Biology For Dummies

Part III It’s a Small, Interconnected World

In this part . . .

Life on Earth comes in all forms, shapes, and sizes. It inhabits almost every environment that people have explored (and even some they haven’t), from the deep, dark caves of the world’s oceans to the hot springs of Yellowstone National Park. All the wonderfully diverse organisms of this planet are connected to each other in fundamental cycles of energy and matter transfer. Prepare to explore the diversity (and interconnectedness) of life on Earth in this part.

Organisms that are successful in obtaining what they need reproduce, creating offspring in their own image. As the Earth changes over time, the requirements for success change, causing shifts in the populations of organisms. In this part, we also explain the connections between living things in space and time.

Chapter 10

In This Chapter

Exploring biodiversity

Getting to know the various forms of life on Earth

As human beings, we’re deeply connected to the living world around us. Yet as we transform the world in order to meet our needs, we’re changing it in ways that make it less hospitable to other species.

This chapter is designed to help you understand why biodiversity is so important to the future of humanity on planet Earth. It’s your chance to get acquainted with the diversity of life around you and discover how biologists organize all of those diverse life-forms into a specific classification system.

Biodiversity: Recognizing How Our Differences Make Us Stronger

The diversity of living things on Earth is referred to as biodiversity. Almost everywhere biologists have looked on this planet — from the deepest, darkest caves to the lush Amazonian rain forests to the depths of the oceans — they’ve found life. In the deepest, darkest caves where no light ever enters, bacteria obtain energy from the metals in the rocks. In the Amazonian rain forest, plants grow attached to the tops of trees, collecting water and forming little ponds in the sky that become home to insects and tree frogs. In the deep oceans, blind fish and other animals live on the debris that drifts down to them like snow from the lit world far above. Each of these environments presents a unique set of resources and challenges, and life on Earth is incredibly diverse due to the ways in which organisms have responded to these challenges over time.

The following sections not only clue you in to the reasons why biodiversity is so important and how human actions are harming it but also how human actions can protect biodiversity moving forward.

Valuing biodiversity

Most people choose to live with species that are a lot like them — other people, dogs, cats, and farm animals, for example. Living things that are different, such as slugs, bugs, and bacteria, may seem annoying, gross, weird, or even scary. On the other hand, some people are fascinated by the diversity of life on Earth and make a habit of watching nature shows on television; visiting zoos, aquariums, and botanical preserves; or traveling to different places on Earth to see unique organisms in their natural habitats.

Whether or not you appreciate the diversity of life on Earth, biodiversity is important — and worth valuing — for the following reasons:

The health of natural systems depends on biodiversity. Scientists who study the interconnections between different types of living things and their environments (see Chapter 11) believe that biodiversity is important for maintaining balance in natural systems. Each type of living thing plays a role in its environment, and the loss of even one species can have widespread effects.

Many economies rely upon natural environments. A whole industry called ecotourism has grown up around tour guides leading people on trips through natural habitats and explaining the local biology along the way.

Human medicines come from other living things. For example, the anticancer drug taxol was originally obtained from the bark of the Pacific yew, and the heart medicine digitalin comes from the foxglove plant.

Biodiversity adds to the beauty of nature. Natural systems have an aesthetic value that’s pleasing to the eye and calming to the mind in today’s technologically driven world.

Surveying the threats posed by human actions

As the human population grows and uses more and more of the Earth’s resources (head to Chapter 11 for the scoop on human population growth), the populations of other species are declining as a direct result. Following are the ways in which human actions pose major threats to biodiversity:

Development is reducing the size of natural environments. People need places to live and farms to raise food. In order to meet these needs, they burn rain forests, drain wetlands, cut down forests, pave over valleys, and plow up grasslands. Whenever people convert land for their own use, they destroy the habitats of other species, causing habitat loss. Even if some natural habitat remains, those patches become small and scattered. This habitat fragmentation has the biggest impact on large animals, such as mountain gorillas and tigers, that need big habitats in which to roam.

Unnatural, human-produced wastes are polluting the air and water. Automobiles and factories burn gasoline and coal, releasing pollution into the air. Metals from mining and chemicals from factories, farms, and homes get into groundwater. After pollution enters the air and water, it travels around the globe and can hurt multiple species, including humans.

