BIOL 2Q04 Lecture Notes - Lecture 18: Intermediate Disturbance Hypothesis, Abies Amabilis, Climax Community
14 Dec 2016
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BIOL 2Q04 | November 23, 2016
SUCCESSIONAL CONT’D
Tolerance Model
− Later successional species are neither inhibited nor aided by species of earlier stages
o Communities are composed of species most efficient in exploiting available resources
o I.e. Pacific silver fir will grow under the canopy of Douglas fir – extremely shade-tolerant (uses
light more efficiently)
▪ Eventually, this late successional specialist prevails if there are no forest fires
The Climax State
− Final stage of succession – stable community
− In equilibrium with physical and biotic environments
− How it stays in equilibrium, 3 hypotheses:
o Monoclimax (Clements)
▪ Every region has one climax community toward which all communities are developing
toward
• I.e. Climax vegetation of the same general type will be produced and stabilized
irrespective of prior site conditions
o Polyclimax (Tansley)
▪ Many different climax communities may be present in an area, they are controlled by:
animals, soil moisture, nutrients, etc.
o Climax Pattern (Whittaker)
▪ The whole natural community is adapted to the whole pattern of environmental
factors in which it exists – soil, climate, fire, biotic factors and wind – prevailing
hypothesis
Chronosequence – Studying Succession
− How do we know succession is working?
o Observing changes overtime (direct)
▪ Glacier Bay: about 1500 years
▪ Yves’ Forest: aout 200 years
o Logging vegetation over the years to be able to paint a picture of what is going on in terms of
succession
o Best approach but takes a longer time
− Indirect: chronosequences (chronoseres) are groups of sites used to compare patterns of diversity
through succession
o Assumption is that different patches are replicates in space of the same process but are
asynchronous
Intermediate disturbance Hypothesis
− Disturane ats to reset the lok
o Starts at time 0
− Patterns of diversity depend on the frequency of disturbance
o Disturbance is the underlying pattern in the community structure – diversity being a product
o High disturbance → diversity will remain low because later successional species never have a
chance to colonize
find more resources at oneclass.com
find more resources at oneclass.com
▪ Animals that also depend on later successional species will not be present
o Low disturbance → diversity will decline as later successional species displace earlier species
▪ Eventually moves to a climax state – governs type of species that survive
o Intermediate disturbance → colonization can occur but competitive displacement is held to a
minimum
Intermediate Disturbance Hypothesis → Disturbance and Diversity: An Ecological Chicken and Egg Problem
− Cause or consequence of biodiversity?
o I.e. Experimental manipulation = decreased invasion success but invasion = increase in
biodiversity
− Intermediate levels are what governs community structure and biodiversity
Heterotrophic Communities – Animals
− “uession also ours…
o I.e. Decomposition of fallen trees, animal carcasses, and droppings by fungi
− As the vertical structure becomes more complex, new species appear
− Depending on the species that are dominating in that community will give rise to other species in the
communities
o More complex with regards to resources and habitat needs as succession takes place
Community Concept
− The structure of communities is the product of complex interaction of patterns and processes
− Species respond to an array of environmental factors that vary over space and time – that leads to slow
changes over time
Energy Flow and Trophic Structure
Community Structure
− The physical structure of a community reflects abiotic and biotic factors
− Form and structure of terrestrial communities are defined primarily by vegetation
o Often named based on the dominant growth forms and their physical structure
▪ I.e. herbaceous woody
− Aquatic communities are defined by features of the abiotic environment
o I.e. salinity, and water depth
− Vegetation dictates the structure of the communities
Organization of Communities – 4 Main Biotic Drivers
− Competition
o Controls diversity and abundance among plants, herbivores, and carnivores
− Predation
o Organizes the community according to: who eats whom:
o Framework of the community is dictated by animals
− Herbivory
o Same as predation – structures the community according to who eats whom
− Mutualism
o Links species – might serve to increase community organization by linking species to the
benefit of all
How to Study Community Structure – Who Eats Whom?
− Once we know the diversity of our community, next: relations between species
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Later successional species are neither inhibited nor aided by species of earlier stages: communities are composed of species most efficient in exploiting available resources. Pacific silver fir will grow under the canopy of douglas fir extremely shade-tolerant (uses light more efficiently: eventually, this late successional specialist prevails if there are no forest fires. Final stage of succession stable community. How it stays in equilibrium, 3 hypotheses: In equilibrium with physical and biotic environments: monoclimax (clements, every region has one climax community toward which all communities are developing toward. Indirect: chronosequences (chronoseres) are groups of sites used to compare patterns of diversity through succession: assumption is that different patches are replicates in space of the same process but are asynchronous. Distur(cid:271)an(cid:272)e a(cid:272)ts to (cid:862)reset the (cid:272)lo(cid:272)k(cid:863: starts at time 0. Intermediate disturbance colonization can occur but competitive displacement is held to a minimum. Intermediate disturbance hypothesis disturbance and diversity: an ecological chicken and egg problem.
