Human Influences on Ecosystems: Food, Pollution, and Conservation
Planet Protector: Save the Ecosystem 🌍♻️
Introduction
1. Introduction
Okay, real talk: this is the chapter where biology stops being about cells and starts being about you and the planet. Every food choice, every plastic bottle, every forest cleared for a farm shows up here as a chain reaction.
And that word, chain, is the whole secret. The exam almost never asks "what is eutrophication?" It asks "how does fertiliser in a lake end up killing the fish?" and wants the steps in order. Get the order right and you basically can't be stopped. Let's save some ecosystems 🌍
And that word, chain, is the whole secret. The exam almost never asks "what is eutrophication?" It asks "how does fertiliser in a lake end up killing the fish?" and wants the steps in order. Get the order right and you basically can't be stopped. Let's save some ecosystems 🌍
2. Growing More Food: Monocultures & Fertilisers
More people = more mouths, but the land isn't getting any bigger. So farmers go big: monocultures (one crop, giant field) and chemical fertilisers (minerals like nitrate to make crops grow fast). Monocultures are efficient because everything is identical, one machine plants, sprays and harvests the lot.
But "everything identical" is the catch. Every plant is genetically the same, so if one gets a disease, they all get it, there's no resistant plant to slow it down, and the whole field can wipe out. It's like a group chat where everyone opens the same dodgy link. Low genetic variety = high risk. 🎯
But "everything identical" is the catch. Every plant is genetically the same, so if one gets a disease, they all get it, there's no resistant plant to slow it down, and the whole field can wipe out. It's like a group chat where everyone opens the same dodgy link. Low genetic variety = high risk. 🎯
Key idea🔑 Key idea: monocultures give huge yields and easy machine farming, BUT genetic uniformity means one disease can destroy the entire crop.
Worked example
Worked Example: The Monoculture Flex
Worked Example: Why Farmers Go Big 🚜
A monoculture is growing a single crop species over a large area. What is an advantage of large-scale monocultures?
- 1Advantages come from everything being the same + on a huge scale, so think efficiency and yield.
- 2Because the whole field is one crop, it can be planted, sprayed and harvested by machine all at once, giving a big yield of one product per area.
- 3Watch the trap answers, "more species", "resists disease", "richer soil" are all things a monoculture reduces, not gives. So the advantage is the efficient, high-yield, machine-friendly farming.
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3. Feeding Animals: Intensive Livestock
Intensive livestock = lots of animals packed into a small, controlled space. Because the animals barely move and stay warm, less of their food energy leaks away as heat and movement, and more goes into growth, so you get more meat from less feed, and cheaper prices.
The downside is the crowding. Diseases rip through packed animals fast, so farmers give antibiotics to the whole herd routinely, not just to sick ones. That drives antibiotic resistance: the antibiotic kills the weak bacteria and leaves resistant ones to multiply (natural selection), and those resistant bacteria can end up in bacteria that infect humans. 💊
The downside is the crowding. Diseases rip through packed animals fast, so farmers give antibiotics to the whole herd routinely, not just to sick ones. That drives antibiotic resistance: the antibiotic kills the weak bacteria and leaves resistant ones to multiply (natural selection), and those resistant bacteria can end up in bacteria that infect humans. 💊
Key idea🔑 Key idea: intensive livestock = more food per space (less energy wasted on movement), BUT routine antibiotics drive antibiotic resistance that can reach humans.
Worked example
Worked Example: The Antibiotic Maths
Worked Example: Percentage Glow-Up 📈
In one country, antibiotic use in livestock rose from 260 mg per kg in 2010 to 400 mg per kg in 2016. What is the percentage increase, to two significant figures?
- 1Percentage increase = (increase ÷ starting value) × 100. First find the increase.
- 2Divide by the starting value (260, NOT 400) and times 100.
- 3Sense-check: 260 up to 400 is a bit more than half again, so just over 50% is spot on. The classic fail is dividing by 400, always divide by where you started.
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4. Wrecking Habitats
A habitat is where an organism lives, and habitat destruction is humans damaging or removing it, the #1 cause of species dying out. Why do we do it? Three big reasons: clearing land (farms, houses, factories), grabbing resources (mining, timber), and pollution.
The main result is a drop in biodiversity, the number of different species in an area. And here's the killer detail everyone gets wrong: biodiversity is about variety, not headcount. A field of a million identical wheat plants has a massive population but almost zero biodiversity. 🌱
The main result is a drop in biodiversity, the number of different species in an area. And here's the killer detail everyone gets wrong: biodiversity is about variety, not headcount. A field of a million identical wheat plants has a massive population but almost zero biodiversity. 🌱
Key idea🔑 Key idea: biodiversity = the NUMBER OF DIFFERENT SPECIES, not the number of individuals. Habitat destruction lowers it fast.
