Population Ecology

Study of populations in relation to environment –Environmental influences on: population density population distribution (dispersion) age structure

Definition of Population: Group of individuals of a single species living in a specific geographic region at the same time

Density: A Dynamic Perspective Determining the density of natural populations is possible, but difficult to accomplish In most cases it is impractical or impossible to count all individuals in a population –How do wildlife biologists approximate populations?

Estimating Wildlife Population Size Defined Populations -complete counts -incomplete counts -indirect counts Undefined Populations Mark and Recapture

Density is the result of a dynamic interaction of processes that add individuals to a population and those that remove individuals from it Births and immigration add individuals to a population. BirthsImmigration PopuIation size Emigration Deaths Deaths and emigration remove individuals from a population. How do these factors Contribute to Population Size?? Births Deaths Immigration Emigration

Patterns of Dispersion Environmental and social factors influence the spacing of individuals in a population

Clumped Dispersion –Individuals aggregate in patches –May be influenced by resource availability and behavior

Uniform Dispersion –Individuals are evenly distributed –May be influenced by social interactions such as territoriality

Random Dispersion Position of each individual is independent of other individuals (c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

Life history traits are products of natural selection Life history traits are evolutionary outcomes –Reflected in the development, physiology, and behavior of an organism

Semelparity: Big Bang –Reproduce a single time and die –putting all available resources into maximizing reproduction at the expense of future life

Iteroparity – Repeated Reproduction –produce offspring repeatedly over time –increased parental care along with enhanced energetic investment per offspring

“Trade-offs” and Life Histories Organisms have finite resources The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents. CONCLUSION Reduced brood size Normal brood size Enlarged brood size Parents surviving the following winter (%) Male Female –Which may lead to trade-offs between survival and reproduction RESULTS –Kestrels: Produce a few eggs? –Can invest more into each, ensuring greater survival Produce many eggs? –Costly but if all survive, fitness is better

More is Better? Some plants produce a large number of small seeds –Ensuring that at least some of them will grow and eventually reproduce

Fewer is Better? Other types of plants produce a moderate number of large seeds –That provide a large store of energy that will help seedlings become established

Demography Study of the vital statistics of a population –And how they change over time Death rates and birth rates Zero population growth –Occurs when the birth rate equals the death rate

Exponential Population Growth Population increase under idealized conditions No limits on growth Under these conditions –The rate of reproduction is at its maximum, called the intrinsic rate of increase

Example-understanding growth Question: I offer you a job for 1 cent/day and your pay will double every day. You will be hired for 30 days. Will you take my job offer? Answer: If you said YES, you will have made $~21 million dollars for 30 days of work. How is this possible?????

1ST DAY OF WORK: 1 cent pay/day 30TH DAY OF WORK: ~10.2 million/day How is this possible????? Amount of Pay/Day # of Days

Exponential Growth Model *Idealized population in an unlimited environment *Very rapid doubling time; steep J curve *r=  N=(b-d)N  t r=instrinsic rate of growth dN dt  r max N

Exponential Growth in the Real World Characteristic of some populations that are rebounding Year 0 2,000 4,000 6,000 8,000 Elephant population –Cannot be sustained for long in any population

Logistic Population Growth A more realistic population model –Limits growth by incorporating carrying capacity

Logistic Population Growth Carrying capacity (K) –Is the maximum population size the environment can support In the logistic population growth model –The per capita rate of increase declines as carrying capacity is reached

Logistic Population Growth –Produces a sigmoid (S-shaped) curve Figure dN dt  1.0N Exponential growth Logistic growth dN dt  1.0N 1,500  N 1,500 K  1, ,000 1,500 2,000 Number of generations Population size (N) dN dt  ( K  N ) K r max N

Time (days) (a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the experimenter maintains a constant environment. 1,000 Number of Paramecium/ml The Logistic Model and Real Populations The growth of laboratory populations of Paramecium –Fits an S-shaped curve

Logistic Growth and The Real World Some populations overshoot K –Before settling down to a relatively stable density Time (days) Number of Daphnia/50 ml (b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size. What type of feedback loop is this?

Logistic Growth and the Real World Some populations –Fluctuate greatly around K Time (years) Number of females (c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model.

The Logistic Model and Life Histories Life history traits favored by natural selection –May vary with population density and environmental conditions

Natural selection (diverse reproductive strategies) a) Relatively few, large offspring (K selected species) b) Many, small offspring (r selected species) (r selected species) (K selected species)

Populations Regulated Biotic and Abiotic Factors Two general questions we can ask about regulation of population growth 1.What environmental factors stop a population from growing? 2. Why do some populations show radical fluctuations in size over time, while others remain stable?

Competition for Resources In crowded populations, increasing population density –Intensifies intraspecific competition for resources ,000 10,000 Average number of seeds per reproducing individual (log scale) Average clutch size Seeds planted per m 2 Density of females (a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases. (b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

Cheetahs are highly territorial –Using chemical communication to warn other cheetahs of their boundaries Many vertebrates and some invertebrates are territorial –Territoriality may limit density

Territoriality: Ocean birds –Exhibit territoriality in nesting behavior

Health Population density –Can influence the health and survival of organisms In dense populations –Pathogens can spread more rapidly

Fluctuations in Population Size Extreme fluctuations in population size –Are typically more common in invertebrates than in large mammals Figure Year , , ,000 Commercial catch (kg) of male crabs (log scale)

Metapopulations and Immigration Metapopulations –Groups of populations linked by immigration and emigration

Immigration- Movement Into a Population High levels of immigration combined with higher survival can result in greater stability in populations Figure Mandarte island Small islands Number of breeding females Year

Population Cycles Many populations undergo regular boom-and-bust cycles Year Lynx population size (thousands) Hare population size (thousands) Lynx Snowshoe hare Influenced by complex interactions between biotic and abiotic factors

Human Populations No population can grow indefinitely and humans are no exception Figure B.C B.C B.C B.C B.C A.D. 0 The Plague Human population (billions) 2000 A.D

Global Carrying Capacity Just how many humans can the biosphere support? Carrying capacity of earth is unknown…. -LGOUT-exp_fresh+div-1r-8-HM ed

Age Structure One important demographic factor in present and future growth trends –Is a country’s age structure, the relative number of individuals at each age

Age structure is commonly represented in pyramids Figure Rapid growth Afghanistan Slow growth United States Decrease Italy Male Female Male FemaleMale Female Age Percent of population 80–84 85  75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 20–24 25–29 10–14 5–9 0–4 15–19 80–84 85  75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 20–24 25–29 10–14 5–9 0–4 15–19

Infant Mortality and Life Expectancy Infant mortality and life expectancy at birth –Vary widely among developed and developing countries but do not capture the wide range of the human condition Figure Developed countries Developing countries Developed countries Developing countries Infant mortality (deaths per 1,000 births) Life expectancy (years)