Zonation refers to changes in community along an environmental gradient. As environmental conditions shift from place to place, community composition shifts with them.

An environmental gradient is a spatial pattern in which an environmental variable changes progressively with distance. It may involve elevation or latitude, tidal level, soil horizons, or distance from a water source. Several variables usually change at the same time, so visible zones shouldn’t automatically be linked to one cause.

Moving up a mountain, for example, may bring lower temperatures and stronger winds, along with thinner soils and altered precipitation. Species tolerate different parts of this combined gradient, which produces recognizable vegetation zones. The same principle applies on a much smaller scale along a shore. As height above the low-water mark increases, so do the duration of exposure to air, desiccation and temperature fluctuation.

The key distinction is straightforward: zonation is spatial, describing ecological change over space, while succession is temporal and describes change over time. Ecological systems vary along both dimensions.

A transect is a sampling line or belt placed across an environmental gradient so that biotic and abiotic variables can be measured systematically along it. This method is useful when there is a clear gradient, as random quadrats scattered across the whole site could miss parts of it.

Designing the investigation

Lay a measuring tape across the gradient—for example, running from wet ground to dry ground. Take samples at fixed distances along it. With a line transect, record organisms that touch the line. A belt transect places quadrats beside or across the line, allowing abundance, frequency or percentage cover to be measured.

At each sampling position:

• record a biotic variable, such as species presence, abundance or percentage cover;
• measure relevant abiotic variables, such as soil moisture, light intensity, temperature, pH or salinity;
• keep the quadrat size and sampling interval the same throughout;
• repeat transects at randomly selected positions to reduce bias and improve reliability;
• use consistent measurement methods and timing.

Results collected at different times of day may not be comparable. Temperature, light and moisture can change quickly. Seasonal variation also matters, so collect biotic and abiotic data close together in time. Where appropriate, repeat the sampling through the year or use data loggers to take continuous measurements.

The investigation should do more than show a change in species distribution. Compare the biological pattern with the abiotic measurements and look for variables that could explain it. A correlation along a transect doesn’t establish causation by itself; unmeasured variables or interactions between variables may also affect the pattern.

Kite diagrams

A kite diagram is a distribution figure in which the width of a symmetrical shape represents the abundance of a species at each point along a transect. Distance along the transect goes on the horizontal axis. Plot abundance or percentage cover equally above and below a central line, then connect adjacent values. Wider sections show high abundance, while narrow sections indicate low abundance or absence.

Kite diagrams show zones and overlaps clearly. Compare them with figures or tables of abiotic data to suggest which environmental conditions restrict each species. Fieldwork results may also be presented as an infographic or poster that explains the method, the observed zones and the likely controlling variables.

Succession is the replacement of one community by another in an area over time due to changes in biotic and abiotic variables. It is a temporal phenomenon, whereas zonation is spatial.

A community doesn’t simply wait to be displaced. The organisms within it alter local conditions. They may trap sediment, build soil, add organic matter, change nutrient availability, create shade or modify moisture. Other species can then colonize. If these later arrivals compete more successfully in the changed conditions, they replace much of the earlier community.

Succession can continue for hundreds of years. A pollen record consists of a sequence of preserved pollen grains found at different depths in sediment or peat, providing evidence of past vegetation. Deeper layers are usually older. Changes in the proportions of pollen types can therefore reveal the long-term replacement of plant communities.

Mapping change through succession

When direct observation over centuries is impossible, secondary evidence can be used instead. Sources may include dated aerial photographs, satellite images, ecological surveys and published datasets. A geographic information system (GIS) is a computer-based system that stores, maps and analyses geographically referenced data. Historical layers can be aligned, then classified into community types. This allows changes in their area and distribution to be reconstructed through time.

Images should use comparable seasons, spatial resolution and classification methods. If they don’t, an apparent ecological change may result from differences in image quality or sampling methods instead. Using secondary data with a mapping database turns succession into a measurable spatial history.

