In looking at nature. . . never forget that every single organic being around us may be said to be striving to increase its numbers.---CHARLES DARWIN, 1859
3-1 EVOLUTION AND ADAPTATION
What Is Evolution? How do populations of organisms adapt to changes in environmental conditions? According to scientific evidence, the answer is through biological evolution, or evolution: the change in a population's genetic makeup (gene pool) through successive generations. Note that populations, not individuals, evolve by becoming genetically different.
According to the theory of evolution, all species descended from earlier, ancestral species. This widely accepted scientific theory explains how life has changed over the past 3.7 billion years (Figure 3-1, p.44) and why life is so diverse today.
Biologists use the term microevolution to describe the small genetic changes that occur in a population. The term macroevolution is used to describe longterm, large-scale evolutionary changes through which (1) new species are formed from ancestral species (speciation) and (2) other species are lost through extinction.
How Does Microevolution Work? A population's gene pool consists of all of the genes (Figure 2-3, p. 18) in its individuals. Microevolution is a change in a population's gene pool over time.
The first step in microevolution is the development of genetic variability in a population. Genetic variability in a population originates through mutations: random changes in the structure or number of DNA molecules in a cell. Mutations can occur in two ways:
. Exposure of DNA to external agents such as radioactivity, X rays, and natural and human-made chemicals (called mutagens)
. Random mistakes that sometimes occur in coded genetic instructions when DNA molecules are copied when a cell divides and an organism reproduces
Mutations can occur in any cells, but only those in reproductive cells are passed on to offspring.
Some mutations are harmless, but most are harmful and alter traits so that an individual cannot survive (lethal mutations). Every so often, a mutation is beneficial. The result is new genetic traits that give that individual and its offspring better chances for survival and reproduction (1) under existing environmental conditions or (2) when such conditions change. Mutations are (1) random and unpredictable, (2) the only source of totally new genetic raw material, and (3) rare events.
What Role Does Natural Selection Play in Microevolution? Natural selection occurs when some individuals of a population have genetically based and heritable traits that increase their chances of survival and their ability to produce offspring. Microevolution of a population by natural selection occurs when (1) a heritable trait is passed from one generation to another and (2) this trait leads to differential reproduction that enables individuals with the trait to leave more offspring than members of the population without the trait.
A heritable trait that enables organisms to better survive and reproduce under a given set of environmental conditions is called an adaptation, or adaptive trait. Structural adaptations include (1) coloration (allowing more individuals to hide from predators or to sneak up on prey), (2) mimicry (looking like a poisonous or dangerous species), (3) protective cover (shell, thick skin, bark, or thorns), and (4) gripping mechanisms (hands with opposable thumbs). Physiological adaptations include the ability to (1) hibernate during cold weather and (2) give off chemicals that poison or repel prey. Behavioral adaptations include the ability to fly to a warmer climate during winter and various interactions between species.
The process of microevolution can be summarized as follows: Genes mutate, individuals are selected, and populations evolve.
3-2 ECOLOGICAL NICHES AND ADAPTATION
What Is an Ecological Niche? If asked what role a certain species such as an alligator plays in an ecosystem, an ecologist would describe its ecological niche, or simply niche (pronounced "nitch"), the species' way of life or functional role in an ecosystem. This includes (1) its range of tolerance for various physical and chemical conditions, such as temperature (Figure 2-16, p. 29), (2) the types and amounts of resources it uses, (3) how it interacts with other living and nonliving components of the ecosystems in which it is found, and (4) the role it plays in the energy flow and matter cycling in an ecosystem (Figure 2-18, p. 31).
The ecological niche of a species is different from its habitat, or physical location, where it lives. Ecologists often say that a niche is like a species' occupation, whereas habitat is like its address.
A species' ecological niche represents the adaptations or adaptive traits that its members have acquired through microevolution. These traits enable its members to survive and reproduce more effectively under a given set of environmental conditions.
Is It Better to Be a Generalist or a Specialist Species?
Broad and Narrow Niches The niches of species can be used to classify them as generalists
or specialists. Generalist species have broad niches. They can (1) live in many
different places, (2) eat a variety of foods, and (3) tolerate a wide range of
environmental conditions. Flies, cockroaches (Spotlight, p. 46), mice, rats, white-tailed
deer, raccoons, coyotes, copperheads, channel catfish, and humans are generalist species.