Worked example
Worked Example: Deforestation's Damage
Worked Example: When the Trees Go 🌲
Besides contributing to global warming, deforestation has other undesirable effects. Name three of these other effects.
- 1Global warming is off the table, so pick from the other three effects of cutting down forests.
- 2(1) Loss of habitat → fewer species / extinction. (2) Soil erosion → no roots to hold the soil, so rain washes the topsoil away. (3) Disrupted water cycle → less transpiration → less rain → floods then drought.
- 3Big trap: don't give the same effect three times ("animals die / plants die / species lost" is all just lost biodiversity). Three different effects = full marks.
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5. When One Species Falls, Others Follow
Here's why habitat destruction hits so much harder than it looks: organisms are wired together in food webs, so knocking out one species topples others. Wreck a habitat and you don't just lose the species living there, you cut the food supply for everything that ate them, and the food supply for everything that ate those.
This is why a "specialist" (an organism that eats only one thing) is in way more danger than a "generalist" that can switch foods. Lose its one food source and it's done, no backup plan. 🁢
This is why a "specialist" (an organism that eats only one thing) is in way more danger than a "generalist" that can switch foods. Lose its one food source and it's done, no backup plan. 🁢
Key idea🔑 Key idea: food webs link everything, so removing one species starves the ones that depended on it. Specialists (one-food eaters) crash hardest.
Worked example
Worked Example: The Food Web Domino Drop
Worked Example: One Down, All Down 🕸️
Habitat destruction removed a herbivore's main food plant. A predator that fed only on that herbivore then also collapsed. Explain why.
- 1Food plant gone → the herbivore loses its food → the herbivore population falls.
- 2The predator only ate that herbivore → as the herbivore drops, the predator loses its food too → with no backup prey, the predator collapses.
- 3The point: the predator got wrecked indirectly, the plants were removed, not the predator, but everything's linked in a food web. A picky eater (one prey only) is way more at risk than a flexible one.
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6. The Eutrophication Chain
This is THE sequence question of the whole course, and the order is literally the marks. When fertiliser or untreated sewage runs into water, it adds nitrate and phosphate ions. Then: algae grow like crazy (an algal bloom) → the bloom blocks light → plants below can't photosynthesise and die → decomposing bacteria feast on the dead plants, multiply and respire → the bacteria use up the dissolved oxygen → the fish suffocate.
The #1 mistake? Saying "the plants use up the oxygen". Nope, it's the decomposing bacteria respiring that strips the oxygen out. Name them. 🎯
The #1 mistake? Saying "the plants use up the oxygen". Nope, it's the decomposing bacteria respiring that strips the oxygen out. Name them. 🎯
Key idea🔑 Key idea: nutrients → algal bloom → light blocked → plants die → DECOMPOSING BACTERIA multiply and respire → oxygen used up → fish die. The bacteria take the oxygen, not the plants.
Worked example
Worked Example: From Fertiliser to Dead Fish
Worked Example: Follow the Chain 🔗
Fertilisers and untreated sewage run off into a lake. Give the correct order of events that leads to fish dying during eutrophication.
- 1Start at the cause: fertiliser/sewage adds nitrate and phosphate ions to the water.
- 2Follow the dominoes in order: algae bloom → bloom blocks light → plants below die → decomposing bacteria multiply and respire → oxygen used up.
- 3End of chain: with the oxygen gone, the fish can't respire, so they die. Any answer where fish die before the oxygen runs out, or the plants (not bacteria) take the oxygen, breaks the chain and loses marks.
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7. Plastic That Never Leaves
Some water pollution isn't about oxygen at all, it's about stuff that just won't break down. Biodegradable materials (like paper) get broken down by decomposers and vanish in weeks. Non-biodegradable plastic can't be broken down, so it hangs around for decades or centuries.
Plastic breaks into tiny microplastics that small animals eat. Then a small fish eats loads of those animals (getting all their plastic), and a big predator eats loads of those fish. The plastic can't be broken down or removed, so it builds up at every level, and the top predator ends up with the most. It's the same reason toxins pile up in top predators. ♻️
Plastic breaks into tiny microplastics that small animals eat. Then a small fish eats loads of those animals (getting all their plastic), and a big predator eats loads of those fish. The plastic can't be broken down or removed, so it builds up at every level, and the top predator ends up with the most. It's the same reason toxins pile up in top predators. ♻️
Key idea🔑 Key idea: non-biodegradable = not broken down by decomposers = lasts forever + builds up along the food chain, hitting top predators hardest.
Worked example
Worked Example: The Plastic Property
Worked Example: Paper vs Plastic 📄
Compared with a biodegradable material such as paper, which property of non-biodegradable plastic makes it particularly harmful in aquatic ecosystems?
- 1Compare the two head to head: paper is biodegradable, decomposers break it down fast. Plastic is non-biodegradable, decomposers can't touch it.
