Disturbance Ecology and Successional Trajectories

An ecological disturbance is a temporary event, such as a fire, storm, flood or human activity, that damages a community and changes the availability of resources, and successional trajectories describe the different paths along which a community recovers afterward. In brief: disturbances differ in type, size, intensity and frequency. The intermediate disturbance hypothesis proposes that species diversity is highest at moderate levels of disturbance, because very frequent disturbance eliminates sensitive species, and very rare disturbance lets a few strong competitors dominate. Recovery can follow one of several paths, and does not always return to the original community.

Disturbances are a normal part of nature, and many ecosystems depend on them. This guide reviews succession and then goes deeper: it describes the features of disturbance, explains the intermediate disturbance hypothesis and the concepts of resistance and resilience, discusses alternative stable states, works through a numerical example of diversity and compares disturbance regimes.

What is an ecological disturbance?

A disturbance is a relatively short-lived event that removes biomass, kills organisms or changes the environment, and so alters the structure of a community. Natural examples are wildfires, hurricanes, floods, volcanic eruptions, landslides, droughts, outbreaks of pests and the death of a large tree. Human-caused examples include logging, ploughing, grazing, mining, urban development and pollution. A disturbance differs from a stress, which is a continuing pressure such as a constant shortage of nutrients.

Disturbances are important in many ecosystems. Some pine species need the heat of fire to open their cones, so the seeds are released, grasslands depend on fire and grazing to stop trees from taking over, and floods renew river floodplains with fertile sediment.

How are disturbances described?

• Type: fire, wind, flood, drought, human clearing and so on.
• Size (extent): from a single fallen tree to millions of hectares.
• Intensity (severity): how much biomass is lost. A light ground fire has a different effect from a crown fire that kills all trees.
• Frequency: how often it occurs. Some grasslands burn every 1 to 5 years, and some forests every 100 to 300 years.
• Return interval and predictability: the average time between events, and how regular it is.
• Timing: the season matters, because it affects species in different stages of their life cycle.

Together, these features form the disturbance regime of a place. Species are adapted to the regime of their habitat, so a change in the regime, such as fire suppression for decades or more frequent fires because of climate change, can alter the community.

What are successional trajectories?

Ecological succession, as explained in an earlier article, is the sequence of changes in a community after a disturbance. The word trajectory refers to the actual path followed. Early models suggested that succession always leads to the same stable climax community. Ecologists now recognize several possible outcomes:

• Convergence: different starting points reach the same community, if the environment is the same.
• Divergence: similar sites follow different paths, depending on which species arrive first (priority effects), on chance and on the nature of the disturbance.
• Arrested succession: recurring disturbances, such as grazing or fire, hold the community in an early stage.
• Alternative stable states: the community may switch to a different state that is self-maintaining and does not return, even if the disturbance stops.

The mechanisms in succession were described by Joseph Connell and Ralph Slatyer in 1977 as facilitation (early species make the environment better for later ones, such as nitrogen-fixing alder on bare ground), tolerance (later species are not helped or harmed by earlier ones, and succeed because they tolerate low resource levels) and inhibition (early species prevent others from establishing until they die or are damaged).

What is the intermediate disturbance hypothesis?

Joseph Connell proposed in 1978 that species diversity is greatest at an intermediate level of disturbance (frequency or intensity). He based this on observations of coral reefs and tropical rainforests.

• High disturbance: only fast-colonizing, tolerant species survive, so diversity is low.
• Low disturbance: a few superior competitors outcompete others (competitive exclusion), so diversity is again low.
• Intermediate disturbance: competition is interrupted before exclusion is complete, and patches at different stages of recovery coexist in a mosaic, so both fast colonizers and good competitors can live in the community.

A graph of diversity against disturbance is a hump-shaped curve. The hypothesis has been supported in some studies, for instance on rocky shores and in streams, but not in others, and it is debated how general it is. It remains a useful concept in conservation, since it suggests that some disturbance, if not too much, may maintain diversity.

What are resistance and resilience?

Resistance is the ability of a community to avoid change when it is disturbed. Resilience is the speed and degree to which a community recovers after a disturbance. Tropical rainforests are often thought to have low resilience to clearing, because their nutrients are held in the vegetation and the thin soils erode, and recovery can take centuries, whereas grasslands and many fire-adapted systems recover quickly. A community that has been pushed past a threshold (a tipping point) may shift to a different state: a coral reef that has been overgrown by algae after overfishing of grazing fish and pollution may remain algal; a shallow lake with too many nutrients may flip from clear water with plants to turbid water with algae, and stay so.

Disturbance regimes compared

Worked example: species diversity after disturbance

An ecologist surveys four rocky shore sites that are disturbed by waves from storms at different frequencies. The table gives the number of individuals of each species, and the Simpson diversity index D = 1 − Σ(n/N)² is calculated for each site.

1. Site A (N = 100): (0.9)² + (0.1)² = 0.81 + 0.01 = 0.82. D = 1 − 0.82 = 0.18. Only two species survive the frequent storms.
2. Site B (N = 100): (0.35)² + (0.30)² + (0.20)² + (0.15)² = 0.1225 + 0.09 + 0.04 + 0.0225 = 0.275. D = 1 − 0.275 = 0.725.
3. Site C (N = 100): (0.85)² + 3 × (0.05)² = 0.7225 + 0.0075 = 0.73. D = 1 − 0.73 = 0.27.

Diversity is highest at the site with intermediate disturbance (D = 0.725), which is consistent with the hypothesis. At site C, with rare disturbance, one species dominates (85 percent of individuals, D = 0.27), and at site A, the frequent storms leave only two species (D = 0.18). With only three sites, this is only an illustration, and real tests would use many sites and statistical analysis.

How is this used in management?

Land managers use controlled burns to reduce the fuel that builds up when fires are suppressed, and to maintain fire-adapted habitats. Grazing regimes and mowing are used to maintain meadows. River managers have considered “environmental flows” that mimic natural floods. Restoration ecologists use the idea of different trajectories to plan: they consider which species to plant, whether to remove invasive plants that could push the community to an unwanted state and whether nature can recover on its own. Disturbance after wildfires and storms is likely to increase with climate change, so understanding resilience is important.

How did the ideas develop?

Frederic Clements (1916) described succession as the development of a climax community, and Henry Gleason (1926) argued for an individualistic view. Joseph Connell’s 1978 paper on the intermediate disturbance hypothesis, and the work of Pickett and White on patch dynamics in the 1980s, shifted the emphasis to disturbance as a normal process. C. S. Holling introduced the concept of resilience in ecology in 1973. The eruption of Mount St. Helens in 1980 provided a natural experiment in primary and secondary succession that has been studied for more than four decades.

Which mistakes are common?

Mistake 1: Treating all disturbance as damaging

Many ecosystems need disturbance to maintain their species and structure.

Mistake 2: Assuming that recovery always leads back to the original community

Trajectories can diverge, and some systems switch to alternative states.

Mistake 3: Mixing up resistance and resilience

Resistance is not changing under disturbance. Resilience is recovering afterward.

Mistake 4: Saying that the highest diversity occurs with no disturbance

According to the intermediate disturbance hypothesis, the highest diversity is found at moderate levels.