Ecology chapter 9

cohort

group of same aged young that grow & survive at similar rates.
used to determine survival & mortality of individuals in a population

age class

consists of individuals of a particular age

life table

provides data on the number of individuals alive in different age classes & the age specific survival & mortality rates in these age classes

survivorship curve

graphical representation of the numbers of individuals alive in a population at various ages

2 types of life tables

cohort life table
static life table

cohort life table

follows a cohort of individuals from birth to death.
used to estimate the age specific probabilities of survival

static life table

data is gathered on the age structure of a given population at one point in time.
for long lived organisms, a snapshot approach is used.

age distribution

reflects survival & mortality patterns.
do not include males in calculating population growth
imbalance in age classes influences population's future: increasing has large number of young, decreasing has few young.

static life tables

provide snapshot of population's age structure from a sample at a given time

Static life table calculations

dx = (nx-1) - nx
nx = number alive at start of previous year
dx = number dying in a given age class or year

survivorship calculation

lx = nx / n0
lx = survivorship
nx = number surviving this year
n0 = number born

mortality rate calculation

qx = dx / nx
qx = mortality rate within each age class
dx = number dying this year
nx = number alive this year

Average number alive calculation

Lx = [nx + (nx+1)] / 2
Lx = average number alive in each age class
nx = number alive this year
(nx+1) = number alive next year

Total years lived calculation

Tx = sum of Lx (L1+L2+L3+.....Lx)
Tx = total years lived
Lx = average number alive in each age class

Life expectancy calculation

ex = Tx / nx
ex = life expectancy - average number of additional age classes an individual can expect to live at each age
Tx = total years lived
nx = number alive this year

limiting factors of static life tables

accuracy limited by assuming that equal numbers of offspring are born each year b/c there's no independent method for estimating birth rate of each age class
methodology difficulties = difficult to accurately assess age structure of population

constructing cohort life table

conduct population censuses frequently but for limited time
use log to express number of individuals

survivorship curve

graphical representation of numbers of individuals alive in a population at various ages.
plot the number of surviving, nx, from any given cohort of young (log scale) against age

determining population loss with log

express nx on log scale to examine rates of change with time not in absolute numbers.
shows difference between starting & ending population sizes

type I curve

rate of loss for juveniles relatively low
most lost later in live b/c older & prone to sickness & predators
few offspring but invest time & resources in raising young
ex:
large mammals
humans

type II curve

fairly uniform death rates over time
ex:
birds
small mammals
reptiles
some annual plants

type III curve

rate of loss for juveniles is relatively high
survivorship curve flattens out for organisms that have avoided early death
most juveniles die or are eaten; few reach favorable habitat & thrive
ex:
fish
marine invertebrates

accuracy in static vs cohort

cohort:
always more accurate b/c in natural world & environmental variation includes years of good & bad climate or food supply
do not have time & resources to follow populations of organisms from birth to death
static:
quicker & cheaper method to get rough idea of how the individuals of a population are distributed among age classes

age specific fertility rate

the proportion of female offspring that are born to females of reproductive age

age specific fertility rate calculation

mx = number of offspring/ number of females at age x
mx = age specific fertility rate

lxmx

represents the contribution of each age class to the overall population growth rate
average number of offspring per female
age specific fertility rate X average number alive in age class

net reproductive rate

overall growth rate per generation is the number of offspring born to all females of all ages

net reproductive rate calculation

R0 = sum of lxmx
R0 = net reproductive rate (overall growth rate)
lx = average number alive in age class
mx = age specific fertility rate

how to tell with Net reproductive rate if population is growing

R0 < 1, population is in decline
R0 = 1, population is at equilibrium
R0 > 1, population will grow

future size of population calculation

N(t+1) = NtR0
N(t+1) = net population size in next generation
Nt = number at generation t
R0 = net reproductive rate

Generation time calculation

T = sum of xlxmx / R0
x = age class
lx = average number alive in age class
mx = age specific fertility rate
R0 = net reproductive rate

generation time vs body size

generally, generation time increases as organismal size increases

Which of the following is true concerning static life tables?

they provide a snapshot a population's age structure at a specific point in time

why do survivorship curves typically express the number of individuals alive during each age class as log10nx, rather than simply nx?

because ecologists are more interested in rate of mortality, rather than the absolute number of survivors

which type of organism is most likely to exhibit a type III survivorship curve?

butterfly

age specific fertility rates are typically based only on females in the population because

it is easier to accurately assess their fecundity compared to males

which of the following is a characteristic of type II survivorship curves?

constant mortality across age classes

which of the following was found to affect the survivorship of fox populations in england?

hunting

which of the following represents the net reproductive rate (R0) of a population?

R0 = sum of lxmx

adolf murie found which of the following to be responsible for the relatively high mortality of both young and old age classes of all mountain sheep in alaska?

predation by wolves

which of the following is generally true regarding cohort life tables?

they are more accurate than static life tables

a population is said to be at its equilibrium size when which of the following is true?

R0 = 1

which of the following was not found by joe connell in his study of barnacle?

R0>1 for barnacle population

how much is a population of mice for which R0 = 1.15 growing?

15%

what is the yearly net reproductive rate for a pack of wolves if the pack contained 62 wolves last generation & consists of 51 individuals this generation?

0.823

dx

number of individuals dying during age class x

px

proportion of individuals surviving age class x

ex

average life expectancy of an individual alive at start of age class x

nx

number of individuals alive at start of age class x

lx

proportion of individuals surviving to the start of age class x

qx

proportion of individuals dying during age class x

Lx

average number of individuals alive during age class x

___ survivorship curves are usually associated with organisms that have high mortality rates in early stages of life

type III

if the net reproductive rate (R0) is equal to .5, what assumptions can we make about the population?

in decline

in population of beavers, in n2=500, n3=300 & n4=200, what is d3?

100

if nt = 100 & r0 = .5, what is n(t+1)?

50

type 1 survivorship cures include

large mammals