National Geographic October 1998 #4. I know this is not the best of scientific journals, but it does give a pretty graphic representation of the human population over time, and what will possibly happen when the population becomes too large.
Population Biology: is about measuring changes in population size and composition and to identify the factors that cause this change.
No population can grow indefinitely. Most populations are relatively stable. Populations that show dramatic increases are usually followed by dramatic decreases.
Characteristics of Populations. (many individuals of one species occupying the same area at the same time)
Important characteristics are Density and dispersion.
Density (number of individuals per unit area)
It is very difficult to count all individuals so normally samples are taken.
count a small plot and multiply by the area size.
estimate by indirect indicators, burrows, tracks or droppings.
mark-recapture method. animals are trapped within boundaries marked, released and retrapped. The number of individuals in a population (N) is estimated by the formula N= (number marked) X (total catch the second time)/number of marked recaptures. This assumes that the marked individuals have the same probability of being trapped as the unmarked individuals. (sometimes they get to liking the bait and the safe house in the trap for the night)
Dispersion ( the pattern of spacing among individuals within a geographical boundary.)
a geographical range is the geographic limits within which the population lives. Remember local densities my vary substantially because not all areas within a range provide equally suitable habitat.
the three general patterns of dispersion are
Clumped- where individuals are aggregated in patches (clumped plants clumped animals. (common)
Uniform pattern- spacing is even due to antagonistic interactions between individuals of population during feeding breeding nesting ect.
random- individual spacing varies in an unpredictable way. When there is no sense of attraction or repulsion among individuals (not common)
Demography- study of factors that affect birth and death rates a study of vital statistics.
(birth and immigration add individuals to a population and death and migration take individuals away)
most populations have an age structure (relative # of individuals of each age in a population) because some individuals may live longer than the time they reproduce, individuals may reproduce several time over many years giving multiple generations.
exception: organisms like annual plants that reproduce and die all together.
each age group will have its own birth and death rate.
Birth rate (fecundity) is greatest for intermediate ages.
death rate- highest in young and old
older populations tend to grow more slowly than populations of younger individuals (more breeders)
Generation time is an important factor related to age structure.
average span of time between the firth of individuals and birth of offspring.
indefinite increases in population size dos not occur.
a population may increase rapidly from a low level but eventually conditions occur to reduce that growth.
a combination of limited resources and other factors will stop the growth of a population.
We can model an ideal population in an unlimited environment.
Change in population size during atime interval =births during time interval-deaths during time internal.
in a large population changes can be modeled using birth rates and death rates.
A change in size in a population of 1000 individuals equals
change in number/change in time= (birth rate)x(population size)-(death rate)x(population size)
rate of change r= birth rate - death rate and is used to determine if a population is growing or decreasing.
zero population growth occurs when death rate and birth rates are equal.
A logistic model of population growth incorporates the concept of carrying capacity.
the above calculations are modeling a population with infinite resources and where the limiting factors have been removed. This is not the case in most populations.
carrying capacity is an environmental property that varies over space and time with the abundance of limiting resources. It represents the maximum stable population size that the particular environment can support offer a relatively long time period.
Population limiting factors
Density dependent factors regulate population growth by varying with the density.
density dependent factors intensifies ad the population increases.
health (increase in disease)
accumulation of toxic metabolic wastes.
Density independent factors are unrelated to population size and effects the same percentage of individuals regardless of size of population.
weather and climate.
in some populations may have more of an effect than density dependent factors.
A mix of density and density independent factors probably limits the growth of most population.
some populations regularly have boom and bust cycles.
K selected (equilibrium populations)
K selected populations are those living at a density near the limits of their resources.
r-selected (opportunistic populations)
r selected populations are more likely to be found in variable environments where population densities fluctuate or where little competition occurs.
Most populations are a mixture of these two.
How do you think the human population is regulated?