Human Biology: Ecology and the Nature of Ecosystems

The Nature of Ecosystems

• Biosphere: Defined as the portion of the Earth that contains living organisms.
• Ecosystem: A specific area of the biosphere where organisms interact with each other and with the physical and chemical environment.
• Niche: The specific role an organism plays in an ecosystem, such as being a producer or a carnivore.

Types of Ecosystems and Biomes

• Biomes: These are types of terrestrial ecosystems, largely defined by climate conditions.
  • Major Terrestrial Biomes: Tropical rain forest, savanna, temperate grasslands, deserts, the taiga, and the tundra.
• Aquatic Ecosystems: Divided into fresh water and salt water (marine) categories.
  • Freshwater Ecosystems: Divided into standing water (lakes and ponds) and running water (rivers and streams). These are further categorized by nutrient levels.
  • Marine Ecosystems: The ocean covers $70\%$ of the Earth's surface.
  • Coral Reefs: Located offshore, these are home to an incredible diversity of marine life.
  • Estuaries: Ecosystems where fresh and saltwater mix; they represent one of the most diverse aquatic environments.

Biotic and Abiotic Components

• Abiotic Components: Nonliving elements of an ecosystem, including nutrients in the soil, water, and weather conditions.
• Biotic Components: Living elements categorized by their method of obtaining energy.
  • Autotrophs (Producers): Use inorganic nutrients and an outside energy source (usually the sun) to produce organic nutrients.
    • Algae: The primary photosynthesizers in freshwater and marine ecosystems.
    • Green Plants: The primary photosynthesizers in terrestrial ecosystems.
  • Heterotrophs (Consumers): Organisms that must consume organic food.
    • Herbivores: Feed directly on producers (e.g., deer, rabbits, caterpillars, and certain birds in terrestrial habitats; protists in aquatic ecosystems).
    • Carnivores: Feed on other animals (e.g., snakes and hawks).
    • Omnivores: Feed on both plants and animals (e.g., humans).
    • Detritus Feeders: Feed on decomposing organic matter (detritus). Examples include earthworms, beetles, termites, and ants.
    • Decomposers: Specialized heterotrophs (bacteria and fungi) that release inorganic substances back into the environment to be reused by plants.

Consumer Classification and Trophic Levels

• Heterotrophs are organized by their position in the food chain relative to producers:
  • Primary Consumers: Herbivores that eat plants (e.g., plant-eating insects).
  • Secondary Consumers: Feed on primary consumers (e.g., insect-eating frogs).
  • Tertiary Consumers: Feed on secondary consumers (e.g., frog-eating snakes).
  • Top Predators: Consumers at the very top level of a food chain.

Energy Flow and Chemical Cycling

• Energy Flow: This process begins when producers absorb solar energy. It is a one-way flow; energy is not recycled but dissipated as heat.
• Nutrient (Chemical) Cycling: Producers take in inorganic chemicals from the environment to make organic nutrients, which then move through the food chain. Decomposers eventually return these chemicals to the soil or atmosphere.
• Efficiency of Energy Transfer:
  • Only about $55\%$ of original energy absorbed by plants is available to the ecosystem.
  • Plants use energy for cellular respiration.
  • Herbivores do not digest all eaten food; some is eliminated as feces, some as metabolic waste (urine), and much is lost as heat during metabolism.
  • Only a small fraction of energy is converted into increased body weight or offspring available to higher levels.

Energy Flow Models: Food Webs and Chains

• Food Web: A complex diagram describing trophic (feeding) relationships.
  • Grazing Food Web: Begins with producers (e.g., oak trees and grass). Insects like caterpillars eat leaves; mice, rabbits, and deer eat leaves and grass; birds and chipmunks eat seeds/nuts; carnivores like hawks and snakes eat these herbivores.
  • Detrital Food Web: Begins with wastes and dead organisms. Detritus is consumed by soil organisms like earthworms, which are eaten by carnivorous invertebrates, then by salamanders and shrews. These webs connect to the grazing web when aboveground carnivores eat detrital web members.
• Food Chain: A diagram showing a single path of energy flow.
  • Grazing Chain Example: $oak\,leaves \rightarrow caterpillars \rightarrow mice \rightarrow hawks$
  • Detrital Chain Example: $detritus \rightarrow earthworms \rightarrow beetles \rightarrow shrews$
• Trophic Level: Composed of all organisms feeding at a particular link in a food chain.
  • $1^{st}\,level$: Producers.
  • $2^{nd}\,level$: Primary consumers.
  • $3^{rd}\,level$: Secondary consumers.

Ecological Pyramids and Biomass

• $10\%$ Rule: Generally, only $10\%$ of the energy at one trophic level is available to the next.
  • If producers have $1,000\,kg$ of biomass, herbivores will have $100\,kg$, first-level carnivores $10\,kg$, and second-level carnivores $1\,kg$.
• Biomass: The number of organisms multiplied by the weight of organic matter in one organism.
  • In terrestrial systems, producer biomass is usually highest.
  • In aquatic ecosystems, herbivore biomass may exceed producer biomass because algae are the sole producers and are consumed rapidly.

Global Biogeochemical Cycles

• Biogeochemical Cycle: Pathways involving both biotic and abiotic components through which chemicals circulate.
• Types of Cycles:
  • Gaseous Cycle: Elements return to/are withdrawn from the atmosphere as gas (e.g., Carbon, Nitrogen).
  • Sedimentary Cycle: Elements are absorbed from soil, passed to heterotrophs, and returned to soil by decomposers (e.g., Phosphorus).
• Key Components:
  • Reservoir: Source of chemicals unavailable to producers (e.g., fossil fuels, minerals in rocks, calcium carbonate shells in ocean sediments).
  • Exchange Pool: Source from which organisms can readily take chemicals (e.g., atmosphere, soil, water).

