Ford Ballantyne

A polymorphism maintained by opposite patterns of parasitism and predation

Although polymorphism is a widespread phenomenon that hasbeen recognized for nearly two centuries, the basic mechanisms maintaining most polymorphisms in nature are unknown,. We present evidence that a polymorphism can be maintained exclusively by balanced selection from two predatory species. For fieldand laboratory experiments, we used the pea aphid, Acyrthosiphon pisum, which occurs as ‘green’ and ‘red’ colour morphs, and two species that attack pea aphids, the parasitoid Aphidius ervi and the predator Coccinella septempunctata. We found that when parasitism rates in the field were high relative to predation rates, the proportion of red morphs increased relative to green morphs, whereas the converse was true when predation rates were high relative to parasitism rates. Detailed laboratory and field studies confirmed that green morphs suffer higher rates of parasitism than red morphs, whereas red morphs are more likely to be preyed on by predators than green morphs are. We present a mathematical model that demonstrates that biased density-dependent parasitism and/or predation on different morphs is sufficient to maintain the colour polymorphism in the population. Our findings support an important role for predation in the maintenance of genetic diversity.

Improved detection of infective endocarditis with transesophageal echocardiography

The incremental advantage of transesophageal echocardiography was determined by comparing results of paired transthoracic and transesophageal echocardiographic examinations performed in 61 patients for evaluation of suspected infective endocarditis. According to clinical and pathologic data, 31 of 61 (51%) patients had finding that were positive for infective endocarditis. Studies were graded as positive or negative for vegetations and were also graded for image quality. The sensitivity of transesophageal echocardiography in detecting vegetations was 88% versus 30% for transthoracic studies (p less than 0.01). For patients with aortic valve infective endocarditis, transesophageal sensitivity was 88% versus 25% for transthoracic sensitivity, because transesophageal echocardiography successfully separated vegetations from chronic valve disease caused by sclerosis or calcification (p less than 0.01). For patients with mitral valve infective endocarditis, transesophageal sensitivity was 100% versus 50% for transthoracic sensitivity, because transesophageal echocardiography distinguished vegetations from myxomatous changes or detected vegetations on prosthetic valves (p less than 0.01). Thus transesophageal echocardiography improves recognition of infective endocarditis, particularly in the presence of underlying valvular disease.

Linking Ecosystem Services, Rehabilitation, and River Hydrogeomorphology

Assignment of values for natural ecological benefits and anthropocentric ecosystem services in riverine landscapes has been problematic because a firm scientific basis linking these to the rivers physical structure has been absent. We highlight some inherent problems in this process and suggest possible solutions on the basis of the hydrogeomorphic classification of rivers. We suggest this link can be useful in fair asset trading (mitigation and offsets), selection of sites for rehabilitation, cost-benefit decisions on incremental steps in restoring ecological functions, and general protection of rivers.

How interactions between microbial resource demands, soil organic matter stoichiometry, and substrate reactivity determine the direction and magnitude of soil respiratory responses to warming

Recent empirical and theoretical advances inform us about multiple drivers of soil organic matter (SOM) decomposition and microbial responses to warming. Absent from our conceptual framework of how soil respiration will respond to warming are adequate links between microbial resource demands, kinetic theory, and substrate stoichiometry. Here, we describe two important concepts either insufficiently explored in current investigations of SOM responses to temperature, or not yet addressed. First, we describe the complete range of responses for how warming may change microbial resource demands, physiology, community structure, and total biomass. Second, we describe how any relationship between SOM activation energy of decay and carbon (C) and nitrogen (N) stoichiometry can alter the relative availability of C and N as temperature changes. Changing availabilities of C and N liberated from their organic precursors can feedback to microbial resource demands, which in turn influence the aggregated respiratory response to temperature we observe. An unsuspecting biogeochemist focused primarily on temperature sensitivity of substrate decay thus cannot make accurate projections of heterotrophic CO2 losses from diverse organic matter reservoirs in a warming world. We establish the linkages between enzyme kinetics, SOM characteristics, and potential for microbial adaptation critical for making such projections. By examining how changing microbial needs interact with inherent SOM structure and composition, and thus reactivity, we demonstrate the means by which increasing temperature could result in increasing, unchanging, or even decreasing respiration rates observed in soils. We use this exercise to highlight ideas for future research that will develop our abilities to predict SOM feedbacks to climate.

Shaking a leg and hot to trot: The effects of body size and temperature on running speed in ants

Abstract 1. Data were compiled from the literature and our own studies on 24 ant species to characterise the effects of body size and temperature on forager running speed.

Nutrient recycling affects autotroph and ecosystem stoichiometry.

Stoichiometric nutrient ratios are the consequence of myriad interacting processes, both biotic and abiotic. Theoretical explanations for autotroph stoichiometry have focused on species’ nutrient requirements but have not addressed the role of nutrient availability in determining autotroph stoichiometry. Remineralization of organic N and P supplies a significant fraction of inorganic N and P to autotrophs, making nutrient recycling a potentially important process influencing autotroph stoichiometry. To quantitatively investigate the relationship between available N and P, autotroph N:P, and nutrient recycling, we analyze a stoichiometrically explicit model of autotroph growth, incorporating Michaelis‐Menten‐Monod nutrient uptake kinetics, Droop growth, and Liebig’s law of the minimum. If autotroph growth is limited by a single nutrient, increased recycling of the limiting nutrient pushes autotrophs toward colimitation and alters both autotroph and environmental stoichiometry. We derive a steady state relationship between input stoichiometry, autotroph N:P, and the stoichiometry of organic losses that allows us to estimate the relative recycling of N to P within an ecosystem. We then estimate relative N and P recycling for a marine, an aquatic, and two terrestrial ecosystems. Preferential P recycling, in conjunction with greater relative P retention at the organismal and ecosystem levels, presents a strong case for the importance of P to biomass production across ecosystems.

