B. Biology: Sustainable Ecosystems and Climate Change · Sustainable Ecosystems and Climate ChangeBiodiversity: Why Variety Keeps Ecosystems Sustainable
B2.4 A1.1 A1.2About 70 minutes
Learning goals- I can describe genetic, species and ecosystem diversity.
- I can measure and compare biodiversity using species richness and evenness.
- I can explain how biodiversity makes ecosystems more stable and resilient.
- I can describe ecosystem services and the role of keystone species.
Words to know
biodiversity
The variety of life in an area: genetic, species and ecosystem diversity.
species richness
The number of different species in an area.
species evenness
How evenly individuals are spread among the species present.
resilience
The ability of an ecosystem to recover after a disturbance such as a fire, drought or disease.
ecosystem services
Benefits people get from ecosystems, such as clean water, pollination, fertile soil and climate regulation.
keystone species
A species that has a much larger effect on its ecosystem than its numbers suggest; removing it changes the whole ecosystem.
quadrat
A square frame of known size used to sample organisms in one spot.
monoculture
A large area planted with only one species or variety.
Remember from earlier grades
In Grade 6 you studied biodiversity and how to classify living things, and in Grade 7 you met keystone species and invasive species. This lesson focuses on how to measure biodiversity and why it makes ecosystems sustainable, one of the factors named in your Grade 9 curriculum.
Let's start with a city street
In the 2000s, many Ontario streets were lined with ash trees. Then an invasive beetle, the emerald ash borer, arrived (first found near Windsor and Detroit in 2002). It kills almost every ash tree it infests. Streets that had only ash trees lost all their shade in a few years. Streets with a mix of maples, oaks, lindens and ashes lost some trees but stayed green.
The mixed streets were more resilient. The same is true of whole ecosystems: variety is protection.
Three levels of biodiversity
- Genetic diversity: differences in genes within one species. Different individuals of the same species can vary in size, colour or disease resistance. A population with more genetic diversity is more likely to have some individuals that survive a new disease or a hotter climate.
- Species diversity: the variety of species in an area. It has two parts:
- Richness: how many species.
- Evenness: how balanced their numbers are.
- Ecosystem diversity: the variety of habitats in a region, such as forests, wetlands, meadows, lakes and rivers. More habitat types support more species.
Ontario has an amazing range: the Carolinian zone in the far south has species found nowhere else in Canada, such as the tulip tree and Kentucky coffee-tree, while the boreal forest and Hudson Bay Lowlands cover the north.
How biodiversity keeps ecosystems sustainable
- More pathways for energy and matter. In a diverse food web, if one prey species declines, predators can switch to another. Energy keeps flowing.
- Insurance (redundancy). Several species may do the same job (such as many kinds of bees pollinating). If one is lost, others can fill in.
- Resilience. Diverse ecosystems usually recover faster from fire, drought, floods or disease, because some species are likely to survive and recolonize.
- Resistance to invaders. When all the niches are filled, invasive species often have a harder time taking over.
- Ecosystem services. Diverse ecosystems give people pollination of crops, clean water (wetlands filter it), fertile soil, flood control, carbon storage, food and medicines.
A monoculture, such as a field of one crop or a street of one tree species, has low biodiversity and is vulnerable: one pest or disease can wipe it out.
Keystone species
Some species matter far more than their numbers suggest. The beaver is a keystone species in Ontario. Its dams create ponds and wetlands that become habitat for fish, frogs, ducks, moose and many plants. Beaver ponds store water during droughts and slow down floods. Remove the beavers, and the dam eventually breaks, the pond drains, and the whole community changes.
Top predators can also be keystone species. Wolves keep deer and moose populations in check, which protects the young trees and shrubs those animals eat.
Measuring biodiversity
Scientists can't count every organism, so they sample:
- A quadrat is a square of known size (often 1 m × 1 m) placed in several random spots. Inside, you count the species and the individuals of each.
- A transect is a line (like a measuring tape) across a habitat; you record organisms at regular intervals to see how they change, for example from a lakeshore into a forest.
To compare two areas, look at richness and evenness together. Two areas with the same number of species are not equally diverse if one species dominates in one of them.
