Low Resource Availability Research Articles

Kernel Set in Maize Hybrids and Their Inbred Lines Exposed to Stress

Heterosis for grain yield in maize (Zea mays L.) has been associated with heterosis for kernel number. The objective of this study was to elucidate physiological traits underlying the superior kernel no. establishment in hybrids in comparison with that in their inbred lines, using the relationship between kernel no. plant−1 (KNP) and plant growth rate during the critical period of approximately 30 d bracketing silking (PGRS). Experiments were performed at the Arkell Research Station near Guelph, ON, Canada, during 2003 and 2004. Maize was grown at three levels of water availability (100, 75, or 60% of daily transpiration) during a period bracketing silking and at two plant densities (6 and 10 plants m−2) without nutrient limitations to generate a range of levels of resource availability plant−1 Kernel no. plant−1 was greater in the hybrids than in their parental inbred lines at all levels of resource availability, which was attributable mainly to a greater kernel set per unit PGRS in the hybrids. Greater kernels set per unit PGRS in hybrids vs. their inbred lines resulted from one or more of the following features: (i) low threshold of PGRS for kernel set, (ii) high kernel set response to PGRS increments at low resource availability plant−1, and (iii) high potential kernel number. Heterosis for kernel set was associated with heterosis for ear growth rate during the critical period for kernel set bracketing silking (EGRs) to varying degrees, and the extent of the association varied with inbred line–hybrid combination and level of resource availability plant−1

Possible fitness consequences of experimentally advanced laying dates in Great Tits: differences between populations in different habitats

Summary In birds, early breeding individuals generally reproduce more successfully than late breeding individuals. The lack of response to this selection could be explained by resource constraints during the egg production period. Parus species can learn from the mismatch experienced between breeding time and nestling food availability and subsequently adjust their breeding time accordingly. In two Great Tit populations, breeding time was manipulated by creating an artificial food peak. This allowed us to study fitness consequences of manipulated breeding time in the following year without the confounding effects of food supplementation. In one population, manipulated females advanced their laying dates in response to the artificial food peak. However, sample sizes were too low to quantify fitness consequences. In the other population, no response to the treatment was found. This difference could be caused by differences in resource availability in early spring between the two habitats. Low resource availability in early spring could also explain the lack of response to selection observed in one population.

Are we eating more than we think? Illegitimate signaling and xenohormesis as participants in the pathogenesis of obesity

Resource utilization may represent a central force driving evolution. A tight link between sensing energy availability and managing energy acquisition and utilization constitutes a common feature among all organisms. While such a link was likely adaptive during prehistoric evolution, modern lifestyles may decouple perceived cues from actual energy availability so as to promote obesity in humans. A particular illegitimate signal is chronic stress, which may shift body phenotype to suit a more conservative state of energy management. In prehistoric times, such a response likely aided survival during periods of low resource availability. However, new sources of chronic stress have emerged that bear little relationship to contextual energy, which is generally abundant in the modern world. In addition, modern techniques of husbandry and agriculture can produce stress in the food chain, such that food itself can act as an illegitimate signal of chronic stress. Obese livestock and unusual fat profiles in farmed fish, meat, and eggs may reflect stress phenotypes. Consumers of stressed foods may sense those signals--a phenomenon known as xenohormesis--and assume the stressed phenotype. This maladaptive process may promote obesity by erroneously biasing hosts towards caloric accumulation in the context of energy abundance. Regional tissue accumulation of fat may indicate local tissue stress. Atherosclerosis may result from stress signals that induce sympathetic bias and regional fat accumulation in vessel adventitia. Medications such as neuroleptics and foods such as diet drinks may generate illegitimate signals by mimicking molecules used for energy management. Implications for the prevention and treatment of dysfunctions related to these derangements are discussed. New strategies for manufacturing biologics by manipulating stress conditions or controlling fatty acid attachments to proteins are envisioned.

