The Ecology Book

by DK

  AFTER

  1991 German biologist Harald Esch describes how muscle “warm-up” plays a role in brood incubation and colony defense as well as flight preparation.

  2012 Using infrared thermography, Spanish zoologist Jose R. Verdu shows how some dung beetle species heat or cool their thorax to improve flight performance.

  Insects are usually described as “cold-blooded,” or ectotherms. Unlike mammals and other “warm-blooded” endotherms, animals that maintain their body temperature at a more or less constant level, insects have a variable body temperature that changes with their environment.

  In the early 19th century, however, British entomologist George Newport discovered that some moths and bees raise the temperature of their thorax (the central part of the body, to which wings and limbs attach) above that of the surrounding air by rapidly flexing their muscles. It is now known that many insects are heterotherms, maintaining different temperatures in different parts of the body, and are sometimes far warmer than the ambient temperature.

  “In insects… the active flight muscles… are, metabolically, the most active tissues known.”

  Bernd Heinrich

  The right temperature

  The main challenge facing insects is how to get warm enough to fly but cool enough not to overheat. German–American entomologist Bernd Heinrich explained in 1974 how moths, bees, and beetles could continue to function by controlling their own temperature. He realized that insects’ thermal adaptations do not differ as much from those of vertebrates as had been thought.

  Most flying insects have higher metabolic rates than other animals but their small body size means they lose heat rapidly, so they cannot keep their temperature constant at all times. The minimum temperature that allows an insect to fly varies from species to species, but the maximum temperature falls within 104–113°F (40–45°C). To prevent overheating, insects can transfer heat from the thorax to the abdomen.

  Many larger flying insects would remain grounded if they were not able to increase the temperature of their flight muscles. These insects “quiver” the muscles that control the upbeat and downbeat of the wings to generate heat before taking off. Once flying, the muscles use large amounts of chemical energy but only some of it is used to beat the wings; the rest becomes more heat. This, combined with the warmth of direct sunlight, means a flying insect risks overheating.

  To solve this problem, many species have a heat-exchange mechanism that shifts excess heat from the thorax to the abdomen, allowing the insect to maintain a steady temperature in its thorax.

  A tortoiseshell butterfly feeds on a dandelion. Most butterflies can angle their wings upward in an attempt to cool down, in a process called behavioral thermoregulation.

  Range of techniques

  By changing the angle of their wings, butterflies control their body temperature. When they are trying to warm up, holding their wings wide open maximizes the amount of sunshine falling on them. When they are trying to cool down, they move into shade or angle their wings upward so that less direct sunlight shines on their surface.

  Other insects use even more remarkable methods to regulate their body temperature. When a mosquito drinks the warm blood of a mammal, this raises its body temperature. To compensate, it produces droplets of fluid that are kept at the end of the abdomen; evaporative cooling of these droplets lowers the insect’s temperature. Dung beetles construct balls of dung in which females lay their eggs. Some dung beetles are able to raise the temperature of their thorax so they can roll heavier balls.

  The range of thermoregulation techniques shows how life forms evolve to better fit their environment. They can also inspire technology: arrays of solar panels angled to track the Sun capture maximum amounts of solar radiation—just like butterfly wings.

  Heat regulation

  This Japanese giant hornet is raiding nursery cells in a bees’ nest in Hase Valley, Japan. Hornets seek to devour the bee larvae inside the cells.

  Honeybees are renowned for controlling the temperature of their hive. When it gets too hot, they ventilate it by using their wings to fan the hot air out of the nest. When it gets too cold, the bees generate metabolic heat by rapidly contracting and relaxing their flight muscles. They also use heat as a defense mechanism. Japanese giant hornets are fierce predators of honeybees. Capable of killing large numbers quickly, they pose a serious threat to bees’ nests. Hornets begin their attacks by picking off single honeybees at the entrance to the hive. However, Japanese honeybees defend themselves with self-generated heat. If a hornet attacks, they swarm around it, vibrating their wings to raise their collective temperature. Since the hornet cannot tolerate a temperature above 114.8°F (46°C) whereas the bees can survive at almost 118.4°F (48°C), the attacker eventually dies.

  See also: Evolution by natural selection • Ecophysiology • Animal ecology • Organisms and their environment

  INTRODUCTION

  When Aristotle wrote about plant and animal species existing for the sake of others, he showed a basic understanding of food chains—as have countless observers of the natural world since ancient Greek times. Arab scholar Al-Jahiz described a three-level food chain in the 9th century, as did the Dutch microscopist Antonie van Leeuwenhoek in 1717. British naturalist Richard Bradley published more detailed findings on food chains in 1718, and in 1859, Charles Darwin described a “web of complex relations” in the natural environment in his book On the Origin of Species. The concept of a food web, with many predator–prey interactions, was then further developed by Charles Elton in his classic Animal Ecology (1927).

  The concept of the ecosystem (“a recognizable self-contained entity”) followed soon after, when in 1935, British botanist Arthur Tansley wrote that organisms and their environment should be considered one physical system. In his Ph.D. thesis, American ecologist Raymond Lindeman expanded on Tansley’s work, positing that ecosystems are composed of physical, chemical, and biological processes “active within a space–time unit of any magnitude.”

