Ecosystems

The boundary between community and ecosystem can be ambiguous, especially in discussions about exotic species.
Invasive species like kudzu can influence various communities within an ecosystem and affect ecosystem processes such as nutrient cycling.
The discussion can either focus solely on the community or expand to include the ecosystem, as the processes involved are fundamentally similar.

Ecological succession refers to the process of directional change in the species structure of a community over time.
Types of Succession:
- Primary Succession: Involves community changes starting from a bare substrate with no previous biological community or soil.
- Secondary Succession: Occurs after a disturbance in an established ecosystem, where soil remains intact.
Successional communities are referred to as seres; a series of seres progresses through succession within an ecosystem.

Takes place in systems that lack biotic influence; characterized by the absence of a biological community and often no soil.
Examples of systems likely to undergo primary succession:
  - Lava flows
  - Areas behind retreating glaciers
  - Newly formed sand dunes
  - Bare rocks
The first organisms to populate these areas belong to the pioneer community, consisting of species capable of colonizing harsh new habitats.
Pioneer species must be adept at surviving challenging conditions, and typical examples include:
- Moss and lichens: These generally colonize bare rock or sandy substrates.

Often possesses adaptations for extreme conditions:
  - Short root systems (e.g., approximately 1 cm long).
  - Capable of gripping bare rock surfaces or adapting to unstable sand.
  - Highly tolerant of drought and nutrient-poor conditions.
Lichens represent a mutualistic relationship between fungi and algae, often featuring nitrogen-fixing cyanobacteria, making them more efficient nutrient users.
Annual plants can quickly produce seeds that endure even in dry years.

Pioneer species initiate the conditions that enable their eventual extinction by modifying the environment:
- For example, lichens and roots may help weather rocks and begin soil formation.
- Consequently, dead plant materials provide sustenance for decomposers, initiating nutrient cycling.
This transitional process may take years to decades.

Following the pioneer stage:
- The harshness of the ecosystem remains but begins to decrease as soil nutrients improve.
- New species may still require adaptations but aren't as specialized as pioneers; they include fast-dispersing species with high reproductive rates but limited competitive abilities, resembling invasive characteristics.
Nutrient cycling becomes more robust, leading to richer and deeper soil, allowing more complex plant-animal interactions.

The final climax sere represents the most competitive species, albeit with slower dispersal rates.
Climax species require specific environmental conditions but will dominate when these conditions arise.
The climax community persists until disturbed, effectively resetting the successional process.

Secondary succession mirrors primary but starts in established ecosystems post-disturbance, with intact soil.
Disturbances can include:
  - Fire
  - Agricultural plowing
  - Natural occurrences like drought or tree falls
The key is that secondary succession does not start from scratch; soil and some nutrients remain.

Following disturbance, the site is populated by a new pioneer community that differs from primary succession; species adapt better to existing soil.
These species are rapid colonizers characterized by high dispersal and reproductive rates, making them poor competitors.
As time progresses, the community evolves into later seres with enhanced competitive abilities and slower life histories.

Climax communities are stable if environmental conditions do not shift.
Changes can be gradual through small environmental variances, but the primary structure often remains constant.
An ecological disturbance (like soil exposure) initiates progression through the seral stages.

After a disturbance, a bare soil may remain for several days to the entirety of the growing season:
- Following this, pioneer plants seed, either from existing seed banks or nearby habitats.
- Over subsequent years, varying species replace the pioneers as competition ensues.

As trees mature and flourish, they overshadow grasses and forbs, with early trees being low-growing and later species competing more effectively for light.
Eventually, a mixed community of trees dominates, forming a multi-layered canopy (the climax community).

Climax community definitions can be complex and context-dependent:
- If natural disturbances are allowed (e.g., regular fires), a different community may emerge termed disclimax.
Disclimax occurs when frequent disturbances inhibit the establishment of potential climax species (hardwood forests in fire-prone grasslands).

Challenges in categorizing disturbances exist; generally defined as any event profoundly altering ecosystem structure.
Repeated disturbances may not reset the successional clock, leading to stable communities with little change.

Succession is an ongoing directional process resulting in shifts in community structures over time:
  - Primary succession begins anew with no prior biological imprint.
  - Secondary succession progresses from an interrupted point in the journey toward maturity without a complete reset of conditions.
  - Early successional species are characterized by high reproductive and dispersal rates but low competitive abilities.
  - Late successional species exhibit opposite traits, with higher competitive capabilities but lower rates of reproduction and dispersal.

While traditionally analyzed in plant communities, succession applies to other taxa but without clear definitions, particularly in animal communities.
In North American grasslands, succession in rodents can be observed, influenced mainly by plant community succession, emphasizing the interconnectedness of these ecosystems.
Small mammal communities adapt relative to the vegetation structure; for instance, if woodland vegetation dominates due to fire suppression, certain rodent species (e.g., white-footed mouse) only establish under requisite tree/shrub community conditions.