Showing papers in "Functional Ecology in 2008"

The use of leukocyte profiles to measure stress in vertebrates: a review for ecologists

Abstract: Summary 1. A growing number of ecologists are turning to the enumeration of white blood cells from blood smears (leukocyte profiles) to assess stress in animals. There has been some inconsistency and controversy in the ecological literature, however, regarding their interpretation. The inconsistencies may stem partly from a lack of information regarding how stress affects leukocytes in different taxa, and partly from a failure on the part of researchers in one discipline to consult potentially informative literature from another. 2. Here, we seek to address both issues by reviewing the literature on the leukocyte response to stress, spanning the taxa of mammals (including humans), birds, amphibians, reptiles and fish. 3. We show that much of the early literature points to a close link between leukocyte profiles and glucocorticoid levels. Specifically, these hormones act to increase the number and percentage of neutrophils (heterophils in birds and reptiles), while decreasing the number and percentage of lymphocytes. This phenomenon is seen in all five vertebrate taxa in response to either natural stressors or exogenous administration of stress hormones. For the ecologist, therefore, high ratios of heterophils or neutrophils to lymphocytes (‘H : L’ or ‘N : L’ ratios) in blood samples reliably indicate high glucocorticoid levels. Furthermore, this close relationship between stress hormones and N : L or H : L ratios needs to be highlighted more prominently in haematological assessments of stress, as it aids the interpretation of results. 4. As with hormone assays, there are challenges to overcome in the use of leukocytes profiles to assess levels of stress; however, there are also advantages to this approach, and we outline each. Given the universal and consistent nature of the haematological response to stress, plus the overwhelming evidence from the veterinary, biomedical and ecological literature reviewed here, we conclude that this method can provide a reliable assessment of stress in all vertebrate taxa.

Sexual selection, sexual conflict and the evolution of ageing and life span

Abstract: Summary 1Classic evolutionary models interpret ageing as a cost of reproduction, but evolutionary research has thus far largely neglected the conceptual links between the evolution of ageing and a key mode of selection on male and female reproductive strategies – sexual selection and sexual conflict. 2We synthesize ideas and evidence linking sex and ageing, and make the case that a focus on this fascinating problem will ultimately lead to a more complete understanding of both the evolution of ageing and the evolution of sexual strategies. 3The primary and secondary differentiation of male and female reproductive strategies is expected to produce sex-specific optima for traits that affect longevity and ageing rate, often favouring a ‘live fast, die young’ strategy in males, relative to females, although numerous exceptions to this pattern are observed and sex-differences in ageing rate, in particular, remain poorly understood. 4Conversely, environmental factors that influence life expectancy or ageing rate can thereby determine the magnitude or even sign of sexual selection. 5Sexual conflict is expected to displace the sexes from their sex-specific life-history optima through sexually antagonistic interactions, as well as sex-specific selection on loci expressed in both sexes. 6Despite the availability of interesting and testable hypotheses linking sexual selection and ageing, relevant empirical studies are remarkably sparse, and the complex relation between sex, mortality rate and ageing remains poorly understood.

Decomposers in disguise: mycorrhizal fungi as regulators of soil C dynamics in ecosystems under global change

Abstract: Author(s): Talbot, JM; Allison, SD; Treseder, KK | Abstract: 1 In the conventional view of soil carbon (C) cycling, mycorrhizal fungi are primarily considered vectors for plant C input to soils However, there is accumulating evidence that mycorrhizal fungi may also contribute to the direct loss of soil C by acting as decomposers, that is by producing extracellular lytic enzymes and metabolizing soil C 2 Most of the evidence that mycorrhizal fungi can act as decomposers comes from studies of ericoid and ectomycorrhizal fungi, although there is increasing experimental evidence for a role of arbuscular mycorrhizal fungi in soil C decomposition Decomposition by mycorrhizal fungi implies that soil C balance is subjected to the ecological factors that affect both plant and fungal symbionts; this interaction has important consequences for how soil C stocks respond to global change 3 In this synthesis, we propose a new model of soil C cycling, including decomposition of soil C by mycorrhizal fungi, and we evaluate how this new integrative model alters our predictions of soil C feedbacks to global change We present three hypothetical mechanisms by which mycorrhizal fungi may metabolize significant quantities of soil C The first hypothesis ('Plan B' hypothesis) is that mycorrhizal fungi metabolize soil C as an alternate C source when supplies of photosynthate from the host plant are low Our second hypothesis ('Coincidental Decomposer' hypothesis) is that mycorrhizal fungi decompose soil C as a consequence of mining soil for organic nutrients The third hypothesis ('Priming Effects' hypothesis) is that mycorrhizal fungi decompose soil C when allocation of plant photosynthate to mycorrhizal roots is high, such that plant C 'primes' the growth and activity of mycorrhizal fungi 4 Further empirical tests of these hypotheses will clarify the role of mycorrhizal fungi in soil C balance and improve our understanding of soil C responses to global change © 2008 The Authors

Measuring senescence in wild animal populations: towards a longitudinal approach

Abstract: Summary 1A major current challenge in ageing research is to understand why senescence rates vary between individuals, populations and species in wild populations. 2Recent studies clearly illustrate that senescent declines in key demographic and life-history traits can be observed in many wild animal systems. 3Here, we summarize the key challenges facing researchers working to understand senescence in the wild. We concentrate on: (i) limited data availability, (ii) the substantial individual heterogeneity typical of wild populations, (iii) incomplete capture histories, and (iv) trade-offs across the life span. 4We discuss analytical methods to overcome these challenges. We advocate the use of Capture–Mark–Recapture models to remove likely bias associated with re-sampling rates of less than one. We also illustrate that ageing trajectories may vary between different traits in wild populations. Wherever possible, researchers should examine ageing patterns in multiple traits. 5Numerous models are available to describe the rate and shape of senescence in free-living populations, but there is currently little consensus regarding which is most appropriate in analyses of wild organisms. 6We argue that only longitudinal studies of marked or recognizable individuals provide reliable sources of information in the study of senescence. Senescence is a within-individual process and only longitudinal studies allow researchers to separate within-individual ageing patterns from between-individual heterogeneity. 7We examine two analytical approaches to measure ageing using longitudinal data from wild populations: a jack-knifing approach, well-suited to modelling survival probability, and a mixed-effects model approach. Both methods control for sources of between-individual heterogeneity to allow more accurate measurement of within-individual ageing patterns.

