Interactions Between Populations Ecology: Communities Interactions Between Populations

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

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

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.)

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.

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.

Symbiosis Parasitism Commensalism Mutualism

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

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

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

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.

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

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

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

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

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)