Ecology - the study of the relations between organisms and theirenvironment (both biological and physical)

Population Ecology - the study of the factors that influence thenumbers and distribution of a species

population - a group of individuals of the same species livingin an area

Some species havelimiteddistributions

Population sizehas an importantinfluence onspecies persistence

Populations often change distributions through time due toclimate change

Some species have increased their range due to chanceintroductions

Population Dispersion

Random distributions result whenindividuals of a species do not interactwith each other positively ornegatively - rare in nature

Uniform distributions result fromcompetition for resources orbehavioral interactions liketerritoriality

Clumped distributions often resultfrom an uneven distribution ofrequired resources or socialinteractions like herding

Metapopulations - distinct populations that interact with each otherthrough dispersal - allow greater overall numbers and persistentrefuges that can be a source of species recolonization of an areaafter local extinction

With increasing human impact onhabitats and the range of manyspecies - the study of thedynamics of metapopulations hasimportant implications for thelong-term conservation of somespecies

Demography - the study of population parameters and statisticsCan help to predict population changePresent distribution of ages, distribution of sexes, survival rates,birth rates all have effects on the characteristics of the populationin the future

Factors that influence population changesex ratio - birth rate is most closely related to number of

females in the populationgeneration time - the average amount of time that it takes for

a female to give birth to another female - shorter generationtimes can lead to faster population growth

age structure - the number of individuals in each age class -number of individuals of reproductive age can influencepopulation growth

Populations maintain stable numbers when the number of birthsin each time period is matched by the number of deaths

Survivorship - the proportion of newborn individuals thatsurvive to a given age - if 50% survive to age 10, then age 10survivorship is 0.5

Survivorship curve - showschanges in mortality ratethrough the life of a groupof individuals

Type I - most mortalitylate in life

Type II - uniformmortality throughout life

Type III - most mortalityearly in life

Reproduction has a price - reduced survival and/or reduced futurereproduction - “Cost of reproduction”

There is a trade-off between the number of offspring producedand the investment in each offspring

Investment each offspring receives influences its chance ofsurvival - large clutches can result in few surviving offspring,small clutches also have few survivors, intermediate sizeclutches may produce the greatest number of survivors - the“optimal clutch size”

Number of reproductive attempts per lifetime

Iteroparous - having many attempts at reproduction - requireslow expenditure each time in order to ensure parental survival

Semelparous - having a single reproduction - maximalreproductive expenditure - “big bang” reproduction - seen wherechance of parental survival is low regardless of expenditure -annual plants in the desert, Pacific salmon

Age at first reproduction is earlier in species that have lowsurvival rates - high survival rates often allow species to delayreproduction to gain experience and resources

The rate of population growth and the size of a population islimited by the environment

The potential growth of populations is large - but most populationsmaintain relatively stable numbers

Two models of population growthExponential growth - the rate of change in population size is

proportional to the number of individuals in the population and the intrinsic rate of increase

Logistic growth - the rate of change in the population is negatively impacted by an increase in the number of individuals in the population - density dependent growth

Exponential growth - the rate of change in population size isproportional to the number of individuals in the population andintrinsic rate of increase

dN/dt = rN

dN/dt - change in numbers per unit timeN - population sizer - intrinsic rate of increaser = birth rate - death rate

Logistic growth - the rate of change in the population is negativelyimpacted by the number of individuals in the population - densitydependent growth

dN/dt = rN(K-N)/K

dN/dt - change in numbers per unit timeN - population sizer - intrinsic rate of increaser = birth rate - death rateK - the carrying capacity

K - the number ofindividuals that a givenarea can supportindefinitely

when N=K, dN/dt = 0

Examples of Logistic Growth

Logistic growth is also called“density dependent” growth

High density populations havefewer resources availablewhich leads to higher mortalityand lower birth rates

high mortality and lower birthrates reduce dN/dt as thepopulation approaches itscarrying capacity

The growth rate of many populations is influenced byenvironmental factors that act independent of density - floods,freezes, storms, droughts, and other disasters - environmentalcontrols of population size often results in population “crashes.”

The effect of unchecked population growth

Other factors can lead to population cycles

Prey numbers can vary because of predator density

Predator numbers can vary because of prey density

Populations that are routinely grow and decline due to densityindependent factors and those that grow in a density dependentfashion have very different adaptations that promote growth andreproduction under those conditions

Populations that routinely grow in a density independent fashionhave adaptations that favor their ability to reproduce rapidly insize before the next population disaster. Such populations arecalled “r-selected” because they have characteristics that resultin a high instrinsic rate of increase.

Populations that routinely grow in a density dependent fashionhave adaptations that favor their ability to survive and reproducewhen the population is at its carrying capacity. Such populationsare called “K-selected” because they have characteristics thatallow them to live in situations where resources are limited - attheir carrying capacity.

Adaptations to density independent and density dependent growth

The human species exhibits many K-selected traits - likely due anancestry of species that lived at or near K - limited by foodavailability, predators, and disease

Technological advances havereduced death rates and increasedbirth rates

r is currently about .014 - a smallnumber, but large N produces alarge dN/dt

Agriculture and technology haveincreased K but recent estimatesplace K at about 6,000,000,000 -about the current population size

In many developing countries the rate of growth ( “r” ) is largertoday than 50 years ago even though birth rates have fallen,because death rates have declined more

Most growth in the human population is occurring in developingcountries - and this trend is expected to increase

The age structure of the human population in developingcountries results in a greater proportion of the population ofchild-bearing age, and a greater proportion that will soon reachreproductive age

The population of the U.S. represents 4% of the total humanpopulation - and we consume 25% of the world’s resources