Community Structure

Question 1

Food web

Answer 1

• the feeding relationships among organisms in a community

- made up of all food chains

Question 2

Food chain

Answer 2

-a single path within a food web

Question 3

Trophic level

Answer 3

-the position of an organisms in a food chain

Question 4

Bottom-up control

Answer 4

• basal trophic levels effect those above

- primary producers limited by abiotic factors (sunlight, water…), thus levels above become limited

Question 5

Top-down control

Answer 5

-abundance and behavior of consumers limits the lower trophic levels

Question 6

Keystone species

Answer 6

-have a disproportionately large effect on a community relative to its abundance

• frequently predators that maintain diversity across trophic levels
• top-down control

Question 7

Trophic cascade

Answer 7

-a series of changes in the abundance of species in a food web

• can be caused by a loss of top predators
• when changes in top-down control cause conspicuous effects two or three links away in a food web

Question 8

Climax community

Answer 8

• more or less less permanent and final stage of a particular succession
• slow rates of change
• species tolerant of competition
• the final stage of the developmental progression of a community

Question 9

Disturbance

Answer 9

• any strong, short-live disruption to a community that changes the distribution of living and/or nonliving resources
• the impact of disturbance is a function of three factors: the type of disturbance, its frequency, and its severity

Question 10

Disturbance regime

Answer 10

-the historic patterns (frequency and extent) of natural processes such as fire, insects, wind, and mass movement that affect the ecosystems and landscapes in a particular area

Question 11

Top predators

Answer 11

-the loss of these predators can cause a trophic cascade

• often more vulnerable to threats
• individuals often require large ares to feed, find mates, and reproduce
• degradation reduces population sizes and species range

Question 12

Succession

Answer 12

• changes in a communities occupying an area after a disturbance or the creation of a new substrate
• changes are often predictable
• at each stage species alter the environment (sunlight, temperature, nutrients)
• existing species affect subsequent species: facilitation, tolerance, inhibition
• two types; Primary and secondary

Question 13

Primary Succession

Answer 13

• on newly exposed landforms without soil or organisms

e. g. glacial retreat; volcanoes

Question 14

Secondary Succession

Answer 14

• are has been partially or completely removed by a disturbance, but where soil seeds, and spores remain
• more common type
  e. g. fire; logging

Question 15

Fredrick Clements

Answer 15

-studied plant succession

said that communities are:

• predictable
• stable, orderly
• determined by climate and species interactions
• species not likely to be found without each other

Question 16

Henry Gleason

Answer 16

said that communities are:

• not predictable or stable
• chance and historical events determine community (e.g. colonization after disturbance)

Question 17

Facilitation

Answer 17

• an existing species makes the conditions better for a subsequent species
• e.g. one species makes the soil more acidic and the next plant that comes in needs acidic soils

Question 18

Tolerance

Answer 18

-the existing species have no effect on subsequent species in the community

Question 19

Inhibition

Answer 19

-the existing species hinders the establishment of a subsequent species

Question 20

Experimental test of whether communities are predictable or not

Answer 20

• 12 identical ponds are filled at the same time with sterilized water and the species that develop are recorded and compared
• At least 20 species were found in most or all of the ponds (Predictable- Clements)
• Each pond has a unique composition (Unpredictable- Gleason)
• Other research shows that the same groups of species don’t always occur together (elevational gradients; pollen cores)
• Generally, Clements view seems too strict
• Chance and history do seem to play a large role in shaping communities

Question 21

What two factors seem to play large roles in shaping communities according to research?

Answer 21

Chance and History

Question 22

Pioneer Species

Answer 22

• species that establish soon after disturbance
• “weedy” : grasses and small plants
• a weed is adapted for growth in disturbed soils
• devote energy to reproduction rather than competitive ability
• high r-max