Microclimatic challenges in global change biology

Despite decades of work on climate change biology, the scientific community remains uncertain about where and when most species distributions will respond to altered climates. A major barrier is the spatial mismatch between the size of organisms and the scale at which climate data are collected and modeled. Using a meta-analysis of published literature, we show that grid lengths in species distribution models are, on average, ca. 10 000-fold larger than the animals they study, and ca. 1000-fold larger than the plants they study. And the gap is even worse than these ratios indicate, as most work has focused on organisms that are significantly biased toward large size. This mismatch is problematic because organisms do not experience climate on coarse scales. Rather, they live in microclimates, which can be highly heterogeneous and strongly divergent from surrounding macroclimates. Bridging the spatial gap should be a high priority for research and will require gathering climate data at finer scales, developing better methods for downscaling environmental data to microclimates, and improving our statistical understanding of variation at finer scales. Interdisciplinary collaborations (including ecologists, engineers, climatologists, meteorologists, statisticians, and geographers) will be key to bridging the gap, and ultimately to providing scientifically grounded data and recommendations to conservation biologists and policy makers.

Interactive effects of rearing temperature and oxygen on the development of Drosophila melanogaster

Although higher temperatures strongly stimulate ectothermic metabolic rates, they only slightly increase oxygen diffusion rates and decrease oxygen solubility. Consequently, we predicted that insect gas exchange systems would have more difficulty meeting tissue oxygen demands at higher temperatures. In this study, Drosophila melanogaster were reared from egg to adult in hyperoxic (40%), hypoxic (10%), and normoxic (21%) conditions and in temperatures ranging from 15 degrees -31.5 degrees C to examine the interactive effect of temperature and oxygen on development. Hyperoxia generally increased mass and growth rate at higher rearing temperatures. At lower rearing temperatures, however, hyperoxia had a very small effect on mass, did not affect growth rate, and lengthened time to eclosion. Relative to normoxia, flies reared in hypoxic conditions were generally smaller (mass and thorax length), had longer eclosion times, slower growth rates, and reduced survival. At cooler temperatures, hypoxia had relatively modest or nonsignificant effects on development, while at higher temperatures, the effects of hypoxia were large. These results suggest that higher temperatures reduce oxygen delivery capacity relative to tissue oxygen needs, which may partially explain why ectotherms are smaller when development occurs at higher temperatures.

The metabolic basis of whole-organism RNA and phosphorus content

Understanding the storage, flux, and turnover of nutrients in organisms is important for quantifying contributions of biota to biogeochemical cycles. Here we present a model that predicts the storage of phosphorus-rich RNA and whole-body phosphorus content in eukaryotes based on the mass- and temperature-dependence of ATP production in mitochondria. Data from a broad assortment of eukaryotes support the model's two main predictions. First, whole-body RNA concentration is proportional to mitochondrial density and consequently scales with body mass to the -1/4 power. Second, whole-body phosphorus content declines with increasing body mass in eukaryotic unicells but approaches a relatively constant value in large multicellular animals because the fraction of phosphorus in RNA decreases relative to the fraction in other pools. Extension of the model shows that differences in the flux of RNA-associated phosphorus are due to the size dependencies of metabolic rate and RNA concentration. Thus, the model explicitly links two biological currencies at the individual level: energy in the form of ATP and materials in the form of phosphorus, both of which are critical to the functioning of ecosystems. The model provides a framework for linking attributes of individuals to the storage and flux of phosphorus in ecosystems.

Interpreting rejections of the beneficial acclimation hypothesis: when is physiological plasticity adaptive?

Although many studies testing the beneficial acclimation hypothesis have rejected it, what these rejections imply about the adaptive value of physiological change remains unclear. Uncertainty arises because the hypothesis focuses on the relative performance of organisms exposed to one environment versus another, whereas the raw material available to evolution is variation in acclimation responses of individual traits. This mismatch is problematic when organisms are exposed to poor environments. In poor environments, the adaptive or maladaptive value of changes in individual traits may be obscured by long-term decrements in organismal condition. A better match between the evolutionary pressures shaping acclimation and the tests used to examine them can be achieved by focusing on the fitness consequences of acclimation changes in individual traits.

