Lecture week 12 (11-7, 11-9) - Notes Population Ecology

Population Ecology

The study of how and why population size changes over time and the effects of population change on the population.

Population Genetics

The study of how populations change genetically over time and how new populations may arise.

Exponential Growth

Population growth that occurs in an unlimited environment with unlimited resources and space, resulting in a rapid increase in numbers over time.

Logistic Growth

Population growth that involves three stages - initial exponential growth, decelerating growth rates, and fluctuations around a carrying capacity (K) of the environment.

Carrying Capacity (K)

The maximum number of individuals that a given environment can support or the point at which the population size is in equilibrium with resources.

not constant

Demography

The study of factors that determine the size and structure of a population over time, including age classes, sex ratio, rates of immigration and emigration, survivorship, mortality, and fecundity.

Helps to better understand how a population changes and to predict how a population will change in the future.

Life Tables

Summaries of the probabilities that an individual age class will survive and reproduce in any given year over its lifetime, used to understand population change and predict future changes.

Based on survivorship per age class.

Survivorship Curves

Graphs that show the log of the number of survivors vs age, categorized into Type I, Type II, and Type III curves based on patterns of survivorship throughout an organism's life.

Fecundity

The number of offspring an individual can have in its lifetime, typically referring to the number of female offspring produced by a female, used in conjunction with life table data to calculate the growth rate of a population (add age- specific fecundity = avg. number of female offspring produced by a female at a certain age)

Net Reproductive Rate

The product of survivorship and fecundity for a specific age class, used to calculate the growth rate of a population. If the sum of net reproductive rates across all age classes is less than 1, the population is getting smaller; if it is equal to 1, the population is staying the same; if it is greater than 1, the population is growing.

Survivorship x fecundity

Exponential Growth Equation

(🔼N/🔼t) = rN

Where r = b - d, rate of population growth

N = population size

Logistic equation

(🔼N/🔼t) = rN((K-N)/K)

What is the issue with the exponential model?

It doesn’t take into account the limit that resources place on populations.

Using the logistic model

If N > K: population rate decreases

If N < K: population rate increases

If N = K: population rate does not change

Problems with the exponential and logistical growth

Some choose growth over reproduction.

They assume all individuals in that population are reproducing the same.

Some die off before they can reproduce.

Cohort

All the individuals that are born at the same time, represented by an age class.

Life table variables

X = the year in consideration

n = the number of cohort remaining in the population

lx = survivorship

dx = mortality

nx = the number remaining for a particular age class

Type I survivorship curve

Large percentage of survivors throughout much of the individuals life time, followed by a rapid decline in individuals within the cohort.

Type II survivorship curve

Relatively constant decline in survivorship throughout the life of the species

Type III survivorship curve

Having a low survivorship early in the life of the organism, followed by a fairly high survivorship throughout the remainder of the lifespan.

Life history

How an organism allocates energy and effort into growing, reproducing, maintaining its body.

Life history trade off

The balancing act between living and growing and reproducing

Summary

High fecundity = low survivorship