A seral community, or sere, is a community occupying one identifiable stage of ecological succession. A pioneer community is the first seral community and contains species able to establish under the difficult initial conditions. A climax community is the relatively stable community reached at the later end of a succession. The word “relatively” is key here: stability doesn’t mean the community is frozen or unaffected by disturbance.

As each sere develops, it changes the environment and affects what can establish next. On bare rock, for instance, early colonists help with physical and chemical weathering. Growing moss traps particles and holds water, while decomposers turn dead material into organic matter. Gradually, a shallow soil forms and larger plants can establish. Their roots deepen and stabilize the soil; they also increase shade and organic inputs. Later, shrubs and trees may outcompete earlier species for light, water or nutrients.

This process is often called facilitation, because organisms create conditions that suit later colonists. Competition still causes much of the replacement. A pioneer species may thrive in exposed, nutrient-poor conditions, yet perform poorly once taller vegetation produces shade and the soil becomes deeper.

The broad pathway is:

1. pioneer species colonize;
2. the community changes abiotic conditions;
3. new species become able to establish;
4. competition alters species dominance;
5. successive seral communities develop until a comparatively stable community is reached.

In reality, succession doesn’t always follow a single, tidy linear route. Even so, this model provides the essential starting point.

Primary successions happen on newly formed substratum where there is no soil or pre-existing community, such as rock newly formed by volcanism, moraines revealed by retreating glaciers, wind-blown sand or waterborne silt. Ecological development therefore begins without an inherited soil community.

The 1980 eruption of Mount St Helens in the United States created and exposed extensive deposits containing very little organic matter. Wind-dispersed microorganisms were among the earliest pioneers, along with plants that could tolerate exposure and nutrient scarcity. In some places, nitrogen-fixing plants increased the amount of biologically available nitrogen and added litter. Soil then began to develop, allowing grasses, shrubs and young trees to establish as later seral communities.

The general pattern is:

• newly exposed substratum retains little water and contains few available nutrients;
• pioneer organisms arrive by wind, water or animals;
• weathering and biological activity start to form soil;
• dead biomass adds organic matter and provides support for decomposers;
• soil becomes deeper, nutrients become more available and habitat complexity increases;
• through competition, later seral communities replace many pioneer organisms;
• eventually, a relatively stable climax community may develop.

Primary succession usually takes a long time because soil must form before many larger plants can survive. Don’t describe it as a process in which “nothing exists”. Microorganisms and mobile organisms may arrive quickly, but initially there is no soil or established community.

Secondary successions happen on bare soil where there has been a pre-existing community, such as a field where agriculture has ceased or a forest after an intense firestorm. What separates secondary succession from primary succession is the survival of soil, rather than the specific cause of the disturbance.

After the 1988 fires in parts of Greater Yellowstone, soil remained across much of the burned landscape. Seeds, roots, soil organisms and nutrients survived too, though their distribution was uneven. Herbs and grasses regenerated first, followed by shrubs and young trees. Burn intensity, surviving organisms and seed availability differed from place to place. As a result, the landscape developed as a mosaic instead of recovering at one uniform rate.

Secondary succession is usually faster than primary succession because some ecological resources are already available:

• developed soil containing stored nutrients;
• decomposers and other soil organisms;
• a seed bank, spores, roots or surviving individuals;
• nearby communities that provide colonists.

Secondary succession doesn’t require a disturbance to remove every organism. The former community must be substantially disrupted, but the soil remains. Abandoned farmland provides another clear example. Short-lived weeds may dominate at first, then perennial grasses and shrubs, with woodland eventually developing where climate, grazing and management allow it.

Succession alters how an ecosystem is structured and how it functions; the change goes beyond its species list. Figures and datasets often show several connected trends.

Energy flow and productivity

Early communities have little producer biomass, so they capture relatively little energy. As plant cover and leaf area grow, more solar energy enters food chains, supporting larger consumer and decomposer communities. Productivity rises sharply through the developing stages. What happens later depends on the balance between production and respiration.

Species diversity

At the pioneer stage, species diversity is usually low because few species can tolerate the initial conditions. It tends to rise as developing soil and vegetation provide more habitats and niches. During later stages, diversity may level off. Some early light-demanding species may also disappear as shading and competition intensify. Describe this as a broad trend rather than an absolute rule that every species persists.