Thermal sensitivity of growth and feeding in Manduca sexta caterpillars

We explore how the thermal sensitivity of organismic performance emerges from the thermal sensitivity of the underlying component processes involved, using growth and feeding of Manduca sexta caterpillars as a model system. We measured thermal performance curves for the short-term rates of growth, consumption, protein (casein) digestion, amino acid (methionine) uptake, and respiration in fifth-instar caterpillars over a biologically realistic temperature range from 14 degrees to 42 degrees C. Growth and consumption rates increased between 14 degrees and 26 degrees C, reached a maximum value near 34 degrees C, and declined rapidly above 38 degrees C. In contrast, protein digestion rate and respiration rate increased monotonically over the entire temperature range, and amino acid uptake rate increased with temperatures up to 38 degrees C and then leveled off between 38 degrees and 42 degrees C. These results suggest that the shape and position of the thermal performance curve for growth rate--in particular the maximum at 34 degrees C and rapid decline above 38 degrees C--was most closely correlated with the thermal sensitivity of consumption rate; the declining growth performance above 38 degrees C was not associated with declines in digestion or uptake rates or with accelerated respiration rates at these temperatures.

Why might they be giants? Towards an understanding of polar gigantism

Beginning with the earliest expeditions to the poles, over 100 years ago, scientists have compiled an impressive list of polar taxa whose body sizes are unusually large. This phenomenon has become known as ‘polar gigantism’. In the intervening years, biologists have proposed a multitude of hypotheses to explain polar gigantism. These hypotheses run the gamut from invoking release from physical and physiological constraints, to systematic changes in developmental trajectories, to community-level outcomes of broader ecological and evolutionary processes. Here we review polar gigantism and emphasize two main problems. The first is to determine the true strength and generality of this pattern: how prevalent is polar gigantism across taxonomic units? Despite many published descriptions of polar giants, we still have a poor grasp of whether these species are unusual outliers or represent more systematic shifts in distributions of body size. Indeed, current data indicate that some groups show gigantism at the poles whereas others show nanism. The second problem is to identify underlying mechanisms or processes that could drive taxa, or even just allow them, to evolve especially large body size. The contenders are diverse and no clear winner has yet emerged. Distinguishing among the contenders will require better sampling of taxa in both temperate and polar waters and sustained efforts by comparative physiologists and evolutionary ecologists in a strongly comparative framework.

Insect eggs protected from high temperatures by limited homeothermy of plant leaves

Virtually all aspects of insect biology are affected by body temperature,and many taxa have evolved sophisticated temperature-control mechanisms. All insects, however, begin life as eggs and lack the ability to thermoregulate. Eggs laid on leaves experience a thermal environment, and thus a body temperature, that is strongly influenced by the leaves themselves. Because plants can maintain leaf temperatures that differ from ambient, e.g. by evapotranspiration, plant hosts may protect eggs from extreme ambient temperatures. We examined the degree to which leaves buffer ambient thermal variation and whether that buffering benefits leaf-associated insect eggs. In particular, we: (1) measured temperature variation at oviposition sites in the field, (2) manipulated temperatures in the laboratory to determine the effect of different thermal conditions on embryo development time and survival, and(3) tested embryonic metabolic rates over increasing temperatures. Our results show that Datura wrightii leaves buffer Manduca sexta eggs from fatally high ambient temperatures in the southwestern USA. Moreover, small differences in temperature profiles among leaves can cause large variation in egg metabolic rate and development time. Specifically, large leaves were hotter than small leaves during the day, reaching temperatures that are stressfully high for eggs. This study provides the first mechanistic demonstration of how this type of leaf-constructed thermal refuge interacts with egg physiology.

Temperature-dependent oxygen limitation in insect eggs

Most terrestrial insect embryos support metabolism with oxygen from the environment by diffusion across the eggshell. Because metabolism is more temperature sensitive than diffusion, embryos should be relatively oxygen-limited at high temperatures. We tested whether survival, development time and metabolism of eggs of a moth, Manduca sexta, were sensitive to experimentally imposed variation in atmospheric oxygen availability(5–50 kPa; normoxia at sea level is 21 kPa) across a range of biologically realistic temperatures. Temperature–oxygen interactions were apparent in most experiments. Hypoxia affected survival more strongly at warmer temperatures. Metabolic rates, measured as rates of CO2emission, were virtually insensitive to hypo- and hyperoxia at 22°C but were strongly influenced at 37°C. Radial profiles of PO2 inside eggs, measured using an oxygen microelectrode, demonstrated that 3-day-old eggs had broad central volumes with PO2 less than 2 kPa, and that higher temperature led to lower PO2. These data indicate that at realistically high temperatures (32–37°C) eggs of M. sexta were oxygen limited, even in normoxia. This result has important implications for insect population ecology and the evolution of eggshell structures, and it suggests a novel hypothesis about insect gigantism during Paleozoic hyperoxia.

