They are affected by many factors, such as competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors.
In crowded populations, increasing population density intensifies competition for resources and results in a lower birth rate.
Decreased reproduction at high population densities
Percentage of juveniles producing lambs
In many vertebrates and some invertebrates, competition for territory may limit density.
Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries.
(a) Cheetah marking its territory
Disease, Predation, & Toxic Wastes
Population density can influence the health and survival of organisms. In dense populations, pathogens can spread more rapidly.
As a prey population builds up, predators may feed preferentially on that species.
Accumulation of toxic wastes can contribute to density-dependent regulation of population size.
For some populations, intrinsic (physiological) factors appear to regulate population size.
The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size.
Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time.
Weather can affect population size over time.
Changes in predation pressure can drive population fluctuations
Number of moose
Number of wolves
Population Cycles: Scientific Inquiry
Some populations undergo regular boom-and-bust cycles.
Lynx populations follow the 10 year boom-and-bust cycle of hare populations.
Three hypotheses have been proposed to explain the hare’s 10-year interval.
Number of lynx