Human Impacts on Biodiversity

Essential Question

How do human activities reduce biodiversity?

Biodiversity is the variety of life, and many human activities are reducing it. In this lesson, you’ll learn the main ways people affect biodiversity, how habitat size relates to the number of species, and how data can show the effects of overharvesting.

Biodiversity and Its Threats

• Biodiversity: the variety of life
• Genetic, species, and ecosystem diversity
• Human activities can reduce it
• Six common threats

Biodiversity is the variety of life. Scientists describe it at three levels: the genetic variety within a species, the number and variety of species, and the variety of ecosystems. Human activities can reduce biodiversity in several ways. Common threats include habitat loss, fragmentation, invasive species, pollution, overharvesting, and climate change. These threats often act together, so a species may face more than one at the same time.

Habitat Loss and Fragmentation

• Habitat loss removes living space
• Fragmentation splits habitat into small pieces
• Smaller areas usually hold fewer species
• Isolated populations can be more vulnerable

Clearing land for farms, cities, and roads is a major cause of habitat loss. It can also cause fragmentation, which splits a large habitat into smaller, isolated pieces. In general, larger areas hold more species than smaller areas, as the sample data in the graph show. Small fragments can support smaller populations, which may be more likely to disappear, and roads and fields can make it hard for animals to move between pieces. The exact numbers vary from place to place, but the general pattern is widely observed.

Invasive Species and Pollution

• Invasive species spread and harm natives
• They may lack natural predators or competitors
• Extra nutrients can cause algal blooms
• Decay of algae can lower oxygen

An invasive species is a species introduced to a new area where it spreads and harms native species or ecosystems. Many species brought to new places do not become invasive, but those that do may compete with native species, eat them, or spread disease, and they may be limited by fewer predators or competitors than in their home range. Pollution also affects biodiversity. For example, fertilizer that runs off into a lake can cause algae to grow rapidly. When the algae die and decay, the process can use up oxygen, which may harm fish and other organisms.

Overharvesting and Climate Change

• Overharvesting: taking species faster than they recover
• Population can fall sharply
• Climate change shifts ranges and timing
• Species respond at different rates

Overharvesting means hunting, fishing, or gathering a species faster than its population can replace itself. The graph shows an illustrative model in which a fish stock is harvested faster than it can recover, so the population drops sharply. Real fisheries and hunted species have declined or collapsed in this way, although the details differ. Climate change adds another pressure. As temperatures and rainfall patterns change, some species shift where they live, and the timing of events such as flowering and migration can change. Species respond at different rates, so interactions between them can be disrupted.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Name the Threat

Use the six threats to biodiversity. Name the main threat in each case: (a) a forest is cleared for farmland and divided by new roads, (b) fertilizer runs off into a lake and algae grow rapidly, (c) a plant brought from another continent spreads and crowds out native plants, (d) a fish is caught faster than it can reproduce.

Show the solution

1. (a) Forest cleared and divided: land is lost and split into pieces, so this is habitat loss and fragmentation.
2. (b) Fertilizer runoff: extra nutrients enter the lake, so this is pollution.
3. (c) Plant spreading from another continent: a non-native species is crowding out natives, so this is an invasive species.
4. (d) Fish caught too fast: the species is taken faster than it can recover, so this is overharvesting.
5. Answer: (a) habitat loss and fragmentation, (b) pollution, (c) invasive species, (d) overharvesting.

Example 2: Habitat Size and Species

Use the graph. Do larger habitat fragments hold more species? Write a claim, evidence, and reasoning.

Show the solution

1. Read the graph: the fragments held 8, 14, 25, and 45 species, from the smallest to the largest.
2. Claim: in these sample data, larger habitat fragments hold more species.
3. Evidence: the 1 ha fragment had 8 species and the 1000 ha fragment had 45 species.
4. Reasoning: larger areas can support more individuals and more kinds of habitat, so more species can persist. This suggests that fragmenting a habitat into small pieces may reduce the number of species.
5. Answer: Yes, larger fragments held more species (8 species in 1 ha, 45 species in 1000 ha).

Example 3: How Much More at 100 ha?

A 10 ha fragment had 14 species and a 100 ha fragment had 25 species. Calculate the percent increase in the number of species. Round to the nearest tenth of a percent.

Show the solution

1. Change: 25 − 14 = 11 species.
2. Percent increase: change ÷ starting value × 100 = 11 ÷ 14 × 100 = 78.6%.
3. Answer: 78.6% increase (11 more species).

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Practice

Name threats to biodiversity, use survey data as evidence, calculate percent changes, and explain how human activities reduce biodiversity. Try it on your own first, then check your work with the answer key.

• Human Impacts on Biodiversity Practice Worksheet (PDF)
• Human Impacts on Biodiversity Practice Answer Key (PDF)

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Lesson Slides

• 3.8 Human Impacts on Biodiversity Slides (PDF)

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Big Ideas

• Biodiversity is the variety of life at the genetic, species, and ecosystem levels
• Habitat loss, fragmentation, invasive species, pollution, overharvesting, and climate change can reduce it
• Larger habitat areas generally hold more species
• Harvesting faster than a population can recover can cause a sharp decline
• Threats often act together

Back to the essential question: human activities reduce biodiversity by destroying and splitting habitat, moving species into new places, polluting ecosystems, harvesting species too fast, and changing the climate. Scientists use data to measure these effects. Next, you’ll look at how people can reduce harm and restore ecosystems.

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