Community ecology Outline Community structure attributes Factors influencing
Community ecology Outline: • • • Community structure: attributes Factors influencing the structure of communities Community dynamics Chapter 16 -18
Community attributes • • # of species Relative abundance of species Nature of species interactions (food webs) Physical structure
Community structure • Species richness (# of species within community) 10
Community structure • Relative abundance (% each species contributes to the total number of individuals)
Stand one Stand two
Simpson’s diversity index • D=0 -1 – 0: high diversity – 1: low diversity • Stand one (Table 16. 1): D=0. 13 • Stand two (Table 16. 2): D=0. 36
Dominance Yellow-poplar
Food webs
Keystone species
Functional groups • • • Feeding level Exploitation of common resources (guilds) Photosynthetic pathway Shade tolerance Life history
Physical structure
Zonation
supratidal intertidal subtidal
Association • Relatively consistent species composition • Uniform general appearance • Distribution that is characteristic of a particular habitat
Organismic concept of communities
Continuum concept of communities
Factors controlling community structure
Fundamental niche
Species interactions
Keystone predation
Apparent competition
Indirect commensalism
Top-down vs. bottom-up control
The number of trophic levels regulates plant, herbivore and carnivore numbers # trophic levels: B/comp: numbers limited by availability of resources (bottom-up regulation). Competition strong, predation weak. T/pred: numbers limited by predation (top-down regulation). Competition weak, predation strong.
Stress tolerance and competition
Smooth cordgrass Salt meadow cordgrass Black needle rush
Environmental heterogeneity
Relationship between the number of plants per 300 m 2 plot beside the hood river, NWT, and an index (ranging from 0 to 1) of spatial heterogeneity in abiotic factors associated with topography and soil. More spatially heterogeneous plots had higher species richness.
Environmental quality
Plant species diversity in a control plot and a fertilized plot in the Parkgrass experiment in Rothamstead, England. Fertilized plots have lower species diversity. The Parkgrass experiment, which began in 1856, is the longest running ecological experiment.
Community stability Types of stability: 1. Resilient community: returns to former state after disturbance 2. Resistant community: changes little in response to disturbance
Community dynamics: Succession
Primary succession Pioneer species Late successional species
Secondary succession
beach grass shrubs pines oak
Primary succession: newly exposed substrate
Dryas sp. An early succession species on glacial moraines in Glacier bay. Dryas is a symbiotic N-fixing plant
Populus trichocarpa Salix arctica After Dryas, cottonwood and willows become established
Alnus incana Alders become the dominant tree after 50 years
“Climax”: mixed spruce-hemlock forest Picea sitchensis Tsuga mertensiana
Secondary succession: after disturbance
Density Autogenic vs. allogenic change
Allogenic environmental change
Species diversity during succession Oak-pine forest
Intermediate disturbance hypothesis
Intermediate disturbance hypothesis High Low
In New Zealand streams, less disturbed sites support more complex communities (i. e. , communities with larger, more connected food webs)
Succession of heterotrophs Bark beetle/woodboring beetle Fungi/bacteria Predatory insects Moss and lichen Invertebrates/ mice/ salamanders
Changes over geologic time
Concept of community revisited
Concept of community revisited
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