1. Ch. 54 Ecosystems

2. Introduction
An ecosystem consists of all the organisms living in a community as well as all the abiotic factors with which they interact.
The dynamics of an ecosystem involve two processes: energy flow and chemical cycling.
Ecosystem ecologists view ecosystems as energy machines and matter processors.
We can follow the transformation of energy by grouping the species in a community into trophic levels of feeding relationships.
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3. 1. Trophic relationships determine the routes of energy flow and chemical cycling in an ecosystem
The autotrophs are the primary producers, and are usually photosynthetic (plants or algae).
They use light energy to synthesize sugars and other organic compounds.
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4. Heterotrophs are at trophic levels above the primary producers and depend on their photosynthetic output.
Fig. 54.1
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5. Herbivores that eat primary producers are called primary consumers.
Carnivores that eat herbivores are called secondary consumers.
Carnivores that eat secondary producers are called tertiary consumers.
Another important group of heterotrophs is the detritivores, or decomposers.
They get energy from detritus, nonliving organic material and play an important role in material cycling.
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6. 2. Decomposition connects all trophic levels
The organisms that feed as detritivores often form a major link between the primary producers and the consumers in an ecosystem.
The organic material that makes up the living organisms in an ecosystem gets recycled.
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7. An ecosystem’s main decomposers are fungi and prokaryotes, which secrete enzymes that digest organic material and then absorb the breakdown products.
Fig. 54.2
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8. 3. The laws of physics and chemistry apply to ecosystems
The law of conservation of energy applies to ecosystems.
We can potentially trace all the energy from its solar input to its release as heat by organisms.
The second law of thermodynamics allows us to measure the efficiency of the energy conversions.
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9. Introduction
The amount of light energy converted to chemical energy by an ecosystem’s autotrophs in a given time period is called primary production.
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10. 1. An ecosystem’s energy budget depends on primary production
Most primary producers use light energy to synthesize organic molecules, which can be broken down to produce ATP; there is an energy budget in an ecosystem.
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11. The Global Energy Budget
Every day, Earth is bombarded by large amounts of solar radiation.
Much of this radiation lands on the water and land that either reflect or absorb it.
Of the visible light that reaches photosynthetic organisms, about only 1% is converted to chemical energy.
Although this is a small amount, primary producers are capable of producing about 170 billion tons of organic material per year.
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12. Gross and Net Primary Production.
Total primary production is known as gross primary production (GPP).
This is the amount of light energy that is converted into chemical energy.
The net primary production (NPP) is equal to gross primary production minus the energy used by the primary producers for respiration (R):
NPP = GPP – R
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13. Primary production can be expressed in terms of energy per unit area per unit time, or as biomass of vegetation added to the ecosystem per unit area per unit time.
This should not be confused with the total biomass of photosynthetic autotrophs present in a given time, called the standing crop.
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14. Different ecosystems differ greatly in their production as well as in their contribution to the total production of the Earth.
Fig. 54.3
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15. 2. In aquatic ecosystems, light and nutrients limit primary production
Production in Marine ecosystems.
Light is the first variable to control primary production in oceans, since solar radiation can only penetrate to a certain depth (photic zone).
Fig
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16. We would expect production to increase along a gradient from the poles to the equator; but that is not the case.
There are parts of the ocean in the tropics and subtropics that exhibit low primary production.
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17. Fig. 54.4
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18. It depends on nutrient availability.
Why are tropical and subtropical oceans less productive than we would expect?
It depends on nutrient availability.
Ecologists use the term limiting nutrient to define the nutrient that must be added for production to increase.
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19. In the open ocean, nitrogen and phosphorous levels are very low in the photic zone, but high in deeper water where light does not penetrate.
Fig. 54.5
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20. Nitrogen is the one nutrient that limits phytoplankton growth in many parts of the ocean.
Fig. 54.6
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21. Nutrient enrichment experiments showed that iron availability limited primary production.
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22. Evidence indicates that the iron factor is related to the nitrogen factor.
Iron + cyanobacteria + nitrogen fixation  phytoplankton production.
Marine ecologists are just beginning to understand the interplay of factors that affect primary production.
Fig. 54.7
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23. Production in Freshwater Ecosystems.
Solar radiation and temperature are closely linked to primary production in freshwater lakes.
During the 1970s, sewage and fertilizer pollution added nutrients to lakes, which shifted many lakes from having phytoplankton communities to those dominated by diatoms and green algae.
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24. This process is called eutrophication, and has undesirable impacts from a human perspective.
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25. Controlling pollution may help control eutrophication.
Experiments are being done to study this process.
Fig. 54.8
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26. On a more local scale, mineral nutrients in the soil can play key roles in limiting primary production.
Scientific studies relating nutrients to production have practical applications in agriculture.
Fig. 54.9
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27. Introduction
The amount of chemical energy in consumers’ food that is converted to their own new biomass during a given time period is called secondary production.
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28. 1. The efficiency of energy transfer between trophic levels is usually less than 20%
Production Efficiency.
