Community Structure and Succession

Core Concepts

Species Richness and Diversity

• Species richness is the number of species in a community.

• Species diversity incorporates both richness and relative abundance (evenness). A community with 10 species, each equally abundant, has higher diversity than a community with 10 species where one dominates 90% of individuals.

• Relative abundance describes the proportion of the community each species represents.

Dominant Species, Keystone Species, and Ecosystem Engineers

Dominant species are the most abundant or have the highest biomass in a community. They often exert strong control over community structure by virtue of their sheer abundance — a dominant tree species determines the light, moisture, and nutrient environment of the forest understory.

Keystone species have effects disproportionate to their abundance. Removing a keystone species causes dramatic changes in community structure. The classic example: the sea star Pisaster ochraceus in Pacific Northwest intertidal zones. When Pisaster was experimentally removed, the dominant mussel (Mytilus) expanded, outcompeting other invertebrates and algae, reducing species richness from ~15 to ~5 species. Pisaster was not abundant, but its predation on mussels prevented competitive exclusion, maintaining diversity. Other examples: sea otters (control sea urchins, preventing kelp forest destruction), beavers (create wetlands through dam-building), wolves in Yellowstone (regulate elk, allowing riparian vegetation recovery).

Ecosystem engineers physically modify the environment, creating, maintaining, or destroying habitat for other species. Examples: beavers (build dams, creating ponds and wetlands), earthworms (mix and aerate soil), corals (build reef structures), elephants (knock down trees, converting woodland to grassland).

Food Chains and Food Webs

A food chain traces a single path of energy transfer: grass → grasshopper → frog → snake → hawk. Each step is a trophic level. A food web is the network of interconnected food chains in a community — most species feed at multiple trophic levels (omnivory), and the web captures the complexity of real feeding relationships.

Trophic structure describes the feeding relationships and energy pathways in a community. Food webs are typically more complex than food chains, with many species occupying intermediate trophic positions.

Disturbance, Resistance, and Resilience

Disturbance is any event that removes organisms or alters resource availability. Disturbances vary in frequency, intensity, and scale: fires, floods, storms, volcanic eruptions, tree falls, grazing, and human activities.

Resistance is a community’s ability to withstand disturbance without changing. A grassland with deep-rooted, fire-adapted plants has high resistance to fire.

Resilience is a community’s ability to recover after disturbance. A forest that regrows quickly after a fire has high resilience. Resistance and resilience are separate properties — a community can be resistant but not resilient, or vice versa.

Succession

Succession is the directional change in community composition over time following a disturbance or the creation of new substrate.

Primary succession occurs on newly exposed surfaces that lack soil: lava flows, glacial moraines, sand dunes, bare rock. The process begins with pioneer species — lichens, mosses, and hardy plants that can establish on bare mineral surfaces. These species accelerate weathering, trap organic matter, and begin soil formation. Over time (decades to centuries), larger plants establish, soil deepens, and the community transitions through a series of stages. Primary succession often takes hundreds to thousands of years to develop a complex community.

Secondary succession occurs after a disturbance that removes the existing community but leaves soil intact: abandoned agricultural fields, forest fires, windthrows. Because soil and a seed bank are already present, secondary succession proceeds much faster than primary succession. Early-successional species (fast-growing, short-lived, high dispersal — weeds, grasses) are replaced by mid-successional species (shrubs, pines) and eventually late-successional species (shade-tolerant hardwoods).

Mechanisms driving succession

• Facilitation: Early species modify the environment, making it more suitable for later species (e.g., nitrogen-fixing plants enriching soil).

• Inhibition: Early species occupy space and resist invasion by later species until they are removed by disturbance or senescence.

• Tolerance: Later species can tolerate lower resource levels and eventually outcompete early species.

The Climax-Community Concept (and Its Limitations)

The traditional concept of a “climax community” — a stable, self-perpetuating endpoint of succession — was an oversimplification. Modern ecology recognizes that:

• Communities are constantly subject to disturbance at various scales.

• “Stable” communities may be in a dynamic equilibrium rather than a static endpoint.

• Multiple stable states may be possible for a given environment.

• Climate change, species invasions, and human activities continually alter successional trajectories.

Rather than a climax, communities are better described as existing in a state of ongoing flux, shaped by the interplay of disturbance, species interactions, and environmental conditions.

ELI-10

Ecosystems are like neighborhoods that change over time. When a new neighborhood is built from scratch on bare rock (primary succession), the first settlers are tough pioneers — lichens and mosses that can live on nothing. They slowly break down the rock, creating the first thin soil. Then grasses move in. Then shrubs. Then trees. Each wave of settlers changes the environment, making it suitable for the next wave.

When an existing neighborhood is damaged but not destroyed — a forest fire that leaves the soil intact (secondary succession) — the recovery is much faster. The soil is already there, seeds are already in the ground, and the process takes years or decades instead of centuries.