Soil depth and nutrient cycling

As rock weathers, particles build up and roots break apart the substratum, making the soil deeper. Litter and dead organisms supply organic matter. Growing decomposer communities release mineral nutrients. Meanwhile, deeper roots and increased vegetation cover limit erosion and retain nutrients more effectively. Nutrient cycling therefore becomes more developed and internally regulated.

When interpreting succession data, link each trend to a process. Increasing soil depth, for instance, doesn’t simply occur alongside the growth of larger plants. Deeper, organic-rich soil provides a better supply of water and nutrients, allowing those plants to establish. The plants then add roots and litter, which reinforces soil development.

An ecosystem’s capacity to tolerate disturbances and maintain equilibrium depends on its diversity and resilience.

Resilience is the capacity of a system to resist damage or recover its structure and functioning after disturbance. Stability is the property of a system that keeps its state within limits or enables it to return towards equilibrium after change. Equilibrium is a system condition in which opposing processes are balanced sufficiently for the overall state to remain broadly steady.

As succession progresses, habitat complexity, species diversity and genetic diversity often increase. More complex food webs may offer alternative feeding relationships. As a result, losing one population doesn’t necessarily break an entire energy pathway. Deeper soils and greater biomass provide larger storages, while established nutrient cycles also help buffer environmental change. Negative feedback can oppose a disturbance and help the system regain a steady state.

However, the relationship isn’t as simple as “more species equals perfect stability.” Species identities and ecological roles matter. So do the severity and frequency of disturbance. Even so, functional overlap and multiple pathways usually give an ecosystem more ways to respond and recover.

Human activity can undo these gains. When primary producers are removed, energy input and available habitat decline. Soil degradation creates harsher abiotic conditions. Simplified food webs and monocultures also remove alternative pathways. Agricultural systems that are disturbed frequently may remain in early seral stages, requiring repeated human inputs because their natural resilience is low.

Succession can therefore increase diversity, strengthening resilience and stability; human interference may reduce all three. If a disturbance exceeds the ecosystem’s resilience, the system may cross a tipping point and settle into a different state instead of returning to its former equilibrium.

Time alone does not control the type of community that succession produces. Climate, geology, soil, landform, disturbance and interactions across trophic levels may all shift its pathway.

• Climate: temperature, precipitation, humidity, wind and seasonality determine which organisms can survive and reproduce.
• Bedrock and parent material: mineral composition shapes soil chemistry and texture. Unusual parent rock can create highly alkaline, nutrient-imbalanced or otherwise extreme soils that favour specialist vegetation.
• Soil properties: depth, drainage, aeration, pH, nutrient availability and water-holding capacity affect plant growth. Poor drainage causes waterlogging, reducing the oxygen available to roots.
• Geomorphology: slope, aspect and landform influence erosion, insolation and drainage. Material is easily lost from steep slopes, which restricts soil development and prevents large plants from becoming established.
• Fire and weather-related events: frequency, intensity and timing affect which organisms survive and how often succession is reset.

Top-down control occurs when consumers or predators affect organisms at lower trophic levels, changing community structure. In Gorongosa National Park, Mozambique, shifts in large-herbivore populations change browsing pressure and the balance between woody vegetation and grassland. Predators at a higher trophic level may also alter herbivore abundance or behaviour, which indirectly affects plant regeneration.

The same regional climate therefore won’t produce an identical climax community everywhere. Even when they are close together, a waterlogged hollow, an exposed ridge and a freely drained valley floor may support different stable communities.

Gross productivity is the rate at which producers convert incoming energy into chemical energy in biomass. Net productivity is the rate at which biomass energy accumulates after respiratory losses from the community have been subtracted. The relationship between them is:

Early succession

Gross productivity starts low because conditions are unfavourable and there are few producers. Community respiration is low too, since little living biomass is present and the consumer and decomposer community is small. Even so, gross productivity exceeds respiration. Net productivity is therefore positive: the system grows and biomass accumulates.

