Variety is a safety net: Every level of biodiversity adds to an ecosystem's resilience. With more species, an ecosystem keeps working when one species is lost, like a team with lots of substitutes.
The points to remember
- More species usually means more resilience.
- Redundancy: several species do the same job, so if one is lost another takes over its niche.
- More food chains and pathways: energy and nutrients can flow another way.
- A variety of species is more likely to include some that resist the change.
- Lose species and the web gets simpler, so one disturbance hits harder and can pass a tipping point.
Remember it as: More threads in the web: cut one and it still holds.
Real example: in the UK, crops and wild flowers are pollinated by about 270 kinds of bee, as well as hoverflies, moths and butterflies. If disease hits honeybees in one year, the other pollinators still carry pollen, so the flowers still make seeds.
Say the word 'resilience': Link every point to resilience in so many words: 'so it recovers after the disturbance'. A point about species that never says why the system is more resilient is not complete.
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The second component is genetic diversity. It decides whether a species can survive something new, such as a disease or a drier climate: some plants survive drought better than others of the same kind.
The points to remember
- Genetic diversity means some individuals survive a new disease, pest or climate.
- The survivors breed, so the population recovers and adapts.
- Low genetic diversity: every individual is equally at risk, so one disease can wipe out the lot.
- Crops grown as one variety (or clones) have almost no genetic diversity.
Remember it as: Same genes, same fate.
Real example: in Ireland in the 1840s, most poor families lived on one potato variety, the Lumper. When potato blight arrived in 1845, nearly every plant was equally at risk. About a million people died.
Genes, then survival, then recovery: Write the chain: more genetic diversity, so some individuals resist the change, so they survive and breed, so the population recovers.
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Size matters too. A large population is a big storage; a small one can vanish after a single bad event.
The points to remember
- Large populations hold bigger storages and more genetic diversity, so they last through bad years.
- Small populations can die out after one disturbance, or just by chance.
- Cut off in a small area: inbreeding, a small gene pool, less disease resistance.
- Crowded together: disease spreads fast, competition rises, one fire can kill most of them.
- Home ranges outside the protected area: animals there are hunted or killed by accident.
- Many small populations of different species beat one huge population of one dominant species.
- Big populations of foundation species (corals, kelp) add resilience; big invasive populations reduce it.
Remember it as: Few, crowded and cut off: one blow can finish them.
Real example: by the mid-1990s only about 20 to 30 Florida panthers were left. Inbreeding caused kinked tails and heart defects. In 1995 eight female pumas from Texas were released to bring in new genes, and numbers have since risen to about 120 to 230 adults.
Cover both, when asked for both: A question on species diversity AND population size needs points on each. Answers about only one of them cannot get full marks.
The third component is habitat diversity. A natural forest with many habitats is compared with a plantation monoculture.
The points to remember
- A mix of habitats gives refuges: species survive a disturbance in one patch and recolonise the rest.
- Natural forest: many tree species of every age, complex food webs, deep leaf litter.
- Plantation monoculture: one tree species, same age, often the same genes.
- So plantations suffer more from pests, disease, storms, fire and drought.
- Fewer habitats and niches mean fewer species in the plantation, so it is less resilient.
- Plant cover shields soil from erosion and keeps it moist; plantations can also exhaust the soil.
Real example: Kielder Forest, England's largest planted forest, is mostly rows of Sitka spruce. In November 2021 Storm Arwen flattened large areas of conifer plantation across northern England and Scotland, where every tree was the same height and equally exposed.
Reading two trends: For a scatter graph, say which sites have more cover or water AND more diversity, then give the reason: more niches, food and shade.
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Farms and fisheries are ecosystems too. Diversity at every level helps food production keep going year after year, which makes it more sustainable.
The points to remember
- Genetic diversity: many crop varieties and wild relatives hold genes for resisting disease and drought.
- Species diversity: polyculture, pollinators and natural pest enemies keep yields steady.
- Habitat diversity: hedges, ponds and buffer strips shelter pollinators and pest-eaters.
- Monocultures give big, cheap harvests but are fragile: one pest or disease can ruin them.
- Diversity can also be protected in seed banks, or added by breeding and genetic modification.
- Crop rotation, organic farming and biological pest control use diversity instead of chemicals.
Diversity helps
- Mixed crops: one failure is not a disaster
- Wild pest-eaters cut pesticide use
- Varied genes resist new diseases
But
- Monocultures feed more people cheaply
- Machines and markets suit one crop
- Farmed salmon escape and dilute wild genes
Real example: in Qingtian, China, farmers keep fish in their rice fields. The fish eat weeds and pests, and their droppings feed the rice, so the farms use far less pesticide and fertiliser. The system is over 1,200 years old.
Name a farm for every point: In a long answer, tie each point to a named system and place, then weigh it against the big yields of monocultures before you conclude.
How this comes up: Paper 2, Section B (a): outline how two components of diversity affect resilience [4].
An English hay meadow holds more than 30 kinds of grass and flower; a nearby ryegrass field is sown with one variety of perennial ryegrass.
Outline how species diversity and genetic diversity make the hay meadow more resilient than the ryegrass field.
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