Specialist species have narrow niches. They may be able to (1) live in only one type of habitat, (2) use only one or a few types of food, or (3) tolerate only a narrow range of climatic and other environmental conditions. This makes them more prone to extinction when environmental conditions change. Examples of specialists are (1) tiger salamanders, which can breed only in fishless ponds so their larvae will not be eaten, (2) red-cocknded woodpeckers, which carve nest holes almost exclusively in old (at least 75 years) longleaf pines, and (3) China's highly endangered giant pandas, which feed almost exclusively on various types of bamboo.
In a tropical rain forest, an incredibly diverse array of species survives by occupying specialized ecological niches in various distinct layers of vegetation exposed to different levels of light (Figure 3-2).
Figure 3-2 Stratification of specialized plant and animal niches in various layers of a tropical rain forest. The presence of these specialized niches enables species to avoid or minimize competition for resources and results in the coexistence of a great variety of species (biodiversity).
The widespread clearing and degradation of such forests is dooming millions of such specialized species to premature extinction.
Is it better to be a generalist than a specialist? It depends. When environmental conditions are fairly constant, as in a tropical rain forest, specialists have an advantage because they have fewer competitors. But under rapidly changing environmental conditions, the generalist usually is better off than the specialist.
What Limits Adaptation? Shouldn't evolution lead to perfectly adapted organisms? Shouldn't adaptations to new environmental conditions allow (1) our skin to become more resistant to the harmful effects of ultraviolet radiation, (2) our lungs to cope with air pollutants, and (3) our livers to become better at detoxifying pollutants? The answer to these questions is no because of the following limits to adaptations in nature:
. A change in environmental conditions can lead to adaptation only for traits already present in the gene pool of a population.
. Even if a beneficial heritable trait is present in a population, that population's ability to adapt can be limited by its reproductive capacity. Populations of genetically diverse species that reproduce quickly-such as weeds, mosquitoes, rats, bacteria, or cockroaches--often can adapt to a change in environmental conditions in a short time. In contrast, populations of species such as elephants, tigers, sharks, and humans, which cannot produce large numbers of offspring rapidly, take a long time (typically thousands or even millions of years) to adapt through natural selection.
. Even if a favorable genetic trait is present in a population, most of the population would have to die or become sterile so individuals with the trait could predominate and pass the trait on. This is hardly a desirable solution to the environmental problems the human species faces.
What Are Two Common Misconceptions About Evolution? There are two common misconceptions about evolution:
. "Survival of the fittest" means "survival of the strongest." To biologists, fitness is a measure of reproductive success not strength. Thus the fittest individuals are those that leave the most descendants.
. Evolution involves some grand plan of nature in which species become progressively more perfect. From a scientific standpoint, no plan or goal of perfection exists in the evolutionary process.
Cockroaches, the bugs many people love to hate, have (1) been around for about 350 million years and (2) are one of the great success stories of evolution. They are so successful because they are generalists.
The earth's 4,000 cockroach species can (1) eat almost anything (including algae, dead insects, fingernail clippings, salts in tennis shoes, electrical cords, glue, paper, and soap) and (2) live and breed almost anywhere except in polar regions.
Some species can go for months without food, survive for a month on a drop of water from a dishrag, and withstand massive doses of radiation. One species can survive being frozen for 48 hours.
They usually can evade their predators and a human foot in hot pursuit because (1) the antennae of most cockroach species can detect minute movements of air, (2) they have vibration sensors in their knee joints, and (3) they possess rapid response times (faster than you can blink). Some even have wings.
They also have high reproductive rates. In only a year, a single Asian cockroach (especially prevalent in Florida) and its young can add about 10 million new cockroaches to the world. Their high reproductive rate also helps them quickly develop genetic resistance to almost any poison we throw at them.
Most cockroaches also sample food before it enters their mouths and learn to shun foul-tasting poisons. They also clean up after themselves by eating their own dead and, if food is scarce enough, their living.
Only about 25 species of cockroach live in homes. However, such species can (1) carry viruses and bacteria that cause diseases such as hepatitis, polio, typhoid fever, plague, and salmonella and
(2) cause people to have allergic reactions ranging from watery eyes to severe wheezing. Indeed, about 60% of the 12 million Americans suffering from asthma are allergic to dead or live cockroaches.
Critical Thinking
If you could, would you exterminate all cockroach species? What might be some ecological consequences of doing this?
