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Chapter 8

Chapter 8. Population Ecology. Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction?. They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback. Figure 8-1.

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Chapter 8

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  1. Chapter 8 Population Ecology

  2. Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? • They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback. Figure 8-1

  3. Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? • Sea otters are an important keystone species for sea urchins and other kelp-eating organisms. Figure 8-1

  4. POPULATION DYNAMICS AND CARRYING CAPACITY • Most populations live in clumps although other patterns occur based on resource distribution. Figure 8-2

  5. Why Live In Clumps? • Provides animals with better protection from predators and population decline • Gives some predator species better chance of getting a meal • Provides temporary groups from mating and caring for young.

  6. Changes in Population Size: Entrances and Exits • Populations increase through births and immigration • Populations decrease through deaths and emigration

  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. • Populations made up of reproductive age will increase

  8. Populations dominated post reproductive age will tend to decrease • Fairly even distribution will stay stable births equal deaths

  9. 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.

  10. Population reaches some size limit due to limiting factors (light, water, living space, nutrients, competitors, infectious diseases) • Growth rate of population decreases as size reaches carrying capacity due to resources dwindling

  11. Growth • Populations typically show 2 types of growth patterns • Exponential growth= Increase at a fixed rate each year no limitation to resources • Logistically= Rapid increase of a population then will level off because of declining resources or competition

  12. 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

  13. Environmental Resistance Carrying capacity (K) Population size (N) Biotic Potential Exponential Growth Time (t) Fig. 8-3, p. 163

  14. 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

  15. Logistic Growth • Rapid exponential growth followed by steady decrease in population until becoming steady • Population slows due to environmental resistance • Example brown tree snake

  16. Overshoot Carrying capacity Number of sheep (millions) Year Fig. 8-4, p. 164

  17. 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

  18. Population overshoots carrying capacity Population Crashes Number of reindeer Carrying capacity Year Fig. 8-6, p. 165

  19. Exceeding carrying capacity due reproductive time lag • Examples: Reindeer introduced to the Bering Sea • Can reduce lands carrying capacity (amount of resources needed to sustain the population) • Develop adaptive traits that reduces environmental resistance • Migrating to other areas

  20. Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size • Over time species may increase their carrying capacity by developing adaptations. • Some species maintain their carrying capacity by migrating to other areas. • So far, technological, social, and other cultural changes have extended the earth’s carrying capacity for humans.

  21. 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

  22. Higher population densities may help sexually reproducing individuals find mates but can also lead to competition • Help shield members from predators • Population density decreases opposite effect happens • Density dependent factors regulate a population at a constant size. (Plagues) • Density independent factors (can kill members of a population

  23. 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. Undisturbed areas • Irruptive: populations explode and then crash to a more stable level. Algae, Insects (Seasonal changes) • Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles. (Figure 8-7) snowshoe hare and lynx • Irregular: erratic changes possibly due to chaos or drastic change.

  24. 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

  25. Case Study: Exploding White-Tailed Deer Populations in the United States • Since the 1930s the white-tailed deer population has exploded in the United States. • Nearly extinct prior to their protection in 1920’s. • Today 25-30 million white-tailed deer in U.S. pose human interaction problems. • Deer-vehicle collisions (1.5 million per year). • Transmit disease (Lyme disease in deer ticks).

  26. REPRODUCTIVE PATTERNS • Some species reproduce without having sex (asexual). • Offspring are exact genetic copies (clones). • Others reproduce by having sex (sexual). • Genetic material is mixture of two individuals. • Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly. • Major advantages: genetic diversity chance of offspring protection.

  27. Sexual Reproduction: Courtship • Courtship rituals consume time and energy, can transmit disease, and can inflict injury on males of some species as they compete for sexual partners. Figure 8-8

  28. Reproductive Patterns • Species use different reproductive patterns to help ensure survival. • R selected= Have many offspring with little parental protection. They produce so many that a few will survive • Reproduce when conditions are favorable • Figure 8-10 • K selected= Have small number of offspring. Offspring more likely develop inside mother’s womb

  29. Reproductive Patterns:Opportunists and Competitors • Larger in size and card for by mother until reproductive age • Tend to do well in competitive conditions • Populations follow a logistic growth curve • Figure 8-10 Figure 8-9

  30. Carrying capacity K K species; experience K selection Number of individuals r species; experience r selection Time Fig. 8-9, p. 168

  31. Reproductive Patterns • r-selected species tend to be opportunists while K-selected species tend to be competitors. Figure 8-10

  32. r-Selected Species Cockroach Dandelion 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 Early successional species Fig. 8-10a, p. 168

  33. K-Selected Species Elephant Saguaro 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 Late successional species Fig. 8-10b, p. 168

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