English for

island biogeography
attempts to explain the relatively high 
species richness found in certain isolated island chains, including the Galápagos Islands that inspired 
the young Darwin. 
Relative species abundance
is the number of individuals in a species relative to 
the total number of individuals in all species within a habitat, ecosystem, or biome. Foundation 
species often have the highest relative abundance of species.
 
 
Figure 168. 
Equatorial latitudes are linked to North and South America's highest species richness for 
mammals.
134
.
133
Clark M.,Choi, J. Douglas M. (2020) 
Biology 2e
. Chapter 44, -Pp.1284-1287OpenStax, 
https://openstax.org/details/books/biology-2e
) 
134
(credit: modification of work by NASA, CIESIN, Columbia University) 

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246 
Keystone Species 
A 
keystone species
is one whose presence is key to maintaining biodiversity within an 
ecosystem and to upholding an ecological community’s structure. The intertidal sea star, 
Pisaster 
ochraceus
, of the northwestern United States is a keystone species (Figure 169). According to studies, 
removing this organism from a community causes populations of their natural prey (mussels) to grow, 
completely changing the species composition and lowering biodiversity. The banded tetra, a tropical 
stream fish that provides almost all of the community's required inorganic nutrient phosphorus, is 
another keystone species. If these fish went extinct, it would have a significant impact on the 
neighborhood. 
 
Figure 169.
The 
Pisaster ochraceus
sea star is a keystone species. (credit: Jerry Kirkhart)