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Department of Biological  and Physical Sciences

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IV. Community Ecology (Who's who and who's not)
 
 

B.  How important are species interactions

2. Patterns in species numbers

Are there predictable patterns among communities, and, if so, can interactions between species explain these patterns?
 
 

Examples of patterns in the number of species within communities
 

Increasing species number in tropical regions:

Theory of island biogeography:

The theory was formalized by MacArthur and Wilson to explain two trends in the number of species found on an islands:

1. Number of species increases with island size (e.g. ants, reptiles, birds, plants)

    S=bA^z

Where:
S = number of species on island
A = Area (size) of island
b = coefficient varying w/ taxa and region
z = slope varying w/ degree of isolation (and dispersal abilities)

 
 
 
 

2. Number of species decreases with distance of island from the mainland
 

 

What causes these trends exist?

Robert MacArthur and E.O. Wilson explain this as dynamic equilibrium between extinction and immigration:

Turnover rate is rate at which one species is lost and another regained when at equilibrium. Why does immigration decrease with increasing species number?  Why does extinction increase with increasing species number?

How do island size and distant from mainland affect these rates?

• immigration of newly colonizing species affected by distant  (Is dispersal more or less likely onto island farther from the mainland?).
• 
  

• extinction of previously established species affected by size (Is the smaller population size of species on small islands  more or less likely to result in extinction?).

 
 
 
 

Other possible causes for trends:
 

• Extinction may be higher on smaller islands

• fewer types of environments/habitats

 
• greater ratio of edge to interior exposing species to greater physical stress and to predators from other habitats


 
• habitat not large enough to contain minimum home range or territory.
• insufficient patchs to maintain non-equilibrium processes of competitors and predator-prey

 
• Immigrations may be greater on larger islands
• larger area to catch species

 
 
 
 

Experimental tests of island biogeography theory:


• Simberloff and Wilson fumigated mangrove islands and observed recolonization of insects:

• Rey performed a similar fumigation of Spartina islands with similar results, but also caged islands to measure extinction in the absence of immigration and confirmed the idea that smaller islands experienced greater extinction rates.

   

 

How could this theory be applied in deciding which areas should be set aside as bioreserves to preserve biodiversity?
 
 
 

MacArthur and Wilson's theory is often referred to as the "Equilibrium" Theory of Island Biogeography.  Why?  Does this mean that species composition (who's there and who's not) is constant after a certain point in time?




 
 
• The Intermediate Disturbance Hypothesis (discussed previously) is another example of explanations that predict patterns in species number across communities. Would you predict that only the number of species is effected by level of disturbance, or that particular species are more or less likely to occur at a particular level of disturbance?