Corticosterone in feathers is a long-term, integrated measure of avian stress physiology

Abstract: 1. Stress has pervasive consequences for the well-being of animals. Currently, understanding how individuals cope with stressors is typically accomplished via short-term quantification of blood glucocorticoids released after activation of the hypothalamic-pituitary-adrenal (HPA) axis. 2. We investigated whether the amount of corticosterone (CORT) deposited in growing feathers provides a long-term, integrated measure of HPA activity in birds using captive red-legged partridges Alectoris rufa as a model species. 3. We examined CORT levels in primary feathers induced to grow at the same time as stress series were performed with a capture and restraint protocol. Plasma CORT titres after stress-induced stimulation, but not baseline values, correlated with feather CORT. Feather levels showed the same pattern as plasma of decline across the breeding season, but more severely. 4. For females, CORT in naturally moulted flank feathers was highly and positively correlated with the number of eggs laid in the previous few months, but not clutch size of the following year. For males, the amount of black on a feather, known to be a social signal, was positively correlated with its CORT level. 5. The analysis of feather CORT is a novel methodology that allows for meaningful interpretations of how individuals respond to environmental perturbations and adjust to life-history stages. 6. The analysis of feather hormones has the unique advantages of allowing for experimentation and sampling at any time of the year with minimal investigator-induced impacts and artefacts, and shows the HPA activity of an individual with a flexible time frame from days to months depending on the length of time taken to grow the feather. As this technique can be applied to living or dead birds, or feathers picked up after moult, it provides the ultimate non-invasive physiological measure of considerable benefit in terms of animal welfare and sampling effort.

The evolutionary ecology of senescence

Abstract: Summary 1Research on senescence has largely focused on its underlying causes, and is concentrated on humans and relatively few model organisms in laboratory conditions. To understand the evolutionary ecology of senescence, research on a broader taxonomic range is needed, incorporating field, and, where possible, longitudinal studies. 2Senescence is generally considered to involve progressive deterioration in performance, and it is important to distinguish this from other age-related phenotypic changes. We outline and discuss the main explanations of why selection has not eliminated senescence, and summarise the principal mechanisms thought to be involved. 3The main focus of research on senescence is on age-related changes in mortality risk. However, evolutionary biologists focus on fitness, of which survival is only one component. To understand the selective pressures shaping senescence patterns, more attention needs to be devoted to age-related changes in fecundity. 4Both genetic and environmental factors influence the rate of senescence. However, a much clearer distinction needs to be drawn between life span and senescence rate, and between factors that alter the overall risk of death, and factors that alter the rate of senescence. This is particularly important when considering the potential reversibility and plasticity of senescence, and environmental effects, such as circumstances early in life. 5There is a need to reconcile the different approaches to studying senescence, and to integrate theories to explain the evolution of senescence with other evolutionary theories such as sexual and kin selection.

Carotenoids are minor antioxidants for birds

Abstract: 1 Dipartimento di Biolog?a Animale edell'Uomo, University Sapienza, V?ale dell'Universit? 32,1-00185 Rome, Italy;2 Division of Neuroanatomy and Behaviour, Institute of Anatomy, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland; and 3Laboratoire de Parasitologie Evolutive, CNRS UMR 7103, 6, Universit? Pierre et Marie Curie, B?t. A, 7?me ?tage, 7 quai St. Bernard, 7 Case 237, F-75252 Paris Cedex 05, France

Reproductive ecology of Drosophila

Abstract: Summary 1. Species of the genus Drosophila reproduce in a wide range of different resources, including fruits, sap, flowers, mushrooms and cacti. Drosophila species and their resources also exhibit considerable variability in geographic distribution. 2. Habitat and resource differences pose enormous challenges for Drosophila species. Host chemistry may include highly toxic compounds and breeding sites may be characterized by extreme abiotic conditions such as high and/or low temperature and humidity. 3. Drosophila reproductive biology, in terms of morphology, physiology, and behaviour, is as variable among Drosophila species as is their resource use. In some species, adults are ready to reproduce upon emergence, whereas one sex or the other in other species may require weeks to become sexually mature. 4. Already a robust system for transmission and population genetic studies, the sequencing of the genomes of 12 diverse Drosophila species now brings the power of genomics to investigators wishing to understand the functional aspects of Drosophila ecology

Dietary protein-quality influences melanization and immune function in an insect.

