Invasive Species
Botkin, D. B., & Keller, E. A. (2014). Environmental science: Earth as a living planet (9th ed.). Hoboken, NJ: John Wiley & Sons, Inc.
Chapter 9: Biological Diversity and Biological Invasions
9.1 What Is Biological Diversity?
Biological diversity refers to the variety of life-forms, commonly expressed as the number of species or the number of genetic types in an area. (We remind you of the definitions of population and species in Chapter 5: A population is a group of individuals of the same species living in the same area or interbreeding and sharing genet- ic information. A species is all individuals that are capable of interbreeding. A species is made up of populations.)
Conservation of biological diversity gets lots of attention these days. One day we hear about polar bears on the news, the next day something about wolves or salmon, or elephants, or whales. What should we do to protect these species that mean so much to people? What do we need to do about biological diversity in general—all the life-forms, whether people enjoy them or not? And is this a scientific issue or not? Is it even partially scientific?
That’s what this chapter is about. It introduces the sci- entific concepts concerning biological diversity, explains the aspects of biological diversity that have a scientific base, distinguishes the scientific aspects from the nonsci- entific ones, and thereby provides a basis for you to evalu- ate the biodiversity issues you read about.
Why Do People Value Biodiversity?
Before we discuss the scientific basis of biodiversity and the role of science in its conservation, we should consider why people value it. There are nine primary reasons: utili- tarian; public-service; ecological; moral; theological; aes- thetic; recreational; spiritual; and creative.2
Utilitarian means that a species or group of species provides a product that is of direct value to people. Public- service means that nature and its diversity provide some service, such as taking up carbon dioxide or pollinating flowers, that is essential or valuable to human life and would be expensive or impossible to do ourselves. Eco- logical refers to the fact that species have roles in their ecosystems and that some of these are necessary for the persistence of their ecosystems, perhaps even for the per- sistence of all life. Scientific research tells us which species have such ecosystem roles. The moral reason for valuing biodiversity is the belief that species have a right to exist, independent of their value to people. The theological rea- son refers to the fact that some religions value nature and its diversity, and a person who subscribes to that religion supports this belief.
The last four reasons for valuing nature and its diver- sity—aesthetic, recreational, spiritual, and creative—have to do with the intangible (nonmaterial) ways that nature and its diversity benefit people (see Figure 9.4). These four are often lumped together, but we separate them here. Aes- thetic refers to the beauty of nature, including the variety of life. Recreational is self-explanatory—people enjoy get- ting out into nature, not just because it is beautiful to look at but because it provides us with healthful activities that we enjoy. Spiritual describes the way contact with nature and its diversity often moves people, an uplifting often perceived as a religious experience. Creative refers to the fact that artists, writers, and musicians find stimulation for their creativity in nature and its diversity
Science helps us determine what are utilitarian, pub- lic-service, and ecosystem functions of biological diversity, and scientific research can lead to new utilitarian benefits from biological diversity. For example, medical research led to the discovery and development of paclitaxel (trade name Taxol), a chemical found in the Pacific yew (Taxus brevifolia) and now used widely in chemotherapy treat- ment of lung, ovarian, breast, and head and neck cancers. (Ironically, this discovery initially led to the harvest of this endangered tree species, creating an environmental con- troversy until the compound could be made artificially.)
The rise of the scientific and industrial age brought a great change in the way that people valued nature. Long ago, for example, when travel through mountains was ar- duous, people struggling to cross them were probably not particularly interested in the scenic vistas. But around the time of the Romantic poets, travel through the Alps be- came easier, and suddenly poets began to appreciate the “terrible joy” of mountain scenery. Thus scientific knowl- edge indirectly influences the 3 nonmaterial ways that people value biological diversity.
9.2 Biological Diversity Basics
Biological diversity involves the following concepts:
• Genetic diversity: the total number of genetic char- acteristics of a specific species, subspecies, or group of species. In terms of genetic engineering and our new understanding of DNA, this could mean the total base- pair sequences in DNA; the total number of genes, ac- tive or not; or the total number of active genes.
• Habitat diversity: the different kinds of habitats in a given unit area.
• Species diversity, which in turn has three qualities:
species richness—the total number of species; species evenness—the relative abundance of species; and species dominance—the most abundant species.
To understand the differences between species rich- ness, species evenness, and species dominance, imagine two ecological communities, each with 10 species and 100 individuals, as illustrated in Figure 9.5. In the first com- munity (Figure 9.5a), 82 individuals belong to a single spe- cies, and the remaining nine species are represented by two individuals each. In the second community (Figure 9.5b), all the species are equally abundant; each therefore has 10 individuals. Which community is more diverse?
At first, one might think that the two communi- ties have the same species diversity because they have the same number of species. However, if you walked through both communities, the second would appear more di- verse. In the first community, most of the time you would see individuals only of the dominant species (elephants in Figure 9.5a); you probably wouldn’t see many of the other species at all. The first community would appear to have rela- tively little diversity until it was subjected to careful study, whereas in the second community even a casual visitor would see many of the species in a short time. You can test the prob- ability of encountering a new species in either community by laying a ruler down in any direction on Figures 9.5a and 9.5b and counting the number of species that it touches.
As this example suggests, merely counting the num- ber of species is not enough to describe biological diver- sity. Species diversity has to do with the relative chance of seeing species as much as it has to do with the actual number present. Ecologists refer to the total number of species in an area as species richness, the relative abun- dance of species as species evenness, and the most abun- dant species as dominant.
The Number of Species on Earth
Many species have come and gone on Earth. But how many exist today? Some 1.5 million species have been named, but available estimates suggest there may be al- most 3 million (Table 9.1), and some biologists believe the number will turn out to be much, much larger. No one knows the exact number because new species are dis- covered all the time, especially in little-explored areas such as tropical savannas and rain forests.4
For example, in the spring of 2008, an expedition sponsored by Conservation International and led by sci- entists from Brazilian universities discovered 14 new spe- cies in or near Serra Geraldo Tocantins Ecological Station, a 716,000-hectare (1.77-million-acre) protected area in the Cerrado, a remote tropical savanna region of Brazil, said to be one of the world’s most biodiverse areas. They found eight new fish, three new reptiles, one new amphib- ian, one new mammal, and one new bird.