One way to understand secondary production is to examine the process in individual organisms.
Fig
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29. If we view animals as energy transformers, we can ask questions about their relative efficiencies.
Production efficiency = Net secondary production/assimilation of primary production
Net secondary production is the energy stored in biomass represented by growth and reproduction.
Assimilation consists of the total energy taken in and used for growth, reproduction, and respiration.
In other words production efficiency is the fraction of food energy that is not used for respiration.
This differs between organisms.
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30. Trophic Efficiency and Ecological Pyramids.
Trophic efficiency is the percentage of production transferred from one trophic level to the next.
Pyramids of production represent the multiplicative loss of energy from a food chain.
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31. Fig
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32. Pyramids of biomass represent the ecological consequence of low trophic efficiencies.
Most biomass pyramids narrow sharply from primary producers to top-level carnivores because energy transfers are inefficient.
Fig a
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33. In some aquatic ecosystems, the pyramid is inverted.
In this example, phytoplankton grow, reproduce, and are consumed rapidly.
They have a short turnover time, which is a comparison of standing crop mass compared to production.
Fig b
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34. Pyramids of numbers show how the levels in the pyramids of biomass are proportional to the number of individuals present in each trophic level.
Fig
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35. The dynamics of energy through ecosystems have important implications for the human population.
Fig
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36. 2. Herbivores consume a small percentage of vegetation: the green world hypothesis
According to the green work hypothesis, herbivores consume relatively little plant biomass because they are held in check by a variety of factors including:
Plants have defenses against herbivores
Nutrients, not energy supply, usually limit herbivores
Abiotic factors limit herbivores
Intraspecific competition can limit herbivore numbers
Interspecific interactions check herbivore densities
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37. 3. Nutrient cycling is strongly regulated by vegetation
Long-term ecological research (LTER) monitors the dynamics of ecosystems over long periods of time.
The Hubbard Brook Experimental Forest has been studied since 1963.
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38. Fig
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39. Preliminary studies confirmed that internal cycling within a terrestrial ecosystem conserves most of the mineral nutrients.
Some areas have been completely logged and then sprayed with herbicides to study how removal of vegetation affects nutrient content of the soil.
In addition to the natural ways, industrial production of nitrogen-containing fertilizer contributes to nitrogenous materials in ecosystems.
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40. 1. The human population is disrupting chemical cycles throughout the biosphere
Human activity intrudes in nutrient cycles by removing nutrients from one part of the biosphere and then adding them to another.
Agricultural effects of nutrient cycling.
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41. After awhile, the natural store of nutrients can become exhausted.
In agricultural ecosystems, a large amount of nutrients are removed from the area in the crop biomass.
After awhile, the natural store of nutrients can become exhausted.
Fig
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42. Recent studies indicate that human activities have approximately doubled the worldwide supply of fixed nitrogen, due to the use of fertilizers, cultivation of legumes, and burning.
This may increase the amount of nitrogen oxides in the atmosphere and contribute to atmospheric warming, depletion of ozone and possibly acid rain.
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43. Accelerated eutrophication of lakes.
Human intrusion has disrupted freshwater ecosystems by what is called cultural eutrophication.
Sewage and factory wastes, runoff of animal wastes from pastures and stockyards have overloaded many freshwater streams and lakes with nitrogen.
This can eliminate fish species because it is difficult for them to live in these new conditions.
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44. 3. Toxins can become concentrated in successive trophic levels of food webs
Humans produce many toxic chemicals that are dumped into ecosystems.
These substances are ingested and metabolized by the organisms in the ecosystems and can accumulate in the fatty tissues of animals.
These toxins become more concentrated in successive trophic levels of a food web, a process called biological magnification.
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45. The pesticide DDT, before it was banned, showed this affect.
Fig
The pesticide DDT, before it was banned, showed this affect.
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46. Fig
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47. Introduction
Conservation biology is a goal-oriented science that seeks to counter the biodiversity crisis, the current rapid decrease in Earth’s variety of life.
Extinction is a natural phenomenon that has been occurring since life evolved on earth.
The current rate of extinction is what underlies the biodiversity crisis.
A high rate of species extinction is being caused by humans.
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48. 1. The three levels of biodiversity are genetic diversity, species diversity, and ecosystem diversity
Fig. 55.1
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49. Loss of genetic diversity.
If a local population becomes extinct, then the entire population of that species has lost some genetic diversity.
The loss of this diversity is detrimental to the overall adaptive prospects of the species.
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50. Loss of species diversity.
Much of the discussion of the biodiversity crisis centers on species.
The U.S. Endangered Species Act (ESA) defines an endangered species as one in danger of extinction throughout its range, and a threatened species as those likely to become endangered in the foreseeable future.
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51. Here are a few examples of why conservation biologists are concerned about species loss.
The IUCN reports that 13% of the known 9,040 bird species are threatened with extinction. That is 1,183 species!!!
The Center for Plant Conservation estimates that 200 of the 20,000 known plant species in the U. S. have become extinct since records have been kept, and another 730 are endangered or threatened.
