1. Organisms and their environments
Chapter 15: Ecosystems and Communities
Organisms and their environments
by Mark Manteuffel, St. Louis Community College

2. Learning Objectives Be able to explain the following:
What are ecosystems?
How do energy and chemicals move through ecosystems?

3. Learning Objectives Be able to explain the following:
How do species interactions influence the structure of communities?
How do communities remain stable or change over time?

4. 15.1–15.2 Ecosystems
have living and nonliving
components.

7. 15.1 What are ecosystems?
A community of biological organisms plus the non-living components with which the organisms interact.

8. Picture a lush garden: some greenery, a bit of debris from rotting wood, and abundant wildlife. Grazing animals abound, while predators feed on other animals and their eggs. Parasites are poised, looking for hosts, and, just below the surface, scavengers find meals among the organic detritus.
But now, imagine that the entire scene gets up and walks away! The “camera” in your mind pulls back to reveal that the entire scene is playing out on the back of a beetle no more than two inches long (Figure 15-1 A small-scale ecosystem).
The host of this mini-ecosystem is a beetle from New Guinea called the large weevil. The weevil is camouflaged from its predators by lichens—fungi and photosynthetic algae living together—while the lichens are given a safe surface on which to live. And the garden of lichens supports a wide range of other organisms, from tiny mites to a variety of other microscopic invertebrates, some free-living and some parasitic.

9. What is important is that the two essential elements of an ecosystem are present: the biotic environment and the physical (abiotic) environment (Figure 15-2 What makes up an ecosystem?).
The biotic environment consists of all the living organisms within an area and is often referred to as a community.
The physical environment, often referred to as the organisms’ habitat, consists of:
the chemical resources of the soil, water, and air, such as carbon, nitrogen, and phosphorus, and
the physical conditions, such as the temperature
Biologists view communities of organisms and their habitats as “systems” in much the same way that engineers might, hence the term ecosystem. Biologists monitor the inputs and outputs of the system, tracing the flow of energy and various molecules as they are captured, transformed, and utilized by organisms and later exit the system or are recycled.
They also study how the activities of one species affect the other species in the community—whether the species have a conflicting relationship, such as predator and prey, or a complementary relationship, such as flowering plants and their pollinators.
Regardless of size, the same principles of ecosystem study apply: observe and analyze organisms and their environments, while monitoring everything that goes into and comes out of the system: salinity (salt level), moisture, humidity, and energy sources.

10. Take-home message 15.1
An ecosystem is a community of biological organisms plus the non-living components in the environment with which the organisms interact.

11. Take-home message 15.1
Ecosystems are found not just in obvious places such as ponds, deserts, and tropical rainforests but also in some unexpected places, like the digestive tracts of organisms or the shell of a beetle.

12. 15.2 A variety of biomes occur around the world, each determined by temperature and rainfall.
Biomes cover huge geographic areas of land or water—the deserts that stretch almost all the way across the northern part of Africa, for example. Terrestrial (land) biomes are defined and usually described by the predominant types of plant life in the area. But looking at a map of the world’s terrestrial biomes, it is clear that they are mostly determined by the weather.

13. Biomes What is the average temperature?
What is the average rainfall (or other precipitation)?
Is the temperature relatively constant or does it vary seasonally?
Is the rainfall relatively constant or does it vary seasonally?
Specifically, when defining terrestrial biomes, we ask four questions about the weather:
What is the average temperature?
What is the average rainfall (or other precipitation)?
Is the temperature relatively constant, or does it vary seasonally?
Is the rainfall relatively constant, or does it vary seasonally?

14. Biomes Temperature and precipitation dictate:
Primary productivity levels
the amount of organic matter produced
The numbers and types of primary producers:
are the chief determinants of the amount and breadth of other life in the region.
By knowing the answers to these questions, it is possible to predict, with great accuracy, the type of biome. This is because temperature and precipitation dictate the primary productivity levels, or the amount of organic matter produced, primarily through photosynthesis.
And the numbers and types of primary producers—the organisms responsible for the primary productivity, such as grasses, trees, and agricultural crops—are, in turn, the chief determinants of the amount and breadth of other life in the region.

