Chapter 6: Population Biology

1. Chapter 6: Population Biology EMMA DEBANY AND DERRIAN DURYEA

2. What’s Population? Population: all members of a single species living in specific area at same time.

3. How Can We Express Population Growth? • exponential growth: theoretical unrestricted increase in populations, no limiting factors. • Exponential growth can be expressed as an equation!

4. The Equation dN: change in # of individuals dt: change in time r: rate of growth—a fraction representing the average individual contribution to population growth. If r is POSITIVE, population increasing. r NEGATIVE population decreasing. r is ZERO, no change, and dN/dt = 0 N: number of individuals in population

5. The Equation Continued Previous equation also known as Biotic potential: the potential of a population to grow if nothing is limiting its expansion.

6. How Many Years Will A Population Take to Double? • If population is growing exponentially (without limits), this equation finds how many years a population will take to double: divide 70 by annual percentage growth = approximate doubling time in years in other words: 70 / x% = doubling time (years) (for instance, a population growing at 35% doubles in 2 years)

7. What About When There are Limits to Growth? Carrying capacity: max. population of any species that can be supported by a particular ecosystem Overshoots: when a population goes over the carrying capacity of its environment--death rates begin to rise past birth rates Population crash: after an overshoot, population decreases as fast OR faster than it grew

9. Growth and Decline of Populations Populations normally cycle through growth and decline Regular Cycles: depend on simply factors (like seasonal algae blooms that depend on light and temperature) Irregular Cycles: depend on complex environmental relationships (like outbreaks of migratory locusts in desert) Irruptive growth: long periods of low population size then a sudden population growth

10. Stable Populations Remember exponential growth? There’s also logistic growth! logistic growth: species grow exponentially when resources unlimited BUT pop. growth slows when carrying capacity of environment is approached (Population will decrease if it exceeds carrying capacity)

11. Formula For Logistic Growth! dN/dt: change in numbers over time r: exponential growth rate N: population size K: carrying capacity

12. Formula For Logistic Growth! (1-N/K): represents relationship between N (pop size) at any given time step and K (# of individuals the environment can support) If N is less than K, 1-N/K will be positive, and means the population is growing (smaller numbers greater than 0 is slow growth, larger numbers faster growth) If N is more than K, 1-N/K will be negative and the population will be decreasing.

13. Logistic vs Exponential Growth Exponential: also known as J Curve Logistic: also known as S curve J curve: theoretical growth without restraint toward biotic potential S curve: stabilization in response to environmental resistance

15. Factors that Limit Populations Populations regulated by internal and externalfactors internal: maturity, body size, hormonal status external: habitat, food availability, interactions with other organisms

16. External Limits: Density Dependent vs Independent density-dependent: limits dependent on population density • food and water • disease, stress, exposure to predators or parasites density-independent: limits not involved with population/density of animals • Example: drought / early frost • Habitat destruction: floods, fires, etc

17. What’s Environmental Resistance? Environmental resistance: environmental factors that tend to reduce population growth rates. Resistance is larger and rate of logistic growth smaller as population approaches carrying capacity

18. K-Adapted? R-Adapted? Whaaaat? R adapted: species that persist by depending on high rate of reproduction and growth • rapid reproduction • High mortality of offspring • Will vershoot carrying capacity and die back K adapted species: reproduce more slowly as they approach the carrying capacity of the environment

19. R-Adapted Species R Adapted Species grow exponentially • Move quickly into disturbed environments • grow rapidly • mature quickly • produce many offspring • do little to care for offspring • depend on sheer numbers and dispersal techniques to ensure some survive

20. R –Adapted Graph

21. K-Adapted Species • Larger • Live longer • Mature slower • Produce fewer offspring in each generation • Fewer natural predators

23. Factors that Increase/Decrease Populations Natality: production of new individuals by birth, hatching, germination, cloning (sensitive to environmental conditions) Successful reproduction tied to: nutritional levels, climate, soil, water conditions, social interactions

24. Fecundity vs Fertility Fecundity: physical ability to reproduce Fertility: measure of the actual number of offspring produced (Because of lack of opportunity to mate, fecund individuals may not contribute to pop growth)

25. Immigration Additions to Populations Methods of immigration: • Wind (seeds, spores, small animals carried distances • Fur/feathers/intestines • Water • Self-transportation (birds fly, fish swim, wolves walk)

