Science-Resource-Guide-(P) Section 2.pdf

Section II Biodiversity: From Local to Global In this section, we will focus on the Earth as a living the plantation may be the same as in an equivalent planet and explore the ways that life is organized area of natural forest, but the dive...

Section II Biodiversity: From Local to Global In this section, we will focus on the Earth as a living the plantation may be the same as in an equivalent planet and explore the ways that life is organized area of natural forest, but the diversity of organisms from local to global scales. You will learn about the will be far less. The number of different species in any evolutionary processes that gave rise to the diversity given place is the most common measure of biological of species on our planet, the biological and physical diversity. Variety of species, however, is not the only factors that regulate biodiversity from genes to species, way biological diversity can be measured. Recall from and how living and nonliving components are linked Section I that biological diversity, or biodiversity, is the together into systems—both small and global. The diversity of all the genes, species, and habitats on Earth. major elements you will cover are: The great number of species on Earth is the result of Evolution and Biodiversity − Genetic variation the large amount of genetic diversity within and among Blue Ridge Academy - Maricopa, CA 6 resulting in environmentally adaptive traits species. Genetic diversity is the variety of genes, the has resulted in a diverse number and types of chemical building blocks that provide the blueprint species. for how every individual organism develops. Current Community Ecology − how populations of estimates are that humans have approximately 30,000 6 species grow, disperse, and interact with other different genes, which can combine to form a virtually populations limitless variety of individuals. No two people, except for identical twins, will have exactly the same 6 Ecosystems − the integration of living and combination of genes. There is even more genetic nonliving system components in specific diversity between individuals of different species. geographic areas Biomes − variation in global patterns of Biodiversity also includes the different ways that 6 temperature, sunlight, and rainfall are key in FIGURE 14 creating geographic regions distinguished by different dominant forms of plants and animals 6 Global Energy and Matter Cycles − global biogeochemical cycles on which all ecosystems depend BIODIVERSITY What Is Biodiversity and Why Does It Matter? A small plot of land or a tiny pond has dozens or hundreds of different kinds of plants and animals that even the untrained eye can distinguish, as well as thousands of different kinds of microscopic organisms. In contrast, a carefully tended lawn or a commercial tree plantation usually supports only a few types of grasses or trees. The total number of living things in A sample of the range of multi-cellular species diversity. 2024–2025 Science Pentathlon Resource Guide 22 groups of species are organized together on the planet. A forest community will contain different species of FIGURE 15 plants and animals than a community of organisms in a desert, a lake, or the ocean bottom. Finally, biodiversity describes different combinations of living and nonliving components in varied environmental systems of inputs, outputs, and feedbacks. Genetic Diversity At the most basic level, species are distinguished from each other by how different their genes are. Genetic diversity is the ultimate source of biodiversity on Earth. The genetic differences between members of the same species can lead to physical variety such as different eye color, leaf arrangement, or beak size; differences between members of various species can result in dramatically different body plans and capabilities. Over generations, blueprints can change so that sometimes the organism of today bears very Two pea plants with the Bb genotype (purple flowers) produce little resemblance to its ultimate ancestor. This change a mix of purple and white colored offspring. Source: CK-12.org is the result of evolution. Evolution occurs when the Blue Ridge Academy - Maricopa, CA genes among groups of individuals within the same species change over time, so that the groups become genotypes: BB, Bb, or bb. The phenotypes produced different enough to be recognized as separate species. by the BB and bb genotypes will differ in some To understand how genetic diversity leads to species way. For example, in pea plants the B allele (and diversity, we need to understand some of the basic therefore the BB genotype) produces purple flowers, principles of genetic diversity. and the bb genotype produces white flowers. The Bb All organisms inherit from their parents the genes genotype contains one allele of each type, and yet pea that provide the blueprint for their development and plants with this genotype are also observed to have function. Some genes contain most, or even all, of purple flowers. In this case, the B allele is said to be the instructions for relatively simple traits. More dominant over the b allele. complex traits, such as body size and shape, are the Taken together, all the different alleles for a particular result of the interaction of more than one gene. All of trait that occur in a population are a pool of genetic these traits, the simple and the complex, are referred diversity. It is impossible to estimate the number of to as an individual’s phenotype, which consists of different alleles for genes of all individuals in any all of an individual’s anatomical, physiological, and natural population, and thus all the potential diversity, behavioral characteristics. All of an organism’s genes but a familiar example can give you some idea of the together comprise its genotype, which is its unique possibilities. All the varieties of Canis familiaris, genetic composition and the code for its phenotype. the domestic dog, share enough genetic material to Genes are chemically made up of the molecule DNA be considered one species. However, by repeatedly and arranged within an organism’s cells on structures breeding individuals with certain desired traits, called chromosomes. breeders have produced dogs as varied as toy poodles Genes frequently have alternative forms that contain and Great Danes. Each different breed is expressing a different instructions for what the gene will produce; different combination of alleles that lead to variations each alternative form is called an allele. If a particular in size, shape, coat texture, color, and so forth. gene in a population has two alleles (we’ll call them There is only one way a new allele can be produced: B and b), an individual in the population who has a mutation, an error made when genetic material two copies of the gene will have one of three possible is copied, can permanently alter the genotype an 2024–2025 Science Pentathlon Resource Guide 23 organism passes on to its descendants. Mutations Expressions of Genetic Diversity occur spontaneously and randomly. Millions of copies The observable characteristics associated with an of genetic material are made within cells, so it’s only organism’s genotype is called its phenotype. In some natural that some mistakes will be made. Mutations simple cases, a phenotype is simply the expression of are even more likely under some environmental a genotype. For instance, a person who inherits the conditions—exposure to anthropogenic chemicals, such genes for blue eyes will have blue eyes whether they as those in tobacco smoke, for instance, or radiation. live in tropical Africa or subarctic Alaska. However, most complex phenotypes (for instance, size) are the Though some mutations will have no effect on an result of the interaction between the genotype and its organism, others are harmful. Most neutral or harmful environment; in other words, phenotype = genotype mutations die with the organism. If the mutation + environment. Two growing pine trees may have occurs in the sex cells that produce offspring, however, identical genotypes coding for height, but if one is the genetic change appears in the next generation. grown in a nutrient-rich environment and the other in And if the mutation is beneficial to the organism in a nutrient-poor environment, the first tree will likely some way, making it more likely that the organism grow to be taller. will survive and pass the traits to its descendants over successive generations, the trait may spread throughout Species Diversity a population, becoming one more allele in the pool of The most common definition of a species is a group genetic diversity. of organisms that is distinct from other groups in If there are many different alleles for a particular morphology (body type), physiology, or biochemical Blue Ridge Academy - Maricopa, CA gene in a population, there will likely be a significant properties and that can breed and produce viable amount of genetic variation among individuals within offspring. Species diversity