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  The team also found evidence that the bats chose flight paths that would allow them to plan two steps ahead, rather like skilled chess players. Not only were the animals maximizing their energy input by targeting multiple prey items, but they were also minimizing their energy output by reducing the distance they flew in pursuit of insects. This behavior fits in well with optimal foraging theory.

  See also: Evolution by natural selection • Predator–prey equations • Competitive exclusion principle • Mutualisms

  IN CONTEXT

  KEY FIGURES

  Roy Anderson (1947–), Robert May (1936–)

  BEFORE

  1662 English statistician John Graunt seeks to classify causes of death in London in Natural and Political Observations made upon the Bills of Mortality.

  1927 Scottish scientists Anderson Gray McKendrick and William Ogilvy Kermack develop an epidemic model for infected, uninfected, and immune individuals.

  AFTER

  1996 American epidemiologist James S. Koopman calls for greater use of computational technologies to simulate disease generation and spread.

  2018 A global team tracks the origins and spread of a fungus devastating frog populations.

  Epidemiology is the study of how disease spreads through a population. Its initial application was to human diseases, but its methods have been recognized as an effective way of modeling populations of other organisms, too.

  Ecologists have long known that the size of an animal or plant population and its growth rate depend on the availability of food, living space, and levels of predation. In the 1970s, British epidemiologist Roy Anderson and Australian scientist Robert May showed how parasites and infections from pathogens such as bacteria and viruses limited the size of a population. In wild sheep, for instance, the chief cause of death is lungworms, while most wild birds die from viral infections.

  In ecology, the effects of disease have wider implications. Up to 40 percent of ocean bacteria are killed each day by viruses. This causes a “viral shunt,” because nutrients that would otherwise flow up the food chain to consumers revert to the bottom of the chain.

  Fatalities in London’s cholera outbreak of 1854 were linked to the central pump; its water was found to have been contaminated with infected sewage from a stricken family.

  Human beginnings

  Epidemiology has its beginnings in the work of physician John Snow, who witnessed a cholera epidemic in the Soho district of London in 1854. At the time, disease was thought to be caused by miasma—a sort of poisonous vapor in the air—that spread from the bodies of the dead and dying. Snow was not the first to question this theory, but he was especially suspicious of it in the case of cholera.

  In 1854, Snow plotted every case of cholera on a map of Soho, and found that afflicted households collected their water from a pump on Broad Street (later renamed Broadwick). He shut down the pump, and the epidemic soon ended. This showed that cholera was a waterborne disease that humans contracted through contaminated food and drink. A decade later, Louis Pasteur’s “germ theory” proposed that diseases, as well as general rotting and decay, were the work of microorganisms.

  British doctor John Snow fought the establishment to gain acceptance for his belief that cholera was waterborne. The medical journal The Lancet finally conceded that he was right in 1866.

  Disease model

  In their 1970s studies, Anderson and May focused first on building a mathematical model to show how a microorganism could affect a population. This led to a set of equations that they hoped would help explain the real-life impact of different kinds of pathogens, from bacteria and viruses to parasitic worms and insect larvae.

  In their model, a number of mice were divided into three groups: susceptible (uninfected) mice, infected mice, and mice that had survived infection and were now immune. Unlike many earlier epidemiological models, the total population was not a fixed number; mice could be added either by reproduction or by additions from other populations. Mice also died from natural causes. In the absence of disease, the total would remain more or less the same, with the rate of added mice balancing that at which other mice died.

  For simplicity, the model assumed that the diseases were transmitted by contact between infected and uninfected mice. Not all infected mice would die, so the model also included a recovery rate. Mice that recovered would be immune, at least initially. Immunity to viruses is more or less lifelong, but it is possible to become susceptible again to the same bacterial infection as time passes. Therefore, the calculations also included a rate of loss of immunity.

  Putting all this together, Anderson and May produced a set of equations to predict the rate of population change in the three initial groups of uninfected but susceptible mice, infected mice, and the immune survivors. These equations could be added together to give the rate of change for the total mouse population.

  From their calculations, they deduced that a disease will persist in a population whose equilibrium point (the rate of new additions, balanced by the natural death rate) is greater than the combined effects of natural mortality, disease deaths, recovery, and transmission rate. While the disease is present, that equilibrium point will be lower than if the population were disease free. If, however, the equilibrium point of a population affected by disease is lower than the combined effects of deaths, recoveries, and rate of transmission, the disease will die out. Once a population is disease free, its equilibrium point will return to its former level.

  A ravaged tree in North Yorkshire, UK, shows the effects of Dutch elm disease, a fungus spread by elm bark beetles accidentally introduced to Europe and America from Asia.

  “Sensibly used, mathematical models are no more and no less than tools for thinking about things in a precise way.”