Variation in population dynamics and movement patterns between two geographically isolated populations of the dusky hopping mouse (Notomys fuscus)

The ecology of the dusky hopping mouse (Notomys fuscus) was studied at Pelican Waterhole in south-west Queensland and Montecollina Bore in north-east South Australia over an eight-year period. Population parameters of N. fuscus differed markedly between the two study sites. Whilst the population at Montecollina Bore exhibited large fluctuations in size, captures at Pelican Waterhole were lower but more consistent. Recaptures between sessions at Montecollina Bore peaked at 60% but no individuals were recaptured at Pelican Waterhole. No evidence of seasonal breeding was recorded at either site. Male N. fuscus captured at Montecollina Bore were significantly heavier (average 31.4 g) than Pelican Waterhole animals (average 26.6 g). Over a 1–4-night period, the maximum linear distance moved by radio-collared individuals was 1.5 km (average 481 m) at Pelican Waterhole and 400 m (average 199 m) at Montecollina Bore. Differences in population dynamics between the two sites are ascribed to food availability and habitat quality. Whilst Pelican Waterhole may constitute a more stable, less degraded environment, Montecollina Bore appears to be defined by periods of either very high or very low resource availability depending on rainfall. The abundance of dingoes and low cat and fox activity may contribute to the persistence of N. fuscus at the two study sites.

Occurrence and abundance of fungus-dwelling beetles (Ciidae) in boreal forests and clearcuts: habitat associations at two spatial scales

Insect material (> 30,000 individuals) reared from the fruiting bodies of wood-decaying Trametes fungi was compared between old-growth boreal forests and adjacent clearcuts in Finland. Sulcacis affinis and Cis hispidus occurred more frequently and were, on average, more abundant in the clearcuts. Interestingly, Octotemnus glabriculus and Cis boleti had a slightly higher frequency of occurrence in the forests, despite lower resource availability. The former also showed a higher average abundance. On average, the cluster size of Trametes fruiting bodies occurring on woody debris was higher in the clearcuts than in the forests and had a positive effect on species occurrence and abundance in these clusters. The independent effect of the macrohabitat (forest or clearcut) underscores the importance of the macrohabitat where specific resources occur, and this may override the positive effects of resource availability.

Brood size and environmental conditions sex‐specifically affect nestling immune response in the European starlingSturnus vulgaris

In sexually size‐dimorphic species, the larger sex can be more sensitive to stressful environmental conditions, often resulting in reduced growth and elevated mortality rates. Development of the immune system is regarded as highly resource dependent, and recent data suggest that nestling passerines experience a possible resource‐based trade‐off between growth and immunity. Given the hypothesized importance of maximizing growth for the larger sex, the corresponding immune system may also exhibit similar sensitivity to limited resources. To better understand how natural variation in brood size and resources might differentially affect growth and immune function in nestlings of a sexually size‐dimorphic species, we examined the relationship between brood size and inter‐sexual differences in cell‐mediated immunity (CMI) and survival in European starlingSturnus vulgarisnestlings where males are larger in both mass and structural size. We hypothesized that male CMI response should be negatively impacted by increasing sibling competition (brood size), especially during periods of low resource availability. In a year of reduced parental provisioning rates and reduced chick growth rates, male offspring exhibited the predicted negative relationship, whereas female CMI response was unaffected. However, in a year of improved provisioning rates and chick growth, neither sex exhibited a negative relationship between immune response and brood size. Thus, natural variation in brood size can affect sex‐specific immunity differently in offspring of a sexually size‐dimorphic passerine. However, this relationship appears resource‐dependent, suggesting that the hypothesized resource‐based trade‐off may be compensated for in years of adequate resource abundance.

Fine root chemistry and decomposition in model communities of north-temperate tree species show little response to elevated atmospheric CO2 and varying soil resource availability

