ENV102: Disturbance in Ecosystems

Learning Objectives

• Understand the role of disturbance in ecosystem functioning: Disturbance is a ubiquitous, inherent, and unavoidable part of any ecosystem, influencing its structure, composition, and processes. It operates across a wide range of spatial and temporal scales.

• Define and explain key components of disturbance regimes: A disturbance regime is characterized by its size, frequency, intensity, and duration.

• Understand the interaction of disturbance and successional models: Disturbance often initiates or resets successional processes, dictating the trajectory of ecosystem change over time.

• Describe key ecological theories:

  • Intermediate Disturbance Hypothesis: Explains how diversity can be maximized at intermediate levels of disturbance.

  • Alternative Stable States: Highlights that ecosystems can exist in multiple stable configurations, and disturbance can trigger shifts between these states. These theories relate to species coexistence, richness, and diversity by explaining mechanisms through which different species can persist and thriving in a dynamic environment.

Succession

Succession refers to the change in an ecosystem with time.

Two Types of Succession

1. Primary Succession: The colonization of 'new' substrates that have not previously supported life. This occurs in environments devoid of soil or existing vegetation.

   • Examples: Volcanic rock flows, newly exposed sand dunes, bare rock exposed by glacial retreat.

2. Secondary Succession: The recolonization of an area after a disturbance has removed existing vegetation but left the soil intact.

   • Examples: Forest regeneration after a wildfire, re-establishment of plant communities after logging, agricultural land abandonment, areas affected by major floods.

Importance of Disturbance to Ecosystems

Disturbances are fundamental to ecosystem dynamics. While they cause changes in ecosystems, the crucial question is "what sort of change, over what scale?"

Effects of Disturbance

Disturbances can have both physical and biological effects:

• Physical Effects:

  • Changes in Nutrients (e.g., release from burnt biomass, soil erosion).

  • Alterations in Temperature (e.g., loss of canopy cover leading to increased ground temperature).

  • Variations in Light availability (e.g., creation of gaps in forest canopy).

• Biological Effects:

  • Mortality of existing organisms.

  • Shifts in Competition dynamics among species.

  • Species turnover, leading to changes in community composition.

Balance of Nature vs. Flux of Nature Paradigms

Historically, ecosystems were viewed through the "balance of nature" paradigm:

• Balance of Nature (Older view):

  • Ecosystems were thought to remain largely unchanged over time, essentially experiencing one equilibrium state.

  • After a disturbance, an ecosystem was expected to return to its original state through a predictable recovery process.

  • The role of chance events was considered minor.

In contrast, modern ecological thought leans towards the "flux of nature" or non-equilibrial paradigm:

• Flux of Nature (Modern view):

  • Ecosystems are now understood to be in constant flux, continuously changing.

  • They are recognized as complex, non-linear systems.

  • Disturbances may cause an ecosystem to shift to an entirely new state, not necessarily returning to its original condition.

  • Multiple stable states are possible for a given ecosystem under similar environmental conditions.

  • The role of chance events and rare disturbances is considered highly significant.

Intermediate Disturbance Hypothesis (IDH)

Proposed by Connell in 1978, the IDH was a pivotal concept that challenged the traditional equilibrium view. It suggests that high diversity in systems like tropical rain forests and coral reefs is a non-equilibrium state. If left undisturbed, these systems would progress toward a low-diversity equilibrium community due to competitive exclusion.

• Core Idea: Maximum species diversity occurs at intermediate levels of disturbance frequency, intensity, or size.

• Mechanisms:

  • Low Disturbance: Allows competitive dominant species to outcompete and exclude other species, leading to lower diversity (e.g., a climax community).

  • High Disturbance: Creates conditions too harsh or frequent for many species to establish or complete their life cycles, leading to lower diversity, with only highly resilient or fast-colonizing species persisting.

  • Intermediate Disturbance: Prevents competitive exclusion by preventing dominant species from monopolizing resources, while also allowing enough time for less competitive species and colonizers to establish and coexist, thus maximizing diversity.

• Successional Stages (Diagrammatically):

  • A. Colonizing: Soon after a frequent or large disturbance.

  • B. Mixed: At an intermediate time or with intermediate disturbance, allowing a mix of species.

  • C. Climax: Long after an infrequent or small disturbance, dominated by competitive species.

Alternative Stable States

Ecosystems can exist in multiple stable states, meaning they can switch from one stable configuration to another if a disturbance pushes them past a certain threshold.

