Question

3) Define a life table and describe each column of a life table? What are the three main types of survivorship curves? What are the differences between r-selected and K-selected species?

Answer 1

A life table is tabular represention which records matters of life and death for a population.It summarizes the likelihood that organisms in a population will live, die, and/or reproduce at different stages of their lives.

Taking the example on Murie's skull collection data. The table is standardized to a population of 1000 sheep. As we walk through the table, we can picture what will happen, on average, to those 1000 sheep—specifically, how many will survive or die in each age bracket.

Let's walk together through the first row of the table. Here, we see that 1000 sheep are born, reach an age of zero. Of those sheep, 54 will die before they reach 0.5 years of age. That makes for a death, or mortality, rate of 54/1000, or 0.054, which is recorded in the far-right column.

Age interval in years	Number surviving at beginning of age interval out of 1000 born	Number dying in age interval out of 1000 born	Age-specific mortality rate—fraction of individuals alive at beginning of interval that die during the interval
0–0.5	1000	54	0.054
0.5–1	946	145	0.1533
1–2	801	12	0.015
2–3	789	13	0.0165
3–4	776	12	0.0155
4–5	764	30	0.0393
5–6	734	46	0.0627
6–7	688	48	0.0698
7–8	640	69	0.1078
8–9	571	132	0.2312
9–10	439	187	0.426
10–11	252	156	0.619
11–12	96	90	0.9375
12–13	6	3	0.5
13–14	3	3	1

By looking at the life table, we can see when the sheep have the greatest risk of death. One high-risk period is between 0.5 and 1 years; this reflects that very young sheep are easy prey for predators and may die of exposure. The other period where the death rate is high is late in life, starting around age eight. Here, the sheep are dying of old age.

A survivorship curve is a representation thatshows what fraction of a starting group is still alive during at each successive age.For different species they have differently shaped survivorship curves. In general, we can divide survivorship curves into 3 types based on their shapes:

• Type I. Humans and most primates have a Type I survivorship curve. In a Type I curve, organisms tend not to die when they are young or middle-aged but, instead, die when they become elderly. Species with Type I curves usually have small numbers of offspring and provide lots of parental care to make sure those offspring survive.

• Type II. Many bird species have a Type II survivorship curve. In a Type II curve, organisms die more or less equally at each age interval. Organisms with this type of survivorship curve may also have relatively few offspring and provide significant parental care.

• Type III. Trees, marine invertebrates, and most fish have a Type III survivorship curve. In a Type III curve, very few organisms survive their younger years. However, the lucky ones that make it through youth are likely to have pretty long lives after that. Species with this type of curve usually have lots of offspring at once—such as a tree releasing thousands of seeds—but don't provide much care for the offspring.

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