The overharvesting of species to provide food and other materials for human consumption is driving some species to near extinction. Because they can reproduce, living things such as trees and fish are considered renewable resources. However, if people harvest these resources faster than they can replace themselves, the numbers of individual trees and fish decline. If too few members of a species remain, then survival of that species becomes very unlikely. Case in point: Many important fisheries, like the Great Banks off the coast of Newfoundland, have actually crashed, which means the population of fish declined to a point that the area is no longer fishable and may never recover.

Human movements around the globe sometimes carry species into new environments. An introduced species is a foreign species that’s brought into a new environment. Introduced species that are very aggressive and take over habitats are called invasive species. Invasive species often have a large environmental impact and cause the numbers of native species (organisms belonging to a particular habitat) to decline; they can also attack crop plants and cause human diseases. One example of an invasive species is water hyacinth, a plant that was introduced into the American South during the 1884 exposition in New Orleans. Water hyacinth spread throughout the waterways of the southeastern United States where it choked rivers and lakes with huge masses of floating plants, slowing water flow, blocking the light for aquatic species, and reducing biodiversity. Maintenance crews in modern-day Florida work constantly to try and weed out water hyacinth in order to keep the state’s rivers and lakes usable for recreation and other species.

Exploring the extinction of species

The combined effects of all the various human actions in Earth’s ecosystems are reducing the planet’s biodiversity. In fact, the rate of extinctions is increasing along with the size of the human population. No one knows for certain how extensive the loss of species due to human impacts will ultimately be, but there’s no question that human practices such as hunting and farming have already caused numerous species to become extinct.

Many scientists believe Earth is experiencing its sixth mass extinction, a certain time period in geologic history that shows dramatic losses of many species. (The most famous mass extinction event is the one that occurred about 65 million years ago and included the extinction of the dinosaurs.) Scientists theorize that most of the past mass extinctions were caused by major changes in Earth’s climate and that the current extinctions (most recently including black rhinos, Zanzibar leopards, and golden toads) began as a result of human land use but may increase as a result of global warming.

The loss in biodiversity that’s currently happening on Earth could have effects beyond just the loss of individual species. Living things are connected to each other and their environment in how they obtain food and other resources necessary for survival. If one species depends on another for food, for example, then the loss of a prey species can cause a decline in the predator species.

The sections that follow introduce you to two classifications of species that biologists are keeping an eye on when it comes to questions of extinction.

Keystone species

Some species are so connected with other organisms in their environment that their extinction changes the entire composition of species in the area. Species that have such great effects on the balance of other species in their environment are called keystone species. As biodiversity decreases, keystone species may die out, causing a ripple effect that leads to the loss of many more species. If biodiversity gets too low, then the future of life itself becomes threatened.

An example of a keystone species is the purple seastar, which lives on the northwest Pacific coast of the United States. Purple seastars prey on mussels in the intertidal zone. When the seastars are present, they keep the mussel population in check, allowing a great diversity of other marine animals to live in the intertidal zone. If the seastars are removed from the intertidal zone, however, the mussels take over, and many species of marine animals disappear from the environment.

Indicator species

One way biologists can monitor the health of particular environments and the organisms that live in them is by measuring the success of indicator species, species whose presence or absence in an environment gives information about that environment.

In the Pacific Northwest region of the United States, the health of old-growth forests is measured by the success of the northern spotted owl, a creature that can only make its home and find food in mature forests that are hundreds of years old. As logging decreases the number and size of these old forests, the number of spotted owls has declined, thereby making the number of spotted owls an indicator of the health, or even the existence, of old-growth forests in the Pacific Northwest. Of course, old-growth forests aren’t just home to spotted owls — they shelter a rich diversity of living things including plants, such as sitka spruce and Western hemlock, and animals, such as elk, bald eagles, and flying squirrels. Old-growth forests also perform important environmental functions such as preventing erosion, floods, and landslides; improving water quality; and providing places for salmon to spawn. If old-growth forests become extinct in the Pacific Northwest, the effects will be far reaching and have many negative impacts on the people and other species in the area.

