Marine Science (0697) | Feeding relationships

🌊 Chapter 4.4: Feeding Relationships in the Marine Environment

Introduction: The Ocean’s Food Fight!

Hello Marine Scientists! This chapter is all about how energy moves through the ocean—who eats whom, and what happens to the energy along the way. Understanding these relationships is fundamental to marine ecology, as it explains why some ecosystems are massive and others are small, and why removing one species can cause the whole system to crash.
Don't worry if these terms seem tricky at first. Think of it like a massive, interconnected cooking process, where the Sun provides the initial heat, and every organism is either a cook, a diner, or a recycler!

1. The Source of Energy

The Ultimate Power Source (4.4.1)

The vast majority of energy that powers biological systems on Earth, including marine ecosystems, comes from one place: the Sun.
Marine organisms that capture light energy (like plants on land) are the starting point for almost every food chain.

Did you know? Even in the deep, dark ocean, where sunlight never reaches, the energy originally came from the surface. The organisms down there rely on food (dead organic matter) sinking from the sunlit waters above!

2. Following the Energy Path: Chains and Webs

What is a Food Chain? (4.4.2)

A food chain shows the transfer of energy from one organism to the next, starting with an organism that produces its own food—the producer. The arrows in a food chain always point in the direction of energy flow (towards the organism that is doing the eating!).

Think of a food chain as a straight line, single path of energy.

Example of a simple marine food chain:
Phytoplankton (Producer) → Zooplankton (Eats Producer) → Small Fish (Eats Zooplankton) → Tuna (Eats Small Fish)

Food Webs: The Realistic View (4.4.3, 4.4.4)

In reality, organisms rarely eat just one type of prey. They have multiple food sources and are themselves eaten by multiple predators.

A food web is a much more complex picture—it is a network of interconnected food chains that represents the true feeding relationships in an ecosystem.

When asked to construct or interpret a food web, remember:

• Identify all the organisms.
• Draw arrows from the organism being eaten to the organism doing the eating.
• The web shows how energy can flow along multiple pathways.

Key Takeaway: Chains are simple diagrams; webs are complex networks that reflect real life.

3. Trophic Levels and Consumer Roles

Trophic Levels: Positions in the Food Chain (4.4.5)

A trophic level describes the position an organism occupies in a food chain, food web, or ecological pyramid.

The Trophic Hierarchy:

1. Trophic Level 1: Producers
These organisms make their own food, usually through photosynthesis (e.g., phytoplankton, seagrasses). They capture the Sun’s energy directly.

2. Trophic Level 2: Primary Consumers (4.4.7)
These are herbivores (4.4.8a). They eat the producers (e.g., zooplankton eating phytoplankton).

3. Trophic Level 3: Secondary Consumers (4.4.7)
These are carnivores (4.4.8b) that eat the primary consumers (e.g., sardines eating zooplankton).

4. Trophic Level 4: Tertiary Consumers (4.4.7)
These are carnivores that eat the secondary consumers (e.g., tuna eating sardines).

An organism like a large shark or an orca might be at Trophic Level 5 (a quaternary consumer), often called the apex predator.

Predators and Decomposers (4.4.9, 4.4.10)

A predator is an animal that captures, kills, and eats another animal, which is its prey. This describes a specific type of feeding interaction (hunting). A shark is a predator; a fish is its prey.

A decomposer is different. It gets its energy by breaking down dead or waste organic material. This includes things like bacteria and fungi. They are essential for cycling nutrients back into the ecosystem.

Analogy: Detritivores (like crabs) are the waste collectors who physically eat the scraps. Decomposers (like bacteria) are the microscopic chemists who break the scraps down into basic chemical parts.

4. Biomass and Energy Loss

Biomass (4.4.11)

Biomass is simply the total mass of living matter in a specific area or at a particular trophic level. It is usually measured as dry mass (mass without water content).

The Energy Tax: Why Food Chains are Short (4.4.12)

When energy is transferred from one trophic level to the next, a huge amount of it is lost—meaning the transfer is highly inefficient. This is why food chains typically only have 4 or 5 levels; there isn't enough energy left to support more animals.

Energy is primarily lost from the food chain in four ways:

1. Respiration: Energy is used by the organism for basic life processes, producing heat that is lost to the environment.
2. Movement: Energy used for swimming, hunting, or finding mates.
3. Excretion: Energy lost in waste products (e.g., faeces or urine) that were not fully digested.
4. Removal or Harvesting: Energy stored in organisms that die without being eaten, or are harvested by humans (fishing). This energy goes to decomposers, not the next trophic level.

Because of these losses, typically only about 10% of the energy from one trophic level is actually passed on to the next.

5. Representing Energy: Ecological Pyramids

Drawing and Interpreting Pyramids (4.4.13)

Ecological pyramids are graphical representations that show the relationship between different trophic levels. They are stacked horizontally, with the producer (Trophic Level 1) always forming the widest base.

1. Pyramid of Numbers

A pyramid of numbers shows the actual number of individual organisms at each trophic level.

• Interpretation: Usually pyramid-shaped (many producers supporting fewer primary consumers, etc.).
• Common Mistake: Sometimes, it can be inverted or look odd if one huge producer supports many tiny consumers (e.g., one large mangrove tree supporting thousands of insects).

2. Pyramid of Biomass

A pyramid of biomass shows the total mass of living matter (biomass) at each trophic level.

• Interpretation: Often pyramid-shaped, showing the massive reduction in living matter supported at higher levels.
• Exception: Can sometimes be inverted in marine environments! For example, a small standing biomass of fast-reproducing phytoplankton can support a larger biomass of slow-growing zooplankton at a snapshot in time. However, over a year, the total amount of biomass produced would still form a true pyramid.

3. Pyramid of Energy

A pyramid of energy shows the total energy content at each trophic level over a fixed period of time (e.g., kJ/m²/year).

• Interpretation: This pyramid must always be pyramid-shaped, meaning the base (producers) is always the widest.
• Why? Because energy is continuously lost at every step (respiration, movement, excretion). The second law of thermodynamics means you cannot create energy, so the amount of energy available at a higher trophic level can never exceed the energy available at the level below it.

Key Takeaway: If you are asked to draw a pyramid that is always upright, choose the Pyramid of Energy.