Population Ecology

1. Population Ecology

2. Populations • Population ecology is the study of populations in relation to the environment • A population is a group of individuals of the same species that live in the same area

3. Population Ecology: Vocabulary • To describe populations, we have to talk about how they are situated • Density: • The number of individuals per unit area/volume • Example: 47 elephants/km2 • Dispersion: • The pattern of spacing among individuals in a population • Clumped • Uniform • Random

5. Environmental conditions are uniform Causes COMPETITION or antagonism between organisms (territoriality) Uniform

6. Clumping • Most common • Reproductive patterns favor clumping • Social behaviors lead to clumping • Optimal density is usually intermediate (medium)

7. No competition No tendency to group/clump Conditions are uniform Rarely happens! Random

8. Estimating Population Size • Rarely, it is possible to count the number of species in a population • Usually, a statistical method is needed to determine population size • The mark-recapture method can be used to estimate the size of a population • Capture, mark, release • Recapture and count • Equation: N = Number marked x Total catch 2nd time Number of marked recaptures

9. Factors That Influence Population Size • There are 3 major factors that influence population size: 1. the number of births 2. the number of deaths 3. the number of individuals that enter or leave a population - immigration: individuals entering an existing population - emigration: individuals leaving an existing population

10. DEMOGRAPHICS • In looking at populations, biologists must also look at demographics (the vital statistics of a population and how they change over time) • Birth rates • Death rates • Life tables • Survivorship curves • Reproduction rates • Growth rates • Age structures

11. DEMOGRAPHICS • One tool in demographics is to estimate the life span of organisms • To do this they often create a life table, age summary of a population • A cohort (group of individuals of the same age) are studied to see what percentage of the population dies • This can determine life expectancy, survivorship at each age category, or male vs. female survivorship

12. Survivorship Curves • A way to represent a life table is a survivorship curve, a plot of the proportion of individuals that survive at each age group • When discussing survivorship curves, there are 3 general categories

13. Survivorship Curves • Type I- live to old age & die (most large mammals) • Type II- constant mortality rate (rodents, lizards, hydra) • Type III- high mortality at young age, but if they survive they live a long life (sea turtles).

14. DEMOGRAPHICS • Another important aspect of demographics is reproductive rates • For obvious reasons, biologists who study reproductive rates only focus on females • Age of fertility • Number of offspring for each age group • Time of year • Spawning cycles • Reproductive tables summarize this data

15. Life Histories • Life history are traits that affect an organism’s schedule of reproduction and survival • Clutch size: • Number of offspring produced at each reproductive episode • Semelparity • A life history in which an organism spends most of its energy in growth and development, expend their energy in one large reproductive effort, and then die • Many insects, annual plants, salmon, etc.

16. Life Histories • Iteroparity • A life history pattern in which organisms produce fewer offspring at a time over a span of many seasons • Example: humans, panda bears, etc.

17. Life Histories (Semelparity vs. Iteroparity) • Many factors contribute to the life history of an organism • Finite resources • Reproduction vs. survival • Number and size of offspring • Paternal investment in offspring

18. POPULATION GROWTH MODELS Different models of how populations grow

19. Formulas off your Cheat Sheet • Rate: dY/dt • Generic expression for change of some variable (Y) over time (t) • Population Growth: dN/dt = (B – D) • The change in population size (N) over time (t) is the same as the birth rate (B) minus the death rate (D) • Exponential Growth: dN/dt =rmax •N • The change in population size over time is equal to the growth rate (rmax) multiplied with the current population size

20. Formulas off your Cheat Sheet • Logistic Growth: • dN/dt =rmax •N • [(K-N)/K] • The growth rate over time is the same as exponential growth adjusted for carrying capacity (K)

21. Types of Population Growth (start here) • One of the biological imperatives is to reproduce and pass on genetic material to succeeding generations. • Yet population growth is controlled by the environment and limited resources • This causes different patterns of population growth

22. Patterns of Population Growth • Exponential Growth: • Occurs in ideal conditions with unlimited resources • J shaped curve • Example: • 1 bacterium (reproducing every 20 minutes) could produce enough bacteria to form a 1-foot layer over the entire surface of the Earth in 36 hours

23. Exponential Growth • Exponential growth is a useful model when studying populations that are introduced into a new, unfilled, environment • Recovery after a catastrophe • Exponential Growth: • dN/dt =rmax •N • rmax is the maximum rate of population growth for the species

24. Patterns of Population Growth • Exponential growth cannot continue indefinitely • It is characteristic of populations who are entering a new environment OR those whose numbers are rebounding from a catastrophic events

