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Population Growth Curves and Mechanisms Species and Population • Organism • Species: genetically similar organisms that reproduce • Population: all members of a species in an area Characteristics of populations: Same species living in same area Age structure: –Prereproductive –Reproductive –Postreproductive •What limits population growth? Biotic Potential vs. Environmental Resistance Biotic Potential an organism’s maximum reproductive rate, provided: • unlimited resources are available (food, water, shelter, etc.) • freedom from inhibiting factors (no predators or disease, etc.) Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Table 6.2 •Biotic potential – capacity for growth •Intrinsic rate of increase (r) •Environmental Resistance – Biotic and Abiotic factors limiting population size •Recruitment - ability to survive environmental resistance and become part of the breeding population •Reproductive strategies •High biotic potential and low recruitment •Low biotic potential and high recruitment r - strategists K – strategists • Transparency R strategists: • Many small offspring • Little /no parental care • Early reproductive age • Most offspring die • Small adults • High growth rate (r) • Population fluctuates wildly above and below carrying capacity (K) • Generalist niche • Low ability to compete • Early successional species K strategists: • Fewer, larger offspring • High parental care • Later reproductive age • Larger adults • Lower population growth rate (r) • Population size fairly stable and close to carrying capacity (K) • Specialist niche • High ability to compete • Late successional species •Growth Curve - Result of biotic potential and environmental resistance. •Exponential growth (J-curve) before environmental resistance factors kick in to reduce population size • Population explosion •Logistic growth (S - curve) Balance between environmental resistance and biotic potential •Carrying Capacity The maximum population a habitat can support without being degraded over the long term. Population Size Carrying Capacity -maximum # individuals of a given species that the environment can support Time Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 6.4 Carrying Capacity J-curve -maximum # individuals of a given species that the environment can support Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 6.4 •Critical number – minimum population required to provide protection and support for members of population Calculating Fertility Rates and Doubling Times (CBR - CDR)/10 = Rate of Increase in percentage To get r (intrinsic growth rate), divide by 100 70/ Rate of Increase = Doubling Time •A population had a growth rate of 1.7 in 2000. In what year would the population be double its current population? •70 / 1.7 = 41 years •2000 + 41 = 2041 Calculating Fertility Rates and Doubling Times: Practice Country CBR CDR Kenya 46 7 Mexico 29 6 USA 17 9 Denmark 12 12 Rate of Increase Doubling Time •Threatened – species whose populations are declining rapidly •Endangered – population is near the critical number Mechanisms of population equilibrium: •Predator – prey dynamics Snowshoe hare Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 6.9 Predator-prey Balance:Wolves and Moose Isle Royale Isle Royale • As Wolves decreased Moose increased. • As Wolves increased the Moose decreased. • Wolves kept the Moose population stabilized until canine parvovirus probably killed off large #’s of Wolves. • Moose prospered until overgrazing of Ash and Aspen caused mass death. Plant Herbivore dynamics: –Herbivores are the natural enemies of plants –Overgrazing Mechanisms of Population Equilibrium: Plant-Herbivore Introduced Species: Rabbits in Australia • Introduced into Australia from England in 1859 • No natural enemies – rabbit population exploded • Overabundant herbivore population devastated natural vegetation (see Fig. 4-11 in text). • Using disease as control measure – why will this procedure fail in the long term? •Eliminating predators upsets basic plant – herbivore relationships. •Competition –Interspecific –Intraspecific • Density dependent factors affecting population growth: –Increasing effect as population increases –Examples •Competition •Disease •Parasites •Stress due to crowding • Density independent factors controlling population: –affect all populations, regardless of density –Most are abiotic •Forest fires •Floods •