The introduction of a ‘replacement’ species to areas that have suffered ecosystem damages due to the extinction of a similar species can greatly improve ecosystem health and species diversity. For instance, introduction of Aldabra giant tortoises (Aldabrachelys gigantea) to an area that had recently lost all large frugivores (animals that eat mostly fruit) that were crucial to seed dispersal of a particular fruit tree saw great improvement in tree abundance after the re-introduction. This special frugivore tortoise’s poop is a great seed disperser AND gut passage actually aids germination of the seeds. 

[Photo by Nick Chudeau and caption by Maggie Creamer]

Griffiths, C. J., Hansen, D. M., Jones, C. G., Zuël, N., & Harris, S. (2011). Resurrecting extinct interactions with extant substitutes. Current Biology, 21(9), 762-765.