by DK
Human traits
As well as these short-term studies, British primatologist and ethologist Jane Goodall conducted field observations over a longer period, studying chimpanzees in Tanzania from 1960 to 1975. Her findings challenged the view that human behavior is totally unique in the animal world, and indicated that chimps are behaviorally closer to people than had generally been assumed. She noted, for example, that chimps display a whole range of facial expressions and other body language to indicate their mood, are toolmakers and users, often behave cooperatively, and sometimes go into battle against rival groups.
IN CONTEXT
KEY FIGURE
Louis Pasteur (1822–95)
BEFORE
1683 Dutch amateur scientist Antonie van Leeuwenhoek uses a microscope to observe bacteria and protozoa.
1796 Edward Jenner carries out the first vaccination, using the cowpox virus to protect against smallpox.
AFTER
1926 American microbiologist Thomas Rivers distinguishes between viruses and bacteria.
1928 While studying influenza, Scottish bacteriologist Alexander Fleming discovers penicillin.
2007 An inventory of all the microbes associated with a healthy human body is completed.
Microbes—bacteria, molds, viruses, protozoa, and algae—are present in every environment, living in soil, water, and air. Some microbes cause disease but most are vital for life on Earth. Among other things, they break down organic matter so that it can be recycled back into the ecosystem.
Trillions of microbes also live on and in the human body. The most common of these microbes are beneficial bacteria, which aid the digestion of food, produce vitamins, and help the immune system find and attack more harmful microbes.
Scientists did not understand microbes until they could see them. The first observations began in the 17th century, using the recently invented microscope. These studies revealed a previously unknown world teeming with microbiotic life. Around the same time, the word “germ,” originally meaning “seed,” was first used to describe these tiny organisms.
“Microbes are the worker bees that perform most of the important functions in your body.”
Dr Robynne Chutkan,
Microbiome expert and author
Fighting disease
Some 17th and 18th-century scientists believed that certain “germs” might cause diseases, but the prevailing view was that such maladies were the spontaneous result of inherent weakness in an organism. It was not until the painstaking laboratory work of the 19th-century French chemist Louis Pasteur that the “germ theory of disease” was proved.
Pasteur began by looking at the alcohol fermentation process. He discovered that sourness in wine was caused by external agents—microbes, or germs. A crisis in the French silk industry, caused by an epidemic among silkworms, then allowed Pasteur to isolate and identify the microorganisms that caused the particular disease.
As he extended germ theory to human disease, Pasteur proposed that germs invade the body and cause specific disorders. Edward Jenner, nearly 100 years before, had shown that a disease could be prevented with the application of a “vaccine”—a virus similar to that of the disease-causing microbe. Pasteur found that an attenuated, or weakened, form of a disease-causing germ, produced in a laboratory and injected into the host animal or human, was particularly effective at enabling the body’s immune system to fight off the disease. At first, Pasteur faced strong opposition and alarm at the prospect, but he was able to develop vaccines for anthrax, fowl cholera, and rabies—the latter involving his first test on a human.
“Where observation is concerned, chance favors only the prepared mind.”
Louis Pasteur
Annihilating germs
The focus later shifted to finding germ-killing agents, or antibiotics, such as penicillin—discovered by Alexander Fleming. A strategy of annihilating microbes has been followed ever since. Yet this “slash and burn” approach has its drawbacks. It kills beneficial microbes as well as harmful ones, and also promotes resistance in bacteria that can ultimately render antibiotics ineffective.
The bacterium Enterococcus faecalis is a microbe found in the gut and bowel of healthy humans. If it spreads to other areas of the body, however, it can cause serious infections.
LOUIS PASTEUR
Born in Dole, France, in 1822, Pasteur was the son of a poor tanner. He was an average student, but he worked hard, obtaining his degree in 1842 and his doctorate in science in 1847. After teaching in various universities, in 1867 he became Professor of Chemistry at the Sorbonne in Paris. His major research interest was the fermentation process. Pasteur discovered that the fermentation of wine and beer was caused by germs—microbes. He also discovered that microbes could be killed by short, mild heat treatment—a process now named after him as “pasteurization.” Pasteur’s “germ theory” led to the wider development of vaccines, which remain a vital method of disease control. In 1887, he established the Pasteur Institute, which opened in 1888 and continues to help prevent and fight diseases.
Key works
1870 Studies on Silk Worm Disease
1878 Microbes: Their Role in Fermentation, Putrefaction, and the Contagion
1886 Treatment of Rabies
See also: Classification of living things • The microbiological environment • The ecosystem
IN CONTEXT
KEY FIGURE
Albert Frank (1839–1900)
BEFORE
1840 German botanist Theodor Hartig discovers a network of filaments on the roots of pine trees.
1874 Hellmuth Bruchmann, a German biologist, notes the “Hartig net” is made of fungal filaments.
AFTER
1937 A.B. Hatch, an American botanist, shows a beneficial relationship between pine trees and mycorrhizal fungus.
1950 Swedish botanists Elias Melin and Harald Nilsson show that plant roots can extract more nutrients from the soil with the aid of mycorrhizae.
