Weitzman’s (1998) seminal work applied the metaphor of Noah’s ark and the related libraries model to the problem of species preservation under budget constraints. In this paper we consider the symbiotic Noah's Ark problem with two types of species: a keystone species and a keystone-dependent species, which relies on the keystone species for survival. The central planner maximizes the expected biodiversity value under budget constraint and obtains the optimal preservation policy. One of Weitzman’s main conclusions was that under an appropriate independence assumption, an optimal policy yields an extreme outcome (almost all species either fully survive or die out). In contrast, we show that our symbiotic model with two types of species generates a unique interior optimal policy. Moreover, we find that under an interior optimal preservation policy, the expenditure on the keystone species' survival is greater than 50% of the given budget.