Assertion: An ecosystem with more variety of species shows higher productivity.
Reason: It allows invasion of alien species.

The correct option is C Assertion is a true statement but the reason is false
An ecosystem is a functional unit of nature in which various biotic components interact with each other and also their physical environment.
David Tilman showed that stability of a community depends on its species richness. He performed long-term ecosystem experiments using outdoor plots in algal and grassland ecosystems. He found that plots with more species showed less year to year variation in total biomass and increased diversity contributed to higher productivity.

A stable ecosystem prevents invasion of alien species. Alien species which are non-native organisms, introduced into a habitat. These species may harm the biotic and abiotic components of the already existing ecosystem. For example, cat fish introduced into a pond will eat all other existing fish and multiply in number. This will drastically reduce the number of prey species and finally result in intraspecific competition between the catfish species which will reduce the productivity of the pond ecosystem.