and Physical Sciences
Community Ecology (Who's who and who's not)
B. How important are species interactions2. 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 = 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
Experimental tests of island biogeography theory:
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?
What is "diversity" as measured by ecologists?
Typically "diversity" in ecology refers not only to species number, but also how evenly the number of individuals per species comprise the total ('eveness').
For example, which is most "diverse" according to the above definition?
Community A with 5 species, each with 100 individuals Community B with 5 species, 400 of one species and the other spp. w/ 25 each Community C with 4 species, each with 125 individuals
Several different indices have been developed so that both species richness and eveness are incorporated into a single community number so that diversity among communities can be compared (for example, the Shannon Diversity Index)
Diversity can also be partitioned into alpha, beta, and gamma diversity.