results from both adaptive the population, as different individuals are more likely and nonadaptive processes. Most new species arise to have a different combination of alleles. Further, large as the genotypes and phenotypes of two or more populations will usually have more genetic variation populations diverge through processes that take than small populations because the more individuals place at the population level, such as the evolution of there are, the higher the likelihood that the population different adaptations in populations living in different will have many different alleles for any one gene. environments or the decrease in migration between populations with a corresponding lack of genetic FIGURE 16 transfer. To understand these processes, let’s begin by looking at some basic aspects of evolution. EVOLUTION As explained earlier, repeated change in genotype over time may result in a variety of phenotypes in a species. While some of these phenotypes will have no effect or an adverse effect on survival, others may help an individual survive in its environment. For example, a frog with longer legs than other frogs in its population may be able to hop faster and will probably have a greater ability to catch food or avoid predators. Thus, the frog with longer legs is more likely than its short- legged neighbors to live long enough to reproduce and pass its genes on to its offspring. Fitness is a measure of the relative viability (ability An individual’s physical and behavioral characteristics (the to survive) and fertility (reproductive success) of an phenotype) is a result of the interaction between genotype and the environment. organism. The more likely that an individual will Source: Emphasis survive and the more offspring it leaves behind, the greater its fitness. The successful survival of the genes 2024–2025 Science Pentathlon Resource Guide 24  FIGURE 17 The number of offspring produced over a lifetime is a main indicator of an individual’s fitness. Source: UC Berkeley: Understanding Evolution of more fit individuals over many, many generations of impairments and usually death, mostly among people offspring can lead to a change in the average phenotype of African descent. At the same time, this allele also of the species. On the other hand, alleles that tend conveys resistance to malaria, one of the most deadly Blue Ridge Academy - Maricopa, CA to be more harmful than beneficial to an individual diseases in Africa. Two copies of the allele are necessary will usually die out along with the individuals in a to produce the disease. Individuals with only a single population that carry them. copy of the allele will not develop the disease but will have some natural protection against malaria and thus a An example how different alleles produce individuals higher likelihood of survival in an environment where with different levels of fitness is the allele for sickle-cell that disease is common. Thus, the sickle-cell allele, disease, which reduces the oxygen-carrying capacity of although fatal to people who carry two copies, has blood and results in many severe mental and physical persisted in the population because those people with FIGURE 17 The geographic correlation between the prevalence of malaria and the allele conveying resistance to the disease. Source: Encyclopedia Brittanica 2024–2025 Science Pentathlon Resource Guide 25 only one such allele have been more likely to survive Adaptation to a Changing Environment and pass the allele on to their descendants. Many organisms can adapt to quickly changing environments by simple changes in their physiological Adaptation Through Natural Selection or behavioral responses; they are tolerant to some The idea that species evolve over time had been degree of environmental change without the need suggested by a number of scientists and philosophers, to evolve new adaptations. For example, the panting but the concept of evolution through natural selection of a dog or the sweating of a human are means to did not become synthesized into a unifying theory until cool off on a hot day, while the dropping of leaves by Charles Darwin (1809−82) was able to put the various plants at the beginning of the dry season is a means of pieces together. During his journey aboard the HMS conserving water. However, over centuries or more, Beagle (1831−36), Darwin made many observations a species’ environment may change so dramatically of phenotypic variation and fitness in a great variety that short-term physiological responses are no longer of species. In addition to existing organisms, he found enough. As environments undergo major changes over evidence of species that had not survived. Darwin a long period of time, species will either die out or questioned why some species survive and some do not. adapt through natural selection. For example, as the During the decades after his journey on the Beagle, he environment in many parts of the world became drier, synthesized his observations and developed them into many plant species evolved similar adaptations to both a more robust theory. That theory, finally published in conserve and acquire scarce water. These adaptations 1859, was called “The Origin of Species by Means of include thick, fleshy leaves with thick waxy surfaces, Natural Selection.” all of which better enable a plant to conserve water. Blue Ridge Academy - Maricopa, CA The key ideas of Darwin’s theory of natural selection The ability of a species to adapt to environmental are: changes will depend greatly on how much, and how Organisms produce many more offspring than fast, that change occurs. Consider a tropical bird that 6 are needed to replace the parents. has a strong beak that enables it to crack seeds with very hard shells. Imagine that over many years the 6 Within any given species, individuals will shells of the seeds become even harder. The birds with express a range of phenotypes. stronger beaks will be more fit at opening the seeds than 6 In a given environment, some phenotypes will individuals with weaker beaks. Thus, those that can enable an organism to survive better than other adapt to the change in their environment (harder shells) phenotypes; in other words, some organisms will be more fit and pass on their strong-beak genes will be more fit. to future generations. However, if the environment The more fit individuals will have a better changes so fast that suddenly only extremely hard- 6 chance of reproducing; they will be selected for shelled seeds are available, there may not be any birds and over time will be well established in the with sufficiently strong beaks to open the shells, and the environment. species will die out. Much of the current environmental change caused by humans is both dramatic and sudden, Darwin’s theory explains why individuals with certain and most species, like our hypothetical tropical bird, traits survive and reproduce. The process of becoming may not be able to adapt in time. most fit or most suited for a particular environment is called adaptation. Species differ in how well they Nonadaptive Evolutionary Processes adapt to a given environment and how well they adapt Evolution can also occur through nonadaptive processes. to a changing environment. There are also variations When individuals from one population migrate to in how long it takes species to adapt. A species with another population, gene flow occurs. If the population more genetic variability will have a wider range of from which the migrants come had more alleles of a phenotypes in any given environment, and thus it particular type, or some unique alleles altogether, and is more likely that a successful phenotype for that if these migrants mated and produced offspring in their environment will exist. new population, they would introduce these new alleles into the population, and new forms of the resulting 2024–2025 Science Pentathlon Resource Guide 26 phenotypes would also be introduced. There is evidence that high rates of gene flow occur in species in which FIGURE 19 there is much dispersal of individuals. However, most populations of a species seem to be relatively isolated from each other and need to adapt to their environment with the genetic variation they possess. As we will see, this genetic isolation of most species is an important issue in the maintenance of biodiversity. Another nonadaptive process, genetic drift, or random change in genotypes among small populations of a species, is known to be an important mechanism in evolution. When a population is small, not all possible alleles will show up in the relatively few offspring that are produced. Imagine a population of only twenty organisms, ten with allele A and ten with allele B. If only two of those individuals mate An illustration of the bottleneck effect. and produce offspring, they may well both have the A Source: azolifesciences.com allele; the B allele will be lost in just one generation. The loss of alleles from one generation to another will result in genetic change and a corresponding change The Pace of Evolution Blue Ridge Academy - Maricopa, CA in phenotypes over time, a form of evolution not How long does evolution take? A significant change associated with fitness or selection. in a species’ genotype, such as an adaptation to a completely different food source, can take many One specific type of genetic drift is the bottleneck hundreds to thousands of years. If we examine the effect, in which a population is drastically decreased in evolution of life from single-celled organisms to size due to hunting, habitat loss, a natural disaster, or primates and human beings, we can see that it has changes in the environment. The event that produces taken a very long time. Smaller scale evolutionary the decrease acts like a bottleneck, reducing the changes can occur over a much shorter time scale. number of different alleles present in the population. (Keep in mind that “long” and “short” in the context of The remaining population will thus have a smaller evolution are very different from our everyday sense of pool of genetic diversity. Environmental scientists time.) who study the genetics of natural populations have discovered that low genetic variation is correlated to all Three factors are particularly important influences on kinds of potential problems, including increased risk of the pace of evolution by natural selection. First is the disease and low fertility. rate of environmental change to which a species must adapt. As we noted earlier, rapid environmental change One well-known example is the cheetah, whose forces populations to evolve quickly to adapt to the populations have dramatically decreased due to new environment or die out. Secondly, if a population hunting and loss of habitat. The drastic reduction in has a large amount of genetic variation that produces cheetah population size over a short period of time traits that make individuals more fit, evolution can resulted in the loss of all the alleles that had been occur quite quickly, but if the population must generate present in the killed animals. Today, the cheetah new genetic variation through the accumulation of population is so small that there is almost no genetic mutations, evolution will proceed much more slowly. variation; individuals are essentially all identical Finally, new adaptive traits may be able to spread more twins. Cheetahs tend to have low fertility (males have quickly in small populations than in larger populations. 70 percent abnormal sperm cells) and high rates of Small populations are certainly more likely to undergo disease, at least among zoo populations where most rapid evolution by nonadaptive mechanisms such as reliable studies have been conducted. It is uncertain if genetic drift and bottlenecks. natural populations have the same reduced fitness. 2024–2025 Science Pentathlon Resource Guide 27  FIGURE 20 A timeline of the evolutionary history of life. Blue Ridge Academy - Maricopa, CA Source: Flexbooks CHANGES IN ENVIRONMENTAL those species that no longer exist. If the environment changes so that a population is no longer adapted to CONDITIONS AND EXTINCTIONS it, the population’s growth rate becomes negative. While we do not know the actual number of species Eventually, the population size will decrease to zero in the world today, we do know that number equals unless the population migrates to a new environment all the species that evolved over time minus all of in which it can succeed or unless it adapts to the 2024–2025 Science Pentathlon Resource Guide 28 changed environment through evolution. In most of years ago do not exist today, and species that exist cases, neither option works. There may be no favorable today are not found in the fossil record. There are some environment close enough for the population to exceptions; the ginkgo tree, which exists in China and migrate to or, if there is, it may already be populated is an ornamental tree in many parts of the U.S., also by species the population cannot successfully compete appears in the fossil record from 60 million years ago. with. In the second case, the environmental change The fossil record has been the source of most of our may be so rapid that the species does not have the time knowledge of extinction as well as much information to evolve new adaptations. Organisms that cannot about the state of the environment today. adapt to environmental change will eventually go extinct; that is, no members of the species will remain Mass Extinctions on Earth. Almost every species that has ever lived on The fossil record tells us that all species eventually Earth has gone extinct—although this can mean that go extinct. Over time, individual species evolve and the lineage is lost to history, it can also mean that that go extinct continually, at random intervals. However, they gave rise to new species before going extinct. there are periods of mass extinction, in which the fossil record reveals that large numbers of species died in a The Fossil Record very short time interval. The greatest mass extinction Most of what we know about the evolution of life on record took place at the end of the Paleozoic Era. is based on fossils, remains of extinct plants and Roughly 90 to 95 percent of marine species and 70 animals that have been preserved in rock. When percent of land vertebrates went extinct during this most organisms die, they decompose fairly rapidly, time. The cause of this mass extinction is thought to be and the elements they contain are recycled; in this related to the shutdown of ocean circulation combined Blue Ridge Academy - Maricopa, CA case, nothing of the organism is preserved. However, with a massive, sustained volcanic eruption. A better sometimes the hard parts of an organism (bones, known mass extinction occurred at the boundary of the shells, teeth, etc.)—and occasionally softer organic Cretaceous and the Tertiary Periods, known as the K-T material—will be buried and protected by mud or boundary. This is the period 65 million years ago when other sediment, and after much time the material many species, including the dinosaurs, went extinct. becomes fossilized, or hardened into rock-like material However, some species survived, including a small that is buried under increasing layers of sediment. rodent-like mammalian species that would give rise to When these layers are uncovered, they reveal a record the human species. of at least some of the organisms that existed at the time those layers were preserved. Because of the way During the 1990s and into the twenty-first century, there layers of earth are deposited on top of each other over has been a growing consensus among biologists that time, the oldest fossilized organisms are found in the the Earth is in the beginning stages of a human-caused deepest layers of the fossil record. mass extinction of species, resulting largely from the destruction of habitat. The current mass extinction could The fossil record is the basis of the geologic time reach the magnitude of the previous five mass extinction scale, which divides time into various intervals periods that have occurred sporadically over the last 450 from the formation of the Earth through the present, million years. The recovery of biodiversity from those with distinctive events, such as the evolution of earlier mass extinctions took from 10 to 100 million multicellular organisms or the extinction of the years; recovering from the present mass extinction could dinosaurs, characterizing the major time intervals. take just as long and would probably require returning Bacteria appear in the fossil record as long ago as large portions of the Earth to their natural state so that 3.5 billion years before the present; multicellular and new species could naturally evolve. Much of the current shelled organisms are visible about 540 million years disagreement about policy among many environmental ago. In general, we can trace identifiable species for scientists and government officials stems from differing one million years (for example, mammalian species) to opinions as to how acute this biodiversity crisis is ten million years (for example, some clams and other and how much science is required to solve it. In the marine species) of the fossil record. For the most part, next section, we will discuss current threats to Earth’s species found in the fossil record from many millions biodiversity. 