In Laos, a new bird, the barefaced bulbul, was discov- ered in 2009, and five new mammals have been discovered since 1992: (1) the spindle-horned oryx (which is not only a new species but also represents a previously unknown genus); (2) the small black muntjak; (3) the giant munt- jak (the muntjak, also known as “barking deer,” is a small deer; the giant muntjak is so called because it has large antlers); (4) the striped hare (whose nearest relative lives in Sumatra); and (5) a new species of civet cat. That such a small country with a long history of human occupancy would have so many mammal species previously unknown to science—and some of these were not all that small— suggests how little we still know about the total biological diversity on Earth. But as scientists we must act from what we know, so in this book we will focus on the 1.5 million species identified and named so far (see Table 9.1).
Discovery of new species continues. In 2012, a previ- ously unknown-to-science species of monkey, Cercopi- thecus lomamiensis, was discovered in the Democratic Republic of Congo (see Figure 9.6).5
All living organisms are classified into groups called taxa, usually on the basis of their evolutionary relation- ships or similarity of characteristics. (Carl Linnaeus, a Swedish physician and biologist, who lived from 1707 to 1778, was the originator of the classification system and played a crucial role in working all this out. He explained this system in his book Systema Naturae.)
The hierarchy of these groups (from largest and most inclusive to smallest and least inclusive) begins with a do- main or kingdom. In the recent past, scientists classified life into five kingdoms: animals, plants, fungi, protists, and bacteria. Recent evidence from the fossil record and studies in molecular biology suggest that it may be more appropriate to describe life as existing in three major do- mains, one called Eukaryota or Eukarya, which includes animals, plants, fungi, and protists (mostly single-celled organisms); Bacteria; and Archaea.4 As you learned in Chapter 6, Eukarya cells include a nucleus and other small, organized features called organelles; Bacteria and Archaea do not. (Archaea used to be classified among Bac- teria, but they have substantial molecular differences that suggest ancient divergence in heritage—see Chapter 7, Figure 7.4.)
The plant kingdom is made up of divisions, whereas the animal kingdom is made up of phyla (singular: phy- lum). A phylum or division is, in turn, made up of classes, which are made up of orders, which are made up of fami- lies, which are made up of genera (singular: genus), which are made up of species.
Some argue that the most important thing about bio- logical diversity is the total number of species and that the primary goal of biological conservation should be to main- tain that number at its current known maximum. An in- teresting and important point to take away from Table 9.1 is that most of the species on Earth are insects (somewhere between 668,000 and more than 1 million) and plants (somewhere between 480,000 and 530,000), and also that there are many species of fungi (about 100,000) and pro- tists (about 80,000 to almost 200,000). In contrast, our own kind, the kind of animals most celebrated on televi- sion and in movies—mammals—number a meager 4,000 to 5,000, about the same as reptiles. When it comes to numbers of species on Earth, our kind doesn’t seem to matter much—we amount to about half a percent of all animals. If the total number in a species were the only gauge of a species’ importance, we wouldn’t matter.
9.3 Biological Evolution
The first big question about biological diversity is: How did it all come about? Before modern science, the diver- sity of life and the adaptations of living things to their environment seemed too amazing to have come about by chance. The great Roman philosopher and writer Cicero put it succinctly: “Who cannot wonder at this harmony of things, at this symphony of nature which seems to will the well-being of the world?” He concluded that “everything in the world is marvelously ordered by divine providence and wisdom for the safety and protection of us all.”6 The only possible explanation seemed to be that this diversity was created by God (or gods).
With the rise of modern science, however, other ex- planations became possible. In the 19th century, Charles Darwin found an explanation that became known as biological evolution. Biological evolution refers to the change in inherited characteristics of a population from generation to generation. It can result in new spe- cies—populations that can no longer reproduce with members of the original species but can (and at least oc- casionally do) reproduce with each other. Along with self- reproduction, biological evolution is one of the features that distinguish life from everything else in the universe. (The others are carbon-based, organic-compound-based, self-replicating systems.)
The word evolution in the term biological evolution has a special meaning. Outside biology, evolution is used broadly to mean the history and development of some- thing. For example, book reviewers talk about the evo- lution of a novel’s plot, meaning how the story unfolds. Geologists talk about the evolution of Earth, which sim- ply means Earth’s history and the geologic changes that have occurred over that history. Within biology, however, the term biological evolution means a one-way process: Once a species is extinct, it is gone forever. You can run a machine, such as a mechanical grandfather clock, forward and backward, but when a new species evolves, it cannot evolve backward into its parents.
Our understanding of evolution today owes a lot to the modern science of molecular biology and the practice of genetic engineering, which are creating a revolution in how we think about and deal with species. At present, sci- entists have at hand the complete DNA code for a num- ber of species, and the list is growing rapidly. In 2012 the complete DNA genome had been determined for more than 20 species, including disease organisms: bacterium Haemophilus influenzae; the malaria parasite; its carrier the malaria mosquito; a fungi, baker’s yeast (Saccharomyces cerevisiae); plants: thale cress (Arabidopsis thaliana); wild mustard (Arabidopsis thalian); rice (Oryza sativa); animals: the fruit fly (Drosophila); a nematode C. elegans (a very small worm that lives in water); the puffer fish (Takifugu rubripes); a tree, the black poplar (Populus trichocarpa), and ourselves—humans.7,8 Scientists focused on these species either because they are of great interest to us or because they are relatively easy to study, having either few base pairs (the nematode worm) or having already well- known genetic characteristics (the fruit fly).
