Population Ecology

Population Ecology

Population Dynamics • Theoretically, if reproduction and mortality rates in a non-mobile population are equal and constant, the number of individuals in the population would remain constant • Natural population are not static • Constantly subject to change and motion because of many variable factors both in the environment and within the organisms themselves

Population Ecology • Study of distribution, density, numbers of individuals and structure(gender, age), rates of birth and mortality, factors that affect growth • Density – number of individuals per unit area (ex. Per acre or hectare) or unit volume (ex. In a column of water)

Population Ecology • Composition – number of individuals, gender and age • Changes result of different factors • Reproduction, invasion, emigration, migration, mortality, and cyclic fluctuations of considerably greater length and magnitude involving several years or more

Obtaining Population Information • Direct data on most population numbers is difficult or impossible to obtain • Rarely able to count the entire population • Count all the individuals in a prescribed area

Techniques of Obtaining Populations • Simple counts • # seals/island, #burrows/area, # wildebeest/herd • Can use aerial photographs to obtain population estimates • Seals and sea lions • Wintering waterfowl and marine birds

Techniques of Obtaining Populations 2) Mark-recapture technique • Capture and mark individuals • Trapping, marking, ID tags, radio transmitters • Recapture at a later point in time • Provide estimate of population size for a given area • Calculation = (total number marked)(total number recaptured)/(number of recapture that were marked)

Techniques of Obtaining Populations • Mark-recapture technique • Example • Initial capture of 50 individuals • Second capture of 100 individuals, 10 of the 100 were marked from the first capture • Estimated population size = 50*100/10 = 500 individuals

Techniques of Obtaining Populations 3) Census techniques • Transect methods • Walk or drive a line (transect) and count the number of individuals at specific locations, evenly distributed along the line • Used for pheasant counts

Distribution • Distribution – way species are organized in an area • Can be due to abiotic factors (rocks, water, the environment) or biotic (species interactions, plants, food sources) • Can look at an individual species or the assemblages of species – description at the community level

Distribution • Type 1 – Uniform or regular or nearly uniform • Possible explanations • Territorial species • Dispersed resources • Telephone poles used as perching sites for birds • Behavioral interactions

Distribution • Type 2 – clumped distribution • Possible explanations • Patchy distribution of resources • Organisms live in groups or close together

Distribution • Type 3 – Random distribution • Possible explanations • Random distribution of resources • Absence of strong attractions or repulsions among individuals of a population • Very uncommon

Demographics • Characteristics of a population that affect growth • Two characteristics that are important • Age structure • Sex ratio • In human population of characteristics are considered • Race, education, marital status, religious beliefs, etc.

Age Structure • Methods – follow a group of individuals from birth to death over time • Construct a life table for the group

Age Structure

Sex Ratio • Rate at which a population may grow can be dependent on the sex ratio • Fewer females – slower rate of population growth • Sex Ratios by age (males per 1000 females

Calculate Rates for Populations • 3 Rates that are looked at • Survivorship – number of individuals that reach the next year of life • Birth – number of individuals born within a designated time frame • Mortality – number of individuals that die each year

Survivorship • Number of survivors/age group • Probability of newborn individuals of a group surviving to particular ages • Yields 3 different curves

Survivorship • Type 1 – high survivorship for most age groups except older individuals • Examples – humans, large mammals, organisms that produce few offspring but provide extensive parental care

Survivorship • Type 2 – constant survivorship rate for most age groups • Examples – some species of birds, lizards, annual plants, invertebrates and rodents

Survivorship • Type 3 – low survivorship early but individuals that do make it live longer • Examples – many species of fish and marine invertebrates, perennial plants, trees, species that produce many young and no parental care

Survivorship Curves

Birth Rates • Also called reproduction rate • Number of individuals born within a certain period of time • Population increase primarily dependent upon reproduction • In order to avoid extinction a species must produce new individuals in numbers sufficient to replace those that die

Reproduction Potential • Maximum number of individuals that a population could produce • Number of new individuals that could be produces is greater than the number that is actually produced • Actual number takes into account survival rate • Actual number could be close to potential • Single young produced once a year by certain large mammals • Actual number could be small fraction of potential • Fish that lay several million eggs

Factors of Reproductive Rates Clutch size – number of young produced per reproductive event Small Large Animals with short life span Large number a year Small mammals – mice, voles Fish – salmon, sturgeon, trout Reptiles – snakes, turtles • Animals with long life span • 1 or occasionally 2 young a year • Large herbivorous mammals – elephants, zebras, cows • Semi-aquatic mammals – seals, walrus • Marine mammals – whales, dolphins