26. Immigration Continued Some ecosystems can be maintained by constant influx of immigrants

27. Mortality/Death Rate How to calculate mortality/death rate: divide the # of organisms that die in a certain time period by the # alive at the beginning of the period X1 / X2

28. But What Is Mortality? • Survivorship: percentage of a cohort that survives to a certain age • Life expectancy: the probable number of years of survival for an individual

29. Life Expectancies in US • Rose during 20th Century • 1900: 47.3 years expectany • 2003: 77.4 years expectancy • Differences between sexes, • races, economic class

30. Life Span • Life span: longest period of life reached by a given type of organism • Most organisms don’t live anywhere near the maximum life span for their species

31. Major factors in early mortality: • Predation • Parasitism • Disease • Accidents • Fighting • Environmental influence (climate, nutrition)

32. Emigration Emigration: the movement of the members out of a population size. When a group/individual immigrates to a new area, they emigrate out of an old area Same techniques used for immigration are used for emigration Can help protect a species if area is overpopulated

33. Population Growth Factors What are the factors that regulate population growth? These factors primarily affect natality and mortality

34. Types of Factors: • Intrinsic: operating within individual organisms or between organisms in the same species • Extrinsic: imposed from outside the population

35. More Types of Factors • Biotic: caused by living organisms (tend to be density-dependent) • Abiotic: caused by nonliving components of the environment (tend to be density-independent)

36. Which Factor is More Important in Regulating Population Dynamics? • Has been much debate • In general, depends on: • the particular species involved • that species’ tolerance levels • The stage of growth and development of organisms involved • Ecosystem where the organisms live • The way combination of factors interact

37. Abiotic Generally Density-Independent • Weather or climate are most important factors • Extreme cold, high heat, drought, excess rain, severe storms also important • Factors don’t always diminish population: • After a rainstorm (an abiotic factor) the desert will flourish • Some forests need fires to bloom

38. Density-Dependent Factors Density-dependent factors reduce population size Decrease natality Increase mortality Result of not only interactions between populations of a community, but also interactions within a population

39. Interspecific Interactions • These interactions occur between species • Predator vs prey • Prey species can also benefit: • Moose are killed by wolves • Old/sick moose are killed off • This strengthens herd of moose as a whole • Also benefits wolves

40. Intraspecific Interactions • These occur within species • Animals in species compete for resources • Population density is low = resources plentiful • Pop. Density high = resources low

41. Stress and Crowding • Stress related diseases: when pop. density is high, organisms have symptoms of this • Too much competition/too close proximity to other organisms • Can affect reproduction, thus lowering population density once more—population fixes itself

42. Case Study 2: THE LOCUSTS • Locust plagues have been tragically destructive throughout history • Ever few decades rain comes to the desert and locusts flourish • This high population density for some reason causes them to stop reproducing, grow longer wings, and swarm desert • The swarms devour hundreds of thousands of plants and all die within a few weeks.

43. Case Study 2: THE LOCUSTING • Locust swarms can affect livelihood of 1/10 of the earth’s population with all the plants/crops they destroy • In 2004 heavy rains in the Sahara cause locust swarm • 28 countries in Africa and Mediterranean were affected • Crop losses reached 100% in some places • This study illustrates power of exponential growth and danger of boom and bust life cycles

44. Conservation Biology Small, isolated populations can undergo declines due to environmental change, genetic problems, or random events

45. Island Biogeography • Island Biogeography: a theory that MacArthur and Wilson came up with in 1967 to explain why islands have fewer species than the mainland • Theory explains that diversity in isolated habitats = balance between colonization and extinction rates

46. Biogeography Continued • Islands have low colonization rates because islands are hard to reach • Limited habitat forces population to be small • Therefore larger islands closer to mainland are more populated and more diverse

47. Genetics = important in survival/extinction of small, isolated populations • Hardy-Weinberg equilibrium: in large populations. If mating is random, no mutations occur, the distribution of gene types will preserve genetic diversity

48. Genetic Drift • Genetic Drift: the gradual changes in gene frequencies (occurs in small populations due to fewer individuals with slight genetic variation being involved in mating)

49. Founder effect/demographic bottleneck: a few members of the species survive a disaster, or colonize a new habitat isolated from other members of species Results in loss of genetic diversity

50. Seals and Cheetahs • Elephant seals were nearly hunted to extinction, but their numbers are now normal again. They are also now all almost nearly identical genetically • All male cheetahs are nearly identical genetically, suggesting they all came from one common male ancestor • This lack of diversity responsible for a low fertility rate an low survival rate of offspring