15. For example, where it is always moist and the temperature does not vary across the seasons, tropical rainforests develop. And where it is hot but with strong seasonality that brings a “wet” season and a “dry” season, savannas or tropical seasonal forests tend to develop. At the other end of the spectrum, in dry areas with a hot season and a cold season, temperate grasslands or deserts develop.
Figure 15-3 (Terrestrial ecosystem diversity) shows examples of the nine chief terrestrial biomes; all are determined, in large part, by the precipitation and temperature levels.

16. Aquatic biomes are defined a bit differently, usually based on physical features such as salinity, water movement, and depth. Chief among these environments are 1) lakes and ponds, with non-flowing fresh water; 2) rivers and streams, with flowing fresh water; 3) estuaries and wetlands, where salt water and fresh water mix in a shallow region characterized by exceptionally high productivity; 4) open oceans, with deep salt water; and 5) coral reefs, highly diverse and productive regions in shallow oceans (Figure 15-4 Aquatic ecosystem diversity).

17. Take-home message 15.2
Biomes are the major ecological communities of earth, characterized mostly by the vegetation present.
Different biomes result from differences in temperature and precipitation, and the extent to which they vary from season to season.

18. 15.6–15.8 Energy and chemicals flow within ecosystems.
All life on earth is made possible because energy flows perpetually from the sun to the earth. 18

19. 15.6 Energy flows from producers to consumers.
The sun is where our pathway of energy flow begins. Most of the energy is absorbed or reflected by the earth’s atmosphere or surface, but about 1% of it is intercepted and converted to chemical energy through photosynthesis.
That intercepted energy is then transformed again and again by living organisms, making about four stops as it passes through an ecosystem. Let’s examine what happens at each stop, known as trophic levels. 19

20. First Stop: Primary Producers
First stop: primary producers. When it comes to energy flow, all the species in an ecosystem can be placed in one of two groups: producers or consumers. Plants (along with some algae and bacteria) are, as we noted earlier, the primary producers. They convert light energy from the sun into chemical energy through photosynthesis. We use another word to describe that chemical energy: food. 20

21. Second Stop: Primary Consumers—the Herbivores
Second stop: primary consumers—the herbivores. Cattle grazing in a field, gazelle browsing on herbs, insects devouring the leaves of a crop plant—these are the primary consumers in an ecosystem, the animals that eat plants. Plant material such as cellulose can be difficult to digest. Consequently, most herbivores—animals that eat plants—need a little help in digesting the plants they eat. Primary consumers, from termites to cattle, often have symbiotic bacteria living in their digestive system. These microorganisms break down the cellulose, enabling the herbivore to harness the energy held in the chemical bonds of the plants’ cells. 21

22. Third Stop: Secondary Consumers—the Carnivores
Third stop: secondary consumers—the carnivores. Energy originating from the sun is converted into chemical energy within a plant’s tissue. The herbivore that eats the plant breaks down the chemical bonds, releasing the energy. This energy fuels the herbivore’s growth, reproduction, and movement, but the energy doesn’t remain in the herbivore forever. Carnivores, such as cats, spiders, and frogs, are animals that feed on herbivores. They are also known as secondary consumers. As they eat their prey, some of the energy stored in the chemical bonds of the carbohydrate, protein, and lipid molecules is again captured and harnessed for their own movement, reproduction, and growth. 22

23. Fourth Stop: Tertiary Consumers—the “Top” Carnivores
Fourth stop: tertiary consumers—the “top” carnivores. In some ecosystems, energy makes yet another stop: the tertiary consumers, or “top carnivores.” These are the “animals that eat the animals that eat the animals that eat the plants.” They are several steps removed from the initial capture of solar energy by a plant, but the general process is the same. A top carnivore, such as a tiger, eagle, or great white shark, consumes other carnivores, breaking down their tissue and releasing energy stored in the chemical bonds of the cells. As in each of the previous steps, the top carnivores harness this energy for their own physiological needs. 23

