In order to survive in the sometimes-harsh environment of earth, both on a daily- and evolutionary-basis, constantly competing with other species for scarce resources, organisms must be able to defend themselves in all realms.
(organisms; to defend; themselves; L:in all realms) (organisms; must be; able to defend themselves in all realms)
organisms organisms
to defend must be
themselves L:in all realms
able to defend themselves in all realms