  Lindeman also conceived the idea of feeding levels, or trophic levels, each of which is dependent on the preceding one for its survival. In 1960, the American team of Nelson Hairston, Frederick Smith, and Lawrence Slobodkin published findings on the factors controlling animals on different trophic levels. They identified both the top-down pressures exerted by predators and the bottom-up pressures exerted by limitations on food supply. Twenty years later, American ecologist Robert Paine wrote of the trophic cascade effect—the way a system is changed by the removal of a key species. He described changes to the food web after the experimental removal of the ocher starfish from an intertidal zone. This predatory echinoderm was shown to be a keystone species, playing a crucial role within its ecosystem.

  Island isolation

  Habitat fragmentation is now a major problem in most terrestrial environments because it leaves specialist organisms isolated. For that reason, research into the biogeography of islands—those surrounded by ocean but also “islands” of distinct habitat surrounded by a very different environment—is so important in ecology. In the US in the 1960s, Edward O. Wilson and Robert MacArthur discovered key factors determining species diversity, immigration, and extinction on islands. James Brown later did similar work on animal populations in isolated patches of forest ridge in California. Such work has showed how to identify species most at risk of extinction due to isolation.

  Stability and resilience

  One major contribution to the understanding of ecosystem dynamics was the concept of the evolutionarily stable state. In the 1970s, British biologist John Maynard Smith used the term evolutionarily stable strategy (ESS) to describe the best behavioral strategy for an animal competing with others living in its vicinity. This strategy depends on how the other animals behave and is determined by the animal’s genetic success—if it makes the wrong decisions, it will not live long and cannot pass on its genes. The overall balance between the evolutionarily stable strategies of all the animals in an ecosystem
is called the evolutionarily stable state.

  Canadian ecologist Crawford Stanley Holling introduced the idea of resilience—how ecosystems persist in the wake of disruptive changes such as fire, flood, or deforestation. A system’s resilience can be seen in its capacity to absorb disturbance, or the time it takes to return to a state of equilibrium after a trauma. Ecologists now understand that ecosystems can have more than one stable state, and that resilient systems are not always good for biodiversity.

  When the populations of many species are declining or becoming locally extinct, ecologists are once more focusing their attention on ecosystem resilience. Many, including French ecologist Michel Loreau, believe that if diversity in an ecosystem is reduced, the whole system will be less likely to resist major impacts such as the effects of climate change. Today, Loreau and others are working toward finding a new general theory that can explain the relationship between ecosystem biodiversity and resilience in order to understand and combat the effects of today’s environmental challenges.

  IN CONTEXT

  KEY FIGURE

  Richard Bradley (1688–1732)

  BEFORE

  9th century Arab scholar Al-Jahiz describes a three-level food chain of plant matter, rats, snakes, and birds.

  1717 Dutch scientist Antonie van Leeuwenhoek observes how haddock eat shrimp and cod eat haddock.

  AFTER

  1749 Swedish taxonomist Carl Linnaeus introduces the idea of competition.

  1768 John Bruckner, a Dutch naturalist, introduces the idea of food webs.

  1859 Charles Darwin writes about food webs in On the Origin of Species.

  1927 British zoologist Charles Elton’s Animal Ecology outlines principles of animal behavior, including food chains.

  All animals must eat other living things in order to receive the nutrients they need to grow and function. A food chain shows the feeding hierarchy of different animals in a habitat. For example, the chain would show that foxes eat rabbits but rabbits never eat foxes. Although there were earlier notions of a hierarchy of animals linked to each other in a food chain, British naturalist Richard Bradley brought more detail to this idea in his book New Improvements in Planting and Gardening (1718). He noted that each plant had its own particular set of insects that lived off it and proposed that the insects in turn received the attentions of other organisms of “lesser rank” that fed on them. In this way, he believed that all animals relied upon each other in a self-perpetuating chain.

  Producers and consumers

  The modern concept of a food chain explains that some organisms produce their own food. These are known as producers, or autotrophs. Plants and most algae fall into this category, normally using the energy of sunlight to convert water and carbon dioxide into glucose, at the same time as releasing oxygen. This process, photosynthesis, is the first step towards creating food. In places where there is no sunlight, organisms producing their own food are called chemotrophs. Those in the deep ocean, for example, get the energy they need from chemicals released by hydrothermal vents.

  Animals that eat producers and creatures that eat other animals are called consumers, or heterotrophs. There may be two, three, or more levels of these in any particular part of the food chain, but there is always a producer at the bottom, and all levels above it are consumers. Animals that only eat plants are herbivores, or primary consumers, and they include cattle, rabbits, butterflies, and elephants. Those that eat only other animals are carnivores, or secondary consumers; these include thrushes, dragonflies, and hedgehogs. In turn, secondary consumers may be eaten by larger predators, or tertiary consumers, such as foxes, small cats, and birds of prey. The animals at the top of their food chain are apex predators. They include consumers such as tigers, killer whales, and golden eagles that are not preyed upon by other animals.

