(4/8) Population Ecology and Life History Traits

Demography

• Demography is the study of factors determining population size and structure over time.
  • It includes age classes, sex ratio, immigration/emigration rates, survivorship, mortality, and fecundity.
  • The goal is to understand and predict population changes.
• Population change is understood via Life tables.

Life Tables

• Life tables summarize the probabilities of survival and reproduction for individuals in different age classes.
• A cohort is a group of individuals born at the same time within a population.
• Life tables track a cohort over time to determine how many survive into each new age class.
• Alternatively, data can be collected on multiple age classes simultaneously (requires age determination).
• Life tables are based on survivorship per age class.
• Variables in a life table:
  • x = year or age class.
  • {n_x} = number of individuals of the original cohort remaining in the population for a particular age class (x).
  • {l_x} = survivorship, percentage of the original cohort surviving to age class (x).
  • {dx} = mortality, percentage of the original cohort that dies in each year ({lx - l{x+1} = dx}).
• Survivorship is crucial for understanding population changes.
• High offspring numbers don't always lead to population explosion due to survivorship rates.
• Survivorship is the proportion of offspring surviving to a particular age.

Survivorship Curves

• Survivorship curves are created by plotting the log of the number of survivors against age.
• Types of survivorship curves:
  • Type I: High survivorship early and middle, followed by a rapid decline late in life (e.g., humans, some plants).
  • Type II: Constant decline in survivorship throughout life (e.g., some birds, many perennial plants).
  • Type III: Low survivorship early in life, followed by high survivorship for the remaining lifespan (e.g., many annual plants, most invertebrates).
• Survivorship curves help pinpoint when organisms are most susceptible to mortality.
• They also help determine peak reproductive output when combined with life tables.

Fecundity

• Fecundity is the number of offspring an individual can have in its lifetime.
• Fecundity and fitness are theoretically related, but fecundity is an actual value, while fitness is a relative term.
• Fecundity typically refers to the number of female offspring produced by a female.
• Life tables usually use female data.
• Age-specific fecundity is the average number of female offspring produced by a female at a certain age.
• Adding age-specific fecundity to survivorship data helps calculate population growth rate.
• Net reproductive rate of an age-class: Survivorship x Fecundity
• Net reproductive rate for the entire population: Sum of (Survivorship x Fecundity) across all age-classes (ignoring immigration/emigration).
  • If the sum is < 1.0, the population is shrinking.
  • If the sum is = 1.0, the population is stable.
  • If the sum is > 1.0, the population is growing.

Life History Traits

• Higher reproduction costs result in higher mortality.
• Organisms allocate energy and effort to growth, reproduction, and maintenance.
• Life history trade-off: Balancing act between living, growing, and reproducing because these functions cannot occur simultaneously (e.g., common lizard).
• Organisms are selected for maximizing fitness over their lifetimes.
• High fecundity is generally associated with low survivorship (e.g., mustard plant).
• Low fecundity is associated with higher survivorship (e.g., coconut palm).