Why is variation beneficial to the species but not necessarily for the individual?

Ex situ conservation is carried out in

Following is the data of four species$\left(S_1-S_4\right)$  present $\left(+\right)$ or absent $\left(-\right)$ in five different habitats ($P,Q,R,S$ and $T$). Which species has the maximum diversity?

Variety of organisms existing on the earth, their interrelationships and their relationship with the environment is called