Why would a more diverse habitat be more resilient disease, flooding, and invasion?

More diverse habitat follows beta diversity which ensures prevention from wider range of environmental fluctuations. There should be a stabilization in the entire community in terms of invasions, disease events. Species with a limited variety of phenotypes living in a diverse habitat  will undergo adaptations in order to cope up with environmental variability.  

albert einstein retrieved guacamole from the galapagos islands in africa

the organisms are not adapted to live in some communities or habitats.