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Populations Biology Describing Populations Researchers study populations’ geographic range, density and distribution, growth rate, and age structure. Describing Populations Population density refers to the number of individuals per unit area. Describing Populations A population’s age structure refers to the number of males and females of each age a population contains. Why would knowing this information be important? Population Growth The factors that can affect population size are the birthrate, death rate, and the rate at which individuals enter or leave the population. Population growth A population will grow if individuals move into its range from elsewhere. This process is called…… Immigration A population may decrease in size if individuals move out of the population’s range. This process is called…… Emigration Exponential Growth In exponential growth, the larger a population gets, the faster it grows. Under ideal conditions, with unlimited resources, a population will grow exponentially. Examples? Human Population Growth is Exponential Logistic Growth Logistic growth occurs when a population’s growth slows and then stops, following a period of exponential growth. Carrying Capacity Carrying capacity is the maximum number of individuals of a particular species that a particular environment can support. Limits to Growth What determines an ecosystems carrying capacity? Limiting factors are factors that control the growth of a population Density-Dependent Limiting Factors Density-dependent limiting factors operate strongly only when population density-the number of organisms per unit area – reaches a certain level. Density-dependent factors include: Competition Predation Herbivory Parasitism Disease Stress from overcrowding Competition The more individuals living in an area, the sooner they use up the available resources. Space and food can be related to one another as many grazing animals compete for territories in which to breed and raise offspring. Predation and Herbivory Predator – Prey Relationships Herbivore Effects Humans as Predators Parasitism and Disease Parasites and disease causing organisms feed at the expense of their hosts, weakening them and often causing disease or death. Stress from Overcrowding Some species fight amongst themselves if overcrowded. Too much fighting can cause high levels of stress which can weaken the bodies ability to resist disease. Density-Independent Limiting Factors Density independent limiting factors affect all populations in similar ways, regardless of population size and density. Unusual weather such as hurricanes, droughts, or floods, and natural disasters such as wildfires, can act as density independent limiting factors.