Protecting biodiversity

Biodiversity increases the chance that at least some living things will survive in the face of large changes in the environment, which is why protecting it is crucial. So what can people do to protect biodiversity and the health of the environment in the face of the increasing demands of the human population? No one has all the answers, but here are a few ideas worth trying:

Keep wild habitats as large as possible and connect smaller ones with wildlife corridors (stretches of land or water that wild animals travel as they migrate or search for food) so organisms that need a big habitat to thrive can move between smaller ones.

Use existing technologies and develop new ones to decrease human pollution and clean up damaged habitats. Technologies that have minimal effects on the environment are called clean or green technologies. Some businesses are trying to use these technologies in order to reduce their impact on the environment.

Strive for sustainability in human practices including manufacturing, fishing, logging, and agriculture. Something that’s sustainable meets current human needs without decreasing the ability of future generations to meet their needs.

The Great Law of the Iroquois says that “people must consider the impact of their actions not just on the current generation, but on future generations that aren’t yet born.” This law is often quoted as: “In our every deliberation, we must consider the impact of our decisions on the next seven generations.” A generation spans roughly 25 years, so if people follow the rule of the Iroquois, they must consider the effects their actions will have 175 years from now.

Regulate the transport of species around the world so that species aren’t introduced into foreign habitats. This includes being careful about the transport of not-so-obvious species. For example, ships traveling from one port to another are often asked to empty their ballast water offshore so they don’t accidentally release organisms from other waters into their destination harbors.

Meet Your Neighbors: Looking at Life on Earth

Life on Earth is incredibly diverse, beautiful, and complex. Heck, you could spend a lifetime exploring the microbial universe alone. The deeper you delve into the living world around you, the more you can appreciate the similarities between all life on Earth — and be fascinated by the differences. The following sections give you a brief introduction to the major categories of life on Earth; we encourage you to check them out for yourself.

Unsung heroes: Bacteria

Most people are familiar with disease-causing bacteria such as Streptococcus pyogenes, Mycobacterium tuberculosis, and Staphylococcus aureus. Yet the vast majority of bacteria on Earth don’t cause human diseases. Instead, they play important roles in the environment and health of living things, including humans. Photosynthetic bacteria make significant contributions to planetary food and oxygen production (see Chapter 5 for more on photosynthesis), and E. coli living in your intestines make vitamins that you need to stay healthy. So when you get down to it, plants and animals couldn’t survive on Earth without bacteria.

Generally speaking, bacteria range in size from 1 to 10 micrometers in length and are invisible to the naked eye. Along with being nucleus-free, they have a genome that’s a single circle of DNA. They reproduce asexually by a process called binary fission. Some bacteria move about by secreting a slime that glides over the cell’s surface, allowing it to slide through its environment. Others have flagella (little whiplike appendages made of protein) that they swish around to swim through their watery homes.

Bacteria have many ways of getting the energy they need for growth and various strategies for surviving in extreme environments. Their great metabolic diversity has allowed them to colonize just about every environment on Earth.

A bacteria impersonator: Archaeans

Archaeans are prokaryotes, just like bacteria. In fact, you can’t tell the difference between the two just by looking, even if you look very closely using an electron microscope, because they’re about the same size and shape, have similar cell structures, and divide by binary fission.

Until the 1970s, no one even knew that archaeans existed; up to that point, all prokaryotic cells were assumed to be bacteria. Then, in the 1970s, a scientist named Carl Woese started doing genetic comparisons between prokaryotes. Woese startled the entire scientific world when he revealed that prokaryotes actually separated into two distinct groups — bacteria and archaea — based on sequences in their genetic material.

The first archaeans were discovered in extreme environments (think salt lakes and hot springs), so they have a reputation for being extremophiles (-phile means “love,” so extremophiles means “extreme-loving”). Since their initial discovery, however, archaeans have been found everywhere scientists have looked for them. They’re happily living in the dirt outside your home right now, and they’re abundant in the ocean.

Because archaeans were discovered fairly recently, scientists are still learning about their role on planet Earth, but so far it looks like they’re as abundant and successful as bacteria.

A taste of the familiar: Eukaryotes

Unless you’re a closet biologist, you’re probably most familiar with life in eukaryotic form because you encounter it every day. As soon as you step outside, you can find a wealth of plants and animals (and maybe even a mushroom or two if you look around a little).