According to the theory of biological evolution, new species arise as a result of competition for resources and the differences among individuals in their adaptations to environmental conditions. Since the environment continually changes, which individuals are best adapted changes over time and space. As Darwin wrote, “Can it be doubted, from the struggle each individual has to ob- tain subsistence, that any minute variation in structure, habits, or instincts, adapting that individual better to the new [environmental] conditions, would tell upon its vigor and health? In the struggle it would have a better chance of surviving; and those of its offspring that inherited the variation, be it ever so slight, would also have a better chance.”
Sounds plausible, but how does this evolution occur? Through four processes: mutation, natural selection, mi- gration, and genetic drift.
The Four Key Processes of Biological Evolution
Mutation
Mutations are changes in genes. Contained in the chromosomes within cells, each gene carries a single piece of inherited information from one generation to the next, producing a genotype, the genetic makeup that is charac- teristic of an individual or a group.
Genes are made up of a complex chemical compound called deoxyribonucleic acid (DNA). DNA in turn is made up of chemical building blocks that form a code, a kind of alphabet of information. The DNA alphabet consists of four letters that stand for specific nitrogen-containing compounds, called bases, which are combined in pairs: (A) adenine, (C) cytosine, (G) guanine, and (T) thymine. Each gene has a set of the four base pairs, and how these letters are combined in long strands determines the genetic “mes- sage” interpreted by a cell to produce specific compounds
The number of base pairs that make up a strand of DNA varies. To make matters more complex, some base pairs found in DNA seem to be nonfunctional—they are not active and do not determine any chemicals produced by the cell. Some of these have been discovered to have other functions, such as affecting the activity of others, turning those other genes on or off. And creatures such as ourselves have genes that limit the number of times a cell can divide, and thus determine the individual’s maximum longevity.
When a cell divides, the DNA is reproduced and each new cell gets a copy. But sometimes an error in reproduc- tion changes the DNA and thereby changes the inherited characteristics. Such errors can arise from various causes. Sometimes an external agent comes in contact with DNA and alters it. Radiation, such as X rays and gamma rays, can break the DNA apart or change its chemical structure. Certain chemicals also can change DNA. So can viruses. When DNA changes in any of these ways, it is said to have undergone mutation.
In some cases, a cell or an offspring with a mutation cannot survive (Figure 9.7a and b). In other cases, the mu- tation simply adds variability to the inherited characteris- tics (Figure 9.7c). But in still other cases, individuals with mutations are so different from their parents that they cannot reproduce with normal offspring of their species, so a new species has been created.
Natural Selection
When there is variation within a species, some individuals may be better suited to the environment than others (see A Closer Look 9.1). (Change is not always for the better. Mutation can result in a new species whether or not that species is better adapted than its parent species to the environment.) Organisms whose biological charac- teristics make them better able to survive and reproduce in their environment leave more offspring than others. Their descendants form a larger proportion of the next generation and are more “fit” for the environment. This process of increasing the proportion of offspring is called natural selection. Which inherited characteristics lead to more offspring depends on the specific characteristics of an environment, and as the environment changes over time, the characteristics’ “fit” will also change. In sum- mary, natural selection involves four primary factors:
• Inheritance of traits from one generation to the next and some variation in these traits—that is, genetic variability.
• Environmental variability.
• Differential reproduction (differences in numbers of offspring per individual), which varies with the environment.
• Influence of the environment on survival and reproduction.
Natural selection is illustrated in A Closer Look 9.1, which describes how the mosquitoes that carry malaria develop a resistance to DDT and how the microorgan- ism that causes malaria develops a resistance to quinine, a treatment for the disease.
As explained before, when natural selection takes place over a long time, a number of characteristics can change. The accumulation of these changes may become so great that the present generation can no longer repro- duce with individuals that have the original DNA struc- ture, resulting in a new species.
Migration and Geographic Isolation
Sometimes two populations of the same species be- come geographically isolated from each other for a long time. During that time, the two populations may change so much that they can no longer reproduce together even when they are brought back into contact. In this case, two new species have evolved from the original species. This can happen even if the genetic changes are not more fit but simply different enough to prevent reproduction. Migration has been an important evolutionary process over geologic time (a period long enough for geologic changes to take place).
Darwin’s visit to the Galápagos Islands gave him his most powerful insight into biological evolution.9 He found many species of finches that were related to a single species found elsewhere. On the Galápagos, each species was adapted to a different niche.9,10 Darwin suggested that finches isolated from other species on the continents even- tually separated into a number of groups, each adapted to a more specialized role. The process is called adaptive radiation. This evolution continues today, as illustrated by a recently discovered new species of finch on the Galá- pagos Islands (Figure 9.9).
More recently and more accessible to most visitors, we can find adapative radiation on the Hawaiian Islands, where a finchlike ancestor evolved into several species, including fruit and seed eaters, insect eaters, and nec- tar eaters, each with a beak adapted for its specific food (Figure 9.10).10,11
Ironically, the loss of geographic isolation can also lead to a new species. This can happen when one population of a species migrates into a habitat already occupied by another population of that species, thereby changing gene frequency in that habitat. Such a change can result, for example, from the migration of seeds of flowering plants blown by wind or carried in the fur of mammals. If the seed lands in a new habitat, the environment may be dif- ferent enough to favor genotypes that are not as favored by natural selection in the parents’ habitat. Natural selection, in combination with geographic isolation and subsequent migration, can thus lead to new dominant genotypes and eventually to new species.
Genetic Drift
Genetic drift refers to changes in the frequency of a gene in a population due not to mutation, selection, or migration, but simply to chance. One way this happens is through the founder effect. The founder effect occurs when a small number of individuals are isolated from a larger population; they may have much less genetic variation than the original species (and usually do), and the characteristics that the isolated population has will be affected by chance. In the founder effect and genetic drift, individuals may not be better adapted to the environment—in fact, they may be more poorly adapted or neutrally adapted. Genetic drift can occur in any small population and may present conservation problems when it is by chance isolated from the main population.
