Human Influences on Ecosystems: Food, Pollution, and Conservation
Eco Chain Reaction: Every Choice Ripples 🌍🔁
Introduction
1. Introduction
Okay, last chapter, and it's a big vibe shift: biology stops being about cells for a sec and becomes 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 "chain" is the whole secret. The exam almost never asks "what is a monoculture?" on its own, it gives you a scenario (fertiliser in a lake, a forest cleared) and wants you to say what happens next, in order. Get the order right and you're basically unstoppable. Let's go save some ecosystems 🌍
And "chain" is the whole secret. The exam almost never asks "what is a monoculture?" on its own, it gives you a scenario (fertiliser in a lake, a forest cleared) and wants you to say what happens next, in order. Get the order right and you're basically unstoppable. Let's go save some ecosystems 🌍
2. Growing More Food: Monocultures
More people = more mouths, but the land isn't getting bigger. So farmers use monocultures (one crop, giant field), chemical fertilisers (minerals to make crops grow fast), and herbicides/insecticides (kill weeds/pests). 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 can all get it, no resistant plant to slow it down, and the whole field can wipe out. 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 farmer grows only wheat across a whole large field, no other crop present. What is this method called, and what's one advantage?
- 1Growing one crop species over a large area is called a monoculture.
- 2Because every plant is the same, machinery can plant, spray and harvest the whole field in one pass.
- 3Watch the trap answers, "more species" or "richer soil" are things a monoculture reduces, not gives. The advantage is efficient, machine-friendly farming.
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3. Feeding Animals: Intensive Livestock
Intensive livestock = lots of animals packed into a small, controlled space. Because they barely move and stay warm, less of their food energy leaks away as heat and movement, more goes into growth, so you get more meat from less feed, and cheaper prices.
The downside is crowding: less land is left as natural habitat for wildlife, and disease spreads fast through packed, similar animals, the same risk as a monoculture. 🐔
The downside is crowding: less land is left as natural habitat for wildlife, and disease spreads fast through packed, similar animals, the same risk as a monoculture. 🐔
Key idea🔑 Key idea: intensive livestock = more food per space (less energy wasted on movement), BUT less land for wildlife and disease spreads easily in crowds.
Worked example
Worked Example: Rice Production Glow-Up
Worked Example: Percentage Power-Up 📈
Rice production was 150 million tonnes in 1960 and 350 million tonnes in 1990. What's the percentage increase, to the nearest whole number?
- 1Percentage increase = (increase ÷ starting value) × 100. Find the increase first.
- 2Divide by the starting value (150, NOT 350) and times 100.
- 3Always divide by where you started. Dividing by the final value (350) instead gives a smaller, wrong answer.
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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? Three reasons: more land used (farms, houses, factories), taking resources (mining, timber), and pollution.
The main result: a drop in biodiversity, the number of different species in an area. 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: a drop in biodiversity, the number of different species in an area. 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 🌲
Which combination correctly describes undesirable effects of deforestation?
- 1Removing trees destroys habitats, so species lose their homes.
- 2No roots to hold the soil, so it erodes and washes away.
- 3Fewer trees means less carbon dioxide removed by photosynthesis, so CO rises.
Big trap: don't give the same effect three times ("animals die / plants die / species lost" is all just lost biodiversity). Three genuinely different effects = full marks: habitat loss, soil erosion, and increased carbon dioxide.
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Answer
5. When One Species Falls, Others Follow
Here's why habitat destruction hits 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 for whatever 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 🕸️
In a forest food chain: oak trees → caterpillars → songbirds → hawks. A large area of oak forest is cleared for housing. What's the effect on this food chain?
- 1Fewer oak trees → less food for caterpillars → caterpillar numbers fall.
- 2Fewer caterpillars → less food for songbirds → songbird numbers fall too.
- 3Fewer songbirds → less food for hawks → hawk numbers fall as well. One change at the bottom travels all the way up the chain, not just the trees are affected.
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6. The Pollution Chain
This is THE sequence question of the whole chapter, 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. This whole chain is called eutrophication.
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 Sewage to Dead Fish
Worked Example: Follow the Chain 🔗
Chicken faeces from a farm wash into a lake. Which option correctly explains the sequence leading to fish dying?
- 1Faeces add nutrients (nitrate/phosphate) to the water, so algae grow fast.
- 2The dense algal growth blocks light, so plants below die.
- 3Decomposing bacteria feed on the dead plants, multiply fast, and respire, using up the dissolved oxygen, so fish can't respire and die.
Any answer with fish dying before the oxygen runs out, or plants (not bacteria) taking the oxygen, breaks the chain and loses marks.
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7. Plastic That Never Leaves
Some water pollution isn't about oxygen, 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.
Plastic breaks into tiny microplastics that small animals eat. 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. ♻️
Plastic breaks into tiny microplastics that small animals eat. 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. ♻️
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?
- 1Paper is biodegradable, decomposers break it down fast.
- 2Plastic is non-biodegradable, decomposers can't touch it, so it's not broken down and persists for a very long time.
- 3That persistence is why it 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, 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. 🐄🌾
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 ☁️
(a) Which gas from burning fossil fuels adds most to the enhanced greenhouse effect? (b) Cattle farming and rice paddies both release a different greenhouse gas, name it and its two sources.
- 1Burning fossil fuels releases carbon dioxide, the main greenhouse gas from combustion.
- 2Cattle release methane through digestion; rice paddies release methane from bacteria in the flooded soil.
- 3Name the gas and the source, "pollution" or "farming" alone is too vague. And don't mix this up 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: Is the Quota Sustainable?
Worked Example: Fish Maths 🐟
A lake's fish population is 10 000 at the start of the year. Breeding adds 1 500 new fish. The current quota is 1 200. A council wants to raise it to 1 800. Is the new quota sustainable?
- 1Compare the current quota (1 200) with the fish added (1 500): 1 200 is less than 1 500, so the population is maintained, sustainable.
- 2Compare the proposed quota (1 800) with the fish added (1 500): 1 800 is more than 1 500, so more fish would be removed than replaced.
- 3So the proposed quota would make the population decline, it is not sustainable. Rule: a quota is sustainable exactly when it doesn't exceed the number of new fish added.
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10. Saving Species From Extinction
When a species is endangered (numbers so low it might go extinct), three things usually caused it: habitat loss, hunting, and pollution. Methods to save it: protected habitats (national parks), laws against hunting, education (people who understand, protect), seed banks (frozen plant seeds), and captive breeding (breed animals in zoos, then release into the wild).
Seed banks suit plants (seeds store for decades). Captive breeding suits animals (they have to be bred alive). And if a species has habitat but too few individuals, breed first; if it has numbers but no habitat, protect the habitat first. 🦏
Seed banks suit plants (seeds store for decades). Captive breeding suits animals (they have to be bred alive). And if a species has habitat but too few individuals, breed first; if it has numbers but no habitat, protect the habitat first. 🦏
Key idea🔑 Key idea: species become endangered from habitat loss, hunting, pollution. Conserve with protected habitats, laws, education, seed banks (plants) and captive breeding (animals) — match the method to what's actually missing.
Worked example
Worked Example: Matching Method to Species
Worked Example: Seeds or Zoos? 🌱🦁
A team wants to conserve an endangered wild wheat species and an endangered wild cat species. Which method suits each, and why?
- 1Plant seeds can be dried and stored for years, so seed banks suit the wheat species.
- 2Animals can't be stored as seeds, they must be bred alive, so captive breeding suits the cat species.
- 3The method follows the organism: plants can be preserved dormant (seeds), animals must be kept alive and bred.
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