Abstract: Summary 1. In insects, cuticular melanization and immune function are strongly dependent on the quantity of dietary protein ingested. However, relatively little is known about the role played by the quality of nitrogenous resources in determining phenotypic variation in the degree of melanization and correlated immunological functions. We explored this issue in a generalist-feeding caterpillar, Spodoptera littoralis , by providing larvae with one of two semi-artificial diets differing in their quality of protein supplement (high-quality casein vs. low-quality zein). 2. Larvae given a high-quality protein diet had higher survival and faster growth rates than larvae on the low-quality protein diet; they also had more heavily melanized cuticles. Two components of constitutive immunity were assayed: lysozyme-like antibacterial activity and phenoloxidase (PO) activity. PO activity was not affected by diet quality, but antibacterial activity was higher for insects on the high-quality diet, providing a potential physiological mechanism for observed survival differences between the two dietary treatments. 3. Analysis of nitrogen conversion efficiency using chemically defined diets indicated that proteinquality had little effect on ingestion rates (i.e. nitrogen acquisition), but that post-ingestive utilization of nitrogen was reduced for larvae on the low-quality protein diet. This result implies that proteinquality had a significant influence on the nitrogen pool potentially available for investment in melanin production and immune function. 4. A split-plot, full-sib family breeding experiment was used to dissect the genetic control of cuticular melanization from the effects induced by dietary treatment. Estimates of broad-sense heritability indicated that the expression of melanization had a significant genetic basis, but there was little evidence for a genotype × environment interaction. 5. These results suggest that nutrition is a key factor that influences insect melanization and mediates its coupling with important physiological functions linked to survival.

Predation risk induces stress proteins and reduces antioxidant defense

Abstract: Summary 1Despite its wide ecological relevance, we know little about the physiological mechanisms underlying the growth vs. mortality by predation trade-off. Here, we test for two costly, potential physiological correlates of the fight-or-flight response that may contribute to the growth reduction under predation risk: induction of stress proteins (Hsp60 and Hsp70) and of antioxidant enzymes (superoxide dismutase, SOD and catalase, CAT), in larvae of the damselfly Enallagma cyathigerum. 2Under predation risk, there was a growth reduction and an increase in oxygen consumption, indicative of the fight-or-flight response. Predation risk did not affect Hsp60 levels but induced an increase in energetically costly Hsp70 levels. 3Under predation risk, levels of SOD remained constant and those of CAT decreased. Together with the increase in respiration, this should inevitably result in oxidative stress. 4Our results suggest that induction of stress proteins may contribute to the partly physiologically mediated growth reduction under predation risk and that oxidative stress is a novel cost of predation risk that may have important long-term negative fitness consequences for the prey. The latter adds to the recent insight that costs of stressors and life-history trade-offs may not always directly operate through increased energy consumption and differential allocation, but, may also work through the increased production of reactive oxygen species.

Soil carbon stocks in experimental mesocosms are dependent on the rate of labile carbon, nitrogen and phosphorus inputs to soils

Abstract: Summary 1. The soil sink for carbon is important in regulating climate and soil fertility. The sink strength is dependent on the balance of soil carbon decomposition and formation. Variation in the rates of these processes to manipulations of resource availabilities under global change, such as elevated atmospheric carbon dioxide, is not explained by soil microbial theory. 2. To investigate disparate responses of soil carbon dynamics in field investigations, to altered carbon, nitrogen and phosphorus availability, we couple fractionation, isotope and mesocosm techniques to quantify soil carbon decomposition and formation under different resource regimes. These regimes involve addition of multiple levels of carbon, nitrogen and phosphorus, alone and in combination. 3. We hypothesize that: (i) there is no net effect of labile carbon input rate on soil carbon stocks because reductions in soil carbon decomposition are offset by reductions in soil carbon formation; (ii) with simultaneous nutrient addition soil carbon stocks will increase because nitrogen will inhibit further soil carbon decomposition, and mitigate reductions in soil carbon formation observed under elevated labile carbon availability alone; (iii) this increase in soil carbon stocks will be a product of greater formation and decreased decomposition of slower-cycling, mineral-associated, soil carbon, whereas less stable, particulate soil carbon will simply turnover faster (due to greater soil carbon decomposition and formation). 4. In contrast to our predictions formation of soil carbon is positively correlated with labile carbon input rates. In addition, nutrient amendment does not interact with carbon amendment to affect total soil carbon contents. However, there are significant interactive effects when the formation and decomposition responses of different soil carbon fractions are considered. For nitrogen alone, its effects on soil carbon fractions follow our hypotheses. However, phosphorus amendment increases decomposition of the soil carbon fraction that constitutes a longer-term sink. 5. Our results highlight the need for rhizodeposition, phosphorus and soil carbon fractions to be explicitly considered when interpreting potential soil organic carbon responses to altered resource availability. In the discussion, we make four recommendations for future investigations to improve our understanding of soil carbon responses to altered carbon, nitrogen and phosphorus availabilities.

New approach for capturing soluble root exudates in forest soils

Abstract: Summary 1Soluble root exudates are notoriously difficult to collect in non-hydroponic systems because they are released in a narrow zone around roots and are rapidly assimilated by rhizosphere microbes. This has substantially limited our understanding of their rates of release and chemical composition in situ, and by extension, their ecological significance. 2Here we describe the advantages and limitations of several commonly employed methods for measuring exudation with respect to their potential adaptability for field use in forest ecosystems. Then, we introduce a novel in situ method for measuring exudation in forest soils, and present preliminary results of the spatial and temporal dynamics of loblolly pine (Pinus taeda L.) exudation at the Duke Forest FACTS-1 site, North Carolina, USA from April 2007 to July 2008. 3Exudation rates varied by an order of magnitude, with the highest rates occurring in late-June 2007 and mid-July 2008, and the lowest rates occurring during late-August 2007. On an annual basis, we estimate pine roots in the upper 15 cm of soil release c. 9 g C m−2 year−1 via this flux, which represents 1–2% of net primary productivity at the site. 4The magnitude of exudation rates did not differ across an N availability gradient but did track general patterns of below-ground C allocation at the site. Exudation was well-predicted by root morphological characteristics such as surface area and the number of root and mycorrhizal tips, further supporting a possible link between root C allocation and exudation. 5Because all methods for estimating exudates introduce experimental artefacts, we suggest that only a limited amount of ecologically relevant information is probably gleaned from a single method. Thus, a complementary suite of experimental approaches will best enable researchers to understand consequences of changing patterns of exudation in the wake of global environmental change.

