2. Population Ecology

3. Chapter Overview Questions What are the major characteristics of populations? How do populations respond to changes in environmental conditions? How do species differ in their reproductive patterns?

4. What are Population Dynamics? Population dynamics are changes that occur in a population in response to environmental stress or changes in environmental conditions They change in: –Size –Density –Dispersion (spacial patterns) –Age distribution

5. What Limits Population Growth? Four variables: –Births –Deaths –Immigration (individuals that move in) –Emigration (individuals that move out) Variables depend on changes in resource availability or on other environmental changes

6. Population Growth Pop = Pop 0 + (b + i) - (d + e) ZPG (zero pop growth) (b + i) = (d + e)

7. Age Structure: Young Populations Can Grow Fast How fast a population grows or declines depends on its age structure. –Prereproductive age: not mature enough to reproduce. –Reproductive age: those capable of reproduction. –Postreproductive age: those too old to reproduce.

8. Biotic Potential Ability of populations of a given species to increase in size. Intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. –Abiotic Contributing Factors: Favorable light Favorable Temperatures Favorable chemical environment - nutrients –Biotic Contributing Factors: Reproductive rate Generalized niche Ability to migrate or disperse Adequate defense mechanisms Ability to cope with adverse conditions

9. Consists of all the factors acting jointly to limit the growth of a population Population size of species in a given place and time is determined by interplay between biotic potential and environmental resistance. Affects the young more than the elderly in a population, thereby affecting recruitment (survival to reproductive age) Environmental Resistance

10. Limits on Population Growth: Biotic Potential vs. Environmental Resistance No population can increase its size indefinitely. –The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. –Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

11. Population Growth Populations show two types of growth –Exponential J-shaped curve Growth is independent of population density –Logistic S-shaped curve Growth is dependent on population density

12. Exponential Growth As early as Darwin, scientists have realized that populations have the ability to grow exponentially All populations have this ability, although not all populations realized this type of growth Darwin pondered the question of exponential growth. He knew that all species had the potential to grow exponentially He used elephants as an example because elephants are one of the slowest breeders on the planet

13. Exponential Growth One female will produce 6 young over her 100 year life span. In a population, this amounts to a growth rate of 2% Darwin wondered, how many elephants could result from one male and one female in 750 years? 19,000,000 elephants!!!

14. Exponential Growth Graph

15. Animation: Exponential Growth PLAY ANIMATION

16. Logistic Growth Because of environmental resistance, population growth decreases as density reaches carrying capacity Graph of individuals vs. time yields a sigmoid or S-curved growth curve Reproductive time lag causes population overshoot Population will not be steady curve due to resources (prey) and predators

17. Video: Logistic Growth PLAY VIDEO

18. Population Dynamics and Carrying Capacity Basic Concept: Over a long period of time, populations of species in an ecosystem are usually in a state of equilibrium (balance between births and deaths) –There is a dynamic balance between biotic potential and environmental resistance

19. Carrying Capacity (K) Exponential curve is not realistic due to carrying capacity of area Carrying capacity is maximum number of individuals a habitat can support over a given period of time due to environmental resistance (sustainability)

20. Exponential and Logistic Population Growth: J-Curves and S-Curves Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. Figure 8-4

21. Exponential and Logistic Population Growth: J-Curves and S-Curves As a population levels off, it often fluctuates slightly above and below the carrying capacity. Figure 8-4

22. What Happens if Population Size Exceeds Carrying Capacity? It will overshoot or exceed the carrying capacity. This occurs because of a reproductive time lag, the period needed for birth rate to fall and death rate to rise in response to resource overconsumption. Population suffers a dieback, or crash

23. Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Figure 8-6

24. Population Density and Population Change: Effects of Crowding Population density: the number of individuals in a population found in a particular area or volume. –A population’s density can affect how rapidly it can grow or decline. e.g. biotic factors like disease –Some population control factors are not affected by population density. e.g. abiotic factors like weather

25. How Does Population Density Affect Population Growth? Density-independent population controls affect a population’s size regardless of density. –Floods, fires, hurricanes, unseasonable weather, habitat destruction, pesticide spraying Density-dependent population controls are limiting factors that have a greater effect as population density increases. Tend to reduce population size by decreasing natality or increasing mortality as population increases. –Competition for resources, predation, parasitism, disease.

26. Types of Population Change Curves in Nature Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. –Stable: fluctuates slightly above and below carrying capacity. –Irruptive: populations explode and then crash to a more stable level. –Cyclic: populations fluctuate and regular cyclic or boom- and-bust cycles. –Irregular: erratic changes possibly due to chaos or drastic change.

27. Role of Predation in Controlling Population Size Some species that interact as predator or prey undergo cyclic changes in their numbers, with sharp increase in their numbers followed by periodic crashes. Bottom-up control occur when species consume food faster than it can be replenished, and have a decrease in quantity and quality of food. Can occur without presence of predators. Examples of top-down control of predators on prey populations include –Wolves controlling deer populations and moose populations –Large predatory fish controlling other fish populations in lakes –Sharks and alligators controlling some fish populations

28. Types of Population Change Curves in Nature Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources. Figure 8-7

29. Animation: Capture-Recapture Method PLAY ANIMATION

30. Reproductive Strategies Goal of every species is to produce as many offspring as possible Each individual has a limited amount of energy to put towards life and reproduction This leads to a trade-off of long life or high reproductive rate Natural Selection has lead to two strategies for species: r - strategists and K – strategists Availability of suitable habitat for individuals of a population in a particular area is what determines its ultimate population size.

31. r - Strategists Are opportunist species Spend most of their time in exponential growth Maximize reproductive life Minimum life Can gain a foothold in changed environmental conditions K

32. r - Strategists Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate – (r) Population size fluctuates wildly above and below carrying capacity – (K) Generalist niche Low ability to compete

33. K - Strategists Competitor species Maintain population at carrying capacity (K) Maximize lifespan Thrive best in ecosystems with fairly constant environmental conditions K

34. K- Strategist Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete

35. Survivorship Curves: Short to Long Lives The way to represent the age structure of a population is with a survivorship curve. –Late loss population live to an old age. –Constant loss population die at all ages. –Most members of early loss population, die at young ages.

36. Survivorship Curves: Short to Long Lives The populations of different species vary in how long individual members typically live. Figure 8-11

37. Animation: Life History Patterns PLAY ANIMATION

38. Video: Kelp Forest (Channel Islands) PLAY VIDEO

39. Video: Salmons Swimming Upstream PLAY VIDEO