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Populations 1 Mr. Shrout 3 Important Characteristics to a Population 1. Geographic Distribution- area inhabited by a population – Cm3- km2 2. Population Density- the number of individuals per unit of area – – – Smaller organisms- higher densities Larger organisms- lower densities Ex. 25.1 deer/mi2 in PA 3. Growth Rate- change in the number of individuals over time – – – Positive= growing Negative= shrinking Zero= constant Distribution Patterns Even Dispersal Random Dispersal Clumped Dispersal Population Size • How can you change the size of a population? – Add: • Births: new individuals are added to the population through reproduction • Immigration: movement of individuals into an area – Animals arrive in new habitat etc.. – Decrease: • Deaths: old individual dies • Emigration: movement of individuals out of a population – Young leave an area as they mature… What determines population growth rate? • Population growth rate – change in population size per unit of time • Births + immigration increase population • Deaths + emigration decrease population • Population growth rate = ((birth+immigration rate) – (death+emigration rate))/ time Growth Patterns • Exponential Growth- individuals reproduce at a constant rate. This occurs under “ideal conditions” with unlimited resources – No predation, unlimited food, unlimited space – Ex. Organisms that reproduce asexually- bacteria – This growth pattern creates a J-shaped curve Exponential Growth • Rememberunlimited resources • Reproducing at maximum rate Growth Patterns • Exponential growth never lasts for very longthe environment cannot support it. • Logistic Growth- populations growth slows or stops as resources become less abundant – How: Births decrease, deaths increase, immigrations decrease, emigrations increase – This is the pattern most often observed Logistic Growth • Creates an S-shaped curve • Carrying Capacity (K): The point around which the growth line becomes flat (growth rate= 0) represents the largest number of individuals that environment can support K is a hard thing to reach As Environment Changes, so does carrying capacity Factors that affect populations • What aspects of the ecosystem can limit the population of a species? • Anything that changes the relationship between births, deaths, immigrations and emigrations will change the overall population size. • A limiting factor = a factor that causes population growth to decrease • Some limiting factors include: – – – – – – Access to resources Competition Predation Parasitism and disease Drought and other climate extremes Human disturbances Limiting Factors • There are 2 main categories of limiting factors: – Density-dependent factors- factor that depends on the size of the population to act – Density-independent factors- affect all populations in similar ways, regardless of population size Density-Dependent Factors • A limiting factor that depends on population size including: – Intra and interspecific Competition – organisms struggle for resources – Predation – predator-prey relationship controls population of both the predator and the prey – Parasitism – control population by weakening/killing host. Less hosts= less parasites as well. – Disease - control population by weakening/killing host • Density dependent factors become limiting only when the population density (number of organisms per unit area) reaches a certain level • These factors operate most strongly when a population is large and dense Competition • As resources become more scarce competition for that resources increases • With greater competition, there is a larger struggle for survival • It becomes more likely that a given individual will not succeed. Intraspecific Competition Predator-Prey Relationship • The number of predators and prey in a system are directly related – – – – Predators increase- Prey decreases Prey decreases- predator decreases Predator decreases- prey increases Prey increases- predator increases Disease Density-Independent Factors • Affect all populations in similar ways, regardless of the population size including: – – – – Unusual weather Natural disasters Seasonal cycles Human activities (damming rivers/clearing forests) • Many species will show a characteristic “crash,” then the population will build back up right away, or stay low for some time • Environments are always changing, and most populations can adapt to a certain amount of change by growing or shrinking in size. • Major upsets in the ecosystem can lead to long-term declines in certain populations (human activities) Density Independent- Extreme Weather Seasonal Human Activities Overexploitation of cod fishery- grand banks