2.1N r- and K strategists (HL) Notes

Variation in Life Cycles

1. All organisms follow a life cycle, from birth, growth, and reproduction to death, yet these cycles differ greatly between species.
2. The key differences lie in reproductive behavior (how often and how much a species reproduces) and lifespan (how long individuals live).
3. These variations are evolutionary responses to the type of environment and availability of resources a species experiences.
4. Species that live in unstable or changing environments often reproduce rapidly and in large numbers, while those in stable environments reproduce slowly and invest heavily in the survival of fewer offspring.
5. This pattern is explained through the r/K-selection theory.

The r/K-Selection Theory

1. The r/K-selection theory explains how species adapt their reproductive strategies according to environmental conditions.
2. It is proposed by MacArthur and Wilson (1967).
   1. The symbol r represents the rate of population increase, which is high for fast-reproducing species.
   2. The symbol K represents the carrying capacity of the environment, the largest population size that available resources can support.

r-Strategists- The Opportunists

1. r-strategists are species that reproduce quickly to exploit short-lived or disturbed environments.
2. Their survival strategy relies on producing large numbers of offspring in a short period rather than ensuring the survival of each individual.

Characteristics of r-strategists

1. High reproductive rate: Produce a large number of offspring to maximize the chance that some survive.
2. Minimal parental care: Offspring are left to survive on their own.
3. Short lifespan: Individuals mature quickly and die young.
4. Small body size: Limited energy investment per individual.
5. Population instability: Populations fluctuate greatly with resource availability.
6. Rapid colonization: Can quickly occupy new or disturbed habitats

K-Strategists-The Competitors

1. K-strategists thrive in stable and resource-limited environments, where competition between organisms is strong.
2. They produce fewer offspring, but each receives significant energy investment, increasing its chances of survival to adulthood.

Characteristics of K-strategists

1. Low reproductive rate: Fewer offspring are produced per reproductive cycle.
2. High parental care: Energy is invested in nurturing, protecting, and feeding offspring.
3. Long lifespan: Individuals grow slowly, reach maturity late, and live for many years.
4. Large body size: High energy and nutrient requirements per individual.
5. Population stability: Numbers remain close to the environment’s carrying capacity (K).
6. Competitive advantage: Strong competitors for limited resources in stable ecosystems.

Environmental Conditions and Succession

1. r- and K-strategists correspond to different stages of ecological succession:
   1. r-strategists dominate in early (pioneer) stages, where environments are unstable and resources are abundant but short-lived.
   2. K-strategists dominate in later (climax) stages, where stability, competition, and resource limitation are key features.

C-Strategists: Intermediate Species

1. Some organisms exhibit traits of both r- and K-strategists, termed C-strategists (for “competitive”).
2. They balance rapid reproduction with moderate parental care and tend to dominate mid-successional stages.

Population Growth Patterns

r-Strategists: Exponential Growth (J-Curve)

1. Populations grow rapidly when conditions are favorable.
2. Resource depletion leads to sudden crashes (boom-and-bust cycles).
3. Typical of unstable ecosystems.

K-Strategists: Logistic Growth (S-Curve)

1. Populations grow slowly, then stabilize near the carrying capacity (K).
2. Density-dependent factors (competition, predation, disease) regulate population size.
3. Common in stable environments.

Survivorship Patterns

1. Type I Curve: High survival in youth; death rates increase with age (e.g., elephants, humans).
2. Type II Curve: Constant mortality throughout life (e.g., birds, reptiles).
3. Type III Curve: High juvenile mortality; few individuals reach adulthood (e.g., fish, insects, plants).

Ecological and Evolutionary Implications

1. Adaptation to Environment

1. r-strategists thrive in disturbed or unpredictable habitats.
2. K-strategists dominate stable, resource-limited habitats.

2. Response to Disturbance

1. After events like wildfires or floods, r-strategists recolonize first.
2. As ecosystems mature, K-strategists replace r-strategists through succession.

3. Biodiversity and Stability

1. The balance of r- and K-species within a community affects ecosystem resilience.
2. Diverse ecosystems usually contain a mixture of both strategies.

4. Conservation Relevance

1. K-species are more vulnerable to extinction (slow reproduction).
2. r-species can recover quickly after disturbance but may become invasive.