Plasticity in Resource Allocation and Nitrogen-use Efficiency in Riparian Vegetation: Implications for Nitrogen Retention

In this work, we summarize our current understanding of the function of riparian zones and describe an investigation of changes in the production per unit nitrogen (N) taken up, or nitrogen-use efficiency (NUE) and resource allocation of a riparian shrub in response to changes in N availability. Empirical work included measuring leaf %N and root-to-shoot ratios (R:S) of individual riparian shrubs (Baccharis salicifolia, or seepwillow) growing at a range of N availabilities in the field and growing in fertilized and unfertilized plots in a field fertilization experiment. In both observational and experimental work, N availability was related positively to %N of plant tissues and negatively to R:S. We used a simulation model to investigate feedbacks between seepwillow responses to and effects on N availability. In the model, plasticity in resource allocation and NUE in response to changes in N led to lower productivity at low N supply and higher productivity and lower retention at high N supply than was observed in plants constrained to a constant %N and R:S. Furthermore, uptake became relatively more important as a retention mechanism when plants responded to high N supply. These feedbacks could have significant effects on N retention by riparian zones in watersheds receiving large fertilizer inputs of N or on ecosystems exposed to high rates of atmospheric N deposition.     

Parasite resistance predicts fitness better than fecundity in a natural population of the freshwater snail Potamopyrgus antipodarum

The cost of males should give asexual females an advantage when in competition with sexual females. In addition, high‐fecundity asexual genotypes should have an advantage over low‐fecundity clones, leading to reduction in clonal diversity over time. To evaluate fitness components in a natural population, we measured the annual reproductive rate of individual sexual and asexual female Potamopyrgus antipodarum, a New Zealand freshwater snail, in field enclosures that excluded competitors and predators. We used allozyme genotyping to assign the asexual females to particular clonal genotypes. We found that the most fecund asexual clones had similar or higher fecundity as the top 10% of sexual families, suggesting that fecundity selection, even without the cost of males, would lead to replacement of the sexual population by clones. Consequently, we expected that the clones with the highest fecundity would dominate the natural population. Counter to this prediction, we found that high annual reproductive rates did not correlate with the frequency of clones in the natural population. When we exposed the same clones to parasites in the laboratory, we found that resistance to infection was positively correlated with the frequency of clones in the population. The correlation between fecundity and parasite resistance was negative, suggesting a trade‐off between these two traits. Our results thus suggest that parasite resistance is an important short‐term predictor of the success of asexual P. antipodarum in this population.     

Influence of adult nutrition on the relationship between body size and reproductive parameters in a parasitoid wasp

Abstract.  1. An important constraint upon life-history evolution in parasitoids is the limit imposed by body size on allocation of limited metabolic resources to different fitness-related physiological functions such as reproduction and survival.
2. The influence of adult nutrition on reproductive and maintenance variables was studied in the synovigenic ectoparasitoid Mastrus ridibundus , and it was determined whether resource allocation to these different functions depends on body size.
3. Over the course of adult life there was a positive relationship between body size and the number of mature eggs in adult females both in the presence and absence of food. However, only in the presence of food did egg maturation rates increase significantly with body size. Starved wasps produced significantly smaller eggs than fed ones, which has not been documented before. Moreover, starved wasps produced fewer offspring than fed wasps, and attacked fewer hosts.
4. The availability of food had a major effect on longevity, with fed females living about 10 times longer than starved ones. There was also a positive relationship between body size and longevity. In starved wasps, this relationship was the same both in the presence and absence of hosts, but in fed wasps there was a positive relationship between body size and longevity in the absence of hosts only. Allocation to initial eggs relative to lifetime progeny production did not decline with body size.
5. The data reveal that in M. ridibundus the trade-off between maintenance and reproduction varies with life expectancy.     

