1. Interactions Between Populations
Ecology: Communities
Interactions Between Populations

2. Properties of Communities
Interactions
-Competition
-Predation
-Symbiosis
Change through succession

3. Competition Relates to Niche Overlap
Degree of competition is proportional to the overlap in ecological niches of competing species
Ecological niche = total way of life of all members of a species
--Habitat
--Physiological needs such as temperature, moisture, nutrients
--Behavior --Interactions with other species

4. One Outcome of Competition: Competitive Exclusion
Grown in Separate Flasks
Grown in the Same Flask
Figure: 39.1
Title:
Competitive exclusion
Caption:
Raised separately with a constant food supply, both Paramecium aurelia and P. caudatum show the S-curve typical of a population that initially grows rapidly, then stabilizes. (b) Raised together and forced to occupy the same niche, P. aurelia consistently out-competes P. caudatum and causes that population to die off. (Modified from G. F. Gause, The Struggle for Existence. Baltimore: Williams & Wilkins, 1934.)

5. Another Outcome of Competition: Resource Partitioning
Figure: 39.2
Title:
Resource partitioning
Caption:
Each of these five insect-eating species of North American warblers searches for food in different regions of spruce trees.

6. Effects of Predation Eliminates old and weak individuals
Regulates numbers of both predators and prey
Coevolution of predator and prey
Regulates diversity of populations in community
“A keystone species plays a major role in determining community structure, one that is out of proportion to its abundance in the community. Removal of the keystone species dramatically alters the community.”
When Pisaster present, 15 species can be sustained, including mussels, limpets, barnacles, chitons. Only 8 species remain after removing Pisaster, mussels (favorite food of Pisaster) and barnacles predominate. “Mussels outcompeted algae and other invertebrates.”
The sea star Pisaster is a keystone species in aquatic communities in the Pacific Northwest.

7. Symbiosis
Parasitism
Commensalism
Mutualism

8. Ecological Succession
Predictable sequence of changes in vegetation type over time after a disturbance
Primary succession: occurs in barren environment with no pre-existing ecosystem
Secondary succession: new community develops after disturbance of existing ecosystem
Pioneer speciesclimax community

9. Succession in Progress
Mount St. Helens after 1980 explosion
Figure: 39.15b
Title:
Succession in progress
Caption:
Left: On May 18, 1980, the explosion of Mount St. Helens in Washington State devastated the pine forest ecosystem on its sides. Right: Twenty years later, life abounds on the once-barren landscape. Because traces of the former ecosystem remained, this is an example of secondary succession.
Same view 20 years later

10. Primary Succession Climax Community Pioneer Species Figure: 39.16
Title:
Primary succession
Caption:
Primary succession as it occurs on bare rock in upper Michigan.
Pioneer Species
Climax Community

11. Succession in a Freshwater Pond
Figure: 39.18
Title:
Succession in a freshwater pond
Caption:
In small ponds, succession is speeded by an influx of materials from the surroundings. (a) In this small pond, dissolved minerals carried by runoff from the surroundings support aquatic plants, whose seeds or spores were carried in by the winds or by birds and other animals. (b) Over time, the decaying bodies of aquatic plants build up soil that provides anchorage for more terrestrial plants. (c) Finally, the pond is entirely converted to dry land.

12. Secondary Succession Climax Community Pioneer Species Figure: 39.17
Title:
Secondary succession
Caption:
Secondary succession as it occurs on a plowed, abandoned southeastern farm field.
Pioneer Species
Climax Community

13. Pioneer Species Climax Community Tolerate environmental extremes
R-selected species predominate
High reproductive rates
Plants with seeds that spread easily
Poor competitors in established communities
Climax Community
Stable community at endpoint of succession
Suited to specific environmental conditions
K-selected species predominate

14. Some Communities Never Reach the Climax Stage
Grasslands – maintained by disturbances (fires & grazing)
Many forests – need fire
Chaparral – needs fire for seed germination

15. Human Activity Favors Weeds and Shrubs
Logging and forest clearing disrupt the climax community
Agricultural land is plowed each season
Overgrazing of grasslands in US Southwest gave advantage to mesquite shrubs

16. Applying Your Knowledge
Primary Succession
Secondary Succession
Pioneer Species
Climax Community
Which one would occur in a region where a glacier has receded, leaving a barren rock surface? (choice 1 or 2)
Which category would include a forest of conifer trees? (choice 3 or 4)
Which category would include lichens? (choice 3 or 4)