Ecology: Cycling of Matter, Population Dynamics, and Human Impact

Timeline of Earth and the Origin of Life

• Earth's Formation ($4.6$ billion years ago): The planet formed, the first oceans appeared, and the atmosphere developed gradually over time.
• Appearance of Organisms ($2.4$ billion years ago):
  • The first life forms were unicellular prokaryotes.
  • Eventually, multicellular organisms evolved from these simpler forms.
• Classification of All Organisms: All living things are categorized based on how they obtain energy:
  • Autotrophs: Organisms that produce their own food (e.g., plants, algae, some bacteria, phytoplankton).
  • Heterotrophs: Organisms that do not make their own food and must consume others (e.g., animals, fungi, most protozoa, most bacteria).

Fundamentals of Ecology

• Definition: Ecology is the branch of biology that studies the relationships and interactions between organisms and their environment. It focuses on the balance between living (biotic) and nonliving (abiotic) factors.
• Ecological Balance: Studies include food chains, ecosystems, biodiversity, and how living things interact with components like air, water, soil, and climate.

Hierarchical Levels of Organization

• Individual (Organism): Any single plant or animal.
• Population: A group of interacting individuals of the same species that occupy the same area at the same time and can interbreed.
• Community: All different populations of various species interacting and occupying the same place (the biotic component of an area).
• Ecosystem: A community of living organisms plus their physical, nonliving (abiotic) environment.
• Biome: A large region of the world characterized by a specific climate and the plants and animals adapted to it.
  • Example: The coral reef biome, which includes warm, shallow, sunlit ocean areas supporting corals, fish, sea turtles, and invertebrates.
  • Other Examples: Deserts, rainforests, tundra, grasslands.
• Biosphere: All parts of the Earth where life exists, including land, water, and the air.

Important Ecological Terminology

• Habitat: The specific place where a population lives.
• Niche: The specific role an organism plays in its environment, including what it eats, where it lives, its predators, its habitat, and its position in the food chain (e.g., a Blue Jay's niche).
• Biotic vs. Abiotic:
  • Biotic: Living factors (e.g., plants, animals, bacteria).
  • Abiotic: Nonliving factors (e.g., air, water, soil, sunlight).

Energy Transfer in Ecosystems

• Producers (Autotrophs): Located at the base of the food chain; they make organic nutrients using photosynthesis.
• Consumers (Heterotrophs): Cannot make their own food and must eat other organisms.
  • Primary Consumers ($1^{\circ}$): Feed directly on producers (herbivores).
  • Secondary Consumers ($2^{\circ}$): Feed on primary consumers.
  • Tertiary Consumers ($3^{\circ}$): Feed on secondary consumers.
• Consumer Types by Diet:
  • Herbivores: Eat only plants.
  • Carnivores: Eat only animals.
  • Omnivores: Eat both plants and animals.
• Decomposers: Organisms (like fungus and bacteria) that break down dead plants, animals, and organic waste, returning nutrients to the ecosystem.
• Food Chains vs. Food Webs:
  • Food Chain: A linear sequence showing how energy is transferred from one organism to another.
  • Food Web: A complex network of multiple interacting food chains that reflects biodiversity and the reality that nothing in nature stands alone.

Trophic Levels and the $10\%$ Energy Rule

• Trophic Level: The position an organism occupies based on how it acquires energy and nutrients.
• The $10\%$ Rule: Only $10\%$ of the energy available at one trophic level is transferred to the next higher level. The remaining $90\%$ is lost as heat or used for metabolic processes.
  • Example Energy Pyramid:
    • Trophic Level 1 (Producers): $10,000\,Kcal$
    • Trophic Level 2 (Primary Consumers): $1,000\,Kcal$
    • Trophic Level 3 (Secondary Consumers): $100\,Kcal$
    • Trophic Level 4 (Tertiary Consumers): $10\,Kcal$
• Efficiency: Plants (producers) have the most energy available; energy transfer is only $10\%$ efficient.

Bioaccumulation and Biomagnification

• Bioaccumulation: The accumulation of toxins within a single organism over its lifetime.
• Biomagnification: The increase in toxin levels as you move up the food chain. Predators accumulate all the toxins (such as methylmercury) present in the bodies of all the prey they consume.
  • Case Study (Tuna): Plankton take up traces of mercury. Small fish eat the plankton, larger fish eat the small fish, and tuna eat those fish. By the time it reaches the tuna, the concentration of mercury is high, which is why humans are cautioned against high tuna consumption.

Symbiotic Relationships

• Mutualism ($+/+$): Both species benefit.
  • Example: Sea anemone and clownfish.
• Commensalism ($+/0$): One species benefits, the other is unaffected.
  • Example: Whale and barnacle.
• Parasitism ($+/\text{-}$): One species benefits, the other is harmed.
  • Example: Dog and tick.
• Competition ($\text{-}/\text{-}$): Multiple species or individuals compete for the same limited resources (food, water, space); the interaction is generally taxing for both.