3-3 SPECIATION, EXTINCTION, AND BIODIVERSITY
How Do New Species Evolve? Under certain circumstances natural selection can lead to an entirely new species. In this process, called speciation, two species arise from one.
The most common mechanism of speciation (especially among animals) takes place in two phases: geographic isolation and reproductive isolation. In geographic isolation, members of the same population of a species become physically separated for long periods. For example, part of a population may migrate in search of food and then begin living in another area with different environmental conditions (Figure 3-3). Populations also may become separated (1) by a physical barrier (such as a mountain range, stream, lake, or road), (2) by a change such as a volcanic eruption or earthquake, or (3) when a few individuals are carried to a new area by wind or water.
The second phase of speciation is reproductive isolation. It occurs as mutation and natural selection change the gene pools of geographically isolated populations. If this process continues long enough, members of the geographically and reproductively isolated populations may become so different in genetic makeup that (1) they cannot interbreed, or (2) if they do, they cannot produce live, fertile offspring. Then one species has become two, and speciation has occurred.
For some rapidly reproducing organisms, this type of speciation may occur within hundreds of years. However, for most species, such speciation takes from tens of thousands to millions of years.
How Do Species Become Extinct? After speciation, the second process affecting the number and types of species on the earth is extinction. When environmental conditions change, a species must (1) evolve (become better adapted), (2) move to a more favorable area (if possible), or (3) cease to exist (become extinct).
Extinction is the ultimate fate of all species, just as death is for all individual organisms. Biologists estimate that 99.9% of all the species that have ever existed are now extinct.
As local environmental conditions change, a certain number of species disappear at a low rate, called background extinction. In contrast, mass extinction is a significant rise in extinction rates above the background level. It is a catastrophic, widespread event (such as climate change) in which large groups of existing species (perhaps 25-70%) are wiped out. Scientists have also identified periods of mass depletion in which extinction rates were much higher than normal but not high enough to classify as a mass extinction. Fossil and geological evidence suggests that there have been two mass extinctions and three mass depletions during the past 500 million years.
A mass extinction or mass depletion crisis for one species is an opportunity for another. The existence of millions of species today means that speciation, on average, has kept ahead of extinction. Evidence shows that the earth's mass extinctions and depletions have been followed by periods of recovery called adaptive radiations, in which numerous new species evolve to fill new or vacated ecological roles or niches in changed environments. Fossil records suggest that it takes 5 million years or more for adaptive radiations to rebuild biological diversity after a mass extinction or depletion.
Figure 3-3 How geographic isolation can lead to reproductive isolation, divergence, and speciation. [Arctic Fox]
How Do Speciation and Extinction Affect Biodiversity? Speciation minus extinction equals biodiversity, the planet's genetic raw material for future evolution in response to changing environmental conditions. In the long-term give-and-take between extinction and speciation, mass extinctions and depletions temporarily reduce biodiversity. However, they also create evolutionary opportunities for surviving species to undergo adaptive radiations to fill unoccupied and new biological roles or niches.
Although extinction is a natural process, much evidence indicates that humans have become a major force in the premature extinction of species. Biologist Stuart Primm estimates that during the 20th century, extinction rates increased by 100-1,000 times the natural background rate. As human population and resource consumption increase over the next 50-100 years, we are expected to take over more and more of the earth's surface and net primary productivity (NPP) (Figure 2-23, p. 35). During this century, this may cause the premature extinction of up to a quarter of the earth's current species. This could constitute a new mass depletion and possibly a new mass extinction.
On our short time scale, such major losses cannot be recouped by formation of new species; it took millions of years after each of the earth's past mass extinctions and depletions for life to recover to the previous level of biodiversity. Genetic engineering cannot stop this loss of biodiversity because genetic engineers do not create new genes. Rather, they transfer existing genes or gene fragments from one organism to another and thus rely on natural biodiversity for their raw material.
We can summarize the 3.7-billion-year biological history of the earth in one sentence: Organisms convert solar energy to food, chemicals cycle, and a variety of species with different biological roles (niches) has evolved in response to changing environmental conditions.
Each species here today represents a long chain of evolution, and each of these species plays a unique ecological role in the earth's communities and ecosystems.
These species, communities, and ecosystems also are essential for future evolution as the earth continues its long history of environmental change.