Dynamics of nutrient uptake strategies: lessons from the tortoise and the hare

Many autotrophs vary their allocation to nutrient uptake in response to environmental cues, yet the dynamics of this plasticity are largely unknown. Plasticity dynamics affect the extent of single versus multiple nutrient limitation and thus have implications for plant ecology and biogeochemical cycling. Here we use a model of two essential nutrients cycling through autotrophs and the environment to determine conditions under which different plastic or fixed nutrient uptake strategies are adaptive. Our model includes environment-independent costs of being plastic, environment-dependent costs proportional to the rate of plastic change, and costs of being mismatched to the environment, the last of which is experienced by both fixed and plastic types. In equilibrium environments, environment-independent costs of being plastic select for tortoise strategies—fixed or less plastic types—provided that they are sufficiently close to co-limitation. At intermediate levels of environmental fluctuation forced by periodic nutrient inputs, more hare-like plastic strategies prevail because they remain near co-limitation. However, the fastest is not necessarily the best. The most adaptive strategy is an intermediate level of plasticity that keeps pace with environmental fluctuations, but is not faster. At high levels of environmental fluctuation, the environment-dependent cost of changing rapidly to keep pace with the environment becomes prohibitive and tortoise strategies again dominate. The existence and location of these thresholds depend on plasticity costs and rate, which are largely unknown empirically. These results suggest that the expectations for single nutrient limitation versus co-limitation and therefore biogeochemical cycling and autotroph community dynamics depend on environmental heterogeneity and plasticity costs.

Production of slow responses in canine cardiac Purkinje fibers exposed to reduced pH

Abstract Isolated canine papillary muscle-false tendon tissue preparations were exposed to reduced pH at a level of 6.0 or 5.2. Effects on the transmembrane potential of Purkinje fibers and ventricular muscle fibers were recorded. Ventricular fibers were not significantly altered by these levels of pH relative to the changes observed in Purkinje fibers. In Purkinje fibers acid perfusion initially caused decreases in maximum diastolic potential, rising velocity and overshoot and an increase in duration of the action potential. Subsequently in many fibers these effects progressed and resulted in an action potential with all of the characteristics of what has been termed the “slow response.” The maximum diastolic potential was 50 ± .9 mV, the overshoot was 20 ± 1.1 mV and the action potential duration was 322 ± 11 ms. Phase 4 depolarization of variable degree usually became evident with the onset of slow responses. In a number of experiments the preparation escaped from the drivestimulus and discharged spontaneously at a more rapidrate. In other experiments suspension of drive stimulation resulted in spontaneous discharge of action potentials generated by the mechanism of phase 4 depolarization. Such excitation propagated and caused excitation of other Purkinje fibers as well as ventricular muscle fibers. At times disturbances of conduction were noted and ranged from simple slowing in conduction of excitation to partial or complete block of conduction between regions of the preparation. Agents known to block the slow response were tested for their ability to block pH-induced responses. MnC1 2 or verapamil depressed the upstroke and overshoot of the slow response and slowed or stopped the spontaneous discharge of action potentials in acid medium. The possible role of pH-induced slow responses in causing cardiac arrhythmias, especially those arising secondary to cardiac ischemia, was considered.

Conflict between dynamical and evolutionary stability in simple ecosystems

Here, we address the essential question of whether, in the context of evolving populations, ecosystems attain properties that enable persistence of the ecosystem itself. We use a simple ecosystem model describing resource, producer, and consumer dynamics to analyze how evolution affects dynamical stability properties of the ecosystem. In particular, we compare resilience of the entire system after allowing the producer and consumer populations to evolve to their evolutionarily stable strategy (ESS) to the maximum attainable resilience. We find a substantial reduction in ecosystem resilience when producers and consumers are allowed to evolve compared to the maximal attainable resilience. This study illustrates the inherent difference and possible conflict between maximizing individual-level fitness and maximizing resilience of entire ecosystems.

Continental-scale decrease in net primary productivity in streams due to climate warming

An increase in stream temperature leads to a convergence of metabolic balance, overall decline in net ecosystem productivity, and higher CO2 emissions from streams, according to analyses of temperature sensitivity of stream metabolism across six biomes.AbstractStreams play a key role in the global carbon cycle. The balance between carbon intake through photosynthesis and carbon release via respiration influences carbon emissions from streams and depends on temperature. However, the lack of a comprehensive analysis of the temperature sensitivity of the metabolic balance in inland waters across latitudes and local climate conditions hinders an accurate projection of carbon emissions in a warmer future. Here, we use a model of diel dissolved oxygen dynamics, combined with high-frequency measurements of dissolved oxygen, light and temperature, to estimate the temperature sensitivities of gross primary production and ecosystem respiration in streams across six biomes, from the tropics to the arctic tundra. We find that the change in metabolic balance, that is, the ratio of gross primary production to ecosystem respiration, is a function of stream temperature and current metabolic balance. Applying this relationship to the global compilation of stream metabolism data, we find that a 1 °C increase in stream temperature leads to a convergence of metabolic balance and to a 23.6% overall decline in net ecosystem productivity across the streams studied. We suggest that if the relationship holds for similarly sized streams around the globe, the warming-induced shifts in metabolic balance will result in an increase of 0.0194 Pg carbon emitted from such streams every year.