Biogeochemical Cycles (Cycling of Matter)

The Elements of Life

The human body is composed of approximately $62\%$ water, $16\%$ protein, $16\%$ fat, $6\%$ minerals, and $1\%$ carbohydrate. Key elements include Oxygen ($65\%$), Carbon ($18\%$), Hydrogen ($9.5\%$), and Nitrogen ($3.2\%$).

The Phosphorus Cycle ($P$)

• Importance: Essential for building DNA, RNA, ATP, cell membranes, and bones.
• The Process:
  1. Weathering of phosphate rocks releases phosphorus into soil and water.
  2. Plants take up phosphorus from the soil.
  3. Phosphorus passes through food webs.
  4. Waste and decomposition return it to sediments.
  5. Over long time scales, geological processes form new rock.

The Nitrogen Cycle ($N$)

• Importance: Required for amino acids, proteins, and nucleic acids. Atmosphere is $78\%$ Nitrogen gas ($N_2$), but it is unusable in that form.
• The Process:
  1. Nitrogen Fixation: Bacteria (often in legume root nodules) or lightning convert $N_2$ gas into Ammonia ($NH_3$).
  2. Nitrification: Nitrifying bacteria convert Ammonia ($NH_3$) into Nitrites ($NO_2^-$) and Nitrates ($NO_3^-$) for plant use.
  3. Assimilation: Animals get nitrogen by eating plants.
  4. Decomposition: Waste returns nitrogen to the soil.
  5. Denitrification: Denitrifying bacteria convert nitrates back into $N_2$ gas to return to the air.

The Carbon Cycle ($C$)

• Importance: Carbon travels from the atmosphere to Earth and back in a closed environment.
• The Process:
  • Photosynthesis: Plants absorb $CO_2$ to produce glucose ($C_6H_{12}O_6$) and Oxygen ($O_2$).
  • Respiration: Animals and plants use oxygen to produce energy (ATP), exhaling $CO_2$ as a byproduct.
  • Storage: Carbon is stored in soil, rocks, coal deposits, and the deep ocean.
  • Human Impact: Burning fossil fuels and deforestation release excess carbon, leading to an unbalanced cycle and global climate change.

Population Ecology

• Density: The number of individuals per unit area or volume (how crowded a population is).
• Dispersion: How individuals are spaced.
  • Clumped: Groups (e.g., schools of fish, elephant herds).
  • Uniform: Evenly spaced, often due to territoriality (e.g., nesting penguins).
  • Random: Unpredictable spacing (e.g., wind-blown seeds).
• Demographics: The statistical study of population traits like age, gender, birth rates, and death rates.
  • Population Pyramids:
    • Large base: Increasing population (many young).
    • Narrow base: Decreasing population (few young).
    • Straight sides: Stable population (similar numbers across ages).

Population Growth Models

• Exponential Growth: Unlimited growth represented by a "J-curve." The birth rate and death rate are constant.
• Logistic Growth: Limited growth represented by an "S-curve." Growth slows as it reaches the environment's limit.
• Carrying Capacity ($K$): The maximum number of individuals that an area's natural resources can support. As a population approaches $K$, competition increases.

Factors Regulating Population Size

• Density-Dependent Factors: Stronger as density increases (e.g., competition for food/space, disease, parasitism, predation).
• Density-Independent Factors: Affect populations regardless of density (e.g., weather events, natural disasters, climate change, human disturbances like pollution).

Human Impact and Global Change

• Human Population Growth: Growth was slow until the 1900s, then transitioned to a near-exponential phase due to better sanitation, medicine, and agricultural improvements. The population reached $8$ billion in $2022$.
• Greenhouse Effect: A natural process where gases (like $CO_2$, $CH_4$, $N_2O$, and water vapor) trap heat to keep Earth livable.
  • Runaway Greenhouse Effect: Humans adding too many gases (via fossil fuels and deforestation) leads to overheating, melting ice, rising sea levels, and extreme weather.
• Algal Blooms: Caused by excess nitrogen and phosphorus fertilizers running into water, leading to rapid algae overgrowth that blocks sunlight and depletes oxygen.
• Overfishing: Some fisheries are exhausted; global catches have decreased as oceans run out of fish. Solutions include "No Fish Zones" like the Papahānaumokuākea Marine National Monument, which shows a "spillover benefit" where fish populations recover and move into fishable waters.
• Specific Ecological Threats:
  • Deforestation: Loss of habitat and contributor to climate change.
  • Acid Rain: Caused by atmospheric pollution reactions.
  • Urbanization: Increases pollution and destroys habitats.
  • Beach Erosion: Caused by coastal development.
  • Waste Lagoons: Such as hog farm waste in North Carolina contaminating water.
  • Invasive Species: Nonnative species outcompeting native wildlife.

Conservation and Personal Action

• The Three Rs: Reduce, Reuse, Recycle (paper, glass, metal, electronics).
• Plastic Reduction: Say no to single-use plastics; use reusable containers.
• Energy Efficiency: Turn off lights, use energy-efficient appliances, and support electric vehicles.
• Restoration: Replant trees to absorb $CO_2$ and participate in coral planting programs.
• Awareness: Share information and inspire others to create a "ripple of change."