Ecological succession denotes the systematic and ordered changes in the species composition of an ecological community over time. Such transitions often lead from a simple initial stage to a more complex, mature stage, ultimately shaping the ecological landscapes we observe.
Understanding Ecological Succession
Types of Succession
- Primary Succession: Commences on bare, lifeless substrates such as areas exposed after glacial retreat, newly formed sand dunes, or emerging volcanic islands.
- Characteristics:
- Starts with pioneer species, typically lichens or certain types of mosses, that are resilient and can tolerate the harsh conditions.
- These pioneers start the soil-building process.
- Characteristics:
Image courtesy of Joshfn
- Secondary Succession: Embarks on areas where an existing community has been disturbed but the soil remains intact, for instance, post-forest fires, after farming, or in abandoned lots.
- Characteristics:
- Initial colonisers are often grasses or small herbaceous plants.
- Existing soil allows for quicker recolonisation.
- Characteristics:
A diagram showing secondary succession.
Image courtesy of Katelyn Murphy
Triggers of Succession
Natural Disturbances
- Fires: Certain ecosystems, like grasslands or certain types of forests, are adapted to regular fire disturbances. Post-fire, the community undergoes predictable stages of succession.
- Floods: Excess water can disrupt plant communities, especially in river floodplains or coastal areas.
- Volcanic Eruptions: Newly formed volcanic islands or areas covered in ash will go through primary succession.
Image courtesy of Forest Service, USDA
Human-Induced Disturbances
- Agriculture: Tilling the land and other agricultural practices can disrupt local plant communities.
- Urban Development: Constructing buildings, roads, or other infrastructures often clears the existing plant community.
- Pollution: Soil or water pollution can kill off sensitive species, leading to changes in the community.
Changes During Primary Succession
Increase in Plant Size
- Initial stages: Dominated by small, hardy pioneer species.
- Intermediate stages: Characterised by the arrival of grasses, ferns, and small shrubs as the soil improves.
- Later stages: Larger shrubs, trees, and a greater variety of plants appear, further enhancing soil quality and allowing newer species to establish.
Growth in Primary Production
- As succession progresses, so does the rate at which plants convert sunlight into organic material, a process known as primary production.
- This enhances energy availability within the ecosystem, supporting a richer food web and a greater diversity of consumers.
Augmentation in Species Diversity
- Pioneer stages: Typically have limited species diversity due to harsh conditions.
- As conditions ameliorate: New niches emerge, enabling a wider range of species to colonise and flourish, thereby increasing biodiversity.
Image courtesy of VectorMine
Cyclical Succession
Cyclical succession is driven by recurrent disturbances:
- For instance, in certain grasslands, regular fires reset the succession, leading to a repeating pattern of community change.
- Unlike linear succession, which aims for a climax community, cyclical succession doesn’t have a defined endpoint.
Climax Communities
Defining Characteristics
- Represents a terminal stage where, in the absence of major disturbances, the community remains relatively stable and self-perpetuating.
- Typically characterised by complex food webs, well-established nutrient cycles, and high species diversity.
Role in Ecosystems
- Serve as reservoirs of biodiversity.
- Offer stability in the face of minor disturbances.
Human Impact on Succession
Alteration of Natural Processes
- Activities such as logging, mining, or dam construction can drastically alter the natural trajectory of succession, often preventing ecosystems from achieving their climax state.
Introducing Non-native Species
- Invasive species can monopolise resources, outcompeting native species and reshaping the direction of succession.
Manipulating Natural Disturbances
- By suppressing fires, redirecting water, or other interventions, humans can interfere with the natural disturbances that drive succession.
Barriers to Climax Communities
- Constant Disturbances: If disturbances happen frequently or with high intensity, ecosystems may be perpetually trapped in early succession stages.
- Human Intervention: Urbanisation, farming, or other land-use practices can create conditions where climax communities cannot establish.
- Climate Change: As the planet warms, the conditions that define local climax communities might shift, necessitating adaptive changes in succession patterns.
FAQ
Soil quality experiences substantial improvements throughout the various stages of succession. In primary succession, the process begins with virtually no soil, just a bare substrate. Pioneer species, through their life cycles, contribute organic matter to this substrate. As these organisms die and decompose, they combine with weathered rock particles to form a rudimentary soil. As succession progresses, more species colonise, adding further organic matter and increasing soil depth. The increased plant cover also means greater leaf litter, which decomposes to enrich the soil. By the time a climax community is reached, the soil is typically deep, fertile, and well-structured, supporting a diverse array of plant and animal life.
Disturbances play a distinct role in both primary and secondary succession. In primary succession, the initial disturbance creates an environment entirely devoid of life and soil, setting the stage for pioneer species to colonise. Examples include volcanic eruptions forming new land or glaciers receding and exposing rock. In secondary succession, disturbances don't obliterate all life or soil. Instead, they disrupt an existing ecosystem to an extent that it needs to regenerate but with the advantage of pre-existing soil. Such disturbances include forest fires, hurricanes, or human activities like farming. Thus, while both types of succession are responses to disturbances, secondary succession typically progresses more rapidly due to the presence of soil.
Several factors can prevent an ecosystem from reaching its climax community. Constant disturbances, such as frequent fires or flooding, can repeatedly reset the succession process, trapping the ecosystem in earlier stages. Human interventions, like agriculture, urbanisation, or other land-use changes, can modify the natural environment to such an extent that a climax community cannot form or sustain. Moreover, the introduction of invasive species can drastically alter an ecosystem's trajectory, preventing it from reaching its potential climax state. Lastly, changing climatic conditions due to global phenomena like climate change can shift the parameters defining local climax communities, necessitating adaptive changes in succession patterns.
Yes, many species have evolved specific adaptations to thrive in environments with frequent disturbances. For instance, certain trees have developed thick bark to resist fires, and their seeds may require heat from fire to germinate. Some grass species are adapted to grow rapidly after being grazed or after a fire. Additionally, many plants in flood-prone areas have developed seeds that can remain dormant until suitable growth conditions arise post-flooding. These adaptations not only allow these species to survive in challenging conditions but, in many cases, also to outcompete other species, making them dominant in their respective ecosystems.
Pioneer species play a foundational role in primary succession because they're the initial colonisers of barren or disrupted environments that lack soil. These species, often lichens or specific mosses, possess traits allowing them to endure harsh conditions. As they grow and die, pioneer species contribute organic material that begins to transform the bare substrate into soil. Over time, this developing soil becomes hospitable to other species. The arrival of new organisms, facilitated by the soil-improving actions of the pioneers, brings more complexity and diversity to the ecosystem, marking the progression of succession stages. In essence, without pioneer species, primary succession would not kickstart.
Practice Questions
Human activities can drastically alter the natural progression of ecological succession. One significant impact is the interruption or resetting of succession. For example, when forests are cleared for agriculture, the area might be dominated by grasses or crops, preventing the forest from reaching its climax community. Another example is urban development; constructing buildings or roads usually entails clearing the existing ecological community. Additionally, introducing non-native or invasive species can also influence succession. These species can outcompete or displace native species, thereby altering the natural trajectory of succession in the affected ecosystem.
Primary succession commences in areas devoid of life, where conditions are inhospitable and no soil is present. It's typically instigated by events like glacial retreats or volcanic eruptions. For example, a new volcanic island may initially be colonised by lichens and mosses, the pioneer species. Over time, these pioneers initiate the soil formation process. Conversely, secondary succession starts in regions where an existing ecological community has been disrupted, but the soil remains. For instance, after a forest fire, grasses and small plants might be the first to recolonise the area, using the nutrients from the ash and the existing soil.
