APES 2.5-2.6: Natural Disruptions and Ecological Succession

Ecological succession

The gradual change in the species composition of an ecological community over time.

What distinguishes primary succession from secondary succession?

Primary succession begins in a newly exposed habitat lacking a preexisting community and usually little or no soil. Secondary succession follows a disturbance that removes organisms but leaves remnants such as soil, nutrients, seeds, or surviving organisms.

Primary succession

Succession beginning on a previously uncolonized surface, such as newly exposed rock, lava, sand, or glacial deposits.

Secondary succession

Succession following disturbance of an established community when soil or other biological legacies remain. It is generally faster than primary succession.

What sequence of processes did classical succession theory identify?

The sequence was nudation, migration, ecesis, competition, reaction, and stabilization. These describe habitat opening, arrival, establishment, competition, environmental modification, and development of a relatively stable community.

Nudation

The creation or presence of a bare area with little or no life or organic matter, often produced by disturbance in secondary succession.

Migration and ecesis in ecological succession

Migration is the arrival of propagules such as seeds, spores, or other colonists. Ecesis is their establishment and initial growth in the habitat.

Seral community

An intermediate community occurring during succession before the development of a climax or other persistent state.

Climax community in classical ecological theory

The final, relatively stable, self-maintaining community of a sere, assumed to remain in equilibrium with its physical and biological environment if major disturbances are absent.

What does it mean for a climax community to have balanced energy and matter budgets?

Gross primary production is approximately balanced by total respiration, and energy captured from sunlight is balanced by energy released through decomposition. Nutrient uptake is also balanced by nutrient return in litter and decomposition, so there is no net annual accumulation of organic matter.

Sere and prisere

A sere is a sequence of communities developing through succession. A prisere is the complete series of seres from a nonvegetated surface to a mature community.

Climatic climax versus edaphic climax

A climatic climax is primarily determined by the regional climate. An edaphic climax is maintained by local substrate conditions such as soil moisture, nutrients, topography, fire, or animal activity.

Disclimax

A persistent community maintained by human activity or domestic animals rather than representing the climatic or edaphic endpoint expected for the site. Overgrazing that maintains a shrub-dominated community where grassland could occur is an example.

Autogenic versus allogenic succession

Autogenic succession is driven by organisms modifying their own environment, such as by adding organic matter or creating shade. Allogenic succession is caused by external forces such as erosion, flooding, fire, climate change, or animal activity.

How can plants create feedback loops during succession?

Plants can alter shade, soil nutrients, organic matter, temperature, moisture, and habitat structure. These changes facilitate some species while excluding others, thereby influencing the next stages of community development.

What factors can cause different sites to follow different successional trajectories?

Initial site conditions, disturbance type and intensity, species interactions, dispersal of colonists, weather during colonization, and long-term climate change can all alter the trajectory.

Dispersal limitation

A condition in which a species cannot reach a site even though the site's environmental conditions would permit it to survive there.

Environmental filtering or establishment limitation

A process in which propagules reach a site but fail to establish because local conditions, such as moisture, temperature, nutrients, or disturbance, are unsuitable.

How do stochastic and deterministic processes both influence succession?

Random events such as disturbance timing, weather, and propagule arrival introduce stochastic variation. Deterministic processes, including environmental filtering, competition, and facilitation, favor some species over others based on ecological conditions.

Why is the classical idea of a single predictable climax community considered too simple?

Disturbances may occur too frequently for a climax to develop, and climate can change during succession. Multiple stable states, feedbacks, chance events, and site-specific interactions can produce several possible endpoints or persistent dynamic states.

Monoclimax, polyclimax, and climax-pattern theories

Monoclimax theory proposes one climate-determined climax. Polyclimax theory allows multiple climax communities controlled by local factors such as soil, topography, fire, and animals. Climax-pattern theory views community composition as a continuously varying response to the total environment.

What general changes are often associated with progression toward a mature community?