24. Chains or Webs? Food chain Food web
Pathway from photosynthetic producers through the various levels of animals
Food web
Involve harvesting energy from multiple stops in the food chain
This path from primary producers to tertiary consumers is called a food chain.  
The food chain pathway from photosynthetic producers through the various levels of animals, though, is a slight oversimplification.
In actuality, food chains are better thought of as food webs because many organisms are omnivores and can occupy more than one position in the chain. When you eat a simple meal of rice and chicken, after all, you’re simultaneously a carnivore and an herbivore. On average, about 30 to 35% of the human diet comes from animal products and the remaining 65 to 70% comes from plant products. Many other animals, from bears to cockroaches, also have diets that involve harvesting energy from multiple stops in the food chain. 24

25. Food Chain
Energy Flow

26. Food Web

27. In every ecosystem, as energy is transformed through the steps of a food chain, organic material accumulates in the form of animal waste and dead plant and animal matter.
Decomposers, usually bacteria or fungi, and detritivores, including scavengers such as vultures, worms, and a variety of arthropods, break down the organic material, harvesting energy still stored in the chemical bonds (Figure 15-13).
Because the decomposers are able to break down a much larger range of organic molecules, they are distinguished from the detritivores.
Both groups, nonetheless, release many important chemical components from the organic material than can eventually be recycled and utilized by plants and other primary producers. 27

28. Energy Flows Losses at every “step” in a food chain
Inefficiency of energy transfers
Energy flows from one stop to the next in a food chain, but not in the way that runners pass a baton in a relay race. The difference is that, at every step in the food chain, much of the usable energy is lost as heat. An animal that eats five pounds of plant material doesn’t convert that into five new pounds of body weight. Not by a long shot.
In the next section, we’ll see how this inefficiency of energy transfers ensures that most food chains are very short. 28

29. Take-home message 15.6
Energy from the sun passes through an ecosystem in several steps known as trophic levels.
First, primary producers convert light energy to chemical energy in photosynthesis.

30. Take-home message 15.6
Herbivores then consume the primary producers, the herbivores are consumed by carnivores, and the carnivores, in turn, may be consumed by top carnivores.

31. Take-home message 15.6
Detritivores and decomposers extract energy from organic waste and the remains of organisms that have died.
At each step in a food chain, some usable energy is lost as heat. 31

32. 15.7 Energy pyramids reveal the inefficiency of food chains.
Why are big, fierce animals so rare? And why are there so many more plants than animals?
The answers are closely related to our earlier observation that an animal consuming five pounds of plant material does not gain five pounds in body weight from such a meal. The actual amount of growth such a meal can support is far, far less. And when that herbivore is consumed by a carnivore, the carnivore, too, can convert only a small fraction of the energy it consumes into its own tissue. The fraction turns out to be about 10%, and it is fairly consistent across all levels of the food chain. 32

33. Biomass 10% rule Where does the rest go?
Expended in cellular respiration or lost as feces
This means that only about 10% of the biomass—the total weight of all the living organisms in a given area—of plants in an ecosystem is converted into herbivore biomass. So the herbivore consuming five pounds of plant material is likely to gain only about half a pound in new growth, while the remaining 90% of the meal is either expended in cellular respiration or lost as feces. Similarly, a carnivore eating the herbivore converts only about 10% of the mass it consumes into its own body mass. Again, 90% is lost to metabolism and feces. And the same inefficiency holds for a top carnivore, as well.
Let’s explore how this 10% rule limits the length of food chains and is responsible for the rarity of big, fierce animals outside your window and across the world. 33

34. The 10% rule in application
In humans, why is vegetarianism more energetically efficient than meat-eating?
Given the 10% efficiency with which herbivores convert plant biomass into their own biomass, how much plant biomass is necessary to produce a single 1200-pound (500- kg) cow? On average, that cow would need to eat about 12,000 pounds (5000 kg) of grain in order to grow to weigh 1200 pounds. But that 1200-pound cow, when eaten by a carnivore, could only add about 120 pounds of biomass to the carnivore, and only 12 pounds to a top carnivore.
That’s a huge amount of plant biomass required to generate only a very tiny amount of our top carnivore, which explains why big, fierce animals are so rare (and why vegetarianism is more energetically efficient than meat-eating). After all, multiply that 5000 kg of grain by several hundred (or thousands, more appropriately) to get an idea of how much plant matter would be necessary to support even a small population of top carnivores: Millions of kilograms of grain can support only a few top carnivores.
The 10% rule in application 34

