Unit 3 - Chapter 2 - Life Tables

Question 1

Answer 1

Population Growth : describes how the # of individuals in a population increases over time

Question 2

POPULATION GROWTH:

What is a Life Table? Why do ecologists use these?

Answer 2

• a tabular representation of the survival and mortality rates of individuals within a population across different age classes or stages. This table is an age-specific account for a species & provides quantitative framework for understanding the demography and life history of populations. (**Remember that another method for collecting data about the age structure of a population is by the ‘Mark/Recapture method’ or counting dead specimens– and a couple others)

USED BECAUSE:
1. Population Dynamics (analyses the dynamics of a species such as how they change in size and structure over a life time AND which age classes have significant impact on population growth or decline)
2. Life History Traits (mortality, longevity, reproductive output, reproductive timing)
3. Modelling (serve as foundation to simulate and predict population dynamics under different scenarios and environmental conditions)
4. Comparitive Studies (can compare with other species to identify patterns, variations, different survival rates, different reproductive strategies)
5. Conservation & Management Efforts (crucial role in providing insights into vulnerability resilience, viability of populations. Identify critical life stages that require conservation actions AND evaluate those actions)

Question 3

POPULATION GROWTH:

Describe how Survivorship Curves work.

Answer 3

• based on data from one population, one environment, one point in time.
• can be used as comparison between times, places, sexes (lx on the life tables). It is survival probability on the y-axis and age or time on the x-axis.
• TYPE 1 CURVE: Low juvenile mortality rates, survival rate high until senescence, mortality occurring predominantly in older age classes. High parental investment, long life spans, relatively stable environments (ex: humans, elephants, some primates)
• TYPE II CURVE : almost equal rates of survival/mortality thoughout life. Constant risk or mortality in life span. (ex: birds, squirrels, rodents)
• TYPE III CURVE : high juvenile mortality rates, high probability of surviving after these early life stages. Numerous offspring, but little parental investment. (ex: fish, invertebrates, plants)

Question 4

Life Table example:

Answer 4

Question 5

POPULATION GROWTH:

Birthrates & Fecundity Tables

Answer 5

• birthrates are usually expressed as births per 1000 individuals of a population per unit of time
• Crude birthrate: # of births during a time period (usually a year) divided by estimated population size at the beginning of the time period, multiplied by 1000.
• Estimate can be improved by separating males & females (if sexually dimorphic) because only females give birth.
• FECUNDITY TABLE: only about the females.
• bx = how many females are born in that age class, so 2.0 are born on average at age 1.
• Gross Reproductive Rate = lxbx (add up all these numbers in the 3rd column, and that’s it). The ANSWER = Ro which is the net reproductive rate – the actual # of females that are produced by a female during her lifespan.
• Ro = 1.4 means she makes 1.4 offpsring on average, so is replacing herself, so population increases.
• KNOW what the lower/higher values of Ro mean (look at the table below)

Question 6

POPULATION GROWTH:

Life Expectancy (e)

Answer 6

• refers to the average # of years an individual is expected to live from the time of it’s birth
• Life Tables can be used to calculate age-specific life expectancies (e_x) – calculate lx (lx = nx/no) first which is the average number of individuals alive during the age interval x to x+1 (survivorship)
• Use lx values then to calculate Tx values (# of years individual is expected to live into the future)
• SEE ALL FORMULAS IN THE 2 PICTURES.

DO NOT MIX UP lx (ON THE OTHER CHART!!!) & Lx.

Question 7

Practice all the calculations mentioned above (ex, tx, lx)