1960 Another Swedish botanist, Erik Björkman, shows that plants pass carbon into mycorrhizal fungi in exchange for phosphate and nitrate.
In 1885, a professor of plant pathology at the Royal College of Agriculture in Berlin named Albert Frank was the first to see a connection between fungi growing on tree roots and the health of the trees. Frank realized that these were not pathological (disease-related)infections but in fact underground partnerships: far from suffering, the trees seemed to benefit from better nutrition. He invented a new term for the partnership—“mycorrhiza,” from the Greek mykes, meaning fungus, and rhiza, meaning root.
Mycorrhizae in action
False truffles are an example of the fungal side of this partnership. Nineteenth-century Prussian botanists had found these fungi under spruce trees, and noticed that each tree root was drawn toward a truffle, and wrapped in a fungal husk. Although they did not know it, the botanists were witnessing a phenomenon that is vital to many ecosystems.
Fungi are typically nourished by a supply of organic matter, from which they extract food by external digestion. A deep layer of forest litter is perfect. They pour digestive chemicals onto their meal and absorb the soluble organic compounds produced through a network of microscopic filaments—hyphae—called a mycelium.
Plants rely on root hairs to absorb water and minerals, such as nitrates and phosphates. But there is a limit to how far plant roots can grow and therefore what quantity of nutrients the root hairs can absorb. The hyphae of mycorrhizae can cover a much wider area, absorbing a much greater amount of minerals. When the fungal hyphae attach to the plant roots, they extend the root system, causing extra nutrients to seep into the plant.
Albert Frank realized that this partnership worked both ways. It was a winning combination for both plant and fungus. In exchange for passing on a share of its minerals, the fungus receives sugar from the plant—made by photosynthesis in the leaves and transported to the roots via the plant’s sap. This boosts the nutrient supply that the fungus derives from dea
d organic matter.
Mycorrhizae on the root of a soybean. In arbuscular mycorrhizae, such as these, the tips of the hyphae form clusters inside the plant’s root cells, optimizing nutrient exchange.
Ancient networks
Fossils of plants dating from 400 million years ago—when vegetation was first spreading across dry land—show traces of fungal threads. This suggests that the mycorrhizal partnership was key to the evolution of terrestrial life. Today, the majority of plant species continue to rely on fungi in this way. Trees supported by mycorrhizae are more resistant to drought and disease, and can even communicate alarm signals by releasing chemicals in response to attack by herbivores. This fungal network connecting trees has been dubbed “the wood-wide web.”
The mutualistic relationship between mycorrhizae and plants is highly evolved. As many as 90 percent of all plant species rely on fungi for nutrients and protection. In return, plants supply the fungi with a vital food source.
“[the fungus] performs a “wet nurse” function and performs the entire nourishment of the tree from the soil.”
Albert Frank
Mycorrhizae as pollution indicators
Weak growth in the russet brittlegill, a mycorrhizal fungus of European and North American spruce forests, can be an early indicator of habitat air pollution.
Mycorrhizal fungi are not only good for the health of plants—they can also act as indicators of the health of the entire environment. Laboratory experiments with these fungi have shown that some grow badly in the presence of toxins, which means that they can be used to detect pollutants in the air or soil. For instance, some fungi fail to grow when exposed to heavy metals such as lead or cadmium, and because different kinds of fungi react differently to environmental change, certain species can be used to identify specific kinds of pollution.
Mycorrhizae are also useful indicators of the health of their native habitat. Many form cauliflower-like growths on tree roots, but these are smaller in polluted soil. The trees themselves may also respond to pollution with weaker shoot growth, but the mycorrhizal response is more acute and serves as a valuable early-warning sign of a habitat in decline.
See also: Evolution by natural selection • Mutualisms • The ecosystem • Energy flow through ecosystems
IN CONTEXT
KEY FIGURES
Charles Elton (1900–91), George Evelyn Hutchinson (1903–91)
BEFORE
Ninth century Arab writer Al-Jaziz introduces the concept of the food chain in Kitab al-Hayawan (Book of Animals), concluding that “every weak animal devours those weaker than itself”.
1917 American biologist Joseph Grinnell first describes an ecological niche in his paper, “The niche relationships of the California Thrasher.”
AFTER
1960 American ecologist and philosopher Garrett Hardin publishes an essay in the magazine Science in which he states that “every instance of apparent coexistence must be accounted for.”
1973 Australian ecologist Robert May publishes Stability and Complexity in Model Ecosystems, in which he uses mathematical modeling to demonstrate that complex ecosystems do not necessarily lead to stability.
The concept of food chains—the idea that all living things are linked through their dependence on other species for their food—dates back many centuries, but it was not until the early 20th century that scientists developed the concept of food chains forming a food web.