Breeding system, pollinator choice and variation in pollen quality in British herbaceous plants

Abstract: Summary 1. Although it is well established that different plant species vary considerably in the quality of pollinator rewards they offer, it is unclear how plant reproductive systems, in particular an obligate dependence on insects for pollination, might influence the evolution of pollinator rewards. Moreover, unlike the interaction between nectar reward and pollinator visitation, we have a limited understanding of the way in which pollen quality influences pollinator foraging behaviour. 2. We quantified the pollen protein and amino acid content for 23 NW European plant species. Pollen quality was compared with breeding system (facultative- vs. obligate insect-pollinated). A subset of 18 plants was sampled from a single habitat. For these we compared the proportion of pollen collection visits made by bumblebees with the quality of pollen offered. 3. We found a significant association between pollen quality and reproductive system; pollen of obligate insect-pollinated species contained higher protein content. We also found a significant relationship with pollinator use; plants most frequently visited by pollen-collecting bumblebees produced the highest-quality pollen. 4. We discuss how the close relationship between pollen quality and bumblebee attraction may have important benefits for plant reproductive success. However, we also show how the disruption of this mutualism can have detrimental consequences for plant and pollinator alike.

What limits insect fecundity? Body size- and temperature-dependent egg maturation and oviposition in a butterfly

Abstract: 1. Large female insects usually have high potential fecundity. Therefore selection should favour an increase in body size given that these females get opportunities to realize their potential advan ...

The role of desiccation tolerance in determining tree species distributions along the Malay–Thai Peninsula

Abstract: Summary 1Patterns of water availability are frequently implicated in local and regional tree species distributions. A major floristic and climatic transition from aseasonal to seasonal evergreen tropical forest is the Kangar–Pattani Line (KPL) in the Indo-Sundaic region of Southeast Asia. We hypothesize that differences in species’ drought tolerance will correspond with their distribution with respect to the KPL. Using a common garden study, we assess the role of differences in physiological drought tolerance traits to geographic distributions for 24 tropical tree species in relation to rainfall seasonality. 2Inherent differences in desiccation tolerance of seedlings were quantified as water potentials and relative water contents (RWCs) below which the plant could no longer support living tissue, and plant water relation parameters were measured using pressure–volume analysis. The relationships among these traits were examined using bivariate trait relationships and a principal components analysis (PCA). The physiological traits contributing most to lethal water potential and RWC were assessed using multiple regression analysis. 3Distribution-related differences in all desiccation tolerance and pressure–volume traits were detected both with and without phylogenetic correction. Widespread species that occurred in seasonally dry forests were able to maintain living tissue at more negative water potentials and lower RWCs than were species restricted to aseasonal forests. Likewise, widespread species demonstrated more negative water potentials at turgor loss, more negative saturated osmotic potential, lower symplastic water fraction and higher bulk modulus of elasticity values. Turgor loss point (TLP) and bulk modulus of elasticity were the best predictors of lethal water potential while symplastic water fraction and bulk modulus of elasticity were included as predictors of lethal RWC. 4Inherent differences in physiological traits contributing to drought tolerance are associated with differences in tropical tree species distributions in relation to rainfall seasonality. These results, combined with lack of support for hypothesized historical factors, strongly implicate climate as a determinant of tree species distributions around the KPL.

Discrimination factors (Δ15N and Δ13C) in an omnivorous consumer : effect of diet isotopic ratio

Abstract: Summary 1Naturally occurring stable isotopes in resources and their consumer allow the estimation of nutritional flows between the two and have been much used to improve our understanding of the nutritional ecology of free-living animals. 2The difference in isotopic composition between an animal and its diet is represented by a discrimination factor. Carbon and nitrogen flows are estimated by calculating the discrimination factors in stable isotope ratios (δ15N and δ13C), which are presumed to be c. 3‰ and 1‰ heavier in the consumer tissues than those in their resources, respectively. 3The discrimination factor is known to vary according to species, tissue, age, growth rates and food quality, but the estimation of discrimination factors is difficult and a fixed discrimination factor is usually used in diet reconstruction. It has also been suggested that discrimination factors could vary linearly with the diet isotopic ratio. If this linear relationship could be demonstrated using regression, this would provide an adequate method for the estimation of discrimination factors. In order to understand how diet isotopic ratios affect the discrimination factor, we investigated the pattern of its change in nitrogen (Δ15N) and carbon (Δ13C) in different tissues (liver, muscle and hair) of an omnivore species, the rat Rattus rattus. We fed captive rats with diets of the same nutritional quality but on different isotopic ratios. 4First, discrimination factors for Δ15N and Δ13C showed great variability, ranging from –1·46‰ to 4·59‰ and from –8·79‰ to 0·64‰, respectively. Discrimination factors depended on both diet isotopic ratio and tissue. 5We also show that isotope ratios in shaved hairs showed a turnover during the first month, and then stabilized during the second month. Using shaved hairs has the potential to be an effective non-lethal method for determining resource shifts in non-specialist consumers. 6Finally, we demonstrated, for all tissues, a decrease of Δ15N and Δ13C with an increased values of δ15N and δ13C, respectively. These relationships allow us to propose a framework to estimate discrimination factors from diet isotopic ratios by means of regression models.

