This species was a micro-endemic of the Kihansi Falls, in the Kihansi Gorge, in the Udzungwa Mountains, eastern Tanzania, at 600-940 m asl. Its global range covered an area of less than two hectares around the Kihansi Falls, and searches for it around other waterfalls on the escarpment of the Udzungwa Mountains have not located any additional populations (Channing et al. 2006).
Habitat and Ecology
This species lived only in soaked herbaceous vegetation in the spray zone of the Kihansi Falls. An artificial gravity-fed sprinkler system is currently in place to mimic the original conditions of the spray zone. The toad breeds by internal fertilization, with females retaining the larvae internally in the oviduct until little toadlets are born. Clutch size varies from 5–13 offspring (Channing et al. 2006).
Formerly it was abundant in a tiny area, with a population of around 17,000 individuals. Reports indicate that the species fluctuated naturally in its population size. The population appeared to be at a high in May 1999, dropping to lower numbers in the course of 2001 and 2002 [total population estimates oscillated at around 11,000 individuals up until March 2002; see Channing et al. 2006], and at a high again in June 2003 [a total population estimate of 20,989 individuals; Channing et al. 2006]. However, subsequently the population went into steep decline, and by mid-January 2004, only three individuals could be seen and just two males were heard calling. There were a few records of calling animals during the rest of 2004, and an unconfirmed report from May 2005 (CBSG 2007), but there were no records of any individuals in subsequent years, despite surveys, leading to the species being listed as Extinct in the Wild in May 2009.
In an effort to re-introduce the species into the wild, Toledo and Bronx Zoos have been conducting an ex situ breeding programme. In June 2012 1,000 toads were involved in release experiments (where some were retained in a captive facility at Kihansi and a few were sent to the gorge). In October 2012 2,000 toads were re-introduced back to the gorge. In October 2013 2,000 toads were involved in several activities (where some were kept at Kihansi captive breeding facility, some were used for release experiments and some were re-introduced back to the gorge). There is a plan to re-introduce another 2,000 toads to the wild probably in October 2014 (W. Ngalason pers. comm. July 2014).
The serious decline and extinction of this species appears tp be related to the Lower Kihansi Hydropower Project, which involved the construction of a dam upstream on the Kihansi River in 2000. The dam cut off 90% of the original water flow to the gorge, thereby hugely reducing the volume of spray, particularly in the dry season, as well as altering the vegetational composition.
An artificial gravity-fed sprinkler system was set up to mimic the natural spray of the Kihansi ecosystem with the remaining water flow. Unfortunately, the sprinkler system was not ready by the time the water was cut off in 2000, and by the time the sprinklers came on nine months later the ecosystem had already dried up (see Krajick 2006). Later, during the dry season in 2003, the artificial sprinkler system failed for a while. Around this time, the fungal disease chytridiomycosis was confirmed in dead animals of this species, and this disease is probably responsible for the final population crash. It is possible that the drought caused by the failure of the sprinkler system resulted in stress to the animals that rendered them susceptible to the disease. There are also reports that the 2003 crash coincided with a brief opening of the dam's floodgates to flush sediments; tests showed that these contained pesticides used in maize farming operations upstream, in concentrations sufficient to kill the toads (Krajick 2006). The impact of predation by safari ants (Dorylus sp.) and crabs (Potomonautes sp.) is unknown (Channing et al. 2006).
This toad is not known from any protected areas. Sufficient minimum bypass flow from the dam is required to maintain the spray habitat and an artificial sprinkler system is in place to provide these conditions - this is especially important in the dry season. Captive breeding is ongoing in Toledo and New York Bronx Zoos, and a captive population was established at the University of Dar es Salaam in 2010. Soft-release experiments, as well as full re-introduction, are ongoing and the reintroduced population is under regular monitoring. It is listed on CITES Appendix I.
IUCN SSC Amphibian Specialist Group 2015. Nectophrynoides asperginis. In: IUCN 2014