Energy Flow and Species Interactions in Ecosystems

Energy & Ecosystems

Organisms & Energy Transfer

Understanding how energy moves through ecosystems is fundamental to ecology. Organisms are organized into trophic structures, which describe their roles in energy transfer and their relationships within food chains and food webs.

• Trophic Structure: The hierarchical levels in an ecosystem, consisting of producers, consumers, and decomposers, that describe how energy flows from one organism to another.

• Producers (Autotrophs): Organisms, such as plants and algae, that convert solar energy into chemical energy through photosynthesis.

• Consumers (Heterotrophs): Organisms that obtain energy by eating other organisms. These include herbivores (primary consumers), carnivores (secondary and tertiary consumers), and omnivores.

• Decomposers: Organisms such as bacteria and fungi that break down dead organic matter, recycling nutrients back into the ecosystem.

• Food Chain: A linear sequence showing how energy and nutrients flow from one organism to another.

• Food Web: A complex network of interconnected food chains in an ecosystem, illustrating the multiple pathways energy can take.

• Energy Flow: Energy enters ecosystems through sunlight and is transferred through trophic levels, but much is lost as heat at each step.

• 10% Rule: Only about 10% of the energy at one trophic level is transferred to the next; the rest is lost, primarily as heat.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed. In ecosystems, energy is transformed from light to chemical energy and then to heat.

• Number of Trophic Levels: The loss of energy at each level limits the number of trophic levels in an ecosystem, typically to four or five.

Example: In a grassland ecosystem, grass (producer) is eaten by a grasshopper (primary consumer), which is eaten by a frog (secondary consumer), which is eaten by a snake (tertiary consumer).

Additional info: The decrease in available energy at higher trophic levels explains why there are more producers than consumers in an ecosystem.

Species Interactions & Population Trends

Species interact in various ways that influence population sizes and ecosystem stability. These interactions, along with environmental factors, determine how populations grow and how ecosystems function.

• Limiting Factor: Any biotic or abiotic factor that restricts the size of a population.

• Logistic Growth Model: Describes population growth that slows as it approaches carrying capacity due to limiting factors.

• Carrying Capacity (K): The maximum population size that an environment can sustain indefinitely.

• Biotic Factors: Living components (e.g., predators, competition, disease) that affect population size.

• Abiotic Factors: Non-living components (e.g., temperature, water, nutrients) that affect population size.

• Keystone Species: A species that has a disproportionately large effect on its ecosystem relative to its abundance. Removal of a keystone species can cause dramatic changes in ecosystem structure.

• Trophic Cascade: Occurs when changes at one trophic level cause ripple effects throughout other levels. Can be top-down (controlled by predators) or bottom-up (controlled by resource availability).

• Top-Down Regulation: Population sizes are controlled by predators at higher trophic levels.

• Bottom-Up Regulation: Population sizes are controlled by the availability of resources at lower trophic levels.

• Green World Hypothesis: Suggests that predators keep herbivore populations in check, preventing them from consuming all plant life.

Example: The removal of sea otters (a keystone species) from kelp forest ecosystems leads to an increase in sea urchin populations, which overgraze kelp and reduce biodiversity—a classic example of a top-down trophic cascade.

Additional info: Logistic growth can be modeled by the equation:

$N_{t+1} = N_t + rN_t \left(1 - \frac{N_t}{K}\right)$

where $N_t$ is the population size at time t, $r$ is the intrinsic rate of increase, and $K$ is the carrying capacity.

Key Vocabulary

• Decomposer

• Decomposition

• Ecosystem

• Energy Flow

• Food Chain

• Food Web

• Herbivore

• Predation

• Consumer

• Producer

• 10% Rule

• Keystone Species

• Limiting Factor

• Logistic Growth Model/Carrying Capacity

• Biotic Factor

• Abiotic Factor

• Trophic Cascade (Top-Down, Bottom-Up)

• Green World Hypothesis

Summary Table: Top-Down vs. Bottom-Up Regulation

Regulation Type	Controlled By	Example
Top-Down	Predators/Consumers	Sea otters controlling sea urchin populations
Bottom-Up	Resource Availability (e.g., nutrients, sunlight)	Plant abundance limiting herbivore populations

Summary Table: Trophic Levels and Energy Transfer

Trophic Level	Type of Organism	Approximate Energy Transferred (%)
Producers	Plants, Algae	100
Primary Consumers	Herbivores	10
Secondary Consumers	Carnivores	1
Tertiary Consumers	Top Carnivores	0.1