- 2So the harmful property is that plastic is not broken down and persists for a very long time in the environment.
- 3That persistence is why it builds up, keeps trapping animals, and fragments into microplastics that enter food chains. The right answer is "not broken down / lasts a long time", not "toxic on contact" or "heavier than water".
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8. Warming the Planet
Greenhouse gases like carbon dioxide and methane trap heat that the Earth gives off, which naturally keeps the planet warm enough to live on. The problem: humans have pumped out way more of them, so more heat gets trapped, temperatures rise, and weather goes haywire, that's the enhanced greenhouse effect and climate change.
Where do they come from? CO from burning fossil fuels and cutting down forests; methane from cattle (their digestion) and rice paddy fields. Methane is present in smaller amounts but is a stronger greenhouse gas per molecule. 🐄🌾
Where do they come from? CO from burning fossil fuels and cutting down forests; methane from cattle (their digestion) and rice paddy fields. Methane is present in smaller amounts but is a stronger greenhouse gas per molecule. 🐄🌾
Key idea🔑 Key idea: extra CO (fossil fuels, deforestation) + methane (cattle, rice) trap more heat → enhanced greenhouse effect → climate change.
Worked example
Worked Example: Name That Gas
Worked Example: Greenhouse Gas Check ☁️
Methane and carbon dioxide both trap heat radiated from the Earth's surface. (a) What term describes this type of atmospheric gas? (b) State one human source of each.
- 1A gas that absorbs and re-radiates the Earth's heat, trapping warmth, is a greenhouse gas. Raising their amount causes the enhanced greenhouse effect.
- 2Carbon dioxide → burning fossil fuels and deforestation. Methane → cattle and rice paddy fields.
- 3Name the gas and a real source, "pollution" or "factories" alone is too vague. And don't confuse this with the ozone layer, totally different problem.
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9. Using Nature Without Running Out
Conservation is protecting and carefully managing nature. The key idea is the sustainable resource, one used so it's replaced and never runs out. Forests and fish are the two examples, and both follow one rule: only take what can be replaced.
For forests: fell only mature trees and replant one for every tree cut, so the total stays constant. For fish: use quotas (catch limits), big net holes (young fish escape to breed), closed seasons, and protected areas. Leave enough to reproduce and you can harvest forever. 🎣
For forests: fell only mature trees and replant one for every tree cut, so the total stays constant. For fish: use quotas (catch limits), big net holes (young fish escape to breed), closed seasons, and protected areas. Leave enough to reproduce and you can harvest forever. 🎣
Key idea🔑 Key idea: sustainable = take no faster than it replaces itself. Forests → replant to match felling; fish → quotas + big mesh + closed seasons + protected areas.
Worked example
Worked Example: Why Keep the Big Fish
Worked Example: Throw the Small Ones Back 🐟
When managing fish stocks as a sustainable resource, what is a reason for harvesting only the larger fish of a species?
- 1Smaller fish are usually younger and haven't bred yet.
- 2So keeping only the big (older) fish and letting the small ones escape (big net mesh) means the young fish survive to grow and breed.
- 3Those young fish then replace the ones you caught, so the population stays stable and the stock stays sustainable. Catch the young ones too early and numbers crash, because too few new fish get made.
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10. Saving Species From Extinction
When a species is endangered (numbers so low it might go extinct), we conserve it, to keep biodiversity, protect future medicines and genes, and because it's the right thing to do. Methods: protected habitats (national parks), laws against hunting, seed banks (frozen plant genes), and captive breeding (breeding in zoos, then releasing into the wild).
The tricky bit: captive breeding usually starts with very few animals, which means low genetic variation. Breeding close relatives (inbreeding) raises the chance of harmful genes showing up, so programmes breed the most unrelated animals and use AI and IVF to keep the gene pool varied. 🧬
The tricky bit: captive breeding usually starts with very few animals, which means low genetic variation. Breeding close relatives (inbreeding) raises the chance of harmful genes showing up, so programmes breed the most unrelated animals and use AI and IVF to keep the gene pool varied. 🧬
Key idea🔑 Key idea: small populations lose genetic variation → inbreeding → harmful genes + can't adapt. Fix it with coordinated breeding, AI and IVF to keep diversity up.
Worked example
Worked Example: Too Few To Breed
Worked Example: The 12-Horse Problem 🐴
A captive breeding programme to save a wild horse began with only 12 individuals. Explain the risks to the species of breeding from such a small number.
- 1Only 12 founders means a tiny gene pool, very low genetic variation (few different alleles).
- 2Breeding among so few forces mating between close relatives (inbreeding), which raises the chance offspring inherit two copies of the same harmful recessive allele, so more are weak, infertile or ill.
- 3Low variation also means the species can't adapt if the environment changes or a new disease hits, so it could still die out. Programmes fight this by breeding unrelated individuals and bringing in outside genes with AI and IVF.
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