Species diversity, total biomass, organism longevity, food-web complexity, decomposer importance, and overall stability often increase. Nutrient-use rates, biogeochemical cycling rates, and net primary productivity may decrease, although these patterns are not universal.

Pioneer species

Early colonizers that are often rapidly growing, highly dispersive, and tolerant of exposed conditions. They can modify the habitat and may later be replaced by stronger competitors or shade-tolerant species.

Why are pioneer trees often replaced by shade-tolerant species in an undisturbed forest?

Pioneer trees commonly require strong sunlight and produce abundant seeds. Once they form a canopy, reduced light at the forest floor prevents many pioneer seedlings from growing while allowing shade-tolerant species to establish.

What are typical stages of primary succession on newly exposed rock?

Microorganisms and organisms such as lichens and mosses colonize first, followed by grasses, shrubs, and eventually trees. Soil formation and increasing habitat complexity allow progressively larger and more diverse organisms to establish.

Why does secondary succession usually proceed more rapidly than primary succession?

Secondary succession begins with existing soil, nutrients, organic matter, seed banks, and sometimes surviving organisms. These biological legacies reduce the time required for colonization and ecosystem development.

How can a wildfire produce secondary succession rather than primary succession?

If the fire removes vegetation but leaves soil, nutrients, seeds, roots, or microorganisms, the site retains biological legacies. Regrowth can therefore begin with grasses and herbs rather than requiring soil formation from bare rock.

Seasonal or cyclic succession

Periodic changes in community composition caused by recurring events or fluctuating species interactions rather than a one-way progression toward a fixed endpoint.

How can animal communities change during succession?

As vegetation and habitat structure become more complex, animal abundance and diversity generally increase. Sparse early communities may contain small arthropods, while later communities can support diverse invertebrates and vertebrates.

Microsuccession

Succession occurring within a small habitat or microbial community. It can result from competition, changes in resources, temperature or water availability, and environmental modifications produced by microorganisms.

How can microbial succession begin before autotrophs colonize a new surface?

Heterotrophs may initially use imported dead organic matter, or necromass, as an energy and nutrient source. This shows that early food webs need not always begin with local photosynthetic producers.

What is the typical classical sequence for succession in an open-water wetland?

Emergent plants colonize the margins, organic matter and sediment accumulate, and open water becomes shallower and more marsh-like. The classical model predicts eventual transition toward drier land and possibly forest, although real wetlands often remain in dynamic states.

Why is wetland succession often nonlinear?

Hydrology, including tides, flooding, drying, and moving water, repeatedly alters the community and sediment environment. Fire and variations in precipitation can also interrupt or redirect the predicted progression toward a forested endpoint.

Why are grasslands not necessarily early-successional communities?

Some grasslands are long-established ecosystems with their own stable species interactions and disturbance regimes. Recovery from agricultural tillage or severe disruption can require centuries, showing that grasslands may represent mature communities rather than temporary stages.

What is the role of disturbance in maintaining ecosystem diversity and structure?

Disturbance can remove dominant organisms, create open patches, and allow pioneer species to colonize. Its frequency, intensity, and spatial pattern can prevent a single climax from forming and maintain a mosaic of successional stages.

Resistance versus resilience

Resistance is an ecosystem's ability to withstand a disturbance with little change. Resilience is its ability to recover after being altered by a disturbance.

How do natural disturbances affect ecosystems?

Natural disturbances such as wildfires, floods, volcanic eruptions, and hurricanes can remove organisms, alter soil and habitat conditions, redistribute nutrients, and create opportunities for colonization. Their effects depend on frequency, intensity, duration, and the ecosystem's resistance and resilience.

How can fire, flooding, volcanoes, and hurricanes influence succession?

A disturbance may destroy some or all of a community and initiate secondary succession if soil or biological legacies remain. Severe volcanic eruptions or lava flows can create bare surfaces and initiate primary succession. Floods and hurricanes may also transport organisms and nutrients while producing new gaps for pioneer species.