The biodiversity in a coral reef ecosystem can be changed by both natural and human activities. What is a natural factor that would most likely decrease the ecosystem biodiversity

the immigration of several new populations of sea urchins into the reef.

Genetic variation can be caused by mutation ( which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis ( which reshuffles alleles whitin an organism's offspring).