IB Syllabus focus:
‘Communities replace each other over time as biotic/abiotic conditions shift; peat pollen records provide evidence. Zonation is spatial; succession is temporal.’
Succession describes how ecosystems transform over time, with communities of organisms gradually replaced as conditions change. This temporal process contrasts with zonation, which involves spatial ecological change.
Temporal Nature of Succession
Succession is a temporal process—it unfolds through time rather than space. In any location, species composition does not remain static. Instead, biotic factors (interactions among organisms such as competition, predation, or facilitation) and abiotic factors (such as light, temperature, moisture, or soil chemistry) shift over time. These shifts influence which species can survive and thrive.
In early stages, environmental conditions may be harsh, limiting the range of possible colonisers. Over time, organisms alter the environment—often making it more suitable for other species. This creates a sequence of community replacement that continues until a relatively stable state is reached.
Defining Key Terms
Succession: The process by which communities of organisms in an ecosystem are gradually replaced over time due to changes in biotic and abiotic conditions.
Succession should not be confused with zonation.
Zonation: The spatial distribution of communities along an environmental gradient (e.g., altitude, tidal range, distance from water).
A clear distinction: succession is temporal change, while zonation is spatial change.
Pollen diagram from a raised bog showing changing abundances of plant taxa with depth (a proxy for time). Such diagrams reconstruct past vegetation and document community replacements through succession. Minor extra taxonomic detail is included beyond the syllabus to illustrate typical outputs. Source.
Evidence of Succession
One of the strongest forms of evidence for long-term ecological succession comes from palaeoecological studies, especially those using peat pollen records.
Peat bogs preserve layers of partially decomposed plant material.
Within these layers, pollen grains remain intact for thousands of years.
By analysing the abundance of different pollen species in successive layers, scientists reconstruct how plant communities have shifted through time.
This evidence confirms that community composition in ecosystems is not fixed but evolves over centuries or millennia.
Processes Driving Succession
Biotic Influences
Competition: As species establish, they may outcompete others for limited resources, driving changes in dominance.
Facilitation: Some species alter conditions to the benefit of others, such as nitrogen-fixing plants improving soil fertility.
Herbivory and predation: These interactions regulate population sizes, affecting which species persist.
Abiotic Influences
Soil development: Weathering, decomposition, and organic matter accumulation improve soil structure, enabling new plant types.
Light availability: Initially high in open ground, but reduced as taller vegetation develops.
Moisture retention: Increases as soil depth and organic content grow.
Together, these influences ensure that succession is not linear but dynamic, responding to continual feedback between organisms and their environment.
Temporal Sequences in Succession
Succession often progresses through identifiable seral stages, each characterised by distinct communities. Although these stages are addressed in detail in later subsubtopics, here it is important to recognise:
Early seral stages: Typically include hardy pioneer species adapted to withstand harsh conditions.
Intermediate stages: Increased diversity as soil and habitat improve.
Later stages: Communities become more complex, often stabilising into a climax community if conditions remain undisturbed.
This time-ordered replacement of communities defines succession as a temporal process, distinct from zonation.
Late seral (mature) spruce–hemlock forest in Glacier Bay, representing the endpoint of a well-documented post-glacial succession. Observed chronosequences here illustrate directional community change over decades to centuries. The source page includes earlier stages (e.g., fireweed, dryas, alder) for context not required by the syllabus. Source.
Succession and Ecosystem Feedback
Succession illustrates how ecosystems are self-modifying systems. Each stage alters conditions in ways that influence future stages. For instance:
Early colonisers may stabilise soil, reducing erosion.
Later arrivals can establish in these stabilised soils, creating shade and changing microclimates.
Over time, such feedback mechanisms create an environment increasingly different from the initial starting point.
These cumulative changes highlight the long-term trajectory of ecosystem development.
Methods of Studying Succession
Besides peat pollen records, ecologists study succession using:
Chronosequences: Comparing sites of different ages that have experienced similar disturbances (e.g., glacial retreat).
Long-term monitoring: Tracking species composition, soil development, and abiotic conditions at a single site over decades.
Modelling: Using ecological models to predict future community dynamics based on current trends.
Such methods reinforce the concept of succession as a temporal continuum driven by interacting ecological forces.
Importance of Distinguishing Temporal and Spatial Change
While both succession and zonation describe changes in ecosystems, they address different dimensions:
Succession: Communities shift through time as conditions evolve.
Zonation: Communities differ across space due to environmental gradients.
Recognising this distinction is crucial for ecological analysis. For example, when studying a salt marsh, zonation explains why plant species vary with tidal height, while succession explains how the marsh vegetation changes over decades.
FAQ
Succession is defined by changes in species composition and ecosystem conditions through time, often spanning decades to centuries.
While spatial variation may exist, the defining feature is that one community replaces another due to shifting conditions. This contrasts with zonation, where different communities are observed simultaneously across space.
Peat bogs are waterlogged and acidic, slowing decomposition. This preserves pollen grains and plant remains exceptionally well.
The layered structure acts as a timeline, with deeper strata representing older periods. This makes peat bogs excellent natural archives of ecological change.
Multiple cores are often taken from different locations in a bog.
Radiocarbon dating provides approximate ages for layers.
Statistical analysis compares pollen abundance across depths to detect trends.
These methods reduce bias and strengthen the reliability of succession reconstructions.
Pollen may travel long distances, sometimes reflecting regional rather than local vegetation.
Decomposition can still occur in some conditions, leading to incomplete records. Identification to species level is often difficult, so data usually represent plant families or genera.
Understanding long-term vegetation change helps conservationists predict how ecosystems may respond to current pressures.
Pollen records highlight natural variability versus human-induced disturbance, guiding restoration strategies and helping set realistic conservation baselines.
Practice Questions
Question 1 (2 marks)
Define succession and explain how it differs from zonation.
Mark scheme:
1 mark for correctly defining succession as the temporal replacement of communities in an ecosystem due to changes in biotic and abiotic factors.
1 mark for stating that zonation is spatial change along an environmental gradient (e.g., altitude, tidal zone), contrasting with succession’s temporal nature.
Question 2 (5 marks)
Using peat pollen records as an example, explain how ecologists gather evidence for ecological succession over long timescales.
Mark scheme:
1 mark for identifying peat bogs as archives of preserved plant material.
1 mark for stating that pollen grains are preserved in peat layers.
1 mark for describing that deeper layers represent older time periods (depth = time).
1 mark for explaining that analysis of pollen percentages in successive layers shows shifts in vegetation composition.
1 mark for linking this evidence to the concept of temporal ecological succession (communities replacing each other over centuries or millennia).