Functional diversity affects decomposition processes in experimental grasslands

Abstract: Summary 1. A number of biodiversity experiments have shown that plant diversity plays a significant role for ecosystem functioning. However, diversity effects on processes involving multi-trophic interactions such as litter decomposition are rather rare. In these experiments, plant diversity is usually categorized into taxonomic units or functional groups. Continuous measures of functional diversity that are based on measurable traits, in contrast, may be a more flexible way to analyse the functional significance of biodiversity. 2. Litter decomposition is a key process in ecosystem biogeochemistry. To understand the consequences of altered biodiversity for ecosystem functioning, it is thus crucial to quantify any potential diversity effects on decomposition processes. 3. I performed several complementary decomposition trials within the BIODiversity and Ecological Processes in Terrestrial Herbaceous ecosystems (BIODEPTH) experiment, which established a gradient of plant species richness and number of functional groups. I hypothesized that decomposi tion rates increase with increasing plant diversity due to non-additive litter mixing effects and more favourable microenvironmental conditions. 4. Decomposition rates of both standard materials and community-specific litter increased with the number of functional groups and with a continuous measure of functional diversity. Species richness, in contrast, had no or rather small positive effects on decomposition. Presence of nitrogen fixing legumes strongly enhanced decomposition, via effects on both litter quality and on the decomposition microenvironment. 5. The predominance of functional diversity rather than of species richness effects suggests that profound knowledge on functional attributes of plant species is the key to understand and to predict biodiversity-decomposition relationships.

Birds use fruit colour as honest signal of dietary antioxidant rewards

Abstract: Summary 1. Fruits are among the most antioxidant-rich foods in nature and thus can be important dietary sources for combating oxidative stress in animals and humans. 2. Because fruits are pigmented by important plant antioxidants such as anthocyanins and carotenoids, frugivores may be able to gauge the antioxidant value of a fruit by its colour. However, although the importance of dietary antioxidants and oxidative stress is increasingly being acknowledged in evolutionary ecology, it is unknown whether animals can use visual signals to detect the presence of antioxidants in their food. 3. We studied the colour and pigment content of 60 bird-dispersed fruits and used an avian eye model to assess the ability of birds to visually discriminate carotenoid and anthocyanin contents of fruits. We then tested whether the frugivorous European blackcap ( Sylvia atricapilla ) preferentially consumes food containing anthocyanins. 4. Fruit colour variation was explained by variation in anthocyanin contents, and birds were capable of discriminating anthocyanin concentrations in fruits based on colour because fruits rich in anthocyanins are black or UV reflecting. However, we found that birds could not use colouration to determine the carotenoid contents of fruits. Ripe fruits contained more anthocyanins than unripe fruits, while there was no difference between ripe and unripe fruits in carotenoid contents. Anthocyanin contents correlated with caloric value in fruits, while there was no such correlation between carotenoids and total energy contents. 5. In food choice experiments, blackcaps selected food containing anthocyanins over food without anthocyanins. 6. In sum, our results indicate that birds actively selected for anthocyanins in their food and that they may use fruit colour as a foraging signal of anthocyanin antioxidant rewards.

Predator odour recognition and avoidance in a songbird

Abstract: Summary 1 Although the ability to detect chemical cues is widespread in many organisms, it is surprising how little is known about the role of chemical communication in avian life histories. Nowadays, growing evidence suggests that birds can use olfaction in several contexts. However, we still do not know the role of bird olfaction in one of the most important determinants of survival, predator detection. 2 Blue tits, Cyanistes caeruleus L., were exposed to chemical cues of: (i) mustelid (predator), (ii) quail (odorous control); or (iii) water (odourless control) inside the nest-box where they were provisioning 8-day-old nestlings. 3 We show that blue tits were able to detect the chemical cues and showed antipredatory behaviours to cope with the risk of predation. Birds delayed their entry to the nest-box, and they perched on the hole of the nest-box and refused to enter more times when they found predator scent than control scents inside the nest-box. In addition, birds decreased the time spent inside the predator-scented nest-box when feeding nestlings. 4 The discovery of the ability of birds to use chemical cues of predators to accurately assess predation may help to understand many aspects of bird life histories that have been neglected until now.

A test of the growth-limitation theory for alpine tree line formation in evergreen and deciduous taxa of the eastern Himalayas

Abstract: 1. Whether the global high elevation tree line phenomenon is associated with a low-temperature-induced limitation of sink activities (i.e. direct impact on meristems and thus, growth) or by a limitation of the trees' carbon source activities (net photosynthesis) still awaits detailed tests across a range of taxa and regions, especially for deciduous species in a short growing season. 2. Here, we test the sink limitation hypothesis in the highest tree lines of Eurasia at altitudes up to 4700 m. We assessed growth and tissue concentrations of non-structural carbohydrates (NSC) as a measure of the carbon source-sink balance in needles and branchwood of Abies, Juniperus (evergreen), Betula and Larix (deciduous). 3. The mean soil temperature in deep shade (a proxy for mean air temperature) across the growing season at tree line in this region is around 6.6 degrees C, which is consistent with the threshold temperature found at the natural climatic limit of the tree life-form worldwide. Mean tree height and stem diameter decreased significantly towards the upper tree line in all species studied. 4. Air temperature measurements at an inverted tree line site (valley bottom) indicate strong and rapid oscillations between nighttime freezing and mild daytime temperatures during late winter, which apparently eradicate Abies and select for Juniperus, offering potential explanations for the inferiority of Abies also at the upper Tibetan tree line. 5. At none of the four altitudinal transects studied did we observe a significant depletion of NSC (carbon limitation) at tree line. Instead, NSC increased in the majority of cases, suggesting direct (meristematic) low temperature constraints of growth. These results for these highest Eurasian tree lines suggest a common mechanism of alpine tree line formation for evergreen and deciduous species.