Worked exampleLet's compare richness and evenness
Lawn: 40 Kentucky bluegrass plants, 2 dandelions, 1 white clover. Meadow: 9 goldenrod, 8 asters, 7 grasses, 9 clover, 7 milkweed. Compare their richness and evenness. Which is more diverse?
- Lawn: 3 species, 43 individuals. Bluegrass makes up 40 ÷ 43 ≈ 93%.
- Meadow: 5 species, 9 + 8 + 7 + 9 + 7 = 40 individuals. The biggest group (goldenrod or clover) is 9 ÷ 40 = 22.5%.
- Richness: meadow 5 > lawn 3.
- Evenness: the meadow's species are close to equal; the lawn is dominated by one species.
The meadow is more diverse: it has higher richness and higher evenness.
Worked exampleLet's predict resilience
A new fungal disease kills all clover. Predict the effect on the lawn and the meadow from the example above.
- Lawn: it loses 1 of 43 plants (about 2%). But the lawn already depends almost entirely on one species, bluegrass. If a bluegrass disease came instead, the lawn would collapse.
- Meadow: it loses 9 of 40 plants (22.5%), but four species remain, including other flowers that feed pollinators. Gaps fill in over time.
- Pollinators that visit clover can switch to asters, goldenrod and milkweed in the meadow, but have almost nothing left on the lawn.
The meadow keeps functioning because of its variety; the lawn is fragile because it is nearly a monoculture.
Hands-on activityQuadrat biodiversity survey (A1.2)
You need: 4 pencils or sticks, 4.5 m of string, a metre stick or measuring tape, a notebook, a phone camera or a field guide (an app like iNaturalist or Seek can help identify plants, with a grown-up's permission).
- Make a 1 m × 1 m quadrat: push the sticks into the ground at the corners of a square measured with the metre stick, and tie the string around them.
- Choose two habitats to compare: for example, a mowed lawn and an unmowed garden edge, park meadow or woodlot edge.
- Place the quadrat in 3 different spots in each habitat. Use a fair way to choose spots (for example, toss a soft object over your shoulder).
- In each quadrat, count the number of different plant types (you don't need names: "plant A with round leaves" is fine) and the number of each type. Count clumps of grass as one plant type with an estimate.
- Record your data in a table. Calculate the average richness for each habitat and the percentage of the most common plant.
- Conclude: which habitat is more diverse? Give two reasons it might support more insects and birds.
What to look for: unmowed areas usually have higher richness and evenness; lawns are often dominated by one or two grasses.
Safety: wear gloves, avoid plants you can't identify that might be poison ivy ("leaves of three, let it be"), and check for ticks after working in long grass.
Just for funScientists have described about 400 000 species of beetles. If you met one new beetle species every single day, it would take you more than 1 000 years to meet them all. Better start today.
Your turn
1Define genetic, species and ecosystem diversity with one example each.
Genetic: variety of genes within one species (e.g., different shell colours in one snail species, or disease resistance in some ash trees). Species: variety of species in an area (e.g., many tree species in a forest). Ecosystem: variety of habitats in a region (e.g., forests, wetlands and lakes in Algonquin Park).
2What is the difference between species richness and evenness?
Richness = the number of species. Evenness = how evenly individuals are spread among those species.
3Give three ways biodiversity makes ecosystems more sustainable.
Any three: more food-web pathways, redundancy (several species doing the same job), faster recovery after disturbance (resilience), resistance to invasive species, ecosystem services like pollination and clean water.
4Why is a monoculture vulnerable?
All the organisms are the same species (often very similar genetically), so one pest or disease can destroy most or all of it.
5Explain why the beaver is called a keystone species.
Its dams create ponds and wetlands that provide habitat for many other species and change water flow. Removing beavers changes the whole ecosystem, an effect much bigger than its numbers.
6Name two ecosystem services that wetlands provide.
Any two: filtering and cleaning water, storing floodwater, habitat for fish and birds, storing carbon, recharging groundwater.
7Quadrat C has 6 species with 10 individuals each. Quadrat D has 6 species: 50, 2, 2, 2, 2, 2. Which is more diverse? Why?
