Dynamic biological processes influence population density, dispersion, and demographics Chapter 53, Section 1

Overview  Population ecology explores how biotic and abiotic factors influence the density, distribution, and size of populations.  A population is a group of individuals of a single species living in the same general area.  Members of a population rely on the same resources, are influenced by similar environmental factors, and are likely to interact with one another.  Populations evolve.  Populations are often described by their boundaries and size.

Density: A Dynamic Perspective  Density : the number of individuals per unit area of volume.  In rare cases, population size and density can be determined by counting all individuals within the boundaries of the population.  In most cases, it is impractical or impossible to count all individuals in a population.  Ecologists use a variety of sampling techniques to estimate density and total population size.  Density is not a static property.  Additions to population due to birth and immigration.  Removal of individuals from a population occurs through death and emigration.

Patterns of Dispersion  Dispersion : the pattern of spacing among individuals within the boundaries of the population.  Within a population’s geographic range, local densities may differ.  Differences in local density provide insight into the environmental associations and social interactions of individuals in the population.  Clumped: individuals are aggregated in clumps.  Uniform: individuals are evenly spaced.  Random: the position of each individual is independent of other individuals.

Demographics: Life Tables  Demography : the study of the vital statistics of populations and how they change over time.  Life tables : age-specific summaries of the survival pattern of a population.  How life tables are made…  The best way to construct a life table is to follow the fate of a cohort, a group of individuals of the same age, from birth until death.  Scientists then determine the number of individuals that die in each age group and calculate the proportion of the cohort surviving from one age class to the next.

Demographics: Survivorship Curves  A graphical method of representing some of the data in a life table is a survivorship curve, a plot of the proportion or numbers in a cohort still alive at each age group.  Type I: curve is flat at the start, reflecting low death rates during early and middle life, and then drops steeply as death rates increase among older age groups.  Type II: curves are intermediate, with a constant death rate over the organism’s life span.  Type III: curve drops sharply at the state, but flattens out as death rates decline for those individuals that survive the early period of die-off.

Demographics: Reproductive Rates  Demographers who study sexually reproducing species generally ignore the males and concentrate of the females in a population because only females produce offspring.  Demographers view populations in terms of females giving rise to new females.  A reproductive table, or fertility schedule, is an age specific summary of the reproductive rates in a population.  It is constructed by measuring the reproductive output of a cohort from birth until death.  Vary considerably by species.