Ecology Study Guide

📖 Core Concepts Ecology – the science of how organisms interact with each other and with abiotic components of their environment. Hierarchical organization – organism → population → community → ecosystem → biome → biosphere. Ecosystem – a dynamic system of biotic communities plus their abiotic environment, linked by energy flow and material cycling. Niche – the multi‑dimensional set of environmental conditions that allow a species to maintain a stable population. Fundamental niche: all conditions that could support the species (no biotic limits). Realized niche: the portion actually occupied after competition, predation, etc. Biodiversity – variation at three levels: genetic, species, and ecosystem. Population growth models – exponential (Malthusian) vs. density‑dependent logistic growth. Trophic structure – autotrophs → primary consumers → secondary → tertiary → decomposers; food webs are networks of many food chains. Keystone species – a species with disproportionately large effects on community structure relative to its abundance. Biogeography & Island Theory – species richness on islands reflects a balance between immigration and extinction rates. r‑ and K‑selection – strategies at opposite ends of the density‑dependence spectrum; r = intrinsic rate of increase, K = carrying capacity. --- 📌 Must Remember Four basic demographic variables: births, deaths, immigration, emigration. Logistic growth equation: $$\frac{dN}{dt}= rN\left(1-\frac{N}{K}\right)$$ where N = population size, r = intrinsic rate of increase, K = carrying capacity. Primary production units: grams of carbon per square meter ( \(g\,C/m^{2}\) ). Island biogeography equilibrium: species number = immigration rate ÷ (immigration + extinction rate). Keystone definition: “disproportionately large number of connections in a food web relative to its biomass.” Fundamental vs. realized niche: fundamental = potential; realized = actual after biotic interactions. r‑selection: high r, low K, rapid growth, many offspring, unstable environments. K‑selection: low r, high K, slower growth, few offspring, stable environments. --- 🔄 Key Processes Logistic population growth Start with small N → exponential rise. As N approaches K, growth slows (negative feedback). Stabilizes at K (carrying capacity). Niche construction Organisms alter physical/chemical environment → feedback on selection pressures (e.g., beaver dams). Metapopulation dynamics Local extinctions → recolonization via dispersers; overall occupancy = \( \frac{c}{c+e} \) (c = colonization rate, e = extinction rate). Island species‑richness equilibrium Immigration declines with island size/isolation; extinction rises with smaller island area. Fire‑driven succession Fire removes biomass → creates a mosaic of successional stages; fire‑adapted species (e.g., serotinous pines) rely on this disturbance. Nutrient cycling Decomposition → mineralization → uptake by plants → incorporation into biomass → return to soil via litterfall. --- 🔍 Key Comparisons Fundamental niche vs. Realized niche Potential vs. actual environmental envelope. Population ecology vs. Ecosystem ecology Focus on abundance & distribution vs. energy & material fluxes. Migration vs. Dispersal Seasonal, reversible movement vs. one‑way permanent departure from natal site. Food chain vs. Food web Linear sequence vs. interconnected network of many chains. r‑selection vs. K‑selection Fast, opportunistic growth vs. competitive, efficiency‑focused growth. Autotroph vs. Heterotroph vs. Detritivore Produces own food vs. consumes other organisms vs. recycles dead organic matter. --- ⚠️ Common Misunderstandings Niche = Habitat – habitat is the where; niche is the how a species fits into that space (including biotic interactions). Logistic growth always sigmoid – Allee effects or time‑lagged density dependence can alter the shape. All top predators are keystone species – keystoneness depends on network impact, not trophic level alone. Island biogeography only about area – isolation (distance to source) is equally crucial. r‑ and K‑strategies are mutually exclusive – many species exhibit a mix of traits depending on context. --- 🧠 Mental Models / Intuition Niche as a multi‑dimensional “hyper‑cube” – each axis = an environmental variable; larger cube = broader niche. Logistic curve = “S‑shaped” – think of a bathtub filling: rapid at first, then slows as it reaches the rim (capacity). Island as a “leaky bucket” – water (species) flows in (immigration) and out (extinction); balance determines level. Energy pyramid – only 10 % of energy moves up each trophic level → explains why many top predators are rare. --- 🚩 Exceptions & Edge Cases Allee effect – populations may decline at low density despite being below K. Keystone engineers – small organisms (e.g., earthworms) can be keystones by altering soil structure. Microbial biomes – microbes dominate many biogeochemical cycles yet are often omitted from traditional “plant‑animal” trophic diagrams. Non‑logistic growth – predator‑prey cycles, seasonal breeding, or resource pulses can produce oscillations rather than a simple S‑curve. --- 📍 When to Use Which Exponential vs. Logistic model – use exponential when resources are effectively unlimited (early colonization); switch to logistic once density‑dependent limits appear. Island theory – apply when assessing species richness on true islands or habitat “islands” (e.g., forest fragments). r‑ vs. K‑selection framework – use r‑selection for opportunistic species in disturbed habitats; K‑selection for competitors in stable, resource‑limited settings. Primary production units – always express in \(g\,C/m^{2}\) when comparing ecosystems or calculating carbon budgets. Keystone identification – look for species whose removal causes disproportionate trophic cascades, not just high biomass predators. --- 👀 Patterns to Recognize S‑shaped population curves → logistic regulation. Decreasing species richness with increasing island isolation → classic island‑biogeography pattern. Top‑down control spikes after keystone loss – sudden increases in lower trophic levels. Fire‑adapted traits (serotiny, thick bark) appearing in fire‑prone biomes. Seasonal migration timing synchronized with resource pulses (e.g., insect emergence). --- 🗂️ Exam Traps Confusing fundamental vs. realized niche – exam answers that define niche solely as “the place a species lives” are wrong. Mixing up primary vs. secondary consumers – herbivores are primary consumers; carnivores that eat herbivores are secondary. Assuming any abundant predator is a keystone – keystone status is about network impact, not abundance. Using “r” as a synonym for “reproductive rate” without the context of intrinsic rate of increase – r in r‑selection is the population growth rate, not just clutch size. Applying the logistic equation without the (1 − N/K) term – missing the density‑dependence factor yields the exponential model. ---