On the most fundamental level, all eukaryotes are quite similar. They share a common cell structure with nuclei and organelles (we cover eukaryotic cells in greater detail in Chapter 4), use many of the same metabolic strategies (explained in Chapter 5), and reproduce either asexually by mitosis or sexually by meiosis (both of which are covered in Chapter 6).

Despite these similarities, we bet you still feel that you’re pretty different from a carrot. You’re right to feel that way. The differences between you and a carrot are what separate you into two different kingdoms. In fact, enough differences exist between eukaryotes to separate them into four different kingdoms:

Animalia: Animals are organisms that begin life as a cell called a zygote that results from the fusion of a sperm and an egg and then divides to form a hollow ball of cells called a blastula. If you’re wondering when the fur, scales, and claws come into play, these familiar animal characteristics get factored in much later, at the point when animals get divided up into phyla, families, and orders (see the “Organizing life into smaller and smaller groups” section later in this chapter for more on these groupings). Although the animal kingdom contains familiar animals such as dogs, cats, lizards, birds, and fish, the defining characteristic of animals must be true for all members of the kingdom — including slugs, worms, and sea sponges.

Plantae: Plants are photosynthetic organisms that start life as embryos supported by maternal tissue. This definition of plants includes all the plants you’re familiar with: pine trees, flowering plants (including carrots), grasses, ferns, and mosses. All plants have cells with cell walls made of cellulose. They reproduce asexually by mitosis (described in Chapter 6), but they can also reproduce sexually. (Flip to Chapter 20 for more on plant structures and life cycles.)

The definition of plants, which specifies a stage where an embryo is supported by maternal tissue, excludes most of the algae, like seaweed, found on Earth. Algae and plants are so closely related that many people include algae in the plant kingdom, but many biologists draw the line at including algae in the plant kingdom.

Fungi: Fungi may look a bit like plants, but they aren’t photosynthetic. They get their nutrition by breaking down and digesting dead matter. Their cells have walls made of chitin (a strong, nitrogen-containing polysaccharide), and they don’t produce swimming cells during their life cycle. Kingdom Fungi includes mushrooms, molds that you see on your bread and cheese, and many rusts that attack plants. Yeast is also a member of kingdom Fungi even though it grows differently (most fungi grow as filaments, but yeast grows as little oval cells).

Protista: Kingdom Protista is defined as everything else that’s eukaryotic. Seriously. Biologists have studied animals, plants, and fungi for a long time and defined them as distinct groups long ago. But many, many, eukaryotes don’t fit into these three kingdoms. A whole world of microscopic protists exists in a drop of pond water. The protists are so diverse that some biologists think they should be separated into as many as 11 kingdoms of their own. But so far no one has pushed to make that happen, which is certainly good news for you because we bet you don’t want to memorize the names of 15 kingdoms of eukaryotes.

What about viruses?

Here’s a riddle for you: What has genetic material, exists by the billions, functions like a living parasite, but isn’t truly a living thing? A virus! That’s right, those nasty little bugs that cause diseases, ranging from the human immunodeficiency virus (HIV) and food poisoning to the common cold and even some forms of cancer, may be the world’s most efficient parasites. But, in the strictest sense of the word, viruses aren’t really alive because they can’t reproduce outside of a host cell.

Unlike living things, viruses aren’t made of cells. They’re just very tiny pieces of DNA or RNA covered with protein as protection. Because they’re so small — a fraction of the size of bacteria — you can’t see them, even with the aid of a light microscope.

No one really knows how viruses evolved. Some biologists think they were originally intracellular parasites that got so good at what they did — being parasitic, that is — that they were able to survive with nucleic acid alone. Others think viruses are cellular escapees, genes that ran away from home but can’t replicate until they return to a specific kind of host cell. Still others think viruses may represent an offshoot of life from its very beginning, before cells evolved. For the details on how viruses attack cells and reproduce, head to Chapter 17.

Climbing the Tree of Life: The Classification System of Living Things

Much like you’d draw a family tree to show the relationships between your parents, grandparents, and other members of your family, biologists use a phylogenetic tree (a drawing that shows the relationships between a group of organisms) to represent the relationships between living things. This “tree of life” allows them to categorize all the diverse organisms that call planet Earth home and organize them into manageable classifications.