Below‐ground carbon flux and partitioning: global patterns and response to temperature

Abstract: 1. The fraction of gross primary production (GPP) that is total below-ground carbon flux (TBCF) and the fraction of TBCF that is below-ground net primary production (BNPP) represent globally significant C fluxes that are fundamental in regulating ecosystem C balance. However, global estimates of the partitioning of GPP to TBCF and of TBCF to BNPP, as well as the absolute size of these fluxes, remain highly uncertain. 2. Efforts to model below-ground processes are hindered by methodological difficulties for estimating below-ground C cycling, the complexity of below-ground interactions, and an incomplete understanding of the response of GPP, TBCF and BNPP to climate change. Due to a paucity of available data, many terrestrial ecosystem models and ecosystem-level studies of whole stand C use efficiency rely on assumptions that: (i) C allocation patterns across large geographic, climatic and taxonomic scales are fixed; and (ii) c. 50% of TBCF is BNPP. 3. Here, we examine available information on GPP, TBCF, BNPP, TBCF : GPP and BNPP : TBCF from a diverse global data base of forest ecosystems to understand patterns in below-ground C flux and partitioning, and their response to mean annual temperature (MAT). 4. MAT and mean annual precipitation (MAP) covaried strongly across the global forest data base (37 mm increase in MAP for every 1 °C increase in MAT). In all analyses, however, MAT was the most important variable explaining observed patterns in below-ground C processes. 5. GPP, TBCF and BNPP all increased linearly across the global scale range of MAT. TBCF : GPP increased significantly with MAT for temperate and tropical ecosystems (> 5 °C), but variability was high across the data set. BNPP : TBCF varied from 0·26 to 0·53 across the entire MAT gradient ( − 5 to 30 °C), with a much narrower range of 0·42 to 0·53 for temperate and tropical ecosystems (5 to 30 °C). 6. Variability in the data sets was moderate and clear exceptions to the general patterns exist that likely relate to other factors important for determining below-ground C flux and partitioning, in particular water availability and nutrient supply. Still, our results highlight global patterns in below-ground C flux and partitioning in forests in response to MAT that in part confirm previously held assumptions.

The evolution of senescence from a comparative perspective

Abstract: Summary 1Comparative studies of ageing address the evolutionary lability of the rate of ageing as an indication of potential for, and constraints on, the extension of life span. 2Experimental studies on ageing have focused on damage induced by reactive oxygen species (ROS) and other stresses, and on the mechanisms to prevent or repair this damage. Research on animal models has revealed genes with large effects on life span. However, the relevance of some animal models to human ageing is unclear and it is not known whether evolved differences in ageing involve such major gene effects. 3Studies on the demography of populations of vertebrates in the wild show that animals suffer from senescence in nature. Variation in the rate of ageing is consistent with evolutionary theory in that senescence is delayed in populations that suffer relatively low extrinsic mortality. 4Populations of longer-lived individuals suffer a higher proportion of ageing-related mortality, and thus stronger selection against early ageing. The presence of ageing-related deaths in these populations suggests a lack of suitable mechanisms that would further extend life span. 5Similar patterns of ageing-related mortality in wild and captive or domesticated populations indicate that most ageing-related death is caused by intrinsic factors, such as tumours and cardiovascular failure, rather than increasing vulnerability to extrinsic causes of mortality. 6Studies of several wild populations of long-lived birds suggest that ageing-related mortality is often catastrophic, with individuals maintaining high levels of condition until shortly before their demise. 7Comparative studies of many species suggest connections between early development and the pattern of ageing later in life, consistent with laboratory studies on variation within individual species. The physiological connections across the life span are not well understood. 8Comparative studies have provided important insights into the ageing process. However, we still lack information on important issues, including the causes of death in natural populations, the relationship of within- and between-population variation in the rate of ageing, the genetic basis of variation in rate of ageing in natural populations, and detailed longitudinal studies of individual health and reproductive success in relation to age at death.

Testing the thermal melanism hypothesis: a macrophysiological approach

Abstract: Summary 1. The thermal melanism hypothesis (TMH) predicts that dark (low skin reflectance) individuals are at an advantage in cool climates as they heat faster and reach higher equilibrium temperatures than lighter (higher reflectance) individuals. However, tests of the TMH have yielded mixed support, especially in ectothermic vertebrates. 2. Most studies to date have been undertaken at small spatial scales or using a few, closely related populations or species. Here, we therefore examine the TMH at large scales in heliothermic lizard species, testing two of its major predictions and a corollary thereof, using standard and phylogenetically corrected analyses. 3. First, we test the prediction that skin reflectance and climate variables such as mean annual temperature (MAT) and global solar radiation are positively related across species. Second, we determine whether a positive relationship exists between skin reflectance and body mass. Third, since physiology, behaviour and morphology should be co-adapted, we test the prediction that skin reflectance and traits of thermal biology are positively related. 4. We find strong support for a positive relationship between skin reflectance and mean annual radiation even after adjusting for phylogeny. Moreover, radiation was a better predictor of skin reflectance than MAT. We also find support for a positive relationship of skin reflectance with body size, although this was non-significant after accounting for phylogeny. 5. Skin reflectance was not related to measures of thermal biology, although confounding effects such as methodological differences could not be ruled out. 6. In summary, this study provides novel support for the TMH operating interspecifically at large geographic scales, suggesting adaptive variation of skin reflectance among lizards.

