Ecosystems and Populations Notes

Ecosystems and Populations

Ecosystems: Living Organisms and Their Environment

• Ecosystem: A community of organisms and the physical environment in which they live.
• Population: A group of individuals of the same species that occupy the same geographic area and interact with each other.
• Community: Populations of all species that occupy the same geographic area and interact with each other.
• Biosphere: All the ecosystems on Earth.

The Dynamic Nature of Populations

• Habitat: The location where a species lives.
  • Possesses certain chemical and physical characteristics favorable for the organism's comfort and survival.
• Geographic Range: The area over which a species is found.
• Limitations on geographic range:
  • Competition for resources
  • Intolerable conditions
  • Physical obstacles

Population Growth Rate and Biotic Potential

• Biotic Potential: The maximum rate of growth of a population under ideal conditions.
  • Determined by:
    • Number of offspring produced by each member
    • Length of time for individuals to reach reproductive maturity
    • Ratio of males to females
    • Number of reproductive-age individuals
• Exponential Growth Curve: Biotic potential usually follows a J-shaped exponential growth curve.
• Exponential Growth Rate: Population doubles repeatedly over similar time periods.
• Rule of 72: Used to estimate the doubling time of a population.
  • Formula: Divide 72 by the % growth rate per year.

Environmental Resistance

• Environmental Resistance: Factors that kill organisms or prevent them from reproducing.
  • Limitations on nutrients, energy, and space
  • Predation by other species
  • Disease
  • Environmental toxins
• No population grows at its full biotic potential indefinitely.
• Carrying Capacity: The population size that the environment can support indefinitely.

Invasive Species Alter Ecological Balance

• Invasive Species: Species not naturally found in an ecosystem.
  • Once introduced, it overtakes and dominates the native species within a community.
  • Natural controls (predators, herbivores, diseases) in its home territory may be absent in the new environment.
  • Examples: Kudzu, zebra mussels, tumbleweeds, scotch broom

Communities: Different Species Living Together

• Communities are composed of many different species.
• Complex relationships exist between species.
  • Intense competition
  • Mutual benefit
• Different species in a community may rely on each other for food, shelter, and protection.

Niches and Competition

• Niche: An organism’s role in a community.
• A well-balanced ecosystem supports a wide variety of species, each with a different niche.
• Overlapping Niches: Niches may overlap, leading to competition between species for limited resources.
• Competitive Exclusion: Occurs when one species completely outcompetes another.

Succession Leads Toward a Mature Community

• Succession: Natural sequence of change in terms of which organisms dominate within a community.
  • Determined by: Growth rates, niches, and kinds of competition.
  • The number of species tends to increase as succession advances.
• Mature Community: Succession ends with the establishment of a mature community that changes very little over time.
  • Most efficient, most varied, most stable.
  • If disrupted, it does not recover readily.

Ecosystems: Communities and Their Physical Environment

• Ecosystem includes both living and nonliving components.
• Biomass: Total living component of an ecosystem.
• Nonliving components include:
  • Chemical elements
  • An essential constant supply of energy

Energy Flows Through Ecosystems

• Energy flow through ecosystems obeys the laws of thermodynamics.
• First Law of Thermodynamics: Energy is neither created nor destroyed; it can change form and be stored.
• Second Law of Thermodynamics: Some energy is wasted when it changes form or is transferred, usually converted to heat.
• Producers: Capture and convert energy; make their own organic molecules.
• Consumers: Acquire organic molecules and energy by consuming other organisms.
• Energy Flow: Sun → Producers → Consumers

Producers and Consumers

• Producers:
  • Autotrophs: "Self-nutritive" or "self-growing".
    • Most are capable of photosynthesis.
    • Terrestrial ecosystems: Plants
    • Aquatic ecosystems: Algae
    • Photosynthesis equation: CO2 + H2O \rightarrow C6H{12}O6 + O2 (using sunlight).
    • Some autotrophs are capable of chemosynthesis.
• Consumers:
  • Heterotrophs: Must consume foods that already contain stored forms of energy (animals, most bacteria, and fungi).
    • Herbivores: Primary consumers; use green plants as an energy source.
    • Carnivores: Secondary or tertiary consumers; use other animals as an energy source.
    • Omnivores: Use either plants or animals as energy sources.
    • Decomposers: Use dead organisms as an energy source.

Food Webs

• Food Web: Depicts complex feeding relationships among producers and consumers in an ecosystem.
  • Often depicted as a chain but is typically more complex and web-like.

