The Ecology Book

by DK

  The desert locust (Schistocerca gregaria) has to eat vast quantities of carbon-rich plants in order to get enough nitrogen and phosphorus to maintain its C:N ratio.

  The Growth Rate Hypothesis

  Malignant lung tissue (seen here) and cancerous colon tissue both had the highest phosphorus content in research exploring the rapid growth rates of tumors.

  Cancer research is one area where stoichiometry is now being employed. Evidence is growing for a theory called the Growth Rate Hypothesis (GRH), which may help explain why some cancerous tumors grow at faster rates than the rest of the body.

  The hypothesis states that organisms with high C:P (carbon:phosphorus) ratios, such as fruit flies, have more ribosomes in their cells, which enables them to grow and reproduce more rapidly. Around half of all phosphorus in an organism is in the form of ribosomal RNA (rRNA); it is present in every cell, creating proteins to build new cells and grow the body. Applying biological stoichiometry, James Elser and his team have shown that fast-growing tumors have a much higher phosphorus content than normal body tissue. Such research may help scientists understand how tumor growth could be controlled.

  See also: Ecophysiology • The food chain • Energy flow through ecosystems • The foundations of plant ecology

  IN CONTEXT

  KEY FIGURE

  Earl Werner (1944–)

  BEFORE

  1966 American ecologist Robert Paine conducts a series of groundbreaking field experiments to highlight the crucial effects of a predator on the community in which it lives.

  1990 Canadian biologists Steven Lima and Lawrence Dill analyzed the decision-making of organisms that are at the greatest risk of being preyed on by other creatures.

  AFTER

  2008 American behavioral biologist and ecologist John Orrock teams up with Earl Werner and others to produce mathematical models to explain the nonconsumptive effects of predatory animals.

  Many descriptions of ecosystems focus on predator–prey interactions in which predators kill and prey are eaten. However, American ecologist Earl Werner and others have shown that the mere presence of a predator affects the behavior of prey.

  Apart from apex predators, all animals must balance the need to sleep, reproduce, and feed with the risks of being eaten. The lethal role of predators is obvious, but their nonlethal (nonconsumptive) role can have an even bigger impact on an ecosystem. Potential prey are forced to change their way of life in order to avoid being killed.

  In 1990, Werner studied the effects of green darner dragonfly larvae on toad tadpoles. He noticed that when the predatory larvae were in the tank, the tadpoles were less active, swam to other parts of the tank, and metamorphosed into adults when they were smaller. The predator had changed the toads’ morphology and their behavior just by being there.

  In 1991, Werner investigated what happened when more than one prey species was involved. In the absence of a predator, bullfrog and green frog tadpoles grew at virtually identical rates. However, when predatory dragonfly larvae were introduced to the tank, both prey species became less active and chose different places in which to swim. The bullfrog tadpoles grew more quickly than they had in a predator-free tank, but the green frog tadpoles decreased their feeding activity and grew more slowly. Werner concluded that for prey species there was a trade-off between the need to grow as fast as possible and the risk of predation. Growing more quickly requires more feeding activity, and this in turn increases the chances of being eaten by a predator. As the larvae’s presence altered the behavior of the prey species differently, the bullfrog’s new behavior gave it a competitive advantage over the green frog by making it bigger.

  Terrestrial animals

  Early studies of nonconsumptive effects (NCEs) were concerned with aquatic organisms under laboratory conditions, but more work has now been done in the wild with land-dwelling animals. German field research published in 2018 focused on lynx and their roe deer prey. When lynx were present, researchers found that the roe deer avoided areas they knew to be high-risk, both during the day and on summer nights when nocturnal predation is more common. The deer treated some grazing areas as out of bounds, presumably due to fear of being attacked by lynx.

  Wherever there are predators, they exert NCEs. They also affect some sessile (nonmoving) species, as well as mobile prey. This can happen when certain dominant competitors are displaced by predators and, in their new habitats, outcompete sessile animals for food. Small fish that are displaced, for example, could outcompete sponges for food.

  A green darner dragonfly laying its eggs in a pond. The larvae that hatch out are predators and have been shown to influence the behavior of their tadpole prey.

  “… species react [to predators] by reducing activity and altering space use.”

  Earl Werner

  See also: Evolution by natural selection • Predator–prey equations • Ecological niches • Competitive exclusion principle • Mutualisms • Optimal foraging theory

  INTRODUCTION

  People have long marveled at the variety of life, celebrating nature’s gifts in prehistoric cave art that dates back 30,000 years or more. In Ancient Greece in the 4th century BCE, Aristotle made an early attempt to classify living organisms; his 11-grade scala naturae (“ladder of life”) placed humans and mammals at the top, and descended through other, more “primitive” animals to plants and then minerals. A thousand years later, the medieval world still considered variations on Aristotle’s system to be valid. There were several reasons for this. Without microscopes, nothing was known of cells and microorganisms. Without the means to explore underwater, science’s knowledge of aquatic creatures was limited, and many parts of the world were still unknown to Western scientists. In keeping with the prevailing ideas of the Catholic Church, the natural world was seen as static and unchanging.

