Welcome to Topic 2: Ecology!

Hello future environmental experts! This chapter is the absolute foundation of Environmental Systems and Societies. Ecology is essentially the study of how living things interact with each other and their environment. Understanding these interactions—how energy flows and how matter cycles—is vital for tackling sustainability challenges later in the course.

Don't worry if some terms feel scientific; we're going to break them down with simple examples and analogies. Let's dive into the living world!

Section 2.1: Ecosystems, Communities, and Niches

Defining the Components of Life

In ESS, we look at life at different organizational levels. It's like zooming out from a single organism to the entire planet.

Key Definitions
  • Population: A group of organisms of the same species living in the same area at the same time.
    (Example: All the deer in a single forest.)
  • Community: All the different populations (different species) living and interacting in the same area.
    (Example: The deer, the wolves, the pine trees, and the mushrooms in that forest.)
  • Habitat: The environment where a species normally lives. This is the organism's "address."
  • Ecosystem: A community of interdependent organisms interacting with the abiotic (non-living) components of their environment.
    (This is the whole system: the community + the rocks, soil, water, and air.)

Understanding the Niche

The concept of a niche is fundamental. It is not just where an organism lives (that's the habitat), but how it lives.

  • Niche: Describes the particular set of abiotic and biotic conditions and resources to which an organism or population responds. Think of the niche as the organism's "job" or "role" in the ecosystem.

A niche includes:

  1. The space it occupies (Habitat).
  2. The time it is active (Diurnal/Nocturnal).
  3. The resources it uses (Food, water).
  4. Its interactions with other species (Predator, prey, competitor).
Real vs. Ideal Niche

There are two types of niches:

  • Fundamental Niche: The *full range* of conditions and resources an organism *could* theoretically use if there were no competition or predators. (The perfect, ideal scenario.)
  • Realized Niche: The *actual* conditions and resources an organism uses due to competition or other limiting factors. (The reality.)

Quick Tip: Competition always forces the Fundamental Niche to shrink down to the Realized Niche.

Section 2.2: Energy Flow and Trophic Levels

Energy in almost all ecosystems starts with the sun. This energy flows through the ecosystem in a one-way path, but matter cycles repeatedly.

Trophic Levels: The Energy Pyramid

Trophic level refers to the position an organism occupies in a food chain.

  • Level 1: Producers (Autotrophs)
    Organisms that produce their own food, usually through photosynthesis (e.g., plants, algae). They convert solar energy into chemical energy.
  • Level 2: Primary Consumers (Herbivores)
    Eat producers (e.g., rabbits, cows).
  • Level 3: Secondary Consumers (Carnivores/Omnivores)
    Eat primary consumers (e.g., snakes, small fish).
  • Level 4: Tertiary Consumers (Top Carnivores)
    Eat secondary consumers (e.g., eagles, sharks).
  • Decomposers & Detritivores: These break down dead organic matter at all levels (e.g., fungi, bacteria, earthworms). They recycle nutrients back into the soil, completing the matter cycle.

The 10% Rule and Energy Loss

When energy moves from one trophic level to the next, a vast amount is lost.

  • Only about 10% of the energy stored in one trophic level is typically transferred and stored in the biomass of the next level.

Where does the other 90% go?

  1. It is lost as heat during respiration (\(R\)).
  2. It is stored in biomass that is uneaten by the next level (e.g., roots, bones).
  3. It is lost in waste products (feces) that are passed to decomposers.
Implications for Food Chains

Because energy decreases so rapidly, food chains rarely have more than 4 or 5 steps. The amount of biomass needed to support a top predator is huge!

Analogy: Imagine trying to fill a massive water tank (the top predator) using a tiny cup (the producer) that loses 90% of its water every time it's passed up the line. You need thousands of initial cups!

Quick Review: Pyramids of Numbers, Biomass, and Energy

We often represent trophic levels using pyramids:

  • Pyramid of Energy: Always wide at the base and narrow at the top, reflecting the 10% rule (energy loss). It is the most reliable pyramid.
  • Pyramid of Biomass: Represents the total mass of organisms at each level. It can sometimes be inverted (e.g., small standing stock of plankton supporting a large population of zooplankton in aquatic systems).
  • Pyramid of Numbers: Represents the number of individuals. Can often be inverted or irregular (e.g., one large tree supporting thousands of insects).

Section 2.3: Measuring Productivity (The Energy Budget)

Productivity is the rate at which energy is converted into biomass. This is a measure of how "busy" and efficient an ecosystem is.

Key Productivity Terms

  • Biomass: The total mass of organic matter in a defined area or trophic level (usually measured in \(g m^{-2}\) or \(J m^{-2} year^{-1}\)).
  • Respiration (\(R\)): The energy used by the organism for maintenance, movement, and heat loss. This energy is lost from the system.

The Two Kinds of Productivity

Don't worry, this concept is just like managing your money!

  1. Gross Primary Productivity (GPP)

    This is the total amount of energy captured by producers (plants) through photosynthesis per unit area per unit time. This is the Gross Income—the full amount of solar energy converted.