For example, bighorn sheep live in the mountains of the southwestern deserts of the United States and Mexico. In the summer, these sheep feed high up in the mountains, where it is cooler, wetter, and greener. Before high-density European settlement of the region, the sheep could move freely and sometimes migrated from one mountain to another by descending into the valleys and crossing them in the winter. In this way, large numbers of sheep interbred. With the development of cattle ranches and other human activities, many populations of bighorn sheep could no longer migrate among the mountains by crossing the valleys. These sheep became isolated in very small groups—commonly, a dozen or so—and chance may play a large role in what inherited characteristics remain in the population.
This happened to a population of bighorn sheep on Tiburón Island in Mexico, which was reduced to 20 animals in 1975 but increased greatly to 650 by 1999. Because of the large recovery, this population has been used to repopulate other bighorn sheep habitats in northern Mexico. But a study of the DNA shows that the genetic variability is much less than in other populations in Arizona. Scientists who studied this population suggest that individuals from other isolated bighorn sheep populations should be added to any new transplants to help restore some of the greater genetic variation of the past.12
Biological Evolution as a Strange Kind of Game
Biological evolution is so different from other processes that it is worthwhile to spend some extra time exploring the topic. There are no simple rules that species must fol- low to win or even just to stay in the game of life. Some- times when we try to manage species, we assume that evolution will follow simple rules. But species play tricks on us; they adapt or fail to adapt over time in ways that we did not anticipate. Such unexpected outcomes result from our failure to fully understand how species have evolved in relation to their ecological situations. Nevertheless, we continue to hope and plan as if life and its environment will follow simple rules. This is true even for the most recent work in genetic engineering.
Complexity is a feature of evolution. Species have evolved many intricate and amazing adaptations that have allowed them to persist. It is essential to realize that these adaptations have evolved not in isolation but in the context of relationships to other organisms and to the en- vironment. The environment sets up a situation within which evolution, by natural selection, takes place. The great ecologist G.E. Hutchinson referred to this interac- tion in the title of one of his books, The Ecological Theater and the Evolutionary Play. Here, the ecological situation— the condition of the environment and other species—is the theater and the scenery within which natural selection occurs, and natural selection results in a story of evolu- tion played out in that theater—over the history of life on Earth.13 These features of evolution are another reason that life and ecosystems are not simple, linear, steady-state systems (see Chapter 4).
In summary, the theory of biological evolution tells us the following about biodiversity:
• Since species have evolved and do evolve, and since some species are also always becoming extinct, biologi- cal diversity is always changing, and which species are present in any one location can change over time.
• Adaptation has no rigid rules; species adapt in response to environmental conditions, and complexity is a part of nature. We cannot expect threats to one species to necessarily be threats to another.
• Species and populations do become geographically iso- lated from time to time, and undergo the founder effect and genetic drift.
• Species are always evolving and adapting to environ- mental change. One way they get into trouble—become endangered—is when they do not evolve fast enough to keep up with the environment.
Now that the manipulation of DNA is becoming routine, some scientists are attempting to bring back once extinct species. This includes some attempts to develop something like dinosaurs by starting with the DNA of chickens, very much as was imagined in the movie Jurassic Park.14 But this process will be a very dif- ferent one from biological evolution; it is better called reverse engineering. Biological evolution takes place without any conscious or purposeful intention on the part of the species or its members. It’s just a conse- quence of the fundamental characteristics of life and its environment. Attempts to reconstruct a dinosaur are conscious and purposeful actions. This distinction be- tween biological evolution and consciousness of reverse engineering is useful as a way to help understand both.
9.4 Competition and Ecological Niches
Why there are so many species on Earth has become a key question since the rise of modern ecological and evolutionary sciences. In the next sections we discuss the answers. They partly have to do with how species interact. Speaking most generally, they interact in three ways: competition, in which the outcome is negative for both; symbiosis, in which the interaction benefits both participants; and predation–parasitism, in which the out- come benefits one and is detrimental to the other.
The Competitive Exclusion Principle
The competitive exclusion principle supports those who argue that there should be only a few species. It states that two species with exactly the same requirements cannot coexist in exactly the same habitat. Garrett Hardin expressed the idea most succinctly: “Complete competitors cannot coexist.”15
This principle is illustrated by the introduction of the American gray squirrel into Great Britain. It was intro- duced intentionally because some people thought it was attractive and would be a pleasant addition to the land- scape. About a dozen attempts were made, the first perhaps as early as 1830 (Figure 9.11). By the 1920s, the American gray squirrel was well established in Great Britain, and in the 1940s and 1950s its numbers expanded greatly. It com- petes with the native red squirrel and is winning—there are now about 2.5 million gray squirrels in Great Britain, and only 140,000 red squirrels, most them in Scotland, where the gray squirrel is less abundant.16 The two species have almost exactly the same habitat requirements.
One reason for the shift in the balance of these species may be that in the winter the main source of food for red squirrels is hazelnuts, while gray squirrels prefer acorns. Thus, red squirrels have a competitive advantage in areas with hazelnuts, and gray squirrels have the advantage in oak forests. When gray squirrels were introduced, oaks were the dominant mature trees in Great Britain; about 40% of the trees planted were oaks. But that is not the case today. This difference in food preference may allow the coexistence of the two, or perhaps not.
The competitive exclusion principle suggests that there should be very few species. We know from our dis- cussions of ecosystems (Chapter 6) that food webs have at least four levels—producers, herbivores, carnivores, and decomposers. Suppose we allowed for several more levels of carnivores, so that the average food web had six levels. Since there are about 20 major kinds of ecosystems, one would guess that the total number of winners on Earth would be only 6 3 20, or 120 species.
Being a little more realistic, we could take into account adaptations to major differences in climate and other environmental aspects within kinds of ecosystems. Perhaps we could specify 100 environmental categories: cold and dry; cold and wet; warm and dry; warm and wet; and so forth. Even so, we would expect that within each environmental category, competitive exclusion would result in the survival of only a few species. Allowing six species per major environmental category would result in only 600 species.
