Natural Resources Ecology - Chapter 8: Life Histories

RNR 316: Natural Resources Ecology C
Chapter 8 Focus: Life Histories
Instructor: Deepak Patel, Fall semester 2025
Indigenous Land Acknowledgement: The University of Arizona acknowledges it is on the land and territories of Indigenous peoples, with Tucson being home to the O’odham and Yaqui tribes. The university aims for sustainable relationships with Native Nations and Indigenous communities through education, partnerships, and community service.

Module 2: Organisms continues with Chapter 8.
Short Quizzes (Chapters 6, 7, 8):
- Available on D2L.
- Opens: Thursday at 12:15 pm.
- Closes: Friday at 11:59 pm.
Individual Phenology Project Handout:
- Due: By Friday.
- Instructions: See D2L (Content $\rightarrow$ Module 0 $\rightarrow$ Phenology Project).
- Submission: Email to your assigned instructor (Taylor or Flurin).

The process of evolution is inherently dependent on genetic variation.
Evolution can proceed through either random processes or selection.
Microevolution: Operates at the population level.
Macroevolution: Operates at the species level and higher levels of taxonomic organization.

Life history traits represent the schedule of an organism’s life.
Life history traits are significantly shaped by trade-offs.
Organisms vary in their reproductive frequency, but all eventually experience senescence.
Life histories are highly sensitive to environmental conditions.

Life History: The schedule of an individual’s life, encompassing events such as birth, growth, reproduction, and death.
Trade-off in Allocation: A fundamental principle describing how an organism invests its finite time and energy among three critical functions:
- Growth
- Reproduction
- Survival
  The goal of this allocation is to achieve maximum fitness.
Driving Factors: Life history strategies are influenced by:
- Extrinsic factors: Environmental conditions (ecology).
- Intrinsic factors: Internal constraints (phylogeny, physiology, development).
Fitness: Generally refers to an organism's reproductive success and the survival of its offspring, contributing to the genetic representation in future generations.

Fecundity: Number of offspring per reproductive episode is $1$.
Parity: Typically 7-9 reproductive episodes over their lifetime.
Parental Investment: Significant time and energy caring for offspring, averaging 5 months.
Longevity/Life Expectancy: $15-20$ years.
Survival Rates:
- Only 19\% survive the first year.
- After the first year, there is an average yearly survival rate of 95\%.
Colony Data (2009-2018):
- Research (
  LaRue et al., 2024, Proc. R. Soc. B
  ) tracks existing and other colonies, showing varying average population sizes (e.g., $<1000$ to $>25000$). This data illustrates the demographics of existing emperor penguin populations.

Emperor Penguin Strategy (Low Fecundity, High Investment):
- Fecundity: $1$ offspring.
- Investment: 64 days of incubation.
- Average Adult Survival: 95\%.
Krill Strategy (High Fecundity, Low Investment):
- Fecundity: Approximately 10,000 offspring.
- Investment: 0 parental investment.
- Average Adult Survival: Less than 30\%.
Conclusion: There is a clear trade-off; organisms either invest heavily in a few offspring, leading to higher individual survival, or produce many offspring with little to no individual investment, relying on sheer numbers for population persistence. Variation in one life history trait (e.g., number of offspring) is often correlated with variation in another (e.g., size of offspring).

Organisms exhibit a continuum of life history strategies, often categorized as r-selected (