Predation Study Guide

📖 Core Concepts Predation – A predator kills and consumes a prey organism (vs. parasites that usually don’t kill). Foraging Cycle – Four sequential phases: Search → Assessment → Capture → Handling. Search Strategies – Sit‑and‑wait (ambush) vs. active/wide foraging; choice depends on prey density and predator energy budget. Marginal Value Theorem (MVT) – Predicts the optimal time to leave a prey patch when the marginal gain from continued searching falls below the average gain of moving to a new patch. Lotka‑Volterra Model – Classic differential‑equation pair that generates predator‑prey cycles: $$ \frac{dN}{dt}= rN - aNP,\qquad \frac{dP}{dt}= b\,aNP - mP $$ where \(N\) = prey density, \(P\) = predator density, \(r\) = prey intrinsic growth rate, \(a\) = attack rate, \(b\) = conversion efficiency, \(m\) = predator mortality. Refuge – Habitat zones where prey are safe from predators; refuges alter effective predation rates and can stabilise dynamics. --- 📌 Must Remember Predator vs. Parasitoid – Predator consumes many prey over its life; parasitoid’s larva consumes a single host and always kills it. Energy Decision Rule – Pursue prey only when energy gain > energy cost (including capture and handling). Size Rule – Larger predators tend to take larger prey; very small prey → low payoff, very large prey → high handling cost or danger. MVT Formula (qualitative) – Leave a patch when: \[ \frac{\text{incremental gain}}{\text{incremental time}} < \frac{\text{average gain}}{\text{average time}} \] Lotka‑Volterra Cycle Period – Approximate period \(T \approx 2\pi \sqrt{\frac{1}{r m}}\) (for simple two‑species case). Refuge Threshold – A refuge fraction > critical value (≈0.5 in many models) eliminates limit‑cycle oscillations. --- 🔄 Key Processes Search Phase Choose sit‑and‑wait if prey are dense & mobile, predator has low metabolic demand. Choose active foraging if prey are sparse or sedentary. Assessment Phase Estimate prey size → compare to predator’s handling capacity. Compute energetic balance (gain vs. cost). Capture Phase Ballistic interception: predict prey trajectory, launch intercept. Pursuit: chase; success hinges on speed (straight runs) or maneuverability (turning prey). Endurance hunting: maintain chase until prey fatigues. Handling Phase Kill/disable (e.g., remove spines, inject venom). Process (tear, chew, swallow). For social hunters, coordinate to subdue large prey. --- 🔍 Key Comparisons Sit‑and‑wait vs. Active Foraging – Ambush (low energy, high prey density) vs Wide search (high energy, low prey density). Predator vs. Parasitoid – Multiple prey over life vs Single host, always lethal. Social vs. Solitary Capture – Cooperative kill of larger prey, shared risk vs Exclusive access to kill, higher individual cost. Lévy Walk vs. Random Walk – Many short steps + occasional long jumps vs Uniform step lengths. --- ⚠️ Common Misunderstandings “All scavengers are predators.” – Scavengers eat dead organisms only; many predators also scavenge opportunistically but are not classified as scavengers. “Micropredators are true predators.” – Fleas, mosquitoes feed on live hosts but rarely kill them; they are usually treated as parasites. “Lotka‑Volterra always predicts stable cycles.” – Real systems deviate because of prey refuges, predator satiation, multiple species, and non‑linear functional responses. “A larger predator can eat any prey.” – Handling constraints (spines, toxins) and risk of injury limit prey size despite predator size. --- 🧠 Mental Models / Intuition Energy Budget Scale – Imagine a bank account: a prey item is a deposit; capture/handling are transaction fees. Proceed only if net deposit is positive. Patch Exploitation as “Gold‑Mine” – Enter a mine (patch) while the ore (prey) is plentiful; leave when the ore rate falls below the average profit of moving to a new mine (MVT). Lévy Walk = “Search & Sprint” – Short, frequent steps = local probing; occasional long sprint = relocation to a new patch. --- 🚩 Exceptions & Edge Cases Refuge Paradox – Small refuges can destabilise dynamics by allowing prey to hide yet still be heavily predated when they leave. Venomous Predators – May target prey larger than typical size because venom pre‑subdues dangerous prey. Camouflage vs. Aposematism – Some predators (e.g., ambush snakes) rely on crypsis, but a sudden bright flash (deimatic display) can startle prey, flipping the usual warning signal logic. --- 📍 When to Use Which Choosing Search Strategy – Use sit‑and‑wait when: (1) prey density > threshold, (2) predator’s metabolic rate low. Use active otherwise. Applying MVT – Calculate marginal gain curve; if gain slope flattens before the average gain line, switch patches. Model Selection – Use simple Lotka‑Volterra for introductory, two‑species, no‑refuge scenarios. Add refuge term (\(f\) = refuge fraction) to the prey equation: \( \frac{dN}{dt}= rN(1-f) - aNP\). Use functional‑response (Holling type II/III) models when predator saturation is evident. --- 👀 Patterns to Recognize High prey density + low predator energy → ambush behavior. Cyclical population data with 10‑year period → classic hare‑lynx dynamics. Presence of spines/venom → likely handling adaptations in predators targeting dangerous prey. Patchy prey distribution → expect patch exploitation and MVT‑guided patch residence times. --- 🗂️ Exam Traps Distractor: “Predators always have higher trophic level than scavengers.” – False; scavengers can be secondary or tertiary consumers. Trap: Assuming “all predators are apex.” – Many predators are mesopredators and subject to intraguild predation. Misleading Choice: “Lévy walk is only used by microorganisms.” – Incorrect; observed in sharks, honeybees, human hunter‑gatherers. Wrong Formula: Using Lotka‑Volterra without noting its assumption of no refuges; exam may ask why real cycles differ. ---