The Water Cycle (Hydrologic Cycle)

• Evaporation: Solar heat causes fresh water to evaporate from the ocean, leaving salt behind.
• Condensation: Gas changes to liquid, forming clouds.
• Precipitation: Water falls as rain, snow, sleet, hail, or fog.
• Transpiration: Evaporation of water specifically from plants.
• Movement on Land: Gravity returns water to the sea.
  • Runoff: Water flowing into streams, lakes, and oceans.
  • Groundwater: Water percolating into the ground.
  • Aquifers: Rock layers containing water; refilled by rainfall/snowmelt.
  • Water Table: The top of the saturation zone.
• Human Interference:
  • Withdrawing water from aquifers exceeding recharge rates (Groundwater Mining).
  • Clearing vegetation/building roads, which increases runoff and prevents percolation.
  • Adding pollutants (sewage/chemicals) that interfere with natural purification.

The Carbon Cycle

• Exchange Pool: The atmosphere (as $CO_2$).
• Terrestrial Process: Plants take up $CO_2$ via photosynthesis; organisms return it via cellular respiration.
• Aquatic Process: $CO_2$ combines with water to produce bicarbonate ion ($HCO_3^-$). This exists in equilibrium with atmospheric $CO_2$.
• Reservoirs:
  • Living and dead organisms.
  • Fossil Fuels: Coal, oil, and natural gas formed from non-decomposed remains.
  • Limestone: Formed from calcium carbonate ($CaCO_3$) shells of marine organisms in sediments.
• Human Interference and Climate Change:
  • Burning fossil fuels and deforestation increase atmospheric $CO_2$ beyond photosynthetic capacity.
  • Greenhouse Gases: $CO_2$, nitrous oxide ($N_2O$ from fertilizers/animal waste), and methane ($CH_4$ from bacterial decomposition in animal guts, e.g., cattle).
  • Greenhouse Effect: Gases block heat from escaping into space, causing Global Warming.
  • Predictions: Temperatures may rise by $2.0$ to $8.1\,^{\circ}F$ by $2100$.
  • Consequences: Sea level rise from melting glaciers, reduced crop yields, and coastal city flooding.

The Nitrogen Cycle

• Atmospheric Nitrogen: $N_2$ makes up $78\%$ of the atmosphere but is unusable by plants.
• Nitrogen Fixation: Conversion of $N_2$ to ammonium ($NH_4^+$).
  • Carried out by cyanobacteria, soil bacteria, and bacteria in legume root nodules.
• Nitrification: Production of nitrates ($NO_3^-$).
  • $N_2 \rightarrow NO_3^-$ (in atmosphere via lightning/physical processes).
  • $NH_4^+ \rightarrow Nitrite\, (NO_2^-) \rightarrow Nitrate\, (NO_3^-)$ (via soil bacteria).
• Assimilation: Plants take up and use $NH_4^+$ and $NO_3^-$ to produce proteins and nucleic acids.
• Denitrification: Conversion of nitrate back to $N_2$ gas by bacteria in anaerobic mud (lakes, bogs, estuaries).
• Human Interference:
  • Fertilizer production doubles the nitrogen fixation rate.
  • Eutrophication: Fertilizer runoff causes algal blooms; when algae die, decomposers deplete oxygen, killing fish.
  • Acid Deposition: Burning fossil fuels releases nitrogen oxides ($NO_x$) and sulfur dioxide ($SO_2$), forming acid rain. This kills trees, reduces crop yields, and corrodes metal/stone.
  • Photochemical Smog: $NO_x$ and hydrocarbons react in sunlight.
  • Thermal Inversion: Pollutants are trapped near Earth under a layer of warm, stagnant air.

The Phosphorus Cycle

• Sedimentary Cycle: Phosphorus does not enter the atmosphere.
• Process: Weathering of rocks releases phosphate ions into soil for plants. Animals eat plants. Death/decomposition returns phosphate to the soil.
• Aquatic Process: Phosphate runs off into water for algae. Eventually, it is trapped in marine sediments until geologic upheaval brings it back to land.
• Human Interference:
  • Mining phosphate for fertilizers and detergents.
  • Animal waste and sewage discharge cause Cultural Eutrophication (overenrichment) of waterways.

Questions & Discussion

• Why can the biosphere be considered a giant ecosystem?
  • The biosphere includes all living organisms on Earth and their interactions with the physical (abiotic) environment on a global scale, functioning as a single unit.
• Explain why autotrophs are called producers and heterotrophs are called consumers.
  • Autotrophs produce organic food from inorganic sources and energy; heterotrophs must consume this pre-made organic food for survival.
• Explain the characteristics used to identify biomes.
  • Biomes are identified primarily by climate (temperature and rainfall) and the specific types of terrestrial vegetation and adaptations found there.
• Distinguish between the movement of energy and chemicals in an ecosystem.
  • Energy flows in one direction and is eventually dissipated as heat; chemicals (nutrients) cycle through the ecosystem and can be reused repeatedly.
• Explain the difference between a grazing food web and a detrital food web.
  • A grazing food web begins with living producers (grass, trees); a detrital food web begins with organic waste and dead matter (detritus).
• Explain why a diagram of trophic levels resembles a pyramid.
  • Because of the $10\%$ rule of energy transfer, each successive level has significantly less energy and usually less biomass than the level below it.
• Discuss the roles of bacteria in the nitrogen cycle.
  • Bacteria are essential for nitrogen fixation ($N_2 \rightarrow NH_4^+$), nitrification (converting ammonium to nitrites and nitrates), and denitrification (returning nitrogen to the atmosphere).