Population Growth & Life Tables

Page 1

LU 4 – POPULATION GROWTH & LIFE TABLES

Population = a group on individuals of the same species

living in the same area.

Ecology = the study of how organisms interact with each

other and their environment.

Population ecology = branch of ecology that studies the

structure and dynamics of populations.

Definitions :

1.

Population ecology = population is a group of

individuals of the same species inhabiting the same area. Living in the same

place at the same time.

2.

Human demography : population is a set of humans

in an area.

3.

Genetics = population is group of interbreeding

individuals of the same species and are isolated from other such groups.

Characteristics of a population

-

Size = no of indiv in the population

-

Density = no of indiv in an area

-

Distribution = spread of indiv in the area

-

Dispersion = how they spread to other locations

: whether in uniform, in clumps, or randomly

-

Age structure = pre-reproductive, reproductive

& post-reproductive

-

Natality = birth rate

-

Mortality = death rate

-

Growth form / growth rate = r – selected / k-

selected.

What controls the size of population ?

1.

Immigration = no of organisms that move into the

area occupied by the population.

2.

Emigration = no of organisms that move out of

the area occupied by the population

3.

Birth rate / natality

4.

Death rate / mortality

Natality and mortality area effected by age composition and

environment. Not all members can survive in an environment and are able to

reproduce.

Density

1.

Crude density = no of indiv per unit of total

volume / space

2.

Ecological density = no of indiv per unit of

habitat space that can be colonised by the population such as 200 trees per

hectare.

Density dependent factor

-

It’s a factor where the population growth rates

is influenced by it.

-

Any factor that limits the population growth

-

Big impact when population size increases

Examples = competition for space, limited food supply, high

concentration of predators, accumulation of toxic wastes, high densities of

population can cause stress, hormone changes & immune protection level,

diseases.

Density – independent factor

-

any factor that reduce the population size by

the same amount, regardless of the size of the population.

-

Happens by chance

-

Weather, climate, natural disasters.

Biotic potential = maximum rate, where a population could

increase under ideal conditions.

Why each species have different biotic potential ?

1.

Variations in the species reproductive span. How

long the individual is capable of reproducing.

2.

Frequency of reproduction = how often the

individual can reproduce. Semelparaous = once in a lifetime. Iteroparous =

multiple times per lifetime.

3.

“Litter size” = how many offsprings born each

time

4.

Survival rate = how many offsprings survive to a

reproductive age.

Exponential Growth = continuous population growth in an

unlimited environment.

dN/dt = rN

dN = change in number

dt = change in time

r = per capita rate of increase / biotic potential

N = number of indic.

dN/dt = rate of population change equals to the per capita

rate of increase times number of individuals

when N is small, growth is small. When N is large, growth is

large. When r is larger, the growth becomes more rapid.

Growth populations overtime = small population, selagi birth

rate > death rate, slightly above, the population grows exponentially when a

characteristic J-curve.

Density dependent growth vs exponential growth.

1.

Population won’t increase forever. If we ignore

interactions between species, the resources will still limit the growth like

food & space.

Limits to population growth

-

Few population can live up to their biotic

potential. Everything is ideal, resources, space, food shelter. But overtime

limiting resources will limit the population growth

-

Population size will reach carrying capacity (K)

-

Logistic growth occurs (S-curve)

Characteristics of Density-dependent Growth

-

Carrying capacity = the max no of indiv. Of a

species that can be sustained by an environment without decreasing the capacity

of the environment to sustain the same amount in the future.

Population growth rate = dN/dT = rN [(K-N)/K] = this is for

logistic growth.

[(K-N)/K] = oppurtunity to grow. R is biotic potential, N is

population size, K is carrying capacity.

Exponential growth

1.

Biotic potential = the maximum rate where a

population can increase under ideal conditions.

2.

R- strategy = J curve

3.

Growth = dN/dt = rN

4.

When the size of population and biotic potential

increase, growth rate increase

5.

When size population and biotic potential

decrease, growth rate decrease.

Logistic Growth

1.

Carrying capacity = the max no of individuals of

a species that can be sustained by an environment without decreasing the

capacity of the environment to sustain the same amount of indiv in the future.

2.

K-strategy = S curve

3.

Growth rate = dN/dt = rN [(K-N)/K]

4.

When population size decrease, growth increase.

5.

When K and r have effect, growth is intermediate

6.

When population size increase, growth decrease.

7.

When population size is the same as carrying

capacity, there is no growth.

POPULATION STRATEGIES = R-STRATEGY COMPARED TO K-STRATEGY

r-strategy = high intrinsic growth rates, it focus on

reproduction. The indiv of the population are not competing with other indiv in

the population.

k-strategy = focuses more on the population near / at the

carrying capacity and these indiv must be able to compete with other indiv in

the population for food and resources.

R strategy characteristics in terms of reproductive

strategies = such as oysters. Very r

strategy. Produce 500million fertilized eggs a year & no parental care.

1.

has a short life

2.

rapid growth

3.

early maturity

4.

many small offsprings

5.

little parental care or protection

6.

Little investment in indiv. Offspring.

K strategy characteristics in terms of reproductive

strategies = such as great apes. Very K strategy. Produce 1 infant every 5 or 6

years and provide extensive parental care.

1.

has a long life

2.

slower growth

3.

late maturity

4.

fewer large offspring

5.

high parental care and protection

6.

high investment in indiv offspring.

LIFE TABLES

1.

shows how survival & reproductive rate vary

with age, population size or life cycle (developmental stage).

2.

Information of birth and death is important for

us to predict trends & future population size.

3.

There is data for mortality & surviorship

curves.

Life tables & survivorship curve

1.

Survivorship curve = a plot of no of indiv from

a hypothecal cohort that will survive to reach diff ages (data from life table)

2.

Survivorship curves can vary = among

populations, between males and females, among cohorts that experience different

environmental conditions.

3.

Has 3 types.

Survivorship curves

Type I = Death rate is high in post reproductive years. /

Die late in life.

Type II = constant mortality rate throughout life span /

uniform rate of decline

Type III = Death rate is high in the early years, but the

few survivors will live long & stay reproductive. / Huge decline in young

-

Type I are humans and other large mammals, also

giant tortoise. High death rate when they get old. K strategy.

-

Type II = birds like seagull and songbird, small

mammals, reptiles. Probabity of death isn’t related to their age.

-

Type III = Aquatic organisms that release

fertilised eggs, invertebrates, plants which are annual and perennial. High

death rate for young indiv. R-strategy.

Reproduction changes with age = proportion of indiv in each

reproductive class can have large effect on population growth.

Expanding = Population momentum, when the youngsters are a

lot and in a big proportion, there is high potential for rapid increase of

births/ natality when these young reach a reproductive age. = bottom heavy

Stable = stationaty phase = mortality is where each class

goes to the next class in approximately the same class as the next class was

at. The population replaces the number of births.

Declining = births has fallen below replacement numbers =

top heavy

-

Average life expectancy rised by about 25 years,

due to modern medicine, better food & better sanitation. Cleaner

environment.

SUMMARY

-

we have to consider human growth pattern.

Because human out compete other species for food water lands etc. so the other

species carrying capacity is deeply effected.

-

Human population size increase, amount of toxic

build up increase as well. Pesticide run off, cargo ship oil spills & acid

rain.

-

Some get benefits from human population = house

sparrows/cockroaches = shelter. Bacteria / amoeba = human as hosts. Lice = live

in humans.

LIFE TABLE = help ecologist & demographers to understand

patters & causes of mortality, predict growth or decline of populations

& manage populations of endangered species.