Introduction: Humans and the Living World

In this chapter, we explore how human activities change the biodiversity of our planet. Biodiversity is a measure of the variety of different species living in an ecosystem. High biodiversity is usually a sign of a healthy ecosystem because it means organisms are less dependent on just one other species for food or shelter.

We will look at how we sometimes harm biodiversity (often by accident!) and what we are doing to fix it through conservation and reforestation. Whether you are aiming for a Grade 5 or a Grade 9, understanding these human interactions is key for your Paper 2 exam.

1. Human Impacts on Biodiversity

Humans interact with ecosystems in many ways. While some are helpful, three main activities often reduce biodiversity: fish farming, introducing non-indigenous species, and eutrophication.

A. Fish Farming

As the human population grows, we need more food. Fish farming involves raising large numbers of fish in cages in the ocean or in ponds. While this provides food, it can damage the local ecosystem:

  • Waste: Uneaten food and fish waste (faeces) sink to the bottom. This can change the chemical balance of the water and lead to eutrophication.
  • Disease: Because the fish are kept so close together, parasites (like sea lice) and diseases spread very quickly. These can escape the farm and infect wild fish populations.
  • Predators: Animals like seals or birds may get tangled in the nets trying to reach the fish.
  • Competition: If farmed fish escape, they may outcompete wild species for food and mates.

B. Non-Indigenous Species

A non-indigenous species is one that does not naturally belong in an ecosystem. Humans often introduce them on purpose (for farming or gardening) or by accident (on ships).

Example: The North American Grey Squirrel was introduced to the UK and has caused the native Red Squirrel population to crash.

They reduce biodiversity because:

  • They may compete with native species for food, water, or space.
  • They might prey on native species that have no natural defences.
  • They can bring new diseases into the environment.

C. Eutrophication

This is a major cause of water pollution, often caused by fertilisers washing off farmers' fields into lakes and rivers. Here is the step-by-step process you need to know for the exam:

  1. Runoff: Nitrates from fertilisers wash into the water.
  2. Algal Bloom: The nitrates cause algae (tiny water plants) to grow very rapidly on the surface.
  3. Blocked Light: The thick layer of algae blocks sunlight from reaching plants at the bottom.
  4. Death: The plants at the bottom cannot photosynthesise, so they die.
  5. Decomposition: Decomposers (bacteria) break down the dead plants.
  6. Oxygen Depletion: These bacteria use up the dissolved oxygen (\(O_2\)) in the water as they respire.
  7. Death of the Ecosystem: There is no longer enough oxygen for fish and other animals, so they suffocate and die.

Quick Review: Biodiversity is about variety. Fish farming, "invader" species, and fertiliser runoff (eutrophication) all tend to lower that variety.

2. Conservation and Reforestation

It’s not all bad news! We can take steps to maintain or increase biodiversity.

Conservation

Conservation is the effort to protect species and their habitats. This can include:

  • Creating protected areas like National Parks where hunting or building is banned.
  • Seed banks to store the seeds of endangered plants so they don't go extinct.
  • Captive breeding programs in zoos to boost animal numbers.

Reforestation

Reforestation is the planting of trees in an area where a forest has been cut down. This increases biodiversity by providing new habitats and food sources for a wide range of insects, birds, and mammals.

Why bother? (The benefits)

Maintaining biodiversity is important because:

  • It protects our food supply (many crops depend on specific insects for pollination).
  • Many medicines are discovered in wild plants.
  • It provides cultural and aesthetic value (people enjoy nature!).
  • It keeps material cycles (like the carbon and nitrogen cycles) balanced.

3. Food Security (Biology Only)

9.11B: Food security means having enough safe and nutritious food to feed a population. There are several biological factors that threaten this:

  • Increasing human population: More people means we need more food.
  • Changing diets: As people in wealthier countries want more meat, more land is used for animal feed rather than human food.
  • New pests and pathogens: These can destroy crops and livestock.
  • Environmental change: Global warming can lead to droughts or floods, making it harder to grow crops.
  • Cost of agricultural inputs: The high price of fertilisers and seeds can make farming difficult in some areas.

Note: To help solve these problems, we use fertilisers and biological control (using one organism to kill a pest), which you can learn more about in the "Natural Selection and Genetic Modification" chapter.

4. Indicator Species (Higher Tier Only)

9.16B: Scientists can use indicator species to judge how polluted an area is without using expensive sensors. These are organisms that are very sensitive to their environment.

Water Pollution Indicators
  • Polluted Water: Bloodworms and sludgeworms can survive in water with very little oxygen. If you find lots of them, the water is likely polluted.
  • Clean Water: Freshwater shrimps and stonefly larvae need lots of oxygen. If you find these, the water is very clean.
Air Pollution Indicators
  • Lichens: These grow on trees and rocks. Some types of lichen cannot grow if there is too much sulfur dioxide (a pollutant) in the air.
  • Blackspot fungus: This fungus grows on rose leaves. It is killed by sulfur dioxide, so if your roses are healthy and "clean," the air might actually be polluted!

Study Tip: Remember that "Indicator" means "to show." These species show us the health of the environment.

Summary Checklist

- Can you define biodiversity? (The variety of species in an area.)
- Can you explain the steps of eutrophication? (Nitrates \(\rightarrow\) Algae \(\rightarrow\) No light \(\rightarrow\) Dead plants \(\rightarrow\) Bacteria \(\rightarrow\) No oxygen \(\rightarrow\) Dead fish.)
- Do you know the problems with fish farming? (Waste, disease, and escapes.)
- (Biology Only) Can you list three threats to food security? (Population growth, pests, climate change.)
- (Higher Tier Only) Which species indicates clean water? (Stonefly larvae or freshwater shrimp.)

If you want to know how we actually measure these populations in the field, check out the chapter on "Fieldwork: quadrats, transects and population estimates".