35. Why are big, fierce animal species so rare in the world?
How much would be required to support an even higher link in the food chain? Ten times as much, so much that there may not be enough land in the ecosystem to produce enough plant material. And even if there were, the area required would be so large that the “top, top carnivores” might be so spread out and so busy trying to eat enough that they would be unlikely to encounter each other in order to mate. Hence, the 10% rule limits the length of food chains. 35

36. We can illustrate the path of energy through the organisms of an ecosystem with an energy pyramid in which each layer of the pyramid represents the biomass of a trophic level. In Figure (Inefficiencies in the transfer of biomass), we can see that, for terrestrial ecosystems, the biomass (in kilograms per square meter) found in the photosynthetic organisms, at the base of the pyramid, is reduced significantly at each step, given the incomplete utilization by organisms higher up the food chain. 36

37. Take-home message 15.7
Energy pyramids reveal that the biomass of primary producers in an ecosystem tends to be far greater than the biomass of herbivores.

38. Take-home message 15.7
Similarly, the biomass transferred at each successive step along the food chain tends to be only about 10% of the biomass of the organisms consumed.
As a consequence of this inefficiency, food chains rarely exceed four levels.

39. 15.8 Essential chemicals cycle through ecosystems.
The recycling of molecules
What is necessary for life? Energy and some essential chemicals top the list.
New energy continually comes to earth from the sun, fulfilling the first need.
And everything else is already here. The chemicals just cycle around and around, using the same pathway taken by energy—the food chain. Plants and other producers generally take up the molecules from the atmosphere or the soil. Then, the chemicals move into animals as they consume plants or other animals, and thus move up the food chain. And, as the plants and animals die, detritivores and decomposers  return the chemicals to the abiotic environment. From a chemical perspective, life is just a continuous recycling of molecules. 39

40. Chemical Reservoirs
Each chemical is stored in a non-living part of the environment.
Organisms acquire the chemical from the reservoir.
The chemical cycles through the food chain.
Eventually, the chemical is returned to the reservoir.
Chemicals cycle through the living and non-living components of an ecosystem.
Each chemical is stored in a non-living part of the environment called a reservoir.
Organisms acquire the chemical from the reservoir, the chemical cycles through the food chain, and eventually, it is returned to the reservoir.
This is a useful overview, but we can get a deeper appreciation of the functioning of ecosystems and the ecological problems that can occur when they are disturbed—particularly by humans—by investigating a few of these cycles in more detail. 40

41. The Three Most Important Chemical Cycles
Carbon
Nitrogen
Phosphorus
Here, we’ll explore the three most important chemical cycles: carbon, nitrogen, and phosphorus. 41

42. Carbon is found largely in four compartments on earth: the oceans, the atmosphere, terrestrial organisms, and fossil deposits. Plants and other photosynthetic organisms obtain most of their carbon from the atmosphere, where carbon is in the form of carbon dioxide (CO2).
As we saw in Chapter 4, plants and some microorganisms utilize carbon dioxide in photosynthesis, separating the carbon molecules from CO2 and using them to build sugars. Carbon then moves through the food chain as organisms eat plants and are themselves eaten (Figure Element cycling: carbon).
The oceans contain most of the earth’s carbon. Here, many organisms use dissolved carbon to build shells (which later dissolve back into the water after the organism dies).
Most carbon returns to its reservoir as a consequence of organisms’ metabolic processes. Organisms extract energy from food by breaking carbon-carbon bonds, releasing the energy stored in the bonds, and combining the released carbon atoms with oxygen. They then exhale the end product as CO2. 42

43. Why are global CO2 levels rising?
Fossil fuels
Fossil fuels are created when large numbers of organisms die and are buried in sediment lacking oxygen. In the absence of oxygen, at high pressures, and after very long periods of time, the organic remains are ultimately transformed into coal, oil, and natural gas. Trapped underground or in rock, these sources of carbon played little role in the global carbon cycle until humans in industrialized countries began using fossil fuels to power various technologies.
Burning coal, oil, and natural gas releases large amounts of carbon dioxide, thus increasing the average CO2 concentration in the atmosphere—the current level of CO2 in the atmosphere is the highest it has been in almost half a million years.
This has potentially disastrous implications, as we will see in Chapter 16. 43