3-4 BIOMES: CLIMATE AND LIFE ON LAND
Why Do Different Organisms Live in Different Places? Biologists have classified the terrestrial (land) portion of the biosphere into biomes ("BYohms"). They are large regions such as forests, deserts, and grasslands characterized by (1) a distinct climate and (2) specific forms of life, especially vegetation, adapted to it.
Why is one area of the earth's land surface a desert, another a grassland, and another a forest? Why do different types of deserts, grasslands, and forests exist?
The general answer to these questions is differences in climate: a region's general pattern of atmospheric or weather conditions over a long period. Average temperature and average precipitation are the two main factors determining a region's climate. Figure 3-4 shows the global distribution of biomes. Figure 3-5 shows major biomes in the United States as one moves through different climates along the 39th parallel.
Figure 3-4 The earth's major biomes-the main types of natural vegetation in different undisturbed land areas-result primarily from differences in climate. Each biome contains many ecosystems whose communities have adapted to differences in climate, soil, and other environmental factors. In some areas, people have altered these biomes by removing or changing much of this natural vegetation for farming, livestock grazing, lumber and fuelwood, mining, and construction.
For plants, precipitation generally is the limiting factor that determines whether a land area is (1) desert (with low precipitation and sparse, widely spaced, mostly low vegetation), (2) grassland (with enough precipitation to support grass but not large stands of trees), and (3) forest (with enough precipitation to support stands of various tree species and smaller forms of vegetation).
Taken together, average annual precipitation and temperature (along with soil type) are the most important factors in producing tropical, temperate, or polar deserts, grasslands, and forests (Figure 3-6, p. 50). On maps such as the one in Figure 3-4, biomes are presented as (1) having sharp boundaries and (2) being covered with the same general type of vegetation. In reality, biomes do not have sharp boundaries and are not uniform. Instead, the types and numbers of plants in a biome vary from one location to another because of variations in (1) local climate (microclimates), (2) soil types, and (3) natural and human-caused disturbances.
Figure 3-7 (p. 50) shows how climate and vegetation vary with latitude (distance from the equator) and altitude (elevation above sea level). If you travel from the equator toward either pole, you will generally encounter colder climates and zones of vegetation adapted to those climates (Figure 3-7, right). Similarly, as elevation above sea level increases, climate becomes colder (Figure 3-7, left). Thus if you climb a tall mountain from its base to its summit, you can observe changes in plant life similar to those you would encounter in traveling from the equator to the earth's poles (Figure 3-7).
What Impacts Do Humans Have on Deserts, Grasslands, and Forests? Figure 3-8 (p. 51) shows some major components and interactions in a temperate desert biome, and Figure 3-9 (p. 52) shows major human impacts on deserts. Deserts take a long time to recover from disturbances because of their (1) slow plant growth, (2) low species diversity, (3) slow nutrient cycling (because of little bacterial activity in their soils), and (4) water shortages. Desert vegetation destroyed by livestock overgrazing and off-road vehicles may take decades to grow back.
Figure 3-5 Major biomes found along the 39th parallel across the United States. The differences reflect changes in climate, mainly differences in average annual precipitation and temperature (not shown).
Figure 3-6 Average precipitation and average temperature, acting together as limiting factors over a period of 30 or more years, determine the type of desert, grassland, or forest biome in a particular area. Although the actual situation is much more complex, this simplified diagram explains how climate determines the types and amounts of natural vegetation found in an area left undisturbed by human activities. (Used by permission of Macmillan Publishing Company, from Derek Elsom, The Earth, New York: Macmillan, 1992. Copyright @ 1992 by Marshall Editions Developments Limited)
Mountain ice and snow
Tundra (herbs, lichens, mosses)
Coniferous Forest
Deciduous Forest
Latitude
Tropical Forest
Tropical Forest
Deciduous Forest
Coniferous Forest
Tundra (herbs, lichens, mosses)
Polar ice and snow
Figure 3-7 Generalized effects of altitude (left) and latitude (right) on climate and biomes. Parallel changes in vegetation type occur when we travel from the equator to the poles or from lowlands to mountaintops.
Figure 3-8 Some components and interactions in a temperate desert ecosystem. When these organisms die, decomposers break down their organic matter into minerals used by plants. Arrows indicate transfers of matter and energy among producers, primary consumers (herbivores), and secondary (or higher-level) consumers (carnivores). Organisms are not drawn to scale.