Although you probably know how your family members are related to each other, biologists have to use clues to figure out the relationships between living things. The types of clues they use to figure out these relationships include

Physical structures: The structures that biologists use for comparison may be large, like feathers, or very small, like a cell wall (flip to Chapter 4 for more on this and other parts of a cell’s structure). For example, organisms with feathers are related to each other and are included in a group called birds.

Biologists consider reproductive structures to be especially important for determining relationships.

Chemical components: Some organisms produce unique chemicals. Bacteria, for example, are the only cells that make a hybrid sugar-protein molecule called peptidoglycan. If biologists encounter an organism that produces this molecule, they can safely group it in with other bacteria.

Genetic information: An organism’s genetic code determines its traits, so by reading the genetic code in DNA, biologists can go right to the source of differences between species. Even organisms that seem incredibly different, like you and the bacteria E. coli, have some traits in common. For example, all cells on Earth contain ribosomes for making proteins, which means biologists can read a gene that has the code for one of your ribosomal proteins and compare it to the gene that has the code for one of E. coli’s ribosomal proteins.

The more characteristics two organisms have in common with each other, the more closely related they are. Characteristics that organisms have in common are called shared characteristics.

Based on structural, cellular, biochemical, and genetic characteristics, biologists can classify life on Earth into groups that reflect the evolutionary history of the planet. That history indicates that all life on Earth began from one original universal ancestor after the Earth formed 4.5 billion years ago. All the diversity of life that exists today is related because it’s descended from that original ancestor.

The next sections break down the various classifications of living things and explain how each one gets its own unique scientific name.

Mastering the domains

You can interpret the degree of relationship between two organisms by looking at their positions on a phylogenetic tree.

Each organism or group being compared has a branch on the tree. One branch represents all the animals, and another branch represents all the plants.

The smaller the distance between two groups, the closer the relationship between them. The distance between animals and plants is a great deal less than the distance between animals and any of the bacteria, so animals are much more closely related to plants than they are to bacteria.

If two branches meet at a common point, then the groups represented by those branches evolved from a common ancestor. Groups with a common point that’s far away separated further back in time than groups with a common point that’s near.

When biologists used genetic information to compare all life on Earth, they discovered that living things fall into three main groups called domains. The three domains of life are

Bacteria: Consisting mostly of single-celled organisms, bacteria are prokaryotic, meaning they lack a nuclear membrane around their DNA (refer to Chapter 4 for the details on prokaryotic cells). Most bacteria have a cell wall made of peptidoglycan.

Archaea: These are single-celled, prokaryotic organisms. In addition to their genetic differences from bacteria, archaeans have some chemical differences, including the fact that their cell walls are never made of peptidoglycan.

Eukarya: Organisms in the Eukarya domain may be single-celled or multicellular; either way, their cells are eukaryotic, meaning they have a nuclear membrane around their DNA (see Chapter 4 for more on eukaryotic cells). This domain contains familiar organisms such as animals, plants, mushrooms, and seaweed.

Organizing life into smaller and smaller groups

Being able to categorize the three largest, and most distantly related, groups of living things on Earth into domains (as explained in the preceding section) is great, but biologists need smaller groups to work with in order to determine how similar different types of organisms are. Hence the creation of the taxonomic hierarchy, a naming system that ranks organisms by their evolutionary relationships. Within this hierarchy, living things are organized into the largest, most-inclusive group down to the smallest, least-inclusive group.

The taxonomic hierarchy is as follows, from largest to smallest. (Note that organisms are placed into each category based on similarities within that particular group of organisms. Whatever characteristics are used to define a category must be shared by all organisms placed into that category.)

Domain: Domains group organisms by fundamental characteristics such as cell structure and chemistry. For example, organisms in domain Eukarya are separated from those in the Bacteria and Archaea domains based on whether their cells have a nucleus, the types of molecules found in the cell wall and membrane, and how they go about protein synthesis.

Kingdom: Kingdoms group organisms based on developmental characteristics and nutritional strategy. For example, organisms in the animal kingdom (Animalia) are separated from those in the plant kingdom (Plantae) because of differences in the early development of these organisms and the fact that plants make their own food by photosynthesis whereas animals ingest their food. (Kingdoms are most useful in domain Eukarya because they’re not well defined for the prokaryotic domains.)