Factors of Reproductive Rate • Number of reproductive episodes per year • Small clutches – usually once per year or every couple years • Long gestation period • Long life span • Middle to Large clutches – multiple times per year • Short gestation period • Short life span • Record example – captive vole produced 17 litters within 1 year

Factors of Reproductive Rates • Number of reproductive episodes per lifetime Semelparity Iteroparity Reproduce many times in life Plants – perennials Examples Humans Vertebrates – birds, reptiles, virtually all mammals, and most fish Invertebrates – most molluscs, many insects • Reproduce one time in life • Plants – annuals • “Big bang” reproduction • Reproductive event usually large and fatal • Examples • Pacific salmon – lay eggs and die • Many insects, squid, octopus, arachnids

Factors of Reproductive Rate • Age of reproductive maturity – how old the animal must be to reproduce • Some species have delayed maturity • Condor – can’t breed until they are about 5 yrs old • Many large mammals – 1-2 years • Voles – breed at 3 to 6 weeks • Some are born pregnant • Species of mite

Factors of Reproductive Rate • Density • High density – may cause decrease in fertility, resulting in shortening of the breeding season and reduction in number of young per litter • Low density – becomes harder to find mates, inbreeding • Age • Effects breeding abilities

Measure/Model Population Growth • Strait counts are hard to achieve • Use mathematical model to predict population size in the future

Model Population Growth

Measure/Model Population Growth • N = population size – total number of individuals in a specific area at a given time • B = number of births • b = birth rate • N = 1000 • B = 34 • b = B/N = 34/1000 = 0.034

Measure/Model Population Growth • D = number of deaths • d = death rate • N = 1000 • D = 16 • d = D/N = 16/1000 = 0.016 • T = time • r = rate of increase • r = b-d

Measure of Population Growth • Change in population size would be the number of births minus the number of deaths in a specific period of time • ΔN/Δt = B-D • This requires us to count number born and number that die in specific period of time • Easier to use rates

Measure of Population Growth • The simplest case – no limitations on growth within the environment • Two things occur • Population displays its intrinsic rate of increase • Population experiences exponential growth

Intrinsic Rate of Population Increase • Rate of growth of a population when population is growing under ideal conditions and without limits • As fast as it possibly can • Difference between birth rate and death rate is maximized • Characteristic of population and not of the environment • Usually can’t be achieved in most environments

Intrinsic Rate of Population Increase • Higher intrinsic rate – grow faster • Lower rate of increase – slower growth • Intrinsic rate – rmax • Influenced by different factors • Age of reproduction maturity • Number of young produced • How well the young survive

Measurements – Unlimited growth • Formula • Nt = N0(er)t • Nt = number of individuals at present time • N0 = number of initial organisms • Produces J shaped curve (called J curve)

Limits on Population Growth • Exponential growth cannot go on forever • Population will eventually run into limits in their environment • Environment has finite amount of resources • Each environment has a carrying capacity • A specific number of individuals the environment can support

Carrying capacity • Environment has finite amount of available resources • Population has to share the available resource • As population increases – more individuals have to share limited resources • Each individual gets an increasingly smaller share • Carrying capacity • Maximum stable population size that a particular environment can support over a long period of time

Carrying Capacity • Symbolized – K • Property of the environment • Vary over space and time • Affected by abundance of limiting resources • If number of individuals exceeds carrying capacity – environment will be destroyed to the point where it can no longer support that number of individuals

Carrying Capacity • As population approaches carrying capacity • Individuals experience either a higher death rate or a lower fecundity • Rate of population growth declines towards zero

Logistic Growth • Logistic growth • Mathematical description that takes into consideration carrying capacity • Employs two parameters • rmax • K • Curve is S-shaped

Logistic Growth • Initially the population grows exponentially at a rate which is determined by rmax • As population size approaches carrying capacity – population growth rate slows • As population gets larger – rate gets slower • Ultimately the rate of growth reaches zero at the carrying capacity

Logistic Growth =

Logistic Growth • Logistic model – density dependent • Rate at which population changes with density of organisms that are currently in the population • Population do not typically display idealized logistic growth seen with the model • Deviation – delayed feedback • Overshoot • Vary up and down around the carrying capacity

Logistic Model

K or r Selected Populations K - selected r-selected Opportunistic populations Species good at growing rapidly in disturbed environments Significantly less capable of maintaining its population at carrying capacity • Equilibrium populations • Species good at maintaining population sizes at carrying capacity

K or r selected Populations • Few populations are either purely r or K selected

Population Ecology