Large desert cities
Soil destruction by vehicles and urban development
Soil salinization from irrigation
Depletion of underground water supplies
Land disturbance and pollution from mineral extraction
Storage of toxic and radioactive wastes
Large arrays of solar cells and solar collectors used to produce electricity
Figure 3-9 Major human impacts on deserts.
Conversion of savanna and temperate grassland to cropland
Release of CO2 to atmosphere from burning and conversion of grassland to cropland
Overgrazing of tropical and temperate grasslands by livestock
Damage to fragile arctic tundra by oil production, air and water pollution, and vehicles
Figure 3-10 Major human impacts on grasslands.
Clearing and degradation of tropical forests for agriculture, livestock grazing, and timber harvesting
Clearing of temperate deciduous forests in Europe, Asia, and
North America for timber, agriculture, and urban development
Clearing of evergreen coniferous forests in North America, Finland, Sweden, Canada, Siberia, and Russia
Conversion of diverse forests to less biodiverse tree plantations
Figure 3-11 Major human impacts on forests.
Figure 3-10 lists major human impacts on grasslands. Because of their thick and fertile soils, temperate grasslands are widely used to grow crops. However, plowing breaks up the complex soil structure and leaves it vulnerable to erosion by wind and water.
Figure 3-11 lists major human impacts on the world's forests. Large areas of the world's temperate forests have been cleared to grow crops and build urban areas. Tropical forests are also being cleared rapidly for agriculture, timber, and mining.
3-5 LIFE IN WATER ENVIRONMENTS
What Are Aquatic Life Zones? The aquatic equivalents of biomes are called aquatic life zones. The major types of organisms found in aquatic environments are determined by the water's salinity (the amounts of various salts such as sodium chloride dissolved in a given volume of water). As a result, aquatic environments are divided into two major types: (1) saltwater or marine (such as oceans, estuaries, coastal marshes, mangrove swamps, and coral reefs) and (2) freshwater (such as lakes, rivers, and inland wetlands).
Most aquatic life zones can be divided into three layers: surface, middle, and bottom. Important environmental factors determining the types and numbers of organisms found in these layers are (1) temperature, (2) access to sunlight for photosynthesis, (3) dissolved oxygen content, and (4) availability of nutrients such as carbon (as dissolved CO2 gas), nitrogen (as nitrate), and phosphorus (mostly as phosphate) for producers.
What Are the Major Saltwater Life Zones? A more accurate name for Earth would be Ocean because saltwater oceans cover about 71% of the planet's surface (Figure 3-12). Figure 3-13 lists important ecological and economic services provided by these marine systems. Despite its ecological and economic importance less than 5% of the earth's global ocean has been explored and mapped with the same level of detail as the surface of the moon and Mars.
Oceans have two major life zones: the coastal zone and the open sea (Figure 3-14, p. 54). Although it makes up less than 10% of the world's ocean area, the coastal zone contains 90% of all marine species and is the site of most large commercial marine fisheries. This zone has numerous interactions with the land and thus human activities easily affect it.
Most ecosystems found in the coastal zone have a very high NPP per unit of area (Figure 2-23, p. 35). This occurs because of the zone's ample supplies of (1) sunlight and (2) plant nutrients (flowing from land and distributed by wind and ocean currents).
Figure 3-12 The ocean planet. The salty oceans cover about 71 % of the earth's surface. About 97% of the earth's water is in the interconnected oceans, which cover 90% of the planet's mostly ocean hemisphere (left) and 50% of its land-ocean hemisphere (right).
Important ecosystems found in the coastal zone include
. Estuaries, which are partially enclosed areas of coastal water where seawater mixes with fresh water and nutrients from rivers, streams, and runoff from land.
. Coastal wetlands, which are land areas covered with water all or part of the year. They include (1) mangrove forest swamps in tropical waters and (2) salt marshes in temperate zones (Figure 3-15, p. 55).
. Coral reefs that form in the shallow coastal zones of warm tropical and subtropical oceans. These beautiful natural wonders are among the world's most diverse and productive ecosystems and are homes for about one-fourth of all marine species.
Figure 3-13 Major ecological and economic services provided by marine systems.