Endophyte symbiosis benefits a rare grass under low water availability

Abstract: Summary 1. Symbiotic relationships with microbes may influence how plant species respond to environmental change. Here, we investigated how a fungal endophyte symbiosis affects the growth and survival of a rare, mid-western United States grass species under altered water availability. In a 12-week long greenhouse experiment, we compared the performance of endophyte-infected and endophytedisinfected grove bluegrass ( Poa alsodes , Poaceae) subjected to two levels of water availability. Gravimetric water measurements indicated that the low water treatment had 24% less soil moisture than the high water treatment. 2. In the high water treatment, the performance of endophyte-infected plants and disinfected plants was similar. However, under low water, endophyte-infected plants had 17% more total biomass than disinfected plants. Despite localization of the endophyte in above-ground plant tissues, effects of the endophyte were stronger below-ground: Under low water, endophyte-infected plants produced 24% more root biomass, but just 14% more shoot biomass than disinfected plants. When water-limited, disinfected plants had 29% greater leaf senescence than endophyte-infected plants. 3. We investigated several plant traits that may underlie benefits of symbiosis. Under low water, disinfected plants had significantly higher relative leaf water content than endophyte-infected plants, consistent with the hypothesis that endophytes cause plants to up-regulate water conservation mechanisms faster in response to drought. There were no strong differences between endophyteinfected and disinfected plants in root morphology, leaf area or water use efficiency, although plants with the endophyte had slightly shorter leaves. 4. Our results suggest that endophyte symbiosis may ameliorate the negative effects of drought stress for grove bluegrass. The heritable transmission of endophytes from plants to seeds provides opportunities for environmental change to influence selection on native grass-endophyte symbioses.

Ageing in a eusocial insect: molecular and physiological characteristics of life span plasticity in the honey bee

Abstract: Commonly held views assume that ageing, or senescence, represents an inevitable, passive, and random decline in function that is strongly linked to chronological age. In recent years, genetic intervention of life span regulating pathways, for example, in Drosophila as well as case studies in non-classical animal models, have provided compelling evidence to challenge these views.Rather than comprehensively revisiting studies on the established genetic model systems of ageing, we here focus on an alternative model organism with a wild type (unselected genotype) characterized by a unique diversity in longevity - the honey bee.Honey bee (Apis mellifera) life span varies from a few weeks to more than 2 years. This plasticity is largely controlled by environmental factors. Thereby, although individuals are closely related genetically, distinct life histories can emerge as a function of social environmental change.Another remarkable feature of the honey bee is the occurrence of reverted behavioural ontogeny in the worker (female helper) caste. This behavioural peculiarity is associated with alterations in somatic maintenance functions that are indicative of reverted senescence. Thus, although intraspecific variation in organismal life span is not uncommon, the honey bee holds great promise for gaining insights into regulatory pathways that can shape the time-course of ageing by delaying, halting or even reversing processes of senescence. These aspects provide the setting of our review.We will highlight comparative findings from Drosophila melanogaster and Caenorhabditis elegans in particular, and focus on knowledge spanning from molecular- to behavioural-senescence to elucidate how the honey bee can contribute to novel insights into regulatory mechanisms that underlie plasticity and robustness or irreversibility in ageing.

Whole ecosystem metabolic pulses following precipitation events

Abstract: Summary 1. Ecosystem respiration varies substantially at short temporal intervals and identifying the role of coupled temperature- and precipitation-induced changes has been an ongoing challenge. To address this challenge we applied a metabolic ecological theory to identify pulses in ecosystem respiration following rain events. Using this metabolic framework, precipitation-induced pulses were described as a reduction in metabolic activation energy after individual precipitation events. 2. We used this approach to estimate the responses of 237 individual events recorded over 2 years at four eddy-covariance sites in southern AZ, USA. The sites varied in both community type (woody and grass dominated) and landscape position (riparian and upland). We used a nonlinear inversion procedure to identify both the parameters for the pre-event temperature sensitivity and the predicted response of the temperature sensitivity to precipitation. By examining multiple events we evaluated the consistency of pulses between sites and discriminated between hypotheses regarding landscape position, event distributions, and pre-event ecosystem metabolism rates. 3. Over the 5-day post-event period across all sites the mean precipitation effect was attributed to 6·1 g CO 2 m ‐2 of carbon release, which represented a 21% increase in respiration over the pre-event steady state trajectory of carbon loss. Differences in vegetation community were associated with differences in the integrated magnitude of pulse responses, while differences in topographic position were associated with the initial peak pulse rate. In conjunction with the differences between sites, the individual total pulse response was positively related to the drying time interval and metabolic rates prior to the event. The quantitative theory presented provides an approach for understanding ecosystem pulse dynamics and helps characterized the dependence of ecosystem metabolism on both temperature and precipitation.

Geographic variation in the response to thermal acclimation in rufous-collared sparrows: are physiological flexibility and environmental heterogeneity correlated?

Abstract: Summary 1It has been proposed that habitats with low productivity, and hence low resource availability, may favour individuals with lower rates of resource use and energy expenditure. In birds, some studies have shown that basal metabolic rate (BMR) and total evaporative water loss (TEWL) are reduced in desert bird species compared to those that live in more mesic areas. However, the contribution of the phenotypic plasticity of birds to this pattern has been poorly studied. 2We evaluated the phenotypic flexibility of BMR, TEWL and organ size in sparrows (Zonotrichia capensis) from three populations along an aridity gradient that differ in environmental temperature, rainfall and seasonality. We also tested the hypothesis that populations from variable environments exhibit more physiological flexibility than populations from stable environments. 3BMR and TEWL in Z. capensis exhibited a positive association with latitude. Additionally, the population from the more stable environment was unaffected by thermal acclimation, but birds from seasonal environments were affected. 4Our results support the hypothesis of the existence of a correspondence between environmental variability and phenotypic flexibility; that is, birds from localities with a drastic climatic seasonality have a greater response in BMR to thermal acclimation than birds from a more stable environment.