The price of safety: food deprivation in early life influences the efficacy of chemical defence in an aposematic moth

Aposematism is the combination of a primary signal with a secondary defence that predators must learn to associate with one another. However, variation in the level of defence, both within and between species, is very common. As secondary defences influence individual fitness, this variation in quality and quantity requires an evolutionary explanation, particularly as it may or may not correlate with variation in primary signals. The costs of defence production are expected to play a considerable role in generating this variation, yet studies of the cost of chemical defence have focused on species that sequester their defences, while studies in species that produce them de novo are scarce. Here we examine effects of resource availability on the production of chemical defences in female wood tiger moths Arctia plantaginis. This species produces defensive fluids when attacked, and advertises this protection with bright colours on its hindwings. Furthermore, the chemicals in these fluids are produced de novo. In order to establish if the production of this chemical protection is costly, and thus resource‐limited, we manipulated resource availability (i.e. food) during larval development and measured its subsequent effects on adult chemical defence. We presented defensive fluids from female moths to wild blue tits, in the absence of any visual cues, to determine their effectiveness against avian predators. Our results demonstrate that the defensive fluids produced by female moths repel birds, and that these defences require resource investment to produce. We found that moths that were resource‐limited during development had less effective chemical defences, despite producing the same volume of defensive fluids as the control moths. As a reduction in available resources negatively influences the effectiveness of the chemical defence, resource availability may explain the variation seen among individuals.     

Parasite diversity drives rapid host dynamics and evolution of resistance in a bacteria‐phage system

Host–parasite evolutionary interactions are typically considered in a pairwise species framework. However, natural infections frequently involve multiple parasites. Altering parasite diversity alters ecological and evolutionary dynamics as parasites compete and hosts resist multiple infection. We investigated the effects of parasite diversity on host–parasite population dynamics and evolution using the pathogen Pseudomonas aeruginosa and five lytic bacteriophage parasites. To manipulate parasite diversity, bacterial populations were exposed for 24 hours to either phage monocultures or diverse communities containing up to five phages. Phage communities suppressed host populations more rapidly but also showed reduced phage density, likely due to interphage competition. The evolution of resistance allowed rapid bacterial recovery that was greater in magnitude with increases in phage diversity. We observed no difference in the extent of resistance with increased parasite diversity, but there was a profound impact on the specificity of resistance; specialized resistance evolved to monocultures through mutations in a diverse set of genes. In summary, we demonstrate that parasite diversity has rapid effects on host–parasite population dynamics and evolution by selecting for different resistance mutations and affecting the magnitude of bacterial suppression and recovery. Finally, we discuss the implications of phage diversity for their use as biological control agents.     

Resource allocation in the red ant Myrmica ruginodis– an interplay of genetics and ecology

Worker‐queen conflicts over reproductive allocation (colony maintenance vs. reproduction) and sex allocation (females vs. males) were examined in two populations of the facultatively polygynous ant Myrmica ruginodis. Plasticity of social organization in the form of two co‐existing social types (microgyna and macrogyna) has a profound effect on reproductive allocation. Workers control sex allocation by biasing sex ratios towards their own interest, but local resource competition (LRC) because of restricted dispersal of microgyna females resulted in male bias in one study population. Colony sex ratios were split and followed the predictions of the split sex ratio theory: single queen colonies with higher relatedness asymmetry (RA) produced more females than multiple queen colonies with lower RA. Single and multiple queen colonies showed similar patterns in most aspects of their reproduction, and reproductive allocation could not be explained by the hypothesis tested. This suggests that reproductive allocation conflict is of minor importance in M. ruginodis.     

A trade-off between sexual signalling and immune function in a natural population of the drumming wolf spider Hygrolycosa rubrofasciata

