Food web Study Guide

📖 Core Concepts Food Web – A network diagram showing who eats whom in a community; combines many food chains. Trophic Level – Position of a species in the feeding hierarchy (autotroph = level 0, herbivore = 1, carnivore = 2, etc.). Trophic Species (Functional Group) – A set of species that share exactly the same predators and prey; treated as one node in a web. Trophic Link – A directed edge representing a feeding relationship between two trophic species. Connectance (C) – Proportion of possible links that are actually realized: $$ C = \frac{L}{S^{2}} $$ where L = number of links, S = number of species (or trophic species). Energy Flow (E) – Total energy moving through a trophic level: $$ E = P + R $$ P = metabolic production, R = respiration. Transfer Efficiency – Roughly 10 % of energy passes to the next level; 80‑90 % is lost as heat/waste. Top‑Down vs. Bottom‑Up Regulation – Predators control lower trophic levels (top‑down) vs. resource quality controls herbivores (bottom‑up). Trophic Cascade – Indirect effect where a change at one level (usually a predator) propagates to non‑adjacent levels, altering plant biomass, etc. Complexity – Often quantified as species richness × connectance; includes heterogeneity, nestedness, modularity. Nestedness – Specialist diets are subsets of generalist diets. Modularity / Compartments – Sub‑groups with strong internal links and weak external links; promote stability. --- 📌 Must Remember Energy loss per transfer ≈ 90 %, leaving 10 % for the next level. Mean chain length = average number of links from basal resource to consumers; typical maximum 3–5 levels due to energy loss. Connectance formula: $C = L / S^{2}$; maximum binary links = $S(S-1)/2$. Top‑down hypothesis = predators limit herbivore abundance (green‑world). Bottom‑up hypothesis = plant quality/defenses limit herbivores. Omnivory = feeding on multiple trophic levels; creates multiple pathways for energy. Detrital web recycles primary production; most material returns to the system as detritus. Weak links in long loops → dampen oscillations → increase stability. Highly connected “hubs”: removal can cause network collapse; compartments buffer this. --- 🔄 Key Processes Determining Trophic Position Gut‑content analysis → direct identification of prey items. Stable‑isotope analysis → infer trophic level from isotope enrichment (e.g., $^{15}$N). Calculating Connectance Count L (realized feeding links). Count S (trophic species). Apply $C = L / S^{2}$. Mean Chain Length (MCL) List every unique food chain in the web. Count links in each chain. Compute arithmetic average of those lengths. Energy Transfer Across Levels Start with primary production (P). At each step multiply by 0.1 (10 % efficiency). Sum losses as heat/waste → $E = P + R$ for each level. Assessing Stability via Modularity Identify clusters of tightly linked species (high interaction strength). Evaluate inter‑cluster link strengths (should be weaker). --- 🔍 Key Comparisons Top‑Down vs. Bottom‑Up Top‑Down: Predator → ↓ herbivore → ↑ plant. Bottom‑Up: Plant quality → ↑/↓ herbivore → cascade upward. Omnivore vs. Specialist Omnivore: Feeds on multiple trophic levels → more pathways, higher network connectance. Specialist: Narrow diet → often a subset of a generalist’s diet (nestedness). Pyramid of Numbers vs. Pyramid of Biomass Numbers: Counts of individuals; usually decreasing upward. Biomass: Dry weight; can invert in aquatic systems where microbes dominate lower levels. Detrital Web vs. Classic Food Web Detrital: Starts with dead organic matter → decomposers → secondary consumers. Classic: Starts with living primary producers → herbivores → predators. --- ⚠️ Common Misunderstandings “Energy flows in cycles.” – Energy is unidirectional; only matter cycles. “All trophic levels have equal biomass.” – Pyramids of energy are always upright; numbers/biomass can vary. “More links always mean more stability.” – Excessive strong links can cause instability; weak links and modularity are stabilizing. “Detritus is waste, not part of the food web.” – Detritus fuels a major secondary production pathway. “Higher connectance always equals higher complexity.” – Complexity = species × connectance; a large web can have low connectance yet be complex. --- 🧠 Mental Models / Intuition “Energy Faucet” – Imagine primary production as water entering a faucet; each level is a leaky pipe that lets out 90 % of the water, leaving only a trickle for the next level. “Network Neighborhood” – Think of a compartment as a cul‑de‑sac in a city: traffic (energy) moves freely inside but rarely exits, protecting the rest of the city from a blockage. “Nested Dinner Plate” – Specialists sit on a small plate that fits entirely on the larger plate of a generalist’s diet. --- 🚩 Exceptions & Edge Cases Inverted Biomass Pyramids – Common in planktonic oceans where microbial biomass exceeds that of higher consumers. High Transfer Efficiency (>10 %) – Occurs in some highly productive, low‑loss systems (e.g., certain kelp forests). Cross‑Boundary Subsidies – Nutrients or organisms moving from adjacent ecosystems can boost productivity beyond what local primary production predicts. --- 📍 When to Use Which Assessing Trophic Position → Use stable isotopes when gut contents are ambiguous; use gut‑content for short‑term diet snapshots. Choosing a Stability Metric → Use connectance for overall complexity; use modularity when evaluating resilience to species loss. Modeling Energy Flow → Apply Lindeman’s trophic‑dynamic approach for energy budgets; use energy pyramids to illustrate directionality. Evaluating Food‑Web Type → If you need quantitative fluxes → build an energy‑flow web. Predicting Chain Length → Consider productivity and habitat size; longer chains in highly productive, large habitats. --- 👀 Patterns to Recognize Many weak links + few strong links → typical stable web. Omnivory + multiple pathways → indicates higher connectance and potential for indirect effects. Nested diet structure → specialists’ prey sets are subsets of generalists’. Inverted biomass pyramid → look for dominant microbial loops or suspended aquatic systems. High modularity → clusters of species that interact mostly among themselves (often taxonomically or habitat‑based). --- 🗂️ Exam Traps “Energy loss per trophic transfer is 80 %.” – The correct figure is 90 % loss (10 % retained). “Higher connectance always equals more stability.” – Only when weak links dominate; strong, dense connections can destabilize. “All detritivores are primary producers.” – Detritivores consume dead material, not live photosynthates. “Pyramids of numbers and biomass always have the same shape.” – They can differ; biomass pyramids may invert in aquatic systems. “Omnivory eliminates trophic cascades.” – Omnivory can dampen or alter cascades, but does not eliminate them. “Mean chain length equals the number of trophic levels.” – Chain length counts links; a 3‑level web can have chain length 2. ---