Population densities influence birth and death rates
Because each additional individual typically makes things worse for other members of the
population in an environment with limited resources, per capita birth and death rates usually
change together with changes in population density density dependent.
Birth rates and death rates may be density dependent for several reasons:
o As a species increases in abundance, it may deplete its food supply.
o Predators may be attracted to areas with high densities of their prey.
o Diseases can spread more easily.
Factors that change per capita birth and death rates in a population independently of its density
are said to be density independent (e.g. drought).
All populations fluctuate less than the theoretical maximum, but the sizes of some populations
fluctuate remarkably little.
Stable populations are seen in species with long lived individuals that have low reproductive
Small, short lived individuals are generally more vulnerable to environmental changes than long-
EPISODIC REPRODUCTION GENERATES POPULATION FLUCTUATIONS
o Some years are better than others for reproductive success.
o Population densities increase following years of good reproductive success and decrease
following years of poor reproduction.
RESOURCE FLUCTUATIONS GENERATE CONSUMER FLUCTUATIONS
o Densities of populations of species that depend on a single or just a few resources are
likely to fluctuate more than those of species that depend on wide variety of resources.
Several factors explain why some species are more common than others
Many factors determine why typical population densities vary so greatly among species, but
four of them—resource abundance, the size of individuals, the length of time a species has lived
in an area, and social organization—exert especially strong influences.
o Species that use abundant resources generally reach higher population densities than
species that use scarce resources.
o Species with small body sizes generally reach higher population densities than species
with large body sizes.
o Some newly introduced species reach high population densities (i.e. species that have
recently escaped from the control of certain factors, or introduced into a region where
their regular predators and pathogens are absent).
o Complex social organization may facilitate high densiti