Ecological Pyramids

• Ecological pyramids depict total biomass or total energy stored at each level of an ecosystem.
  • Producers capture approximately 2% of the energy in sunlight.
  • Only about 10% of the energy from a lower level is available to the next higher level.
  • Lower levels of the ecological pyramid support consumer populations.
  • Consumers at any level depend critically on the populations of consumers directly below them.
  • A small amount of energy available to tertiary consumers depends on energy transfers at all levels below them.

Human Activities Disrupt Ecological Pyramids

• Humans as primary consumers: Eating plants.
• Humans as secondary consumers: Eating meat.
• Utilizing only 10% of energy that would be available from plants.
• Modern farming practices exclude other species from their place in the food web and their place in the ecological pyramid.

Biogeochemical Cycles

• Biogeochemical Cycles: Include living organisms, geologic events, and weather events.
• Molecules and elements cycle between three different pools:
  • Biomass (living organisms)
  • Exchange pool (water, soil, atmosphere)
  • Reserve: Large, but hard-to-access pool of nutrients

The Water Cycle

• Water cycles between the atmosphere, the ocean, and land.
• Human activities affect the water cycle:
  • Loss of wetlands
  • Water pollution

The Carbon Cycle

• Carbon Cycle: Largely a gaseous cycle.
• Carbon in living organisms is exchanged with atmospheric carbon dioxide.
• Closely tied to photosynthesis and aerobic respiration.
• Human activity affects the carbon cycle:
  • Accelerating the rate of CO_2 production by burning fossil fuels.
  • CO_2 increase is responsible for global warming.

The Nitrogen Cycle

• Nitrogen: Essential component of proteins and nucleic acids.
• Atmosphere: Largest reservoir of nitrogen.
• Nitrogen Fixation: Converts atmospheric nitrogen to ammonium.
  • Performed by legumes (peas, alfalfa, soybeans).
• Nitrification: Converts ammonium to nitrate.
• Denitrification: Converts nitrates back to nitrogen gas (N_2).

The Phosphorus Cycle

• Phosphorus never enters the atmosphere; it is a sedimentary cycle.
• Human activity disrupts the balance of this cycle.
  • Creates increased runoff of phosphorus into aquatic ecosystems.
  • Runoff may cause excessive algal growth (blooms).
  • Decomposers feeding on dead algae may take up so much oxygen for metabolism that other organisms may suffocate (eutrophication).

Human Population Growth

• Most of human history:
  • Stable population, never exceeding 10 million people until about 4,000 years ago.
• 4,000 years ago:
  • Began slow increase in human population.
  • Development of agriculture decreased environmental resistance and increased carrying capacity.
• 300 years ago:
  • Industrial Revolution.
  • Rapid growth in human population.

Factors Contributing to Human Population Growth

• Factors that reduced environmental resistance and/or increased biotic potential:
  • Agricultural development
  • Plant and animal domestication
  • Improved medical care (vaccines, antibiotics)
  • Improved transportation
  • Improved housing
  • Advances in communication

Current Trends in Human Population Growth

• Currently experiencing explosive growth.
• Unanswered questions:
  • How high is the human carrying capacity of Earth?
  • Can the carrying capacity be raised by future scientific discoveries?
  • Do humans have the capacity to limit population growth before the carrying capacity is reached?

Zero Population Growth

• Growth Rate: Calculated as \frac{births/year - deaths/year}{total population}.
• Current human population growth rate: 1.1%/year.
• To reach zero population growth, must decrease the birth rate.
• Fertility Rate: Number of children per woman.
• Replacement Fertility Rate: 2.1 children per woman.

Population Projections

• If replacement fertility rate is achieved by 2050, the population would stabilize at about 9.6 billion.
• Estimates range from 8.3 to 10.9 billion.
• The difference in these estimates is only one child per couple.

Population Age Structure and Economic Development

• Age Structure: The number of people in each age group within a population.
• Demographic Transition: Progressive changes in the age structure of a population as a country undergoes industrial and economic development.
• More Industrialized Countries (MICs):
  • Europe, North America, Australia, Japan.
  • Roughly the same number of people in pre-reproductive, reproductive, and post-reproductive groups.
  • Predicts a more stable population.
• Less Industrialized Countries (LICs):
  • Africa, Latin America, Asia.
  • Pyramid shape of age structure.
  • Much of the population is younger than reproductive age.
  • Predicts a population continuing to expand.

Population Growth Predictions

• Over the next 50 years, population growth will be most rapid in the less industrialized countries, the countries least able to provide for their citizens.Population structure and ponulation