Community Structure & Disturbance Flashcards
Stability
the tendency of a community to maintain a relatively constant composition and number of species in the face of disturbance
Non equilibrium model
Communities are subject to constant change as a result of disturbance and ecological succession <- the norm for most communities
Disturbance
Disturbance influences species diversity and
composition – directly + via resource availability
Disturbance - any event that changes community composition, removes organisms, and alters resource availability.
Fragility
A community is FRAGILE
when it remains unchanged by minor disturbance but is dramatically altered by major disturbances
Robustness
A community is
ROBUST when it remains largely unchanged even during major disturbances
Resilience
Resilient communities
rapidly return to their former structure following disturbance
Resistance
Resistant communities
undergo relatively little change in the face of disturbance
Impacts of disturbance are not always negative
Can create opportunities
for some species
(which have not previously
been able to establish)
e.g. less competitive species
Small scale disturbances
can enhance habitat
PATCHINESS
– Helps maintain diversity
– Abiotic heterogeneity
High levels of disturbance
reduce diversity
o Stress exceeding the tolerance of many species
o High frequency disturbance prevents slow colonisers/growers from establishing
Low levels of disturbance
Reduce diversity
o Competitively dominant species exclude other species
o Disturbance may have a huge influence on local biodiversity
The intermediate disturbance hypothesis
Moderate levels of disturbance create
conditions that foster high diversity
Ecological succession
- Disturbed areas colonised by a variety of species which are in turn replaced by others, and so on
- Changes in community
composition most apparent
after disturbance removes
existing vegetation - Volcanic eruptions,
glaciers, winds, floods, fire
Primary succession
– When a new, lifeless area is colonised (e.g. Volcanic island, glacial moraine)
* Initial life forms:
o Prokaryotes
o Lichens and mosses the first multi- cellular arrivals
* Easily transported via windblown spores
Colonisation
Arrival and establishment are related to three key processes:
Facilitation
* Early arrivals may facilitate the arrival of others (e.g. via fertilising the soil)
Inhibition
* Early species may inhibit the arrival of others so
successful colonisation occurs in spite of early arrivals
Tolerance
* Early arrivals may neither facilitate nor inhibit the
arrival of later species
Secondary succession
– Disturbance, clears most vegetation, but soils remain intact (e.g. Yellowstone fires)
* Area will often return to its former state
* Areas cleared for agriculture
– If left can return to the previous climax community
– Rainforests of central America
– Secondary forest – regrown after logging