Seasonal mortality and the effect of body size : a review and an empirical test using individual data on brown trout

Abstract: Summary 1. For organisms inhabiting strongly seasonal environments, over-winter mortality is thought to be severe and size-dependent, with larger individuals presumed to survive at a higher rate than smaller conspecifics. Despite the intuitive appeal and prevalence of these ideas in the literature, few studies have formally tested these hypotheses. 2. We here tested the support for these two hypotheses in stream-dwelling salmonids. In particular, we combined an empirical study in which we tracked the fate of individually-marked brown trout across multiple seasons and multiple years with a literature review in which we compiled the results of all previous pertinent research in stream-dwelling salmonids. 3. We report that over-winter mortality does not consistently exceed mortality during other seasons. This result emerged from both our own research as well as our review of previous research focusing on whether winter survival is lower than survival during other seasons. 4. We also report that bigger is not always better in terms of survival. Indeed, bigger is often worse. Again, this result emerged from both our own empirical work as well as the compilation of previous research focusing on the relationship between size and survival. 5. We suggest that these results are not entirely unexpected because self-sustaining populations are presumably adapted to the predictable seasonal variation in environmental conditions that they experience.

Why two kinds of stamens in buzz‐pollinated flowers? Experimental support for Darwin's division‐of‐labour hypothesis

Abstract: Summary 1. Most animal-pollinated flowering plants offer nectar as a reward for their pollinators. Some 20 000 species, however, offer only pollen and rely on pollen-foraging bees for pollination. This creates a dilemma since pollen grains contain the male gametes and should be protected from becoming bee food. Darwin was the first to hypothesize that a ‘division-of-labour’ among stamens could solve this dilemma, with some stamens providing pollen as food, others providing pollen for fertilization. This hypothesis can only be tested if pollen grains from the two sets of stamens can be distinguished and their fates determined to the point of attachment on the stigma. 2. We tested Darwin’s hypothesis in Melastoma malabathricum (Melastomataceae), a pollen-only flower with conspicuously differentiated stamens, an inner set that is short and yellow and an outer set that is much longer and purple with small yellow spurs. Pollen release is through terminal anther pores. Taking advantage of different exine patterns on the pollen produced by the two sets of stamens, we carried out a series of experimental manipulations to compare pollinator foraging behaviour and pollen pathways from anthers to stigmas. 3. The results demonstrate that in spite of all 10 stamens being buzzed simultaneously by the carpenter bees that pollinate M. malabathricum , pollen from the purple ‘fertilization’ stamens is dramatically more likely to land on stigmas than pollen from the yellow ‘feeding’ stamens. Removal experiments showed that the yellow ‘feeding’ stamens also attracted pollinators from a distance. Flowers that had their anther pores plugged received fewer buzzing bouts per visit, indicating that bees assessed the amount of pollen received per bout. Surprisingly, there was no significant difference in pollen loads on stigmas of flowers that had their anthers plugged and stigmas of controls, demonstrating the efficiency of vector-assisted cross-pollination and the lack of vector-assisted self-pollination. 4. The unexpectedly precise placement of pollen grains even with buzz pollination, with a large proportion of the grains deposited out of the bees’ grooming reach, helps explain the evolutionary persistence of pollen as a reward in spite of the bees’ ability to assess the amount of pollen received during foraging bouts. Together, these results strongly support Darwin’s division-of-labour hypothesis.

Ecological specialization and the evolution of a specialized caste in Cephalotes ants

Abstract: Summary 1 Ecology can have a profound influence on social evolution. However, the role of ecology in the evolution of specialized altruistic phenotypes or ‘castes’, a recurrent theme in insect societies, is poorly understood. 2 Cephalotes ants nest in pre-existing arboreal cavities, and extant species span four incremental steps in the evolution of a soldier caste that is morphologically specialized for blocking cavity entrances. I address the hypothesis that ecological specialization, defined as specialized use of cavities with entrances close to the area of one ant head, has selected for a morphologically and behaviourally specialized soldier in Cephalotes. This is done with comparative studies of four Cephalotes species, with one representative for each of the four character states of soldier evolution. 3 By quantifying nesting ecology, nest defence and foraging behaviour, I provide strong support for two key predictions of the focal hypothesis. First, the least specialized nesting ecology is seen in the species with the ancestral state of no-soldier. Thus, more individuals are needed to block the average entrance than in other species, and the variation in entrance area is also greater. Second, a systematic increase in ecological specialization (lower mean and variance of entrance area), and behavioural specialization of soldiers, is associated with each of the three transitions to more specialized soldier morphology. This pattern culminates in the consistent use of cavities with an entrance area equal to one soldier head, and a strict soldier-only blocking defence, in the species representing the most specialized and derived soldier morphology. All species used some entrances equal to one ant head, thus it was specifically the specialization on such nests that was associated with increased morphological and behavioural specialization of the soldier. 4 Overall, theses findings provide strong comparative support for the hypothesis that ecological specialization has played a key role in the evolution of a specialized soldier caste in Cephalotes. This pattern is opposite to classic theoretical prediction that a broader resource base should select for caste evolution. The idea that ecological specialization could be a general force in caste evolution is explored, with critical comparative and experimental tests discussed.