🌾World Biogeography Unit 7 – Ecological Succession & Community Assembly

Key Concepts & Definitions

• Ecological succession involves the gradual and orderly process of change in an ecosystem's species structure over time
• Primary succession occurs in an area that has not previously been occupied by a community (bare rock, lava flows, glacial retreat)
• Secondary succession takes place following a disturbance that reduces an already established ecosystem (forest fires, logging, hurricanes)
• Pioneer species are the first to colonize a disturbed or newly exposed area and initiate the succession process
• Climax community represents the final stage of succession, where the ecosystem reaches a stable state and is self-sustaining
• Sere refers to the entire sequence of communities that successively occupy an area during ecological succession
• Facilitation occurs when early successional species modify the environment, making it more suitable for later successional species to establish
• Inhibition happens when early successional species hinder the establishment of later successional species through competition or allelopathy

Types of Ecological Succession

• Autogenic succession is driven by biotic factors and interactions within the community itself (facilitation, competition, herbivory)
• Allogenic succession is influenced by external abiotic factors, such as changes in soil composition, climate, or topography
• Primary autogenic succession involves the establishment of a community on a newly exposed surface devoid of soil or vegetation (volcanic islands, sand dunes, glacial moraines)
• Secondary autogenic succession follows a disturbance that removes or disrupts an existing community without destroying the soil (abandoned agricultural fields, clear-cut forests)
• Allogenic primary succession is initiated by external factors that create new substrates for colonization (lava flows, landslides, retreating glaciers)
• Allogenic secondary succession is triggered by external disturbances that alter the physical environment (climate change, human interventions, natural disasters)

Stages of Succession

• Pioneer stage marks the beginning of succession, where hardy, fast-growing species (lichens, mosses, grasses) colonize the bare substrate
• Early successional stage is characterized by the establishment of herbaceous plants and small shrubs that can tolerate the harsh conditions
• Mid-successional stage sees the development of a more complex community structure, with the appearance of larger shrubs and early-successional tree species
• Late successional stage is dominated by long-lived, shade-tolerant tree species that form a dense canopy
• Climax stage represents the final, self-sustaining community that is in equilibrium with the environment and resistant to change
• Seral stages are the intermediate communities that develop during the succession process, each with distinct species composition and structure
• Seral communities are typically less diverse and more unstable compared to the climax community
• The duration and characteristics of each stage depend on factors such as climate, soil type, and the pool of available species

Factors Influencing Succession

• Climate, particularly temperature and precipitation patterns, determines the rate and trajectory of succession by influencing species growth and survival
• Soil properties, such as nutrient availability, pH, and moisture content, affect the establishment and performance of plant species
• Topography, including elevation, slope, and aspect, creates microclimates that support different successional pathways
• Disturbance regime, characterized by the frequency, intensity, and scale of disturbances, shapes the succession process and community structure
• Biological interactions, such as competition, facilitation, and herbivory, drive the replacement of species and the development of community structure
• Seed dispersal and colonization from nearby areas determine the pool of species available for succession and the rate of community assembly
• Human activities, such as land use change, pollution, and resource extraction, can alter the course and outcomes of succession
• Historical factors, including past disturbances and land use, leave long-lasting legacies that influence the current successional trajectory

Community Assembly Processes

• Dispersal limitation refers to the constraints on species' ability to reach and colonize new sites, affecting the pool of available species for succession
• Niche-based processes, such as environmental filtering and niche differentiation, determine which species can establish and coexist based on their functional traits and resource requirements
• Neutral processes, such as ecological drift and random colonization events, can shape community assembly in the absence of strong niche-based processes
• Priority effects occur when the order and timing of species arrival influence the subsequent community structure and successional trajectory
• Biotic interactions, including competition, facilitation, and mutualism, shape the assembly process by determining species coexistence and dominance
• Assembly rules describe the non-random patterns and processes that govern the organization of ecological communities during succession
• Environmental heterogeneity promotes diverse community assembly by providing a range of niches and microhabitats for species to occupy
• Functional diversity, rather than species richness, is often a better predictor of ecosystem functioning and stability during community assembly

Case Studies & Examples

• The volcanic island of Surtsey, Iceland, demonstrates primary succession following its emergence from the ocean in 1963, with the gradual establishment of plants, insects, and birds
• The retreat of the Mendenhall Glacier in Alaska has exposed new terrain for primary succession, with pioneer species like fireweed and Sitka alder colonizing the bare soil
• Yellowstone National Park's recovery after the 1988 wildfires showcases secondary succession, with lodgepole pine forests regenerating and supporting diverse wildlife
• Abandoned agricultural fields in the eastern United States undergo secondary succession, with old-field plant communities gradually giving way to shrublands and forests
• The restoration of tallgrass prairies in the Midwest involves managing succession through prescribed burns and selective plant introductions
• Mount St. Helens' ecological recovery following the 1980 eruption provides insights into primary succession on a large scale, with lupines playing a key role in soil development
• The Rothamsted Broadbalk experiment in England, running since 1843, demonstrates long-term succession and the effects of agricultural practices on plant communities
• The Oostvaardersplassen nature reserve in the Netherlands showcases the role of large herbivores in shaping succession and maintaining diverse habitats

Ecological Models & Theories

• The Clementsian model proposes that succession is a predictable, orderly process leading to a stable climax community determined by regional climate
• The Gleasonian model emphasizes the individualistic nature of species' responses to the environment, resulting in a more stochastic and variable succession process
• The Relay Floristics model suggests that early successional species facilitate the establishment of later successional species through environmental modification
• The Initial Floristic Composition model argues that all species are present from the beginning of succession, with their relative abundances changing over time
• The Intermediate Disturbance Hypothesis posits that moderate levels of disturbance promote species diversity by preventing competitive exclusion
• The Resource Ratio Hypothesis relates the availability of limiting resources to the outcomes of succession and community assembly
• The Assembly Rules concept aims to identify the deterministic processes and constraints that shape community structure during succession
• The Neutral Theory of Biodiversity and Biogeography emphasizes the role of stochastic processes, such as dispersal limitation and ecological drift, in community assembly

Implications for Conservation & Management

• Understanding succession is crucial for predicting ecosystem responses to disturbances and climate change, informing conservation and restoration strategies
• Incorporating knowledge of successional stages and trajectories can guide the selection of appropriate species for ecological restoration projects
• Managing disturbance regimes, such as fire and grazing, can be used to maintain diverse habitats and support different successional stages
• Protecting and connecting natural areas can facilitate species dispersal and colonization, enhancing the resilience of ecosystems to disturbances
• Monitoring successional processes can provide insights into the effectiveness of conservation interventions and the recovery of degraded ecosystems
• Considering the role of ecosystem engineers and keystone species in succession can inform targeted conservation efforts
• Integrating successional theory with landscape ecology can help prioritize conservation actions and optimize the spatial arrangement of protected areas
• Recognizing the potential for alternative stable states and novel ecosystems can guide adaptive management approaches in a changing world