That just isn’t the case. How did so many different species survive, and how do so many coexist? Part of the answer lies in the different ways in which organisms in- teract, and part of the answer lies with the idea of the ecological niche.
Niches: How Species Coexist
The ecological niche concept explains how so many spe- cies can coexist, and this concept is introduced most easily by experiments done with a small, common insect—the flour beetle (Tribolium), which, as its name suggests, lives on wheat flour. Flour beetles make good experimentalsubjects because they require only small con- tainers of wheat flour to live and are easy to grow (in fact, too easy; if you don’t store your flour at home properly, you will find these little beetles happily eating in it).
The flour beetle experiments work like this: A specified number of beetles of two species are placed in small containers of flour—each con- tainer with the same number of beetles of each species. The containers are then maintained at various temperature and moisture levels— some are cool and wet, others warm and dry. Periodically, the beetles in each container are counted. This is very easy. The experimenter just puts the flour through a sieve that lets the flour through but not the beetles. Then the experimenter counts the number of beetles of
each species and puts the beetles back in their container to eat, grow, and reproduce for another interval. Eventually, one species always wins—some of its individuals continue to live in the container while the other species goes extinct. So far, it would seem that there should be only one species of Tri- bolium. But which species survives depends on temperature and moisture. One species does better when it is cold and wet, the other when it is warm and dry (Figure 9.12).
Curiously, when conditions are in between, some- times one species wins and sometimes the other, seeming- ly randomly; but invariably one persists while the second becomes extinct. So the competitive exclusion principle holds for these beetles. Both species can survive in a com- plex environment—one that has cold and wet habitats as well as warm and dry habitats. In no location, however, do the species coexist.
The little beetles provide us with the key to the co- existence of many species. Species that require the same resources can coexist by using those resources under differ- ent environmental conditions. So it is habitat complexity that allows complete competitors—and not-so-complete competitors—to coexist because they avoid competing with each other.17
The flour beetles are said to have the same ecologically functional niche, which means they have the same profession—eating flour. But they have different habitats. Where a species lives is its habitat, but what it does for a living (its profession) is its ecological niche. Suppose you have a neighbor who drives a school bus. Where your neighbor lives and works—your town—is his habitat. What your neighbor does—drive a bus—is his niche. Similarly, if someone says, “Here comes a wolf,” you think not only of a creature that inhabits the northern forests (its habitat) but also of a predator that feeds on large mammals (its niche).
Understanding the niche of a species is useful in as- sessing the impact of land development or changes in land use. Will the change remove an essential requirement for some species’ niche? A new highway that makes car travel easier might eliminate your neighbor’s bus route (an es- sential part of his habitat) and thereby eliminate his pro- fession (or niche). Other things could also eliminate this niche. Suppose a new school were built and all the chil- dren could now walk to school. A school bus driver would not be needed; this niche would no longer exist in your town. In the same way, cutting a forest may drive away prey and eliminate the wolf’s niche.
Measuring Niches
An ecological niche is often described and measured as the set of all environmental conditions under which a species can persist and carry out its life functions. It is illustrated by the distribution of two species of flatworm that live on the bottom of freshwater streams. A study of two species of these small worms in Great Britain found that some streams contained one species, some the other, and still others both.18
The stream waters are cold at their source in the mountains and become progressively warmer as they flow downstream. Each species of flatworm occurs within a specific range of water temperatures. In streams where species A occurs alone, it is found from 6° to 17°C (42.8°– 62.6°F) (Figure 9.13a). Where species B occurs alone, I is found from 6° to 23°C (42.8°–73.4°F) (Figure 9.13b). When they occur in the same stream, their temperature ranges are much narrower. Species A lives in the upstream sections, where the temperature ranges from 6° to 14°C (42.8°–57.2°F), and species B lives in the warmer downstream areas, where temperatures range from 14° to 23°C (57.2°–73.4°F) (Figure 9.13c).
The temperature range in which species A occurs when it has no competition from B is called its fundamen- tal temperature niche. The set of conditions under which it persists in the presence of B is called its realized tempera- ture niche. The flatworms show that species divide up their habitat so that they use resources from different parts of it. Of course, temperature is only one aspect of the environ- ment. Flatworms also have requirements relating to the acidity of the water and other factors. We could create graphs for each of these factors, showing the range within which A and B occurred. The collection of all those graphs would constitute the complete Hutchinsonian description of the niche of a species.
A Practical Implication
From the discussion of the competitive exclusion principle and the ecological niche, we learn something important about the conservation of species: If we want to conserve a species in its native habitat, we must make sure that all the requirements of its niche are present. Conser- vation of endangered species is more than a matter of put- ting many individuals of that species into an area. All the life requirements for that species must also be present— we have to conserve not only a population but also its habitat and its niche.
9.5 Symbiosis
Our discussion up to this point might leave the impres- sion that species interact mainly through competition— by interfering with one another. But symbiosis is also important. This term is derived from a Greek word mean- ing “living together.” In ecology, symbiosis describes a relationship between two organisms that is beneficial to both and enhances each organism’s chances of persisting. Each partner in symbiosis is called a symbiont.
Symbiosis is widespread and common; most animals and plants have symbiotic relationships with other species. We, too, have symbionts—microbiologists tell us that about 10% of our body weight is actually the weight of symbiotic microorganisms that live in our intestines. They help our digestion, and we provide a habitat that supplies all their needs; both we and they benefit. We become aware of this intestinal community when it changes—for example, when we take antibiotics that kill some of these organisms, changing the balance of that community, or when we travel to a foreign country and ingest new strains of bacteria. Then we suffer a well-known traveler’s malady, gastrointestinal upset.
