1. Environmental Science: Toward a Sustainable Future Richard T. Wright
Chapter 4
Ecosystems: Populations and Succession
PPT by Clark E. Adams

2. Factors That Contribute to Ecosystem Change
Dynamics of natural populations
Mechanisms of population equilibrium
Mechanisms of species adaptation
Ecosystem response to disturbance
Lessons to learn

3. Dynamics of Natural Populations
Population growth curves
Biotic potential versus environmental resistance
Density dependence and critical number

4. Biotic Potential and Environmental Resistance

5. Population Equilibrium
A dynamic balance between births and deaths.
Births
Deaths

6. Population Growth Curves

7. Population Growth Curves
Reproductive strategies:
Many offspring with
low parental care
Few offspring with
high parental care
J-shaped growth curve
S-shaped growth curve

8. Population Dynamics
Environmental resistance: combination of biotic and abiotic factors that may limit population increase
Predators, competitors, disease
Adverse weather, limited food/nutrients

9. Biotic Potential and Environmental Resistance

10. Density Dependence and Critical Numbers
Factors of environmental resistance are either:
density-independent: effect does not vary with population density; e.g., adverse weather
density-dependent: effect varies with population density; e.g., infectious disease
Critical number: the lowest population level for survival and recovery

11. Mechanisms of Population Equilibrium
Predator–prey dynamics
Competition
Interspecific
Intraspecific
Introduced species

12. Predator–Prey Balance: Wolves and Moose

13. Lessons to Be Learned about Predator–Prey Balance
Absence of natural enemies allows a herbivore population to exceed carrying capacity, which results in overgrazing of the habitat.
The herbivore population subsequently crashes.
The size of the herbivore population is maintained so that overgrazing or other overuse does not occur.

14. Plant–Herbivore Dynamics
Reindeer on St. Matthew Island
No regulatory control (predation) on herbivores
Went into exponential growth pattern
Overgrazed habitat
Massive die-off of herbivores

15. Mechanisms of Population Equilibrium: Plant–Herbivore
Compare the predator–prey with plant–herbivore methods of controlling the size of the herbivore population.
How would the herbivore population growth curve look if diseases or predators were used as the control mechanism?

16. Keystone Species
A single species that maintains biotic structure of the ecosystem
Pisaster ochraceus: a starfish that feeds on mussels, keeping them from blanketing the rocks

17. Competition: Intraspecific
Territoriality: defense of a resource against individuals of the same species
Examples of wolves and songbirds
Results in priority access and use of resources
How do wolves and songbirds establish territory?

18. Competition: Interspecific
Grasslands contain plants with both fibrous roots and taproots
Coexist by accessing resources from different soil levels

19. Introduced Species Rabbits in Australia (next slide)
Chestnut blight in United States
Japanese beetles, fire ants, gypsy moths in United States
Water hyacinth, kudzu, spotted knapweed, purple loosestrife (see Fig in text) in United States

20. Rabbits Overgrazing in Australia

21. Introduced Species
Why have introductions of nonnative and exotic species resulted in a degradation of ecosystems? (Think in terms of environmental resistance and biotic potential.)
An example of the answer to this question is given in the next slide.

22. Introduced Species: Rabbits in Australia
Introduced into Australia from England in 1859
No natural enemies – rabbit population exploded
Overabundant herbivore population devastated natural vegetation (see Fig in text).
Using disease as control measure – why will this procedure fail in the long term?

23. Mechanisms of Species Adaptation
Change through natural selection
Selective pressure determines which organisms survive and reproduce and which are eliminated.

24. NATURAL SELECTION: For? or Against?
Recipe for Change
GENES
ENVIRONMENT
ADAPTATIONS
NATURAL SELECTION: For? or Against? +

25. Adaptations to the Environment

26. The Limits of Change
Adapt
Move (migrate)
Die (extinction)

27. Vulnerability of different organisms to
environmental changes

28. Prerequisites for Speciation
Original population must separate into smaller populations that do not interbreed with one another.
List some ways this might happen.
Separated populations must be exposed to different selective pressures.
Example: arctic and gray fox (next slide)

29. Speciation: Foxes

30. Speciation: Galápagos Finches

31. Ecosystem Responses to Disturbance
Ecological succession
Disturbance and resilience
Evolving ecosystems

32. Equilibrium Theory
Ecosystems are stable environments in which the biotic interactions among species determine the structure of the communities present.

33. Succession and Disturbance
Ecological succession: transition between biotic communities
Primary: no previous biotic community
Secondary: previously occupied by a community
Aquatic: transition from pond or lake to terrestrial community

34. Primary Succession

35. Primary Succession
Mosses invade an area and provide a place for soil to accumulate.
Larger plants germinate in the new soil layer, resulting in additional soil formation.
Eventually shrubs and trees will invade the area.

36. Secondary Succession

37. Aquatic Succession

38. Disturbance and Resilience
Removes organisms
Reduces populations
Creates opportunities for other species to colonize

39. Fire and Succession

40. Ground Fire

41. Fire and Succession
Fire climax ecosystems: dependent upon fire for maintenance of existing balance; e.g., grasslands, pine and redwood forests
What significance does this have for humans and where they live?

42. Resilience in Ecosystems

43. Resilience Mechanisms after a Forest Fire
Nutrient release to soil
Regrowth by remnant roots and seeds
Invasions from neighboring ecosystems
Rapid restoration of energy flow and nutrient cycling

44. Lessons to Learn
Managing ecosystems
The pressure of population

45. Managing Ecosystems
Protecting and managing the natural environment to maintain the goods and services vital to human economy and survival.

46. The Pressures of Population
What is the carrying capacity for the human population on Earth?
How will the human ecological footprint impact on nature’s goods and services?

47. Carrying Capacity and Overshoot

48. End of Chapter 4