AP Biology Notes Outline Chapter 53: Population Ecology

CONCEPT 1: Population Ecology

Population ecology is the study of populations in relation to the environment. It includes environmental influences on population density and distribution, age structure, and variations in population size. A population is a group of individuals of the same species living in the same general area

CONCPET 2: Density and Dispersion

Density is the number of individuals per unit area or volume. Dispersion is the pattern of spacing among individuals within the boundaries of the population. Population density results from interplay of processes that add individuals and those that remove them from the population. Immigration and birth add individuals whereas death and emigration remove individuals.

Environmental and social factors influence the spacing of individuals in a population. The three patterns of dispersion are:

1. Clumped – individuals aggregate in patches as a result of cooperativity or resource dispersion

2. Uniform – individuals are evenly distributed as a result of social interactions such as territoriality

3. Random – position of each individual is independent of other individuals

AP Biology Notes Outline Chapter 53: Population Ecology

CONCPET 3: Demography

Demography is the study of the vital statistics of a population and how they change over time. Death rates and birth rates are of particular interest to demographers

A life table is an age-specific summary of the survival pattern of a population. Data in a life table can be represented graphically by a survival curve. Curve usually based on a standardized population of 1000 individuals and the X-axis scale is logarithmic.

• Type 1 – high survivorship until old age...typical of animals that produce few young but care for them well.

• Type 2 – constant proportion of individuals die at each age...fairly steady death rate throughout life (predation).

• Type 3 – experience high mortality as larvae but decreased mortality later in life...typical of individuals that produce large numbers of offspring but do not provide parental care.

Some species exhibit semelparous, or “big-bang” reproduction. These species reproduce once and die (bamboo, salmon, century plant). Semelparous reproduction is often an adaptation to erratic climatic conditions. Suitable breeding conditions occur rarely and organisms devote all their resources to reproduction when conditions are good (e.g. century plant). Also occurs when an organism’s chances of reproducing again are so low that it is better to commit all resources to a single bout of reproduction (e.g. Salmon).

Some species exhibit iteroparous, or repeated reproduction and produce offspring repeatedly over time (i.e. humans, cats, birds). Iteroparous reproduction occurs when organisms have good prospects of reproducing in the future (i.e., they are long-lived). This is characteristic of larger organisms and those that experience more stable environmental conditions.

Quantity v. Quality of Offspring:Organisms face tradeoffs between the number and quality of young they can produce because they have only a limited quantity of resources to invest. The choice is basically between a few large or many small offspring. Dandelions and coconuts produce dramatically different sized seeds. Salmon produce hundreds to thousands of eggs whereas albatrosses produce only one egg every 2 years.

AP Biology Notes Outline Chapter 53: Population Ecology

CONCEPT 4: Population Growth

Population growth occurs when birth rate exceeds death rate. The equation for population growth is

ΔN/Δt = bN-dN• N = population size• b is per capita birth rate• d is per capita death rate. • ΔN/Δt is change in population N over a small time period t.

The per capita rate of population increase is symbolized by r.• r = b-d.

r indicates whether a population is growing (r >0) or declining (r<0). Ecologists express instantaneous population growth using calculus. Zero population growth occurs when the birth rate equals the death rate r = 0. The population growth equation can be expressed as

Exponential Population Growth describes population growth in an idealized, unlimited environment.Results in a J-shaped curve. Exponential growth cannot be sustained for long in any population. A more realistic population model limits growth by incorporating carrying capacity.

AP Biology Notes Outline Chapter 53: Population Ecology

Carrying Capacity (K) is the maximum population size the environment can support. In the logistic population growth model the per capita rate of increase declines as carrying capacity is approached. We construct the logistic model by starting with the exponential model and adding an expression that reduces the per capita rate of increase as N increases. The logistic growth equation includes K, the carrying capacity (number of organisms environment can support). Logistic model produces a sigmoid (S-shaped) population growth curve.

AP Biology Notes Outline Chapter 53: Population Ecology

CONCEPT 5: K and r Reproductive Strategies

K -selection , or density-dependent selection, selects for life history traits that are sensitive to population density.Produce relative FEW offspring that have a GOOD chance of survival. Parental care is high in k-selected species, and

r -selection , or density-independent selection, selects for life history traits that maximize reproduction. High reproductive rate is the chief determinant of life history. Many offspring that receive little or no parental care characterize this reproductive strategy.

CONCEPT 6: Limits to Population Growth

Limiting Factors are any factors that cause population growth to decrease.• Density Dependent factors – include disease, competition, parasites and food. These have an

increasing effect as the population increases.• Density Independent factors – affect all populations regardless of their density (numbers). Most are

abiotic factors such as temperature, storms, floods, droughts and habitat destruction.

Populations are regulated by a complex interaction of biotic and abiotic influences:• In density-independent populations birth rate and death rate do not change with population density. For

example, in dune fescue grass environmental conditions kill a similar proportion of individuals regardless of density.

• In contrast in density-dependent populations birth rates fall and death rates rise with population density. Density-dependent population regulation much more common than density- independent

Organisms in a population constantly compete for resources. When numbers are low, resources are plentiful, but when overcrowding occurs populations compete. Competition is density dependent, overcrowding can lead to increased aggression, decreased fertility, decrease in parental care, and decrease in ability to fight disease.

• Interspecific is between organisms of different species but in same habitat• Intraspecific is between organisms of same species in same population in same habitat

AP Biology Notes Outline Chapter 53: Population Ecology

CONCEPT 7: Demography and Human Population Growth

Demography is the study of human population growth characteristics. It looks at growth rate, age structure, and geographic distribution. Using demography, we can tell if population is growing by looking at the difference between the birth rate and the death rate.

In US, death rate is declining, life expectancy is increasing, and fertility rate is decreasing. The human population rate increased slowly until about 1650, then began to grow exponentially. Some argue that we have exceeded our carrying capacity. Approximately 33% of the human population died as a result of the plague around 1300.

The global human population has grown almost continuously throughout history, but it skyrocketed after the Industrial Revolution. The rate has slowed in recent decades, mainly as a result of decreased birth rates throughout the world.

Age structure diagram- a population profile that graphs the numbers of people in different age groups in the population. Age structure diagrams not only predict a population’s growth trends, but they can also illuminate social conditions.

AP Biology Notes Outline Chapter 53: Population Ecology