Separate diversity from a species count

More than one level of diversity

Genetic diversity concerns inherited variation within populations and species. Species diversity concerns the variety and relative abundance of species. Habitat or ecosystem diversity concerns the variety of ecological settings and processes across a landscape. One species list cannot fully measure all three. A species-rich site may still contain a population with little genetic variation, and a uniform habitat may support many transient visitors.

Richness counts the number of species represented. An abundance-sensitive index additionally responds to their relative proportions. State which quantity you use before comparing sites, because a larger count and a more balanced community are different claims.

A transparent index, not a magic score

Our synthetic community gives one focal species 55% of individuals and three others 15% each. The sum of squared proportions is 0.55²+3×0.15²=0.37. Its complement, 1−sum(p²), is 0.63. For population proportions, this is the probability that two independent draws with replacement belong to different species.

Keep four species but make all shares 25%. The complement is 1−4×0.25²=0.75. Keep four species but set the focal share to 85% and the others to 5% each: the index falls to 0.27. Richness did not change. A real sample estimator may require a finite-sample correction; this lab uses assigned population proportions so the formula and interpretation are explicit.

The focal species is not necessarily the most abundant when there is only one other species. The controls describe a focal share, not a guaranteed dominance ranking. All proportions sum to one, and every represented species has positive share throughout the supported grid.

Sampling can change apparent diversity

A larger sampling area or greater effort often detects more species. Rare, seasonal, nocturnal or cryptic organisms may be missed. Compare equal effort, comparable habitat coverage and identification methods before treating observed richness as a biological difference. A second survey finding a new species may reveal earlier non-detection rather than immigration.

Design a stratified sampling plan across habitat types, with repeated observations and explicit rules for uncertain identifications. Record effort and detection limitations. More samples improve information but do not automatically remove systematic bias, such as surveying only accessible roadsides.

Transfer and interpretation

Site A has four equally abundant species; Site B has six species but one accounts for 85% and the remaining five each for 3%. B has greater richness, but its index is 1−(0.85²+5×0.03²)=0.273, below A’s 0.75. Both rankings can be correct because they measure different properties.

Do not infer conservation priority from this index alone. Endemism, threatened populations, ecological roles, habitat connectivity and cultural values may matter. A high index can include nonnative species, and an invasive species can alter functions even before a richness count falls. Explain the purpose of the measurement and what additional evidence would change the decision.

Variation, bottlenecks and selective habitat loss

A bottleneck reduces a population so severely that surviving individuals may carry only part of its former genetic variation. Subsequent population growth does not automatically restore lost variants. More genetic variation can provide more alternatives on which selection acts when conditions change, but it does not guarantee that a useful variant exists. Likewise, species diversity can support recovery when species respond differently and some sustain functions after others decline; species identity and disturbance severity still matter.

In an original inherited-marker packet, a population initially contains variants A, B, C and D. The only survivors carry A and B, and their descendants reach the original population size without mutation or immigration. Predict marker richness: it remains two, not four. Change the packet so immigrants bring C: three variants are now represented, but this does not establish adaptation to a specified stress. Distinguish abundance recovery, genetic recovery and functional recovery.

Habitat loss can disproportionately remove specialists whose required resources disappear, and species needing large territories may lose viable space even when small generalists remain. This is a conditional vulnerability pattern, not a universal extinction order. Suppose a fictional specialist needs an intact reed patch and a territorial species needs 20 contiguous hectares. Conversion leaves 30 hectares total in six isolated 5-hectare patches, with no reeds. Explain why total area alone does not meet either requirement. If one 25-hectare connected reed patch remains instead, both stated habitat constraints improve, but food, reproduction and actual occupancy still need evidence.

Source scope and evidence boundary

Original explanation, synthetic data and practice aligned with AP Environmental Science topic 2.1. Framework checked 30 October 2026: official topic map and Fall 2026 clarifications. The adaptation wording distinguishes inherited population changes over generations from individual need. Public numerical feedback is not independent assessment; virtual captures do not establish supervised fieldwork or actual ecosystem outcomes.

Exact-clause reconciliation checked 31 October 2026 using the official framework and Fall 2026 corrections. Added packets are original and observational or virtual only.