The open sea is divided into three vertical zoneseuphotic, bathyal, and abyssal-based primarily on the penetration of sunlight (Figure 3-14). This vast volume contains only about 10% of all marine species. Average NPP per unit of area is quite low in the open sea (Figure 2-23, p. 35) except at an occasional equatorial upwelling, where currents bring up nutrients from the ocean bottom. Overall productivity in the open sea is low because (1) sunlight cannot penetrate the lower layers and bring about photosynthesis and (2) the surface layer normally has fairly low levels of nutrients for phytoplankton, which are the main photosynthetic producers of the open ocean. However, because the open sea covers so much of the earth's surface (Figure 3-12), it makes the largest contribution to the earth's overall NPP.
What Are the Major Freshwater Life Zones? Although freshwater lakes, streams, rivers, and inland wetlands occupy only 1% of the earth's surface, they provide important ecological and economic services (Figure 3-16, p. 56).
Lakes are large natural bodies of standing fresh water formed when precipitation, runoff, or groundwater seepage fills depressions in the earth's surface. Lakes normally consist of distinct zones (Figure 3-17, p. 56), which provide habitats and niches for different species.
Ecologists classify lakes according to their nutrient content and primary productivity. A newly formed lake generally has a small supply of plant nutrients and is called an oligotrophic (poorly nourished) lake (Figure 3-18, top, p. 57). Such nutrient-poor lakes (1) are often deep and (2) have crystal-clear blue or green water because their low NPP supports few algae and other producers.
Over time, sediment washes into an oligotrophic lake, and plants grow and decompose to form bottom sediments. A lake with a large or excessive supply of nutrients (mostly nitrates and phosphates) needed by producers is called a eutrophic (well-nourished) lake (Figure 3-18, bottom, p. 57). Such nutrient-rich lakes typically (1) are shallow, (2) have a high NPP, and (3) have murky brown or green water with very poor visibility because of their high content of algae and other producers. Many lakes fall somewhere between the two extremes of nutrient enrichment and are called mesotrophic lakes.
Figure 3.14 Major life zones in an ocean. (Actual depths of zones may vary.)
Precipitation that does not sink into the ground or evaporate is surface water. It becomes runoff when it flows into streams. The land area that delivers runoff, sediment, and dissolved substances to a stream is called a watershed, or drainage basin. In many areas, streams begin in mountainous or hilly areas that collect and release water falling to the earth's surface as rain or snow. The downward flow of water in a river system from mountain highlands to the sea takes place in three different aquatic life zones (Figure 3-19, p. 57). Because of different environmental conditions in each zone, a river system is a series of different ecosystems with different average depths, flow rates, dissolved oxygen levels, temperatures, and aquatic species.
As streams flow downhill, they become powerful shapers of land. Over millions of years the friction of moving water levels mountains and cuts deep canyons, and the rock and soil the water removes are deposited as sediment in low-lying areas.
Inland wetlands are lands covered with fresh water all or part of the time (excluding lakes, reservoirs, and streams) and located away from coastal areas. They include (1) marshes dominated by grasses, (2) prairie potholes, which are depressions carved out by glaciers, (3) swamps dominated by trees and shrubs, and (4) floodplains, which receive excess water during heavy rains and floods.
Some of these wetlands are covered with water year round. Others, called seasonal wetlands, usually are underwater or soggy for only a short time each year. They include prairie potholes, floodplain wetlands, and bottomland hardwood swamps. Some stay dry for years before being covered with water again.
Figure 3-15 Some components and interactions in a salt marsh ecosystem in a temperate area such as the United States. When these organisms die, decomposers break down their organic matter into minerals used by plants. Arrows indicate transfers of matter and energy between consumers (herbivores) and secondary (or higher-level) consumers (carnivores). Organisms are not drawn to scale.
What Are the Impacts of Human Activities on Aquatic Systems? Despite the ecological and economic importance of marine systems (Figure 3-13) and freshwater systems (Figure 3-16), these aquatic ecosystems are under severe stress from human activities. These harmful impacts include the following:
Species Loss and Endangennent
. At least 1,200 marine species have become extinct in the past few hundred years mostly because of human activities.
. About one-third of all known fish species and onehalf of all known freshwater fish species are threatened with extinction during this century as a result of human activities such as (1) overfishing, (2) habitat destruction and degradation, and (3) pollution.
Marine Habitat Loss and Degradation
. Half the world's original coastal and inland wetlands have been filled in for agriculture and urban development since 1800.
