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Community Dynamics Part 1

by: Jesse McDonald

Community Dynamics Part 1 Biology 286

Marketplace > Purdue University > Biology > Biology 286 > Community Dynamics Part 1
Jesse McDonald

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About this Document

These notes cover the beginning to community dynamics and correspond to lecture 21
Introduction to Ecology and Evolution
Dr. Josh Springer
Class Notes
community dynamics, Biology, Ecology, evolution
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This 4 page Class Notes was uploaded by Jesse McDonald on Tuesday April 12, 2016. The Class Notes belongs to Biology 286 at Purdue University taught by Dr. Josh Springer in Spring 2016. Since its upload, it has received 11 views. For similar materials see Introduction to Ecology and Evolution in Biology at Purdue University.


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Date Created: 04/12/16
iClicker Question:  Predator populations usually lag in size and mimic what occurs in prey population sizes.  This statement is:  A. True   B. False     True predators do not necessarily kill their prey immediately and may feed off of it for a  long time before the kill. This statement is:  A. True   B. False     Community Dynamics     Quantify Communities:  ● Important for understanding diversity  ○ Communicating results to other ecologists  ○ Such as restoration, wildlife management, evolutionary implications over long  periods of time.     Community Structure:  ● Species in a community have many different types of interactions.  ● Specific attributes of a community include:  ○ Species number and relative abundance  ○ Physical structure:  ■ Usually defined by plant growth forms  ○ Interactions among species  ○ EVOLUTION of communities as well, not just individual species.     Community defined by species composition:  ● Individuals of each species in a community can be counted or estimated  ● A more meaningful measure isrelative abundance​→ the proportion of each species  relative to the total number of individuals of all species living in the community    i​ ni​   ● P​i ​roportion of individuals of species i   ● N​i ​umber of individuals of species i   ● N ​  total number of individuals of all species    ● Two features help us define community structure:  ○ Species richness ​S) ­ the number of species in the community  ○ Species evenness  ­ how equally individuals are distributed among the species  ■ Simpson’s Diversity Index refers to 3 closely related indicates that  consider both species number and relative abundance.  ● Simpson’s index (D) ­ probability that two randomly selected  individuals from the community will belong to the same species  2  D = Σpi​ ● P​= proportion of total individuals in community  i ​ represented by species i (relative abundance)  ● Value ranges between 0 and 1  ● Because this range of values seems to be counterintuitive:  ○ D = 1 with no diversity (with one species, the probability  that both selected will be the same is one)  ○ D approaches zero with higher diversity  ● Simpson’s index of diversity = 1 ­ D  ○ The value increases with species diversity  ○ This index represents the probability that two individuals  randomly drawn from a community will belong to different  species.  ● Shannon index ​is a widely used index of diversity that also considers  species richness and evenness  ○ This index is symbolized by H and is computed:  ○ H = ­Σ(pi​l​i​ ○ P​i​ the proportion of the total individuals in the community  represented by species  ​ ○ Ln = natural log  ○ Minimum value = 0 (one species represent)  ○ Maximum value = ln S (all species in equal numbers)  ● This maximum value, Hmax​S can be used to calculate the index of  species evenness:  ○ E​H​ H ÷ max  ○ The values range from 0 to 1 (complete evenness with all species  equally abundant)  Dominance can be defined by a number of criteria:  ● Abundance alone is not always a sufficient measure of dominance  ● In a forest:  ○ There may be more small understory trees  ○ But the fewer large trees have most of the biomass  ● In a deciduous forest in Virginia:  ○ Relative abundance ­ 60% of the trees are red maple and dogwood  ○ Relative biomass ­ 60% of the biomass is in white oaks, which account for 9% of  the relative abundance.    Keystone species influence community structure disproportionately to their numbers:  ● Keystone species function in a unique and significant way within a community  ○ Their effect is much greater and disproportionate to their numerical abundance  ● The role of a keystone species may be to create or modify habitats, or influence  interactions among other species.  ● The removal of a keystone species can lead to:  ○ Changes in community structures  ○ Loss of biodiversity  ● The coral​culina arbuscul(live off the coast of eastern North America) is a keystone  species.  ○ It’s the only coral in the region with a structurally complex, branching shape.  ○ Creates a habitat for 300 species of invertebrates that live among its branches.  ○ Many species complete much of their life cycle within the coral.   ● Keystone herbivores can modify the community through their feeding activities.  ○ African elephants in savannas of southern Africa:  ■ Feed mainly on woody plants (browse)  ■ Are destructive feeders, often uprooting, breaking and destroying the  shrubs they eat.  ○ This reduction of tree and shrub density favors growth and reproduction of  grasses  ■ This community change benefits grazing herbivores, but not the  elephants.   ● Predators can be keystone species in communities also.  ● Sea otters are a keystone predator in kelp bed communities of the pacific northwest.  ○ Kelp beds are habitat for many species  ○ Sea urchins feed on kelp → sea otters feed on sea urchins → killer whales feed  on sea otters  ○ Sea otter populations have declined as a result of killer whale predation ­  keystone predator removed.  ○ Sea urchin population has increased, and it has overgrazed the kelp, reducing  habitat for other species.  Food webs describe species interactions:  ● Intermediate species­ either herbivores (H) or carnivores (C ) that feed on other  species and are the prey of other species (may also be omnivores)  ● Top predators (P) ­ feed on intermediate and sometimes basal species (omnivores) but  are not preyed upon themselves.  ● Study of a Caribbean marine food web: total of 3313 trophic interactions among 249  species  ● Food web could be divided into five compartments based on:  1. Differences in body size  2. Range of prey sizes selected  3. Uses of shore vs. offshore habitats  4. Associated predators  ● Any two species in a food web are linked by a single arrow (from prey to predator)  ● Community dynamics do not only involve direct species interactions  ○ A predator may reduced competition between two prey species by keeping the  population size of both species below the carrying capacity for each  ● These indirect effects must be included in an analysis of community structure.    Zonation is spatial change in community structure:  ● Zonation is the change in physical and biological structures of communities as seen  when moving across the landscape  ● From the base to the summit of the Siskiyou Mountains at the California/Oregon border  ○ Dominant tree species richness  ■ From 17 to 9  species from lower to mid­elevation   ■ Only 3 species at 1920­2140 meters  ○ The insects, birds, and small mammals also change.     Defining boundaries between communities is often difficult:  ● The example of forest zonation in the Siskiyou Mountains, moving up the mountainside  through elevational zones, covers a relatively short distance.  ● In a larger area, differences in community structure increase.   ● Patterns of forest zonation in Great Smoky Mountains National Park  ○ Zonation pattern is complex, including elevation, slope position, and exposure.  ○ Communities are named for their dominant tree species.   


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