Biology 20C - Fall 1998

Biology 20C - Fall 1998

ECOLOGY AND EVOLUTIONARY BIOLOGY

Lecture 20 - Population Density; Demography

Abundance: Population size (N) is usually expressed as density (= number/area or number/vol).

In an empirically defined population, the data can be extrapolated to a conceptual population by assuming that density in the area under study is representative of the density throughout the entire population. Changing density then reflects changing abundance (i.e. population growth).

Absolute density ( N_a ): This is a precise quantitative measure of the numbers of individuals (e.g. rabbits: n km^-2; snails: n m^-2; bacteria n mm^-3). This is possible only when the area or volume can be defined accurately, and individuals can be detected and counted accurately.

Relative density ( N_r ): This is some index of biological activity of the population that is known or assumed to be proportional to the absolute density (i.e. N_r = cN_a ). Assuming the value of "c" is constant (although) usually unknown, proportional changes in N_r should be identical to those in N_c. Possible indices of relative density are limited only by the investigator's imagination, but include such things as:

	number of coyote tracks per hectare
	number of mouse burrows per 100 m²
	area of leaves eaten by insects per 100 cm²
	number of bird territories per acre
	number of male frogs calling per 100 m of shoreline
	concentration of photosynthetic pigments (of phytoplankton) per cm³
	weight of rabbit fecal pellets per m²

Relative density can also be measured in terms of the effort required to detect the organisms:

	time to catch 50 trout
	volume of water filtered to get 1 gram of phytoplankton
	number of birds seen in 1 hour of observations
	weight of jellyfish caught in 1 hour of trawling

Measuring density:

1. Census	Complete count of all individuals in entire population	Abs. N

2. Sampling	- Point sampling (0-dimensional)	Rel. N
	- Transects (1-dimensional)	Rel. N
	- Quadrats (2- or 3-dimensional)	Abs./Rel. N

3. Mark-release-recapture (Lincoln Index) - dilution methods		Abs./Rel. N
	N = No. marked and released X No. in second sample No. of marked individuals in second sample

Cohort: A group of individuals born at the same time, and hence, of the same age. (Demographic studies are commonly done on cohorts).

Age-specific mortality ( d_x ): The probability that an individual aged "x" will die at age "x" (i.e. before reaching "x+1")

Age-specific survivorship ( l_x ): The probability that an individual will survive from birth until age "x". Expressed as survivorship curves with l_x plotted against age; curves are usually scaled between 1.0 (at birth, when x = 0, l₀ = 1.0) and 0.0 (l_max = 0.0 after last member of cohort dies)

Type I	Good juvenile survival; high adult mortality	e.g. mammals, annual plants
Type II	Constant proportion dying at every age	e.g. adult birds
Type III	High juvenile mortality; good adult survival	e.g. invertebrates, trees

Age-specific fecundity ( b_x ): The average number of female young born to each female of age "x" (assuming a constant sex ratio). Expressed as curves of mean average number of females young plotted against mothers age (x). B_x curves always have pre-reproductive and reproductive phases. There may also be a post-reproductive phase. Phases are separated by:

	a (alpha)	= age of first reproduction
	w (omega)	= age of last reproduction

	Semelparity	Each female breeds only once during her life
	Iteroparity	Each female may breed many times during her life

	Continuous breeding	Each female breeds frequently (without seasonality)
	Discrete breeding	Each female has distinct breeding / non-breeding seasons

	Synchronized breeding	All females breed at the same time
	Non-synchronized breeding	Females breed independently of others