Another important kind of symbiotic interaction occurs between certain mammals and bacteria. A reindeer on the northern tundra may appear to be alone but carries with it many companions. Like domestic cattle, the reindeer is a ruminant, with a four-chambered stomach (Figure 9.14) teeming with microbes (a billion per cubic centimeter). In this partially closed environment, the respiration of microorganisms uses up the oxygen ingested by the reindeer while eating. Other microorganisms digest cellulose, take nitrogen from the air in the stomach, and make proteins. The bacterial species that digest the parts of the vegetation that the reindeer cannot digest itself (in particular, the cellulose and lignins of cell walls in woody tissue) require a peculiar envi- ronment: They can survive only in an environment without oxygen. One of the few places on Earth’s surface where such an environment exists is the inside of a ruminant’s stomach.3 The bacteria and the reindeer are symbionts, each providing what the other needs, and neither could survive without the other. They are therefore called obligate symbionts.
Crop plants illustrate another kind of symbiosis. Plants depend on animals to spread their seeds and have evolved symbiotic relationships with them. That’s why fruits are so edible; it’s a way for plants to get their seeds spread, as Henry David Thoreau discussed in his book Faith in a Seed.
A Broader View of Symbiosis
So far we have discussed symbiosis in terms of physiologi- cal relationships between organisms of different species. But symbiosis is much broader, and includes social and behavioral relationships that benefit both populations. Consider, for example, dogs and wolves. Wolves avoid human beings and have long been feared and disliked by many peoples, but dogs have done very well because of their behavioral connection with people. Being friendly, helpful, and companionable to people has made dogs very abundant. This is another kind of symbiosis.
A Practical Implication
We can see that symbiosis promotes biological diversity, and that if we want to save a species from extinction, we must save not only its habitat and niche but also its sym- bionts. This suggests another important point that will become more and more evident in later chapters: The attempt to save a single species almost invariably leads us to conserve a group of species, not just a single species or a par- ticular physical habitat.
9.6 predation and parasitism
Predation–parasitism is the third way in which species interact. In ecology, a predator–parasite relation is one that benefits one individual (the predator or parasite) and is nega- tive for the other (the prey or host). Predation is when an or- ganism (a predator) feeds on other live organisms (prey), usually of another species. Parasitism is when one organ- ism (the parasite) lives on or within another (the host) and depends on it for existence but makes no useful contribu- tion to it and may in fact harm it.
Predation can increase the diversity of prey species. Think again about the competitive exclusion principle. Suppose two species are competing in the same habi- tat and have the same requirements. One will win out. But if a predator feeds on the more abundant species, it can keep that prey species from overwhelming the other. Both might persist, whereas without the predator only one would. For example, some studies have shown that a moderately grazed pasture has more species of plants than an ungrazed one. The same seems to be true for natural grasslands and savannas. Without grazers and browsers, then, African grasslands and savannas might have fewer species of plants.
A Practical Implication
Predators and parasites influence diversity and can increase it.
9.7 how geography and geology Affect Biological Diversity
Species are not uniformly distributed over the Earth’s surface; diversity varies greatly from place to place. For instance, suppose you were to go outside and count all the species in a field or any open space near where you are reading this book (that would be a good way to begin to learn for yourself about biodiversity). The number of species you found would depend on where you are. If you live in northern Alaska, Canada, Scandinavia, or Siberia, you would probably find a significantly smaller number of species than if you live in the tropical areas of Brazil, Indo- nesia, or central Africa. Variation in diversity is partially a question of latitude—in general, greater diversity occurs at lower latitudes. Diversity also varies within local areas. If you count species in the relatively sparse environment of an abandoned city lot, for example, you will find quite a different number than if you count species in an old, long-undisturbed forest.
The species and ecosystems that occur on the land change with soil type and topography: slope, aspect (the direction the slope faces), elevation, and nearness to a drainage basin. These factors influence the number and kinds of plants, and the kinds of plants in turn influence the number and kinds of animals.
Such a change in species can be seen with changes in elevation in mountainous areas like the Grand Can- yon and the nearby San Francisco Mountains of Arizona (Figure 9.15). Although such patterns are easiest to see in vegetation, they occur for all organisms.
Some habitats harbor few species because they are stressful to life, as a comparison of vegetation in two areas of Africa illustrates. In eastern and southern Africa, well- drained, sandy soils support diverse vegetation, including many species of Acacia and Combretum trees, as well as many grasses. In contrast, woodlands on the very heavy clay soils of wet areas near rivers, such as the Sengwa River in Zimbabwe, consist almost exclusively of a single spe- cies called Mopane. Very heavy clay soils store water and prevent most oxygen from reaching roots. As a result, only tree species with very shallow roots survive.
Moderate environmental disturbance can also increase diversity. For example, fire is a common disturbance in many forests and grasslands. Occasional light fires produce a mosaic of recently burned and unburned areas. These patches favor different kinds of species and increase overall diversity. Table 9.2 shows some of the major influences on biodiversity. Of course, people also affect diversity. In general, urbanization, industrialization, and agriculture decrease diversity, reducing the number of habitats and simplifying habitats. (See, for example, the effects of agri- culture on habitats, discussed in Chapter 11.) In addition, we intentionally favor specific species and manipulate populations for our own purposes—for example, when a person plants a lawn or when a farmer plants a single crop over a large area.
Most people don’t think of cities as having any ben- eficial effects on biological diversity. Indeed, the develop- ment of cities tends to reduce biological diversity. This is partly because cities have typically been located at good sites for travel, such as along rivers or near oceans, where biological diversity is often high. However, in recent years we have begun to realize that cities can contrib- ute in important ways to the conservation of biological diversity.