  An age of discovery

  The age of great expeditions of discovery revealed previously uncharted regions and their animals and plants. In his History of the Animals (1551–58), Swiss physician and naturalist Conrad Gesner included some of the recent finds from the New World and the Far East, as well as relying on classical literature. The five-volume work reflected his division of animals into mammals; reptiles and amphibians; birds; fish and aquatic animals; and snakes and scorpions.

  The invention of the microscope also had a major impact. English scholar Robert Hooke was quick to adopt this new technology: his book Micrographia (1665) inspired others to do likewise. Able to view specimens magnified to 50 times their actual size, he made meticulous drawings of microscopic life, and also coined the term “cell” after examining plant fibers. Hooke also suggested a living origin for fossil fragments found in rocks.

  Classifying variety

  English vicar John Ray’s History of Plants (1686–1704) was the botanical equivalent of Gesner’s earlier work, listing some 18,000 species in three huge volumes. Ray also produced a biological definition of a species, remarking that “one species never springs from the seed of another.”

  Swedish botanist Carl Linnaeus, the “father of taxonomy,” first published Systema Naturae in 1735, but it is the 10th edition from 1758 that founded the modern zoological naming system. Two volumes of Linnaeus’ work are devoted to plants and animals, which he divided into classes, orders, genera, and species. The binomial system, in which every species is given a generic name followed by a specific name, is still in use today. Linnaeus also wrote a third volume on rocks, minerals, and fossils.

  Species concepts

  Building on Darwin’s theory of evolution by means of natural selection, German-American evolutionary biologist Ernst Mayr cemented the biological concept of species in his Systematics and the Origin of Species (1942). He argued that a species is not just a group of morphologically similar individuals, but a population that can breed only among themselves. Mayr went on to explain how if groups within a species become isolated from the rest of the population, they may start to differ from the rest, and over time, through
genetic drift and natural selection, may even evolve into new species.

  Modern technological advances, including electron microscopy and mitochondrial DNA analysis, have revealed much information—some of it surprising—about the number of species and the relationships between them. In 1966, striving to reflect the intricacies of evolution, German entomologist Willi Hennig proposed a new taxonomic system of clades—groups of organisms based on a common ancestor. In the 1970s, American biologist Carl Woese classified all life into three new domains. As of 2018, about 1.74 million extant plant and animal species have been described, but estimates of the total number range from 2 million to 1 trillion.

  The threat to diversity

  By the late 20th century, however, alongside a growing knowledge of the scale and critical role of biodiversity—and of how evolution can destroy species as well as create them—American ecologist E.O. Wilson and others made the world aware that human activity was responsible for causing a rapid acceleration in the extinction rate. Some have even warned that Earth could be on the verge of a sixth mass extinction. Many policies are now being proposed to counter this, including the protection of biodiversity hotspots.

  IN CONTEXT

  KEY FIGURE

  Aristotle (c. 384–322 BCE)

  BEFORE

  c. 1500 BCE Different properties of plants are recognized by ancient Egyptians.

  AFTER

  8th–9th centuries CE Islamic scholars of the Umayyad and Abbasid dynasties translate many of Aristotle’s works into Arabic.

  1551–58 Conrad Gessner’s History of Animals classifies the animals of the world into five basic groups.

  1682 John Ray publishes his History of Plants, which lists more than 18,000 species.

  1735 Carl Linnaeus devises a system of binomial names, the first consistent classification of organisms, according to which he names every species listed in his Systema Naturae.

  From the beginning of recorded history, people have attempted to identify organisms according to their uses. Egyptian wall paintings from c. 1500 BCE show, for example, that people understood the medicinal properties of many plants. In the text History of Animals, written in the 4th century BCE, the Greek philosopher and scholar Aristotle made the first serious attempt to classify organisms, studying their anatomy, life cycles, and behavior.

  “If any person thinks the examination of the rest of the animal kingdom an unworthy task, he must hold in like disesteem the study of man.”

  Aristotle

  Features of classification

  Aristotle divided living things into plants and animals. He further grouped about 500 species of animals according to obvious anatomical features, such as whether they had blood, were “warm-blooded” or “cold-blooded,” whether they had four legs or more, and whether they gave birth to live offspring or laid eggs. He also noted whether animals lived in the sea, on land, or flew in the air. Most significantly, Aristotle used names for his groupings that were later translated into the Latin words “genus” and “species”—terms that are still used by modern taxonomists to this day.

  Aristotle placed animals in a scala naturae (ladder of nature), with 11 grades distinguished by their mode of birth. Those in the top grades gave birth to live, hot, wet offspring; those in the lower grades to cold, dry eggs. Humans were at the very top of the scale, with live-bearing tetrapods (four-legged creatures), cetaceans, birds, and egg-laying tetrapods lower down. Aristotle placed minerals on the bottom grade of his scale, with plants, worms, sponges, larva-bearing insects, and hard-shelled animals on the levels above.