Developing succession

As producer cover, leaf area and biomass increase, gross productivity rises. Consumer and decomposer populations also grow, raising community respiration. Net productivity commonly stays positive and may reach its highest level during these active growth stages.

Climax community

Gross productivity may remain high because producers are abundant. At the same time, the large standing biomass and substantial consumer and decomposer populations cause high respiratory losses. As $R$ approaches $GP$, $NP$ approaches zero. Photosynthesis hasn’t stopped; production is approximately balanced by respiration, leaving little net accumulation of ecosystem biomass.

$r$-strategist species are those that produce large numbers of offspring so they can colonize new habitats quickly and make use of short-lived resources; $K$-strategist species tend to produce a small number of offspring, which increases their survival rate and enables them to survive in long-term climax communities. The two strategies sit at opposite ends of a continuum; they aren’t completely separate categories.

Pioneer habitats tend to be disturbed, exposed and short-lived. To succeed there, species must reach new sites quickly and reproduce before conditions shift or competitors become established. Rapid development, effective dispersal and large numbers of offspring suit $r$-strategists. Each offspring has a relatively low chance of survival, but sheer numbers increase the likelihood that some will establish.

In climax communities, competition is stronger and interactions are more stable and long-lasting. $K$-strategists put more resources into fewer offspring, which raises each offspring’s probability of survival. This generally makes them better suited to maintaining populations in crowded communities over long periods.

The letters refer to reproductive strategies, not to particular seral stages. During intermediate succession especially, a community may include species showing a mixture of both strategies.

The traditional succession model describes a site moving towards one predictable, stable climax community. That view has been challenged. Ecosystems are dynamic, disturbance is unavoidable, and human influence has often continued for so long that the supposedly natural baseline is hard to identify.

An alternative stable state is one of two or more persistent ecosystem conditions that can occur under broadly similar external environmental conditions. Similar sites may be pushed towards different states by random events, colonization differences, disturbance history or feedback mechanisms. After a state becomes established, its own feedbacks may keep it in place.

The Vera wood-pasture hypothesis

The Vera wood-pasture hypothesis challenges the assumption that, without people, much of temperate Europe would necessarily become continuous closed-canopy forest. Large primary consumers, it proposes, could suppress tree regeneration in some places by grazing, browsing and trampling. As these animals moved and affected different areas, they could create a shifting mosaic of woodland, scrub and open grassland.

This debate matters because the evidence allows more than one interpretation. Pollen records can show that tree species were present in a region, but they don’t always reveal whether the vegetation was closed forest or an open mosaic. Centuries of farming, hunting and species removal have also shaped modern landscapes, so finding an untouched comparison is difficult.

Climax communities may still occur. However, the idea that succession always leads to one inevitable endpoint is often too simple. A community can remain relatively stable, while its state depends on chance events, consumers, disturbance and historical human influence.

A plagioclimax is a community maintained when human activity diverts or repeatedly interrupts the natural progression of succession. Not every human-altered ecosystem is a plagioclimax. Intervention must prevent or redirect the expected successional pathway.

In the South Downs of England, livestock graze the chalk grassland and remove young shrubs and tree seedlings. Without this continued grazing, the open grassland would tend towards scrub and woodland under the local climate. If grazing stopped for long enough, taller plants could become established. They would change the shade and soil conditions, restarting succession towards a different community.

Human activities that produce a plagioclimax include:

• grazing by domesticated livestock;
• regular cutting or mowing;
• controlled burning;
• cultivation and repeated harvesting;
• drainage or irrigation;
• removal of top carnivores, which can increase herbivore pressure and suppress tree regeneration.

Frequency and intensity determine the outcome. One disturbance may initiate secondary succession, while repeated management can keep the ecosystem in an early or intermediate sere. People may value the community for farming, recreation or conservation, but it usually needs continued intervention to remain in that state.

Removing top carnivores completely shows how humans can have an indirect effect. Herbivore populations or browsing activity may increase, preventing tree seedlings from becoming established. A trophic pathway then maintains the open vegetation rather than direct clearance by people.