Zonation, Succession
& Change in Ecosystems

Key insight: Spatial zonation can mirror temporal succession — the sequence of communities you see along a beach from water to dunes often reflects the same stages you'd see over time during primary succession on newly formed land.

Biotic factors: Species identity, abundance, % cover, population density
Abiotic factors: Temperature, light intensity, soil moisture, pH, salinity, wind speed — measured at each sampling point along the transect

Kite diagram: Species A widens near the water's edge; Species B peaks mid-shore; Species C dominates the upper shore — visually showing zonation.

• Pioneer species (lichens, mosses) are the first to colonize bare rock
• They secrete acids that chemically weather rock → begin soil formation
• When they die and decompose, organic matter accumulates → shallow soil layer
• This thin soil is enough for grass seeds to germinate → succession advances

• 1963: Volcanic island emerges from the ocean — bare lava rock, no soil, no life
• 1965: First lichens and mosses colonize the rock surface
• 1970s: Soil develops; grasses and flowering plants establish
• 2000s: Shrubs and small trees; nesting seabirds bring nutrients via guano
• Today: 60+ plant species, self-sustaining ecosystem — still progressing toward climax

Surtsey is a UNESCO World Heritage Site — a living laboratory of primary succession.

• May 2016: Massive wildfire burns ~590,000 hectares of boreal forest; 90,000 people evacuated
• Pioneer stage: Fireweed, grasses, jack pine seedlings (fire-adapted species) colonize within 1 year
• Intermediate: Shrubs (willow) and perennial herbs establish; biodiversity increases
• Late stages: Black spruce, white spruce, aspen re-establish; boreal forest expected to recover over ~60–80 years

Why faster than primary? Soil, seed bank, and root systems survived the fire — the ecosystem had a head start.

Resilience: The ability of an ecosystem to absorb disturbance and recover to its original state.

Stability: The ability to remain relatively unchanged. Includes two components:

• Resistance: Withstanding disturbance without change
• Resilience: Recovering after change has occurred

• Before (1926–1995): Wolves removed → elk populations exploded → overgrazed willow and aspen along rivers → riparian habitat degraded → fewer songbirds, beavers, and fish
• After reintroduction (1995): Wolves reduced elk → vegetation recovered → riverbanks stabilized → beavers returned → new ponds created → biodiversity surged
• This is a trophic cascade: Top predator influences the entire food web, shaping the plant community and even the physical landscape.

Why Does NPP Approach Zero at Climax?

• The ecosystem accumulates massive biomass over centuries → enormous respiratory demand
• Dead matter accumulates → decomposers respire heavily → recycle nutrients
• Net community production (NCP) = 0 means the system is in dynamic equilibrium — not dead, but balanced
• Energy still flows through the system — it just doesn't accumulate as net new biomass

• There is no single predetermined climax for any given set of climate conditions
• Multiple stable communities can exist under similar environmental conditions
• Different successional paths may arise from different starting points or random events (historical contingency)
• Evidence: Lake core samples and peat bogs show multiple vegetation types at the same site over time — not a linear march to one climax

Frans Vera proposed that post-Ice Age Europe was not covered by dense, closed-canopy forest as traditionally assumed. Instead:

• The landscape was a mosaic of open grasslands, scattered trees, and wooded pastures
• Large herbivores (aurochs, tarpan) maintained open areas through grazing and browsing
• This challenges the assumption that without humans, Europe would be entirely forested
• Implications for rewilding — restoring large herbivores may create more diverse, open landscapes

Not universally accepted, but has sparked significant debate and research into historical ecology.

A plagioclimax community is a stage in succession that is maintained by regular human disturbance, preventing the natural progression to a climax community. These communities are typically less diverse than natural climax communities.

• Temporary: If human activity stops (e.g. farmland abandoned), succession resumes toward climax
• Permanent: If the ecosystem's resilience has been exceeded (e.g. soil completely degraded, seed bank destroyed), it may never recover to its original state — an alternative stable state
• The more diverse and resilient the ecosystem, the more resistant it is to permanent diversion

• Zonation (space) ↔ Succession (time): Spatial patterns often mirror temporal stages — seashore zones = successional stages viewed across a gradient.
• Pioneer → Seral → Climax: Each stage modifies the environment to facilitate the next through competition.
• Diversity → Resilience: More species = more functional redundancy = faster recovery from disturbance.
• NPP → 0 at climax: Not zero productivity — it means GPP = R. Dynamic equilibrium.

• Graph GP and NP through succession — GP rises then plateaus; NPP starts high then approaches zero.
• Compare r- and K-strategists with specific examples for each trait (offspring number, life span, parental care).
• Explain trophic cascades using Yellowstone wolves or similar examples.
• Discuss why "the climax community" is a simplification — alternative stable states, Vera hypothesis.
• Distinguish temporary vs permanent plagioclimax based on soil/seed bank integrity and ecosystem resilience.