The Living World: Biodiversity

How this unit is tested

What you have to know

13 practice questions

1. Which measure of biodiversity accounts for both the number of species present and how evenly individuals are distributed among those species?
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   Answer. A species diversity index

   Species richness counts only the number of species, and evenness measures relative abundance alone; a diversity index (like the Shannon index) combines both into a single value, which is why it is considered the more complete biodiversity measure.
2. According to island biogeography theory, which island would be predicted to reach equilibrium with the highest species richness?
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   Answer. A large island close to the mainland

   Large islands have lower extinction rates because they support bigger populations and more habitat variety; islands close to the mainland have higher immigration rates because colonizers arrive more easily. Combining both effects maximizes equilibrium richness.
3. Explain why the immigration rate of new species onto an island decreases as the island approaches its equilibrium number of species.
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   Answer. As more species already occupy the island, fewer species from the mainland pool remain that have not yet colonized, and existing species increase competition for available niches, making successful new establishment less likely.

   Immigration rate is highest when an island is nearly empty because almost any arriving species can find an open niche; as species accumulate, the pool of non-resident species shrinks and competitive exclusion becomes more likely, driving immigration toward zero at equilibrium.
4. A fish species survives in water from 5°C to 30°C but reproduces successfully only between 15°C and 25°C. What do the 5–15°C and 25–30°C ranges represent on a range-of-tolerance graph?
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   Answer. The zones of physiological stress

   Outside the optimal reproductive range but still within the survivable range, organisms experience physiological stress — they can live but growth and reproduction are reduced. Only temperatures below 5°C or above 30°C would fall in the zone of intolerance.
5. Describe one natural disturbance and explain how it can increase biodiversity in an ecosystem over time.
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   Answer. A wildfire clears mature, dominant vegetation and opens the canopy, allowing sunlight to reach the ground; this lets shade-intolerant pioneer species colonize, creating a mosaic of habitat types and successional stages that supports a greater variety of species than a single uniform mature stand.

   Disturbances prevent any one species from permanently dominating and reset patches of the landscape to earlier successional stages, producing habitat heterogeneity across the ecosystem, which generally supports higher overall species richness than an undisturbed climax community alone.
6. Which statement best distinguishes primary succession from secondary succession?
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   Answer. Primary succession occurs where no soil or prior life exists, while secondary succession occurs where soil and some organisms remain after a disturbance

   The presence or absence of soil is the defining difference: primary succession (e.g., on new volcanic rock) must start with pioneer species like lichens that build soil, while secondary succession (e.g., after a wildfire or on abandoned farmland) starts from existing soil and proceeds much faster.
7. In primary succession on bare volcanic rock, lichens and mosses are typically the first colonizers. What role do these pioneer species play in enabling later succession stages?
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   Answer. They break down rock and accumulate organic matter, gradually forming a thin layer of soil that later plants can root in

   Pioneer species must tolerate extreme conditions with no soil; through weathering and the accumulation of dead organic matter, they create the soil substrate that grasses, shrubs, and eventually trees require to establish, driving the community toward a climax stage.
8. A species has a high reproductive rate, produces many small offspring, provides little parental care, and has a short lifespan. This life history strategy is best described as:
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   Answer. r-selected

   r-selected species prioritize rapid reproduction to exploit unstable or disturbance-prone environments, unlike K-selected species, which invest heavily in fewer offspring and thrive near carrying capacity in stable environments.
9. Explain how habitat fragmentation caused by human development relates to the predictions of island biogeography theory.
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   Answer. Fragmentation breaks a continuous habitat into smaller, isolated patches that function like islands surrounded by unsuitable land; smaller and more isolated fragments have lower immigration rates and higher extinction rates than the original continuous habitat, predicting reduced species richness in each fragment.

   Because island biogeography links species richness to area and isolation, applying it to fragments explains why breaking a large forest into small patches divided by roads or farmland tends to reduce biodiversity even if total remaining habitat area is similar, partly due to increased edge effects and isolation.
10. Using the species-area relationship $S = cA^{z}$ with $z = 0.3$, if a nature reserve's area is increased by a factor of 8, by approximately what factor does the expected species richness increase?
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    Answer. About 1.9 times

    Since $S_2/S_1 = (A_2/A_1)^{z}$, the ratio is $8^{0.3} \approx 1.87$. Increasing area eightfold only increases richness by about 87%, illustrating the decelerating nature of the species-area curve.
11. Define ecological tolerance and explain how a limiting factor affects a species' geographic distribution.
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    Answer. Ecological tolerance is the range of an environmental condition (such as temperature, pH, or moisture) within which a species can survive and reproduce; a limiting factor is a specific condition that falls outside a population's optimal or tolerable range, restricting where the population can grow or persist even if other conditions are favorable.

    Even one limiting factor outside a species' tolerance range — for example, soil moisture too low for a plant despite ideal temperature and light — can prevent establishment in an otherwise suitable habitat, which is why distribution maps often track a single limiting variable rather than overall habitat quality.
12. Explain why an ecosystem with higher biodiversity is generally more resilient to disturbance than one with low biodiversity.
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    Answer. Higher biodiversity provides functional redundancy, meaning multiple species can perform similar ecological roles, so the loss of one species is less likely to collapse an ecosystem function; greater genetic and species diversity also increases the chance that some individuals or species can tolerate or adapt to the disturbance.

    In a low-diversity system, losing a single dominant species can eliminate an entire ecological function (like pollination or decomposition) with no substitute, whereas a diverse food web has multiple pathways for energy and nutrients to flow, buffering the system against disruption.
13. A grassland experiences a moderate-intensity fire that kills above-ground vegetation but leaves the root systems and soil largely intact. What type of succession will most likely follow, and why?
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    Answer. Secondary succession, because the soil and some organisms (such as roots and seed banks) survive the fire, allowing recovery to proceed faster than it would on bare substrate

    The key indicator is that soil and biological legacies remain after the disturbance; this distinguishes it from primary succession, which would require starting from bare rock or newly formed substrate with no existing soil or organisms.

What people get wrong

1. Assuming extinction rate rises with island area — it's the opposite: larger islands support larger, more resilient populations and more habitat diversity, so extinction rate decreases as area increases. Always tie island size to extinction, and distance to immigration.
2. Misclassifying ecosystem services, especially calling pollination or water purification 'provisioning' — these are regulating services because they maintain a process rather than supply a direct product. Ask 'does this give me a good, or does it maintain a condition?' before classifying.
3. Confusing primary and secondary succession by focusing on the disturbance type instead of soil presence. A fire on farmland is secondary succession because soil remains; a lava flow is primary succession because no soil exists. Check for soil, not just 'was there a disaster.'
4. Treating the zone of physiological stress as if organisms cannot survive there at all. Organisms can survive in this zone but show reduced growth or reproduction; only outside the tolerance limits (zone of intolerance) do they die. Read range-of-tolerance graphs by labeling three zones, not two.
5. Reversing r- and K-selected traits under exam time pressure. Anchor the pair to a real example (dandelions/insects = r; elephants/whales = K) rather than memorizing the letters alone.
6. Forgetting that succession increases biodiversity and structural complexity only up to the climax community, then assuming it keeps increasing indefinitely. Climax communities can persist in relatively stable, high-diversity states until the next disturbance restarts the process.