Rising atmospheric [CO2] has the potential to alter soil carbon (C) cycling by increasing the content of recalcitrant constituents in plant litter, thereby decreasing rates of decomposition. Because fine root turnover constitutes a large fraction of annual NPP, changes in fine root decomposition are especially important. These responses will likely be affected by soil resource availability and the life history characteristics of the dominant tree species. We evaluated the effects of elevated atmospheric [CO2] and soil resource availability on the production and chemistry, mycorrhizal colonization, and decomposition of fine roots in an early- and late-successional tree species that are economically and ecologically important in north temperate forests. Open-top chambers were used to expose young trembling aspen (Populus tremuloides) and sugar maple (Acer saccharum) trees to ambient (36 Pa) and elevated (56 Pa) atmospheric CO2. Soil resource availability was composed of two treatments that bracketed the range found in the Upper Lake States, USA. After 2.5 years of growth, sugar maple had greater fine root standing crop due to relatively greater allocation to fine roots (30% of total root biomass) relative to aspen (7% total root biomass). Relative to the low soil resources treatment, aspen fine root biomass increased 76% with increased soil resource availability, but only under elevated [CO2]. Sugar maple fine root biomass increased 26% with increased soil resource availability (relative to the low soil resources treatment), and showed little response to elevated [CO2]. Concentrations of N and soluble phenolics, and C/N ratio in roots were similar for the two species, but aspen had slightly higher lignin and lower condensed tannins contents compared to sugar maple. As predicted by source-sink models of carbon allocation, pooled constituents (C/N ratio, soluble phenolics) increased in response to increased relative carbon availability (elevated [CO2]/low soil resource availability), however, biosynthetically distinct compounds (lignin, starch, condensed tannins) did not always respond as predicted. We found that mycorrhizal colonization of fine roots was not strongly affected by atmospheric [CO2] or soil resource availability, as indicated by root ergosterol contents. Overall, absolute changes in root chemical composition in response to increases in C and soil resource availability were small and had no effect on soil fungal biomass or specific rates of fine root decomposition. We conclude that root contributions to soil carbon cycling will mainly be influenced by fine root production and turnover responses to rising atmospheric [CO2], rather than changes in substrate chemistry.

Physiological and morphological correlates of among-individual variation in standard metabolic rate in the leopard frog Rana pipiens

Rates of standard metabolism (SMR) are highly variable among individuals within vertebrate populations. Because SMR contributes a substantial proportion of an individual's energy budget, among-individual variation in this trait may affect other energetic processes, and potentially fitness. Here, we examine three potential proximate correlates of variation in SMR: organ mass, serum T4 thyroxine and relative mitochondrial content, using flow cytometry. Body-mass-adjusted kidney mass correlated with SMR, but liver, heart, small intestine and gastrocnemius did not. Thyroxine correlated with SMR, as did mitochondrial content. These results suggest several novel proximate physiological and morphological mechanisms that may contribute to among-individual variation in SMR. Variation in SMR may be maintained by diverse environmental conditions. Some conditions, such as low resource availability, may favor individuals with a low SMR, through small organ size, or low thyroxine or mitochondrial content. Other conditions, such as high resource availability, may favor individuals with a high SMR, through large organ size, or high thyroxine or mitochondrial content.

Evolution of juvenile growth rates in female guppies ( Poecilia reticulata ): predator regime or resource level?

Recent theoretical and empirical work argues that growth rate can evolve and be optimized, rather than always being maximized. Chronically low resource availability is predicted to favour the evolution of slow growth, whereas attaining a size-refuge from mortality risk is predicted to favour the evolution of rapid growth. Guppies (Poecilia reticulata) evolve differences in behaviour, morphology and life-history traits in response to predation, thus demonstrating that predators are potent agents of selection. Predators in low-predation environments prey preferentially on small guppies, but those in high-predation environments appear to be non-selective. Because guppies can outgrow their main predator in low- but not high-predation localities, we predict that predation will select for higher growth rates in the low-predation environments.However, low-predation localities also tend to have lower productivity than high-predation localities, yield-ing the prediction that guppies from these sites should have slower growth rates. Here we compare the growth rates of the second laboratory-born generation of guppies from paired high- and low-predation localities from four different drainages. In two out of four comparisons, guppies from high-predation sites grew significantly faster than their low-predation counterparts. We also compare laboratory born descendants from a field introduction experiment and show that guppies introduced to a low-predation environment evolved slower growth rates after 13 years, although this was evident only at the high food level. The weight of the evidence suggests that resource availability plays a more important role than predation in shaping the evolution of growth rates.

