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📝 Estimated number of species on Earth (9 MCQs)

📖 From Campbell Biology • 1. Evolution and the theme of Biology and Scientific Inquiry • 9 questions available

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What is Estimated number of species on Earth?

Definition:
The estimated number of species on Earth refers to the total count of distinct living organisms, including both described and undescribed species, which is a dynamic figure based on scientific surveys, statistical models, and taxonomic efforts, with current estimates ranging from 5 to 30 million, but only about 1.2 to 2 million species have been formally described and cataloged.

Working:
These estimates are made using methods such as extrapolation from taxonomic rates, species-area relationships, and DNA barcoding, with the species-area relationship described by the equation $S = cA^z$ , where $S$ is the number of species, $A$ is the area, and $c$ and $z$ are constants, and more recent estimates using molecular methods suggest that arthropods, bacteria, and deep-sea organisms represent a large proportion of undiscovered species.

Example:
A simple example is the estimate of insect species, where about 1 million have been described, but estimates suggest there may be 5 to 10 million total, with a single study in a tropical forest canopy finding dozens of new beetle species, illustrating how undiscovered species are abundant in understudied regions and habitats.

Reason:
Knowing the estimated number of species is important for biodiversity conservation, as it highlights the gaps in our knowledge, sets priorities for taxonomy and exploration, and underscores the urgency of preserving habitats before unknown species are lost, which is critical for ecosystem functioning and potential biotechnological resources.

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📝 All Estimated number of species on Earth MCQs

Q1. A researcher wants to estimate the total number of species on Earth but can directly identify only a fraction of them. Which conclusion is most scientifically defensible when the estimate is much larger than the number of formally described species?

A.The estimate must be wrong because undiscovered species cannot be counted

B.The estimate can be reasonable if it is based on sampling, extrapolation, and assumptions about undiscovered diversity ✅

C.Every undiscovered organism should automatically be classified as a separate species

D.The number of described species should always equal the estimated total

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Scientists can estimate total species richness without having identified every species by using representative sampling and mathematical extrapolation. Such estimates depend on assumptions and uncertainty, so a larger estimated total does not automatically indicate an error.

Q2. Two surveys examine the same tropical forest. Survey A samples 10 sites and identifies 420 species, while Survey B samples 40 sites and identifies 910 species. Why might Survey B provide a better basis for estimating regional species richness?

A.More sites can capture species missed by limited sampling ✅

B.Larger samples always produce exactly correct species counts

C.Survey B proves that every species occurs equally often

D.Sampling fewer sites eliminates rare species from the estimate

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Increasing the number of sampling sites generally increases the chance of detecting rare or spatially restricted species. Survey B therefore provides broader information about diversity, although additional sampling does not guarantee complete discovery.

Q3. A model estimates 8 million species globally, but a revised model estimates 20 million after including poorly sampled microorganisms. Which interpretation is strongest?

A.The first estimate was necessarily fabricated

B.The second estimate demonstrates that all microorganisms are new species

C.The revised estimate may reflect improved sampling assumptions and recognition of previously underrepresented diversity ✅

D.The number of species on Earth must have increased by 12 million

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: A change in an estimate does not necessarily mean biological diversity changed. Improved sampling, new taxonomic information, or better modeling can substantially alter estimates, especially for groups that are difficult to detect.

Q4. A student argues: 'Scientists have described about 2 million species, so there cannot be more than 2 million species on Earth.' What is the main flaw in this reasoning?

A.It assumes described species represent the complete global diversity ✅

B.It assumes species are found only in terrestrial environments

C.It ignores that some species are very large

D.It assumes every species has identical genetic variation

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The number of formally described species is a documented count, whereas the total number of species includes organisms that have not yet been discovered or formally described. Confusing these quantities produces an unjustified conclusion.

Q5. A biodiversity study produces the following cumulative species counts as sampling effort increases: 10 samples → 180 species, 20 → 290, 40 → 365, 80 → 405. What does this pattern most strongly suggest?

A.Species discovery is approaching a plateau ✅

B.Species discovery is increasing exponentially without limit

C.Sampling has no relationship with species detection

D.The ecosystem contains exactly 405 species

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The increasingly smaller gains in newly detected species suggest a leveling-off pattern. However, reaching a plateau in a sample does not prove that the entire ecosystem contains exactly that number because rare species may remain undetected.

Q6. A graph shows that estimated global species richness rises sharply when the number of sampled habitats increases from 1 to 20, then rises more slowly from 20 to 100 habitats. Which explanation best accounts for the pattern?

A.Early habitats reveal many distinct species, while later habitats increasingly contain species already detected ✅

B.Later sampling destroys species and therefore lowers biodiversity

C.The graph proves that only the first 20 habitats contain organisms

D.Species become genetically identical after 20 habitats are sampled

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Rapid early increases are expected when different habitats contribute many previously undetected species. As sampling expands, overlap among habitats usually increases, so each additional habitat contributes fewer unique species to the cumulative estimate.

Q7. Researchers compare two methods for estimating total species richness. Method X relies heavily on easily observed large animals, while Method Y includes soil microbes, fungi, insects, and organisms from several habitat types. Which method is likely to produce a more comprehensive estimate?

A.Method X, because large animals are easier to identify

B.Method X, because difficult organisms should be excluded from estimates

C.Method Y, because it reduces taxonomic and habitat sampling bias ✅

D.Both methods must produce identical estimates

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: A comprehensive estimate should account for groups and habitats that differ in detectability and abundance. Focusing mainly on conspicuous organisms can substantially underestimate total diversity, whereas broader sampling reduces some forms of bias.

Q8. A model predicts 15 million species using data from well-studied temperate ecosystems. A second model predicts 30 million using additional data from tropical forests and understudied microorganisms. Why might the second estimate be higher even if both models use valid statistical procedures?

A.It incorporates sources of diversity that were underrepresented in the first dataset ✅

B.Statistical models always produce larger values when more data are added

C.Tropical forests automatically contain exactly twice as many species

D.Microorganisms cannot be included in biodiversity estimates

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Valid models can yield different estimates when they use different datasets and assumptions. Adding poorly sampled habitats and highly diverse groups can reveal patterns that were absent from the first model, increasing the estimated global richness.

Q9. An investigator estimates total species richness by multiplying the average number of species found per sampled habitat by the estimated number of habitats worldwide. Which assumption is most important for judging whether this approach is reliable?

A.All habitats must contain exactly the same species

B.The sampled habitats must reasonably represent the diversity and species richness of habitats being extrapolated to ✅

C.Every species must have the same population size

D.Only common species should be included in the samples

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: This extrapolation depends critically on representativeness. If sampled habitats differ systematically from unsampled habitats, multiplying an average by the global habitat count can produce serious bias. Variation in richness must therefore be incorporated or justified.

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