1. Community Ecology Part 2
organism
population
community
ecosystem
biosphere

2. Characterizing a community
Community structure
species diversity
how many different species
composition
dominant species
most abundant species or highest biomass (total weight)
keystone species
key role
strong effect on composition of the community

3. Species diversity greater diversity = greater stability
Greater biodiversity offers:
more food resources
more habitats
more resilience in face of environmental change

4. The impact of reduced biodiversity
compare these communities
suburban lawn
agricultural “monoculture”
“old field”
In the early 1970's, a new corn genotype was released in the US; the "Texas male sterile" (TMS). Hybrids that contained this new characteristic had many desirable properties, and growers were excited about them, planting them over miles and miles of corn acreage in US. They were, of course, bred to be resistant to the most common corn diseases. However, they were not resistant to a previously unimportant strain of a fungal disease; the southern corn leaf blight (caused by the fungus Helminthosporium maydis). Ninety percent of the corn sowed in the US in 1970 contained the TMS trait and also shared genetic susceptibility to this pathogen. The fungus encountered all this acreage of susceptible host and wiped out one fourth of the US corn crop in 1970, a loss of over one billion dollars in production! If the corn acreage hadn't been such a monoculture, the fungus wouldn't have been able to spread as rapidly, as it would have encountered barriers of genetically resistant plants.
Irish potato famine
1970 US corn crop failure

5. Keystone species Influential ecological role
Pisaster ochraceous
Influential ecological role
exert important regulating effect on other species in community
keystone species increases diversity of habitat
Sea star
diversity increases
diversity decreases
mussels out-compete other species
Washington coast

6. Keystone species Sea otter is a keystone predator in North Pacific
What is the impact of the Orca whale?
Structure of a community may be controlled bottom-up by nutrients or top-down by predators

7. Keystone species Beaver is a keystone species in Northeast & West
dams transform flowing streams into ponds creating new habitat

8. Ecological succession
Sequence of community changes
transition in species composition over time
years or decades
usually after a disturbance
Mt. St. Helens

9. Primary succession
Begins with virtually lifeless area without soil, then…
bacteria
lichens & mosses
grasses
shrubs
trees
make soil {

10. Secondary succession
Existing community cleared, but base soil is still intact
burning releases nutrients formerly locked up in the tissues of tree
the disturbance starts the process of succession over again

11. Succession of species pioneer species compete well in high sunlight
lichens & mosses
grasses
more shade tolerant species
climax forest
shade tolerant species
stable community
bushes & small trees
trees

12. What causes succession?
Tolerance
early species are weedy r-selected
tolerant of harsh conditions
Facilitation & Inhibition
early species facilitate habitat changes
change soil pH
change soil fertility
change light levels
allows other species to out-compete

13. Climax forest Plant community dominated by trees
Representing final stage of natural succession for specific location
stable plant community
remains essentially unchanged in species composition as long as site remains undisturbed
birch, beech, maple, hemlock
oak, hickory, pine

14. Climax forest
taiga
The species mix of climax forest is dependent on the abiotic factors of the region
solar energy levels
temperature
rainfall
fertility & depth of soil
temperate deciduous forest birch, beech, maple, hemlock

15. Disturbances as natural cycle
Disturbances are often necessary for community development & survival
release nutrients
increases biodiversity
fire climax forests
increases habitats
rejuvenates community

16. Jack Pine
Fire climax species

17. When people don’t learn ecology!
Building homes in fire climax zones
preventing fires makes next year’s fire much worse!

18. Don’t blow your top! Ask Questions!