The pioneer of this thinking was British zoologist Charles Elton, whose book Animal Ecology (1927) describes what he called the “food cycle”. He later went on to develop theories that encompassed more complex interactions between animals and the environment—insights that underpin modern animal ecology. He likened our knowledge of individual plant and animal species to the cells in a beehive—each “cell” of knowledge is important in its own right, but by putting them all together something much more than the sum of the parts is created—the “beehive” of ecology.
Nowadays, the study of animal ecology focuses on how animals interact with their environment, the roles played by different species, why populations rise and fall, why animal behavior sometimes changes, and the impact of environmental change on animals. The principle underlying the work of animal ecologists is that there is generally a balance in nature, so if the population of a given species grows too large it will be regulated, most often by a lack of food. However, relationships between organisms and their environment change from place to place and through time.
A food web is a graphic depiction of the feeding connections between different species within an ecological community. This example illustrates the relationships within a marine ecosystem, in which killer whales are the apex predators and phytoplankton are the primary producers.
Chain of dependence
In Animal Ecology, Elton outlined the key principles of the study of animal communities: food chains and webs, food size, and ecological niches. Each food chain and web, he asserted, is dependent on producers: plants and algae that support plant-eating consumers (herbivores). These herbivores in turn support one or more levels of meat-eating consumers (carnivores). Large carnivores generally eat smaller animals, but because small animals reproduce more quickly, their numbers are able to support the larger predators.
Competition for food is very tight near the top of a food web. Although apex (top) predators, such as big cats and large birds of prey, have no natural predators, this often means that they have to defend territories against rivals of their own species to ensure there is enough food for themselves and their offspring.
“Every animal is closely linked with a number of other animals living around it—and these are largely food relations.”
Charles Elton
Forecasting the effects of climate change
A lone polar bear surveys the sea for prey from a piece of floating ice in the Arctic. The shrinking area of sea ice in the region threatens this species’ survival.
Ecologists examine changes to animal populations and distribution and apply climate change models to forecast how these will change further in the future, over the course of 5, 10, 50, or more years. In the Arctic, for example, where average temperatures are rising more rapidly than anywhere else, the sea ice is contracting. As a result, polar bears have to travel farther in search of ice where they can catch seals, rest, and mate. The farther they swim, the more energy they burn. As the sea ice declines, the polar bears starve. Scientists monitor their numbers and movements and compare this data with changes in sea ice.
The polar bear plays a vital part in the ecology of the Arctic. As an apex predator and keystone species, it must have access to seals, which are its almost exclusive diet. The number of seals regulates the density of polar bears, while polar bear predation in turn regulates the density and reproductive success of seals.
Food size
One of Elton’s most important points was the notion that food chains exist primarily because of the principle of food size. He explained that every carnivorous animal eats prey between upper and lower limits. Predators are physically unable to catch and consume other animals above a certain size because they are not large enough, strong enough, or skillful enough. That is not to say that predators cannot kill and eat larger animals than themselves; a weasel can easily kill a larger rabbit because it is more aggressive. However, an adult lioness, one of the world’s top predators, is not capable of killing a healthy adult African elephant. Likewise, a dragonfly larva on the bottom of a pond may be able to prey on a small tadpole, but it would not be able to eat an adult frog.
Animals may be capable of killing much smaller prey, but it is simply not worth the effort. Wolves hunt medium-sized or large mammals such as elk. If those mammals disappear from their environment, they find it hard to catch sufficient numbers of smaller animals such as mice to sustain them; the energy they use finding small prey is greater than the energy they gain by consuming them.
Plants cannot run away or fight back, so different consi
derations apply to herbivores when it comes to food size. There is a maximum size of seed that a given finch, for example, can fit in its bill, so larger finches have an advantage over smaller species. Similarly, individual species of hummingbirds can drink nectar only from flowers up to a certain size, depending on the length of their bill.
A spider traps a damselfly, demonstrating that the principle of food size can be modified by the comparative aggression and strength of the predator and its prey.
Ecological niches
An animal or plant’s niche is its ecological role or way of life. For American zoologist Joseph Grinnell, working in the early decades of the 20th century, an organism’s niche was defined as its habitat. He studied birds called thrashers in California and observed how they fed, nested, and hid from predators in the dense undergrowth of the chaparral shrubland. However, a niche is more complex than simply the place where an organism lives. Oxpeckers and buffalo share exactly the same habitat—open grassland – but their requirements for survival are very different: the buffalo graze on the grasses, while oxpeckers derive their food from the ticks they peck from the buffalos’ hide.
Charles Elton explored the concept of ecological niches in more depth. For him, food was the primary factor in defining an animal’s niche. What it ate and what it was eaten by were crucial. Depending on the habitat, a particular niche could be filled by a different animal. Elton cited the example of a niche that was filled by birds of prey that hunted small ground-dwelling animals such as mice and voles. In a European oakwood, that niche would be filled by Tawny Owls, while on open grassland Kestrels would fill the role.