A species geographical range is the total area where it naturally occurs. Species with narrow ranges face higher extinction risk. IUCN criterion B: extent of occurrence less than 20,000 sq km AND area of occupancy less than 2000 sq km qualifies as threatened. Wide-range species benefits: multiple populations as refugia, higher total population size, often generalist habitats, more genetic diversity. Narrow range species: any single catastrophe can eliminate entire species, limited genetic diversity, cannot easily shift range with climate change. Example: Dodo (Mauritius only) vs House sparrow (worldwide).

Endemic species: found ONLY in a specific area, nowhere else. Palaeoendemics: ancient species with formerly wider range now restricted. Neoendemics: recently evolved species. India endemic examples: Lion-tailed macaque, Nilgiri tahr, Purple frog (Western Ghats only), Indian giant squirrel. These face high extinction risk - any habitat loss = potential global extinction. Western Ghats has 5000+ endemic plant species and 139 endemic amphibian species.

Biogeography: study of species distribution across space and time. Wallace's Line: sharp faunal boundary between Asian and Australian biogeographic regions. Vicariance: separation of populations by physical barrier leading to allopatric speciation. Dispersal: movement to new areas. Island biogeography (MacArthur and Wilson 1967): species richness on islands = balance between immigration and extinction. Larger islands and islands closer to mainland have more species. Species-area relationship: S = CA^z.

Gene flow: movement of alleles between populations. Wide-range species: extensive gene flow, higher genetic diversity. Narrow-range species: limited gene flow, increased genetic drift, inbreeding. Peripheral populations (range edges): smaller, more isolated, more genetically differentiated - important evolutionary reservoirs for adaptation to future climate change. Range shifts: species shifting ranges in response to climate change. Wide-range species better able to shift. Mountaintop narrow-range species cannot shift ranges.

Interspecific interactions: Competition (-/-): both harmed. Gause competitive exclusion: two species cannot occupy same niche indefinitely. Predation (+/-): predator benefits, prey harmed. Mutualism (+/+): both benefit (mycorrhizae, pollinators). Commensalism (+/0): one benefits, other unaffected. Parasitism (+/-): parasite benefits without immediately killing host. Keystone species: disproportionately large effect relative to abundance. Sea otter controls sea urchins, maintains kelp forests. Wolves in Yellowstone caused trophic cascade.

Primary succession: on bare substrate (bare rock after volcanic eruption). Pioneer species first (lichens, mosses). Gradual soil formation. Eventually climax community. Secondary succession: where community existed but was disturbed (fire, flood). Soil present, faster recovery. Seral stages: herbaceous weeds, shrubs, pioneer trees, climax forest. During succession: species diversity, biomass, stability increase. Nutrient cycling becomes more complex.

10% rule: only ~10% of energy transferred from one trophic level to next (Lindeman efficiency). Food chains and food webs. GPP (Gross Primary Productivity): total organic matter fixed. NPP = GPP - plant respiration = energy available to consumers. Nutrient cycling: Carbon cycle (photosynthesis, respiration, decomposition), Nitrogen cycle (fixation by Rhizobium/Azotobacter, nitrification, denitrification), Phosphorus cycle (no gaseous phase).

Minimum viable population (MVP): smallest population with >95% survival probability for 100 years. Below MVP: Allee effects, inbreeding depression, genetic drift. For large mammals: MVP typically 1000-10,000 individuals. Effective population size (Ne): always less than actual N. Ne/N typically 0.1-0.25. Habitat corridors: connect isolated patches, allow dispersal and gene flow. Metapopulation: network of partially isolated populations connected by dispersal.