  Roy Anderson and Robert May

  Matching the real world

  Anderson and May needed to show that their model was an accurate predictor of a real-life population. They did so by using data from a study of laboratory mice infected with the bacterial disease pasteurellosis; the data included the impact on the population of adding individuals at different rates. The observed data confirmed their predictions, so the two scientists were able to consider the effects of hypothetical values. They found, for instance, that when the rate of added mice was highest, the disease had the greatest impact on population numbers. This suggests that species with high reproductive rates (introducing large numbers of uninfected offspring) are most likely to have endemic diseases within the population, and show depressed numbers compared with species that breed more slowly. They also explored the differing effects on populations of diseases of different intensities.

  Unlike endemic diseases, in which the population’s level of infection remains consistent, epidemics appear in populations when the growth rate of all infected and uninfected members is low compared to the death rate caused by the disease. Infection numbers rise sharply to a maximum, then drop away. Epidemics also occur when a disease is not particularly deadly but slows the population growth rate; this has occurred with human diseases such as measles and chickenpox.

  “Diseases such as measles and rubella, with short infections and lasting immunity, will tend to exhibit epidemic patterns.”

  Roy Anderson

  Applying the theory

  The characteristics of disease and its effects on animal and plant populations are of increasing ecological importance. Food producers, for example, benefit from studies into the nature of parasites and the dynamics of diseases that can affect crops and livestock. Conservationists also employ epidemiology to predict how exotic diseases and invasive parasites might affect fragile ecosystems.

  A pathogen strikes when it finds a suitable host in an environment that favors infection, as shown where the circles intersect. For instance, diarrheal diseases spread quickly among sick people in unsanitary conditions.

  The role of drought in plant diseases

  A summer drought produces only sparse growth of young barley plants
. Lack of moisture and too much heat reduce their resistance to fungi that attack their roots.

  Like other disease-causing agents, a plant pathogen (disease-causing agent) needs a supply of susceptible individuals to infect. Periods of drought slow the rate of plant reproduction and growth, thereby reducing the prevalence of disease.

  Aridity, however, also weakens plants and makes them susceptible to pathogens that thrive in dry conditions. These include various forms of fungi that attack the leaves of grain crops, legumes, and fruits. These fungi are adapted to survive in a dormant state as hardened microscopic bodies in soil. They can exist for many years in dry soil, but when the soil becomes wet, the fungi must find a host within a few weeks or die. They do not necessarily kill their host. Recent research into chickpeas suggests that although infections from such fungi do increase during a dry spell, the mortality rate of the affected plants goes down during a drought.

  See also: The microbiological environment • Microbiology • The ubiquity of mycorrhizae • Biodiversity and ecosystem function

  IN CONTEXT

  KEY FIGURE

  Knut Schmidt-Nielsen (1915–2007)

  BEFORE

  1845 The explorer Alexander von Humboldt reveals that plants facing similar ecological factors also have many analogous features.

  1859 Charles Darwin argues that organisms evolve because they are adapting to changed ecological conditions.

  AFTER

  1966 Australian biochemists Marshall Hatch and Charles Slack explain that the most widespread plants are the ones that photosynthesize most efficiently.

  1984 Peter Wheeler, a British scientist, suggests that human bipedalism—the ability to walk on two legs—evolved as a thermoregulatory adaptation that reduces the body’s exposure to direct sunlight.

  The central principle of Darwinian evolution is that all organisms, from simple bacteria to complex mammals, are adapted by natural selection to survive in a particular niche and habitat. Ecophysiology—for which Knut Schmidt-Nielsen’s book Animal Physiology (1960) was a vital inspiration—is the study of an organism’s anatomy and how it functions (its physiology), as well as how these characteristics relate to the challenges posed by its environment. It shows how the anatomy of an animal or plant is linked to its ability to survive, and to its distribution, abundance, and fertility. Ecophysiology now plays an important role in helping scientists understand how the stresses created by climate change impact on both wild ecosystems and cultivated environments.

  “From a physiological viewpoint, freshwater is no more freely available in the sea than in the desert.”

  Knut Schmidt-Nielsen

  Managing temperature

  Ecophysiology has revealed a number of specific adaptations for different environments. For example, animals that live in colder regions tend to have larger bodies and smaller legs, ears, and tails than related species living in warmer climes. A larger body has a smaller surface-area-to-mass ratio, and therefore loses heat more slowly, while smaller appendages reduce exposure to frostbite.

  In the most extreme cold, the feet of a warm-blooded animal are at risk of becoming frozen to the ground. Mammals in Arctic regions such as musk oxen and polar bears are adapted for life in these conditions by having thick hairs to insulate their feet.

  In the Antarctic, the undersides of penguins’ feet are insulated by a thick layer of fat. Penguins also have a heat-exchange (or counter-current) mechanism in their legs. The warm blood arriving from the body is cooled to near 32°F (0°C) by the chilled blood arriving from the feet, which warms to body temperature in the process.