Relative importance of nymphal and adult resource availability for reproductive allocation in Melanoplus sanguinipes (Orthoptera: Acrididae)

A better understanding of the relative effects that nymphal and adult resource availabilities have on reproductive allocation is needed, given the potentially important implications for the prediction of grasshopper population dynamics. I conducted a field experiment to examine the importance of nymphal and adult resource availabilities on reproductive allocation in the grasshopper Melanoplus sanguinipes (Fabricius). Female nymphs were reared on either high or low nymphal resource availability, with watered and ambient-level adult resource treatments. Adult resource treatments were initiated below the carrying capacity, to allow an examination of food limitation effects on reproductive allocation, while minimizing strong density dependent effects. Although nymphs reared with ad libitum resources had higher adult body mass, nymphal resource availability did not affect reproductive allocation or adult survival. Female M. sanguinipes grasshoppers responded to the increased host plant availability in the watered treatment, by increasing reproduction allocation over the course of the experiment. M. sanguinipes females did not reduce vitellogenesis in response to decreased adult resource availability, but completed development of a lower percentage of ovarioles that initiated development. Although resource dynamics in semi-arid grasslands can change rapidly during grasshopper development, there were no direct effects of limited resource availability for developing nymphs that subsequently affected per capita reproduction of adults.

Fungal translocation - creating and responding to environmental heterogeneity

It has been known for a long time that fungi may transport substances in their hyphae. Experiments using radioactive tracer isotopes have greatly expanded our knowledge about translocation and have revealed that many fungi may circulate resources throughout their mycelia. This article outlines a conceptual framework for when and where net-translocation of resources takes place. Effects of substrate qualities, mycelial growth and activity as well as interactions with living plant hosts and other microorganisms are discussed and exemplified with experimental data. It is concluded that translocation from more or less remote parts of the mycelium enables fungi to colonise substrates with a low initial resource availability and to actively increase the resource availability in the substrates, turning the colonising mycelium from a resource sink into a source. Thus, translocation not only occurs in response to environmental heterogeneity, but also allows fungi to create heterogeneity in their growth environment.

The role of niche breadth, resource availability and range position on the life history of butterflies

We analysed the relationship between three life history characteristics (mobility, length of flight period and body size) and niche breadth (larval host plant specificity and adult habitat breadth), resource availability (distribution and abundance of host plants) and range position (distance between the northernmost distribution record and southernmost point of Finland) of the butterfly fauna of Finland. The data is based on literature and questionnaires. Often in across species studies phylogeny may create spurious relationships between life‐history and ecological variables. We took the phylogenetic relatedness of butterfly species into account by analysing the data with phylogenetically independent contrasts (CAIC method). Butterfly mobility was positively related to the niche breadth, resource availability and range position. The length of the flight period was negatively related to the range position, indicating that the species at the northern edge of their distribution range have shorter flight period than species which are further way from the range edge. After controlling for the phylogenetic relatedness we found no significant correlations between body size and niche breadth, resource availability or range position. We suggest that the relationship between the length of the flight period and range position may arise as a consequence of lower hatching asynchrony in edge species as a result of lower environmental variance in larval growth conditions. Our results on the mobility suggest that there is selection pressure towards lower migration rate in species that have restricted niche breadth, low resource availability and in species that are on the northern edge of their geographical distribution range. In such species, selection against mobile individuals is likely to result from the decreased probability of finding another habitat patch suitable for egg laying.

Relative allocation to horn and body growth in bighorn rams varies with resource availability

Males may allocate a greater proportion of metabolic resources to maintenance than to the development of secondary sexual characters when food is scarce, to avoid compromising their probability of survival. We assessed the effects of resource availability on body mass and horn growth of bighorn rams (Ovis canadensis) at Ram Mountain, Alberta, Canada over 30 years. The number of adult ewes in the population tripled during our study, and the average mass of yearling females decreased by 13%. We used the average mass of yearling females as an index of resource availability. Yearling female mass was negatively correlated with the body mass of rams of all ages, but it affected horn growth only during the first three years of life. Yearly horn growth was affected by a complex interaction of age, body mass, and resource availability. Among rams aged 2–4 years, the heaviest individuals had similar horn growth at high and at low resource availability, but as ram mass decreased, horn growth for a given body mass became progressively smaller with decreasing resource availability. For rams aged 5–9 years, horn growth was weakly but positively correlated with body mass, and rams grew slightly more horn for a given body mass as resource availability decreased. When food is limited, young rams may direct more resources to body growth than to horn growth, possibly trading long-term reproductive success for short-term survival. Although horn growth of older rams appeared to be greater at low than at high resource availability, we found no correlation between early and late growth in horn length for the same ram, suggesting that compensatory horn growth does not occur in our study population. Young rams with longer horns were more likely to be shot by sport hunters than those with shorter horns. Trophy hunting could select against rams with fast-growing horns.