  The food chain does not break when plants and animals die. Detritus feeders (detritivores) prey on the remains, recycling nutrients and energy for the next generation of producers to use.

  An apex predator, such as the bronze whaler shark, has no natural predators. In the temperate waters of the ocean off South Africa the shark can find vast quantities of sardines to eat.

  “Each species has a specific place in nature, in geographic location and in the food chain.”

  Carl Linnaeus

  Food webs

  Observers after Bradley suggested that animals were not simply part of a food chain, but a larger and more complex “food web” that comprises all the food chains in a location. This idea was put forward by Dutch naturalist John Bruckner in 1768, and later taken up by Charles Darwin, who called the variety of connected feeding relationships between species a “web of complex relations”.

  RICHARD BRADLEY

  Born around 1688, noted British botanist Richard Bradley gained patrons after writing a Treatise of Succulent Plants at the age of 22. With no university education, he was nonetheless elected a Fellow of the Royal Society and later became the first professor of botany at Cambridge.

  Bradley’s research interests were wide-ranging, including fungal spore germination and plant pollination. In some cases, Bradley was ahead of his time; he argued, for example, that infections were caused by tiny organisms, visible only with a microscope. His investigations into the productivity of rabbit warrens and fish lakes led to his theories about predator–prey relations. Bradley died in 1732.

  Key works

  1716–27 The History of Succulent Plants

  1718 New Improvements in Planting and Gardening

  1721 A Philosophical Account of the Works of Nature

  See also: Predator–prey equations • Mutualisms • Keystone species • Optimal foraging theory • Animal ecology • The ecosystem • Trophic cascades • Ecological resilience

  IN CONTEXT

  KEY FIGURE

  Arthur Tansley (1871–1955)

  BEFORE

  1864 George Perkins Marsh, an American conservationist, publishes Man and Nature, which hints at the concept of ecosystems.

  1875 Austrian geologist Eduard Suess proposes the term “biosphere.”

  AFTER

  1953 American ecologists Howard and Eugene Odum develop a “systems approach” to studying the flow of energy through ecosystems.

  1956 American ecologist Paul Sears highlights the role of ecosystems in recycling nutrients.

  1970 Paul Ehrlich and Rosa Weigert warn of potentially destructive human interference in ecosystems.

  British biologist Arthur Tansley was the first to insist that communities of organisms in a particular area had to be seen in a wider context, including the nonliving elements of that area. Tansley argued that in a given region, all the living organisms and their geophysical environment together form a single, interactive entity. Borrowing a concept from engineering, he saw the network of interactions as a dynamic, physical system. On the suggestion of his colleague Arthur Clapham, he coined the word “ecosystem” to describe it.

  This idea had been developing long before Arthur Tansley published his influential paper on the subject in 1935. As early as 1864, conservationist George Perkins Marsh, in his book Man and Nature, had identified “the woods,” “the waters,” and “the sands” as different types of habitat. He examined how the relationship between them and the animals and plants that lived in them could be upset by human activity.

  Interconnected systems

  By the 20th century, the idea had taken hold that these and other environments could be understood as discrete entities, with distinctive interactions between the living and inert elements within them. In 1916, American ecologist Frederic Clements built on this idea in his work on plant succession, referring to a “community” of vegetation as a single unit, and using the term “biome” to describe the whole complex of organisms inhabiting a given region.

  Tansley envisaged ecosystems as being made up of biotic (living) elements and abiotic (nonliving) elements such as energy, water, nitrogen, and soil minerals, which are essential to the functio
ning of the systems as a whole. The biotic components within an ecosystem not only interact with one another, but also with the abiotic parts. Thus, within any given ecosystem, the organisms adapt to both the biological and physical elements of the environment. The different types of ecosystem can be defined by their physical environments. There are four categories of ecosystem: terrestrial, freshwater, marine, and atmospheric. These can be further subdivided into various types according to different physical environments and the biodiversity within them. Terrestrial ecosystems, for example, can be subdivided into deserts, forests, grasslands, taigas, and tundras.

  Tropical coral reefs are some of the most diverse ecosystems of all, full of fish, sea turtles, crustaceans, mollusks, and sponges, as well as corals.

  Dynamic feedback

  Tansley’s most important insight was that these discrete communities of living and nonliving components form dynamic systems. In a terrestrial ecosystem, for example, the organisms interact to recycle matter: plants absorb carbon dioxide (CO2) from the atmosphere and nutrients from the soil to grow. These plants release life-sustaining oxygen into the atmosphere by respiration, and provide food for animals. Animal excreta and dead matter also release carbon, and provide material to be decomposed by bacteria and fungi, in turn providing soil nutrients for plants.

  Arthur Tansley also argued that these internal processes within an ecosystem conform to what he described as “the great universal law of equilibrium.” Self-regulating, these processes have a natural tendency toward stability. The cycles within an ecosystem contain feedback loops that correct any fluctuations from a state of equilibrium.