AS

Food Webs and Energy Flow in Ecosystems

Food Webs: Energy Flux Through Ecosystems

Energy Fundamentals

  • Energy Definition: The ability to do work.

    • Potential Energy: Stored energy. Examples include energy in chemical bonds, concentration gradients, or positional energy.

    • Kinetic Energy: Energy of motion or actively doing work. Examples include electrical energy, heat, and the movement of molecules or organisms.

  • First Law of Thermodynamics: Energy can change forms (e.g., kinetic to potential or vice versa), but it cannot be created or destroyed. The total amount of energy in an isolated system remains constant.

Trophic Levels & Energy Flow

  • Trophic Level Definition: The feeding level of an organism, based on its role in the flow of energy through ecological communities.

  • Producers (Autotrophs):

    • Organisms that produce their own food, typically through photosynthesis (using solar energy) or chemosynthesis.

    • Convert kinetic energy (e.g., solar energy) into potential energy (chemical energy stored in organic molecules).

    • Autotroph: Means "self-feeder" (Auto = self, troph = feeder).

    • Average Net Primary Productivity (ANPP): The amount of energy accumulated by producers in a given region during photosynthesis.

      • Typically expressed as grams of organic carbon per square meter per year (g ext{ }C/m^2/yr).

      • Depends on various environmental factors: available sunlight, water, nutrients, and temperature.

      • Global productivity patterns show significant variation, with high productivity in areas like algal beds, reefs, tropical wet forests, and estuaries, and low productivity in open oceans, tundras, and deserts.

  • Consumers (Heterotrophs):

    • Organisms that obtain energy by consuming other organisms.

    • Convert potential energy (from the food they eat) back into kinetic energy (for movement, heat, and other metabolic activities).

    • Heterotroph: Means "other-feeder" (Hetero = other, troph = feeder).

    • Types of Consumers within a Food Chain & Web:

      • Primary Consumers (Herbivores): Feed exclusively on producers (e.g., grasshoppers eating flowering plants and grass).

      • Secondary Consumers (Carnivores): Eat the flesh of other animals (primary consumers).

      • Tertiary Consumers (Carnivores): Eat other carnivores (secondary consumers).

      • Quaternary Consumers: Eat tertiary consumers.

  • Food Chains vs. Food Webs:

    • Food Chain: A simplified, linear representation of energy transfer, showing a single pathway (e.g., plant
      ightarrow herbivore
      ightarrow carnivore).

    • Food Web: A more realistic and complex representation showing all interconnected feeding relationships within an ecosystem, illustrating multiple pathways of energy flow. Food webs provide a comprehensive view of trophic relationships and often omit decomposers for simplicity in some diagrams.

  • Decomposers/Detritivores:

    • Organisms that obtain energy by breaking down dead organic matter (detritus) and waste products.

    • Detritivores: A diverse group including worms, insects (e.g., millipedes, pillbugs), and scavengers (e.g., vultures) that consume wastes and dead remains.

    • Decomposers (e.g., Bacteria and Fungi): Digest food externally, releasing nutrients back into the soil and water, which are then available for producers. They play a crucial role in nutrient recycling.

Trophic Energy Transfer and Efficiency

  • Energy Transfer Efficiency: The transfer of energy from one trophic level to the next is not 100% efficient. Much energy is lost at each step.

  • Second Law of Thermodynamics: States that the transfer of energy is inefficient, leading to an increase in entropy (disorder) in a system. In biological systems, this means a portion of energy is always lost as heat during metabolic processes.

  • The 10% Rule: On average, only about 10\% of the energy from one trophic level becomes incorporated into the biomass of the next trophic level.

    • Example (Ecological Energy Pyramid): If primary producers have 10,000 J of energy, primary consumers will assimilate about 1,000 J, secondary consumers 100 J, and tertiary consumers 10 J. This means 1,000,000 J of sunlight may lead to only 10,000 J in primary producers.

  • Where Energy Goes When "Lost": The energy not transferred to the next trophic level is primarily:

    • Lost as Heat: During metabolic activities (respiration) of organisms at each trophic level (as per the second law of thermodynamics).

    • Used for Maintenance: Energy is expended for daily life functions like movement, growth, and reproduction.

    • Enters the Decomposer Food Web: Undigested waste products and dead organisms from all trophic levels become detritus, providing energy for decomposers.

