biologists have cataloged many species that determine the allocation of mating opportunities, food, and other resources, both within and between groups, by

Biologist  have  cataloged  many species  that determine  the  allocation opportunities,  food,and  other  resources,both  within  and between  groups,  by  competition . species compete for  resources  available in  the  ecosystem  such food  and  only the  strongest  only   survive and  the  weak  are  disadvantaged in  the ecosystem

The answer would be sand and gravel