Phylum: Phyla separate organisms based on key characteristics that define the major groups within the kingdom. For example, within kingdom Plantae, flowering plants (Angiophyta) are in a different phylum than cone-bearing plants (Coniferophyta).

Class: Classes separate organisms based on key characteristics that define the major groups within the phylum. For example, within phylum Angiophyta, plants that have two seed leaves (dicots, class Magnoliopsida) are in a separate class than plants with one seed leaf (monocots, class Liliopsida).

Order: Orders separate organisms based on key characteristics that define the major groups within the class. For example, within class Magnoliopsida, nutmeg plants (Magnoliales) are put in a different order than black pepper plants (Piperales) due to differences in their flower and pollen structure.

Family: Families separate organisms based on key characteristics that define the major groups within the order. For example, within order Magnoliales, buttercups (Ranunculaceae) are in a different family than roses (Rosaceae) due to differences in their flower structure.

Genus: Genera separate organisms based on key characteristics that define the major groups within the family. For example, within family Rosaceae, roses (Rosa) are in a different genus than cherries (Prunus) thanks to differences in their flower structure.

Species: Species separate eukaryotic organisms based on whether they can successfully reproduce with each other. You can walk through a rose garden and see many different colors of China roses (Rosa chinensis) that are all considered one species because they can reproduce with each other.

Think of how biologists organize living things like how you might organize your clothing. In your first round of organizing, you might make groups of pants, shirts, socks, and shoes. From there, you might go into the shirt group and organize your shirts into smaller groups, such as short-sleeved versus long-sleeved shirts. Then perhaps you’d organize them by type of fabric, then color, and so on. At some point, you’d have very small groups with very similar articles of clothing — perhaps a group of two short-sleeved, button-down, blue shirts, for example. All of your clothing would be organized in a hierarchy, from the big category of clothing all the way down to the small category of short-sleeved, button-down, blue shirts.

People have lots of funny sayings to help them remember the taxonomic hierarchy. Our favorite, because it’s so hard to forget, is Dumb Kids Playing Chase On Freeways Get Squished. The first letter of each word in the sentence represents the first letter of a category in the taxonomic hierarchy. If you don’t like this particular saying, just search the Internet for “taxonomic hierarchy mnemonic,” and you’ll find many more.

All life on Earth is related, but relative position within the taxonomic hierarchy demonstrates the degree of that relationship. For instance, you and a carrot are both in domain Eukarya, so you definitely have some things in common, but you have more characteristics in common with organisms that are part of the animal kingdom.

Table 10-1 compares the classification, or taxonomy, of you, a dog, a carrot, and E. coli.

Of the organisms listed in Table 10-1, you have the most in common with a dog. You’re both animals possessing a central nervous chord (phylum Chordata), and you’re both mammals (class Mammalia), which means you have hair and the females of your species make milk. However, you also have many differences, including the tooth structure that separates you into the order Primates and a dog into the order Carnivora. If you compare yourself to a plant, you can see that you have certain features of cell structure that place you together in domain Eukarya, but little else in common.

Two organisms that belong to the same species are the most similar of all. For most eukaryotic organisms, members of the same species can successfully sexually reproduce together, producing live offspring that can also reproduce. Bacteria and archaea don’t reproduce sexually, so their species are defined by chemical and genetic similarities.

Playing the name game

When biologists discover a new organism, they give it a scientific name. They’ve been doing this for hundreds of years according to a system developed by Swedish naturalist Carl Linnaeus in the 1750s. Linnaeus created a classification system that included some of the categories, such as kingdom and class, that are still used today in the taxonomic hierarchy. Linnaeus also proposed the system of binomial nomenclature that modern biologists use to give every type of living thing a unique name that has two parts.

In binomial nomenclature, the first part of an organism’s name is the genus, and the second part is the species name, or specific epithet. The rules for using binomial nomenclature are as follows:

The genus is always capitalized.

The species name is never written without the genus, although the genus can be abbreviated by just the first letter.

Both the genus and species should be italicized or underlined to indicate that the name is the official scientific name.

According to these rules, humans may be correctly identified as Homo sapiens or H. sapiens.