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Chapter 4: Population Biology Population • group of organisms, all of the same species, that live in a specific area How fast do populations grow? • Not linear growth – so it is not a straight line • Graph- starts to increase slowly, then resembles a Jshaped curve • Initial increase slow, b/c # of organisms that reproduce is small • Increases b/c the total # of individuals that can reproduce has increased Is Growth Limited? • J-shaped growth curve illustrates exponential population growth • Exponential Growth – means that as a population get larger, it also grows at a faster rate What Can Limit Growth? Population growth does have limits • Limiting Factors – food, disease, predators, or lack of space will Cause population growth to SLOW • Under these conditions the population may stabilize in a S-shape growth curve Carrying Capacity • the number of organisms of one species that an environment can support indefinitely • Developing population – more births than deaths and the population increases until the carrying capacity is reached or passed • When a population overshoots the carrying capacity, limiting factors come into affect • Deaths begin to exceed births and the population falls below the carrying capacity Reproduction Patterns • Life-History Pattern- organisms reproduction pattern • Slow life-history pattern – slow rate of reproduction and produce relatively few offspring – Ex. Elephant • Fast life-history pattern – reproduce rapidly and produce many offspring in a short period of time – Ex. Mosquito Rapid Life- History Patterns • Common among organisms in changeable or unpredictable environments • Organisms have: – – – – Small body size Mature rapidly Reproduce early Short life span • reproduction rises rapidly then decline when environment becomes unsuitable • population survives and will reproduce again when the environment is favorable Slow Life- History Patterns large species that live in more stable environments – Elephants, Bears, Whales, Humans, and plants (trees) • reproduce and mature slowly • long-lived • maintain population sizes at or near carrying capacity Density Factors and Population Growth 3 Patterns of Dispersal: 1. Random 2. Clumped 3. Uniform Density-Dependent Factors • • • • • disease competition predators parasites food • increasing affect as the population increases • Example- corn fields • Crops grow close together • Disease can spread rapidly and kill off the whole crop Density-Independent Factors can effect most populations regardless of their density • Most are abiotic factors • • • • • • • Volcanic eruptions Temperature Storms Floods Drought Chemical Pesticides Major habitat destruction Usually affect smaller organisms more – Example – Mosquitoes – severe winters kill the adults of most species Population Size • limited by abiotic and biotic factors • controlled by various interactions among organisms that share a community • Predation – can effect population size in minor and major ways • Locusts eat acres of lettuce on a farm • Brown snake introduced into Guam, there were no native predators, and it preyed freely on native birds – drastic effect on the population size • predator- prey relationships are known to experience cycles or changes in their numbers over periods of time • prey population increases and there is more food for the predator, so the predator population increases • predator population increase and predation increase, therefore – the prey population declines. • With less food, the predator population decrease. • Cycle starts again Competition within a Population • density dependent factor • few individuals compete for resources, usually no problems • When population increase and the demand exceeds the supply, the population size decreases Crowding and Stress • populations become crowded, individuals exhibit stress • stress symptoms – – – – Aggression decrease in parental care decreased fertility decreased resistance to disease • all have negative effects on a population Human Population • Demography – study of human population size – density and distribution – movement – Birth and death rates Human Population Growth • Humans change their environment • Developed methods for producing more food • infant mortality rate decreased • clean water • All enable people to live longer and produce more offspring • Population Grows Calculating Growth Rate • Birthrate – number of live births per 1000 population in a given year • Death rate – number of deaths per 1000 population in a given year • Immigration – movement of individuals into a population • Emigration – movement of individuals out of a population Calculate a population’s growth rate by • (Birthrate + Immigration Rate) – (Death rate + Emigration Rate) = Population Growth Rate • PGR = (B + I) – (D + E) • Immigration and emigration are not always accurate use: • PGR = Birthrate – Death rate • Positive Growth Rate – unless the growth rate becomes negative the population continues to grow, just slower • Doubling Time – time needed for a population to double in size • Age Structure – refers to the portions of the population that are in the different age levels