The Ecology Book

by DK

  Elton also argued that an animal could not only tolerate a certain set of environmental conditions, but could also change them. The tree-felling and stream-damming activity of beavers is one of the most dramatic examples, creating habitats for fish in dammed pools, woodpeckers in dead trees, and dragonflies around pool margins.

  The Sword-billed Hummingbird, a native of South America, has a long bill, which enables it to suck up nectar from the long flowers of Passiflora mixta, a species of passionflower. As it feeds, it spreads the plant’s pollen.

  “Observation of species in the wild convinces me that the existence and persistence of species is vitally bound up with environment.”

  Joseph Grinnell

  Niches and competition

  British-born zoologist G. Evelyn Hutchinson, working at Yale University from the 1950s to the 1970s, examined all the physical, chemical, and geological processes at work in ecosystems and proposed that any organism’s role in its niche includes how it feeds, reproduces, finds shelter, and interacts with other organisms and with its environment. For example, each species of trout—and other fish—has its own range of water salinity, acidity, and temperature that it can tolerate, as well as a range of prey and river- or lake-bed conditions. This makes some better competitors than others, depending on the conditions of the habitat in which they live. Seen as the father of modern ecology, Hutchinson inspired other scientists to explore how competing animals use their environment in different ways.

  An animal or plant’s niche width comprises the whole range of factors it requires to allow it to thrive. Brown rats, raccoons, and starlings are examples of animals with a broad niche width in that they are able to survive in a wide variety of conditions. Such species are called generalists. Other animals have narrow requirements. For example, koalas depend almost entirely on eucalyptus leaves, and Hyacinth Macaws in the Pantanal region of Brazil eat virtually nothing but the hard fruits of two species of palm trees—these are specialists.

  Animals rarely occupy the whole of their niche width, owing to competition between species. Part of the habitat requirement of North American bluebirds is dead trees with old woodpecker holes in which they lay their eggs and raise their young. Although suitable holes are common in many forests, bluebirds cannot occupy all these holes because they are often out-competed by more aggressive starlings. Therefore their realized niche—the places they actually occupy—is not as extensive as their potential (or fundamental) niche.

  Many animals share some aspects of their niche, but not others. This is called niche overlap. If different species live in the same habitat and have similar lifestyles, they will be in competition but they may be able to live in close proximity if some aspects of their behavior or diet differ. This arrangement is known as niche partitioning. For example, various anole lizards on Puerto Rico successfully occupy the same areas because they select perching locations in different parts of trees.

  There are limits to niche overlap. When two animals with identical niches live in the same place, one will drive the other to extinction. This concept—the competitive exclusion principle—was outlined by Joseph Grinnell in 1904 and developed in a paper published by Russian ecologist Georgy Gause in 1934, becoming known as Gause’s law.

  A true specialist, a koala bear requires 2.5 lb (1 kg) of eucalyptus leaves a day. This species is found in the wild only in Australia, where eucalyptus is common.

  “Different species press against one another, like soap bubbles, crowding and jostling, as one species acquires … some advantage over another.”

  G. Evelyn Hutchinson

  Pyramid of numbers

  Charles Elton used a pyramid as a way of graphically representing the different levels in a food chain, with the producers at the bottom, the primary consumers on the level above, and so on. Often, the primary consumers—insects, in particular—will outnumber the producers, but the higher levels of consumers will become less numerous toward the top of the pyramid. This system does not take account of parasites; fleas and ticks on mammals and birds will far outnumber the total of all the vertebrates in an ecosystem.

  In 1938, German-born animal ecologist Frederick Bodenheimer modified Elton’s pyramid of numbers to produce a pyramid of biomass that represented the amount of living matter in a given area at every level. This took into account the fact that some organisms are much larger than others, but because it showed comparative biomasses at a fixed point in time, it produced anomalies. For example, in a pond, the mass of the phytoplankton producer (microscopic organisms that are the foundation of the aquatic food web) may not be as great as the mass of the fish consumers at a particular point in time, so the pyramid will be inverted. However, phytoplankton reproduce quickly when conditions, such as sunlight and nutrients, are right. Over time, the mass of the phytoplankton will far outweigh that of the fish.

  Ecological pyramids represent quantifiable data in an ecosystem. Numbers show the population size of individual species in a trophic level; biomass, their relative presence; and energy, who eats what and how much.

  “The basic process in trophic dynamics is the transfer of energy from one part of the ecosystem to another.”

  Raymond Lindeman

  Microscopic organisms, including these diatoms, form a significant part of all ecological pyramids. Their huge numbers and rapid reproduction provide mass and energy for the species higher up the pyramid.

  Trophic pyramids

  American ecologist Raymond Lindeman proposed a pyramid of energy, called the trophic pyramid, showing the rate at which energy is transferred from one level to the next as herbivores eat plants, and predators eat herbivores. An organism’s trophic level is the position it occupies in a food chain. Plants and algae are at trophic level 1, herbivores at level 2, and the first level of predators is at 3. It is rare for there to be more than five levels. Plants convert the sun’s energy into stored carbon compounds, and when a plant is eaten by a herbivore, some of the energy transfers to the animal. When a predator eats the herbivore, it receives a smaller amount of that energy, and so on.