About 20% of the known freshwater species of fish in the world have become extinct or are seriously threatened.
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52. Since 1900, 123 freshwater vertebrate and invertebrate species have become extinct in North America, and hundreds more are threatened.
Harvard biologist Edward O. Wilson has compiled a list called the Hundred Heartbeats Club, a list of species that number fewer than one hundred and are only that many heartbeats from extinction.
Fig. 52.2
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53. Extinction of species may be local, but it may also be global.
Several researchers estimate that at the current rate of destruction, over half of all plant and animal species will be gone by the end of this new century.
Extinction of species may be local, but it may also be global.
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54. Loss of ecosystem diversity.
The local extinction of one species, like a keystone predator, can affect an entire community.
Some ecosystems are being erased from the Earth at an unbelievable pace.
For example, an area the size of the state of West Virginia is lost from tropical forests each year.
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55. 2. Biodiversity at all three levels is vital to human welfare
Why should we care about biodiversity?
Benefits of species diversity and genetic diversity.
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56. The loss of species also means the loss of genes.
Biodiversity is a crucial natural resource, and species that are threatened could provide crops, fibers, and medicines for human use.
The loss of species also means the loss of genes.
Biodiversity represents the sum of all the genomes on Earth.
Fig. 55.3
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57. One large scale experiment illustrates how little we understand ecosystem services.
Fig. 54.4
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58. The experiment failed and had to be stopped after 15 months.
Biosphere II attempted to create a closed ecosystem, and had a forest with soil, miniature ocean, and several other “ecosystems.”
In 1991, eight people entered and were supposed to be isolated for two years.
The experiment failed and had to be stopped after 15 months.
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59. 3. The four major threats to biodiversity are habitat destruction, introduced species, overexploitation and food chain disruption
Habitat destruction.
Human alteration of habitat is the single greatest cause of habitat destruction.
The IUCN states that destruction of physical habitat is responsible for the 73% of species designated extinct, endangered, vulnerable, or rare.
About 93% of the world’s coral reefs have been damaged by humans.
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60. Habitat destruction has also caused fragmentation of many natural landscapes.
Fig. 55.5
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61. This can also lead to species loss.
Fig. 55.6
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62. Introduced species.
Introduced species are those that humans move from native locations to new geographic regions.
The Nile perch was introduced into Lake Victoria as a food fish, but led to the extinction of several native species.
Fig. 57.7a
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63. There are many examples of how exotic species have disrupted ecosystems.
Fig. 57.7
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64. The great auk was overhunted and became extinct.
Overexploitation.
This refers to the human harvesting of wild plants and animals at rates that exceed the ability of those populations to rebound.
The great auk was overhunted and became extinct.
Fig. 55.8
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65. The bluefin tuna is another example of an over- harvested species.
The African elephant has been overhunted and the populations have declined dramatically.
The bluefin tuna is another example of an over- harvested species.
Fig. 55.9
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66. Disruption of food chains.
The extinction of one species can doom its predators, but only if the predator feeds exclusively on this prey.
Much of the evidence for secondary extinctions of larger organisms due to loss of prey is circumstantial.
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67. How small is too small for a population?
How small does a population have to be before it starts down the extinction vortex?
Minimum viable population size (MVP).
The MVP is the smallest number of individuals needed to sustain a population.
Population viability analysis (PVA) is a method of predicting whether or not a species will survive over time.
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68. 1. Edges and corridors can strongly influence landscape biodiversity
Edges are the boundaries between ecosystems and within ecosystems.
Fig a
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69. Edges have their own communities of organisms.
The proliferation of edge species has positive or negative affects on a community’s biodiversity.
Fig b
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70. These can sometimes be artificial.
A movement corridor is a narrow strip or series of small clumps of good habitat connecting typically isolated patches.
These can sometimes be artificial.
Movement corridors can promote dispersal and reduce inbreeding in declining populations.
Fig
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71. 2. Conservation biologists face many challenges in setting up protected areas
They apply ecological research in setting up reserves or protected areas to slow the loss of biodiversity.
Governments have set aside about 7% of the world’s land in various types of reserves.
Much of the focus has been on biodiversity hot spots, areas with exceptional concentration of endemic species and a large number of threatened or endangered species.
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72. Fig
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73. A small area in Costa Rica provides an example.
Fig
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74. However, the continued high rate of human exploitation of ecosystems leads to the prediction that less than 10% of the biosphere will be protected as nature reserves.
The Florida scrub jay inhabits areas that have nearly been replaced by housing developments.
Fig
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75. 4. Restoring degraded areas is an increasingly important conservation effort
Restoration ecology applies ecological principles in developing ways to return degraded areas to natural conditions.
Biological communities can recover from many types of disturbances, through a series of restoration mechanisms that occur during ecological succession.
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76. Fig
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77. Bioremediation is the use of living organisms to detoxify polluted ecosystems.
Restoration ecologists use various types of organisms to remove many different types of toxins from ecosystems.
Fig
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