Wallace’s Realms and Biotic Provinces
As we noted, biological diversity differs among continents, in terms of both total species diversity and the particular species that occur. This large-scale difference has long fascinated naturalists and travelers, many of whom have discovered strange, new (for them) animals and plants as they have traveled between continents. In 1876 the great British biologist Alfred Russel Wallace (co-discoverer of the theory of biological evolution with Charles Darwin) suggested that the world could be divided into six biogeographic regions on the basis of fundamental features of the animals found in those areas.19 He referred to these regions as realms and named them Nearctic (North America), Neotropical (Central and South America), Palaearctic (Europe, northern Asia, and northern Africa), Ethiopian (central and southern Africa), Oriental (the Indian subcontinent and Malaysia), and Australian. These have become known as Wallace’s realms (Figure 9.16). Recognition of these worldwide patterns in animal species was the first step in understanding biogeography—the geographic distribution of species.
In each major biogeographic area (Wallace’s realm), certain families of animals are dominant, and animals of these families fill the ecological niches. Animals filling a particular ecological niche in one realm are of different genetic stock from those filling the same niche in the other realms. For example, bison and pronghorn antelope are among the large mammalian herbivores in North America. Rodents such as the capybara fill the same niches in South America, and kangaroos fill them in Australia. In central and southern Africa, many species, including giraffes and antelopes, fill these niches.
This is the basic concept of Wallace’s realms, and it is still considered valid and has been extended to all life-forms,20 including plants (Figure 9.16b)21 and in- vertebrates. These realms are now referred to as biotic provinces.22 A biotic province is a region inhabited by a characteristic set of taxa (species, families, orders), bound- ed by barriers that prevent the spread of those distinctive kinds of life to other regions and the immigration of for-
eign species.9 So in a biotic province, organisms share a common genetic heritage but may live in a variety of en- vironments as long as they are genetically isolated from other regions.
Biotic provinces came about because of continen- tal drift, which is caused by plate tectonics and has pe- riodically joined and separated the continents (see the discussion in Chapter 7).23 The unification (joining) of continents enabled organisms to enter new habitats and allowed genetic mixing. Continental separation led to ge- netic isolation and the evolution of new species.
This at least partially explains why introducing spe- cies from one part of the Earth to another can cause problems. Within a realm, species are more likely to be related and to have evolved and adapted in the same place for a long time. But when people bring home a species from far away, they are likely to be introducing a species that is unrelated, or only distantly related, to native species. This new and unrelated “exotic” species has not evolved and adapted in the presence of the home species, so ecological and evolutionary adjustments are yet to take place. Sometimes an introduction brings in a superior competitor.
Biomes
A biome is a kind of ecosystem, such as a desert, a tropical rain forest, or a grassland. The same biome can occur on different continents because similar environments provide similar opportunities for life and similar constraints. As a result, similar environments lead to the evolution of organisms similar in form and func- tion (but not necessarily in genetic heritage or internal makeup) and similar ecosystems. This is known as the rule of climatic similarity. The close relationship between envi- ronment and kinds of life-forms is shown in Figure 9.17.
In sum, the difference between a biome and a biot- ic province is that a biotic province is based on who is related to whom, while a biome is based on niches and habitats. In general, species within a biotic province are more closely related to each other than to species in other provinces. In two different biotic provinces, the same eco- logical niche will be filled with species that perform a spe- cific function and may look very similar to each other but have quite different genetic ancestries. In this way, a biotic province is an evolutionary unit.
Convergent and Divergent Evolution
Plants that grow in deserts of North America and East Africa illustrate the idea of a biome (see Figure 9.18). The Joshua tree and saguaro cactus of North America and the giant Euphorbia of East and Southern Africa are tall, have succulent green stems that replace the leaves as the major sites of photosynthesis, and have spiny projections, but these plants are not closely related. The Josh- ua tree is a member of the agave family, the saguaro is a member of the cactus family, and the Euphorbia is a member of the spurge family. The ancestral differences between these look-alike plants can be found in their flowers, fruits, and seeds, which change the least over time and thus provide the best clues to the genetic history of a species. Geographically isolated for 180 mil- lion years, these plants have been subjected to similar climates, which imposed similar stresses and opened up similar ecological opportunities. On both continents, desert plants evolved to adapt to these stresses and po- tentials, and have come to look alike and prevail in like habitats. Their similar shapes result from evolution in similar desert climates, a process known as convergent evolution.
Another important process that influences life’s ge- ography is divergent evolution. In this process, a popu- lation is divided, usually by geographic barriers. Once separated into two populations, each evolves separately, but the two groups retain some characteristics in com- mon. It is now believed that the ostrich (native to Africa), the rhea (native to South America), and the emu (na- tive to Australia) have a common ancestor but evolved separately (Figure 9.19). In open savannas and grasslands, a large bird that can run quickly but feed efficiently on small seeds and insects has certain advantages over other organisms seeking the same food. Thus, these species maintained the same characteristics in widely separated areas. Both convergent and divergent evolution increase biological diversity.
People make use of convergent evolution when they move decorative and useful plants around the world. Cities that lie in similar climates in different parts of the world now share many of the same decorative plants. Bougainvillea, a spectacularly bright flowering shrub originally native to Southeast Asia, decorates cities as distant from each other as Los Angeles and the capital of Zimbabwe. In New York City and its outlying suburbs, Norway maple from Europe and the tree of heaven and gingko tree from China grow with native species such as sweet gum, sugar maple, and pin oak. People intentionally introduced the Asian and European trees.
9.8 Invasions, Invasive Species, and Island Biogeography
Ever since Darwin’s voyage on The Beagle, which took him to the Galápagos Islands, biologists have been curious about how biological diversity can develop on islands: Do any rules govern this process? How do such invasions happen? And how is biological diversity affected by the size of and distance to a new habitat? E.O. Wilson and R. MacArthur established a theory of island biogeography that sets forth major principles about biological invasion of new habitats,24 and as it turns out, the many jokes and stories about castaways on isolated islands have a basis in fact.
• Islands have fewer species than continents. The two sources of new species on an island are migration from the mainland and evolution of new species in place.
• The smaller the island, the fewer the species, as can be seen in the number of reptiles and amphibians in vari- ous West Indian islands (Figure 9.20).
