Chapter 44.3: Age-Structured Population Growth

Age Structure

 How many individuals are in each age group

• Aids ecologist in predicting if a population will grow or shrink in the future → Some ages reproduce more or less 

  • Eg, tons of young adults = large growth or tons of youth = large growth in the future 

Measurement of Age Structures:

• Divide the population into age classes 

• Age determination methods:

1. Direct anatomical markers: Where there is a known method for checking the age precisely 

2. Size Proxies: where size correlates with its age

• Visual Cues of populations 

Visual cues in age strcutures

• Growing populations: Pyramid shape graph → more young than old

• Stable population: Even shape → equal in all age groups 

• This can be skewed with external factors

  • Eg, fishing → takes older fish leaving only the young that cant survive 

Demography

Study of the size, structure and distribution of a population over time 

Life Tables

tracks how many individuals survive at each stage of life

• Cohort: all individuals born at the same time

• Survivorship: proportion of that cohort still alive at each stage

Reproductive patterns and environmental predictability

• R-Strategists:

• K-Strategists:

Survivorship Curves

 uses information from life tables to show how survival probability changes with age

1. Type I: Most survive early and then die later in life

• High parental care; few offspring

2. Type II: Consistent death rate

• Death equally likely at all ages

3. Type III: Many die young and a few make it to adulthood

• Many offspring, little parental care

R-Strategists:

• Traits: many offspring, little parental care, fast growth, early maturity

• Environment: unpredictable or high juvenile mortality

  • Selection favors high intrinsic rates of increase (r ≈ r_max)

• Survivorship type: Type III

  • Eg Fish

K-Strategists:

• Traits: few offspring, large parental investment, slower development, longer lifespan

• Environment: predictable with populations often near carrying capacity K

• Survivorship type: Type I

  • Eg Humans

Trade-offs: Resource Allocation

• Finite energy/resources must be allocated to…

1. Growth

2. Maintenance

3. Reproduction 

• When allocation increases in on function it reduces resources available for the others → hence the name trade-off

Flexible allocation

• In galapagos lava lizards

  • Adjust the allocation of fat and protein for egg production based on rainfall and prey availability 

    • Dry years → larger eggs and less amount of eggs with larger hatchlings 

    • High predation → more and small eggs 

    • No resources → No reproduction and focus on survival/growth

Life History

• Typical pattern of resource investment across an organisms lifetime 

• Allocation patterns are evolved traits shaped by natural selection 

• Shows at what stages which function is prioritized 

Phenotypic and Physiological Plasticity

The ability of a single genotype to produce different phenotypes (observable characteristics) and physiological states in response to different environmental conditions