There are many organisms that sometimes reproduce asexually and sometimes reproduce sexually (even snakes and sharks!).
There are also a fair number of organism that reproduce sexually but have a sex ratio of offspring that is heavily influenced by environmental conditions (e.g. water temperature with high temperatures favoring males, low temperatures favoring females and ideal temperatures favoring a mix, or vice versa, in some turtles, fish and amphibians).
Is there any scientific reason that there couldn't be an organisms that acts like K-strategists in some circumstances, and r-strategists, in others, using different reproduction modes as the circumstances merited, or that use both simultaneously? (Obviously, one way to show that this is viable would be with a real life example, if there is one.)
For example, maybe an organism would ordinarily have a K-strategy form of reproduction in good environmental conditions, but in the presence of extreme environmental stresses might reproduce by creating r-strategy spores in a manner that caused the death of the parent.