  Gazelles in Africa use a similar counter-current system to cool their body temperature. They are able to chill the blood entering their head, giving them an advantage over their predators, who often overheat. Camels have a heat-exchange system in their nasal cavity, which reduces the amount of water lost in their breath. Hot, dry air is inhaled and mixes with moisture inside the nose before traveling to the lungs. The exhaled air is much cooler than the air outside, so the moisture it carries condenses in the nose. This creates the cool, damp conditions needed to chill the next in-breath.

  Emperor penguins survive freezing Antarctic temperatures thanks in part to the way their bodies have evolved to adapt to the harsh environment.

  Future challenges

  Today ecophysiology is becoming increasingly focused on plants, fungi, and microbes. Like animals, they have to adapt to survive—and studying them offers the possibility of vital discoveries for commercial and conservation purposes.

  KNUT SCHMIDT-NIELSEN

  Knut Schmidt-Nielsen grew up in the Norwegian town of Trondheim. His interest in the way animal physiology related to habitat was inherited from his grandfather who, years before Knut’s birth, had released thousands of flounder (marine fish) hatchlings into a freshwater lake. Although the fish thrived, they were unable to breed because their reproductive physiology was adapted for life in salt water.

  Schmidt-Nielsen joined Duke University, North Carolina, in 1954. He built a climate-controlled space for keeping desert animals, where he considered the anatomy of camels, gerbils, and other species able to live for long periods without water. He also investigated the respiratory systems of birds and the buoyancy of fish. His 1960 textbook Animal Physiology is still a classic work.

  Key works

  1960 Animal Physiology

  1964 Desert Animals

  1972 How Animals Work

  1984 Scaling

  1998 The Camel’s Nose: Memoirs of a Curious Scientist

  See also: Evolution by natural selection • Ecological niches • Competitive exclusion principle • Ecological stoichiometry

  IN CONTEXT

  KEY FIGURES

  Robert Sterner (1958–), James Elser (1959–)

  BEFORE

  1840 German biologist and chemist Justus von Liebig asserts that the limitations on agriculture productivity are primarily chemical.

  1934 US oceanographer Alfred Redfield measures the atomic ratio of carbon, nitrogen, and phosphorus (C:N:P) in plankton and seawater, and finds it to be relatively consistent in all oceans. The Redfield Ratio soon becomes a benchmark for such research in all habitats.

  AFTER

  2015 In “Ocean stoichiometry, global carbon, and climate,” Robert Sterner highlights inconsistencies in C:N:P ratios in phytoplankton, which absorb more atmospheric carbon in low-nutrient, low-latitude ocean surface waters and adjust their ratios accordingly.

  Every living organism—from tiny ocean algae to a mighty redwood—is made up of chemical elements in varying ratios. Ecological stoichiometry considers the balance of these elements, and how the ratios change during chemical reactions. Studying such ratios throws light on the way the living world operates, revealing how organisms obtain the nutrients and other chemicals they require for life from the resources in their environment.

  The field of ecological stoichiometry was comprehensively described for the first time by American biologists Robert Sterner and James Elser; in Ecological Stoichiometry (2002), they used mathematical models to demonstrate the application at every level, from molecules and cells to individual plants and animals, populations, communities, and ecosystems.

  “Individual organisms also show differences in stoichiometry during their life cycles. Young organisms may have different compositions from older ones …”

  Robert Sterner and James J. Elser

  Key chemicals

  In ecological research, the three main elements examined are carbon (C), nitrogen (N), and phosphorus (P), because each plays a vital role. Carbon is a basic building block of all life and an important part of many chemical processes. Nitrogen is a major constituent of all proteins, while phosphorus is crucial for cell development and storing energy.

  An organism’s C:N:P ratio is not necessarily consistent. Plants have a variable ratio: they can adjust the balance of their elements according to their environment. For instance, the proportion of carbon in their c
hemical makeup can rise on a particularly sunny day because more photosynthesis occurs—the process by which they take carbon dioxide from the air and use the sun’s energy to convert it into the nutrients they require.

  Higher up the food chain, animals have largely fixed C:N:P ratios, so they must deploy various mechanisms to deal with any imbalances of chemicals entering the body. If an insect or animal herbivore is getting too much carbon from its plant diet, for instance, it may adjust its digestive enzymes and excrete it, store it as fats, or raise its metabolic rate to burn it off, breathing out the excess carbon as CO2. Overuse of such mechanisms to redress a high imbalance can, however, affect fitness, growth, and reproduction. An animal that eats other animals has less work to do, because its prey’s C:N:P ratio closely matches its own. However, the size of its prey population is still determined by the plants in its environment because plants with a high carbon ratio can only support a small food chain of consumers.

  A locust eats grass that may contain six times as much carbon as it needs. To get the right balance, it excretes carbon or breathes it out as CO2. Locusts are widely used in research because they are easy to breed.

  Understanding our world

  Food chains are one area of study; ecological stoichiometry covers just about everything and all the links in between. By discovering how the chemical content of organisms shapes their ecology, scientists are also learning how environments can be better managed. Their findings may significantly influence the future of life on Earth.