What is biodiversity, why does it matter, and what threatens it?
The components of biodiversity (genetic diversity, species diversity and ecosystem diversity), the measurement of species diversity from richness and relative abundance, the threats posed by human activity, and the meaning and causes of mass extinction.
An SQA Higher Biology answer on biodiversity and mass extinction, covering genetic, species and ecosystem diversity, how species diversity is measured from richness and relative abundance, the threats from human activity, and the meaning and causes of mass extinction.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this key area is asking
The SQA wants you to describe the three components of biodiversity, explain how species diversity is measured from richness and relative abundance, describe the threats human activity poses to biodiversity, and explain the meaning and causes of mass extinction.
The components of biodiversity
- Genetic diversity is the number and frequency of alleles within a population of a species. Low genetic diversity, for example in a small population, makes a species less able to adapt to change and more likely to suffer from inbreeding.
- Species diversity is the number of different species in an area together with how common each one is.
- Ecosystem diversity is the number of distinct ecosystems or habitats within an area, such as woodland, river and grassland in one region.
Measuring species diversity
This matters because two communities with the same number of species can be very different: one where all species are common is far more diverse, and usually more stable, than one where a single species dominates and the rest barely survive.
Threats to biodiversity
Human activity reduces biodiversity through several pressures, which often act together:
- Habitat loss and fragmentation, for example from clearing forests for farming or building.
- Overexploitation, such as overfishing or overhunting, which removes species faster than they can reproduce.
- Pollution, which degrades habitats and poisons organisms.
- Introduced (invasive) species, which compete with, prey on or bring disease to native species that have no defences against them.
- Climate change, which shifts the temperature and rainfall conditions species depend on, forcing them to move or die out.
Low genetic diversity also leaves small, isolated populations vulnerable, as they cannot adapt easily and may suffer inbreeding, which can push a struggling species towards extinction.
Mass extinction
There have been five major mass extinctions in the past, caused by events such as major climate change, huge volcanic activity or asteroid impacts; the most famous wiped out the dinosaurs about 66 million years ago. Many scientists argue that current rates of species loss, driven largely by human activity, are high enough to represent a sixth mass extinction, this time caused by people rather than by a natural catastrophe.
Examples in context
Example 1. Low genetic diversity in the cheetah. Cheetahs passed through a population bottleneck thousands of years ago, leaving them with very low genetic diversity. As a result, individuals are so genetically similar that they struggle to adapt to new diseases and suffer reduced fertility. This shows why genetic diversity, not just species numbers, is a vital component of biodiversity and why small populations are so vulnerable.
Example 2. Invasive species on islands. When rats and cats were introduced to oceanic islands, many native birds that had evolved without such predators were driven to extinction, because they had no defences against them. These losses illustrate how introduced species threaten biodiversity, and how human activity can trigger extinctions rapidly. The accumulation of such losses worldwide is part of the evidence for a current, human-driven sixth mass extinction.
Try this
Q1. Name the three components of biodiversity. [1 mark]
- Cue. Genetic diversity, species diversity and ecosystem diversity.
Q2. Explain why a community with high species richness can still have low species diversity. [2 marks]
- Cue. If one species dominates and most others are rare, the relative abundance is uneven, so diversity is low.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20184 marksDescribe the three components of biodiversity, and explain why species richness alone does not fully describe species diversity.Show worked answer →
A 4-mark answer needs the three components plus the richness limitation.
Genetic diversity is the number and frequency of alleles within a population of a species. Species diversity is the number of different species in an area together with their relative abundance. Ecosystem diversity is the number of distinct ecosystems or habitats within an area.
Species richness alone does not fully describe species diversity because diversity also depends on relative abundance (how evenly the individuals are spread among the species). A community can have high richness but low diversity if one species dominates and the rest are rare.
Markers reward the three components correctly described and the point that relative abundance, as well as richness, determines diversity.
SQA Higher 20214 marksTwo woodlands each contain 5 species of tree. In woodland A one species makes up 90 percent of the trees; in woodland B the 5 species are present in roughly equal numbers. Determine which woodland has the higher species diversity and explain your answer, then state two human activities that threaten biodiversity.Show worked answer →
This applies the richness and relative abundance idea.
Step 1. Both woodlands have the same species richness (5 species each), so richness cannot separate them.
Step 2. Diversity also depends on relative abundance. In woodland A one species makes up 90 percent, so abundance is very uneven, giving low diversity. In woodland B the species are roughly equal, so abundance is even, giving high diversity.
Step 3. So woodland B has the higher species diversity, because both richness and relative abundance are high.
Two human activities that threaten biodiversity are, for example, habitat loss (clearing land) and overexploitation (overfishing or overhunting); pollution, invasive species and climate change are also valid.
Markers reward the conclusion that woodland B is more diverse, the reason based on even relative abundance, and two valid threats.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)