44. Global CO2 levels are rising in general, but they also exhibit a sharp rise and fall within each year.
Although, on average, the level of CO2 in the environment is increasing, there is a yearly cycle of ups and downs in the CO2 levels in the northern hemisphere. This cycling is due to fluctuations in the ability of plants to absorb CO2.
Many trees lose their leaves each fall and, during the winter months, relatively low rates of photosynthesis lead to low rates of CO2 consumption, causing an annual peak in atmospheric CO2 levels.
During the summer, leaves are present, sunlight is strong, and photosynthesis (consuming CO2) occurs at much higher levels, causing a drop in the atmospheric CO2 level.
Why? 44

45. Nitrogen is necessary to build all amino acids, the components of every protein molecule, as well as the precursors of other nitrogen-containing molecules—all critical to life. Like carbon, the chief reservoir of nitrogen is the atmosphere. But even though more than 78% of the atmosphere is nitrogen gas (N2), for most organisms, this nitrogen is completely unusable. The problem is that atmospheric nitrogen consists of two nitrogen atoms bonded tightly together, and these bonds need to be broken to make the nitrogen usable for living organisms.
Only through the metabolic magic (chemistry, actually) of some soil-dwelling bacteria, the nitrogen-fixers, can most nitrogen enter the food chain. These bacteria chemically convert or “fix” nitrogen by attaching it to other atoms, including hydrogen, producing ammonia and related compounds. These compounds are then further modified by other bacteria into a form that can be taken up by plants and used to build proteins.
And once nitrogen is in plant tissues, animals acquire it in the same way they acquire carbon: by eating the plants. Nitrogen returns to the atmosphere when animal wastes and dead animals are broken down by soil bacteria that convert the nitrogen compounds in tissue back to nitrogen gas. 45

46. Nitrogen is like a bottleneck limiting plant growth.
Fertilizers
Because it is necessary for the production of every plant protein, and because all nitrogen must first be made usable by bacteria, plant growth is often limited by nitrogen levels in the soil. For this reason, most fertilizers contain nitrogen in a form usable by plants. 46

47. Every molecule of ATP and DNA requires phosphorus
Every molecule of ATP and DNA requires phosphorus. Virtually no phosphorus is available in the atmosphere, though. Instead, soil serves as the chief reservoir. Like nitrogen, phosphorus is chemically converted—“fixed”—into a form usable by plants (phosphate)  and then absorbed by their roots.
It cycles through the food chain as herbivores consume plants and carnivores consume herbivores. As organisms die, their remains are broken down by bacteria and other organisms, returning the phosphorus to the soil.
The pool of phosphorus in the soil is also influenced by the much slower process of rock formation on the sea floor, its uplifting into mountains, and the eventual weathering of the rock, releasing its phosphorus. 47

48. Like nitrogen, phosphorus is often a limiting resource in soils, constraining plant growth. Consequently, fertilizers usually contain large amounts of phosphorus. This is beneficial in the short run, but it can have some disastrous unintended consequences. As more and more phosphorus (and nitrogen) is added to soil, some of it runs off with water and ends up in lakes, ponds, and rivers. In these habitats, also, it acts as a fertilizer, making spectacular growth possible for algae. But eventually, the algae die and sink, creating a huge source of food for bacteria. The bacteria population increases and can use up too much of the dissolved oxygen, causing fish, insects, and many other organisms to suffocate and die.
This process of excess nutrients leading to rapid growth of algae and bacteria, followed by large-scale die-offs, is called eutrophication. It is increasingly a problem in both small and large bodies of water, affecting more than half of the lakes in Asia, Europe, and North America. Lake Erie, on the U.S.-Canadian border, for example, has experienced eutrophication as a result of all the phosphorus- and nitrogen-containing waste water that drains into it from the extensive surrounding farmlands. Given the lower use of fertilizers in South America and Africa, eutrophication is less common there. 48

49. Take-home message 15.8
Chemicals essential to life—including carbon, nitrogen, and phosphorus—cycle through ecosystems.

