Reice 1994 - Nonequilibrium determinants Flashcards
What are the 2 historical approaches to community structure?
1 - Community composition determined by environment, which sets range for colonization, reproduction, survival. Taxons results from random colonization and envr variability.
2 - Equilibrium theory
Interspecific
Between species interactions
Intraspecific
Within species interactions
Equilibrium theory
- Systems in steady-state equilibrium
- Equilibrium maximizes biodiversity
- Environment constant
- Biotic interactions are key determinant
- Species composition relative abundances stable through time overall
- Due to biotic interactions among members
- System returns to original structure after perterbation
What is the key determinant of community structure in equilibrium theory?
- Biotic interactions
- Community is result of competitive and predator-prey relationships
- Coexistence and spatial heterogeneity still important for diversity, but still fall w/in equilibrium
Shortcomings of equilibrium theory
- Prediction of interspecific-interactions don’t appear to hold in nature
- Coexistence of quite similar species and lack of demonstrable effects both of suspected competitors on each other and of predators on their prey distributions
What do non equilibrium theories emphasize?
- Environmental disturbance and spatial heterogeneity as factors that encourage colonization and species diversity
- Ex. Surge in diversity after fires
What happens to individual organisms with predictable vs. erratic disturbances?
- Predictable = biota adapt
- Erratic = loss of individuals
What concepts do non equilibrium theories include?
- Disturbance theory
- Patch dynamics
- Supply-side ecology
- High diversity and coexistence of species attributed to process of disturbance and recruitment
- tate, rarely achieved due to frequent disturbance followed by recovery
How is equilibrium viewed in a non equilibrium theory?
- Equilibrium is unusual state, rarely achieved due to frequent disturbance followed by recovery
How does disturbance and patches relate to one another?
- Disturbance creates patches
- But disturbance is also a product of the patches themselves
- Underlying control of community structure
What does the underlying physical-chemical environment determine in non-equilibrium theory?
- Determines frequency and magnitude of disturbance
- Influences time course of recovery
- Community structure
What does stochastic recruitment in a heterogenous, disturbed, patchy environment result in?
- High overall species diversity
What does disturbance and loss of individuals allow for in an ecosystem?
- Allows for creation of open spaces, recolonization and greater diversity
Environmental crunch
- Fluctuations in environment, temporal variability
- Affects relative importance of competition in communities over time
- Scarce resources and competition becomes more important
Features of temporal variability
- Seasonality of rainfall in tropics
- Temp fluctuations in arctic
- Temporal variation allows for coexistence of trophically equivalent species, one better under one set of conditions, and vice versa
How does coexistence of tropically equivalent species occur?
- Temporal variability resulting in envr fluctuations
- Leads to fluctuating competitor advantages, perhaps by season
- Mechanism greatest impact in disturbed ecosystems, ‘ecosystem crunch’
Simple definition of disturbance
Any relatively discrete event in time that disrupts ecosystems, community, or population structure, and changes resources, availability of substratum, or the physical environment
- Physical event, not the biotic outcome of the event
- Physical force that damages natural systems and removes organisms
What is the 1st impact of disturbance?
- Removal of organisms
What is the relationship of spatial scale with disturbance perception
- Large scales and long temporal scales make disturbances appear normal/predictable
- Smaller scales and durations lead to a random perception of disturbance
Is predation a considered a disturbance?
- No, Even though it removes individuals
- Predation is intrinsic to nature and prey can adapt to it
What is the disturbance associated with envr enrichment?
- Addition of nutrients or manure is not the disturbance
- It is the loss of dissolved oxygen that is the disturbance that leads to loss of organisms like fish
What is community structure made of?
- Species richness, number of species
- Species evenness, distribution of individuals among species
Which level of disturbance leads to maximum species richness?
- Intermediate disturbance (magnitude, intensity, and freq)
- Some resident species persist along with colonizers
- Too frequent and dominant competitors reduce and colonizers dominate, = reduced richness
Successional sequences after disturbance
- Different disturbances remove different species to different degrees
- Therefore succession is unique to the community
- Disturbance is reinitializing step in successional development, restructures whole ecosystem, not just a population process
Huston’s model of dynamic equilibrium for community structure
- trade-offs btwn pop growth rates, competitive exclusion rates, and freq of pop reductions
- Short disturbance interval = poorer competitors persist b/c competitive exclusion doesn’t have enough time to take place, = increased richness
- Diversity determined not by competition but by influence of the envr on outcome of species interactions
What is the return interval of disturbance roughly proportionate to?
- Proportionate to the lifespan of the dominant organisms (space holders) in a system
- Disturbed frequently relative to life history of dominant species
- Trees live hundreds of years, freq of fire or wind disturbances is shorter than that
- Disturbance return interval may be critical for life-history characteristics
What does the removal of individuals lead to?
- The opportunity for new species to colonize and change community structure
- New physical and niche space is created
- Larger magnitude = more individuals removed = more open space
What are the 3 mechanisms of recolonization of newly available niche space after disturbance?
- Regrowth (survivors, trees grow from stubs, coral polyps regenerate)
- Migration (from adjacent patches)
- Recruitment (individuals from outside proximate system - wind blown seeds, egg laying)
How can patchiness affect disturbance?
- Mitigate or intensify disturbance
- Patches have differential susceptibility to disturbance
- Disturbance enhances patchiness, whereby one patch sustains more population loss than other, therefore patches will be differentially open to recolonization
- Frequency of disturbance is result of physical patchiness of envr
How do patches opened by disturbance experience recruitment
- Recruitment pool diversity is varied, in space and time
- Patches develop distinctly different communities b/c only a portion of pool reaches each patch
- Patch community consists of random subset of populations available for recruitment
- Nearby identical patches that receive recruits from same pool may not get same species or diversity
- Recruits are in right place at right time to move into available space
