BIOL 1911 Chapter Notes - Chapter 54.3: Herbaceous Plant
29 Apr 2018
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54.3 – Diversity influences species diversity and
composition
• “ ” F ,
o ’
o Focus on competition as a key factor determining community composition and maintaining stability in
communities
o S = ’
• F. E. Clements
o Argued for the balance of nature view in the 1900s
o Believed that the community of plants at a site had only one stable equilibrium, a climax community controlled
solely by climate
o Biotic interactions caused the species in the community to function as an integrated unit – like a superorganism
▪ Based on the observation that certain species of plants are consistently found together
• A. G. Tansley
o Challenged the concept of a climax community
o Argued that differences in soils, topography and other factors created many potential communities that were
stable with a region
• H. A. Gleason
o Communities are a chance assemblage of species found together because they have to have similar abiotic
requirements (temperature, rainfall, soil type)
o Disturbances – any event that changes a community by organisms from it or altering resource availability –
keeps many communities from reaching a state of equilibrium in species diversity or composition
• Nonequilibrium model
o Describes most communities as constantly changing after disturbance
o Even relatively stable communities can be rapidly transformed into nonequilibrium communities
Characterizing Disturbance
• Types of disturbances and their frequency and severity vary among communities
• Ex: storms, fire, flood
• High level of disturbance: result of frequent and intense disturbance
• Low disturbance levels; either low frequency or low intensity of disturbance
• Intermediate disturbance hypothesis
o Moderate levels of disturbance foster greater species diversity than do high or low levels of disturbance
o High levels of disturbance reduce diversity by creating environmental stresses that exceed the tolerances of
many species or by disturbing the community so often that slow-growing or slow-colonizing species are
excluded
o Low levels of disturbance can reduce species diversity by allowing competitively dominant species to exclude
less competitive ones
o Intermediate levels of disturbance can foster greater species diversity by opening up habitats for occupation by
less competitive species
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