AP Biology · Lesson 3

Biodiversity and disruptions to ecosystems

Know Know it

Biodiversity includes the variety of genes within species, the number of species and their relative numbers, and the variety of ecosystems. Diversity of species has two parts: richness, how many species there are, and evenness, how equally individuals are spread among them.

Simpson’s diversity index = 1 - the sum of (n/N) squared, where n is the number of individuals of one species and N is the total for all species. The index runs from 0 (one species) toward 1. More species, and a more even spread, both raise it.

Ecosystems with more diversity are usually more resilient: they recover faster after a disturbance, because some species can take over the roles of others that are lost. A population with more genetic variation is more likely to include individuals that survive a new disease or a changing climate. A keystone species strongly shapes the diversity of its community; removing it can cause a large drop in the number of species.

Ecosystems are disrupted by natural events, such as volcanoes, fires and droughts, and by people. Habitat loss is the largest threat to many species. Invasive species spread quickly in new areas, often because their natural predators, parasites or diseases were left behind. Eutrophication starts when fertilizer or sewage adds nutrients to water: algae bloom, then die, and decomposers use up the dissolved oxygen as they break the algae down. Burning fossil fuels raises carbon dioxide in the air, warming the climate and making the oceans more acidic.

After a disturbance, communities change over time by succession. Primary succession begins on bare rock with no soil; secondary succession begins where soil remains, as after a fire or on an abandoned field.

Trap Two communities can tie in richness and differ in diversity

Counting species is not enough. Two plots with the same four species can have very different Simpson’s index values if one species makes up most of the individuals in one plot. Always use the proportions.

Worked example Easy

Two fictional meadows each have the same four plant species. In meadow X, each species makes up 25 percent of the plants. In meadow Y, one species makes up 85 percent and each of the others 5 percent. Which meadow is more diverse, and why?

  1. Meadow Y, because one species is very successful there
  2. Neither, because both meadows have the same four species
  3. Meadow Y, because a dominant species makes a community stable
  4. Meadow X, because its species are evenly representedAnswer

How to solve it

  1. Richness is the same: four species in each meadow.
  2. Evenness differs: X is 25 percent each, Y is 85, 5, 5 and 5 percent.
  3. The more even community, X, is more diverse.

Why each choice is right or wrong

  • A. Incorrect. One species dominating meadow Y makes it less even, which lowers its diversity.
  • B. Incorrect. Equal richness is only half the measure; evenness differs, so their diversity differs.
  • C. Incorrect. Dominance by one species lowers evenness and diversity; it does not make the meadow more diverse.
  • D. Correct. Both have four species, but X is perfectly even, so it is more diverse.

Explain Explain it

Task verbs: calculate, predict

For a Simpson’s index problem, find each species’ share n/N, square each share, add the squares, and subtract the sum from 1. Show every share.

To predict the effect of a disruption, name what changes first, then follow it through the food web or the nutrient cycle one link at a time, and say what happens to diversity.

Worked example Medium

A fictional plot holds 100 plants: 40 of species A, 30 of species B, 20 of species C and 10 of species D. What is its Simpson’s diversity index?

  1. 0.30
  2. 0.70Answer
  3. 0.75
  4. 0.40

How to solve it

  1. Shares: 40/100 = 0.4, 30/100 = 0.3, 20/100 = 0.2, 10/100 = 0.1.
  2. Squares: 0.16 + 0.09 + 0.04 + 0.01 = 0.30.
  3. Index = 1 - 0.30 = 0.70.

Why each choice is right or wrong

  • A. Incorrect. 0.30 is the sum of the squared shares, 0.16 + 0.09 + 0.04 + 0.01; it still has to be subtracted from 1.
  • B. Correct. The squared shares are 0.16, 0.09, 0.04 and 0.01, which add to 0.30, and 1 - 0.30 = 0.70.
  • C. Incorrect. 0.75 is the index for four species in exactly equal numbers, but these four species are not equal.
  • D. Incorrect. 0.40 is just the share of species A, the most common species, not a diversity index.

Calculate Your turn

Two fictional meadow plots each hold 80 plants of the same two species. Plot 1 has 60 of species A and 20 of species B. Plot 2 has 40 of each. Calculate Simpson’s diversity index for each plot, using index = 1 - the sum of (n/N) squared and showing each share and its square. Then say which plot is more diverse and why, even though both have two species.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Calculates the index for plot 1 as 1 - (0.75 x 0.75 + 0.25 x 0.25) = 1 - 0.625 = 0.375
  • Calculates the index for plot 2 as 1 - (0.5 x 0.5 + 0.5 x 0.5) = 1 - 0.5 = 0.5
  • Says plot 2 is more diverse because its plants are spread more evenly between the two species

Model answer

Plot 1: the shares are 60/80 = 0.75 and 20/80 = 0.25. The squares are 0.5625 and 0.0625, which add to 0.625, so the index is 1 - 0.625 = 0.375. Plot 2: each share is 40/80 = 0.5, and 0.5 squared is 0.25, so the squares add to 0.25 + 0.25 = 0.5 and the index is 1 - 0.5 = 0.5. Plot 2 is more diverse. Both plots have the same two species, but plot 2 splits its plants evenly between them, while plot 1 is dominated by species A.

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