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Lecture 9

Biology Lecture 9 Article 2.docx

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University of Toronto St. George
Scott Browning

BIOLOGY LECTURE 9 Succession: A Closer Look o Volcanoes, glaciers, sand dunes, storms, agriculture, fire… All initiate succession! (Summary) o Multiple, complex mechs can all interacy to result in predictable patterns of change in communities over time  Succession refers to directional, predictable changes in community structure over time  Change due to shifts in presence of different species as time passes  Most often thought about in terms of plant, but shift in the pops of other orgs also need to be considered Role of Disturbance  Plant communities: succession begins when an area is made partially or completely devoid of beg because of a disturbance  Large extreme disturbances result in slow succession because of complete mortality of all living things and loss of soil = PRIMARY SUCCESSION  Large, low intensity disturbance (plowing) = moderate amounts of succession where species can survive in soil and quickly recolonize after a disturbance = SECONDARY *Early successional communities: establish themselves relatively quickly after disturbance *Late successional communities: establish themselves much later Class Perspective of Succession:  Cowles (#1) described the chronosequence of vegetation along sand dunes, moving from bare sand beach to grasslands to mature forests  *Chronosequence = “spate-for-time” substitution where you can predict temporal patterns often based on area gradient representing diff ages of succession = often used to study succession = DIAGRAM Clements: Predictable change in veg time  Proposed concept of climax state for communities (final, permanent end stage of succession)  Communities are assemblage of characersitic plants that define an eco, after disturbance, any eco would eventually return to its characteristic assemblage of species… will return to climax community  Ideas of extreme predictability of succession made SUPER-ORG idea: all species of climax community work together to keep a stable composition  Idea that ecosystem could self-form/self-renew into a stable climax community  Idea of climax community still good today, but not super org  Gleason was like, no way dawg, and argued that communities were individualistic: only the fortuitous assembly of species and no such thing as climax state  Recognized that environ had important role in regulating species assemblages, and that community changes weren’t that predictable  Recognition of random aspects of community assembly liked by neutral models of biodiversity Patterns and Mechanisms of Succession  Successional are less predictable than what Clements thought  Several of his predicted patterns are true Ex) species diversity tend to increase with successional age of an ego  Mt. St. Helens eruption, return of plant life had steady increase in diversity over time Odum: described several predictable diffs between early + ate succession Ex) Early succession tend to have: o Smaller plant biomass, o Short plant longevity, faster rates of soil nutrient consumption, o Reduce role for decomp orgs, o More open and rapid cycling, o Higher rates of net primary productivity, o Lower stability o Lower diversity Bazzaz: characterized early and late successional systems based on physiology of plants assc with these stages Ex) Early successiona
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