--- title: "Biogeography: Why a Species Is Here and Not There" description: "A species' range reflects environmental tolerance, dispersal, interactions and history. Species-area models estimate richness under stated assumptions. Island-equilibrium theory relates immigration an" canonical: https://lightmysky.com/learn/science/biogeography-why-a-species-is-here-and-not-there-mt_sRi0FPcK8r source: https://lightmysky.com/learn/science/biogeography-why-a-species-is-here-and-not-there-mt_sRi0FPcK8r.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Biogeography: Why a Species Is Here and Not There A species' range reflects environmental tolerance, dispersal, interactions and history. Species-area models estimate richness under stated assumptions. Island-equilibrium theory relates immigration and extinction to richness, without explaining every feature of real islands. Subject: Science · Area: Ecology & Conservation · Ages 20 to 21 Page: https://lightmysky.com/learn/science/biogeography-why-a-species-is-here-and-not-there-mt_sRi0FPcK8r ## Ready when they can - Separate a range limit set by physiology from one set by dispersal or by a competitor - State the species-area relationship and use it to estimate loss from habitat reduction - Explain what the equilibrium theory of island biogeography predicts and what it leaves out ## Lesson: Explaining species ranges Biomes describe broad conditions; species ranges need more detail. A tolerance range describes conditions an organism can withstand. Conditions supporting survival may differ from those supporting reproduction, and life stages can differ. Physiological tolerance, dispersal, interactions and history all help explain where a species persists. Field abundance does not directly measure tolerance: habitat availability, competitors and sampling affect it too. Conditions inside a narrow tolerance may still occur over a wide area. Responses need not be bell-shaped, and field peaks need not mark physiological optima. An absence may reflect unsuitable conditions, failure to arrive, or interactions such as competition and predation. Past events can also matter. Compare predictions before choosing a cause. Island models add scale: greater isolation tends to reduce immigration, while smaller area often increases the risk of local extinction. **Example.** A controlled introduction can test whether an absent species can establish. If introduced organisms survive and reproduce without changing local competitors or habitat, blocked arrival becomes more plausible. This does not prove arrival was the only cause. Use permits and biosecurity safeguards; do not move organisms between habitats yourself. Vital rates measure survival, growth and reproduction. Long-term measurements across sites can show where populations perform well or poorly. Stable rates do not by themselves prove genetic adaptation; inherited change needs separate evidence. Differences between years or sites can also reflect weather, habitat and other interacting factors. **Example.** A species-area model is S = cA^z: S is richness, A is area, and c and z are fitted constants. With unchanged constants, S_new/S_old = (A_new/A_old)^z. For illustrative z = 0.5, use the square root, the nonnegative number that squares to the area ratio. If area becomes 1/9 of its original value, predicted richness becomes 1/3, since 1/3 times 1/3 = 1/9. This is not a guaranteed or immediate extinction count; habitat quality, connectivity and time matter. *(drawing: Immigration and local extinction occur together. Equal gains and losses keep richness stable. The basic model omits speciation and many local differences.)* **Recap.** Test range-limit explanations separately. Species-area estimates depend on assumptions. Island equilibrium concerns richness, which can stay stable while species identities change. *Original lesson and exercises written for this learning dataset, including the species-area calculation.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Biomes and What Draws Their Boundaries](https://lightmysky.com/learn/science/biomes-and-what-draws-their-boundaries-mt_LpO2gwRYHd) ## Optional connections - [Measuring Diversity: Richness, Evenness and Turnover Between Sites](https://lightmysky.com/learn/science/measuring-diversity-richness-evenness-and-turnover-between-sites-mt_d_iIiIArWZ) ## Opens up - [Demography: life tables, age structure and what they predict](https://lightmysky.com/learn/science/demography-life-tables-age-structure-and-what-they-predict-mt_raWfdLLACU) - [Global Change Ecology: Ranges, Phenology and Regime Shifts](https://lightmysky.com/learn/science/global-change-ecology-ranges-phenology-and-regime-shifts-mt_tJ7XpAADMF) - [Conservation: Small Populations, Fragmentation and Reserve Design](https://lightmysky.com/learn/science/conservation-small-populations-fragmentation-and-reserve-design-mt_yOl2NGoLeb)