  Published in 1942, Lindeman’s Ten Percent Law explains that when organisms are consumed, only about 10 percent of the energy transferred from them is stored as flesh at the next trophic level. The energy model creates a more realistic picture of the condition of an ecosystem. For example, if the biomass of weed and fish in a pond is the same, but the weed reproduces twice as fast as the fish, the energy of the weed would be shown to be twice as large. Also, there are no inverted pyramids—there is always more energy in the lowest trophic level than the one above. Assessing energy transfer, however, requires a lot of information about energy intake, as well as the number and mass of organisms.

  Tench feed on snails, which graze on periphyton—a mixture of microbial organisms that cling to plants. By reducing the number of snails, tench increase the periphyton biomass.

  Future thinking

  Relationships between organisms and their environment change from place to place and through time. Global climate change is one example of environmental factors that will increasingly affect animal communities. Some changes have already taken place, but one of the challenges of ecological thinking in the future is to forecast others.

  Snowshoe hare and lynx population cycles

  A Canadian lynx captures a snowshoe hare, its preferred prey. When hares are plentiful, a lynx will eat two every three days.

  In Canada’s boreal forests, the favored prey of lynx are snowshoe hares. Charles Elton examined the relationship between the populations of these two species, using data covering the period 1845–1925. When hares are numerous, lynx hunt little else. After their population reaches its peak density, the hares struggle to find enough plant food. Some starve, while others are weakened and are more easily caught by predators, including lynx, which feed very well for a time. When hare numbers continue to fall, this affects the lynx. They are forced to hunt less nutritious prey, such as mice and grouse.

  As th
ey struggle to find enough to eat, lynx produce smaller litters or even stop breeding altogether. Some starve to death. A decline in the lynx population sets in one or two years after the hare population has bottomed out, a cycle that repeats every eight to eleven years.

  See also: Keystone species • The food chain • The ecosystem • Energy flow through ecosystems • Trophic cascades

  IN CONTEXT

  KEY FIGURE

  David Lack (1910–73)

  BEFORE

  1930 British geneticist Ronald Fisher combines Gregor Mendel’s work on genetics with Charles Darwin’s theory of natural selection, and argues that the effort spent on reproduction must be worth the cost.

  AFTER

  1948 David Lack extends his theory of optimal clutch size in birds to include litter size in mammals.

  1954 Lack develops his food limitation hypothesis further in The Natural Regulation of Animal Numbers, to encompass birds, mammals, and some insect species.

  1982 Tore Slagsvold proposes the nest predation hypothesis, which states that clutch size is related to the likelihood of the nest being attacked.

  Why do some birds lay more eggs than others? For example, Blue Tits lay nine eggs, Northern Flickers six, and Robins four. In the 1940s, British ornithologist and evolutionary ecologist David Lack proposed an explanation that rapidly gained support. He argued that the clutch size (number of eggs laid) was not controlled by the female’s ability to lay eggs, since birds can lay many more eggs than they typically do. This fact can be demonstrated by replacement experiments, in which eggs are removed from a nest; the bird will re-lay repeatedly to compensate for the loss.

  Instead, Lack said, the number of eggs laid by any species has evolved to fit with the food supply available. In other words, nature favors clutch sizes that correspond to the maximum number of young that the parents are likely to be able to sustain. So, if a pair of birds can only find enough food to feed six chicks, but the female has laid 12 eggs, those young will be hungry and may starve. If she has laid just one egg, although the chick will be raised successfully, most of the available food will have been unused. So neither the 12-egg nor the one-egg scenarios are good reproductive strategies; instead, laying six eggs offers the best chance of raising the most offspring.

  This theory became known as the food limitation hypothesis, or Lack’s principle, and it was later generalized by him and others to cover litter size in mammals and clutch size in fish and invertebrates.

  Blue Tit nests contain an average of nine eggs, although the females can lay many more. David Lack proposed that the clutch size is determined by the likely amount of available food.

  “Clutch size increases with increasing latitude and day length because … a longer day enables the parents to find more food.”

  David Lack

  The “latitude trend”

  Lack’s hypothesis also suggested an answer to another puzzle: why most bird species have bigger clutches at higher latitudes. On average, birds near the equator lay about half the number of eggs laid by the same species in the far north. This “latitude trend” could be explained by a greater availability of food during the long day-length of summer compared with the shorter day-length in the tropics.

  However, other factors may also apply. Higher mortality rates in high latitudes—where winters are harsh—may have led to the evolution of large clutch sizes. This is because the chances of survival until the next breeding season are low, and the reduced population results in more food being available for the survivors next season.

  In 1982, Tore Slagsvold, a Norwegian evolutionary ecologist, advanced the nest predation hypothesis, which proposes that high rates of nest predation result in smaller clutches. If a nest with many chicks is found by a predator, more work by the parent birds will have been wasted than if the nest contained fewer chicks. Also, parents raising a large clutch are more likely to be seen by predators, because of the extra activity. Some ecologists have argued that the relative abundance of predators in the tropics has been more important than food supply in the evolution of small clutch sizes at low latitudes.

