Populations

1. 42 Populations

2. Chapter 42 Populations • Key Concepts • 42.1 Populations Are Patchy in Space and Dynamic over Time • 42.2 Births Increase and Deaths Decrease Population Size • 42.3 Life Histories Determine Population Growth Rates

3. Chapter 42 Populations • Key Concepts • 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • 42.5 Immigration and Emigration Affect Population Dynamics • 42.6 Ecology Provides Tools for Conserving and Managing Populations

4. Chapter 42 Opening Question • How does understanding the population ecology of disease vectors help us combat infectious diseases?

5. Concept 42.1 Populations Are Patchy in Space and Dynamic over Time • Populations: groups of individuals of the same species • Humans have long been interested in understanding species abundance: • To increase populations of species that provide resources and food • To decrease abundance of crop pests, pathogens, etc.

6. Concept 42.1 Populations Are Patchy in Space and Dynamic over Time • Population density—number of individuals per unit of area or volume • Population size—total number of individuals in a population • Counting all individuals is usually not feasible; ecologists often measure density, then multiply by the area occupied by the population to get population size.

7. Concept 42.1 Populations Are Patchy in Space and Dynamic over Time • Abundance varies on several spatial scales. • Geographic range—region in which a species is found • Within the range, species may be restricted to specific environments or habitats. • Habitat patches are “islands” of suitable habitat separated by areas of unsuitable habitat.

8. Figure 42.1 Species Are Patchily Distributed on Several Spatial Scales

9. Concept 42.1 Populations Are Patchy in Space and Dynamic over Time • Population densities are dynamic—they change over time. • Density of one species population may be related to density of other species populations.

10. Figure 42.2 Population Densities Are Dynamic

11. Concept 42.2 Births Increase and Deaths Decrease Population Size • Change in population size depends on the number of births and deaths over a given length of time. • “Birth–death” or BD model of population change:

12. Concept 42.2 Births Increase and Deaths Decrease Population Size • Population growth rate (change in size over one time interval):

13. Concept 42.2 Births Increase and Deaths Decrease Population Size • Change in population size can be measured only for very small populations that can be counted, such as zoo animals. • To estimate growth rates, ecologists keep track of a sample of individuals over time.

14. Concept 42.2 Births Increase and Deaths Decrease Population Size • Per capita birth rate(b)—number of offspring an average individual produces • Per capita death rate(d)—average individual’s chance of dying • Per capita growth rate (r) = (b – d) = average individual’s contribution to total population growth rate

15. Concept 42.2 Births Increase and Deaths Decrease Population Size • If b > d, then r > 0, and the population grows. • If b < d, then r < 0, and the population shrinks. • If b = d, then r = 0, and population size does not change.

16. Concept 42.3 Life Histories Determine Population Growth Rates • Demography:study of processes influencing birth, death, and population growth rates • Life history: timing of key events such as growth and development, reproduction, and death during an average individual’s life • Example: Life cycle of the black-legged tick

17. Figure 42.3 Life History of the Black-Legged Tick

18. Concept 42.3 Life Histories Determine Population Growth Rates • A life history shows the ages at which individuals make life cycle transitions and how many individuals do so successfully: • Survivorship—fraction of individuals that survive from birth to different life stages or ages • Fecundity—average number of offspring each individual produces at different life stages or ages

19. Table 42.1

20. Concept 42.3 Life Histories Determine Population Growth Rates • Survivorship can also be expressed as mortality: the fraction of individuals that do not survive from birth to a given stage or age. • Mortality = 1 – survivorship

21. Concept 42.3 Life Histories Determine Population Growth Rates • Survivorship and fecundity affect r. The higher the fecundity rate and survivorship, the higher r will be. • If reproduction shifts to earlier ages, r will increase as well.

22. Concept 42.3 Life Histories Determine Population Growth Rates • Life histories vary among species: how many and what types of developmental stages, age of first reproduction, frequency of reproduction, how many offspring they produce, and how long they live. • Life histories can vary within a species. For example, different human populations have different life expectancies and age of sexual maturity.

23. Concept 42.3 Life Histories Determine Population Growth Rates • Individual organisms require resources (materials and energy) and physical conditions they can tolerate. • The rate at which an organism can acquire a resource increases with the availability of the resource. • Examples: Photosynthetic rate increases with sunlight intensity; an animal’s rate of food intake increases with the density of food

24. Figure 42.4 Resource Acquisition Increases with Resource Availability—Up to a Point

25. Concept 42.3 Life Histories Determine Population Growth Rates • Principle of allocation • Once an organism has acquired a unit of some resource, it can be used for only one function at a time, such as maintenance, growth, defense, or reproduction. • In stressful conditions, more resources go to maintaining homeostasis. • Once an organism has more resources than it needs for maintenance, it can allocate the excess to other functions.

