Explain what biodiversity is and why it matters for ecosystem stability, and describe how organisms are classified into a hierarchy of groups based on shared characteristics and evolutionary relationships (MA STE HS-LS4-5, HS-LS2-7 supporting).

What this topic is asking

The Massachusetts STE framework (HS-LS4-5 and the ecosystem standards) asks you to explain how changes in conditions affect biodiversity, and to connect the variety of life to its evolutionary origins. On the High School Biology MCAS, this is tested by comparing the biodiversity of ecosystems, explaining why biodiversity supports stability, and reasoning about how organisms are classified. The crosscutting concepts are stability and change (biodiversity and ecosystem resilience) and patterns (classification reflecting relationships).

What biodiversity is

Biodiversity is a measure of variety, and the MCAS uses it to compare ecosystems: an area with many different species has higher biodiversity than one with only a few. Biodiversity arises over long periods through speciation and natural selection (see speciation and population genetics), which is why it belongs in the evolution module as well as ecology.

Why biodiversity matters for stability

A central MCAS idea is that higher biodiversity tends to make an ecosystem more stable and resilient:

• With more species, there are more feeding relationships and more alternative food sources, so a food web has built-in backups.
• If one species declines or disappears, other species can fill its role, so the ecosystem is less likely to collapse.
• Genetic variety within a species means a population is more likely to include individuals that can survive a new disease or a change in conditions.

So biodiversity is a kind of insurance: variety lets an ecosystem absorb change. A low-biodiversity ecosystem is more fragile, because the loss of a single key species can have large effects. This connects to ecosystem dynamics in ecosystem structure and organization.

What threatens biodiversity

The MCAS expects you to name human activities that reduce biodiversity:

• Habitat destruction (clearing forests, draining wetlands, urban development).
• Pollution (of air, water, and soil).
• Overhunting and overfishing (removing species faster than they can recover).
• Introducing invasive species that outcompete or prey on native species.
• Climate change, which shifts the conditions species are adapted to.

Each reduces the number of species or the genetic variety within them, lowering an ecosystem's resilience. This links to the human-impact topic in human impact on ecosystems.

How organisms are classified

Scientists organize the variety of life by classification: sorting organisms into a hierarchy of nested groups (such as kingdom, phylum, class, order, family, genus, species). The basis for grouping is shared characteristics, increasingly including molecular (DNA) similarities. Crucially, modern classification reflects evolutionary relationships: organisms placed in the same small group share more characteristics and a more recent common ancestor than organisms in different large groups. So classification is not arbitrary; it mirrors the branching pattern of the phylogenetic trees from common ancestry and phylogeny.

Try this

Q1. Define biodiversity. [2]

• Cue. The variety of living things in an area, including the number of different species and the genetic variation within them.

Q2. Explain why higher biodiversity tends to make an ecosystem more stable. [2]

• Cue. More species means more feeding relationships and alternatives, so if one species declines others can fill its role, making the ecosystem more resilient.