  “Laying a clutch which will result in a smaller brood than … could be fed and reared successfully … confers advantages.”

  Tore Slagsvold

  Siblicide and the Blue-footed Booby

  Blue-footed Boobies are driven to siblicide by genetic factors. The murder of a sibling can benefit the perpetrator while also ensuring the survival of the entire species.

  Blue-footed Boobies are seabirds native to the Pacific Ocean. They get their food from the ocean, but come to rocky shores and cliffs to breed. The female lays two eggs, roughly five days apart, so that by the time the second chick hatches, the first one has already grown considerably. When food is plentiful, the parents can find enough to feed both offspring until they fly the nest (fledge). However, when food is scarce, the larger chick will peck its junior sibling to death. The older chick can then get more food, and is more likely to fledge. If it does not murder its sibling when food is scarce, both chicks may starve.

  This behavior, based exclusively on the availability of food, is called “facultative siblicide.” In contrast, masked boobies practice “obligate siblicide”—the first-hatched chick nearly always kills its brother or sister, regardless of how much food is available.

  See also: Animal ecology • Animal behavior • The food chain • The ecosystem • Ecological resilience

  IN CONTEXT

  KEY FIGURES

  Konrad Lorenz (1903–89), Nikolaas Tinbergen (1907–88)

  BEFORE

  1872 Charles Darwin’s The Expression of the Emotions in Man and Animals posits that behavior is instinctive and has a genetic basis.

  1951 Nikolaas Tinbergen’s The Study of Instinct lays down the foundations and theory behind ethology, the study of animal behavior.

  AFTER

  1967 Desmond Morris, a British zoologist, brings ethology to bear on human behavior in his popular book The Naked Ape.

  1976 British evolutionary biologist Richard Dawkins publishes The Selfish Gene, describing how most of an animal’s behavior is designed to pass on its genes.

  Any dog owner will describe the companionable and loyal relationship they enjoy with their pet. The Austrian zoologist Konrad Lorenz set out to explain this behavior in Man Meets Dog (1949). He described the behavior of dogs and other pets as substantially innate, “instinctive activity,” as opposed to behavior learned through conditioning. Lorenz proposed that such hard-wired behavior helped the animal survive as a species. For example, a domestic dog’s loyalty to its human master originates in the natural behavior of its wild ancestors, which were loyal to the pack leader because this had benefits in terms of hunting success and safety.

  Ducklings imprinting is an example of instinctive behavior that can be manipulated—to make them imprint on humans or even inanimate objects.

  Field experiments

  Lorenz was not alone in his theories. Other biologists working in the field included fellow Austrian Karl von Frisch and Dutch biologist Nikolaas Tinbergen, who studied animals in their natural environments. Until then, most animal behavior studies had taken place in laboratories or artificial settings, so the behavior witnessed was not entirely natural. Studying animals in the wild had its own challenges, particularly when devising rigorous field experiments that could be repeated, so that the findings could be recognized as facts, not anecdotes.

  The term “ethology” was coined by American entomologist William Morton Wheeler in 1902 to describe the scientific study of animal behavior. Ethologists study animals in their natural habitats, combining laboratory studies and fieldwork in order to describe an animal’s behavior in relation to its ecology, evolution, and genetics.

  Ethologists found that in certain situations, an animal will have a predictable behavioral response. They called this a “fixed action pattern” (FAP). A FAP has set characteristics. It is species-specific; it is repeated in the same way every time
and is not affected by experience. The triggers for the behavior (“sign stimuli”) are highly specific and may involve a color, pattern, or sound. For example, male sticklebacks respond aggressively when another male enters their streambed patch. Ethologists suggest this is triggered by seeing the male’s red underbelly.

  Nikolaas Tinbergen found that some an artificial sign stimuli work better than the real thing. He investigated the begging behavior of herring gull chicks, which peck at a red spot on the parent gull’s beak to make it regurgitate food. He found that chicks will also peck at a model of the gull’s beak, yet when they were offered a narrow red pencil with three white lines at the end, the chicks pecked at this even more enthusiastically. Tinbergen called this a “supernormal stimulus,” showing that instinctive animal behavior can be manipulated artificially.

  KONRAD LORENZ

  Born in Vienna, Austria, Lorenz was enthralled by animals from an early age and kept fish, birds, cats, and dogs. The son of an orthopedic surgeon, he studied medicine at Vienna University, graduating in 1928, and gained his Ph.D. in zoology in 1933. His numerous pets became the first subjects of his studies. Lorenz is perhaps best known for describing the phenomenon known as “imprinting.” This is when a newly hatched chick bonds with the first thing it sees (usually its parent) and will follow it around. The behavior, seen in ducks and other birds, as well as mammals, is instinctive and occurs shortly after birth. Lorenz demonstrated the theory by quacking like a duck at newly hatched ducklings. He soon had a flock of ducklings that followed him everywhere.