2024–2025 Science Pentathlon Resource Guide 29  FIGURE 21 Blue Ridge Academy - Maricopa, CA Five mass extinction events have occurred over various geologic time periods, and we may currently be experiencing a sixth mass extinction event. Source: National Geographic HUMAN ACTIVITY AND their activities change animal and plant habitats. In addition, humans can cause extinction through habitat BIODIVERSITY modification and fragmentation, the introduction of Human activity can alter biodiversity in a variety nonnative species, and other ways. of ways. Species diversity can be reduced by direct removal of a species by humans (overexploitation), as Habitat Fragmentation when overhunting by European sailors stopping at the Like the tropical rainforest example just discussed, Indian Ocean island of Mauritius in the seventeenth human activity may fragment a large tract of land century helped to exterminate the Dodo, a large into smaller pieces through the construction of roads, flightless bird used as an easy source of meat. (The housing developments, or shopping or industrial introduction of animals that competed with the dodo centers. Fragmentation of habitats has a number for food resources was also a significant factor.) Other of major impacts on biodiversity. Fragmentation direct removals such as fishing and overharvesting reduces the area of contiguous habitat, which can of plants can also directly reduce biodiversity. create barriers to the normal movement of a species Humans can also affect biodiversity indirectly when for purposes such as feeding, mating, and migrating. 2024–2025 Science Pentathlon Resource Guide 30 New housing developments can cause habitat destruction and/ or fragmentation. While many species have minimal space requirements and thrive in fragmented areas, others—such as the mountain lion, wolf, and tiger—require large tracts of relatively uninhabited, undisturbed land. Blue Ridge Academy - Maricopa, CA Secondly, fragmentation generates more habitat that is along an edge. This consequence of fragmentation stems from basic geometry: a greater number of smaller tracts of land will contain more edge, even if the total area of land is identical to an unfragmented parcel of land. Edge habitat is in proximity to other kinds of habitat; in a forest, for example, the edge habitat is that area adjacent It is believed that the zebra mussel, a native of the Caspian to clearings or other nonforested areas. Increased edge Sea in Asia, entered the Great Lakes during the 1980s; since habitat will change the species composition of the then, its population has grown exponentially there and habitat overall. For example, raccoons and skunks may throughout the eastern United States and Canada. increase in number along forest edges. Since there is now more edge closer to interior portions of land, these or not—from one part of the globe to another. Species species will be more easily able to penetrate further into that are not native to an area but are introduced, the forest, where they may eat plants, small mammals, deliberately or accidentally, by people are called bird eggs, and other organisms that normally would be exotic species. In the process of moving themselves relatively safe in the forest interior. and their belongings from place to place, people have accidentally transported many species and diseases Third, habitat fragmentation will divide a population to new continents and new environments. The ballast into several smaller populations. Gene flow between water in ships often contains many species from the these smaller populations will usually be greatly point of origin that get distributed in every port the ship reduced with the result that they will become genetically visits. It is believed that the zebra mussel, a native of the isolated and will likely lose genetic variation through Caspian Sea in Asia, entered the Great Lakes this way genetic drift. during the 1980s; since then, its population has grown exponentially there and throughout the eastern United The Introduction of Exotic Species States and Canada. With no known predators in North Before the advent of widespread human travel, the America, it has depleted food supplies, clogged water free movement of many species was impeded by large intake valves, and caused many other problems. bodies of water, deserts, mountains, and other land and water barriers. However, for quite some time, people When an exotic species enters a new environment, have been capable of transporting species—knowingly it may encounter an unexploited resource that it can 2024–2025 Science Pentathlon Resource Guide 31 rapidly utilize. In the new environment, the organisms wide variety of conditions determine which species that keep it under control in its native environment are and communities flourish in some habitats and not usually not present, nor are there naturally occurring others. For terrestrial systems, the availability of light predators. While a fair number of exotic species do not from solar radiation, the temperature of the air and thrive in their new environments, some do survive and soil, the amount of precipitation, soil type, and quantity occasionally are successful enough to replace the native of nutrients such as nitrogen or phosphorus are usually species that are more vulnerable to local pathogens the major conditions that determine the presence or and predators. Once an exotic species is successful in absence of certain species. These conditions vary with its new environment, it is rare for it to ever be brought latitude and elevation. In aquatic systems, in addition under control. to temperature and solar radiation, gradients in the amount of oxygen dissolved in the water, salinity, and LINKING BIODIVERSITY AND acidity also play a role. These conditions vary with the depth of the water, the location in a stream (upstream, EVOLUTION TO ECOLOGY midstream, downstream), latitude, elevation, and many The Ecological Perspective other aspects of the habitat. Evolutionary history provides the first step in understanding why some species are found or not For each environmental condition, there is a range found in various regions. However, to fully explore within which a species can live. Under optimal patterns of biodiversity, we need to know how conditions, individuals will thrive: they will survive, species distribution and abundance is limited by grow, and reproduce. As conditions become less current environmental conditions. This is the domain optimal—for example if temperatures become Blue Ridge Academy - Maricopa, CA of ecology, the study of the relationship between significantly higher or lower than the ideal—individuals organisms and their environment. The environment may survive and grow, but not reproduce. If conditions includes other individuals in an organism’s population, become even less desirable, individuals may survive, other populations of plants and animals with which an but not grow or reproduce. Finally, if conditions become organism and its population interact, and the abiotic even worse, individuals may not survive, and the species environment—the physical and chemical factors that won’t exist in that environment. The range within which influence life. “Relationships” include such factors a given species will exist is called its range of tolerance. as the adaptation of an individual’s physiology to Over time, adaptations evolved through natural selection environmental extremes, the killing and eating of prey have allowed some organisms to survive, grow, and by predators, and the flow of carbon through biotic and reproduce under any environmental condition on Earth. abiotic components of the environment. Resources To study such a diversity of natural phenomena, Resources are those aspects of the environment that ecologists have traditionally divided the science into individuals use to stay alive—food, water, light, and a hierarchy from individuals to large-scale ecological oxygen. Unlike environmental conditions, resources systems of interacting biotic and abiotic components. are consumed and thereby become unavailable to Different ecological processes occurring at these other organisms. The amount of available resources is various levels will have different effects on biodiversity. important at all levels of ecology. Individuals require We will begin our exploration at the individual level. sufficient resources to grow and reproduce. Populations The ecology of the individual is concerned with an require sufficient resources to maintain a size that organism’s ability to live in the environment. Three will prevent extinction. And communities require things are critical to an individual’s survival in a sufficient resources to maintain several different particular area: the abiotic environmental conditions; the species in one habitat. Habitats with a large amount availability of resources such as food and water; and a of resources are generally able to maintain both many place, or habitat, for the individual to live. individuals within each species and a large number of different species, but, surprisingly, in many habitats Environmental Conditions there is little direct correlation between the amount of Conditions are the chemical or physical factors in the resources and the number of species. The explanation environment that influence survival and growth. A for this rests with the varied abilities of individual 2024–2025 Science Pentathlon Resource Guide 32  own oxygen-rich microhabitats in the soil surrounding them. Another way that organisms adapt to their changing environments is to change the way they allocate energy. COMMUNITY ECOLOGY Populations of different species interact with one another in three general ways: interspecific competition, predation, and mutualism. Interspecific Competition Interspecific