  While Aristotle’s system of classification was rudimentary, it was based largely on first-hand observations, many of which were made on the island of Lesvos. He recorded things that noone else had described, including that young dogfish grew inside their mothers’ bodies, male river catfish guard eggs, and octopuses can change color. Most of his observations were good—and some were confirmed only centuries later.

  An octopus blends in with its surroundings. The ability of these creatures to change color was one of Aristotle’s many accurate observations.

  The great chain of being

  Despite its limitations, Aristotle’s method of classification heavily influenced every later attempt at grouping animals and plants until the 18th century. Medieval Christianity developed his scala naturae as a “great chain of being,” with God at the top of a strict hierarchy, humans and animals beneath, and plants at the bottom.

  The Swiss doctor Conrad Gessner wrote the first modern register of animals—also called History of Animals—in the mid-16th century. This monumental five-volume work was based on classical sources but included newly discovered species from East Asia. It covered the main animal groups as Gessner saw them: live-bearing quadrupeds (mammals); egg-laying quadrupeds (reptiles and amphibians); birds; fish and aquatic animals; and snakes and scorpions. In 1682, the English naturalist John Ray produced the equivalent register for botany with his History of Plants. Within little more than 50 years, the classification of living things would be completely transformed by Carl Linnaeus’s Systema Naturae.

  ARISTOTLE

  Aristotle was born in Macedonia, ancient Greece. Both his parents died when he was young, and he was raised by a guardian. Aged 17 or 18, Aristotle joined Plato’s Academy in Athens, where he studied for 20 years, writing on physics, biology, zoology, politics, economics, government, poetry, and music. Later, he traveled to the island of Lesvos with a student named Theophrastus to study the island’s botany and zoology. Much of his History of Animals was based on observations he made there. Aristotle taught both the future scholar Ptolemy and King Alexander the Great. In 335 BCE, he established his own school at the Lyceum in Athens. After Alexander’s death in 322 BCE, Aristotle fled the city, and died on the island of Euboea in the same year.

  Key works

  4th century BCE

  History of Animals

  On the Parts of Animals

  On the Generation of Animals

  On the Movement of Animals

  On the Progression of Animals

  See also: The microbiological environment • A system for identifying all nature’s organisms • Biological species concept • Microbiology • Animal behavior • Island biogeography

  IN CONTEXT

  KEY FIGURE

  Robert Hooke (1635–1703)

  BEFORE

  1267 English philosopher Roger Bacon discusses the use of optics for looking at “the smallest particles of dust” in his Opus Majus Volume V.

  1661 Microscopic drawings by English architect Christopher Wren impress Charles II, who commissions more drawings from Robert Hooke.

  AFTER

  1683 Dutch amateur scientist Antonie van Leeuwenhoek uses a microscope to observe bacteria and protozoa, and publishes his findings with the Royal Society of London.

  1798 Edward Jenner, an English physician and scientist, develops the world’s first vaccine—for smallpox—and publishes An Inquiry into the Causes and Effects of the Variolae Vaccinae.

  Leafing through the pages of Micrographia, a 17th-century reader would have been astonished. Here, in English scientist Robert Hooke’s seminal 1665 book, were many detailed illustrations of structures previously hidden from the human eye due to their minuscule size. Hooke’s microscope magnified things by a factor of fifty, but the accuracy of his drawings also owes much to his painstaking approach. Hooke would make numerous sketches from many different angles before combining them into a single image.

  Although it is not known for certain who developed the first microscopes, they were certainly in use by the 1660s. The early instruments were unreliable—due to the difficulty of making the lenses—and scientists had to be inventive and work around the problem. At first, Hooke had difficulty seeing his specimens clearly, so he invented an improved light source, named a “scotoscope.”

  Hooke’s book is more than just an accurate representation of what he saw through the lens; it also theorizes on what th
e images reveal about the workings of the organisms he studied. For example, when looking at a wafer-thin specimen of cork, Hooke saw a honeycomb-like pattern, the elements of which he described as “cells”—a term that is still used today.

  “… in every little particle… we now behold almost as great a variety of Creatures, as we were able before to reckon up in the whole Universe itself.”

  Robert Hooke

  Microscopic marvels

  Micrographia inspired many other scientists to investigate the microscopic world. Following notes and diagrams from Hooke’s book, Dutch scientist Antonie van Leeuwenhoek was able to construct his own microscopes. He achieved magnifications of more than 200 times actual size.

  Van Leeuwenhoek examined samples of rainwater and stagnant pondwater and marveled at the multitude of life he saw there. He identified single-celled protozoa, naming them “animalcules,” and went on to discover bacteria. He also made many observations of human and animal anatomy, including blood cells and sperm.

  While van Leeuwenhoek examined water samples, fellow Dutchman Jan Swammerdam was placing insects under his own microscope. He published records of all manner of insects depicted in the finest detail and uncovered much about their anatomy. Swammerdam’s most influential work was Life of the Ephemera (1675), which recorded in great detail the life cycle of the mayfly.