Characteristics of Populations Three important characteristics of a population are its: geographic distribution population density growth rate Population Growth Three factors can affect population size: the number of births the number of deaths the number of individuals that enter or leave the population A population can grow when its birthrate is greater than its death rate. Immigration, the movement of individuals into an area Emigration, the movement of individuals out of an area Under perfect conditions with unlimited resources, a population will grow exponentially. Copyright Pearson Prentice Hall Exponential Growth As resources become less available (no more space, food, water, etc.), the growth of a population slows or stops. Called carrying capacity Copyright Pearson Prentice Hall Are humans experiencing exponential or logistic growth? Do you think humans are close to the Earths carrying capacity? Why? Copyright Pearson Prentice Hall In the context of populations, a limiting factor is a factor that causes population growth to decrease. Can be: Density-dependent Density-independent Density-Dependent Factors A limiting factor that depends on population size is called a density-dependent limiting factor. Density-dependent limiting factors include: competition predation parasitism Disease Most impact with larger populations Competition When populations become crowded, organisms compete for food, water space, sunlight and other essentials. Competition can also occur: between members of same species (intraspecific) between members of different species (interspecific) IntraspecificInterspecific Predation Populations in nature are often controlled by predation. Or Predator Prey Relationship Copyright Pearson Prentice Hall Wolf and Moose Populations on Isle Royale Parasitism and Disease Parasites can limit the growth of a population. zombie_roaches_and_other_tales_of_parasiteszombie_roaches_and_other_tales_of_parasites Density-Independent Factors Density-independent limiting factors affect all populations in similar ways, regardless of the population size. unusual weather natural disasters seasonal cycles certain human activitiessuch as damming rivers and clear-cutting forests