  2. Net Primary Productivity (NPP)

    This is the amount of energy left over *after* the producers have used some for their own respiration (\(R\)). This energy becomes the available biomass for the next trophic level. This is your Net Income (what you actually get to spend).

The relationship is shown by the critical formula:

\[NPP = GPP - R\]

Secondary Productivity (Consumers)

The same idea applies to consumers, though they get energy from eating, not photosynthesis:

  • Gross Secondary Productivity (GSP): The total energy assimilated by consumers (total food eaten and absorbed).
  • Net Secondary Productivity (NSP): The energy stored in the consumer's biomass (growth and fat) after respiration losses.

Key Takeaway: Highly productive ecosystems (like tropical rainforests and algal beds) can support more complex food webs and higher biomass because they have a large NPP.

Section 2.4: Biomes and Limiting Factors

What is a Biome?

A biome is a collection of ecosystems sharing similar climatic conditions and therefore having similar vegetation types. Biomes are global scale ecosystems.

Distribution and Climate

The distribution of biomes is largely determined by two critical abiotic factors:

  1. Temperature: Affects metabolic rates and water availability.
  2. Precipitation (Rainfall): Determines water availability for plants.

Other factors like altitude, sunlight, soil, and ocean currents also play a role.

Did You Know?

Temperature and precipitation create the famous Holdridge Life Zone chart, which predicts what biome will occur based solely on these two variables.

Limiting Factors

A limiting factor is a resource or environmental condition that restricts the growth, abundance, or distribution of an organism or population.

In terrestrial (land) biomes, the main limiting factors are often temperature, light, and nutrients (like nitrates or phosphates).

In aquatic (water) ecosystems, the main limiting factors are often light (which decreases with depth), salinity, and dissolved oxygen.

Key Takeaway: When you analyze an ecosystem, always identify the specific factors that are holding back growth. For example, in the desert, water is the primary limiting factor, while in the deep ocean, light is the limiting factor.

Section 2.5: Biogeochemical Cycles

While energy flows, chemical elements necessary for life (matter) must be constantly recycled. These cycles involve biotic components (bio) and geological components (geo) interacting chemically.

The Carbon Cycle

Carbon is the backbone of all organic life (biomass). The key processes moving carbon between its main stores (sinks) are:

  1. Photosynthesis: Removes CO\(_2\) from the atmosphere and stores it in plant biomass. (Transfer from atmosphere to biosphere).
  2. Respiration: Releases CO\(_2\) back into the atmosphere as organisms use stored energy. (Transfer from biosphere to atmosphere).
  3. Decomposition: Decomposers break down dead material, releasing CO\(_2\) into the atmosphere/soil.
  4. Combustion (Burning): Natural (fires) and anthropogenic (human-caused) burning releases CO\(_2\) from biomass or fossil fuel stores.
  5. Weathering and Sedimentation: Slow geological processes locking carbon into rocks (long-term sink).
Human Impact on the Carbon Cycle

Humans significantly increase the flux (movement rate) of carbon from long-term stores into the atmosphere by:

  • Burning Fossil Fuels: Releases carbon stored for millions of years (geosphere) directly into the atmosphere, increasing the greenhouse effect.
  • Deforestation: Removes carbon sinks (trees) and often involves burning, accelerating the transfer of carbon from the biosphere to the atmosphere.

The Nitrogen Cycle

Nitrogen is essential for proteins and DNA but is often a limiting factor because plants cannot directly use the abundant gaseous nitrogen (\(N_2\)) in the atmosphere.

This cycle requires specialized bacteria, making it complex. The main steps are:

  1. Nitrogen Fixation: Gaseous \(N_2\) is converted into usable forms (ammonia/ammonium) by nitrogen-fixing bacteria (often in legume roots) or by lightning.
  2. Nitrification: Bacteria convert ammonia into nitrites (\(NO_2\)) and then into nitrates (\(NO_3\)), which are easily absorbed by plants (assimilation).
  3. Assimilation: Plants absorb nitrates and incorporate the nitrogen into their biomass.
  4. Ammonification (Decomposition): Decomposers break down dead matter, releasing nitrogen back into the soil as ammonia.
  5. Denitrification: Bacteria convert nitrates back into atmospheric \(N_2\), completing the cycle.
Human Impact on the Nitrogen Cycle

The largest human impact comes from the Haber-Bosch process, which allows us to artificially fix nitrogen (create synthetic fertilizer).

This leads to:

  • Eutrophication: Excess nitrogen fertilizer washes into waterways, causing excessive algal growth, depleting oxygen, and killing aquatic life.
  • Acid Rain: Emissions from burning fossil fuels release nitrogen oxides, which contribute to acid deposition.

Quick Ecology Review and Encouragement

You've covered the heart of how ecosystems function!

Remember that the key conceptual link in this topic is systems: Energy is an open system (flows through and out), but matter is a closed system (cycles internally).

Mnemonic for Nitrogen Cycle: Feel Nice And Drink Delicious Nitrogen:

Fixation, Nitrification, AssImilation, Decomposition, Denitrification.

Keep these fundamental processes clear in your mind, and you will be well-prepared to analyze the human impacts we discuss in later ESS topics!