• The farther the island is from a mainland (continent), the fewer the species (Figure 9.21).25
Clearly, the farther an island is from the mainland, the harder it will be for an organism to travel the distance, and the smaller the island, the less likely that it will be found by individuals of any species. In addition, the smaller the island, the fewer individuals it can support. Small islands tend to have fewer habitat types, and some habitats on a small island may be too small to support a population large enough to have a good chance of surviving for a long time. Generally, the smaller the population, the greater its risk of extinction. It might be easily extinguished by a storm, flood, or other catastrophe or disturbance, and every species is subject to the risk of extinction by preda- tion, disease (parasitism), competition, climatic change, or habitat alteration.
A final generalization about island biogeography is that over a long time, an island tends to maintain a rather constant number of species, which is the result of the rate at which species are added minus the rate at which they become extinct. These numbers follow the curves shown in Figure 9.21. For any island, the number of species of a particular life-form can be predicted from the island’s size and distance from the mainland.
The concepts of island biogeography apply not just to real islands in an ocean but also to ecological islands. An ecological island is a comparatively small habitat separated from a major habitat of the same kind. For example, a pond in the Michigan woods is an ecological island relative to the Great Lakes that border Michigan. A small stand of trees within a prairie is a forest island. A city park is also an ecological island. Is a city park large enough to support a population of a particular species? To know whether it is, we can apply the concepts of island biogeography.
Biogeography and People
Benefits of Biological Invasions
We have seen that biogeography affects biological diversity. Changes in biological diversity in turn affect people and the living resources on which we depend. These effects extend from individuals to civilizations. For example, the last ice ages had dramatic effects on plants and animals and thus on human beings. Europe and Great Britain have fewer native species of trees than other temperate regions of the world. Only 30 tree species are native to Great Britain (that is, they were present prior to human settlement), although hundreds of species grow there today.
Why are there so few native species in Europe and Great Britain? Because of the combined effects of cli- mate change and the geography of European mountain ranges. In Europe, major mountain ranges run east– west, whereas in North America and Asia the major ranges run north–south. During the past 2 million years, Earth has experienced several episodes of conti- nental glaciation, when glaciers several kilometers thick expanded from the Arctic over the landscape. At the same time, glaciers formed in the mountains and ex- panded downward. Trees in Europe, caught between the ice from the north and the ice from the mountains, had few refuges, and many species became extinct. In contrast, in North America and Asia, as the ice ad- vanced, tree seeds could spread southward, where they became established and produced new plants. Thus, the tree species “migrated” southward and survived each episode of glaciation.16
Since the rise of modern civilization, these ancient events have had many practical consequences. As we mentioned earlier, soon after Europeans discovered North America, they began to bring exotic North American species of trees and shrubs into Europe and Great Britain. These exotic imports were used to decorate gardens, homes, and parks and formed the basis of much of the commercial forestry in the region. For example, in the famous gardens of the Alhambra in Granada, Spain, Monterey cypress from North America are grown as hedges and cut in elaborate shapes. In Great Britain and Europe, Douglas fir and Monterey pine are important commercial timber trees today. These are only two examples of how knowledge of biogeography— enabling people to predict what will grow where based on climatic similarity—has been used for both aesthetic and economic benefits.
Why Invasive Species Are a Serious Problem Today
The ease and speed of long-distance travel have led to a huge rate of introductions, with invasive pests (includ- ing disease-causing microbes) arriving from all around the world both intentionally and unintentionally (Table 9.3 and Figure 9.22). Table 9.3 shows the number of plant pests intercepted by various means in 2007 by the U.S. gov- ernment. The majority of interceptions—42,003—were at airports, ten times more than maritime interceptions, which before the jet age would have accounted for most of them. Passenger ships arrive at fewer locations and far less frequently than do commercial aircraft today. Bear in mind that the 42,003 were just those intercepted—no doubt many passed undetected—and that these are only for pests of plants, not for such things as zebra mussels dumped into American waters from cargo ships. Accord- ing to the USDA, the present situation is not completely controllable.
Another major avenue of species invasions has been the international trade in exotic pets, like the Burmese python. Many of these pets are released outdoors when they get to be too big and too much trouble for their owners.
The upshot of this is that we can expect the in- vasion of species to continue in large numbers, and some will cause problems not yet known in the United States.
SUMMArY
• Biological evolution—the change in inherited characteristics of a population from generation to generation—is responsible for the development of the many species of life on Earth. Four processes that lead to evolution are mutation, natural selection, migration, and genetic drift.
• Biological diversity involves three concepts: genetic di- versity (the total number of genetic characteristics), habi- tat diversity (the diversity of habitats in a given unit area), and species diversity. Species diversity, in turn, involves three ideas: species richness (the total number of species), species evenness (the relative abundance of species), and species dominance (the most abundant species).
• About 1.5 million species have been identified and named. Insects and plants make up most of these spe- cies. With further explorations, especially in tropical areas, the number of identified species, especially of in- vertebrates and plants, will increase.
• Species engage in three basic kinds of interactions: competition, symbiosis, and predation–parasitism.
Each type of interaction affects evolution, the persis- tence of species, and the overall diversity of life. It is important to understand that organisms have evolved together, so predator, parasite, prey, competitor, and symbiont have adjusted to one another. Human inter- ventions frequently upset these adjustments.
• The competitive exclusion principle states that two spe- cies that have exactly the same requirements cannot co- exist in exactly the same habitat; one must win. The reason more species do not die out from competition is that they have developed a particular niche and thus avoid competition.
• The number of species in a given habitat is determined by many factors, including latitude, elevation, topog- raphy, severity of the environment, and diversity of the habitat. Predation and moderate disturbances, such as fire, can actually increase the diversity of species. The number of species also varies over time. Of course, peo- ple affect diversity as well.
Botkin, D. B., & Keller, E. A. (2014). Environmental science: Earth as a living planet (9th ed.). Hoboken, NJ: John Wiley & Sons, Inc.
