Biodiversity in Ecosystems (College Board AP® Environmental Science): Study Guide

Components of biodiversity

What is biodiversity?

• Biodiversity can be thought of as a study of all the variation that exists within and between all forms of life

• Biodiversity looks at the range and variety of habitats, species and genes within a particular region

• It can be assessed at three different levels:

  • The genetic variation within each species or population (genetic diversity)

  • The number of species and their relative abundance (species diversity)

  • The number and range of different habitats (habitat diversity)

• Biodiversity is very important for the resilience of ecosystems

  • This is because biodiversity allows them to resist changes in the environment and avoid ecological tipping points

Genetic diversity in populations

What is genetic diversity?

• Genetic diversity is the diversity of genes found within different individuals of a species

• Individuals of the same species will have the same set of genes

  • However, these genes can take a variety of different forms

• This makes it possible for genetic diversity to occur between populations of the same species

• Genetic diversity within a single population also occurs

Importance of genetic diversity

• Genetic diversity is important as it can help the species adapt to changes in the environment (sometimes referred to as environmental stressors)

  • This can help them to survive these changes

  • This could include changes in biotic factors such as:

    • New predators

    • New pathogens

    • Competition with other species

  • Or the changes could be abiotic factors such as:

    • Changes in temperature, humidity and rainfall

• Populations with higher genetic diversity are more resilient to environmental stressors

Population bottlenecks

• A population bottleneck occurs when the size of a population is significantly reduced

  • This is usually due to an event, such as a natural disaster, disease outbreak, or human activity

• This sudden reduction leads to a loss of genetic diversity, as fewer individuals mean fewer genetic traits are passed on

  • Example: Overhunting of northern elephant seals reduced their population to about 20 individuals in the late 19th century, resulting in very low genetic diversity today

Effects of population bottlenecks

• Reduced ability to adapt to environmental changes

  • This increases vulnerability to extinction

• Higher risk of inbreeding, which can lead to harmful genetic mutations

• Limited recovery potential for populations under ongoing stress

• Example: The cheetah population has suffered multiple bottlenecks, leaving it with genetic uniformity that hampers its ability to resist diseases or adapt to habitat changes

Species diversity in ecosystems

What is species diversity?

• The greater the number of species in an ecosystem and the more evenly distributed the number of organisms are among each species, then the greater the species diversity

• Ecosystems with high species diversity are usually:

  • More stable than those with lower species diversity

  • More resilient to environmental changes

  • More likely to recover from disruptions, such as natural disasters or human activities

• For example, in the pine forests of Florida, the ecosystem is dominated by one or two tree species

  • If a new pathogen targets one of the two dominant species of trees, then the whole population could be wiped out and the ecosystem it is a part of could collapse

Habitat diversity in ecosystems

• This is the range of different habitats within a particular area, ecosystem or biome

• If there are a large number of different habitats within an area, then that area is likely to have high biodiversity

  • A good example of this is a coral reef

  • They are very complex with lots of microhabitats and niches to be exploited

• If there is only one or two different habitats, then an area has low biodiversity

  • Large sandy deserts typically have very low biodiversity

  • The conditions are basically the same throughout the whole area

Effects of habitat loss

• Loss of habitat leads to:

  • Decline in specialist species that rely on specific conditions

  • Decline in generalist species that thrive in a variety of conditions

  • Reduction in species with large territorial needs, such as apex predators

Species richness & evenness

What is species richness?

• Species richness is the number of species in a community or defined area

  • In some cases, it can be a useful measure to compare the biodiversity of different areas

• However, in other cases, species richness can be a misleading indicator of diversity

  • This is because it does not take into account the number of individuals of each species

• Once the abundance of each species in an area has been recorded, the results can be used to calculate the species diversity for that area

What is species evenness?

• Species diversity looks at the number of different species in an area but also the species evenness

  • Species evenness is the evenness of abundance across the different species (i.e. their relative abundances)

Species richness vs species diversity

• Species diversity is a much more informative measurement than species richness

• Conservationists often favor the use of species diversity as it takes into account both species richness and evenness

• For example:

  • Area 1 and Area 2 both contain four tree species

  • However, Area 2 is actually dominated by one species and in fact, one of the species is very rare (only one individual)

  • Although the two areas have exactly the same species richness, Area 1 has a higher species evenness (and therefore a higher overall species diversity) than Area 2

  • This example illustrates the limitations of using just species richness on its own