Never just one threat: Most ecosystems face several human impacts at the same time. These impacts are increasing, and together they do more damage than each would do alone.
The points to remember
- Most ecosystems face several human impacts at once: habitat loss, pollution, overharvesting, invasive species, climate change.
- These impacts are increasing, as the human population and its use of resources grow.
- Combined, their effect is amplified: greater than the sum of each acting alone.
- One impact lowers resilience, so the next does more damage and recovery is slower.
- So tackling one threat alone may not save an ecosystem: the impacts must be managed together.
Remember it as: One plus one makes more than two.
Real example: the Great Barrier Reef, Australia, lost about half of its coral cover between 1985 and 2012. Scientists traced the loss to three impacts at once: cyclone damage, outbreaks of coral-eating crown-of-thorns starfish, and bleaching in hot years, while run-off from farms clouded the water.
Name more than one impact: When you describe a real ecosystem, list every impact it faces, then say how they act together.
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Why is the damage greater together? Because each impact weakens the ecosystem, so it is less able to cope with the next.
The points to remember
- Climate change lowers resilience: stressed, weakened species resist other threats less well.
- Invasive species then spread faster and do more harm than they would alone.
- Fragmentation blocks escape: species cannot move to cooler places as the climate warms.
- Overharvesting removes predators or grazers that would keep a pest or an invader in check.
- Pollution plus warming: warm water holds less oxygen, so eutrophication kills more fish.
Real example: on the Great Barrier Reef, floods wash fertiliser from sugar cane farms into the sea, which feeds the young of the crown-of-thorns starfish. After bleaching has already killed much coral, the starfish eat a larger share of what is left, and the weakened reef has too little time to recover before the next heatwave.
Show the link: Listing threats side by side is not enough. Say how one makes another worse: 'warming weakens the corals, so the starfish outbreak kills a larger share'.
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How do scientists show that people are the cause? One way is a transect laid at 90° to the source of the impact.
The points to remember
- Lay a transect at 90° to the source of the impact: a footpath, a road, an outfall pipe, a mine.
- Place quadrats at regular intervals (e.g. every 2 m) and record each species and its abundance (percentage cover or numbers).
- Work out species richness or a diversity index (Simpson's reciprocal index) for each quadrat.
- Measure an abiotic factor too (trampling, soil compaction, salinity, light), to link the change to its cause.
- Repeat several transects, placed at random along the source, for reliability.
- If diversity rises with distance from the source, the human activity is the likely cause.
Real example: on Snowdon, Wales, about 600,000 walkers a year trample the paths. Quadrats along transects at 90° to a path show few, tough grasses at the edge and many more species a few metres away, where the soil is not packed down.
Why at 90°?: A transect at 90° to the source crosses the gradient of the impact, from strongest to weakest, so any change in diversity can be linked to distance from the source.
The second method compares the same place before and after the activity. Other techniques add evidence, and each has a reason.
The points to remember
- Before and after: sample the same area before the activity and after, with random quadrats within transects.
- Compare with a control site the activity does not reach, sampled at the same time of year.
- Moving animals: mark-release-recapture (the Lincoln index), because quadrats miss animals that move.
- Indicator species show the level of impact: stonefly larvae need clean water; sludge worms tolerate pollution.
- A diversity index combines richness and evenness, so sites and years can be compared fairly.
- Biodiversity also includes habitat and genetic diversity: map the habitats, and sample genes where possible.
- Always give the reason for each technique: evidence of cause, reliability, a fair comparison.
Remember it as: Near and far, before and after: compare, and say why.
Transect at 90° to the source
- Samples near and far at the same time
- Shows a gradient of impact
- Good for a footpath, road or pipe
Before and after
- Samples the same place twice
- Shows change over time
- Needs data from before the activity
Real example: on the Elwha River, USA, two dams were removed in 2011-2014. Scientists had sampled fish and river invertebrates before removal, and sampled the same stretches after: salmon soon returned upstream.
Purpose, not detail: In a [7] on techniques, name a real ecosystem and explain why each technique is used. Extra detail of the method, or what the results might be, adds nothing.
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How this comes up: Paper 2, Section B (b): explain how ecological techniques can study the effect of a human activity on the biodiversity of a named ecosystem [7].
Coastal sand dunes at Studland Bay, England, are crossed by paths used by thousands of visitors each summer.
Explain how ecological techniques could be used to study the effects of trampling on the biodiversity of the dunes.
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