Population Ecology Chapter 45

Overview: Earth’s Fluctuating Populations To understand human population growth, we must consider general principles of population ecology

Population ecology = study of populations relative to environment, including environmental influences on density and distribution, age structure, and population size Population = group of individuals of a single species living in the same general area

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 Environmental and social factors influence spacing of individuals in a population

Clumped distribution may be influenced by resource availability (living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young) Uniform distribution may be influenced by social interactions such as territoriality Random distribution—the position of each individual is independent of other individuals

Demography Study of theory and statistics behind population growth and decline N = size of the population

Demographic Statistics Birth rate = number of offspring produced per time period Death rate = number of deaths per time period Sex ratio = proportion of males and females in a population Generation time = time needed for individuals to reach reproductive maturity

Demographic Statistics Age structure = statistic that compares the relative number of individuals in the population from each age group Immigration rate = rate at which individuals relocate into a given population Emigration rate = rate which individuals relocate out of a give population

Population Growth Growth rate of a population is the amount by which a population’s size changes in a given time.

Population Growth and Size Biotic potential = maximum growth rate of a population given unlimited resources, unlimited space, and lack of competition or predators Rate varies from species to species Carrying capacity (k) = maximum number of individuals that a population can sustain in a given environment without destroying the habitat

Limiting Factors Control Population Sizes Density-dependent factors come into play when population approaches and/or passes the carrying capacity Food supplies, waste products, population-crowding diseases

Limiting Factors Control Population Sizes Density-independent factors have nothing to do with the population size Floods, droughts, earthquakes, other natural disasters and weather conditions

Population Growth Exponential Growth = population grows as if there are no limitations as to how large it can get (biotic potential) A population increases slowly at first (the "lag phase") and then grows increasingly rapidly as time passes (the "log phase"). When numbers are low, a doubling does not produce much addition to the population, but as numbers increase, each successive doubling adds larger and larger increments.

Population Growth Logistic Growth = population growth slows to zero and population size tends to stabilize because of environmental resistance (limiting factors)

Point of Maximum growth (K/2)

Exponential Growth Rate Equation Determining Population Growth per unit time (G): where: G = Population Growth per unit time r = net population growth rate per individual per unit time N = number of individuals

Logistic Growth Rate Equation The S-shaped (sigmoid) curve that shows the effect of environmental resistance upon population growth can be represented by the following equation, often referred to as the logistic equation: where K is the carrying capacity (maximum value of N for a given set of environmental conditions)

Carrying Capacity Number of generations Population size (N) K = 1,500 2,000 1,000 500 15 10 5 Logistic growth Exponential growth = 1.0N dN dt 1,500 – N

Survivorship Patterns Survivorship is the percentage of an original population that survives to a given age Involves study of cohort population – individuals of same age Species differ with respect to mortality patterns Extreme forms: K versus r

Survivorship curves Type I Mortality rises in postreproductive years Type II Mortality constant throughout life Type III Mortality low after establishment

Life History Strategies K-selected populations are of a roughly constant size whose members have low reproductive rates. Offspring require extensive postnatal care until sufficiently matured (humans) R-selected populations experience rapid growth Offspring are numerous, mature rapidly, and require little postnatal care (bacteria)