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