BIOLOGY 20 ECOSYSTEMS: Ecological Pyramids

Ecological Pyramids

Ecological pyramids are graphical representations used in biology, specifically within the study of ecosystems, to illustrate the relative amounts of material or energy present at different trophic levels within a food chain or food web.

• Purpose: To visually demonstrate how quantities like number of organisms, biomass, or energy change as one moves up the trophic levels.
• Structure: The dimensions of each part (or level) of the pyramid are drawn proportionally to the amount of material or energy at that specific trophic level.
• Types: There are three primary types of ecological pyramids:
  1. Pyramid of Numbers
  2. Pyramid of Biomass
  3. Pyramid of Energy

Pyramid of Numbers

This type of pyramid represents the total number (n) of individual organisms present at each trophic level. Each level may encompass multiple species.

• Characteristics:
  • It can be inverted or spindle-shaped (narrow at the bottom, wider in the middle, and narrow again at the top).
  • The shape depends on the specific structure of the food chain and the relative sizes of the organisms at the base.
• Reasons for Inversion/Spindle Shape:
  • Inverted: A single, large producer (e.g., a tree) can support a vast number of primary consumers (e.g., insects). Similarly, a single host organism can be home to numerous parasites (often tertiary consumers).
  • Spindle-shaped: This occurs when a producer level supports fewer primary consumers, which in turn support an even greater number of secondary or tertiary consumers.
• Examples:
  • Inverted: One primary producer (e.g., a large tree) $\rightarrow$ many herbivores $\rightarrow$ fewer carnivores. Or, one host organism $\rightarrow$ numerous parasites.
  • Spindle-shaped: Producers $\rightarrow$ Herbivores $\rightarrow$ Carnivore (fewer carnivores than herbivores, but more herbivores than producers if the producer is very large).

Pyramid of Biomass

This pyramid illustrates the dry mass of living organic material present at each trophic level. Biomass is typically measured in units like grams per square meter ($gm/m^2$) or kilograms per square kilometer ($kg/km^2$).

• Characteristics:
  • It may be inverted.
  • Inverted pyramids of biomass are common in aquatic ecosystems.
• Reasons for Inversion:
  • In aquatic environments, producers (e.g., phytoplankton) often have very short lifespans and high reproductive rates. At any given moment, their total biomass may be less than the biomass of the primary consumers (e.g., zooplankton) that they support, because the producers are consumed almost as quickly as they reproduce.
• Examples:
  • Upright: Grass ($2.0925 \times 10^7 kg/km^2$) $\rightarrow$ Snowshoe hare ($20,925 kg/km^2$) $\rightarrow$ Red fox ($2100 kg/km^2$) $\rightarrow$ Wolf ($4200 kg/km^2$) (Note: The wolf example shows an anomaly in this particular upright example, where the tertiary consumer's biomass is higher than the secondary consumer's. This might be a specific ecosystem characteristic or an illustrative example, but general rule is decrease).
  • Inverted in an Aquatic Ecosystem: Producers ($4-6 gm/m^2$) $\rightarrow$ Herbivores ($8 gm/m^2$) $\rightarrow$ Carnivores ($12 gm/m^2$) – where the biomass increases at higher trophic levels.

Pyramid of Energy

This pyramid depicts the total amount of chemical energy stored at each trophic level. Energy is typically measured in kilojoules ($kJ$) or kilocalories ($kcal$).

• Characteristics:
  • It is always an upright pyramid.
  • This is a fundamental principle of ecology, reflecting the laws of thermodynamics.
• Reason for Always Being Upright:
  • Energy transfer from one trophic level to the next is inherently inefficient. A significant portion of energy is lost at each transfer, primarily as heat or through metabolic processes, meaning there is less energy available at successively higher trophic levels.
• Examples Illustrating Decreasing Energy Flow:
  • Producers: $1,000,000 kcal$
  • Primary Consumers: $100,000 kcal$
  • Secondary Consumers: $10,000 kcal$
  • Tertiary Consumers: $1000 kcal$
  • Another example: Producers ($10,000 kcal$) $\rightarrow$ Primary Consumer ($1000 kcal$) $\rightarrow$ Secondary Consumer ($100 kcal$) $\rightarrow$ Tertiary Consumer ($10 kcal$).

The 10% Rule

This ecological rule quantifies the energy transfer efficiency between adjacent trophic levels.

• Principle: On average, only about $10\%$ of the chemical energy from one trophic level is successfully transferred and incorporated into the biomass of the next higher trophic level.
• Calculation: To determine the energy transferred to the next level, you can use the formula:
  $\text{Energy in next level} = \text{Energy in 1st level} \times 0.10$
• Significance: This rule explains why food chains typically have only three to five trophic levels; there simply isn't enough energy remaining to support more levels.

Where Does the Lost Energy Go?

The remaining approximately $90\%$ of energy at each trophic level is not transferred to the next level due to various biological processes and thermodynamic losses.

• Metabolic Activities: Organisms utilize a large portion of the ingested energy to perform essential life functions for their own survival, including:
  • Finding food and water
  • Seeking shelter
  • Escaping predators
  • Reproduction (mating)
• Waste Products: Energy is lost from organisms in the form of chemical bonds within waste materials:
  • Carbon dioxide ($CO_2$) exhaled during respiration
  • Urine
  • Feces
• Heat Radiation: A significant amount of energy is radiated out from organisms, particularly mammals and birds (endotherms), in the form of heat due to metabolic processes required to maintain body temperature.