Life in the Frequency Domain: the Biological Impacts of Changes in Climate Variability at Multiple Time Scales

Over the last few decades, biologists have made substantial progress in understanding relationships between changing climates and organism performance. Much of this work has focused on temperature because it is the best kept of climatic records, in many locations it is predicted to keep rising into the future, and it has profound effects on the physiology, performance, and ecology of organisms, especially ectothermic organisms which make up the vast majority of life on Earth. Nevertheless, much of the existing literature on temperature-organism interactions relies on mean temperatures. In reality, most organisms do not directly experience mean temperatures; rather, they experience variation in temperature over many time scales, from seconds to years. We propose to shift the focus more directly on patterns of temperature variation, rather than on means per se, and present a framework both for analyzing temporal patterns of temperature variation and for incorporating those patterns into predictions about organismal biology. In particular, we advocate using the Fourier transform to decompose temperature time series into their component sinusoids, thus allowing transformations between the time and frequency domains. This approach provides (1) standardized ways of visualizing the contributions that different frequencies make to total temporal variation; (2) the ability to assess how patterns of temperature variation have changed over the past half century and may change into the future; and (3) clear approaches to manipulating temporal time series to ask "what if" questions about the potential effects of future climates. We first summarize global patterns of change in temperature variation over the past 40 years; we find meaningful changes in variation at the half day to yearly times scales. We then demonstrate the utility of the Fourier framework by exploring how power added to different frequencies alters the overall incidence of long-term waves of high and low temperatures, and find that power added to the lowest frequencies greatly increases the probability of long-term heat and cold waves. Finally, we review what is known about the time scales over which organismal thermal performance curves change in response to variation in the thermal environment. We conclude that integrating information characterizing both the frequency spectra of temperature time series and the time scales of resulting physiological change offers a powerful new avenue for relating climate, and climate change, to the future performance of ectothermic organisms.

There is plenty of room at the bottom: microclimates drive insect vulnerability to climate change

Climate warming impacts biological systems profoundly. Climatologists deliver predictions about warming amplitude at coarse scales. Nevertheless, insects are small, and it remains unclear how much of the warming at coarse scales appears in the microclimates where they live. We propose a simple method for determining the pertinent spatial scale of insect microclimates. Recent studies have quantified the ability of forest understory to buffer thermal extremes, but these microclimates typically are characterized at spatial scales much larger than those determined by our method. Indeed, recent evidence supports the idea that insects can be thermally adapted even to fine scale microclimatic patterns, which can be highly variable. Finally, we discuss how microhabitat surfaces may buffer or magnify the amplitude of climate warming.

Dietary phosphorus affects the growth of larval Manduca sexta

Although phosphorus has long been considered an important factor in the growth of diverse biota such as bacteria, algae, and zooplankton, insect nutrition has classically focused on dietary protein and energy content. However, research in elemental stoichiometry has suggested that primary producer biomass has similar N:P ratios in aquatic and terrestrial systems, and phosphorus-rich herbivores in freshwater systems frequently face phosphorus-limited nutritional conditions. Therefore, herbivorous insects should also be prone to phosphorus limitation. We tested this prediction by rearing Manduca sexta larvae on artificial and natural (Datura wrightii leaves) diets containing varying levels of phosphorus (approximately 0.20, 0.55, or 1.2% phosphorus by dry weight). For both artificial and natural diets, increased dietary phosphorus significantly increased growth rates and body phosphorus contents, and shortened the time to the final instar molt. Caterpillars did not consistently exhibit compensatory feeding for phosphorus on either type of diet. The growth and body phosphorus responses were not explicable by changes in amounts of potassium or calcium, which co-varied with phosphorus in the diets. Concentrations of phosphorus in D. wrightii leaves collected in the field varied over a range in which leaf phosphorus is predicted to affect M. sexta's growth rates. These results suggest that natural variation in dietary phosphorus is likely to affect the growth rate and population dynamics of M. sexta, and perhaps larval insects more generally.