competition occurs when individuals of different species share a limiting resource in the Salt marshes produce extremely high levels of biomass and act same area. A limiting resource is the one on which as a storage tank for large amounts of nutrients caught from a population depends and which exists in low, and the flow of rivers. usually variable, quantities, such as food, oxygen, or organisms to use resources under various abiotic space. As the limiting resource decreases, so does the environmental conditions. size of the population which depends upon it. Species can share food sources as long as those sources are not One example is the large salt marshes that extend along their limiting resource. If the food source, or any other the eastern coast of the United States from Georgia Blue Ridge Academy - Maricopa, CA resource, limits the growth and reproduction of the to Cape Cod. Salt marshes produce extremely high species, it cannot be shared: one species will succeed, levels of biomass and act as a storage tank for large and the other will go extinct, a principle known as amounts of nutrients caught from the flow of rivers. competitive exclusion. This high level of nutrients makes salt marshes an important nursery for many fish and shellfish species, Competition among species is not limited to animals. as well as a source of energy for adjacent aquatic and Plant ecologists have shown several instances of marine habitats. However, the vegetation within salt the effects of one plant species on the distribution marshes, while abundant, is not diverse; usually only a and abundance of other species. For example, wild few grass species make up more than 95 percent of the oat (Avena fatua) is an abundant weed in the Great total marsh biomass. These plant species dominate the Plains of North America that competes for space and salt marsh because they can process the nutrient-rich resources with agricultural crops such as flax, wheat, resources under the extreme environmental conditions. and barley. Wild oats outcompete crop plants because their seeds ripen earlier, permitting oat seedlings to The environmental conditions in a salt marsh include start growing before the other species. Only human the increase in salt concentrations that results from the intervention allows the agricultural crops to exist. evaporation of water during summer low tide and the rapid decrease in salt concentrations during flooding Closely related to the competitive exclusion principle is or heavy rains. This means that plants must be able the concept of niche. Because of its use in nonscientific to tolerate both high- and low-salt concentrations and language, the term is often confused with habitat, a dramatic switch between the two. In addition, salt the range of environments in which a species occurs. marsh plants must be able to survive the low oxygen In ecology, niche is commonly defined as the role of levels in the soil, a result of the high level of microbial an organism within a community—what it does and decomposition that takes place in a salt marsh. This how it lives. Many ecologists rephrase the competitive microbial activity also produces large amounts of toxins exclusion principle by stating that two populations as a side-effect, yet another harsh abiotic environmental that fill the same niche, such as by feeding on the condition. The salt marsh grasses survive because of same limited resource, cannot coexist. However, if several adaptations, including special tissues that allow these same two populations partition the resources so them to concentrate and excrete excess salt and air that their niches do not overlap completely, they can chambers in their roots that allow them to produce their coexist. (See Figure 22.) 2024–2025 Science Pentathlon Resource Guide 33  FIGURE 22 Blue Ridge Academy - Maricopa, CA Different species of warblers minimize competition by using different parts of trees. Source: “Resource partitioning among five species of warblers feeding in North American spruce trees.” Biology Forums Gallery. Predation establishes how energy flows from one population Most people think of predation only as a carnivore, such to another within a community and an ecosystem. as a wolf or a lion, killing and eating another animal. In Predators limit the growth of prey populations both the broadest sense, predation is the use of one species as by consuming them and by changing their behavior. a resource by another species. This definition allows us Increases or decreases in the prey population can to include two other species interactions: also lead to increases or decreases in the number of predators, for example, through an increase in the 6 Herbivory, in which animals eat plants, seeds, number of offspring they produce. (See Figure 23.) or fruits Environmental scientists have demonstrated such 6 Parasitism, in which animals, plants, fungi, predator-prey cycles in simple laboratory systems. or bacteria feed on or use as a habitat another However, most of these systems lead to the extinction organism, causing injury but usually not death of the prey, followed by extinction of the predator, unless some other factor is added to balance the fact Predation is an important cause of natural selection that the predator, if able, will eat all available prey. and evolution and, in all its forms, is the process that 2024–2025 Science Pentathlon Resource Guide 34  The most common type of mutualism involves FIGURE 23 interaction between plants and animals. Probably the single most important type of mutualistic interaction is the relationship between plants and their pollinators, such as birds and insects, since many plant species depend upon pollination for their reproduction and survival. Some pollinators pollinate many different species of plants, and many plant species are pollinated by many different species of animals. In these cases, the mutualistic interaction between any particular pair of plant and animal species is weak. In symbiotic Predator-prey oscillations. mutualism, by contrast, one animal species pollinates Source: Flexbooks only one plant species, and the plant is pollinated only by that one animal species. For example, there are about nine hundred species of fig trees, and almost Mutualism every one is pollinated by one particular species of fig The third major type of population interaction is wasp. These types of mutualistic interactions are most mutualism. Mutualistic interactions are those that likely due to resource partitioning in the evolutionary increase the survival probability or reproduction of past. both species. Though the term mutualism may lead ECOLOGICAL COMMUNITIES Blue Ridge Academy - Maricopa, CA some people to imagine species helping each other in a cooperative sense, ecologists see it more as “reciprocal So far, we have seen how the interaction between exploitation” since each species is using the other two populations affects the survival and growth of to benefit itself. If the self-benefit to one population each individual population. However, many of the becomes too low, the interaction will no longer be of most important ecological processes occur at a level value and will no longer provide an adaptive advantage of organization higher than the population (or two to either species. interacting populations)—the ecological community. A community is any assemblage of populations in a particular area or habitat. Community ecology studies groups of populations living in the same area. Food Webs A food web summarizes the species that make up a community and the ways they are linked by various predator-prey interactions to form pathways of energy flow. Food webs operate like food chains, but since they include all the species in a feeding relationship, they are much more complex, as you can see in Figure 24, which shows the Greater Yellowstone ecosystem food web. In an aquatic food web, the photosynthesizers are are primarily multicellular algae and single-celled phytoplankton. Single-celled animals (zooplankton) feed on the phytoplankton, and herbivorous fish eat the algae. Carnivorous fish prey upon zooplankton, insects, and herbivorous fish and are, in turn, eaten by the secondary carnivores— tarpons (the largest fish species in the lake) and several A hummingbird drinking nectar from a flower while also bird species. serving as a pollinator is an example of mutualism. CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=763031 2024–2025 Science Pentathlon Resource Guide 35  FIGURE 24 Blue Ridge Academy - Maricopa, CA Greater Yellowstone ecosystem food web highlighting only some of the many species in the ecosystem. Source: https://yellowstoneinfo.weebly.com/food-web.html Energy and Trophic Levels primary producers such as plants and algae that use The feeding positions in a food chain or web are called the process of photosynthesis to create food from trophic levels, from the Greek word for nourishment, sunlight, carbon dioxide, and water. Producers do not trophe. The lowest trophic level is occupied by rely on other organisms for