---
title: "Biogeography: Why a Species Is Here and Not There"
description: "A species range is set by tolerance, by dispersal and by history, and the three explanations make different predictions. Island patterns, where richness tracks area and isolation, are the cleanest pla"
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: 2026-09-12
---

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# Biogeography: Why a Species Is Here and Not There

A species range is set by tolerance, by dispersal and by history, and the three explanations make different predictions. Island patterns, where richness tracks area and isolation, are the cleanest place to test them.

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: Why a species lives here and not there

Every species survives and breeds only inside its tolerance range, from sweet spot to edge limits. Graph residents against oxygen or heat and you get a bell: most cluster at the peak, few linger at the edges, none live past them. Narrow curves, like the shiner's oxygen band, mean restricted ground; wide ones, like the topminnow's, mean broad ground.

An absence has three usual suspects: physiology cannot take it, a barrier blocks arrival, or a rival or predator holds the ground. Transplant past the edge to test them: if newcomers thrive, tolerance was never the wall.

**Example.** Warming moves each plant's zone, and moss campion from New Mexico to Alaska is the test case. Researchers track vital rates, survival, growth, and reproduction, across sites and elevations over many years against the changing climate. Holding rates mean the plant adapts in place; failing rates mean it must shift ground.

**Tip.** Islands show the pattern cleanest: richness tracks area and isolation. Shrink a habitat and the species-area link forecasts the loss. Equilibrium theory says arrivals balance losses on each island, though real islands always leave parts of the story out.

**Recap.** Tolerance, barriers, and rivals each predict a different response, so test absences before blaming physiology.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Measuring Diversity: Richness, Evenness and Turnover Between Sites](https://lightmysky.com/learn/science/measuring-diversity-richness-evenness-and-turnover-between-sites-mt_d_iIiIArWZ)
- [Biomes and What Draws Their Boundaries](https://lightmysky.com/learn/science/biomes-and-what-draws-their-boundaries-mt_LpO2gwRYHd)

## 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)