Quadrat C. Both have richness 6, but C is perfectly even, while D is dominated by one species (50 of 60, about 83%).
More practice: warm-up
A square frame of known size used to sample organisms.
2What does resilience mean for an ecosystem?
Its ability to recover after a disturbance.
3Name the invasive beetle that kills ash trees in Ontario.
4Which region of Ontario has species found nowhere else in Canada?
The Carolinian zone (far southern Ontario).
5Give one example of a crop monoculture.
Examples: a cornfield, a soybean field, a wheat field, a lawn of one grass.
More practice: on your own
1Why is genetic diversity important when a new disease arrives?
Some individuals may carry genes that make them resistant. They survive and reproduce, so the population can recover. With little genetic diversity, all individuals may be equally vulnerable.
2A survey of a woodlot finds 12 tree species, but 85% of the trees are sugar maples. Discuss its biodiversity.
Its richness is fairly high (12 species), but its evenness is low because sugar maple dominates. A disease or pest of sugar maples could seriously harm the woodlot.
3Why do scientists sample several quadrats placed at random rather than one quadrat in the spot that looks most interesting?
Random, repeated samples give a fair and representative picture of the whole area and reduce bias. One hand-picked quadrat might be unusual.
4Bumblebees, honeybees, sweat bees and hover flies all pollinate apple blossoms in an Ontario orchard. Explain how this redundancy helps the orchard.
If one pollinator declines (from disease, pesticide or a cold spring), the others still pollinate, so the orchard still produces apples. Many pollinators = insurance.
5If wolves were removed from a large park, predict a chain of effects.
Deer and moose would increase (less predation), eat more young trees and shrubs, so forest regrowth would slow, and animals that need those shrubs (some birds, hares) might decline. The ecosystem's balance would shift.
Challenge
1A city must replant 1 000 street trees after the emerald ash borer. Suggest a planting rule based on biodiversity and justify it.
Rule (example): no single species makes up more than about 10% of the trees, and no single genus (e.g., all maples) more than about 20%. Mix native species suited to the site. Justification: high species and genetic diversity means one pest or disease can only kill a small fraction, so the urban forest stays resilient. (Many cities use rules like this.)
2Explain how biodiversity connects to the flow of energy and the cycling of matter.
A diverse community has many producers capturing energy in different conditions and seasons, many consumers with alternative food sources (so energy keeps flowing if one link is lost), and many decomposers that break down different materials, so nutrients keep cycling. Losing species can break pathways and slow both processes.
3Design an investigation to test whether mowing reduces plant biodiversity in a park. Include your question, method and how you will make it fair.
Question: Does mowing reduce plant species richness? Method: choose a mowed and an unmowed area with the same sunlight, soil and slope; place 5 random 1 m² quadrats in each; count species and individuals; calculate average richness and the percentage of the most common species. Fair: same quadrat size, same number of samples, same day and same person identifying plants; repeat later in the season.
Quiz
1. Different shell colours within one snail species are an example of...
2. Two quadrats each have 5 species. In which is evenness highest?
3. Why are diverse ecosystems usually more resilient?
4. Which is an example of an ecosystem service?
5. Which best describes a keystone species?
6. A street of only ash trees loses all its trees to one beetle. This shows the risk of...
Big idea (B2.4, biodiversity): biodiversity exists at genetic, species and ecosystem levels. Greater biodiversity gives ecosystems more pathways, redundancy and resilience, and provides ecosystem services that people depend on. The quadrat survey is a real field method (A1.2), and identifying and researching species builds A1.1.
How to help: Do the quadrat survey together; it can be done in a backyard or park in 30 to 45 minutes. Identification apps are useful, but describing plants by features ("plant with toothed leaves") is just as good for counting richness.
Common difficulties:
- Thinking richness alone measures diversity. Evenness matters too.
- Confusing genetic and species diversity. Genetic diversity is within one species.
- Assuming lawns are "natural". Lawns are mostly managed monocultures.
Safety: check for ticks after time in long grass, and avoid poison ivy.
Sources: emerald ash borer detection in 2002 (Canadian Food Inspection Agency); Carolinian species examples (Carolinian Canada); beetle species numbers are commonly cited estimates of described species.