Why are there so many people?! Population Growth Why are there so many people?!

Changes in Population Size Four Factors INCREASE POPULATION SIZE Immigration: Movement of individuals INTO a population from another population Births DECREASE POPULATION SIZE Emigration: Movement of individuals OUT of a population and into another population Deaths

Population Growth Equation B = Births D = Deaths N = population size r = growth rate Change in population size = births – deaths = B – D Can also be expressed as per capita (average per individual) r = (births – deaths) / population size r = (B – D) / N

Practice! If a population size of 1000 individuals had 60 births and 10 deaths over a one year period, what is the population growth rate? If a population has a per capita birth rate of .5, what would be the number of births expected for a population of 300? r = (60 – 10)/1000 = 5/100 = .05 B = .5 x 300 = 150 individuals

Exponential Growth (J shaped) All population members have unlimited resources and are free to reproduce population increases dramatically over a period of time Characteristic of some populations that are introduced to new or unfilled environments or whose numbers were reduced by a catastrophic event and are rebounding

Logistic Growth (S shaped) Population begins with a brief period of slow growth followed by a brief period of exponential growth before leveling off at a stable size. Carrying capacity (K): The maximum stable population size that a particular environment can support When N is small: growth is approximately exponential As N reaches K growth slows When N = K, growth = 0

Review… Exponential Vs. Logistic Growth and Carrying Capacity What is the carrying capacity for this population? 1,500 individuals

Does logistic growth really happen? In some cases… Yes! Other times…not exactly…

Life History Traits K-selection: organisms live and reproduce around K, and are sensitive to population density. Population size remains relatively constant Produce a small number of relatively large offspring that require extensive parental care Example: elephants r-selection: organisms exhibit high rates of reproduction and occur in variable environments in which population densities fluctuate well below K. Include opportunistic species such as grass, insects Invade a habitat, quickly reproduce, and then die. Offspring are small, mature quickly and require little, if any parental care.

Density Dependent- Limiting Factors Density-dependent factors increase their affect on a population as population density increases. This is a type of negative feedback. Examples: Limited resources Competition Predation/ Territoriality Waste/ disease

Density Independent- Limiting Factors Density-independent factors: are unrelated to population density, and there is no feedback to slow population growth. Examples: Unusual weather Natural disasters Human Activities

More about density-dependent limiting factors… 1. Resource Limitation Resource limitation in crowded populations can stop population growth by reducing reproduction.

2. Intraspecific competition for food can also cause density-dependent behavior of populations. 3. Territoriality, defense of a space, may set a limit on density. 4. Predation may also be a factor because it can cause mortality of prey species.

5. Waste accumulation is another component that can regulate population size. In wine, as yeast populations increase, they make more alcohol during fermentation. However, at an alcohol percentage of approximately 13% they begin to die. 6. Disease can also regulate population growth, because it spreads more rapidly in dense populations.

Population dynamics reflect a complex interaction of biotic and abiotic influences Carrying capacity can vary.

Some populations have regular boom-and-bust cycles There are populations that fluctuate greatly. A good example involves the lynx and snowshoe hare that cycle on a ten year basis.

What about humans? Is this exponential or logistic growth? The human population has been growing almost exponentially for three centuries but cannot do so indefinitely!

Age Structure The relative number of individuals of each age. Diagrams can reveal a population’s growth trends, and can point to future social conditions.

Earth’s Carrying Capacity Predictions of the human population vary from 7.3 to 10.7 billion people by the year 2050. Will the earth be overpopulated by this time?!