however few in kind, yet as the number even in a well-stocked pond is small in comparison with the number of species inhabiting an equal area of land, the competition between them will probably be less severe than between terrestrial species;
(species; inhabiting; an equal area of land) (the competition between them; will probably be; less severe than between terrestrial species)
species the competition between them
inhabiting will probably be
an equal area of land
less severe than between terrestrial species