2.5E Climax communities and plagioclimax (HL) Notes

1. In ecological succession, the reproductive strategies of species play a crucial role in their ability to colonize new habitats and survive in different community stages.
2. The concepts of r-strategy and K-strategy describe two ends of a continuum of reproductive strategies that are adapted to different environments, such as pioneer communities versus climax communities.
   1. r-strategist species thrive in pioneer or disturbed ecosystems, where resources are temporarily abundant but unpredictable.
   2. K-strategist species dominate in climax communities, where the environment is stable but competition is high.

r-strategist Species

1. Adapted for rapid colonization and population growth in unstable or disturbed habitats.
2. Found in early successional stages, such as areas recovering from volcanic eruptions, fires, or floods.
3. Typically small-bodied, short-lived, and capable of producing large numbers of offspring.

Characteristics

1. High reproductive rate (many small offspring).
2. Early maturity and short generation time.
3. Little or no parental care.
4. Small body size and short lifespan.
5. Populations fluctuate widely in size.
6. Thrive in environments with frequent disturbance or abundant but short-lived resources.
7. Controlled primarily by density-independent factors such as temperature, storms, or drought.

Ecological Role

1. r-strategists are the first colonizers of bare or disturbed environments.
2. They exploit available resources rapidly before competition intensifies.
3. They improve environmental conditions for later species (e.g., add organic matter, stabilize soil).

K-strategist Species

1. K-strategists are species adapted to stable, long-term environments such as mature forests or climax ecosystems.
2. The “K” refers to the carrying capacity (K) of the environment, the population size that resources can sustain.
3. Their strategy is based on quality over quantity, ensuring higher survival rates per offspring.

Characteristics

1. Few, large offspring.
2. Late maturity and longer lifespan.
3. High parental investment (care, protection, feeding).
4. Stable population sizes near carrying capacity.
5. High competitive ability and occupy specific niches.
6. Controlled mainly by density-dependent factors such as food availability and predation.

Ecological Role

1. K-strategists dominate mature, stable ecosystems where competition for limited resources is intense.
2. Their success depends on efficiency, cooperation, and resilience rather than rapid reproduction.
3. They maintain population sizes near the ecosystem’s carrying capacity (K), ensuring long-term equilibrium.

Ecological Implications of r/K Strategies

1. r-strategists dominate early succession, rapidly stabilizing disturbed areas and preparing them for more complex communities.
2. K-strategists establish dominance later, maintaining equilibrium through competition and resource optimization.
3. Both strategies are essential for ecosystem renewal and resilience across time.

Comparison between r-strategists and K-strategists

Challenges to the Concept of a Climax Community

1. Traditionally, ecologists viewed succession as a predictable process ending in a single, stable climax community, where species composition remains constant until disturbed.
2. However, this concept has been challenged in modern ecology.

The Traditional Concept

1. A climax community is the final, stable stage in ecological succession.
2. It is characterized by maximum biodiversity, complex food webs, and a dynamic equilibrium between energy input and output.
3. Traditionally, it was believed that ecosystems progress in a predictable and linear path toward a single, stable climax state (often a mature forest).

Modern Challenges to the Concept

1. Ecologists now recognize that ecosystems are dynamic, and no single, fixed “final” stage exists.
2. The climax community concept has been challenged due to several observations:
   1. Ecosystem variability: Natural systems experience constant change due to disturbances (fires, storms, floods).
   2. Alternative stable states: Similar environmental conditions can support different community outcomes based on historical events or random disturbances.
   3. Human influences: Anthropogenic activity prevents ecosystems from reaching a natural climax.
   4. Biotic interactions: Grazing, predation, and competition can alter community composition indefinitely.

The Vera Wood-Pasture Hypothesis

1. Proposed by Frans Vera (2002), this hypothesis challenges the idea that Europe's natural climax community was a dense, closed-canopy forest.
2. Key Argument: Large herbivores (e.g., aurochs, bison, deer, and wild horses) maintained open wood-pasture landscapes, characterized by mosaics of grasslands, shrubs, and scattered trees.
3. Grazing and trampling by these herbivores prevented total forest closure and maintained a dynamic balance between open and wooded areas.

Key Implications

1. Challenges the idea that climax communities are always forested.
2. Suggests that primary consumers (herbivores) can determine vegetation structure.
3. Demonstrates alternative stable states caused by biotic influences, not just abiotic factors.

Alternative Stable States and Random Events

1. Ecosystems can stabilize in different configurations depending on:
   1. Initial species composition.
   2. Random disturbances (e.g., drought, fire, pest outbreak).
   3. Presence or absence of keystone species.
2. These alternative pathways result in diverse ecosystem outcomes, even under similar climatic conditions.

Human Activity and Plagioclimax

1. Human activities can significantly influence the progression of ecological succession, often leading to the creation of a plagioclimax.
2. This typically occurs when humans alter or prevent the natural recovery of ecosystems, often by introducing grazing, deforestation, or removing top predators.
3. The plagioclimax can be characterized by simplified ecosystems that are artificially maintained in a particular state by human actions.

Causes of Plagioclimax

1. Agriculture and Grazing

1. Continuous plowing and grazing halt succession at early stages.
2. Overgrazing by sheep or cattle prevents tree saplings from maturing, keeping the grassland permanent.
3. Fertilizer use alters soil chemistry, favoring fast-growing grasses and weeds.

2. Deforestation

1. Logging or land clearance removes mature forest cover, resetting succession repeatedly.
2. In tropical rainforests, deforestation followed by burning creates grasslands with lower biodiversity and altered microclimates.

3. Burning (Controlled Fires)

1. Fire is used to maintain certain vegetation types (e.g., heather moorlands).
2. Prevents forest regrowth by destroying saplings and promoting fire-adapted species.

4. Urbanization and Footpath Erosion

1. Continuous trampling compacts soil, destroys vegetation, and halts colonization by plants.
2. Common in tourist zones, dunes, and hiking trails.

5. Removal of Top Carnivores

1. Elimination of predators (e.g., wolves, lions) allows herbivore overpopulation, suppressing tree regeneration.
2. Prevents transition from grassland to forest.
3. Creates simplified, less resilient ecosystems.