. About 27% of the world's coral reefs are gone or seriously threatened, and 70% could be gone within 50 years.
. At least 35% of the world's coastal mangrove forest swamps have disappeared, mostly because of clearing for coastal development, growing crops, and aquaculture shrimp farms.
. Almost 70% of the world's beaches are eroding rapidly because of coastal developments and a rising sea level (caused mostly by global warming).
Figure 3-16 Major ecological and economic services provided by freshwater systems.
Figure 3-17 The distinct zones of life in a fairly deep temperate zone lake.
Figure 3-18 An oligotrophic, or nutrient-poor, lake (top) and a eutrophic, or nutrient-rich, lake (bottom). Mesotrophic lakes fall between these two extremes of nutrient enrichment. Nutrient inputs from human activities can accelerate eutrophication.
Overfishing
. About 75% of the world's 200 commercially valuable marine fish species are either overfished or fished to the limit.
. Overfishing can lead to depletion and extinction of species such as sea turtles, dolphins, and other marine mammals that are unintentionally caught as bycatch.
Figure 3-19 A river system consists of three zones in which water flows downhill from mountain highlands to the sea. They are the (1) source zone containing mountain (headwater) streams, (2) transition zone containing wider, lower-elevation streams, and (3) flood plain zone containing rivers, which empty into the ocean.
Nonnative Species
. Hundreds of nonnative species have been deliberately or accidentally introduced into coastal waters, lakes, and wetlands throughout the world. These bioinvaders displace or cause the extinction of native species and disrupt ecosystem functions.
According to aquatic scientists, the scientific investigation of aquatic systems is a poorly funded research frontier whose study could result in immense ecological and economic benefits.
3-6 POPULATION DYNAMICS AND CARRYING CAPACITY
What Limits Population Growth? Four variables-births, deaths, immigration, and emigrationgovern changes in population size. A population gains individuals by birth and immigration and loses them by death and emigration:
Population change = (Births + Immigration) - (Deaths + Emigration)
Populations vary in their capacity for growth, also known as the biotic potential of the population. The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. Generally, individuals in populations with a high intrinsic rate of increase (1) reproduce early in life, (2) have short generation times (the time between successive generations), (3) can reproduce many times (have a long reproductive life), and (4) have many offspring each time they reproduce.
Some species have an astounding biotic potential. For example, without any controls on its population growth, the ancestors of a single female housefly could (1) total about 5.6 trillion flies within about 13 months and (2) cover the earth's entire surface within a few years.
However, this is not a realistic scenario because no population can grow indefinitely. In the real world, a rapidly growing population reaches some size limit imposed by (1) a shortage of one or more limiting factors (such as light, water, space, or nutrients) or (2) too many competitors or predators. There are always limits to population growth in nature.
Environmental resistance consists of all the factors acting jointly to limit the growth of a population. Together biotic potential and environmental resistance determine the carrying capacity (K), the number of individuals of a given species that can be sustained indefinitely in a given space (area or volume).
What Is the Difference Between Exponential and Logistic Population Growth? A population with essentially unlimited access to resources grows exponentially. Exponential growth starts out slowly and then proceeds faster and faster as the population increases. Plotting the number of individuals against time yields a I-shaped exponential growth curve (Figure 3-20a).
Logistic growth involves (1) exponential population growth when there is a steady decrease in population growth with time as the population encounters environmental resistance and approaches the carrying capacity of its environment and levels off. After leveling off, a population with this type of growth typically fluctuates slightly above and below the carrying capacity. A plot of the number of individuals against time yields a sigmoid, or S-shaped, logistic growth curve (Figure 3-20b). Figure 3-21 shows the logistic growth of the sheep population on the island of Tasmania, south of Australia, in the early 19th century.
Figure 3-20 Theoretical population growth curves. (a) Exponential growth, in which the population's growth rate increases with time. Exponential growth occurs when resources are not limiting and a population can grow at its intrinsic rate of increase (r). Exponential growth of a population cannot continue forever because eventually some factor limits population growth. (b) Logistic growth, in which the growth rate decreases as the population gets larger. With time, the population size stabilizes at or near the carrying capacity (K) of its environment and results in a sigmoid (5-shaped) population growth curve.
Figure 3-21 Logistic growth of a sheep population on the island of Tasmania between 1800 and 1925. After sheep were introduced in 1800, their population grew exponentially because of ample food. By 1855, they overshot the land's carrying capacity. Their numbers then stabilized and oscillated around a carrying capacity of about 1.6 million sheep.
