Transcript Populations
Population Dynamics Exponential Growth and Doubling Times Exponential Growth: Growth at a constant rate of increase per unit of time Geometric Growth: Same as exponential growth – Sequence of growth follows a geometric pattern of increase Arithmetic Growth: Growth increases at a constant amount per unit of time 6.4 J curve: Growth curve produced by a constant rate of growth – – Represents theoretical unlimited growth It represents the biotic potential Biotic Potential The maximum reproductive rate of an organism having no limiting factors. If all the individuals in the population survived and reproduced at the maximum rate. It is a reference value allowing one to determine if the observed growth rate is close to the biotic potential. Population Oscillations and Irruptive Growth Dieback: When death rates begin to surpass birth rates Overshoot: The extent to which a population exceeds the carrying capacity of its environment Malthusian (Irruptive) Growth: Pattern of population explosion followed by a population crash Growth to a Stable Population Logistic Growth: Constantly changing rate – When growth slows as populations approach the carrying capacity of the environment Environmental Resistance: Factors that tend to reduce population growth rates 6.7 S curve: Population growth and stabilization in response to environmental resistance Carrying capacity The maximum number of individuals of a certain species that an environment can support. It reflects the limits imposed on population growth by finite space and finite resources. Lifestyle greatly affects carrying capacity It is estimated that 10-15 acres are necessary to sustain one person with an affluent lifestyle To support the 5.6 billion humans at such a lifestyle would require 3 times as much productive land In these terms, the earth can support only 1.8 billion people Environmental resistance can include any of the following. food runs out waste accumulates and becomes toxic living space runs out over-crowding makes the population an easy target for parasites and predators disease predation Calculating % Growth Rates (GR) Population Growth for a Specific Location % GR= (birth +immigration)- (deaths+emmigration) 100 % Global Population Growth Rate % growth rate = (birth rate %-death rate percent) i.e. 6 births/100 people each year 4 deaths/100 people each year this is a rate of increase of 2% Calculating Doubling Time DT = _____70_____ % growth rate 70 is a demographic constant Factors that Increase or Decrease Populations Natality , Fecundity and Fertility Immigration and Emigration Mortality and Survivorship Age Structure Natality, Fecundity, and Fertility Natality: Production of new individuals by birth, hatching, germination, or cloning – Main source of addition to most biological populations Fecundity: Physical ability to reproduce “Potential” Fertility: Measure of actual number of offspring produced “Actual or Realized” Migration Immigration: movement of organisms into a new ecosystem Emigration: movement of organisms out of an ecosystem **Migration is not a factor when determining Global Population Growth Rates Mortality and Survivorship Mortality: Death rate – Determined by dividing number of organisms that die in a certain time period by the number alive at the beginning of the period 6.9 Survivorship: Percentage of a cohort that survives to a certain age Life Span vs Life Expectancy Life Span: Longest period of life reached by a given type of organism 6.10 Life Expectancy: The probable # of years of survival for an individual of a given age Age Structure Population Momentum: Large number of prereproductive individuals – Rapid increase in natality once youngsters reach reproductive age 6.11 Factors That Regulate Population Growth Density-Dependent: Effects are stronger or a higher proportion of the population is affected as population density increases Density-Independent: The effect is the same or a constant proportion of the population is affected regardless of population density More….. Intrinsic: Factors operating within individual organisms or between organisms in the same species Extrinsic: Imposed from outside the population Biotic: Caused by living organisms Abiotic: Caused by nonliving components of the environment Characteristics of contrasting reproductive strategies Extrinsically Controlled Growth Intrinsically Controlled Growth Short Life Long Life Rapid growth Slower growth Early maturity Late maturity Little parental care or protection High parental care and protection Little investment in individual offspring High investment in individual offspring Adapted to unstable environment Adapted to stable environment Pioneers, colonizers Later Stages of succession Niche generalists Niche specialist Prey Predators Regulated mainly by extrinsic factors Regulated mainly by intrinsic factors Low trophic levels High trophic levels Density-Independent Factors Factors that affect natality of mortality independently of populations density abiotic Density-Dependent Factors Factors that reduce population size by decreasing natality or increasing mortality – Tend to be biotic Interspecific Interactions: Two species compete for the same environmental resources in an ecosystem 6.12 – May be beneficial or neutral, such as mutualism Intraspecific Interactions – – Individuals within a population compete for resources Territoriality is an example Stress and Crowding – Stress Shock: A loose set of physical, psychological, and/or behavioral changes thought to result from the stress of excess competition and extreme closeness to other members of the same species