Ecosystem resilience depends strongly on biodiversity. Diverse ecosystems typically resist disturbance better and recover faster because multiple species and interactions can buffer change when conditions shift.

What “diversity” means in this context

Biodiversity can refer to the variety of living components present and how evenly they are represented. In this subtopic, the key idea is that having more component parts (species, functional roles, interactions) generally improves ecosystem performance when environments change.

Core idea from the syllabus

• Ecosystems with fewer component parts and low diversity tend to be less resilient when faced with environmental change (for example, temperature shifts, drought, altered nutrient inputs, storms, or new pathogens).

Resilience and environmental change

Resilience describes how an ecosystem responds when disrupted.

Resilience is often discussed alongside two related response features:

• Resistance: how little an ecosystem changes during a disturbance (smaller immediate impact).

• Recovery: how quickly the ecosystem returns to prior levels of structure and function (faster return after impact).

Why more diversity often increases resilience

Low-diversity ecosystems may rely on a small set of species or interactions; if those are disrupted, there may be no effective substitutes. Higher diversity can increase resilience through several mechanisms.

Functional redundancy (“backup” roles)

This figure contrasts communities with high functional diversity but low redundancy versus communities with both high diversity and high redundancy. It visualizes the idea that when multiple species share similar functional traits, ecosystem processes can be maintained even if one species is lost. In AP Biology terms, redundancy can buffer productivity and nutrient cycling against disturbance by providing “backup” contributors. Source

• Different species can perform similar ecological roles (for example, multiple plant species contributing primary production, or multiple decomposers breaking down detritus).

• If one species declines under new conditions, other species with overlapping functions may maintain ecosystem processes (productivity, decomposition, nutrient retention).

• Low diversity reduces the chance that a “backup” species is present, so function is more likely to drop sharply.

Response diversity (different tolerances within a function)

Even when species share a functional role, they may respond differently to change.

• Within a function (e.g., pollination, nitrogen fixation, grazing), species can differ in heat tolerance, drought tolerance, disease resistance, or timing of activity.

• Environmental change may reduce some species but spare others, stabilizing the overall process.

Stabilising species interactions and food web pathways

Ecosystems are networks of interactions (competition, predation, mutualisms).

This food-web network diagram maps many consumer–resource links within a salt marsh community, with nodes representing species and edges representing feeding interactions. The dense connectivity illustrates how energy and matter can move through multiple pathways rather than a single linear chain. This helps explain why simplified systems with fewer connections can be more vulnerable to disruptions that propagate through the network. Source

Diversity can:

• Provide multiple interaction pathways for energy and matter movement, so the loss of one interaction has a smaller effect.

• Reduce the likelihood that a single disrupted interaction causes cascading failures across the system. In contrast, simplified systems may have fewer pathways, increasing vulnerability when conditions shift.

Statistical “portfolio” effect

• If species fluctuate differently over time, their combined contribution to ecosystem function can be more stable than any single species alone.

• Low diversity increases the chance that many individuals respond similarly to a disturbance, producing larger swings in ecosystem-level outcomes.

Why low-diversity ecosystems can be less resilient

Ecosystems with few component parts often show:

• Greater sensitivity to environmental change because key functions depend on fewer species.

• Lower buffering capacity against disturbances, increasing the magnitude of change.

• Slower recovery if recolonisation is limited or if key species are lost and cannot be replaced quickly.

• Higher risk of regime shifts, where the ecosystem reorganises into a different, persistent state (for example, a shift from clear-water to algal-dominated conditions), especially if feedbacks reinforce the new state.

This conceptual diagram illustrates how gradual environmental forcing (here, nutrient inputs) can push an ecosystem toward a threshold until resilience is exceeded and the system shifts abruptly into a new stable state. The “ball-and-basin” framing emphasizes that returning to the original state may not be as simple as reversing the original change because feedbacks can stabilize the new regime. This supports the idea that some disturbances can trigger persistent ecosystem reorganization rather than straightforward recovery. Source

What counts as “component parts”

To connect directly to the syllabus wording, “component parts” can include:

• Species and their population sizes

• Functional groups (organisms performing similar roles)

• Trophic connections (feeding relationships)

• Mutualisms and other interactions that support reproduction, survival, and nutrient cycling

When many of these parts are missing (low diversity), the ecosystem has fewer ways to maintain function during change.

Evidence students should be able to reason from

In AP Biology-style reasoning, support for the diversity–resilience relationship typically comes from patterns such as:

• After a disturbance, more diverse communities showing smaller declines in productivity or nutrient retention.

• Faster return of ecosystem function when more species are present to recolonise or compensate.

• Greater variability in ecosystem function over time in simplified communities.

These claims should be expressed as cause-and-effect: less diversity → fewer compensating species/interactions → larger functional disruption and/or slower recovery.