Marginal Distributions of Population and Land Area

*The marginal distributions of the bivariate density
functions shown in the previous figure. Both frequency histograms (filled
bars) and empirical cumulative distributions (solid curves) are shown for
(a) number of people by elevation; (b) occupied land area by elevation;
(c) number of people by population density and (d) occupied land area by
population density.*

Marginal distributions of both population and occupied land area as functions
of elevation and population density can be derived from the HAD and LAD
(above). The marginal distribution of people as a function of population
density reveals that 55% of the world's population lives at densities between
100 and 1000 people/km2. The number of people diminishes gradually as population
density decreases below 100 people/sq.km and drops rapidly as population
density increases above 1000 people/sq.km. The marginal distribution of
people as a function of elevation reveals that approximately 1.96 x 109
people, or 34% of world population, live within 100 m of sea level. At elevations
> 800 m, log10 of population falls linearly with increasing elevation
but at elevations below 800 m, log10 of population increases concavely upward.
Thus globally, as elevation drops from 800 m towards sea level, the number
of inhabitants increases faster than exponentially while the occupied (and
total) continental land area increases almost linearly. Consequently, average
population density increases with decreasing elevation below 800 m.