<P> Species richness map (birds) </P> <Li> <P> Species richness map (mammals) </P> </Li> <P> Species richness map (mammals) </P> <P> There are various ways to determine the distribution pattern of species . The Clark--Evans nearest neighbor method can be used to determine if a distribution is clumped, uniform, or random . To utilize the Clark--Evans nearest neighbor method, researchers examine a population of a single species . The distance of an individual to its nearest neighbor is recorded for each individual in the sample . For two individuals that are each other's nearest neighbor, the distance is recorded twice, once for each individual . To receive accurate results, it is suggested that the number of distance measurements is at least 50 . The average distance between nearest neighbors is compared to the expected distance in the case of random distribution to give the ratio: </P>

Which of the following is a chemical factor that can limit the distribution of species