Keystone species are a central idea in ecology because they show how a single species can structure entire communities. Understanding their roles helps explain why some ecosystems resist change while others rapidly destabilise.

Keystone species and why they matter

Core concept: disproportionate impact

A keystone species can be relatively low in abundance yet strongly influences ecosystem diversity and overall stability by controlling species interactions, resource availability, and habitat conditions.

This “keystone” idea highlights that ecosystems are not only shaped by the most abundant organisms; they can be shaped by species that regulate critical ecological pathways.

Ecosystem stability (how AP Bio uses the term)

In AP Biology contexts, ecosystem stability refers to an ecosystem’s tendency to maintain its structure and functioning when conditions change. Keystone species contribute to stability by preventing extreme swings in population sizes and by supporting diverse community composition.

Mechanisms by which keystone species maintain diversity and stability

1) Top-down population regulation

Robert Paine’s classic intertidal experiment shows how removing the keystone predator (Pisaster ochraceus) triggers a trophic cascade. The figure contrasts the community food web and species richness before starfish removal versus 1 year and 5 years after, illustrating how mussels expand and crowd out other species. It visually reinforces the idea that predator control can prevent competitive dominance and maintain diversity. Source

Many keystone species act as keystone predators, limiting competitive dominants and preventing competitive exclusion that would otherwise reduce diversity.

• Predation can keep a fast-growing prey or dominant competitor from monopolising space or resources.

• By reducing the abundance of a dominant species, predators can indirectly allow multiple species to coexist.

• This tends to stabilise communities by dampening boom–bust dynamics in lower trophic levels.

2) Resource or habitat modification

Some keystone species function as ecosystem engineers, physically altering the environment in ways that create or maintain niches for many other species.

A three-stage landscape diagram illustrates how beaver dam-building can convert a narrow stream corridor into a broader wetland mosaic. By slowing and spreading water, the engineered habitat supports more microhabitats (open water, marsh edges, riparian vegetation) that different species can use. This is a visual example of how physical habitat modification can increase niche availability and support ecosystem stability. Source

• Building, burrowing, or reshaping habitat can increase habitat heterogeneity (more microhabitats).

• Increased niche availability can raise species richness and strengthen food web complexity.

• Structural habitat changes can buffer environmental fluctuations (e.g., moisture retention, refuges from predators), supporting stability.

3) Mutualistic or interaction “hubs”

Certain keystone species sustain stability by enabling key interactions that many other organisms depend on.

• If a species provides an essential service (e.g., a critical food resource at limiting times), many populations can depend on it.

• When such a species declines, multiple dependent species may also decline, reducing diversity and simplifying the community.

• Simplified communities often show less stable dynamics because fewer interactions remain to absorb disturbances.

Recognising keystone effects in real ecosystems

Evidence ecologists look for

Because “disproportionate effect” is comparative, keystone status is inferred from community-level outcomes.

• Removal or reduction of the suspected keystone causes a large shift in community composition, often including sharp declines in species diversity.

• Addition or recovery of the species restores interaction networks and increases community complexity.

• The effects are indirect as well as direct: changes can propagate through multiple species via food webs and interaction chains.

Why the impact is disproportionately large

Keystone species often control “bottleneck” processes:

• Access to limiting resources (space, prey, nesting sites)

• Persistence of key habitat features

• Strength of dominant competitors

• Connectivity of interaction networks (many species respond to one species’ presence)

Keystone species and ecosystem stability: what changes when they are lost?

Community-level consequences

When a keystone species is removed, ecosystems often shift toward reduced diversity and lower stability.

• Dominant species can increase rapidly, suppressing other populations.

• Food web connections may weaken as specialist interactions disappear.

• Population fluctuations may become more extreme because regulatory controls are lost.

Stability is not guaranteed by diversity alone

Keystone species help explain why two ecosystems with similar diversity can differ in stability: the identity and ecological role of particular species can matter as much as species count.