Populations

Populations Are Patchy in Space and Dynamic over Time Population—all the individuals of a species that interact with one another within a given area at a particular time. Humans have long been interested in understanding species abundance: to increase populations of species that provide resources and food and to conserve species for ethical and aesthetic reasons to decrease abundance of crop pests, pathogens, etc.

Populations Are Patchy in Space and Dynamic over Time Population size—total number of individuals in a population Counting all individuals is usually not possible Organisms could always be moving There could be too many of them They could be too widespread

Populations Are Patchy in Space and Dynamic over Time In order to overcome these population counting problems Ecologists often Measure density, then multiply by the area occupied by the population to get population size. Sample an area and take an average Population = Total Marked X Total in Second Sample ________________________________________ Total Marked in Second Sample

Populations Are Patchy in Space and Dynamic over Time Population density Number of individuals per unit of area or volume How crowded a population is Can Increase the population density by: Increasing the amount of organisms Decreasing the area

Populations Are Patchy in Space and Dynamic over Time Population densities are dynamic—they change over time. Density of one species population may be related to density of other species populations.

Births Increase and Deaths Decrease Population Size Demography: The study of Population Dynamics Growth Rate (r): Amount by which a population’s size changes in a given amount of time APPLY THE CONCEPT Births increase and deaths decrease population size

Births Increase and Deaths Decrease Population Size Change in population size depends on the number of births and deaths over a given time. “Birth–death” or BD model of population change: Nt+1- Number of individuals in a population sometime in the future Nt- Number of individuals now in a population B- Number that are born D- Number that die APPLY THE CONCEPT Births increase and deaths decrease population size

Births Increase and Deaths Decrease Population Size If b > d, the population grows. If b < d, the population shrinks. If b = d, the population size does not change.

Extinction and Recolonization Affect Population Dynamics The BIDE model of popultion growth adds the number of immigrants (I) and emigrants (E) to the BD growth model.

Life Histories Determine Population Growth Rates Individual organisms require resources (materials and energy) and physical conditions they can tolerate. Rate at which an organism can acquire resources increases with the availability of the resources.

Populations Grow Multiplicatively, but Not for Long Population growth is multiplicative (exponential)—an ever-larger number of individuals is added in each successive time period. In additive growth, a constant number (rather than a constant multiple) is added in each time period.

Populations Grow Multiplicatively, but Not for Long Multiplicative growth has a constant doubling time. The time it takes a population to double in size can be calculated if r is known. APPLY THE CONCEPT Multiplicatively growing populations have a constant doubling time

Populations Grow Multiplicatively, but Not for Long Exponential Growth Populations do not grow multiplicatively for very long. Logistic Growth Growth slows and reaches a more or less steady size

Populations Grow Multiplicatively, but Not for Long When r = 0, the population size stops changing—it reaches an equilibrium size called carrying capacity, or K.

Populations Grow Multiplicatively, but Not for Long Density Independent Factors Reduces the population no matter what size it is Weather Floods Fires Hurricanes

Populations Grow Multiplicatively, but Not for Long Density Dependent Factors Reduction in a population which is triggered by an increase in density As the population grows and becomes more crowded, birth rates tend to decrease and death rates tend to increase. Disease Resource limitation Predation

Populations Grow Multiplicatively, but Not for Long The human population is unique. It has grown at an ever-faster per capita rate, as indicated by steadily decreasing doubling times. Technological advances have raised carrying capacity by increasing food production and improving health.

Populations Grow Multiplicatively, but Not for Long Many believe that the human population has now overshot its carrying capacity for two reasons: Technological advances and agriculture have depended on fossil fuels, a finite resource. Climate change and ecosystem degradation have been a consequence of 20th century population expansion.

Populations Grow Multiplicatively, but Not for Long If the human population has indeed exceeded carrying capacity, ultimately it will decrease. We can bring this about voluntarily if we continue to reduce per capita birth rate or decrease life expectancy LINK Table 32.1 Voluntary reduction of birth rates can be achieved by the use of contraceptive methods