Some species have an outsized impact. A keystone species is like the chief engineer of a community — not necessarily numerous, but if you remove it, the whole system changes. Pull the sea stars out of a tide pool, and mussels take over everything, crowding out other species. Bring wolves back to Yellowstone, and they change elk behavior, which lets riverside trees grow back, which changes the river’s shape. One species, huge ripple effects.

Food webs connect everything. A grasshopper eats grass, a frog eats the grasshopper, a snake eats the frog, a hawk eats the snake. But real food webs are messier — hawks also eat snakes, frogs also eat insects that are not grasshoppers, and so on. It is a web, not a chain.

ELI Example

Succession is like rebuilding a neighborhood after a disaster. Primary succession is building on a brand-new volcanic island — first come the hardy pioneers (lichens), then the early settlers (grasses), then the first families (shrubs), and finally the established residents (trees). It takes centuries. Secondary succession is rebuilding after a fire that left the foundations (soil) intact — much faster, because you are not starting from bare rock.

Keystone species are the neighborhood’s critical infrastructure — the grocery store or power plant. It might be a small building, but if it closes, the whole neighborhood reorganizes. Remove wolves from a forest, and deer overgraze everything. Return them, and the system shifts back toward balance.

Do Not Confuse

• Primary vs. Secondary Succession: Primary = no soil (bare rock, lava). Secondary = soil present (after fire, abandoned farm). Secondary is faster.

• Species Richness vs. Diversity: Richness = count of species. Diversity = richness + evenness (relative abundance). A community with ten equally abundant species has higher diversity than one where one species dominates.

• Resistance vs. Resilience: Resistance = ability to withstand change. Resilience = ability to recover after change. A concrete building resists earthquake damage (resistant). A tent collapses easily but can be set up quickly (resilient).

High-Yield Memory Anchors

• Species richness = number of species. Diversity = richness + evenness.

• Keystone species: disproportionate effect relative to abundance. Example: sea star Pisaster, wolves, beavers.

• Primary succession = no soil (bare rock → pioneers → soil formation → complex community). Secondary succession = soil intact (faster).

• Food chain = single path. Food web = interconnected network. Trophic levels: producers → consumers → decomposers.

• Climax community concept is oversimplified. Communities are dynamic, subject to disturbance and ongoing change.

Quick Check

Q1: Which of the following best describes primary succession?

A) Regrowth of a forest after a fire

B) Colonization of a newly formed volcanic island

C) Recovery of a grassland after a drought

D) Replacement of weeds by shrubs in an abandoned field

Q2: An ecologist removes a rare predatory snail from experimental tide pools and observes that species richness drops from 12 to 4 species. What type of species is the snail, and what mechanism explains the loss of diversity?

Q3: Compare the roles of facilitation and inhibition in driving succession. Provide an example of each.

Quick Check Answers

A1: B. Colonization of a newly formed volcanic island. Primary succession begins on substrate lacking soil. A new volcanic island is bare rock. Choices A, C, and D describe secondary succession (soil present).

A2: The snail is a keystone species — rare but with a disproportionate effect on community structure. The mechanism: the snail preys on the competitively dominant species (likely a mussel or barnacle) that would otherwise monopolize space, outcompeting other species. By keeping the dominant competitor in check, the snail prevents competitive exclusion and maintains high species richness. Removing the keystone predator allows the dominant competitor to expand, excluding subordinate species and collapsing diversity.

A3: Facilitation: Early-successional species modify the environment in ways that benefit later species. Example: nitrogen-fixing alder trees colonize nutrient-poor glacial till, enriching the soil with nitrogen. This improves conditions for spruce trees, which eventually replace the alder. Inhibition: Early species occupy space and resist invasion, slowing the arrival of later species. Example: a dense mat of grasses and weeds in an abandoned field prevents tree seedlings from establishing by shading them and competing for water. Only when the grass mat is disrupted (by disturbance or localized death) can trees establish. Both mechanisms can operate simultaneously in the same successional sequence.

Chapter Summary

Communities are structured by species richness, diversity, relative abundance, and trophic relationships. Keystone species and ecosystem engineers have disproportionate effects on community structure. Succession is the directional change in community composition over time — primary (no soil) and secondary (soil intact) — driven by facilitation, inhibition, and tolerance. Communities are dynamic, shaped by disturbance and ongoing change rather than converging on a static climax.

Common Mistakes

• “Succession always ends at a stable climax community.” Succession is ongoing. Disturbances reset the clock. Climate change shifts the “target.” Communities are dynamic, not static endpoints.

• “Keystone species are always top predators.” Keystone species can be predators (sea stars, wolves), herbivores (beavers — ecosystem engineers), mutualists (pollinators, mycorrhizal fungi), or plants (nitrogen-fixing trees in early succession). The defining feature is disproportionate ecological impact, not trophic position.