resource availability, notably lack of ample solar radiation due to strong interference at higher density plots, is exceedingly costly. This was most clearly exhibited by a sharp increase in relative energy partitioning to a single propagule in response to the increased density, the relative energy cost to a single propagule (RA) increasing from one to seven-fold. The results obtained in this study coincide well with the findings made in other plants, e.g., Helianthus annuus, Oryza sativa , and Coix ma-yuen , with the same experimental designs.     

Proximate factors affecting the larval life history of Acanthocephalus lucii (Acanthocephala)

The growth and eventual size of larval helminths in their intermediate hosts presumably has a variety of fitness consequences. Therefore, elucidating the proximate factors affecting parasite development within intermediate hosts should provide insight into the evolution of parasite life histories. An experimental infection that resulted in heavy intensities of an acanthocephalan (Acanthocephalus lucii) in its isopod intermediate host (Asellus aquaticus) permitted the examination of parasite developmental responses to variable levels of resource availability and intraspecific competition. Isopods were infected by exposure to egg-containing fish feces, and larval infrapopulations were monitored throughout the course of A. lucii development. The relative rate of parasite growth slowed over time, and indications of resource constraints on developing parasites, e.g., crowding effects, were only observed in late infections. Consequently, the factors likely representative of resource availability to larval parasites (host size and molting rate) primarily affected parasite size in late infections. Moreover, at this stage of infection, competitive interactions, gauged by variation in worm size, seemed to be alleviated by greater resources, i.e., larger hosts that molted more frequently. The relatively rapid, unconstrained growth of young parasites may be worse for host viability than the slower, resource-limited growth of larger parasites.