H3+Density+and+Density+Independence

=**H3 DENSITY AND DENSITY INDEPENDENCE**=

• Density= # of individuals per unit area, per volume, or per unit of habitat • Densities vary depending on the organism (density of soil arthropods will be different than density of deer) • Techniques for calculating density depend on how easily species can be counted • For more conspicuous animals/plants, a total count is taken • For less conspicuous animals, where only a small proportion of the present species can be sighted/trapped, density must be estimated • Relative density= estimates numbers per sampling effort, e.g. fish caught per rod per day
 * DENSITY**

• Numerous, small organisms: sifted out of habitat and counted (plankton) • Less numerous, more motile organisms: • A group of o’ is trapped, marked, then released into wild population • Another trap is set - catching random sample of marked + unmarked • Proportion of marked to unmarked in trap assumed to be the same proportion of marked to unmarked in wild • (# trapped/# recaptured) x # originally marked = Total # in population
 * ESTIMATING DENSITY – MARK RELEASE RECAPTURE**

• When birth rates increase with density while death rates decrease, rates are density dependent • Birth rates + death rates that do not change with density are density independent • With no immigration, a population will continue to increase unless either the per capita birth rate or death rate is density dependent • Allee effect= maximal population growth often occurs at moderate densities • High densities-> resource limitation • Low densities-> difficult to mate + transfer gametes
 * DENSITY DEPENDENCE**

• Equilibrium population density (equivalent to carrying capacity K)= per capita death rate exactly balances the per capita birth rate (one must be density dependent – if both are, the density will fluctuate) • There is no ONE birth rate or death rate for a given density, rather a “carrying capacity range” of values dependent on environmental conditions • S-shaped population curve= as density increases, birth and/or death rates adjust, reducing rate of increase and slowing pop. growth until the carrying capacity is reached
 * EQUILIBRIUM POPULATION DENSITY**



• Three types of density dependence: • Exactly compensating: # of survivors will be constant, regardless of starting density • Overcompensating: # of survivors falls as the starting density rises • Undercompensating: # of survivors rises as the starting density rises
 * COMPENSATION AND DENSITY DEPENDENCE**