The field of ecological immunology is ultimately seeking to address the question ‘Why is there variation in immune function?’ Here, we provide experimental evidence that costs of ubiquitous sexual signals are a significant source of variation in immune function. In the mating season, males of the wolf spider Hygrolycosa rubrofasciata drum against dry leaves while wandering around the habitat searching for receptive females. According to a previous study, the male metabolic rate during the drumming increases 22‐fold compared to the resting metabolic rate. In the present study, we examined whether investment in costly courtship drumming decreases male immune function in a wild population of H. rubrofasciata. We induced males to increase their drumming rate by introducing females in proximity. As estimates of male immune function, we used lytic activity and encapsulation rate. Lytic activity estimates the concentration of antimicrobial peptides in haemolymph, which have been shown to play an important role in defence against bacteria, viruses and fungi. Encapsulation is an important defence mechanism against nematodes and insect parasitoids, but it also plays a role in defence against viruses. Our results show that males with nonarbitrarily increased investment in drumming rate had considerably lower lytic activities than control males. Also, there was a tendency for males with nonarbitrarily increased investment in drumming rate to have lower encapsulation rates than control males. This study provides experimental evidence for the first time, to our knowledge, that there are direct immunological costs of sexual signalling in natural populations. Therefore, immunological costs of sexual signals may provide significant phenotypic variation to parasite‐mediated sexual selection.     

Distribution of Daphnia in a trade-off between food and temperature: individual habitat choice and time allocation

1. In a thermally stratified water column with a deep‐water algal maximum, Daphnia face a trade‐off between food (high fecundity) and temperature (fast development). Recent studies showed that Daphnia populations move up and down the entire water column to take advantage of both, but the proportion of time allocated by individuals to the epilimnion, metalimnion and hypolimnion with their specific food and temperature conditions is not yet known. 2. In a system of 1 m deep, vertical perspex tubes, I established three temperature gradients with 2, 5 and 10 °C differences between the surface (epilimnion) and the bottom layer (hypolimnion). Algae were added to the hypolimnion to simulate a deep‐water algal maximum. 3. The migration behaviour of individual neonate and egg‐bearing Daphnia hyalina × galeata was monitored in order to measure the proportions of time the individuals allocated to the different vertical habitats and to assess the frequency of their shifts between epilimnion and hypolimnion. 4. Neonates stayed continuously at the surface, taking advantage of the higher temperature, possibly because feeding was less important for them because of egg yolk reserves. In contrast, egg‐bearing females spent more time feeding in the hypolimnion when the temperature gradient was weak, but also migrated into the epilimnion to take advantage of the higher temperature. In the steepest temperature gradient, the egg‐bearing females either shifted between epilimnion and hypolimnion, or dwelled constantly in the metalimnion with intermediate conditions.     

Sex and hatching order modulate the association between MHC‐II diversity and fitness in early‐life stages of a wild seabird

Genes of the major histocompatibility complex (MHC) play a pivotal role in parasite resistance, and their allelic diversity has been associated with fitness variations in several taxa. However, studies report inconsistencies in the direction of this association, with either positive, quadratic or no association being described. These discrepancies may arise because the fitness costs and benefits of MHC diversity differ among individuals depending on their exposure and immune responses to parasites. Here, we investigated in black‐legged kittiwake (Rissa tridactyla) chicks whether associations between MHC class‐II diversity and fitness vary with sex and hatching order. MHC‐II diversity was positively associated with growth and tick clearance in female chicks, but not in male chicks. Our data also revealed a positive association between MHC‐II diversity and survival in second‐hatched female chicks (two eggs being the typical clutch size). These findings may result from condition‐dependent parasite infections differentially impacting sexes in relation to hatching order. We thus suggest that it may be important to account for individual heterogeneities in traits that potentially exert selective pressures on MHC diversity in order to properly predict MHC–fitness associations.     

The Shape of the Female Fitness Curve for Cynoglossum officinale: Quantifying Seed Dispersal and Seedling Survival in the Field

Abstract: When hermaphroditic plants shift their sex allocation to produce more seeds and less pollen, it is frequently assumed that the female component of fitness is strictly proportional to the number of seeds produced. However, if producing more seeds results in more competition between seedlings, the female fitness gain curve levels off with high investments in seeds. The shape of this curve is relevant for sex allocation theory, but rarely have data been collected. For Cynoglossum officinale we described the relationship between the number of seeds produced on the mother plant and the number and weight of seedlings in September of the following year. As expected, around large plants of C. officinale more seedlings were retrieved after the germination period in March. The seedlings of large plants were dispersed over an area similar to that of small plants. As a result, seedlings around large plants had a significantly higher chance to have a neighbouring sibling within one dm². Survival and growth of single or grouped (density > 2) seedlings did not differ significantly. As a result, total dry weight of seedlings in September was a linear function of the number of seeds on the parent plant. Our data indicate a linear female fitness gain curve.     

