Class 13

Life History Sensitivity to Environment

• Life history traits (growth rate, age at first reproduction, number of offspring, lifespan) are highly sensitive to environmental changes.

• This sensitivity allows organisms to adapt and optimize fitness in fluctuating conditions.

• Key aspects: allocation of resources towards growth, reproduction (number/size of offspring, breeding frequency), and aging.

Importance of Photoperiod

• Definition: Duration of light and dark periods within a 24-hour cycle.

• Significance: Profoundly influences timing of biological events, especially reproductive cycles, due to predictable seasonal changes.

  • Example: Oat grama (Bouteloua curtipendula) flowers when exposed to ~13 hours of photoperiod.

• Organisms may use photoperiod, temperature, or both cues.

• Implication: Anthropogenic climate change can disrupt synchronization between species, leading to ecological mismatches due to altered temperature and subtle light cues.

Clicker Question on Photoperiod

• Students address photoperiod's role in life history decisions, seasonal changes, and ecological consequences.

Circadian Rhythms and Seasonal Affective Disorder (SAD)

• Environmental light cues regulate daily circadian rhythms and broader physiological processes.

• Light regulation impacts mood, potentially leading to SAD.

  • SAD: Depressive symptoms typically in autumn/winter, linked to reduced natural light.

  • Reduced light disrupts melatonin production and serotonin levels, affecting mood and sleep.

• Personal Anecdote: Comparison of Syracuse (cloudy) vs. Reno (sunny) highlights light's impact on mental well-being and circadian regulation.

Developmental Responses to Predation

• Example: Tree frog embryos (Agalychnis callidryas) sense predators (e.g., cat-eyed snake, Leptodeira septentrionalis) while still in eggs.

• Mechanism: Predator secretions/waste products act as chemical cues (kairomones), triggering premature hatching to escape.

• Core Concept: Illustrates evolutionary significance of reproductive timing plasticity, balancing early escape with developmental readiness.

Snail Reproductive Strategies in Presence/Absence of Predators

• Experiment: Freshwater snails (Potamopyrgus antipodarum) exposed to crayfish predators vs. no predators.

• Findings:

  • Without predators: Earlier reproduction, lower average initial offspring weight.

  • With predators: Delayed reproduction, larger mass at first reproduction (likely investing in growth for survival).

• Conclusion: Despite variations, overall lifetime egg production and reproductive success often remain consistent, suggesting compensatory strategies.

Hatching Success Correlation with Precipitation

• Analysis: Hatching success rates of leatherback sea turtles (Dermochelys coriacea) vs. rainfall.

• Graph Details:

  • Y-Axis: Annual hatching success percentage (0-100%).

  • X-Axis: Time, correlated with precipitation data (blue dots).

• Observations: Higher rainfall years generally coincide with increased hatching success.

• Takeaway: Correlation does not imply causation; mechanistic data needed (e.g., humidity effects, sand temperature, fungal growth) to prove direct impact.

Growth Rates Related to Food Supply in Frogs

• Food availability significantly impacts frog tadpole growth rates.

  • High food: Rapid growth, earlier metamorphosis.

  • Low food: Slower growth, extended larval period.

• Conceptual Question: Trade-offs between growth and reproduction (time to reach optimal weight vs. time to reproduce).

• Ecological Challenges: Organisms balance optimal growth for survival and reaching reproductive maturity before mortality risks.

Seasonal Mismatches and Climate Change

• Rising temperatures due to climate change lead to seasonal mismatches in interdependent relationships (e.g., pollinators and flowering plants).

• Phenology Definition: Scientific study of timing of recurring biological events influenced by seasonal climate changes.

• Observations: Graphs show earlier plant flowering times correlated with increased global average temperatures.

• Consequences: If plants flower earlier due to warming while pollinators still cue to photoperiod, it leads to desynchronization, negatively affecting plant reproduction and pollinator populations.

Human Impact on Life History

• Example: Selective fishing practices exert artificial selection pressure.

• Effect: Targets larger, older individuals, leading to fish maturing at smaller size and younger age over generations.

• Studies: Drive genetic changes favoring earlier, smaller reproduction, impacting population dynamics and ecosystem health.

Trade-offs in Life History Strategies

• Organisms face fundamental energy allocation trade-offs.

  • Example: Hadrosaur: Produced relatively large eggs (~1.5 pounds), altricial hatchlings requiring extensive parental care.

  • Trade-off: High cost per offspring but potentially higher individual survival rates with intensive care.

  • Example: Black widow spiders (Latrodectus mactans): Hundreds of tiny eggs with limited/no parental care.

  • Implication: High mortality risk for most offspring, but sheer quantity increases overall reproductive success and probability of some surviving in unpredictable environments.

Reproductive Strategies

• Overview of Types:

  • Sexual Reproduction:

  • Fusion of gametes (sperm, egg) from two parents.

  • Offspring have unique genetic combinations.

  • Asexual Reproduction:

  • Genetic contributions from a single parent.

  • Offspring are generally genetically identical clones.

  • Vegetative Reproduction (Pants):

  • New individuals from non-reproductive tissues (roots, stems, leaves).

  • Examples: cuttings, tubers (potatoes), runners (strawberries).

  • Parthenogenesis:

  • Offspring develop from an un-fertilized egg.

  • Observed in insects, lizards, some snakes (e.g., Leiopython albertisii facultative parthenogenesis).

Pros and Cons of Sexual vs. Asexual Reproduction

• Sexual Reproduction:

  • Pros: Promotes genetic diversity, enhances adaptability to changing environments, facilitates rapid evolutionary change.

  • Cons: More energetically costly (mate search, courtship, gamete fusion), risks of poor genetic combinations or STDs.

• Asexual Reproduction:

  • Pros: Highly efficient (no mate needed), rapid population growth, successful gene combinations passed intact.

  • Cons: Limited genetic variability, reduced adaptability to novel environmental challenges (pathogens, climate shifts).

The Red Queen Hypothesis

• Concept: Host organisms must constantly evolve and adapt, rapidly, to maintain fitness against co-evolving parasitic threats and antagonistic relationships.

• Sexual reproduction is crucial, as it generates genetic variation, allowing offspring with new gene combinations to resist rapidly evolving pathogens/parasites.

• This continuous co-evolutionary arms race necessitates ongoing adaptation just to survive ("It takes all the running you can do, to keep in the same place.")