POPULATION

Dynamics of Natural Population Populations of most species remain more or less constant. Population Equilibrium: deaths = births

Dynamics of Natural Population Population Growth Curves Exponential increase results in Population Explosion J-shaped curve (only happens when a disturbance occurs—foreign species, habitat change, removal of predator) Growth is independent of population density Also called geometric growth

Dynamics of Natural Population Doubling Time How long it takes to double a population Doubling time (years) = 70/annual % growth Annual % growth = (BR-DR)/10 Example BR = 20 DR = 5 Annual % growth = (20-5)/10 = 1.5 Doubling time = 70/1.5 = 47 years

Dynamics of Natural Population When a population exceeds the carrying capacity a population crash or dieback occurs Overshoot: extent to which a population exceeds the carrying capacity This pattern of population explosion then crash is called irruptive or Malthusian growth

Dynamics of Natural Population Population Growth Curves Logistic S-shaped curve (population levels off and continues in a dynamic equilibrium) Growth is not independent of population density A more stable population

BIOTIC POTENTIAL Number of offspring that a species may produce Recruitment: survival through earthy growth stages to become part of the breeding population

REPRODUCTIVE STRATEGIES Malthusian “Strategies” Produce massive numbers of young, but leave survival to nature High biotic potential, but low recruitment (high mortality) so population may not increase Examples: insects, rodents, marine invertebrates, parasites and “weeds” R-strategists

REPRODUCTIVE STRATEGIES Logistic “Strategies” Produce less young, but care for them until they can compete for resources themselves Usually are larger, live longer, mature more slowly and have fewer natural predators Examples: wolves, elephants, whales and primates K-strategists

Factors that Increase or Decrease Populations Natality: number of individuals added through reproduction Fecundity: physical ability to reproduce Fertility: measure of the actual number of offspring produced Immigration: organisms introduced into a new ecosystem

Factors that Increase or Decrease Populations Mortality: or death rate Number organisms that die in time period divided by number alive at beginning of period Survivorship: percentage of a cohort that survives to a certain age Life Expectancy: probable number of years of survival for an individual of a given age Emigration: members leave a population

Life Expectancy In United States 1900—47.3 years 2004—77.6 years Often varies with sex and race Difference due to genetics, access to medical care, occupations, diet and behavior Life Span: longest period of life reached by a given type of organism

Survivorship Curves Late Loss: K-strategists that produce few young and care for them until they reach reproductive age thus reducing juvenile mortality Constant Loss: typically intermediate reproductive strategies with fairly constant mortality throughout all age classes Early Loss: r-strategists with many offspring, high infant mortality and high survivorship once a certain size and age Can also have early and late loss with high survival during reproductive years

Factors that Regulate Population Growth Various factors regulate population growth, usually by affecting natality or mortality Intrinsic: operating within individual organisms or between organisms in the same species Extrinsic: imposed from outside the population

Factors that Regulate Population Growth Biotic Factors: Ability of animals to migrate or if seeds to disperse, to similar habitats Ability to adapt to and invade new habitats Defense mechanisms Resistance to adverse conditions and disease Abiotic Factors: Light, temperature, nutrients, etc.

Environmental Resistance Combination of all the biotic and abiotic factors that may limit a population’s increase Replacement Level: when recruitment (young) = adults that have died

Carrying Capacity The maximum population a habitat can support (sustainable) If population greatly exceeds carrying capacity, it will undergo a J-curve crash Population reaches max and then starts to decline because of lack of resources

Density Dependence/Critical # Population Density: number of individuals per unit of area Density Dependent: factors that change based on population density Population increases, environmental resistance becomes more intense Density Independent: factors not affected by population Abiotic factors

Factors that Regulate Population Growth Density-Dependent Factors: Most are results of interactions between populations of a community (predation) but some are within a population 1)Interspecific Interactions -predation helps prevent population overshoot -predator/prey populations often oscillate known as Lotka-Volterra model -mutualism and commensalism

Factors that Regulate Population Growth 2) Intraspecific Interactions -population of a species can increase to carrying capacity but then start to compete with each other for resources -territoriality (usually between members of the same species)

Factors that Regulate Population Growth 3) Stress and Crowding -when population densities get high, organisms often exhibit symptoms of what is called stress shock or stress-related diseases -physical, psychological and/or behavioral changes may occur if too much competition and too close proximity to other members of the same species

Factors that Regulate Population Growth Density-Independent Factors: Tend to be abiotic components Examples: weather or climate Abiotic factors can be beneficial—rainfall in deserts, fires in grasslands These factors operate without regard to the number of organisms involved

Density Dependence/Critical # Populations may not recover from low #s Critical Number: minimum number of individuals a species needs to stay a healthy population If population goes below this it will probably become extinct.

Mechanisms of Population Equilibrium Predator-prey Dynamics Herbivores and predators, Parasites, Plant-herbivore dynamics and Keystone species Competition Intraspecific (same species) and Interspecific (different species) Introduced Species

Island Biogeography Diversity in isolated habitats is a balance between colonization and extinction rates Smaller islands farther from mainland have less colonization and fewer individuals of a species Larger islands closer to mainland tend to have greater diversity

Conservation Genetics Hardy-Weinberg equilibrium In large populations, if mating is random, no mutations occur and there is no gene in-flow or selective pressure for or against particular traits, random distribution of gene types will occur Different alleles will be distributed in the offspring in the same ratio they occur in the parents and genetic diversity is preserved

Conservation Genetics In small, isolated populations immigration, mortality, mutations or chance mating events involving only a few individuals can greatly alter the genetic makeup of the population Gradual changes in gene frequencies due to random events is called genetic drift

Conservation Genetics Founder Effect or Demographic Bottleneck occurs when just a few members of a species survive a catastrophic event or colonize new habitat geographically isolated from other members of the same species

Population Viability Analysis Minimum viable population size number of individuals needed for long-term survival of rare and endangered species

Metapopulations A collection of populations that have regular or intermittent gene flow between geographically separate units Some species exist in several distinct habitats and individuals occasionally move among these, mating with existing animals or recolonizing empty habitats