26. Figure 42.5 The Principle of Allocation

27. Concept 42.3 Life Histories Determine Population Growth Rates • In general, as average individuals in a population acquire more resources, the average fecundity, survivorship, and per capita growth rate increase.

28. Concept 42.3 Life Histories Determine Population Growth Rates • Life-history tradeoffs—negative relationships among growth, reproduction, and survival • Example: A species that invests heavily in growth early in life cannot simultaneously invest heavily in defense. • Environment is also a factor: if high mortality rates are likely, it makes sense to invest in early reproduction.

29. Concept 42.3 Life Histories Determine Population Growth Rates • Species’ distributions reflect the effects of environment on per capita growth rates. • A study of temperature change in a lizard’s environment, combined with knowledge of its physiology and behavior, led to conclusions about how climate change may affect survivorship, fecundity, and distribution of these lizards.

30. Figure 42.6 Climate Warming Stresses Spiny Lizards (Part 1)

31. Figure 42.6 Climate Warming Stresses Spiny Lizards (Part 2)

32. Figure 42.6 Climate Warming Stresses Spiny Lizards (Part 3)

33. Concept 42.3 Life Histories Determine Population Growth Rates • Laboratory experiments have also shown the links between environmental conditions, life histories, and species distributions. • Example: Quantifying life history traits of two species of grain beetles in different temperature and humidity conditions explained distributions of these species in Australia.

34. Figure 42.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions

35. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • Population growth is multiplicative—an ever-larger number of individuals is added in each successive time period. • In additive growth, a constant number (rather than a constant multiple) is added in each time period.

36. In-Text Art, Chapter 42, p. 873 (2)

37. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • Charles Darwin was aware of the power of multiplicative growth: • “As more individuals are produced than can possibly survive, there must in every case be a struggle for existence.” • This ecological struggle for existence, fueled by multiplicative growth, drives natural selection and adaptation.

38. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • Multiplicative growth with a constant r has a constant doubling time. • The time it takes a population to double in size can be calculated if r is known.

39. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • Populations do not grow multiplicatively for very long. Growth slows and reaches a more or less steady size:

40. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • r decreases as the population becomes more crowded; r is density dependent. • As the population grows and becomes more crowded, birth rates tend to decrease and death rates tend to increase. • When r = 0, the population size stops changing—it reaches an equilibrium size called carrying capacity, or K.

41. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • K can be thought of as the number of individuals that a given environment can support indefinitely. • When population density reaches K, an average individual has just the amount of resources it needs to exactly replace itself. • When density <K, an average individual can more than replace itself; when density >K, the average individual has fewer resources than it needs to replace itself.

42. Figure 42.8 Per Capita Growth Rate Decreases with Population Density

43. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • Spatial variation in environmental factors can result in variation of carrying capacity. • Temporal variation in environmental conditions may cause the population to fluctuate above and below the current carrying capacity. • Example: the rodents and ticks in Millbrook, New York

44. Figure 42.2 Population Densities Are Dynamic

45. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • Environmental changes affected fecundity of the Galápagos cactus ground finches: • When females were 7 and 8 years old, they produced no surviving young, and survivorship dropped. • Low food availability during these years resulted from a severe drought in 1985. • When the females were 5, a wet year produced abundant food and high fecundity.

46. Table 42.1

47. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • The human population is unique. It has grown at an ever-faster per capita rate, as indicated by steadily decreasing doubling times. • Technological advances have raised carrying capacity by increasing food production and improving health.

48. Figure 42.9 Human Population Growth

49. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • In 1798 Thomas Malthus pointed out that the human population was growing multiplicatively, but food supply was growing additively, and predicted that food shortages would limit human population growth. • His essay provided Charles Darwin with a critical insight for the mechanism of natural selection. • Malthus could not have predicted the effects of technology such as medical advances and the Green Revolution.

50. Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change • Many believe that the human population has now overshot its carrying capacity for two reasons: • Technological advances and agriculture have depended on fossil fuels—a finite resource. • Climate change and ecosystem degradation have been a consequence of 20th century population expansion.