FOLIAR NUTRITION, SITE QUALITY, AND TEMPERATURE INFLUENCE FOLIAR CHEMISTRY OF TALLOWWOOD (EUCALYPTUS MICROCORYS)

The distribution of marsupial folivores in Australian eucalypt forests is extremely patchy and, although distribution patterns are known to be partly determined by nutritional factors, the role of plant secondary metabolites (PSMs) has been little studied. We investigated variation in the foliar concentrations of five constituents: total phenolics (TP); the foliar component bound by polyethylene glycol (PEGBC); the formylphloroglucinol compound, sideroxylonal (a strong deterrent of mammalian herbivory); the terpene, cineole; and nitrogen (N). These constituents were analyzed using near infrared spectroscopy in 501 individual tallowwood (Eucalyptus microcorys) trees at 42 sites across a broad geographic range in eastern Australia. We tested three hypotheses: (1) trees with lower foliar N produce higher PSM concentrations; (2) PSM concentrations are greater at colder sites; and (3) PSM concentrations are higher at sites with low resource availability. Hypothesis 1 was true for all PSMs considered and was the result of variation that occurred within sites. This is consistent with the carbon/nutrient balance hypothesis and with the hypothesis that PSMs protect leaves from photodamage. Hypothesis 2 was upheld for all constituents. This was consistent with both the growth–differentiation balance hypothesis, and with the hypothesis that PSMs represent an evolutionary adaptation to greater risks of photodamage and frost damage in cold environments. Hypothesis 3 was rejected, with one PSM, sideroxylonal, being affected by resource availability in a manner opposite to that predicted. The optimal defense hypothesis offers an explanation for this result (as sideroxylonal concentrations increased along with the apparent nutritional quality of the foliage, possibly to provide defense against higher rates of herbivory). We found that trees associated with koala fecal pellets had lower concentrations of sideroxylonal and cineole. We discuss this finding and further implications for the herbivores of Eucalyptus of the patterns of plant defense that we detected.

Diet Selection by the Fossorial Rodent Ctenomys mendocinus Inhabiting an Environment with Low Food Availability (Mendoza, Argentina)

The seasonal diet of Ctenomys mendocinus was determined in a habitat of the Andean Precordillera, through microhistological analyses of stomach contents and in relation to food availability. Diet included 23 genera of plants and only 6 of them were consumed with frequencies higher than 4%. The grasses Stipa and Elymus comprised about 80% of the diet. A high proportion of aerial plant material was registered throughout the year suggesting a tendency to forage on the surface. Dietary selectivity was evidenced by a preferential consumption of grasses and avoidance of shrubs. Moreover, most dietary items frequently used were eaten in proportions that differed from their availabilities. This non-opportunistic feeding behavior, specialized in a few dietary items, did not agree with the one expected for a subterranean rodent inhabiting an environment with severe climatic conditions, low food availability and patchy distribution of food resources.

The relationship between relative growth rate and susceptibility to aphids in wild barley under different nutrient levels.