  • Case Study: Silver Springs, Florida (Annual Energy Flow Data from the 1970s)

    • Only a small fraction of incoming sunlight (e.g., 1,700,000 kcal/m²/yr) is captured by primary producers (20,810 kcal/m²/yr).

    • Transfer Efficiency Calculations:

      • Primary producers to Primary consumers: (3,368 \text{ kcal/m}^2/ ext{yr}) / (20,810 \text{ kcal/m}^2/ ext{yr}) \approx 16\%

      • Primary consumers to Secondary consumers: (383 \text{ kcal/m}^2/ ext{yr}) / (3,368 \text{ kcal/m}^2/ ext{yr}) \approx 11\%

      • Secondary consumers to Tertiary consumers: (21 \text{ kcal/m}^2/ ext{yr}) / (383 \text{ kcal/m}^2/ ext{yr}) \approx 5\%

    • The remaining energy is largely lost to respiration, heat, or transferred to decomposers (5,060 kcal/m²/yr).

  • Case Study: Hubbard Brook Forest LTER, NH

    • Only 0.8\% of incoming solar energy (1,254,000 kcal/m²/yr) is captured by photosynthesis.

    • Of this captured energy:

      • 45\% supports net primary production (growth).

      • 55\% is lost to maintenance activities or as heat.

      • 11\% enters the consumer food web.

      • 34\% enters the decomposer food web as dead material.

  • Implications of Energy Transfer Efficiency:

    • Limits to Trophic Levels: The significant energy loss at each step limits the number of trophic levels in an ecosystem, typically to 3-5 levels, because there isn't enough energy to support more.

    • Human Diet Impact: Supporting a population as vegetarians requires less energy in the food web compared to supporting them as meat-eaters, as fewer trophic levels are involved, reducing energy loss.

    • Ecological Pyramids: The decreasing energy at successive trophic levels results in biomass and numbers pyramids, where the base (producers) is typically much larger than the top (tertiary consumers).

Biomagnification (Biological Magnification)

  • Definition: The increasing accumulation and concentration of toxic chemicals in organisms at higher trophic levels within a food web.

  • Mechanism: Persistent toxins, which are not easily metabolized or excreted, accumulate in the tissues of organisms. When a predator consumes many prey, it ingests and accumulates the toxins from all those prey, leading to higher concentrations at successively higher trophic levels.

  • Examples and Human Health Impacts:

    • DDT (Dichlorodiphenyltrichloroethane): A pesticide that biomagnified, causing reproductive problems in top predators like peregrine falcons and bald eagles. It was also found in human mother's milk, raising health concerns.

    • PCBs (Polychlorinated Biphenyls): Industrial chemicals that biomagnify. In humans, exposure can cause skin rashes, ocular lesions, irregular menstrual cycles, lowered immune response, liver damage, cancer, birth defects, and poor cognitive development.

    • Fishing Guidelines: Regulatory bodies (e.g., Wisconsin DNR) issue safe-eating guidelines for fish due to biomagnification of pollutants (e.g., mercury, PCBs), advising different consumption limits for various fish species based on human demographics (e.g., women of childbearing age, children).

Benefits of Biodiversity to Humans

  • Definition: Biodiversity refers to the variety of life on Earth at all its levels, from genes to ecosystems.

  • Direct Benefits:

    • Drugs and Medicines: More than half of all prescription drugs contain natural products or are derived from natural compounds.

    • Food: As many as 80,000 edible wild plant species could be utilized by humans, providing a vast genetic resource for agriculture.

  • Ecological Benefits (Ecosystem Services):

    • Natural processes essential for life, including:

      • Soil formation and stabilization.

      • Air and water purification.

      • Flood protection and regulation.

      • Nutrient cycling.

      • Solar energy absorption.

      • Pest control.

      • Food production.

      • Community stability and resilience against disturbances.

  • Aesthetic and Cultural Benefits:

    • Spiritual Significance: Many cultures and religions find spiritual value in nature and its diversity.

    • Recreation: Biodiversity supports a wide range of wildlife-related recreation, such as hunting, fishing, bird watching, and scuba diving. The U.S. Fish and Wildlife Service estimates Americans spend $156.3 billion annually on wildlife recreation, involving 40\% of the population.

    • Ecotourism: Can be an important form of sustainable economic development in biodiverse regions.

  • Existence (Intrinsic) Value: The belief that biodiversity has inherent worth, independent of its utility to humans. Organisms and ecosystems have a right to exist for their own sake.