Chapter 1

Photo by Peter Batson/Image Quest Marine.

In the early 1800s, scientists hypothesized that deep ocean waters were devoid of life. Although unable to explore the deepest regions of the ocean at that time, they knew that sunlight could not penetrate depths greater than 275 m. Without sunlight there can be no photosynthesis, and without photosynthesis there can be no plants or algae to serve as food for other organisms. The cold temperatures and extreme pressures of deep ocean waters were also thought to contribute to the absence of deep-sea life. Given that ocean depths can exceed 10,000 m, it was reasonable to hypothesize that the deepest areas of the ocean could not support life.

As exploration continued throughout the nineteenth century, scientists began to collect organisms from ever greater ocean depths and their ideas about the limits of life began to change. In an 1873 expedition, scientists aboard the British research ship HMS Challenger dragged a large, open-sided, heavy box that was suspended from long ropes behind the ship across the floor of the Atlantic Ocean. This box—known as a dredge—sampled the sea floor in different parts of the ocean at depths of up to 4,572 m. The scientists were astonished to discover nearly 5,000 previously unknown species. When it became clear that life flourished at depths beyond the penetration of light, scientists were forced to reject their earlier hypothesis that no life existed in the deep ocean waters.

After discovering this rich abundance of deep-sea life, scientists were faced with the need to understand how it could exist. The lack of light suggested that deep-sea organisms were somehow sustained by energy that did not come from photosynthesis on the ocean floor. Scientists had observed that the surface waters of the ocean produced a steady descent of tiny particles that were produced by the death and decomposition of organisms living in the surface waters of the ocean. These particles are known as “marine snow.” In addition to marine snow, large organisms, such as whales, occasionally died and fell to the ocean floor. Scientists hypothesized that marine snow and the remains of large organisms such as whales provided the energy needed to sustain organisms in the depths of the ocean.

A deep-sea vent. In some regions of the ocean floor, hot water containing sulfur compounds is released from the ground. The sulfur compounds provide energy for chemosynthetic bacteria, which then serve as food for many other species that live near the vents, including these rust-colored tube worms (Tevnia jerichonana) that have been stained orange by iron compounds emitted from the vents.

Photo by Peter Batson/Image Quest Marine.

In the 1970s, scientists were finally able to send submersibles—small manned submarines—to take a first-hand look at the deepest ocean areas. Their discoveries were shocking. They not only confirmed that much of the ocean floor supported living organisms, but that areas near openings in the floor of the ocean, which later came to be known as hydrothermal vents, contained a great diversity of deep-sea species. Hydrothermal vents release plumes of hot water with high concentrations of sulfur compounds and other mineral nutrients. A tremendous diversity of species surrounded these hydrothermal vents, including tubeworms, clams, crabs, and fish. Indeed, the total amount of life at these depths rivaled that seen in some of the most diverse places on Earth. It became clear that the amount of energy contained in the descending organic matter—the marine snow—was not sufficient to support such a diverse and abundant set of life forms. That earlier hypothesis now had to be rejected.

How could so much life exist at the bottom of the ocean? That this life existed near the hydrothermal vents suggested that the vents were somehow responsible. Scientists had known for a long time that some species of bacteria could obtain their energy from chemicals rather than from the Sun. The bacteria use the energy in chemical bonds, combined with carbon dioxide (CO₂), to produce organic compounds—a process known as chemosynthesis—similar to the way that plants and algae use the energy of the Sun and CO₂ to produce organic compounds through photosynthesis. Based on this knowledge, scientists hypothesized that the hot vents, which release water with dissolved hydrogen sulfide gas and other chemicals, provided a source of energy for bacteria and that these bacteria could be consumed by the other organisms living around the vents. After several years of investigations, scientists found that the immediate area around the hot vents contained a group of organisms known as tubeworms, which can grow to more than 2 m long. These animals have no digestive system, but possess specialized organs that house vast numbers of chemosynthetic bacteria that live in a symbiotic relationship with the tubeworms. The tubeworms capture the sulfide gases and CO₂ from the surrounding water and pass these compounds to the bacteria, which then use the sulfide gases and CO₂ to produce organic compounds. Some of these organic compounds are passed to the tubeworms, which use them as food. These bacteria also represent a food source for many of the other animals that live near the vents. In turn, these bacteria-consuming animals can be consumed by larger animals, such as fish.

The story of the deep-sea vents reveals how scientists work: They make observations, devise hypotheses, test the hypotheses to confirm or reject them, and, if a hypothesis is rejected, devise a new hypothesis. As you will see throughout this chapter and subsequent chapters, science is an ongoing process that often leads to fascinating discoveries about how nature works.

The story of deep-sea vents offers an excellent introduction to the science of ecology. Ecology is the scientific study of the abundance and distribution of organisms in relation to other organisms and environmental conditions. The word ecology is taken from the Greek oikos, meaning “house,” and thus refers to our immediate surroundings, or environment.

Although Charles Darwin never used the word ecology in his writings, he appreciated the importance of beneficial and harmful interactions among species. In his book, On the Origin of Species, published in 1859, Darwin compared the large number of interactions among species in nature to the large number of interactions among consumers and businesses in human economic systems. He described species interactions as “the economy of nature.”

In 1870, the German zoologist Ernst Haeckel gave the word a broader meaning:

By ecology, we mean the body of knowledge concerning the economy of nature—the investigation of the total relations of the animal both to its organic and to its inorganic environment; including above all, its friendly and inimical relation with those animals and plants with which it comes directly or indirectly into contact—in a word, ecology is the study of all the complex interrelationships referred to by Darwin as the conditions of the struggle for existence.

The word ecology came into general use in the late 1800s. Since that time, the science of ecology has grown and diversified. Professional ecologists now number in the tens of thousands and have produced an immense body of knowledge about the world around us. Ecology is an active, modern science that continues to yield fascinating new insights about the environment and our impact on it. As we saw in the chapter opening story about life in the ocean depths, science is an ongoing process through which our understanding of nature constantly changes. Scientific investigation uses a variety of tools to understand how nature works. This understanding is never complete or absolute, but changes constantly as scientists make new discoveries. At the same time, rapid growth of the human population and the increasing sophistication of technological advances have caused major changes in our environment, frequently with dramatic consequences. With the knowledge that ecologists provide through their study of the natural world, we are in a better position to develop effective policies to manage environmental concerns related to land use, water, natural catastrophes, and public health.

This chapter will start you on the road to thinking like an ecologist. Throughout this book, we will consider the full range of ecological systems—biological entities that have both their own internal processes and yet interact with their external surroundings. Ecological systems exist at many different levels, ranging from an individual organism to the entire globe. Despite tremendous variations in size, all ecological systems obey the same principles with regard to their physical and chemical attributes and the regulation of their structure and function.

We begin this journey by examining the many different levels of organization for ecological systems, the physical and biological principles that govern ecological systems, and the different roles species play in ecological systems. Once we understand these basics of ecological systems, we will consider the many approaches to studying ecology and then consider the importance of understanding ecology when faced with the wide variety of ways that humans affect ecological systems.