Community ecology Study Guide

📖 Core Concepts Ecological community – all populations of different species living together in the same place at the same time. Synecology (community ecology) – studies how those species interact, how they are distributed, how many there are, and how abiotic factors (temperature, soil pH, etc.) shape those patterns. Niche – the “job” a species performs (its resource use, habitat, activity time). Two species can coexist when they occupy different niches (niche partitioning). Guild – a group of species that exploit the same class of resources in a similar way; members of a guild compete directly. Trophic level – position in a food chain/web (primary producer → primary consumer → secondary/tertiary consumer → apex predator). Foundation species – create or modify the physical environment, influencing many other species (e.g., coral reefs). Keystone species – exert a disproportionate influence on community structure relative to their abundance (often top predators). Ecosystem engineer – modifies habitat and creates new resources (e.g., beaver dams). Theoretical frameworks – Holistic (organismic) theory: community as a super‑organism with tight, co‑evolved integration. Individualistic (continuum) theory: each species responds independently to gradients; communities are overlapping, not sharply bounded. Neutral theory: species are functionally equivalent; composition driven by stochastic birth–death (ecological drift) balanced by speciation/immigration. --- 📌 Must Remember Niche partitioning → reduces competition because intraspecific competition > interspecific competition. Number of niches ≈ maximum species richness in a community. Keystone removal → top‑down trophic cascade (e.g., wolves in Yellowstone). Energy flow: only 10 % of energy transfers to the next trophic level (ecological inefficiency). Competition types: Interference – direct antagonism (e.g., lion chases hyena). Exploitative – indirect via resource depletion (symmetric vs. size‑asymmetric). Apparent – indirect via shared predator. Predation categories: specialist vs. generalist; kill‑and‑consume vs. parasitism. Mutualism → both partners benefit (e.g., rhizobia‑legume nitrogen fixation). Commensalism → one benefits, the other neutral (epiphytic orchids). Amensalism → one harmed, the other neutral. Parasitism → parasite benefits, host harmed (mosquito‑Plasmodium). Neutralism – rare; true lack of effect is uncommon. --- 🔄 Key Processes Lotka–Volterra predator–prey dynamics $$\frac{dN{\text{prey}}}{dt}= rN{\text{prey}} - aN{\text{prey}}N{\text{pred}}$$ $$\frac{dN{\text{pred}}}{dt}= b aN{\text{prey}}N{\text{pred}} - dN{\text{pred}}$$ \(r\): prey intrinsic growth rate; \(a\): attack rate; \(b\): conversion efficiency; \(d\): predator mortality. Niche partitioning sequence Identify overlapping resource axes → differentiate by time, space, diet, or size → reduce direct competition. Trophic cascade (top‑down) Remove keystone predator → herbivore surge → primary producer decline. Community assembly under neutral theory Random birth–death events + speciation/immigration → species abundance distribution follows a zero‑sum multinomial. --- 🔍 Key Comparisons Holistic vs. Individualistic Holistic → community as integrated super‑organism, discrete boundaries. Individualistic → species assemble independently along gradients, boundaries are fuzzy. Keystone vs. Foundation species Keystone: strong impact despite low abundance (often predator). Foundation: shape environment physically; can be abundant (often primary producers). Exploitative vs. Interference competition Exploitative – indirect, via resource depletion. Interference – direct aggression or territoriality. Specialist vs. Generalist predators Specialist – narrow prey range, more vulnerable to prey fluctuations. Generalist – broad diet, more stable populations. --- ⚠️ Common Misunderstandings “All competition is exploitative.” Interference (direct aggression) also limits populations. “Keystone species must be abundant.” By definition, they have a large effect relative to their abundance. “Neutral theory means ‘no interactions.’” It assumes functional equivalence, not the absence of interaction. “Mutualism is always equal benefit.” Benefit can be asymmetric (e.g., plants may gain more than pollinators). --- 🧠 Mental Models / Intuition “Niche‑space is a hotel with limited rooms.” Each species needs a distinct “room” (resource/condition); once rooms fill, newcomers can’t coexist unless they remodel (partition). “Community as a tapestry.” Holistic view sees tightly woven threads; individualistic view sees loosely placed patches that overlap. “Energy ladder loses 90 % per rung.” Visualize a leaking bucket: each step up the ladder is a smaller bucket of energy. --- 🚩 Exceptions & Edge Cases Size‑asymmetric exploitative competition – large individuals monopolize resources, breaking the assumption of symmetric resource use. Apparent competition – species may appear to compete but are actually linked through a shared predator; removing the predator removes the “apparent” effect. True neutralism – virtually never observed because indirect effects (e.g., habitat alteration) usually exist. --- 📍 When to Use Which Identify the driver of community pattern → If species show tightly coupled distributions and co‑evolution → apply Holistic theory. If species abundances change gradually along an environmental gradient → use Individualistic theory. When species appear ecologically equivalent and stochastic processes dominate → invoke Neutral theory. Choose interaction type → Observe direct aggression → label interference competition. Observe resource depletion without direct contact → label exploitative competition. Observe shared predator effects → label apparent competition. --- 👀 Patterns to Recognize Co‑occurrence of multiple predators → likely a keystone or top‑down control situation. High species richness in a structurally complex habitat → presence of foundation species or ecosystem engineers. Gradual species turnover along a moisture gradient → supports individualistic (continuum) theory. Flat species‑abundance distribution with many rare species → may indicate neutral processes. --- 🗂️ Exam Traps Distractor: “Keystone species are always apex predators.” – False; keystones can be any trophic level (e.g., sea otters controlling urchins). Distractor: “Apparent competition is a form of exploitative competition.” – Incorrect; it is indirect via a shared predator. Distractor: “Neutral theory predicts deterministic community composition.” – Opposite; neutral theory emphasizes stochasticity. Distractor: “All guild members are closely related.” – Wrong; guilds may include unrelated species that converge on similar resource use. Distractor: “Foundation species are rare.” – Not necessarily; many are abundant primary producers. ---