Chapter 5 - Density-dependence

Abstract

Publisher Summary This chapter discusses the regulation of population density of birds. Where habitats are not disturbed, most bird species remain relatively stable in abundance over long periods of years. Their breeding numbers may fluctuate from year to year, but between limits that are restricted compared with what their reproductive and mortality rates would allow. This implies that their breeding numbers are regulated in some way, by factors that act to curb the rate of increase as these numbers rise and to curb the rate of decrease as the numbers fall. Without the operation of stabilizing “density-dependent” mechanisms, populations could increase or decrease without bounds. Following a change in environment, some populations may indeed increase or decrease for many years. An increase in population may lead to a new and higher mean level and a decrease in population may lead to a lower level or extinction. Density dependence in any demographic parameter, whether births or deaths, immigration or emigration, can be brought about by competition for resources such as food, nest sites, or territorial space. It can also be brought about by natural enemies if predators kill an increasing proportion of prey individuals as the prey density rises, or if parasites infect a greater proportion of host individuals as the host density rises.