The endemic rice rats have lost more species than any other group of vertebrates in the Galapagos, a decline believed to have been caused by introduced black rats (Rattus rattus), mice and feral cats (2) (3) (4) (5). The Santiago Galapagos mouse is the only one of the remaining species to occur alongside the black rat, which is known to have been present on Santiago since at least 1835, when Charles Darwin collected it during his visit to the island (2) (5). However, the coexistence of the two species is not stable. The black rat is larger, more aggressive and more dominant than the Santiago Galapagos mouse, with studies showing it displaces the smaller endemic species at food sources and has a negative impact on its population (4) (5).
Although two new populations of the Santiago Galapagos mouse were found in 2005 (1) (6), the species has been lost from parts of its historical range (5), and its small, highly restricted population apparently owes its continued survival to areas of Opuntia cacti (4). These cacti provide a rich source of food, water and nesting sites, particularly in the dry season when alternative resources are hard to find. However, the black rat appears not to feed on Opuntia, giving the Santiago Galapagos mouse a vital refuge from competition (4) (5) (9). In addition, the particularly arid local climate at this site allows the more drought-resistant Santiago Galapagos mouse to survive while the black rat population crashes during years of drought (4) (10). The biggest threat to the Santiago Galapagos mouse now comes from climate change, with a predicted increase in rainfall that could lead to increased vegetation and food supply, allowing black rat populations to explode, while at the same time decimating the Opuntia vegetation on which the Santiago Galapagos mouse now depends (4) (10).