What Happens If the Population Size Exceeds the Carrying Capacity? The populations of some species do not make such a smooth transition from exponential growth to logistic growth. Instead, such a population (1) uses up its resource base temporarily (for example, by eating more plants or animals than can be replenished) and (2) temporarily overshoots, or exceeds, the carrying capacity of its environment. This overshoot occurs because of a reproductive time lag, the period needed for the birth rate to fall and the death rate to rise in response to resource overconsumption.
In such cases, the population suffers a dieback, or crash, unless the excess individuals switch to new resources or move to an area with more favorable conditions. Such a crash occurred when reindeer were introduced onto a small island off the southwest coast of Alaska (Figure 3-22).
Humans are not exempt from population overshoot and dieback. Ireland experienced a population crash after a fungus destroyed the potato crop in 1845. About 1 million people died, and 3 million people emigrated to other countries.
Technological, social, and other cultural changes have extended the earth's carrying capacity for the human species. For example, we have increased food production and used large amounts of energy and matter resources to make normally uninhabitable areas of the earth habitable. However, there is growing concern about how long we will be able to keep doing this on a planet (1) with a finite size and resources and (2) a human population whose size (Figure 1-1, p. 2) and per capita resource use are growing exponentially.
How Do Species Reproduce? Reproductive individuals in populations of all species engage in a struggle for genetic immortality by trying to have as many members as possible in the next generation carrying their genes. Species use different reproductive patterns to help ensure their survival.
At one extreme are species that reproduce early and put most of their energy into reproduction. They (1) have many (usually small) offspring each time they reproduce, (2) reach reproductive age rapidly, (3) have short generation times, (4) give their offspring little or no parental care or protection to help them survive, and (5) are short lived (usually with a life span of less than a year). Species with this reproductive pattern overcome the massive loss of their offspring by producing so many unprotected young that a few will survive to reproduce many offspring to begin the cycle again.
Examples are algae, bacteria, rodents, and most insects (Spotlight, p. 24). Such species tend to be opportunists. They reproduce and disperse rapidly when conditions are favorable or when a new habitat or niche becomes available.
Changed environmental conditions from disturbances can allow opportunist species to gain a foothold. However, once established, their populations may crash because of (1) changing or unfavorable environmental conditions or (2) invasion by more competitive species. Therefore, most opportunist species go through irregular and unstable boom-and-bust cycles in their population size.
Figure 3.22 Exponential growth, overshoot, and population crash of reindeer introduced to a small island off the southwest coast of Alaska. When 26 reindeer (24 of them female) were introduced in 1910, lichens, mosses, and other food sources were plentiful. By 1935, the herd's population had soared to 2,000, overshooting the island's carrying capacity. This led to a population crash, with the herd plummeting to only 8 reindeer by 1950.
At the other extreme are competitor species that (1) put fairly little energy into reproduction, (2) tend to reproduce late in life, (3) have few offspring with long generation times, and (4) put most of their energy into nurturing and protecting their young until they reach reproductive age.
Typically the offspring of such species (1) develop inside their mothers (where they are safe), (2) are fairly large, (3) mature slowly, and (4) are cared for and protected by one or both parents until they reach reproductive age. This reproductive pattern results in a few big and strong individuals that can compete for resources and reproduce a few young to begin the cycle again.
Such species tend to maintain their population size near the carrying capacity of their environment. Their populations typically follow a logistic growth curve (Figures 3-20b and 3-21). Examples are (1) most large mammals (such as elephants, whales, and humans), (2) birds of prey, and (3) large and long-lived plants (such as the saguaro cactus, oak trees, redwood trees, and most tropical rain forest trees). Many of these species, especially those with long generation times and low reproductive rates (such as elephants, rhinoceroses, and sharks) are prone to extinction. The reproductive patterns of most species fall somewhere between these two extremes.
The reproductive pattern of a species may give it a temporary advantage, but the availability of suitable habitat for individuals of a population in a particular area determines its ultimate population size. Regardless of how fast a species can reproduce, there can be no more dandelions than there is dandelion habitat and no more zebras than there is zebra habitat in a particular area.
Nothing in biology makes sense except in the light of evolution. THEODOSIUS DOBZHANSKY