Oxygen in egg masses: interactive effects of temperature, age, and egg-mass morphology on oxygen supply to embryos

Embryos of many marine invertebrates are encased in gelatinous masses for part or all of development. Because gel and intervening embryos retard oxygen flux, such a life-history mode profoundly affects partial pressures of metabolic gases surrounding embryos. However, little is known about relationships between egg-mass structure and the opportunities and constraints imposed on structure by metabolic gas transport. We examined the effects of four factors (temperature, embryo age, embryo density and egg-mass size) on the metabolism of egg masses using both natural egg masses of a nudibranch and artificial egg masses made from sand dollar embryos and low-melting point agarose. Both temperature and embryo age strongly affected metabolic rates of nudibranch embryos. For embryos of a given age (stage), rates of oxygen consumption roughly doubled between 12 and 21°C; from early cleavage to the veliger stage, consumption rose two- to fourfold, depending on temperature. Oxygen profiles in egg masses showed that advanced embryonic age,and to a lesser extent high temperature, both led to steeper oxygen gradients into egg masses. Egg masses containing advanced embryos at 21°C had very low central oxygen levels. Small-diameter artificial masses (2 mm diameter)had virtually no internal oxygen gradients regardless of embryo density or temperature, while medium (4 mm) and large diameter (10 mm) artificial masses had oxygen profiles that depended strongly and interactively on embryo density and temperature. Together, our data on natural and artificial egg masses suggest that (i) multiple factors have strong effects on metabolic rate; (ii)rates of oxygen transport are relatively invariant with temperature in simple,artificial systems but may vary more strongly with temperature in natural egg masses; and (iii) the four factors – temperature, embryo age, embryo density and egg-mass size – interact in important ways bearing on egg mass design.

A simple mathematical model is developed to provide a quantitative means of estimating primary and interactive effects of the different factors. We also show that in T. diomedea the gel itself is the main barrier to oxygen transport into egg masses, and that the metabolic activity of embryos increases substantially when embryos are artificially released from the capsules that contain them within the gel mass.

Patterns and mechanisms of growth of fifth-instar Manduca sexta caterpillars following exposure to low- or high-protein food during early instars

For many insect herbivores, variation in protein availability is a pervasive part of the environment. I explore how variable protein availability affects growth rates of fifth-instar Manduca sexta caterpillars and how growth is related to behavior and physiology. Groups of larvae were reared on low- or high-protein artificial diets (5.9% and 17.7% casein by dry weight, respectively) and then transferred in the fifth instar to the same or opposite diet. During or after the 24-h period following transfer, I measured growth rate, consumption rate, growth efficiency, midgut proteolytic activity, and masses of midgut contents and tissues. Fifth-instar caterpillars reared in earlier instars on high-protein diet grew about 20% more rapidly over 24 h than did caterpillars reared on low-protein diet. This growth pattern appears to be caused by differences in consumption and growth efficiency: caterpillars reared on high protein consumed more food, and used it more efficiently, than did caterpillars reared on low-protein diet. Over the short term (24 h), in contrast, fifth instars that received low-protein diet grew as rapidly as caterpillars that received high-protein diet. Increased (compensatory) consumption appears to be the primary mechanism by which caterpillars consuming low-protein food maintained growth rates.

Effects of dietary variation on growth, composition, and maturation of Manduca sexta (Sphingidae: Lepidoptera)

Most studies linking dietary variation with insect fitness focus on a single dietary component and late larval growth. We examined the effects of variation in multiple dietary factors over most life stages of the sphingid moth, Manduca sexta. Larvae received artificial diets in which protein, sucrose, and water content were varied. The relationship between larval size, growth and consumption rates differed significantly across diets. Larvae on control and low-sucrose diets grew most rapidly and attained the largest pupal and adult sizes. Conversely, larvae on low-water and low-protein diets initially grew slowly, but accelerated in the fifth instar and became pupae and adults comparable to control animals in size. There were no fundamental differences in protein:carbohydrate consumption patterns or strategies among experimental diets and larval instars. However, inadequate dietary water appeared to be more important for early than late instar larvae. Larvae on all artificial diets showed increasing fat content throughout all stages, including wandering and metamorphosis. Compensatory feeding among low-water and low-protein larvae was correlated with significantly higher fat content in larvae, pupae and adults, whereas low-sucrose animals were substantially leaner than those on the control diet. These differences may have strong effects on adult physiology, reproduction, and foraging patterns.

High elevation insect communities face shifting ecological and evolutionary landscapes

Climate change is proceeding rapidly in high mountain regions worldwide. Rising temperatures will impact insect physiology and associated fitness and will shift populations in space and time, thereby altering community interactions and composition. Shifts in space are expected as insects move upslope to escape warming temperatures and shifts in time will occur with changes in phenology of resident high-elevation insects. Clearly, spatiotemporal shifts will not affect all species equally. Terrestrial insects may have more opportunities than aquatic insects to exploit microhabitats, potentially buffering them from warming. Such responses of insects to warming may also fuel evolutionary change, including hitchhiking of maladaptive alleles and genetic rescue. Together, these considerations suggest a striking restructuring of high-elevation insect communities that remains largely unstudied.