r/K selection theory describes two ends of a reproductive spectrum. r-strategists produce many offspring with little parental investment (most die young); K-strategists produce few offspring with heavy investment (survival rate high).
Mice vs. elephants is the textbook example. But the axes are continuous and independent: you can have high investment without low fecundity (seabirds), or low investment without high fecundity.
The theory has been extended (sometimes recklessly) beyond biology. The underlying logic — more investment per offspring means fewer can be supported — is sound. The details depend heavily on the specific environment.
Related: keystone-species, island-biogeography