Community Structure Flashcards
Food web
- the feeding relationships among organisms in a community
- made up of all food chains
Food chain
-a single path within a food web
Trophic level
-the position of an organisms in a food chain
Bottom-up control
- basal trophic levels effect those above
- primary producers limited by abiotic factors (sunlight, water…), thus levels above become limited
Top-down control
-abundance and behavior of consumers limits the lower trophic levels
Keystone species
-have a disproportionately large effect on a community relative to its abundance
- frequently predators that maintain diversity across trophic levels
- top-down control
Trophic cascade
-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
Climax community
- 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
Disturbance
- 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
Disturbance regime
-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
Top predators
-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
Succession
- 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
Primary Succession
- on newly exposed landforms without soil or organisms
e. g. glacial retreat; volcanoes
Secondary Succession
- are has been partially or completely removed by a disturbance, but where soil seeds, and spores remain
- more common type
e. g. fire; logging
Fredrick Clements
-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
Henry Gleason
said that communities are:
- not predictable or stable
- chance and historical events determine community (e.g. colonization after disturbance)
Facilitation
- 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
Tolerance
-the existing species have no effect on subsequent species in the community
Inhibition
-the existing species hinders the establishment of a subsequent species
Experimental test of whether communities are predictable or not
- 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
What two factors seem to play large roles in shaping communities according to research?
Chance and History
Pioneer Species
- 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