古尔班通古特沙漠南缘3种生活型草本植物生物量分配及相关生长关系

为了解古尔班通古特沙漠不同生活型草本植物的生物量分配特征,通过取样调查当地34种草本植物的生物量研究表明：（1）采集到的每个物种样本的总生物量大多小于4 g,其中多数个体（大于80%）的地上与地下生物量分别在2和0.5 g以内,根冠比主要集中在0.25以内;（2）类短命植物向地下部分分配较多生物量,根冠比较大,集中分布在1.67左右;短命和一年生长营养期草本向地下部分配的生物量远小于类短命植物,两者的根冠比较小,主要集中在0.15左右;（3）短命植物地下与地上部生物量间为等速生长关系,而类短命植物和一年生长营养期草本植物地下与地上生物量间为异速生长关系,其中类短命植物随个体增大向地下部分配生物量的比例增多,而一年生长营养期草本则相反。综上所述,短命、类短命和一年生长营养期草本植物地下与地上生物量间具有不同的分配特征,与各自独特的生活史特征相一致。     

Costs and benefits of host feeding in the parasitoid wasp Trichogramma turkestanica

Females of the parasitoid wasp Trichogramma turkestanica Meyer (Hymenoptera: Trichogrammatidae) generally host feed after ovipositing on the first egg of Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) they encounter. We measured the impact of host feeding on the fecundity and longevity of females, in absence of host or food, and on the fitness of their progeny. We also determined if the frequency of host feeding is influenced by the humidity level at which T. turkestanica females developed. Host feeding increased egg production by 70% but decreased female longevity. This impact of host feeding on the longevity of females is probably due to the allocation of carbohydrates to egg production at the expense of somatic maintenance. Humidity did not influence the occurrence or duration of host feeding. The size of individuals developing in eggs on which females host fed was smaller, indicating that their fitness was affected.     

Resource allocation in a social wasp: effects of breeding system and life cycle on reproductive decisions

Organisms must make important decisions on how to allocate resources to reproduction. We investigated allocation decisions in the social wasp Vespula maculifrons to understand how social insects make reproductive choices. We first determined how annual colonies apportioned resources to growth and reproduction by analysing developing brood. In contrast to expectations, colonies invested in both growth (workers) and reproduction (males) simultaneously. In addition, colonies showed evidence of producing males in pulses and reversing their reproductive choices by decreasing investment in males late in the season. This reversal is consistent with theory suggesting that colonies decrease production in males if fitness of late emerging males is low. To further investigate reproductive decisions within colonies, we determined if the male mates of multiply-mated queens varied in their reproductive success over time. Sperm use by queens did vary over time suggesting that male success may depend on sperm clumping within the female reproductive tract. Finally, we tested if colony sex ratio conformed to expectations under kin selection theory that nestmate relatedness would positively correlate with investment in new queens if workers controlled sex allocation. Surprisingly, the proportion of queens produced by colonies was negatively correlated with nestmate relatedness, suggesting that allocation may be shaped by advantages arising from increased genetic diversity resulting from multiple mating by queens. Overall, our study suggests that the reproductive decisions of colonies are flexible and may depend both on environmental cues arising from energetic needs of the colony and genetic cues arising from mating behaviours of queens.     

