2.5 Zonation, success HLion and change

Zonation is a spatial change in community composition along a gradient such as elevation, tidal exposure, moisture or salinity.

As conditions change, species occur where their tolerances and interactions allow survival. Describe the gradient first, then identify which abiotic limit changes with position.

Moving up a shore reduces immersion time; algae near the water may be replaced by salt-tolerant plants higher up.

Ordered community change that tracks a measurable spatial gradient.

Zonation is across space; change through years is succession.

A line transect samples communities at fixed positions along a suspected gradient so biotic patterns can be compared with abiotic measurements.

Place quadrats at matched distances, record abundance or presence, and measure the relevant variable such as moisture, pH, salinity or light. Co-change supports a relationship but does not prove one cause.

Along a shore, algae cover falls as distance above water rises while immersion time is recorded at each quadrat.

A transect tests whether a species-distribution pattern tracks the proposed environmental gradient rather than distance alone.

A kite diagram shows pattern, not causation; confounding variables and sampling bias still matter.

Succession is community replacement over time in one area; zonation is community difference across space at a given time.

Succession can be driven by organisms changing soil, shade and nutrients, plus disturbance. Zonation is read along a spatial gradient such as height or moisture.

A bare field becoming shrubland over 20 years is succession; different plant bands from beach to dune are zonation.

The time dimension; without repeated observations, a spatial snapshot may only show zonation.

Do not infer succession just because communities differ; ask whether the same place changes through time.

A sere is a temporary community stage; each stage modifies conditions that allow a later community to establish.

Pioneers weather rock, trap particles and add organic matter. Deeper soil, shade, moisture and nutrients then change competition so grasses, shrubs and larger plants can replace earlier stages. In the classical model, successive seres continue until a relatively stable climax community is reached; Objective 2.5.12 later examines why that endpoint is uncertain.

Lichens on bare rock add organic matter; grasses root in the developing soil, then shrubs alter light and moisture for tree seedlings.

Each sere changes soil, nutrients, shade or water, altering which species can colonize and compete in the next stage.

Succession is not a fixed species shopping list; disturbance and site conditions can change the pathway.

Primary succession starts on newly exposed substratum with no soil and no established community.

Pioneers tolerate harsh conditions and help build soil from mineral particles and organic matter. The decisive test is starting soil, not how recently the site formed.

A retreating glacier exposes bare moraine; lichens and microbes begin soil formation before larger plants establish.

This is primary succession because the starting surface lacks soil; nearby colonists do not change that classification.

‘New’ alone is insufficient—check whether soil or biological legacies remain.

Secondary succession follows disturbance when soil, nutrients or living legacies remain.

Abandoned fields and many burned forests can retain seed banks, roots, microbes and nearby colonists, so recovery need not rebuild soil from bare rock. Rate depends on disturbance severity.

After a grassland fire, surviving soil and roots allow regrowth much sooner than a newly formed volcanic surface.

Measure surviving soil, organic matter, roots or seed bank; fire alone does not decide the category.

Secondary does not mean ‘fast in every case’; severe disturbance can remove most legacies.

Succession usually increases biomass, soil development and food-web complexity, but productivity and diversity need not rise monotonically.

New niches can raise diversity, while late dominance can reduce it. Productivity rises as producers establish, then respiration may catch up; nutrient cycling gains stores and pathways.

Diversity may peak in a middle stage with pioneers and later colonists together, then fall when one canopy species dominates.

State the measured variable and stage; do not assume every trend is always upward.

More biomass does not automatically mean more diversity or higher net productivity.

Resilience is the capacity to recover after disturbance; diversity can support it by providing overlapping functions, but outcome depends on disturbance conditions.

Assess magnitude, frequency, duration and which functions were lost. A diverse system may resist change or recover through response variety; simplification can remove those options.

After one species declines, several pollinators can keep pollination going; after repeated drought removes all drought-sensitive species, recovery may fail.

Greater diversity can add functional options that support resilience and stability, but extreme or repeated disturbance can still push the ecosystem beyond recovery.

Stability and resilience are related but not identical; a system can resist change yet recover poorly after collapse.

Succession is shaped by interacting climate, substrate, disturbance and consumer controls—not climate alone.

Bedrock and soil affect pH, nutrients and water; slope and drainage affect development; fire, flood, drought and storms reset stages; consumers alter vegetation from the top down.

Heavy grazing removes tree seedlings, keeping a site open even where climate could support woodland.

Check substrate, disturbance history and consumer pressure alongside climate.

A climate-only prediction ignores the processes that can reset or maintain an alternative state.

Net productivity is gross production minus community respiration: NP = GP − R.

Early succession has sparse producers and low respiration, so NP can be strongly positive. Later GP may be high, but plants, consumers and decomposers respire more, bringing NP toward zero.

Early in succession, GP is low but community respiration is lower, so positive NP allows biomass to accumulate. Later, GP may be high, but respiration by a larger community nearly balances it, so NP approaches zero.

A climax community can contain a large biomass while NP is near zero because gross production is balanced by community respiration.

Productivity is a rate, not stored biomass; do not infer NP from stock size alone.

r-strategy favours many small offspring and rapid colonization; K-strategy favours fewer, better-invested offspring under stronger competition and stable resources.

Use the environment, not a species label: short-lived disturbance and open niches favour rapid reproduction, while crowded long-lived populations favour competitive investment. Real species lie on a continuum.

Wind-dispersed annuals can colonize bare ground quickly; slow-growing trees invest in fewer offspring in a mature forest.

Recently disturbed habitat tends to favour r-like traits because empty space and short-lived opportunities reward rapid reproduction and colonization.

‘r’ and ‘K’ are endpoints of a continuum, not permanent boxes for every species or life stage.

The classic climax model predicts one stable endpoint, but real ecosystems may remain dynamic or settle into multiple persistent states.

Chance, disturbance, herbivores and human influence can redirect succession. Alternative stable states persist under similar broad conditions, so compare evidence rather than assuming one endpoint.

A grazed woodland mosaic and a closed forest may both be plausible outcomes depending on large-herbivore pressure and disturbance history.

Evidence for the challenge includes multiple persistent states under similar broad conditions, or strong evidence that disturbance and feedback prevent one fixed endpoint.

Rejecting the classic model does not mean succession is random; it means endpoint and pathway require evidence.

A plagioclimax is a community maintained because human activity arrests or redirects succession.

Name the intervention, the transition it blocks, and the community maintained. Repeated grazing or mowing can remove tree seedlings; stopping the intervention may allow a different stage to develop.

Annual mowing keeps grassland open by preventing shrubs and trees from establishing.

Shrub or woodland succession may resume, subject to soil, climate and other disturbances.

Plagioclimax is not ‘unnatural’ in every value judgement; it is a descriptive term for human-maintained succession state.