Why do larger islands closer to the mainland support more species, and what does this teach us about habitat fragments?
Topic 2.3 Island Biogeography: explain how island size and distance from the mainland determine species richness, and apply the theory to habitat fragments.
A focused answer to APES Topic 2.3, covering the theory of island biogeography, the effects of island size and distance, immigration and extinction rates, endemism, and its application to habitat fragmentation, with a worked island-comparison question.
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What this topic is asking
The College Board (Topic 2.3) wants you to explain the theory of island biogeography: how an island's size and its distance from the mainland determine how many species it holds. You must understand the balance between immigration and extinction rates, and apply the theory to modern habitat fragments, which behave like islands.
The theory
Two physical features of an island shift this balance: its size and its distance from a source of colonists.
Size and distance
- Size mainly affects the extinction rate (smaller islands lose species faster).
- Distance mainly affects the immigration rate (more distant islands gain species more slowly).
Endemism
Endemism makes islands biodiversity hotspots but also makes their species especially vulnerable, because an endemic species lost on its only island is lost everywhere (extinct).
Applying the theory to habitat fragments
The most important modern use of island biogeography is to habitat fragmentation. When a large continuous habitat is broken into patches (for example a forest cut into fragments by farms and roads), each fragment behaves like an island in a hostile "sea" of altered land. The theory predicts that smaller, more isolated fragments hold fewer species and lose species over time, because they have higher extinction rates and lower immigration than a large continuous habitat. This is why fragmentation is one of the main drivers of biodiversity loss, and why conservation favors large, connected reserves linked by wildlife corridors that restore immigration between patches. Topic 2.3 thus turns an idea developed for real islands into a practical tool for predicting how human land use erodes the biodiversity that Topic 2.1 established as valuable.
Try this
Q1. Identify which island feature most affects the immigration rate of new species. [1 point]
- Cue. Distance from the mainland (nearer islands have higher immigration).
Q2. Explain why a small, isolated forest fragment loses species over time. [2 points]
- Cue. It acts like a small, distant island: small size raises extinction rates and isolation lowers immigration, so the equilibrium number of species is low and species are lost.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)4 marksSection II (FRQ). (a) Explain how the size of an island affects its species richness. (b) Explain how the distance of an island from the mainland affects its species richness. (c) Describe how the theory of island biogeography applies to a forest fragment surrounded by farmland. (d) Define an endemic species and explain why islands often have many of them.Show worked answer →
A 4-point FRQ on island biogeography.
(a) Explain (1 point): larger islands support more species because they offer more habitat area, more resources and more niches, and have lower extinction rates.
(b) Explain (1 point): islands closer to the mainland have higher species richness because organisms can reach them more easily (higher immigration rate), while distant islands receive fewer colonists.
(c) Describe (1 point): a forest fragment acts like an island in a sea of farmland; smaller and more isolated fragments support fewer species and lose species over time, just as small, distant islands do.
(d) Define and explain (1 point): an endemic species is found naturally in only one place; islands have many because their isolation allows populations to evolve separately into unique species.
Markers reward linking size to area and lower extinction, distance to immigration, the island analogy for fragments, and the endemism definition with an isolation-driven reason.
AP 2020 (style)1 marksSection I (multiple choice). According to the theory of island biogeography, which island would be expected to have the greatest species richness? (A) Small and far from the mainland (B) Large and far from the mainland (C) Small and near the mainland (D) Large and near the mainland. Justify your choice.Show worked answer →
A 1-point MCQ on island biogeography. The answer is (D).
A large island offers more habitat and lower extinction rates, and a near island receives more colonists (higher immigration). Combining both, a large, near island has the highest species richness. (A) is the lowest (small, low immigration, high extinction); (B) and (C) each have only one favorable factor. The trap is considering size or distance alone rather than both together.
Related dot points
- Topic 2.1 Introduction to Biodiversity: describe the three levels of biodiversity and explain how genetic and species diversity contribute to ecosystem resilience.
A focused answer to APES Topic 2.1, covering genetic, species and habitat diversity, species richness and evenness, the value of genetic diversity, bottlenecks and resilience, with a worked diversity-comparison question.
- Topic 2.2 Ecosystem Services: describe the four categories of ecosystem services and explain how the disruption of ecosystems affects the services they provide.
A focused answer to APES Topic 2.2, covering provisioning, regulating, cultural and supporting ecosystem services, examples of each, their economic value, and how disruption reduces them, with a worked valuation question.
- Topic 2.4 Ecological Tolerance: describe the range of tolerance of organisms and explain how tolerance limits determine the distribution and survival of species.
A focused answer to APES Topic 2.4, covering the range of tolerance, optimum range, zones of stress, limits of tolerance, the law of tolerance and how tolerance varies between species and life stages, with a worked tolerance-curve question.
- Topic 2.5 Natural Disruptions to Ecosystems: describe natural disruptions to ecosystems and explain their short-term and long-term effects on populations and biodiversity.
A focused answer to APES Topic 2.5, covering periodic, episodic and random natural disruptions, fire, drought, storms, volcanism, plate tectonics and climate change, their short- and long-term effects, and ecosystem recovery, with a worked disturbance-analysis question.
- Topic 2.7 Ecological Succession: distinguish primary and secondary succession, describe how communities change over time, and explain the roles of pioneer, keystone and indicator species.
A focused answer to APES Topic 2.7, covering primary and secondary succession, pioneer species, the path to a climax community, keystone and indicator species, and the effects of succession on biomass and biodiversity, with a worked succession-sequencing question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)