Density
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Young Black-throated
Blue Warbler that just left its nest (photo by M. Cline). |
One of the oldest questions in ecology
is: Why don’t population sizes keep on increasing? For example,
if an average pair of American Robins can raise four young each
year and each bird can breed for three years, then every two
birds would produce twelve in their lifetimes! If those reproductive
rates continued, pretty soon the world would be teeming with
robins. Yet scientists have recognized that those demographic
rates don’t continue, and either life-spans get shorter
or reproductive rates get lower when population sizes are very
large. Populations have been observed to grow quickly when numbers
are low and conditions are good, but population growth then
slows once there are so many individuals that conditions for
each individual become worse. This process is called density-dependence,
and it is thought to be a very important process in regulating
- or controlling - the size of natural populations. Density dependence
stabilizes a population at its carrying capacity, which is the
population size that can be stable over time.
Population density is the number
of individuals in a given area, so the term density-dependence
refers to the fact that the demographic rates vary in relation
to density. When density is low, and the population is below
its carrying capacity, each individual has plenty of the resources—food,
water, nest sites—it needs. Individuals in these good
conditions can survive longer and/or breed more successfully,
so the population grows. Yet once the population is large, more
individuals have to compete for the same amount of resources,
so the average share for each one is lower. This leads to lower
survival or reproductive success, so the population can stabilize
at the carrying capacity. Finally, when there are more individuals
than the environment can support, death rates will be high or
reproduction will be unsuccessful, causing the population to
decline back to the carrying capacity.
In this module we'll explore how predators, food and density influence the reproductive success of Black-throated Blue Warblers. But first we'll learn a thing or two about the biology of the species and the place where they were studied.
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