Biology 106 Lecture Topic # 22 Population Growth and Regulation 1/2
I. Ecology = study of relationships among organisms and their environment, can be studied at many levels
1. Population
2. Community level - poplns of different organisms
3. Ecosystem - a community in addition to abiotic factors interacting
4. Biomes - major terrestrial assemblages of organisms that occur over wide geographic area
II. Population Characteristics
1. Size small vs. large
n
inbr depr
vigor
pr{extinction}
genetic variation
pr{genetic drift}
2. Density = # indiv.'s/ unit area
3. Dispersion = describes spatial pattern among organisms in 3 basic ways:
1. Randomly spaced = no correlation between location of individuals
2. Evenly spaced = distance among individuals is greater than expected by chance
3. Clumped = most common in nature, distance among individuals is smaller than due to chance
4. Demography = the statistical study of populations. A quantitative description of the number of indiv.'s in a popln and has predictive value of whether popln size remains stable or increases or decreases over time.
Assuming the age structure remains the same
A stable popln: births + immigration deaths + emigration An increasing popln: births + immigration deaths + emigration
A decreasing popln: births + immigration deaths + emigration
Because poplns of organisms vary in size, density, dispersion, and demography, each occupies a particular place and plays a role in its ecosystem. The role = ___________
III. Population Growth
- major attribute of a population is its ability to _______
- unlimited popln growth is exponential (i.e., the larger the popln, the faster it grows)
- In nature - most poplns remain ________________ in # regardless of the # of offspring produced
- Darwin and Wallace partly based their independent discovery of natural selection on these observations ( Malthusian theory)
- when any organisms reproduction is unchecked, they would soon cover the earth
- most poplns have potential to greatly increase their size, and do so when conditions allow
- So why don't populations commonly experience exponential growth????
IV. Factors limit population growth:
A. Mortality and Survivorship
r = Intrinsic rate of increase of popln and is dependent upon:
*ages of organisms in popln
*reproductive performance of different aged organisms
B. Age Distribution - varies by spp and region and across a species range ex.,
C. Sex Distribution - vary by spp and region depending on the mating system, ex.,
D. Generation Time - the time period in between successive generations
ex. ,
V. Human Population
- our ultimate popln size is controlled by our environment
- humans have exponentially expanded their popln growth by technical innovations
ex.s, -
Population pyramids = graphic representation of popln age, sex, and size changes over time
- view historical trends in demography
- growing popln: many prereprod and reprod indivs
- declining popln: few prereprod and reprod indivs
- often more females than males in popln
VI. Mathematical approach to popln growth
A. Exponential growth: (no limits) not frequently experienced by a popln in nature
Growth=(avg. birth rate - avg. death rate) x N
dN/dt = rN
where: N = # indivs in popln
dN/dt = rate of change of #'s in popln
r = intrinsic/innate rate of increase without any imposed limits (b-d). Difficult to calculate
- i.e., colonization of new area, or unlimited resources for a period of time
In any given environment if:
r is _______ then popln growing
r is _______ then popln constant in size
r is _______ then popln decreasing
Must use life tables to calculate r
Life Table =
Demography =
However, R, is the actual rate of popln increase and can be calculated
R = (Births + Iimmigration) - (Deaths + Emigration)
if = 0 + 10 - 5
= 5
then R of popln ???
B. Logistic Growth = Limited Population Growth
Carrying capacity ( K) of a popln is a fact of life because resources are limited
Popln grows to its carrying capacity in a given environment, and stabilizes
*graphically referred to as a sigmoidal curve
K is a dynamic value, not static, the value will oscillate around the mean due to minor variations in resources
dN/dt = r N (K-N)
K
dN/dt = growth rate
r = intrinsic increase
N = # indivs present at any one time
K = carrying capacity
As population reaches K it may experience:
1. Density Dependent Effects - magnified by an increase in popln size
ex. competition basis of natural selection
2. Density Independent Effects - operate regardless of popln size
ex.
*Most poplns regulated by interaction between density dependent and density independent effects.
VII. Life History Strategies - Two cases represent the extremes of a continuum of life history strategies
r K
many annuals, aphids, mice coconut palms, whales, elephants
Many animals are a combination of r and K strategies and shift from one extreme to the next depending on environmental conditions, i.e. opportunists
VIII. Survivorship Curves - graphic representation
Survivorship = the percentage of original popln that survives to a given age
Three basic types of survivorship curves:
1. Type I
2. Type II
3. Type III