What happens in competition between two species?

The correct answer is Option (2) → The fitness of one species is lowered in the presence of another.

Populations of different species interact in several ways. In competition, both species lose (–, –). It is not only between closely related species, and it can happen even when food and space are plenty, if one species interferes with the feeding of the other. So the book defines competition by its result: the fitness of one species, measured by its intrinsic rate of increase (r), is significantly lower in the presence of the other.

Option (1) Both species benefit from the interaction. — Incorrect. When both species benefit, the interaction is mutualism (+, +), as in lichens and mycorrhizae. In competition both lose.

Option (2) The fitness of one species is lowered in the presence of another. — Correct. This is the definition of competition. The other species lowers its fitness (its ‘r’), whether by using up the resource or by interfering.

Option (3) Both species increase their fitness in the interaction. — Incorrect. A rise in fitness of both partners is again mutualism, the opposite of competition.

Option (4) One species benefits without affecting the other. — Incorrect. One species gaining while the other is neither harmed nor helped is commensalism (+, 0), as with an orchid growing on a mango branch.

NCERT Extract:
Population Interactions – “… Both the species benefit in mutualism and both lose in competition in their interactions with each other.”
Population Interactions – “… Secondly, resources need not be limiting for competition to occur; in interference competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, even if resources (food and space) are abundant. Therefore, competition is best defined as a process in which the fitness of one species (measured in terms of its ‘r’ the intrinsic rate of increase) is significantly lower in the presence of another species.”