Succession Flashcards
Ecological succession
A form of growth and development of an ecosystem as a whole
progressive change in species composition, ecosystem structure and functioning along time (usually ) following a disturbance
Primary succession
what happens after a volcanic explosion for instance, when the process begins in a virtually lifeless area.
Secondary succession
what happens when an existing community has been cleared by a disturbance, but soil is left largely intact, for instance deforestation.
Process of primary succession
Building an ecosystem from the bottom-up
Determinism
1- Producers
2- Consumers
3- Predators
Stochasticity
-Immigration
-Colonisation
-Extinction
Pioneer species
liverworts, mosses, fireweed, Dryas shrubs and willow
Disturbance
any relatively discrete1 event in space and time that disrupts2 ecosystem, community, or population structure and changes resources, substrate, or the physical environment (Pickett and White 1985)
1- Discrete in time: cannot be a chronic stress or background environmental variablity
2- Cause a notable change (perturbation) in the state of the system
Classic Clementsian succession
An orderly return to pre-disturbance community composition:
Pioneer community -> Climax community
Facilitation: Earlier communities alter habitat so that plants in succeeding communities can invade
The role of randomness in succession
Gleason 1926 – communities as sum of individual behaviour (individualistic view) coexisting species are the result of their similarities and adaptations succession as stochastic process
Communities are random aggregation of species, thus;
Succession not seen as deterministic
Stochastic
having a random probability distribution or pattern that may be analysed statistically but may not be predicted precisely.
Relay floristics
similar to Clement’s model, early colonists facilitate replacement by later colonists.
Initial floristic composition
all species are present from very start, main difference in success is due to growth rates and lifespan.
Gradient in time
species turnover in time due to life-history traits related to adaptation to different biotic and abiotic conditions. Early occupants inhibit colonisation of later occupants.
Resource ratios
species turnover is driven by a shift in resource availability from high to low.
Shade tolerance
trade-off between shade tolerance and other life history traits (e.g growth rate, dispersal ability).
Lines of theories to explain succession
Relay floristics
Initial floristic composition
Gradient in time
Resource ratios
Shade tolerance
Phases of secondary succession in a tropical forest
Stand initiation phase
Stem exclusion phase
Understory reinitiation stage
Stand initiation phase - 0-10 years
Germination of seed bank and newly dispersed seeds
Re sprouting of remnant trees
Colonisation by shade-intolerant and shade-tolerant pioneer species
Rapid height and diameter growth of woody species
High mortality of herbaceous old-field colonising species
High rates of seed predation
Stem exclusion phase - 10-25 years
Canopy closure
Recruitment of shade-tolerant seedlings, saplings and trees
High mortality of short-lived, shade -intolerant trees
Development of canopy and understory tree strata
Understory re initiation stage
Formation of canopy gaps
Development of spatial aggregations of tree seedlings
Increased heterogeneity in understory light availability
Intermediate disturbance theory
Low levels of disturbance (very stable communities) allow the loss of species through: competitive exclusion of some species
Constantly high levels of disturbance (stressful habitats) prevent an increase in richness due to low population numbers/higher population extinction rates
Intermediate levels of disturbance (stress) prevent competitive exclusion, but do not reduce the population sizes so low that species go extinct by chance
How do you test intermediate disturbance theory
Temporal sequences
Spatial samples at different age or stage
Control for other factors (elevation, geology, additional disturbance…)
Management of succession
Conservation of habitats frequently involves the management of succession.
If particular species and habitats present at earlier stage of succession are of conservation interest.
Thus deliberate interference to prevent the ecosystem developing further, maintaining what is known as a plagioclimax.
Coppicing
Traditionally used for production of wood for fences/baskets with taller trees for construction, e.g. hazel/sweet chestnut and other species
Much conservation management involves interrupting natural succession (applications)
Species richness peaks with intermediate disturbance
Recruitment limitation and inhibition by resident vegetation
Sparce seed bank
Seed predation
Seedling predation
Inhibition by resident vegetation
Which stages of succession can we accelerate
Seedling establishment of bird- and bat-dispersed, shade-tolerant trees Development species
High mortality of short-lived, shade-intolerant pioneer trees
Formation of canopy gaps
Increased heterogeneity in understory light availability
Important measures of terrestrial ecosystems
Successional changes in species diversity
Above ground biomass
Above ground litter production
Net ecosystem production
Nutrient pools
Nutrient cycling
