Disturbance and Ecological Succession in Ecosystems

Conceptual Framework and Definitions

Disturbance: A relatively discrete event in time that removes organic material, including living plant biomass and soil. It disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment.
Succession: A directional change in ecosystem structure and functioning after a disturbance, characterized by species replacements over time. It represents the process of community recovery.

Properties of the Disturbance Regime

The impact and subsequent recovery of an ecosystem are determined by the specific characteristics of the disturbance regime:

Severity: The amount of organic material removed ( $0\,\%$ to $100\,\%$ ). This is the primary determinant of whether succession is primary or secondary.
Intensity: The energy released per unit of area/time (e.g., the temperature of a fire or the wind speed of a storm).
Frequency: How often the disturbance occurs (rare vs. frequent).
Type: The specific damaging agent, such as fire, glaciers, storms, or disease outbreaks.
Size/Pattern: The spatial extent, ranging from small patches to entire landscape-level events.
Timing: The temporal context, such as whether a disturbance occurs shortly after a previous one or after a long period of stability.

Primary vs. Secondary Succession

Succession is categorized based on the severity of the initial disturbance and the state of the remaining environment.

Primary Succession

Initial State: Starts from parent material with no organic soil.
Severity: Extreme; removes almost all living organisms.
Colonization: Occurs from seed sources that are typically located far away.
Examples: Retreating glaciers, volcanic eruptions, and shifting floodplains.

Secondary Succession

Initial State: Commences with organic soil already present.
Severity: Less severe; some organisms and biological legacies remain intact.
Colonization: Plants can re-sprout from existing roots or germinate from a seed bank already present in the soil.
Examples: Forest fires, hurricanes, agricultural clearing, and pest outbreaks.

Severity Thresholds for Successional Types

$0 \rightarrow 50\,\%$ Severity: Generally associated with steady-state maintenance or minor changes (e.g., herbivory, low-intensity fire).
$50 \rightarrow 90\,\%$ Severity: Triggers secondary succession (e.g., intense fire, agricultural clearing, flooding, mining, and wars).
$90 \rightarrow 100\,\%$ Severity: Triggers primary succession (e.g., glaciers and volcanoes).

Life-History Traits and Seed Dispersal Trade-offs

Succession is heavily influenced by the trade-offs between dispersal, growth, and longevity.

Seed Size and Dispersal:
- Small Seeds: Can disperse over long distances but have fewer internal energy reserves for establishment.
- Large Seeds: Disperse only short distances but provide more resources for the seedling.
Primary Succession Colonizers: Typically characterize by small seeds to facilitate long-distance arrival on bare substrates.
Secondary Succession Colonizers: Possess a wide range of seed sizes and generally exhibit fast growth rates.
Late Successional Species: Characterized by large seeds, slow growth rates, and high longevity.
RGR (Relative Growth Rate) vs. Stress Tolerance Trade-off:
- Secondary Colonizers: Generally high RGR, lower stress tolerance.
- Primary Colonizers: Specialized for high stress tolerance in harsh, barren environments, often with lower RGR compared to secondary colonizers in optimal conditions.

Case Study: Floodplain Primary Succession (Tanana River, Alaska)

Succession in this environment follows a predictable sequence of dominant plant types:

Willow: Fast-growing shrubs; light-seeded early colonizers.
Alder: Nitrogen-fixing shrub; highly productive.
Poplar: Deciduous tree.
Spruce: Slower-growing evergreen conifer tree; long-lived.

Life History Parameters of Tanana River Species

Species	Seed Rain ( $seeds\,m^{-2}\,yr^{-1}$ )	Max Stem Age ( $years$ )
Willow	$231 \pm 47$	$47 \pm 0.5$
Alder	$5 \pm 2$	$20 \pm 0.4$
Poplar	$129 \pm 12$	$102 \pm 4.3$
Spruce	$8 \pm 2$	$128 \pm 7.1$

Key Finding: Early colonizers (Willow) produce massive amounts of seeds but have shorter life spans. Later dominants (Spruce) produce fewer seeds but live much longer and eventually dominate through longevity and competition.

Species Interactions and Herbivory

Interactions Between Plants

Facilitation: Common early in succession. Early species modify the environment in ways that benefit later species (e.g., adding organic matter or fixing nitrogen).
Competition (Inhibition): Becomes more dominant later in succession. Established species may inhibit the growth of others through root competition or light competition.

The Role of Herbivores

General Rule: Fast-growing early successional plants are often more palatable (higher quality) to herbivores.
Speeding Succession: Mammalian herbivores, such as moose, can accelerate succession by preferentially browsing and removing fast-growing species (like Willows), thereby facilitating the establishment of later species (like Alders).
Browsed vs. Unbrowsed: In the Tanana River, the Alder to Willow ratio is significantly higher in areas with moose browsing compared to unbrowsed areas.

Variation in Succession: Predictability and Legacy

Glacial Retreat and Predictability (Glacier Bay, AK)

Classical View: Succession is driven by sequential facilitation:
1. Mosses/Lichen add organic matter to create soil.
2. Dryas and Alder fix Nitrogen.
3. Alder litter acidifies the soil, favoring spruce.
4. Slower decomposition and higher soil organic carbon increase water holding capacity for hemlock.
Modern Critique (Chris Fastie, 1995): Tree ring data showed spruce were older than expected on sites deglaciated before 1840. Proximity to relict forests allowed early dispersal. Succession is not always a single pathway; it depends heavily on the spatial arrangement of seed sources (dispersal).

Biological Legacies (Mt. St. Helens)

Animals can significantly alter successional speed. Pocket gophers survived the 1980 eruption underground. Their burrowing activity brought buried seeds and soil to the surface, speeding up the recovery of the community.

Biodiversity and the Intermediate Disturbance Hypothesis

Low Disturbance: Competitive exclusion reduces diversity as the most dominant competitors take over.
High Disturbance: High mortality rates decline diversity as only a few specialized or resistant species can survive.
Intermediate Disturbance: Diversity is maximized because there is a balance between the disruption of competition and the prevention of total mortality.
Successional Diversity: Diversity generally increases with time since a disturbance but may decline late in succession due to intense competition.

Variation in Ecosystem Function: Productivity and Nutrients

Net Primary Productivity (NPP)

Early Phase: NPP increases rapidly due to fast-growing species and initial nitrogen inputs.
Mid-Succession: Forest productivity often peaks here.
Late Phase: NPP typically declines after canopy closure due to:
- Replacement by slower-growing species.
- Decreased nutrient availability as nutrients become bound in Soil Organic Matter (SOM) and humus.

Nutrient Cycling and Soil Development

Nitrogen (N): N availability increases early due to fixation (e.g., Alders). Late in succession, N availability decreases because mosses and slow-decomposing conifer litter bind nitrogen.
Cation Exchange Capacity (CEC): As colonizers allow carbon (C) to accumulate in the SOM, soils develop a higher CEC, which improves the retention of cation nutrients.
Secondary Succession Advantage: Nutrient retention is generally higher in secondary succession than primary succession because the pre-existing soil organic matter provides immediate retention mechanisms (microbial uptake and chemical fixation). If plants or microbes do not take up nutrients after a disturbance, they are lost through leaching.