Ecosystem Study Guide

📖 Core Concepts Ecosystem – All living organisms (biotic) plus the non‑living environment (abiotic) that interact as a unit. Biotic components – Plants, animals, microbes, fungi. Abiotic components – Soil, water, climate, nutrients, light. Primary production – Conversion of inorganic carbon (CO₂) into organic matter via photosynthesis. Gross Primary Production (GPP) – Total carbon fixed by all photosynthesizing organisms. Net Primary Production (NPP) – GPP − plant respiration; the carbon available to other trophic levels. Net Ecosystem Production (NEP) – GPP − total ecosystem respiration (plants + microbes). When NEP > 0 the ecosystem stores carbon. Energy flow – Sunlight → primary producers → herbivores → carnivores → detritivores; each step loses ∼90 % of energy (10 % rule). Nutrient cycles – Nitrogen and phosphorus are recycled internally; carbon cycles in and out via photosynthesis and respiration. Disturbance & Succession – Discrete events (fire, flood) reset structure; primary succession starts on bare substrate, secondary succession follows milder disturbance. Resilience vs. Resistance – Resistance: ability to stay near equilibrium after a shock. Resilience: ability to absorb shock, reorganize, and retain core functions. Ecosystem services – Benefits humans obtain: provisioning, regulating, cultural, and supporting. --- 📌 Must Remember GPP ≈ 2 × NPP (about half of GPP is respired by plants). NEP = GPP − (Plant + Microbial Respiration). Limiting factors for photosynthesis: light, leaf area, CO₂, water, temperature. Nitrogen fixation supplies 80 % of anthropogenic N fluxes (fertilizers, combustion, deposition). Phosphorus limitation increases with ecosystem age, especially in tropical soils. Decomposition rate ↑ with temperature & moisture, but slows in water‑logged or very dry soils. Keystone species have outsized effects relative to their abundance; dominant species shape function by sheer biomass. Resistance ≠ Resilience – a system can be highly resistant but not resilient (or vice‑versa). Primary vs. secondary succession: primary = no soil/seed bank; secondary = soil/seed bank present. --- 🔄 Key Processes Photosynthetic Carbon Fixation Light captured → electron transport → CO₂ → carbohydrate. Plant respiration returns ½ of GPP as CO₂ → yields NPP. Nitrogen Cycle N₂ fixation → ammonium (NH₄⁺) → nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻) → plant uptake → mineralization (organic N → NH₄⁺) → denitrification (NO₃⁻ → N₂ gas). Phosphorus Release Weathering of rock → soluble P → plant uptake (often aided by mycorrhizae) → litter → mineralization → re‑uptake or loss via runoff. Decomposition Litter → microbial colonization → enzymatic breakdown → CO₂ + mineral nutrients released. Successional Recovery Disturbance → seed bank/colonizers → early‑successional species → modify environment (soil, light) → allow later‑successional species → eventual climax (or alternative stable state). --- 🔍 Key Comparisons GPP vs. NPP vs. NEP GPP: total carbon fixed. NPP: carbon left for herbivores after plant respiration. NEP: ecosystem‑wide carbon balance (includes microbial respiration). Primary vs. Secondary Succession Primary: starts on bare rock/soil; long‑term development. Secondary: uses existing soil & seed bank; faster recovery. External vs. Internal Factors External: climate, latitude, precipitation (not altered by ecosystem). Internal: competition, decomposition, species composition (both influence and are influenced by ecosystem). Provisioning vs. Regulating Services Provisioning: tangible goods (food, timber). Regulating: processes that maintain conditions (climate regulation, water purification). Keystone vs. Dominant Species Keystone: low abundance, high functional impact. Dominant: high biomass, drives function by sheer mass. --- ⚠️ Common Misunderstandings NEP = NPP – false; NEP subtracts all respiration, not just plant respiration. All nutrients are supplied from outside – most mineral nutrients (N, P, K) cycle internally; only carbon has large external flux. Higher species richness always = higher productivity – true only up to a threshold; additional similar species add little benefit. Microcosm results always scale up – small‑scale experiments may miss whole‑ecosystem feedbacks. Resilience = quick recovery – resilience also includes re‑organization to retain core functions, not just speed. --- 🧠 Mental Models / Intuition Ecosystem bathtub – Inflow: sunlight, water, nutrients; Outflow: CO₂, heat, leached nutrients. The water level (carbon pool) rises when inflow > outflow (positive NEP). Energy pyramid – Visualize 10 % energy transfer at each trophic step; the base (producers) must be large to support higher levels. Feedback loop – Think of decomposition as a “re‑cycling pump”: faster pump → more nutrients → potentially higher NPP, but also more CO₂ release. --- 🚩 Exceptions & Edge Cases Moisture extremes – Very wet soils become anaerobic → slow decomposition, increased methane, reduced N mineralization. Phosphorus in old tropical soils – Extremely low availability despite high organic matter; mycorrhizal fungi become critical. Disturbance frequency – Very frequent fires can prevent woody species from establishing, locking ecosystem in a grassland state (alternative stable state). Anthropogenic nitrogen dominance – In heavily fertilized/agricultural systems, N fixation by microbes becomes negligible compared with synthetic inputs. --- 📍 When to Use Which Carbon budgeting – Use GPP when you need total photosynthetic input; use NPP for biomass available to herbivores; use NEP to assess net ecosystem carbon storage. Nutrient management – Target nitrogen fixation or fertilizer addition when N is limiting; focus on mycorrhizal inoculation or phosphatase application when P is limiting. Restoration planning – Apply primary succession techniques (soil creation, pioneer species) on bare sites; use secondary succession (seed‑bank enhancement) on partially disturbed lands. Experimental scale – Choose ecosystem‑level experiments for realistic interaction studies; use mesocosms for mechanistic hypothesis testing; reserve microcosms for rapid screening of microbial processes. --- 👀 Patterns to Recognize Temperature‑moisture‑decomposition – Warm, moist, well‑aerated soils → fast decomposition → high nutrient turnover. Disturbance‑succession mosaics – Landscapes often contain patches at different successional stages after a disturbance; look for a patchwork of early‑ vs. late‑successional species. 10 % energy rule – Expect steep drop‑off in biomass/energy from one trophic level to the next. Thresholds in species richness – Productivity rises sharply with the first few functionally distinct species, then plateaus. Keystone impact – Removal of a single keystone (e.g., sea otter) can cause trophic cascades visible as dramatic shifts in prey populations. --- 🗂️ Exam Traps “NEP equals NPP” – distractor that ignores microbial respiration. “All ecosystems are carbon sinks” – many mature forests are carbon neutral (NEP ≈ 0) or even sources. “Higher precipitation always means higher NPP” – water may be abundant, but nutrient limitation (especially P) can cap productivity. “All keystone species are abundant” – keystones are defined by impact, not abundance. “Disturbance always reduces ecosystem function” – moderate, periodic disturbance can increase diversity and resilience. “Phosphorus cycles quickly like nitrogen” – P cycles are much slower; weathering controls supply. ---