nutrition. Higher trophic 2024–2025 Science Pentathlon Resource Guide 36 levels are made up of consumers that cannot make next (Figure 25). Only about 1 percent of the energy their own food and must eat other organisms to obtain of the Sun is transferred to the primary producer. energy. Primary consumers such as grasshoppers, Roughly 10 percent of the energy available at one caterpillars, and termites are herbivores that feed trophic level is passed along to the consumers in the directly on the producers. Secondary consumers next level, and the rest is released as heat. This loss are organisms that feed on the primary consumers. of energy limits the amount of biomass, or weight of Examples of secondary consumers include robins that biological material, that can be supported at a given feed on earthworms, as well as frogs that feed on flies. trophic level. Notice how as we move up the ecological Many ecosystems can support higher trophic levels pyramid from lower to higher trophic levels, both the including tertiary consumers, such as snakes and owls, available energy and biomass decrease. that feed on secondary consumers. Keystone Species Ecological pyramids can be used to show the amount A keystone species is one that, because of its position in of energy that transfers from one trophic level to the the food web or some other population interaction, plays FIGURE 25 Blue Ridge Academy - Maricopa, CA Energy pyramids highlight the decrease in biomass and loss in energy as we move up from one trophic level to the next. Source: Sciencefacts.net 2024–2025 Science Pentathlon Resource Guide 37  FIGURE 26 Blue Ridge Academy - Maricopa, CA A keystone species is one that has a dramatic effect on the health and diversity of an ecosystem. Source: sciencenotes.org a role in the community that is far more important than 6 Ecosystem engineers serve a community its relative abundance would suggest (Figure 26). This by creating or maintaining habitats for means that species that are the most abundant species other species. The North American beaver in a community or the major energy producers, while is a prime example of this type of keystone vital to the health of a community, are not considered species. Although they make up only a small keystone species. The keystone species concept was percentage of the total biomass of the North developed to explain the sometimes unexpected effect American forest, beavers transform streams of removing a relatively rare species from the food into ponds, creating a new habitat for pond- web. Scientists have identified three types of keystone adapted plant and animal species. species: predators, ecosystem engineers, and mutualists. Mutualists are two or more species that interact 6 Predators can be a keystone species within a for each other’s benefit. This type of keystone 6 community by controlling the population of species is exemplified by the mycorrhizal their prey. For example, sea otters maintain fungi on and in the roots of many tree species. kelp forest ecosystems by preying on sea Increasing the tree’s ability to extract nutrients urchins. Without sea otters, sea urchins can from the soil, mycorrhizae play a critical role overpopulate the sea floor and consume kelp in the growth of tree species, which in turn forests that provide cover and food for other provide most of the resources and niches for marine organisms. other members of a forest community. 2024–2025 Science Pentathlon Resource Guide 38  how biotic and abiotic changes create a diversity of ecosystems, as well as how changes at the ecosystem level produce variation in lower levels of biodiversity. Ecosystem Boundaries Many aquatic ecosystems such as lakes, ponds, and streams are relatively easy to identify in nature because the ecosystem’s boundary corresponds to the boundary between land and water. In most cases, however, determining where one ecosystem stops and another begins can be difficult. Environmental scientists usually estimate the boundary of terrestrial ecosystems by the range of the populations that make up the biological The North American beaver functions as a keystone species community or by particular ecological processes. Often within a community by creating or maintaining habitat for the boundary of an “ecosystem,” such as a national other species. park or reserve, is set according to administrative rather than scientific criteria. Knowing the boundary of an MAJOR ASPECTS OF ECOSYSTEMS ecosystem makes it easier to identify the biotic and An ecosystem is “a spatially explicit unit of the abiotic components that make up the system. Earth that includes all of the organisms, along with all components of the abiotic environment within The Biotic Components of Ecosystems Blue Ridge Academy - Maricopa, CA its boundaries.” This definition highlights the three The types of species within an ecosystem will important aspects of an ecosystem: influence how energy flows through the system. The conversion of energy from producers to the different 6 The ecosystem’s boundary—where does one levels of consumers can be modeled as a pyramid in ecosystem end and another begin? which the amount of energy or biomass at each level 6 The biotic component—the individuals, of the ecosystem is represented by the relative size of populations, and communities that live within that part of the pyramid. Energy pyramids will look the ecosystem relatively constant from ecosystem to ecosystem; 6 The abiotic (physical and chemical) most energy is always found at the producer level component, including temperature, water, and decreases as we move up the pyramid. Biomass salinity, soil structure, and mineral nutrients. pyramids, however, vary from ecosystem to ecosystem depending upon the characteristics of the populations The ecosystem is the first level in the hierarchy of making up the various trophic levels, as well as the biodiversity that is self-contained. In contrast to physical and chemical structure of the ecosystem itself. individuals, populations, and communities, which depend upon other organisms for food and the The Impact of Ecosystem Change on Its physical environment for optimal conditions and habitats in which to live, an ecosystem contains all Biotic Components the living and nonliving parts required for long-term All ecosystem-level processes are subject to change. existence. However, an ecosystem is not just the Certain kinds of ecosystem-level changes can be components within it and the boundary around it. It characterized as a disturbance, a process in which is also the processes occurring within it. One of the physical, chemical, and some biological agents cause major processes is the flow of energy from the Sun the relatively rapid injury or death of organisms and through the abiotic and biotic components. Cycling of the damage or collapse of the biotic component of the materials is also central. So many different materials ecosystem. Some natural disturbances are hurricanes, cycle through an ecosystem—water, carbon, nitrogen, ice storms, and natural forest fires; anthropogenic and phosphorus, to name only a few—and they are disturbances include clearcutting of forests, agriculture, all so important to the global environment as well as and air pollution. The slow invasion of a lake by an to any individual ecosystem. Here, we will focus on introduced species is not usually called a disturbance, but is rather considered a perturbation, a much broader 2024–2025 Science Pentathlon Resource Guide 39 term that refers to any kind of change to the normal or the species would return by the following year. A less equilibrium value in a system. The gradual increase in resilient ecosystem might not return to pre-drought temperature by approximately 1°C that has occurred on conditions for many years, if ever. Earth in the last one hundred years is a perturbation. A 2°C increase within one decade would most likely be BIOMES characterized as a disturbance. Biomes and Global Biodiversity Resilience is the rate at which an ecosystem returns to Biomes are major regions of differing vegetation and its original state after some sort of disturbance causes a wildlife types. Environmental scientists have correlated change. A highly resilient ecosystem would return to its the presence and extent of ten major types of terrestrial original state rapidly; a less resilient system more slowly. biomes with a region’s mean annual temperature and For example, if a severe drought were to eliminate mean annual precipitation (Figures 27 and 28). half the species in a highly resilient ecosystem, all FIGURE 27 400 Blue Ridge Academy - Maricopa, CA Tropical Annual precipitation (cm) rainforest 300 t or es a inf ter p era m Te 200 Tropical seasonal Temperate forest/ seasonal forest savanna 100 Boreal forest Woodland/ shrubland ra nd / Subtropical nd te grassla Tu Tempera ert desert cold des -10 0 10 20 30 Average annual temperature (°C) Average annual precipitation and temperature correlated with different biomes. Source: OpenOregon Educational Resources 2024–2025 Science Pentathlon Resource Guide 40 FIGURE 28 2 Tropical rainforest (evergreen) The wettest and warmest biome is the tropical rainforest, such as those in the Amazon and in western and central Africa. Key elements: High plant and animal diversity. Ecosystem productivity is high, but much of the ecosystem’s energy and nutrients are tied up in the vegetation, and the soils are often extremely poor in mineral nutrients. 