50. Take-home message 15.8
They are usually captured from the atmosphere, soil, or water by growing organisms; passed from one trophic level to the next as organisms eat other organisms; and returned to the environment through respiration, decomposition, and erosion.

51. Take-home message 15.8
These cycles can be disrupted as human activities significantly increase the amounts of the chemicals utilized or released to the environment.

52. 15.10. Each species’ role in a community is defined by its niche.
Within a society, most humans seem to find their niche. Each person plays a particular role, defined by the nature of his or her work, activities, and interactions with others. Other species do the same thing. Within their communities—geographic areas defined as loose assemblages of species, sometimes interdependent, with overlapping ranges—each species has its own niche. 52

53. More than just a place for living, a niche is a complete way of living.
We can define an organism’s niche in terms of the ways in which the organism utilizes the resources of the environment. More than just a place for living, a niche is a complete way of living.
In other words, an organism’s niche encompasses: 1) the space it requires, 2) the type and amount of food it consumes, 3) the timing of its reproduction (its life history), 4) its temperature and moisture requirements, and virtually every other aspect that describes the way the organism uses its environment .
Figure A way of living. 53

55. No two species occupy the same niche.

56. Take-home message 15.10
A population of organisms in a community fills a unique niche, defined by the manner in which they utilize the resources in their environment.
It is common for species to find themselves competing with other species for parts of a niche. When this occurs, generally, one of the species is restricted from its full niche. We’ll examine next the various possible outcomes when such niche overlap occurs. 56

57. Take-home message 15.10
Organisms do not always completely fill their niche; competition with species that have overlapping niches can reduce their range. 57

58. 15.11 Competition can be hard to see, but it still influences community structure.
Some species, particularly closely related species, have similar niches. This can lead to conflict as they try to exploit the same resources.

59. Almost invariably, when the fundamental niches of two species overlap, competition occurs. This competition doesn’t last forever, though. Inevitably, one of two outcomes occurs: competitive exclusion or resource partitioning.
In competitive exclusion, two species battle for resources in the same niche until the more efficient of the two wins and the other species is driven to extinction in that location (“local extinction”). In the 1930s, this was demonstrated in simple laboratory experiments using Paramecium, a single-celled organism. Populations of two similar Paramecium species were grown either separately or together in test tubes containing water and their bacterial food source. When grown separately, each species thrived. When grown together, though, one species always drove the other to extinction.
Resource partitioning is an alternative outcome of niche overlap. Individual organisms and species can adapt to changing environmental conditions, and resource partitioning can result from an organism’s behavioral change or a change in its structure. When this occurs, one or both species become restricted in some aspect of their niche, dividing the resource. In other experiments with Paramecium, for example, one of the two species was replaced with a different species. As in the initial experiment, either species thrived when grown alone. But when the two species were grown together in the same test tube, they ended up dividing the test tube “habitat.” One species fed exclusively at the bottom of the test tube, and the other fed only at the top. Simple behavioral change made coexistence possible.
Figure Overlapping niches: competitive exclusion and resource partitioning. 59

60. In many situations, resource partitioning is accompanied by character displacement, an evolutionary divergence in one or both of the species that leads to a partitioning of the niche. A clear example occurs among two species of seed-eating finches on the Galápagos Islands. On islands where both species live, their beak sizes differ significantly. One species has a deeper beak, better for large seeds, while the other has a shallower beak, better for smaller seeds, and they do not compete. On islands where either species occurs alone, beak size is intermediate between the two sizes.
Figure Allowing organisms to divide resources. 60

61. Take-home message 15.11
Populations with completely overlapping niches cannot coexist forever.
Competition for resources occurs until one or both species evolve in ways that reduce the competition, through character displacement, or until one becomes extinct in that location. 61

62. 15.15–15.16 Communities can change or remain stable over time.
A pair of colorful ochre starfish, keystone predators within intertidal zones
in the Pacific Ocean.