The Resource Availability Hypothesis (RAH) states that plants with a low Relative Growth Rate (RGR) and high levels of defence against herbivores or pathogens are favoured in habitats with low resource availability, whereas plants with a high potential RGR and low levels of defence are favoured in environments with high resource availability. High levels of defence are expected to result in lower reproduction and/or growth of the herbivores or pathogens. To test this hypothesis, four accessions of each of nine natural Hordeum spontaneum (wild barley) populations were grown in a climate chamber under two levels of nutrient supply. Susceptibility to Schizaphis graminum (greenbug) was quantified by placing a single adult greenbug on each plant and measuring its realised fecundity after 8 days. Data on potential RGR were available from a previous experiment. No support for the RAH was found. The correlation between potential RGR and greenbug reproduction was not significant, neither at the high nor at the low level of nutrient supply. Furthermore, on average plants grown under high and low nutrients did not differ in susceptibility. However, accessions-within-populations differed in the way susceptibility was affected by nutrient supply, and most accessions had a higher susceptibility under nutrient-poor conditions. It could be that these accessions differed in the spectrum of secondary metabolites they produced. Whatever the cause, the genetic variation for the reaction in susceptibility to nutrient supply suggests that selection could act in favour of more or less plasticity in plants without any apparent change in potential RGR.

Feedstock cost analysis of corn stover residues for further processing

In this paper, we evaluate the costs for collecting, handling, and hauling corn stover to an ethanol conversion facility. We estimate costs for a conventional baling system at varying levels of feedstock demand or conversion facility size. Our results generally indicate that stover can be collected, stored, and hauled for about $43.10–51.60/dry ton using conventional baling equipment for conversion facilities ranging from 500 to 4000 dry tons/day. The cost difference between facility sizes is due to transportation. Transportation, collection and baling, and farmer payments account for over 90% of total delivered costs. These estimates are based on average corn stover resource availability assumptions and are inclusive of all costs including farmer payments. Under conditions of high resource availability costs can be lowered by $6–10/dry ton. Delivered costs increase considerably under low resource availability conditions.

Energy metabolism and thermoregulation in Chinchilla brevicaudata

Chinchilla brevicaudata lives at 3500–5000 m, with high ambient temperatures during the day but cold at night. In this Andean habitat there is also low availability of food and water resources. Physiological attributes that may minimize their energetic cost as well as the water requirements are: (1) Low values of basal metabolic rate (67.2%) and thermal conductance (51.0%) compared to predicted values. (2) The aerobic metabolic expansivity was 5.1, while the calculated theoretical critical lethal temperature was extremely low (−67.8°C). (3) The energetic cost for maintenance of water balance was 85.3% of the predicted value for xeric rodents of similar size.

Woody vegetation resource changes around selected settlement along aridity gradient in the Kalahari, Botswana

This paper investigated changes in economically important woody vegetation resources around seven settlements located along an aridity gradient in the Kalahari. Available data at different rainfall levels were analysed for different vegetation aspects. Total woody plant density and number of species at each site increased with increasing rainfall level, while individual species density differed at each site irrespective of rainfall, due to other factors, for example, land use as indicated by high stump densities around old and high population settlements. Drought resulted in high mortality of certain species, increasing availability of fuelwood resource in the form of dead wood. However, prolonged drought from climatic changes could result in browse shortage, a vital resource in the Kalahari. There was generally low wood resource availability throughout the studied gradient, without any distinct relation to rainfall, reflecting the influence of other factors such as soils, land use and natural variability.

Covariation in leaf and root traits for native and non-native grasses along an altitudinal gradient in New Zealand.

Across 30 grassland sites in New Zealand that ranged from native alpine grasslands to low elevation improved pastures, there were consistent patterns of leaf and root traits and significant differences between native and non-native grasses. Plants of high altitude sites have low N concentrations in both their leaves and roots, have thick leaves and roots, yet no differences in tissue density or photosynthetic water use efficiency when compared to plants of low altitude sites. Both the leaves and roots of the low altitude plants were enriched in (15)N relative to the plants of higher altitude, indicating that the low-N set of traits is associated with a more closed N cycle at high altitude. A second independent set of correlations shows that plants of wetter habitats have lower photosynthetic water use efficiency (more negative partial differential (13)C) and lower leaf and root tissue density than the plants of drier sites. For both leaves and roots, plants of native species consistently had traits associated with lower resource availability: lower N concentrations, denser tissues, more negative partial differential (15)N, and more positive partial differential (13)C than non-native species. If root %N is correlated with root longevity as has been shown in other systems, root longevity may be able to be predicted from simple measurements of leaf %N, though a hysteresis in the relationship between leaf and root N concentrations may make prediction of high longevity roots difficult.