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Population Ecology 1 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 2 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) 3 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 4 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 5 Techniques of Obtaining Populations 1) Simple counts 1) # seals/island, #burrows/area, # wildebeest/herd 2) Can use aerial photographs to obtain population estimates 1) Seals and sea lions 2) Wintering waterfowl and marine birds 6 Techniques of Obtaining Populations 2) Mark-recapture technique 1) Capture and mark individuals 1) Trapping, marking, ID tags, radio transmitters 2) Recapture at a later point in time 1) Provide estimate of population size for a given area 3) Calculation = (total number marked)(total number recaptured)/(number of recapture that were marked) 7 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 8 Techniques of Obtaining Populations 3) Census techniques 1) Transect methods 1) Walk or drive a line (transect) and count the number of individuals at specific locations, evenly distributed along the line 2) Used for pheasant counts 9 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 10 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 11 Distribution • Type 2 – clumped distribution • Possible explanations – Patchy distribution of resources – Organisms live in groups or close together 12 Distribution • Type 3 – Random distribution • Possible explanations – Random distribution of resources – Absence of strong attractions or repulsions among individuals of a population • Very uncommon 13 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. 14 Age Structure • Methods – follow a group of individuals from birth to death over time – Construct a life table for the group 0-9 Number of Survivors 11 10-19 10 1 20-29 8 2 30-39 7 1 40-49 5 2 50-59 3 2 60-69 2 1 70-79 2 0 80-89 1 1 90-99 100+ 0 0 1 0 Age Class Number of Deaths 0 Mortality rate 0.000 = 0/11 0.090 = 1/11 0.200 = 2/10 0.125 = 1/8 0.286 = 2/7 0.400 = 2/5 0.330 0.000 0.500 1.000 1.00 15 Age Structure 16 Sex Ratio • Rate at which a population may grow can be dependent on the sex ratio • Sex Ratios by age (males per 1000 females – Fewer females – slower rate of population growth 17 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 18 Survivorship • Number of survivors/age group • Probability of newborn individuals of a group surviving to particular ages • Yields 3 different curves 19 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 20 Survivorship • Type 2 – constant survivorship rate for most age groups • Examples – some species of birds, lizards, annual plants, invertebrates and rodents 21 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 22 Survivorship Curves 23 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 24 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 25 Factors of Reproductive Rates Clutch size – number of young produced per reproductive event Small Large • 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 • Animals with short life span • Large number a year • Small mammals – mice, voles • Fish – salmon, sturgeon, trout • Reptiles – snakes, turtles 26 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 27 Factors of Reproductive Rates • Number of reproductive episodes per lifetime Semelparity • 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 Iteroparity • Reproduce many times in life • Plants – perennials • Examples – Humans – Vertebrates – birds, reptiles, virtually all mammals, and most fish – Invertebrates – most molluscs, many insects 28 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 29 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 30 Measure/Model Population Growth • Strait counts are hard to achieve • Use mathematical model to predict population size in the future 31 Model Population Growth Immigrate Birth Population size Emigration Death 32 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 33 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 34 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 35 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 36 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 37 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 38 Measurements – Unlimited growth • Formula • Produces J shaped curve (called J curve) 140 120 Population size – Nt = N0(er)t – Nt = number of individuals at present time – N0 = number of initial organisms Unlimited exponential growth 100 80 60 40 20 0 0 2 4 Time 6 8 39 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 40 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 41 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 42 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 43 Logistic Growth • Logistic growth – Mathematical description that takes into consideration carrying capacity – Employs two parameters • rmax •K – Curve is S-shaped 44 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 45 Logistic Growth 𝑁𝑡 = 𝑁0 −𝐾 𝑁0 + 𝐾−𝑁0 ∗𝑒 −𝑟𝑡 46 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 47 Logistic Model 48 K or r Selected Populations K - selected • Equilibrium populations • Species good at maintaining population sizes at carrying capacity r-selected • Opportunistic populations • Species good at growing rapidly in disturbed environments – Significantly less capable of maintaining its population at carrying capacity 49 K or r selected Populations • Few populations are either purely r or K selected r-selected K-selected Organism size Small Large Energy used to make each individual Low High # offspring produced Many Few Timing of maturation Early Late (with much parental care) Life expectancy Short Long Lifetime reproductive events One More than one Survivorship curve Type III Type I or II 50 r-selected curves 51 K-selected curves 52 Population Regulation • Density-dependent factors – Based on competition within the species – Help determine carrying capacity – Factors • • • • • • • Food Mates Increased rates of be coming prey and parasitism, Stress and behavioral problems Nesting habitat Waste buildup Water 53 Population Regulation • Density-independent factors – Occur to same extent regardless of population size – Factors • Weather and climate – Drought, typhoon, hurricane, excessive rain or snow • Geological disturbances – Earthquake, tidal wave, volcanic eruption 54 Cycles • Regular fluctuation in density • Fluctuates on annual cycle – High just after reproduction – Low just before 55 56 Cycles • Long term cycles – periods of 3- 10 years • Possible causes – Hormonal changes – Change in food quality – Lag in response to predator population density – An adaptation to reduce predation • Lemming (4 yr) • Cicadas (13-17 yr) 57 58 59 60 Irregular Cycles • Changes without regular patterns • Can be due to strong weather or climate events 61