Study Notes: Energy Flow, Productivity, and Food Webs

1.8 Primary Productivity

• PP basics (conceptual):

  • PP = rate of photosynthesis by all producers in an area over a period of time.

  • Since photosynthesis leads to growth, PP can be thought of as the amount of plant growth in an area over a period.

  • Higher PP correlates with more plant growth and more food/shelter for animals.

  • Ecosystems with high PP tend to be more biodiverse (greater species variety).

  • PP drives energy input into trophic webs and supports biomass at higher trophic levels.

• Practical implications and real-world relevance:

  • Environmental factors that boost PP (e.g., light, nutrients, water) support higher biodiversity and ecosystem services (food, climate regulation).

  • In aquatic systems, depth and light quality influence which algae and aquatic plants dominate.

  • Understanding PP helps explain why some ecosystems (e.g., reefs, temperate rainforests) are hotspots of productivity and biodiversity.

• Factors shaping PP across biomes (Page 6 content):

  • Trends in productivity: wetter environments, higher temperatures, and higher nutrient availability tend to increase NPP (net primary productivity).

  • Shortages in water, nutrients, or suitable temperatures reduce NPP.

  • Exercise prompts: predict the most productive vs. least productive terrestrial and aquatic biomes.

  • Examples:

  • Desert: low water and nutrients → low PP.

  • Tundra: low temperature with limited liquid water → low PP.

  • Open ocean: low nutrient availability → relatively low PP.

• Connecting concepts (brief):

  • PP sets the energy budget for an ecosystem; higher PP generally supports higher biodiversity.

  • PP is the starting point for energy flow through trophic levels and for biomass accumulation.

1.9 & 1.10 Trophic Levels & The 10% Rule

• The 10% rule (core idea):

  • Only about 10% of the energy at one trophic level becomes available to the next level.

  • Approximately 90% of energy is used for metabolic processes or lost as heat as you move up each level.

  • This creates a pyramid-shaped distribution of biomass and energy across trophic levels.

• Conservation concepts (thermodynamics and matter):

  • First law of thermodynamics (energy conservation): energy cannot be created or destroyed; it changes form.

  • Biogeochemical cycles demonstrate conservation of matter (C, N, H2O, P) across ecosystems.

  • Food webs demonstrate conservation of energy; energy is transferred from producers to consumers with losses at each step.

  • Example: When a rabbit eats a leaf, the leaf’s chemical energy (glucose) is transferred to the rabbit and stored in biomass (fat/muscle).

  • Second law of thermodynamics (entropy and dissipation): every energy transfer loses usable energy as heat; this underpins the 10% rule.

• Biomass and energy transfer (Page 12–14 content):

  • Biomass transfer follows the 10% rule: only about 10% of biomass at one level can be supported at the next level.

  • Typical numerical illustration (conceptual):

  • Producers ≈ 1000 kg → Primary ≈ 100 kg → Secondary ≈ 10 kg → Tertiary ≈ 1 kg.

  • Biomass at the next level is obtained by dividing the current level’s biomass by 10:
    $E<em>{ ext{next}} = \frac{E</em>{ ext{current}}}{10}.$

  • A related numerical example (to illustrate a common classroom problem):

  • If producers generate 100,000 J, energy to tertiary would be
    $E_{ ext{tertiary}} = 100{,}000 \times (0.10)^3 = 100\ \text{J}.$

• Practical implications and applications:

  • The efficiency of energy transfer limits the number of trophic levels and the total biomass that can be supported at higher levels.

  • This explains why large apex predators require vast habitats and abundant lower-level biomass; energy bottlenecks constrain population sizes.

  • Real ecosystems show variations in efficiency depending on ecosystem structure and consumer diets.

1.11 Food Chains and Food Webs

• Food web basics: energy and matter flow through an ecosystem from organism to organism.

  • Arrows in food webs indicate the direction of energy flow (the predator/consumer gains energy from the prey).

  • When an organism is eaten, matter (C, N, H2O, P) and energy (glucose, muscle tissue) are transferred to the predator.

• Food chain vs. food web: structural differences

  • Food chain: a single linear path of energy transfer (e.g., grass → hare → owl).

  • Food web: multiple interlocking food chains; organisms can occupy different trophic levels in different interactions.

  • Examples from the transcript:

  • Grass → hare → owl (where hare is a primary consumer and owl is a tertiary consumer).

  • Grass → grasshopper → robin → owl (grass → primary consumer → secondary consumer → tertiary consumer).

• Interactions and trophic cascades: ecological ripple effects

  • Food webs show how changes in population size of one species ripple through the web.

  • Trophic cascade example: increase in python population → decrease in frog and rat populations → increase in grasshopper population → decrease in corn production.

  • Classic cascade example: decline in wolves → deer population increases → overgrazing → decline in trees.

• Summary takeaway:

  • Primary productivity sets the energy budget for ecosystems. GPP represents total energy captured; RL reduces that energy through metabolism; NPP is the energy available to support herbivores and higher trophic levels. Energy transfer between levels is inefficient, with roughly 10% passing to each successive level, shaping the structure of food chains and webs and driving biodiversity and ecosystem dynamics.

1.8 Primary Productivity

• PP basics:

  • PP = rate of photosynthesis/plant growth by producers.

  • Higher PP correlates with more plant growth, food, shelter, and biodiversity.

  • Drives energy input into trophic webs.

• Practical implications:

  • Environmental factors (light, nutrients, water) boost PP, enhancing biodiversity and ecosystem services.

  • Explains productivity hotspots (e.g., reefs, rainforests).

• Factors shaping PP:

  • Wetter, warmer, and nutrient-rich environments increase Net Primary Productivity (NPP).

  • Shortages of water, nutrients, or suitable temperatures reduce NPP.

  • Examples: Desert, Tundra, Open Ocean have low PP due to limiting factors.

• Connecting concepts:

  • PP sets an ecosystem's energy budget and supports biodiversity.

  • It's the starting point for energy flow and biomass accumulation.

1.9 & 1.10 Trophic Levels & The 10% Rule

• The 10% rule:

  • ~10% of energy transfers from one trophic level to the next.

  • ~90% is lost as metabolic use or heat.

  • Creates a pyramid of biomass and energy.

• Conservation concepts:

  • First law of thermodynamics: energy is conserved, changes form.

  • Second law of thermodynamics: energy transfers lose usable energy as heat (explains 10% rule).

  • Matter (C, N, H2O, P) is conserved in biogeochemical cycles.

• Biomass and energy transfer:

  • Biomass transfer also follows the ~10% rule.

  • Example: If producers generate $100{,}000\ \text{J}$, tertiary consumers receive $100\ \text{J}$ ($100{,}000 \times (0.10)^3$).

• Practical implications:

  • Limits trophic levels and biomass at higher levels.

  • Explains apex predators' need for vast habitats due to energy bottlenecks.

1.11 Food Chains and Food Webs

• Food web basics:

  • Shows energy/matter flow through an ecosystem.

  • Arrows indicate energy direction (consumer gains from prey).

• Food chain vs. food web:

  • Food chain: single, linear energy transfer path (e.g., grass
    hare
    owl).

  • Food web: multiple interlocking food chains; organisms can occupy different trophic levels.

• Interactions and trophic cascades:

  • Population changes in one species ripple through the web (trophic cascade).

  • Example: Decline in wolves
    deer increase
    overgrazing
    tree decline.

• Summary takeaway:

  • NPP sets ecosystem energy budgets.

  • Inefficient energy transfer (~10%) structures food chains/webs, driving biodiversity and dynamics.