3 Tropical dry (seasonal) forest Some forests in the tropics experience a pronounced dry season. Blue Ridge Academy - Maricopa, CA Key elements: Deciduous trees, which drop their leaves and flower during the dry season, are common. Productivity and diversity of both plant and animal species per meter are less than in tropical rainforests. 4 Temperate rainforest Tall coniferous trees are the dominant form in temperate zone rainforests such as the U.S. Pacific Northwest. Key elements: Mild winters, heavy rain, and frequent fog are the main factors creating optimal conditions for trees that are frequently 60−70 meters high. Productivity is roughly half that found in tropical rainforests. Soils tend to be rich in organic matter. 2024–2025 Science Pentathlon Resource Guide 41  5 Temperate broadleaf forest These forests occur in regions of moderate rainfall and high seasonal temperature variation and include deciduous-dominated forests in the eastern U.S., southern Canada, Europe, and eastern Asia. Key elements: Productivity is similar to that of temperate rainforests. Because most plants shed their leaves, a thick leaf litter will decompose into a rich soil. Both plant and animal diversity are much lower than in the tropics. 6 Boreal coniferous forest As temperature decreases, the dominant deciduous vegetation in areas of moderate to high rainfall are forests almost exclusively of conifers, primarily spruces and firs that are 10−20 meters high. Blue Ridge Academy - Maricopa, CA Key elements: Several large mammal species, such as moose, bear, wolf, and Siberian tiger are found in these forests. Productivity is roughly one-third that of tropical rainforests, with low plant species diversity. Yearly weather variation results in dramatic yearly variation in seed production, which causes dramatic fluctuations in bird and other animal populations. Low temperatures and chemicals in foliage result in low leaf litter decomposition and relatively poor soils. 7 Temperate grassland When precipitation decreases to the point that there is not enough water to support dense forests, vegetation shifts to grasslands. Key elements: Grasslands are often called prairies in the U.S. and steppes in central Asia. Productivity is usually about one-third of that found in tropical rainforests. Organic matter accumulates in this biome because decomposition of dead vegetation is limited by low precipitation rates, resulting in rich agricultural land. 2024–2025 Science Pentathlon Resource Guide 42  8 Tropical Scrub Forest and Savanna Savanna is most common in dry tropical regions of Africa, where rainfall ranges from 10−150 cm/year and is seasonal; during the driest three to four months of the year, there may be less than 5 cm per month. Key elements: Portions of this biome contain scrub vegetation, which is small and stunted due to limited nutrients and a short growing season. Migrating herds of herbivores, such as wildebeests, follow the rain and move across this biome. Fire and grazing are responsible for generating and maintaining the savanna biome. Productivity and species diversity per square meter are significantly less than in tropical rainforests. 9 Mediterranean Found in countries bordering the Mediterranean Sea, as well as California (where it is known as chaparral), this biome comprises dry areas that receive most of their rain in the winter, before the temperatures rise enough to permit plant growth. Blue Ridge Academy - Maricopa, CA Key elements: Vegetation is made up mostly of dense, woody shrubs and small trees. Leaves tend to be small, leathery, and waxy—adaptations that help retain water. Fires are frequent, and many trees and shrubs have evolved fire- resistant bark to protect themselves. Several bird species and small mammals such as jackrabbits, kangaroo rats, and chipmunks can be found, along with mule deer and several species of lizards. 10 Desert Usually defined as areas receiving less than 25 cm of precipitation per year, desert biomes cover a fairly broad temperature and latitude range. Key elements: Although commonly considered hot, there are cold deserts in places such as Mongolia and Montana. Because of its low precipitation, Antarctica is classified as a desert. Most deserts are characterized by sandy or rocky soil. Sparsely spaced shrubs and grasses are common. Desert productivity ranges from 0 to roughly 5 percent of that found in tropical rainforests. Many desert species have evolved adaptations to the lack of water. 2024–2025 Science Pentathlon Resource Guide 43  11 Tundra Tundra occurs in the arctic region beyond the tree line, the upper limit of tree growth at high latitude or elevation. Key elements: Vegetation consists primarily of grasses and grass-like sedges, lichens, and dwarf forms of trees. The soil (permafrost) is frozen all year round, though it thaws to a depth of 0.5−1 meters during the brief summer growing season. Mean productivity in the tundra regions is low, normally between 5 percent and 10 percent of what is found in tropical rainforests. Rodent species, such as lemmings, can be abundant, but their populations undergo dramatic fluctuations correlated with variation in resources. Though bird populations can be abundant in summer, most species will migrate south during the long winters. Ten Terrestrial Biomes Blue Ridge Academy - Maricopa, CA Without the variety of large plants used to characterize so most deep-water organisms must migrate toward terrestrial biomes, aquatic regions have not been the surface for food or wait for material to descend subdivided into as many different types and are to them—with an exception being the volcanic vent not traditionally known as biomes. However, a few communities found on the ocean bottom. distinguishable types can be identified in the two major aquatic systems—freshwater and marine. A major limiting factor of marine ecosystems is that while light near the surface is sufficient for Freshwater systems can be divided into flowing (rivers photosynthesis, the nutrient content of these waters and streams) and standing (ponds and lakes) waters can be quite low. Therefore, species abundance and with a relatively low salt concentration. Plants and diversity near the surface is relatively low. In general, animals that live on or near the bottom of rivers and the peak species diversity in oceans occurs at depths of streams are known as the benthic community. Lakes about 2,000−3,000 meters, an area of relative stability and ponds contain both a benthic community and where descending food material tends to fall. Major an open-water community dominated by the major exceptions to this general rule are areas in which there energy producer, phytoplankton—single-celled algae are major upwellings (some coastal and open ocean living in areas of the lake or pond with sufficient areas where winds blowing across the ocean surface light for photosynthesis. Phytoplankton are fed upon will push some water away and allow nutrient-rich by the primary consumer, zooplankton, which are water below to rise to the surface) or coastal waters small animals, mostly crustaceans. Lake and pond where nutrients wash in from the land. Most of the ecosystems are influenced by thermoclines, abrupt world’s major fisheries are found in waters near the changes in the temperature of water with depth that surface that are mixed with nutrient-rich waters, prevents the mixing of the layers of water. permitting high levels of productivity and supporting vast species abundance and diversity. The ocean covers about 71 percent of the Earth, making it larger than all terrestrial biomes combined. The final types of distinct regions are wetlands, which The uneven distribution of light and nutrients, coupled are transitional areas between the strictly terrestrial with the variation in depth, currents, and shoreline and and aquatic. Salt marshes, bogs, swamps, and bottom characteristics, results in many different types intertidal areas are examples of wetlands. There are of communities and sub-ecosystems. Below 100−200 three broad types of wetlands: a marine wetland (the meters, there is not enough light for photosynthesis, intertidal region); an estuarine wetland, which is where 2024–2025 Science Pentathlon Resource Guide 44 salt and fresh water mix at the mouths of rivers, and living organisms is water. Before we can understand freshwater wetlands, which make up 91 percent of all how individual elements cycle, we need the framework wetlands in the continental United States. Freshwater of the water cycle, the movement of water through the wetlands include bogs, marshes, swamps, and atmosphere and over the surface of the Earth. peatlands and differ from open waters (lakes, ponds, and