Biogeography Study Guide

📖 Core Concepts Biogeography – the study of where species and ecosystems occur in space and through time. Ecological vs. Historical Biogeography – short‑term (community interactions) vs. long‑term (speciation, extinction, plate movements). Drivers of Distribution – speciation, extinction, continental drift, glaciation cycles, sea‑level change, and dispersal barriers. Dispersal – movement of individuals (by migration, wind, water, animals) that connects populations and shapes gene flow. Allopatric Speciation – new species arise when a geographic barrier splits a population. Vicariance – the formation of a barrier that divides a once‑continuous population, often leading to speciation. Geodispersal – removal of a barrier, allowing previously isolated biotas to mix. Endemism – species confined to a single area; a key metric for conservation priority. Biogeographic Hierarchy – Realms (ecozones) → Bioregions → Ecoregions → Floristic/Zoogeographic regions. Island Biogeography Theory (MacArthur & Wilson, 1967) – species richness on islands is a balance of immigration and extinction, driven by island area and isolation. Phylogeography – combines molecular systematics with geographic data to infer the history of populations, especially on islands. 📌 Must Remember Buffon’s Law – similar climates host different species; environment alone does not dictate identical biotas. Species‑Area Relationship: $S = cA^{z}$ (where S = species count, A = area, c & z are constants). Larger islands/support larger habitats host more species. Immigration Rate ↓ with distance from source; Extinction Rate ↓ with increasing island area. Continental Drift created the ancient supercontinent Pangea; its breakup explains many modern distribution patterns. Wallace Line – a sharp faunal break separating Asian and Australian biotas, exemplifying the power of geographic barriers. ENM/SDM – models that predict where a species can live based on its environmental preferences; used for climate‑change forecasts. Endemic Area = region where a species occurs nowhere else → high conservation value. Phylogenetic Code – a naming system based on evolutionary relationships; increasingly influencing biogeographic classifications. 🔄 Key Processes Allopatric Speciation Geographic isolation → genetic drift & divergent selection → reproductive isolation → new species. Island Species‑Area Dynamics (MacArthur & Wilson) Immigration (I): $I = I0 e^{-d}$ (decreases with distance d). Extinction (E): $E = E0/A$ (decreases with island area A). Equilibrium species richness where $I = E$. Range Shift under Climate Change Gather occurrence data → build ENM/SDM → project onto future climate layers → identify expansion, contraction, or novel habitats. Phylogeographic Inference Sample DNA across range → construct haplotype network → map lineages onto geography → infer historical dispersal/vicariance events. Protected‑Area Design Using Biogeography Identify endemic‑rich regions → overlay with threat maps → prioritize corridors that connect isolated habitats. 🔍 Key Comparisons Allopatric Speciation vs. Vicariance Allopatric: barrier forms after a population is already split; focuses on process of speciation. Vicariance: pre‑existing barrier splits a continuous population; emphasizes historical separation. Geodispersal vs. Dispersal Geodispersal: large‑scale removal of a barrier (e.g., land bridge formation). Dispersal: movement of individuals across existing barriers (e.g., birds crossing ocean). ENM vs. SDM ENM: models the fundamental niche based on environmental variables. SDM: projects the niche onto geographic space, producing predicted distribution maps. Realms vs. Bioregions vs. Ecoregions Realms: broadest, continent‑scale biotic divisions. Bioregions: subdivisions within realms defined by species composition. Ecoregions: finer units adding ecological/physiognomic criteria. ⚠️ Common Misunderstandings “Species‑area law means bigger islands always have more species.” Ignoring isolation leads to over‑prediction; both area and distance matter. “Dispersal = migration.” Dispersal includes passive mechanisms (wind, water, animal vectors), not just active movement. “Endemic = rare.” – Endemics can be abundant locally; rarity is a separate concept. “ENM predictions are definitive future ranges.” – Models depend on climate variables chosen and assume niche conservatism; they are projections, not certainties. 🧠 Mental Models / Intuition “Island as a bathtub” – water (species) flows in (immigration) and drains out (extinction). The faucet size (distance) and tub size (area) set the steady‑state water level (species richness). “Barrier as a wall” – think of a wall that can be built (vicariance), knocked down (geodispersal), or partially opened (dispersal). The timing of wall construction vs. population split determines whether you call it vicariance or allopatric speciation. “Niche as a climate suit” – a species “fits” a climate suit; ENM finds the suit’s measurements, SDM tries the suit on a map. 🚩 Exceptions & Edge Cases Island “rescue effect” – high immigration can suppress extinction even on small islands, deviating from the simple $E = E0/A$ rule. Long‑distance dispersal (LDD) – rare events (e.g., bird‑carried seeds) can colonize distant islands, breaking the distance‑decay expectation. Human‑mediated range shifts – introductions can create “artificial” dispersal pathways not accounted for in natural models. Phylogenetic code conflicts – when a fossil taxon’s name under the Phylogenetic Code clashes with traditional stratigraphic units, naming conventions may need compromise. 📍 When to Use Which Predicting climate‑driven range changes → use ENM/SDM with future climate scenarios. Estimating species richness on an island → apply the species‑area equation and consider immigration distance (MacArthur‑Wilson model). Determining the cause of a disjunct distribution → evaluate vicariance (plate‑tectonic history) vs. long‑distance dispersal (rare LDD events). Designing a protected‑area network → prioritize endemics and biogeographic realms/bioregions with high phylogenetic distinctiveness. Testing historical population splits → conduct phylogeographic analyses using molecular data. 👀 Patterns to Recognize Sharp faunal breaks (e.g., Wallace Line) → look for deep water or tectonic barriers. High endemism + small area → typical of isolated islands or mountain “sky islands.” Range contraction coinciding with glacial maxima → signals climate‑driven historical shifts. Phylogenetic clustering within a region → suggests vicariance rather than recent dispersal. 🗂️ Exam Traps Distractor: “Larger islands always have lower extinction rates.” Why tempting: Oversimplifies the species‑area relationship. Correct: Extinction declines with area but can be offset by high immigration (rescue effect) or intense competition. Distractor: “Allopatric speciation only occurs on islands.” Why tempting: Island examples are iconic. Correct: Any geographic barrier (mountain ranges, rivers) can create allopatric conditions. Distractor: “ENM equals SDM.” Why tempting: Terms are often used interchangeably. Correct: ENM models the niche; SDM maps the niche onto space. Distractor: “Vicariance is always caused by continental drift.” Why tempting: Plate tectonics is the classic example. Correct: Vicariance can result from river captures, sea‑level changes, or even human‑built barriers. Distractor: “Endemic species are always endangered.” Why tempting: Conservation focus on endemics. Correct: Endemic status alone does not dictate risk; population size and threats matter.