63. 15.15. Many communities change over time.
Human “progress” and development can completely transform an environment—turning a patch of desert into Las Vegas, for example. Urban landscapes, too, can obliterate any signs of the nature that was once there. This is why it can be surprising (and heartening) to observe what happens when humans abandon an area. Little by little, nature reclaims it. The area doesn’t necessarily recover completely, and change is slow. Still, this process is almost universal and virtually unstoppable. Nature responds similarly to other disturbances, too, from a single tree falling in a forest, to a massive flood or fire, to massive volcanic eruptions.
The process of nature reclaiming an area and of communities gradually changing over time is called succession. It is defined specifically as a change in the species composition over time, following a disturbance. There are two types of succession. Primary succession is when the process starts with no life and no soil. Secondary succession is when an already established habitat is disturbed but some life and some soil remain. 63

64. Primary succession Primary succession can take thousands or even tens of thousands of years, but it generally occurs in a consistent sequence. It always begins with a disturbance that leaves an area barren of soil and with no life. Frequently, the disturbance is catastrophic. The huge volcanic eruption on Krakatoa, Indonesia, in 1883, for example, completely destroyed several islands and wiped out all life and soil on others. Primary succession has also begun, in a less dramatic fashion, in regions where glaciers have retreated, such as Glacier Bay, Alaska. Although succession does not occur in a single, definitive order, several steps are relatively common.
An important feature of the colonizers seen in the earliest stages of primary succession is that, while they are all good dispersers, able to move away from their original home (hence their early arrival to a newly available locale), they are not particularly good competitors. That’s why they are gradually replaced.
Ultimately, it is the longer-living, larger species that tend to outcompete the initial colonizers and persist as a stable and self-sustaining community, called a climax community. The specific species present in the climax community depend on physical factors such as temperature and rainfall.
Figure Species composition of a community changes over time. 64

65. Secondary Succession Much faster than primary succession
life and soil are already present
Secondary succession Secondary succession is much faster than primary succession because life and soil are already present. Rather than the thousands or even tens of thousands of years that primary succession may take, secondary succession can happen in a matter of centuries, decades, or even years.
It frequently begins with organisms colonizing the decaying remains of dead organisms. It may involve fungi establishing themselves in the decaying trunk of a tree that has fallen, and these being replaced over time by different species of fungi.
Or secondary succession may begin with weeds springing up in formerly cultivated land that is left untended. If the weeds are allowed to grow, they eventually are outcompeted and replaced by perennial species, and then shrubs, and eventually larger trees. The process is similar to primary succession, but with a head start.
Ultimately, if undisturbed, secondary succession also leads to establishment of a stable, self-sustaining climax community. 65

68. Take-home message 15.15
Succession is the change in the species composition of a community over time, following a disturbance.
In primary succession, the process begins in an area with no life and no soil. 68

69. Take-home message 15.15
In secondary succession, the process occurs in an area where life is already present.
In both types, the process usually takes place in a predictable sequence. 69

70. THREATS TO BIODIVERSITY & THE ENVIRONEMNT

71. BIODIVERSITY
Biodiversity is the variety of different types of life found on earth. It is a measure of the variety of organisms present in different ecosystems.
~ UNITED NATIONS ENVIRONMENT PROGRAM

72. Invasive Species
A non-native species that is imported or introduced to a native habitat.
Problems:
No natural predators
Outcompete native species for resources
Examples:
Zebra mussels (animal)
Purple Loosestrife (plant)

73. Examples

74. Habitat Destruction
Through land development, habitats can be completely destroyed or fragmented.
Both lead to a decrease of habitats for native species.

75. Habitat Fragmentation

76. Pollution Two types of pollution:
Point source: a specific point of polluting air or water (ex: pipe draining into a river from a factory)
Non-point source: no specific point is located but many widespread points (ex: car exhaust)

77. Point source

78. Non-point Source

79. the accumulation of toxins through the trophic levels of a food chain.
Bio magnification
the accumulation of toxins through the trophic levels of a food chain.

80. Ecological “HOT SPOT”
Areas in the world with large amounts of biodiversity but the habitats and species are endanger of extinction due to human impact.

81. NO MORE NOTES EVER!!!