21 Ch. and 22 Community Structure Community a spatially defined area that includes all populations in that area complex interactions between populations
Community Characteristics Biodiversity and biological structure o Species richness the number of species present in a community o Relative abundance the proportion of the total number of individuals in a community belonging to that species Typically a few species are common and most are rare
Species diversity the number of species present in a community AND the relative abundance Looks at the evenness of species o
o Dominants a single of a few species that predominate within a community Influence community in terms of biomass, space, energy flow, and nutrient cycling o Keystone species a species whose activities have a significant role in determining community structure. Small biomass in community Example: starfish in lab Beavers- changes environment and community by changing the landscape
Trophic structure o Food chain-a series in which species are listed in the order of what they eat.
o Trophic level defined o Position occupied on the food chain o Food web o Numerous food chains on it o Highly interwoven, with links representing many interactions
Trophic structure o o o o Primary producers Base of the food chain Produce their own energy through photosynthesis Ex-plants, algae
o o o o
Primary consumers Herbivores Obtain energy through primary producers Ex-deer, elephants
o o o o
Secondary consumers Carivores Obtain energy through primary cons Ex-wolf
o o o o
Tertiary consumers Carivores Energy through secondary consumers Ex-snakes, eagles
o Decomposers o Energy through dead organic material o Example-bacteria, earth worms, fungi o Trophic cascade influence of consumers or producers on populations that are two or more trophic levels removed Top-down control-higher trophic levels determine the size of the trophic levels them. below Bottom-up control-size of trophic level is determined by the rate of production of its food.
Top down------Beavers- sea urchant(up)-kelp(down) Bottom up---------kelp(down)-sea urchent(down)-beavers(down) or kelp(up) sea urchent(up) beavers(up)
Species Interactions o Symbiosis Mutualism Both species benefit Example-clown fish can hide in anemone with protects both
Commensalism One species benefits, the other unaffected Example-amora fish sticks to shark, benefits by getting sharks scraps but shark is not affected.
Parasitism One species benefits, other is negatively affected. Example-cowbird puts eggs in other birds nests and takes a different egg.
Physical structure and Growth form o Vertical structure Canopy Primary site of photsyntehsis Density influences other layers Open canopy-more dense understory and shrub layers-more dry Closed canopy-poorly developed understory-higher moisture
lower canopy/understory Made up of sapling trees Depressed growth from shade Made up of shade tolerant trees
Shrub Similar to understory Smaller sized woody vegetation
Herbaceous Dependent upon soil moisture and nutrient conditions Influenced by the density of the canopy Sunlight wild flowers
forest floor, or ground layer decomposition occurs nutrients are released back into soils
o o o o o o o
Communities described based on vegetation Forest can be: Deciduous Coniferous Mixed Named based on dominant species: Oake-hickory, pine, beech
o Zonation
o Temporal impacts on communities o Not only change over space, but also change over time.
o Primary succession o New community on a previously unoccupied site. o Newly exposed sand or soil.
o Secondary succession o The formation of a new community after some sort of disturbance.
o o o o
Climax community Endpoint of the successional sequence Community has reached steady state Under its environmental conditions