rivers) by having water at or near the soil surface The Water Cycle for most of the year, but rarely more than two meters The water cycle is the driver of biogeochemical cycling deep. Some wetlands, like salt marshes, are highly on Earth. Water in the atmosphere falls to Earth as rain productive and are important nesting and feeding sites or snow. When water comes in contact with vegetation for many animal species, including migratory birds. or soil, three things may happen to it: It may return to the atmosphere by evaporation, BIOGEOCHEMICAL CYCLES 6 or—after being taken up by plant roots—it may Elements continually cycle within the biosphere and return to the atmosphere through transpiration, between the biosphere, soils, and water as plants and the loss of water from the stomates (openings) in animals grow, die, and decompose. Some of these leaves during photosynthesis. The combination cycles, known collectively as biogeochemical cycles, of evaporation and transpiration is called have been altered by human activities that release evapotranspiration. excess amounts of an element into the atmosphere, soil, or water. These changes can have significant 6 It may infiltrate, or penetrate, the soil and enter effects on ecosystems, landscapes, and the global the groundwater system, the water that fills the spaces in rocks and sediments below the soil. Blue Ridge Academy - Maricopa, CA system. For example, the release of nitrogen and phosphorus from agricultural fertilizers can result in 6 It may move across the land surface in rivers the over-fertilization of natural ecosystems. and streams or as runoff, rainfall draining from the land into waterbodies or sinking into the One of the main agents responsible for dissolving soil. and transporting the chemical elements necessary for FIGURE 29 The water cycle. Source: NOAA 2024–2025 Science Pentathlon Resource Guide 45  FIGURE 30 Blue Ridge Academy - Maricopa, CA The carbon cycle. Source: NOAA Eventually, streams and groundwater reach the ocean, as sugars and other food. To do this, they remove which is the ultimate reservoir of water on Earth. carbon dioxide from the atmosphere (by terrestrial Water in the ocean evaporates and forms clouds, which plants) and the ocean (by phytoplankton) and can be released as precipitation over the oceans or incorporate it into plant material such as leaves, roots, blown over land, and then the cycle will begin again and shoots. This process is called carbon fixation. (Figure 29). Because evaporation from the oceans is crucial to the water cycle, and energy from the Sun Plant carbon is returned to the atmosphere when plants drives evaporation, solar energy is the main energy respire at night, and when animals and microorganisms, source for the water cycle. which ultimately derive their nourishment from plants, respire. When organisms die, the carbon that was part of The Carbon Cycle the live biomass pool becomes part of the dead biomass Four processes run the carbon cycle—photosynthesis, pool and decomposes in the soil or ocean, allowing its respiration, decomposition, and combustion (Figure carbon and other elements to continue the cycle. A very 30). As you have already learned, green plants on land small fraction of organic matter present in the biosphere and phytoplankton in the oceans have the ability to gets buried in sediments before it can decompose and convert solar energy into chemical energy that is stored can be fossilized and eventually may turn into coal, oil, 2024–2025 Science Pentathlon Resource Guide 46 and natural gas. Hence, these compounds are called words, in the absence of human activity, the global fossil fuels. Combustion, as occurs in forest fires or the carbon cycle is approximately in a steady state. burning of fossil fuels, also releases carbon back into the environment. In fact, human activities have a major influence on the amount of carbon cycling at both the ecosystem and The greatest amount of carbon on Earth is tied up in global levels. The best known and most significant carbonate rock like limestone and organic matter in human alteration of the carbon cycle is the burning sedimentary rocks such as shale, but this abiotic pool of fossil fuels. Every time a person consumes coal, does not cycle very rapidly. There is also a very large natural gas (methane), oil, gasoline, or other products pool of carbon stored in the oceans, but it shows only a derived from petroleum, fossil carbon that was not part very small annual net gain. Despite this slow exchange, of the contemporary carbon biogeochemical cycle is the chemistry of the oceans is starting to change, and released to the environment. the oceans are becoming more acidic as a result of the extra carbon dioxide in the atmosphere. The movement The net destruction of vegetation through cutting and of carbon between the atmospheric and biospheric burning without replacement by similar vegetation pools is the most important path in the carbon cycle has also created a steadily increasing amount of because it cycles rapidly. In the absence of human carbon moving from the lithosphere (the buried coal, disturbance, the carbon exchange between land plants oil, and natural gas) and biosphere to the atmosphere and soils is in equilibrium with the atmosphere; there and oceans. Normally, if a forest is cut down and a is no net flux of carbon between these pools. In other new forest quickly begins to replace it, the net flux of Blue Ridge Academy - Maricopa, CA FIGURE 31 The nitrogen cycle. Source: Physicalgeography.net 2024–2025 Science Pentathlon Resource Guide 47 carbon to the atmosphere is not that great, especially use the organic matter as a food source and give off over the course of a few decades. In recent decades, ammonium cations. Ammonium is, in turn, converted large areas of forest, particularly tropical forest, have to nitrite (NO2−) and then to nitrate in a two-step process been converted to pastures, grassland, and crop lands, called nitrification. Nitrite is of minor importance in and the trees they contained have been burned and natural ecosystems, though it can be toxic to human not replaced, a practice known as slash-and-burn infants; nitrate, however, is an important plant nutrient. agriculture. In addition to destroying a great deal of biodiversity, the net destruction of forest adds Nitrate is susceptible to leaching—where an element significant amounts of carbon to the atmosphere. or molecule is washed out of soil by moving water— which occurs in almost all ecosystems. If a site The Nitrogen Cycle is disturbed, perhaps by logging or other human Nitrogen is critical for life on Earth because it is activities, nitrate leaching can be substantial and one of the elements in amino acids, which are basic can have a significant impact on rivers and streams. components of all organisms. In terrestrial biomes, the A high accumulation of nitrate in wet soils can lead movement of nitrogen from the atmosphere to plants, to denitrification, the natural conversion of nitrate through many transformations within the soil, and then to the gas nitrous oxide (N2O), which is emitted back into the atmosphere makes the nitrogen cycle to the atmosphere. Since N2O is a greenhouse gas, one of the more interesting and complex cycles among denitrification has important implications for the all the elements. (A similar process occurs in aquatic environment. Though denitrification is a natural ecosystems as well.) process, anthropogenic contributions to nitrate leaching can impact denitrification rates. Blue Ridge Academy - Maricopa, CA Of the macronutrients, nitrogen occurs in the highest concentration in plants, and in many terrestrial systems, Because nitrogen is often the limiting element in nitrogen is the limiting element for plants. Although terrestrial systems, the nitrogen cycle—and in the atmosphere is 78 percent nitrogen, most plants particular, the conversion of organic nitrogen to cannot use the dominant form of atmospheric nitrogen, nitrate—is extremely important in the regulation of N2 gas. Only organisms capable of nitrogen fixation, net primary productivity and plant growth in many the conversion of N2 gas to a plant-available form, ecosystems. The nitrogen cycle is a complex cycle with ammonium (NH4+), can make direct use of atmospheric significant effects on pollution and productivity. nitrogen. Cyanobacteria in water and soils and bacteria and fungi associated with certain legumes, such as peas, SECTION II SUMMARY and certain trees, such as alders, are nitrogen fixers. 6 The term biodiversity refers to the diversity of Virtually all other species must acquire nitrogen from all the genes, species, and habitats on Earth. intermediate sources, such as the soil. 6 Ultimately, all biodiversity derives from genetic diversity. Individuals inherit from their parents Most plants use nitrogen that has already been converted the genes that make up their unique genotype. to a mineral form. How does this conversion occur? Varying combinations of alleles, the alternative Atmospheric nitrogen is fixed