25. Patterns of Population Growth • Logistic Growth: • Pattern of population growth which takes into account the effect of population density on population growth • Occurs when resources become more scarce • Characterized by an S-shaped curve

26. Patterns of Population Growth • dN/dt =rmax•N • [(K-N)/K] • Carrying capacity (K): • The maximum number of individuals that a particular environment can support over a long period of time • Determined by such limiting factors as crowding and food resources • Graph levels off at carrying capacity • K-selected populations (equilibrial populations) live near or at the carrying capacity

27. Carrying Capacity

28. K-strategists (Life history) • Density stays near carrying capacity. • Large, slow growing organisms • Small population sizes • Long life span; slow maturation • Few young/small clutch size • Reproduce late in life • Parental care • Most large mammals; endangered species

29. r- strategists (Life history) • Grow exponentially when environmental conditions allow; when conditions worsen, population size plummets. • Short life span • Reproduce early in life • Many offspring/large clutch size • Usually small in size • Little or no parental care • Bacteria, some plants, insects

30. Environmental Factors Abiotic and biotic influences on population size

31. Limiting Factors • There are a number of factors that limit the size of populations: • Density-dependent limiting factors (depends on the size of the population) • Density-independent limiting factors (does not matter the size of the population)

32. Density-Dependent Limiting Factors • The effect of density-dependent limiting factors intensifies as the population increases • Intraspecific competition • Food, space, etc. • Territoriality • Predation • Waste build up • Disease (if caused by pathogen/contagious)

33. Density-Independent Limiting Factors • The occurrence and severity of density-independent limiting factors are unrelated to population size • Climate • Disease (if not caused by pathogen/not contagious) • Pollution

34. The Interaction of Limiting Factors • Density-dependent and density-independent limiting factors often work together to regulate the size of a population • Deer in snowy winter • Starve from lack of food (density-dependent) • Severity of winter/depth of snow determines access to food (density-independent)

35. Population Dynamics • Population dynamics is the study of the environmental factors that cause variations in the population size • Looks at fluctuations in population over time to examine stability • Immigration and emigration also affect population • Metapopulations are when you have several interconnected populations

36. Boom-and-bust cycle • Another phenomena that affects population are predator-prey relationships. • Each population is interdependent and causes a boom-and-bust cycle • The prey population increases which causes the predator population to increase • The prey are over hunted and their population crashes • This causes the predator population to crash • Now, with fewer predators, the prey population can again increase (recovery gives a geometric increase)

37. Boom-and-Bust Cycles

38. Human Population Growth

39. Increase in Human Population • Agricultural Revolution - Major period of population growth began when humans started to cultivate crops and domesticate animals • Industrial Revolution – Improved food production and distribution • Health Care – germ theory lead to improved hygiene, better waste removal and water treatment

40. Decrease in Human Population • Plague – disease that greatly reduces the size of population (Black Plague in 1300’s reduced the population in England by 50%) • Famine –a severe food shortage causing starvation and death (Potato Famine of 1840’s/China 1870-1890) • War – death by combat, disease, cut off from food supply (Germany 1618-1648/WWI/WWII)

41. Human Population Growth • The human population is unlike any other organism • Since about 1650, we have remained in an exponential population increase • Population increases by about 201,000 people/day worldwide

42. Human Population Growth

43. Human Population Growth • Even though there is a tremendous increase in human population, it is not evenly distributed around the globe • Regional areas have different population trends • Some regions have stable regional human populations (birth rate is the same as death rate) • Other regions show incredible growth rates

44. Industrialized Nations • An emerging nation usually has a very high birth rate, but also a high death rate (disease, lack of modern medical treatment, famine) • An industrialized nation usually has a low death rate, but also a low death rate • Moving from an emerging nation to an industrialized nation is known as a demographic transition

45. Human Population Growth • In the 1950s, mortality rates began to rapidly drop (advances in medicine and sanitation) • Yet, the birth rates have not always dropped • Has caused a huge increase in population in some nations • About 80% of the world’s population lives in emerging nations

46. Age Structure • One way to determine human population growth is to look at the nations’ age structure, relative number of individuals at each age • By looking at the age structure of a population, you can determine the population growth

47. Age-Structure Diagrams

48. Human Population Growth • Implications of exponential human population growth: • Lack of food supplies • Lack of space • Lack of natural resources (metals, fossil fuels, etc) • Lack of sites for waste disposal • Ecologists cannot agree on a carrying capacity for Earth (2 – 40 billion) • Are we going to reach carrying capacity through individual choices and/or government programs? OR • Is Earth’s population going to “level off” as a result of mass deaths?