1. Communities and Biodiversity Lecture 15 Chapters in Text: 16, 26 Ecosystem structure and stability Community Succession

2. Food Web
Represent various species interactions:
Trophic (feeding) levels provides functional classification:
Basal Species – feed on none, are fed upon
Intermediate
Top predators
Guilds:
Divisions of trophic level
Species which exploit similar resources in similar fashion
Community as complex assembly of guilds
May be only distantly related

4. Species richness impacted by feeding relationships
Food Webs
Three types:
Connectedness Webs: feed relationships
Energy Flow webs: connections reflect energy flow relationships
Functional Webs: emphases on niche effect - role of species in structure of web
Species richness impacted by feeding relationships
Predators  diversification of web
Keystone Consumer concept

5. Functional Groups w/i Communities
Based on niche aspects
Conditions as Shade tolerant, drought addapted
C3, C4, CAM photosynthetic
Nature of reproductive cycles
Interoparous
Selemparous
Annual/perennial

6. Species dominance – Impact on Nature of Ecosystem Dominant Species
Most common species
May include size and numerical factors
Generally group species – dominant tree, shrub, fish etc.
Keystone species
Exerts disproportionate impact on a community relative to abundance
Modifies physical environment/structure on which many other species depend

8. Consistency: resistance to change imposed by outside factors
Food Web Stability
Consistency: resistance to change imposed by outside factors
Resilience: ability to return to reference state after disturbance
Example: grassland Ecosystem Change with increased precipitation
Immediate productivity gain
Long-term effect:
Returns to original productivity level
Species composition changes
New altered steady-state

9. Status about which a community functions More than one
Steady States
Status about which a community functions
Variations due to minor perturbations
More than one
Established with major outside change
Climatic change
Removal of major prey item
Introduction of major/keystone predator
Example: prairie plant community enforced by fire
Removal of fire 
Introduction of new grazers  ?

10. Views of Community Structure
Holistic concept:  Closed community
Clements – community structure determined by vegetation
Organisms considered in relation to their role (niche)
Associations are a consequence of evolutionary processes
Individualistic concept:  Open community
Gleason – associations are a coincidence
Community functions as a result of sum of individual actions

11. Ecotone: area of transition – spp at edge of distribuiton
Closed community
Species occur in unique, associated sets
Discrete boundaries – little overlap – distribution spatially determinant
Open community
Community members may occur in various associations
Indeterminate spatial distribution
Ecotone: area of transition – spp at edge of distribuiton

13. Continuum concept - Whittaker
Species distributed along environmental gradient
Gradient analysis: abundance of species plotted against 1+ env. condit
Examples:
Mountain slope
Gradients of temperature and precipitation
Eastern Hardwood Forest
North south gradient: temperature
East-west: precipitation

15. Disturbance: an event that changes a community Primary succession
Occurs where no soil exists when succession begins
Rare: usually soil is present!
The first plants to appear from a pioneering community
Secondary succession
Begins in an area where soil remains after a disturbance
Passes through phases, each known as a sere
Each sere and the ultimate climax community varies with physical conditions

16. Old Field Succession: Dwight Billings
Early species to invade: ‘weedy’ or r-selected species
Do not compete well for resources, high reproductive rate
Shift to k-selected species
Changes in nature of habitat favor species which reproduce successfully at or near carrying capacity

18. Early Stages of Old Field Succession
Pioneer Species – tolerant of soils with
Low nutrient/humus availability
Low water holding capacity
High temperature flux
Annuals  biennials
Add organic matter

19. Later Stages in Old Field Succession
Invasion by perennials – shrubs and trees
Increasing levels of soil organic matter
Water retaining capacity
Deeper root systems of perennials tap water and nutrients at greater soil depths
Increasing competition for sunlight
Forest development: Early species: soft woods (largely rapidly growing conifers)
Creation of microclimate
Shift to slower growing hardwoods + Shade tolerant species

20. Communities are in constant flux
Climax Ecosystem?
Communities are in constant flux
Tends to convey idea of stasis, rigid successional sequence
Succession leads to ‘Mature Ecosystem’
Successional events slow
Modification of community structure still happens - Nonequilibrium model
Moderate disturbance
Climatic changes
Disease pressure
Communities are constantly changing as they are buffeted by disturbances

21. Fire as an important force in maintenance of ecosystem vitality
Smoky Bear – A Dangerous Reactionary?
Fire as an important force in maintenance of ecosystem vitality
Is a significant disturbance in most terrestrial ecosystems
Is often a necessity in some communities
(a) Before a controlled burn. A prairie that has not burned for several years has a high propor- tion of detritus (dead grass).
(b) During the burn. The detritus serves as fuel for fires.
(c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.

22. Nutrients tied up in dead material
Fire suppression:
Nutrients tied up in dead material
Hotter fires, considerably more damage
Fire Ecology
Heat of fire
Time since last burn
Succession:
Fire followers
Smoke induces germination
Frequency of fire