• Example: A system might shift from coral dominance to algal dominance due to specific disturbances (or compound disturbances). Even if the disturbance is removed, the system might not revert to the coral-dominated state.

• Paine et al. (1998) highlighted that:

  • A single small disturbance event might see the system return to its original state (classic paradigm).

  • A compound disturbance (multiple disturbances or interacting pressures) can cause the system to change states.

  • A single disturbance event whose intensity exceeds a critical threshold can also push the system into a new stable state.

Types of Disturbance, Defining Regimes

The disturbance regime is critical for understanding ecosystem change and recovery. It is defined by:

• Size: The spatial extent of the disturbed area.

• Frequency: How often disturbances occur over a given period.

• Intensity: The severity or magnitude of the disturbance.

• Duration: How long the disturbance event lasts.

Recovery and Scale

The time required for an ecosystem to recover from a disturbance (recovery time) is often related to the scale and intensity of the disturbance. Generally, large-scale, high-intensity disturbances require longer recovery times.

• Relationship between Scale and Frequency: Often, large-scale disturbances (e.g., floods, wildfires) are less frequent, while small-scale disturbances (e.g., animal diggings) are more frequent.

Examples of Natural Disturbance Regimes

Small and Frequent Disturbances

These disturbances are localized and occur relatively often, contributing to micro-heterogeneity within an ecosystem.

• Examples:

  • Diggings of animals (e.g., pigs, wombats, rodents).

  • Building of ants' nests, which alters soil structure and nutrient distribution.

  • Small-scale erosion and localized water flow.

  • Individual tree deaths, creating small canopy gaps.

Large and Infrequent Disturbances

These events are broad-scale and occur less often but can have profound impacts on ecosystem structure and succession.

• Examples:

  • Severe storms and cyclones, causing widespread defoliation and treefall.

  • Tornadoes, creating strong localized destruction.

  • Major floods, reshaping river channels and inundating vast areas.

  • Wildfire, burning large tracts of land and influencing nutrient cycles and species composition.

  • Significant droughts, leading to widespread mortality and changes in vegetation composition, such as the 2010-2011 drought which severely impacted ecosystems.

Humans as an Agent of Disturbance

Ecosystems possess a natural resilience, allowing them to recover from even large, intense natural disturbances. However, human activities often push ecosystems over the edge into states from which they cannot recover, leading to ecosystem degradation.

"Human Flux" in the Natural World

• The rate and scale of human-induced disturbances often exceed the capacity of ecosystems to recover naturally.

Changed Disturbance Regimes due to Anthropogenic Activities

Human actions drastically alter natural disturbance regimes, affecting:

• Intensity, frequency, size, and duration of disturbance events.

• Broad-scale clearing: Extensive deforestation, particularly in tropical regions. For example, between 2000-2005, Brazil accounted for 48\% of tropical deforestation, Indonesia for 13\%, and other tropical countries for 39\% (Source: mongabay.com, Hansen et al. (2008)). The Kellerberrin Study Area in Western Australia (an area of 1600 ext{ km}^2) provides a stark visual example of progressive land clearing from PRE 1920 to 1984.

• Loss of native mammals: Can remove ecosystem engineers that perform small-scale, frequent disturbances (e.g., diggings), altering soil aeration and seed dispersal.

• Fire suppression or exclusion: Historically, frequently occurring low-intensity fires maintained open understories. Suppression leads to fuel accumulation, resulting in less frequent but far more intense and destructive wildfires. Conversely, fuel reduction fires can shift regimes towards frequent, low-intensity burns that may not mimic natural fire cycles.

• River regulation: Construction of dams and other structures changes natural flow and flooding regimes, impacting riparian and floodplain ecosystems by altering nutrient transport, sediment deposition, and species breeding cues.

Conclusion

• Successional patterns and trajectories are highly variable, influenced by specific ecosystems and communities.

• Succession and Disturbance are intimately linked: Disturbance is a primary driver of successional processes.

• Disturbance is not an 'if', but a 'when, where, how much': It's an inevitable and integral aspect of ecological systems.

• Disturbance plays a major role in promoting species persistence and coexistence, often explained by theories like the Intermediate Disturbance Hypothesis, by preventing competitive exclusion or creating niches for different life history strategies.

• Anthropogenic activities can introduce entirely novel disturbance types or significantly alter historic modes of disturbance, with profound implications for ecosystem health and stability.