Latitudinal trends in foliar oils of eucalypts: Environmental correlates and diversity of chrysomelid leaf‐beetles

Eucalypts are characterized by their oleaginous foliage, yet no one has considered the universality of oil expression or its ecological associations and implications for biodiversity. Published literature on the oils of 66 eucalypts was combined with geographic distribution information contained in the Australian National Herbarium (ANHSIR) database to investigate continent‐scale changes in oil yield and composition. The exposure to fire and rainfall of each eucalypt was considered in reference to Walker's data on fire frequency and Australian Bureau of Meteorology 97‐year records of rainfall variability. Host collection records for 69 species of chrysomelid leaf‐beetle were collated from entomologists to consider patterns of association with a subset of 16 eucalypts. Eucalypts endemic to the seasonally arid, sub‐tropical to tropical climates of northern Australia have less oleaginous and aromatic leaves than species endemic to the mesic, temperate climates of the southern parts of the continent. Maximum oil yield and the concentrations of cineole and pinene were positively correlated with minimum fire interval but not with rainfall variability. Low oil contents in more northerly distributed species may facilitate persistence in highly fire‐prone habitats. There were no patterns in the diversity of chrysomelid leaf‐beetles with either the oil yield or the concentrations of 1,8‐cineole or α‐pinene in their hosts. When taken in consideration with the apparent strategy of eucalypts to tolerate insect herbivory, current evidence augurs against high concentrations of cineole or pinene acting alone as antibiotic plant secondary metabolites.     

Parasite infections in a social carnivore: Evidence of their fitness consequences and factors modulating infection load

There are substantial individual differences in parasite composition and infection load in wildlife populations. Few studies have investigated the factors shaping this heterogeneity in large wild mammals or the impact of parasite infections on Darwinian fitness, particularly in juveniles. A host's parasite composition and infection load can be shaped by factors that determine contact with infective parasite stages and those that determine the host's resistance to infection, such as abiotic and social environmental factors, and age. Host–parasite interactions and synergies between coinfecting parasites may also be important. We test predictions derived from these different processes to investigate factors shaping infection loads (fecal egg/oocyte load) of two energetically costly gastrointestinal parasites: the hookworm Ancylostoma and the intracellular Cystoisospora, in juvenile spotted hyenas (Crocuta crocuta) in the Serengeti National Park, in Tanzania. We also assess whether parasite infections curtail survival to adulthood and longevity. Ancylostoma and Cystoisospora infection loads declined as the number of adult clan members increased, a result consistent with an encounter‐reduction effect whereby adults reduced encounters between juveniles and infective larvae, but were not affected by the number of juveniles in a clan. Infection loads decreased with age, possibly because active immune responses to infection improved with age. Differences in parasite load between clans possibly indicate variation in abiotic environmental factors between clan den sites. The survival of juveniles (<365 days old) to adulthood decreased with Ancylostoma load, increased with age, and was modulated by maternal social status. High‐ranking individuals with low Ancylostoma loads had a higher survivorship during the first 4 years of life than high‐ranking individuals with high Ancylostoma loads. These findings suggest that high infection loads with energetically costly parasites such as hookworms during early life can have negative fitness consequences.     

Pollen feeding,resource allocation and the evolution of chemical defence in passion vine butterflies

Evolution of pollen feeding in Heliconius has allowed exploitation of rich amino acid sources and dramatically reorganized life‐history traits. In Heliconius, eggs are produced mainly from adult‐acquired resources, leaving somatic development and maintenance to larva effort. This innovation may also have spurred evolution of chemical defence via amino acid‐derived cyanogenic glycosides. In contrast, nonpollen‐feeding heliconiines must rely almost exclusively on larval‐acquired resources for both reproduction and defence. We tested whether adult amino acid intake has an immediate influence on cyanogenesis in Heliconius. Because Heliconius are more distasteful to bird predators than close relatives that do not utilize pollen, we also compared cyanogenesis due to larval input across Heliconius species and nonpollen‐feeding relatives. Except for one species, we found that varying the amino acid diet of an adult Heliconius has negligible effect on its cyanide concentration. Adults denied amino acids showed no decrease in cyanide and no adults showed cyanide increase when fed amino acids. Yet, pollen‐feeding butterflies were capable of producing more defence than nonpollen‐feeding relatives and differences were detectable in freshly emerged adults, before input of adult resources. Our data points to a larger role of larval input in adult chemical defence. This coupled with the compartmentalization of adult nutrition to reproduction and longevity suggests that one evolutionary consequence of pollen feeding, shifting the burden of reproduction to adults, is to allow the evolution of greater allocation of host plant amino acids to defensive compounds by larvae.