Separate richness, evenness and sampling completeness

Mechanism before a label

Biodiversity includes genetic, species and ecosystem variation. Species richness counts distinct species, while evenness describes how abundance is distributed among them. Two communities can have equal richness but very different dominance patterns.

A diversity index combines selected aspects of the abundance distribution. Here Simpson diversity is defined explicitly as1−sum(p_i²), where p_i is each species’ proportion in the sample. Other conventions use the same name for related but different quantities, so state the formula.

Sampling effort, detectability and taxonomic identification affect estimated diversity. Repeated samples, equal effort and a species-accumulation curve help assess completeness. A larger index is not a complete conservation ranking and does not substitute for threatened-species or habitat information.

Test resilience rather than infer it from richness alone

Ecosystem resistance describes how little a measured property changes during disturbance; resilience concerns its recovery afterward. Diverse species with different responses can help sustain or restore functions when one component declines. Low functional diversity can leave fewer alternatives, but richness alone does not guarantee recovery: species roles, interactions, producers, abiotic conditions and the disturbance all matter. A keystone’s disproportionate influence is not captured by counting it as one interchangeable species. Ecosystem resistance and resilience, checked 31 October 2026.

Original changed record: Two fictional communities each have four species and baseline production 100 units. After the same stipulated disturbance each falls to 60. At the declared later observation, community A reaches 95 and B reaches 65. Compare immediate resistance and observed recovery. Does equal richness imply equal resilience or prove that diversity caused the difference?

Check function, time and cause

Both show the same 40-unit initial decline, so this measured resistance comparison is equal. A recovers 35 units and B only 5 by the stated time; A has stronger observed recovery for this function and interval. Equal species counts did not fix the outcome. Species identity, response diversity, environmental differences and replicated trajectories are needed before assigning the difference causally to diversity or generalizing to every ecosystem property.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Community sample	Value (individuals)
Species A	25
Species B	25
Species C	25
Species D	25

Equal-effort fictional sample with 100 individuals.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Separate richness, evenness and sampling completeness

Supplied synthetic evidence: Equal-effort fictional sample with 100 individuals.

Community sample: reference condition

Quantity	Value	Unit
Species A	25	individuals
Species B	25	individuals
Species C	25	individuals
Species D	25	individuals

Task: If a second site was sampled for twice as long and yielded more species, what prevents a direct richness comparison?

The result describes this sample, not every unsampled species.

Compare with the conditional interpretation

Richness4;1−4×0.25²=0.75.

Both samples contain four named species. In the second sample squared proportions sum0.7225+0.0075=0.73, so defined diversity is 0.27 rather than0.75.

Case index for this investigation

0. Distinguish directional responses, activity and fitness
1. Track energy flow and distinguish production from standing biomass
2. Estimate abundance with sampling assumptions and uncertainty
3. Compare exponential and density-limited growth models
4. Use per-capita evidence to identify density dependence
5. Infer direct and indirect interactions from controlled contrasts
6. Separate richness, evenness and sampling completeness
7. Connect disturbance, nutrient cycles and evidence-based intervention

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Change one ecological assumption at a time

Bounded synthetic arithmetic practice, not a fitted ecosystem model or physical investigation. Scratch controls are not saved or graded. Record your first prediction and selected outputs in the lesson notebook.

Species 1 countSpecies 2 countSpecies 3 countSpecies 4 count

N=100; observed richness=4. Proportions=0.250, 0.250, 0.250, 0.250. Defined Simpson diversity1−Σp²=0.7500.

Change to85,5,5,5. Explain why richness stays the same while evenness changes. Equal sample effort and detectability remain necessary for interpreting comparisons. Zero-count categories are not observed species.

Reset practice

Worked reasoning

Both samples contain four named species. In the second sample squared proportions sum0.7225+0.0075=0.73, so defined diversity is 0.27 rather than0.75.

Richness4;1−4×0.25²=0.75.

Apply to a new case

If a second site was sampled for twice as long and yielded more species, what prevents a direct richness comparison?

Compare your reasoning with the worked response

Unequal effort and detection can inflate observed richness. Standardize effort or use an appropriate coverage/rarefaction comparison, and preserve uncertainty and habitat differences.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

The result describes this sample, not every unsampled species. The lower value reflects dominance under this formula, not necessarily lower conservation priority.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 8.6. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.