This species is threatened by: a) habitat destruction and decline in quality, specifically the loss of roosting and feeding trees; b) invasive alien plants that reduce flowering and fruiting of trees and animal species, namely monkeys, that destroy food resources, especially fruits; c) cyclones; d) persecution in response to conflict over fruit crops; e) illegal hunting; and f) accidental deaths e.g., electrocution on powerlines. These threats commonly interact.
a). Habitat destruction and decline in quality:
There are no data on threats to specific roost sites, although there has been a progressive net loss of larger native trees within protected forests (Florens et al. 2017b). Losses include the commonly used roost tree Labourdonnaisia glauca. Decline of large trees is probably even faster in the more invaded forest (which consist of about two thirds of the remaining ‘native’ forests). Threats to bats at roost sites in Mauritius (e.g., hunting, cyclones, human disturbance) can impact many roosts at one time and in no pattern. In the case of an intense cyclone, a very large number of roosts can be damaged at the same time, and the roosts damaged will depend on details of the movement of the cyclone and its strength.
Forest habitats suitable for roosting and important for feeding, particularly at certain times of year, have seen major decline of what is already a habitat much depleted in both area and quality. Native forests are highly fragmented (Safford 1997, Florens 2013) and dominated by invasive alien plants, particularly in the wet forests (Florens 2008, Florens et al. 2016). The total extent of forest cover in Mauritius is c. 25% of total land area, but much of this is planted forests and shrubland (14%). Only about 5% is native forest (Hammond et al. 2015), of which only one third (1.7%) is has more than 50% native canopy cover. The remaining native forest (3.3%) has less than 50% native canopy cover (Safford 1997, Ministry of Agro-Industry and Food Security 2015). In 2013, the extent of native forest with > 50% plant canopy cover, so of “reasonable quality”, was estimated at around 2,600 ha (Ministry of Agro-Industry and Food Security 2015). While the significant holdings by government have shown little decline in recent years, natural habitat on private holdings has declined, and that decline is likely to continue. Illegal habitat destruction has also been recorded in Mauritius, even in protected areas, including some prime foraging habitats like the Cabinet Nature Reserve (Florens 2013). Declining habitat quality in the remaining protected areas is also of great concern. Density of large trees (>10 cm diameter) has roughly halved in about 70 years even in lesser invaded and protected native forests (Florens et al. 2017b). This equates to a rate of loss of about 5% large tree density per generation of P. niger, with likely consequences for the island’s P. niger carrying capacity. Although potential foraging area for bats has increased following the wider introduction of fruit farms, especially for litchi and mango, these are only available for a short period of the year (October-December). Most of the year, the bats would not survive without forest foraging. Moreover, foraging on fruit crops has brought the species into conflict with people and precipitated persecution by individuals and government-implemented mass culls.
b). Invasive plant and animal species:
Invasion of native forests by alien plants is of conservation relevance to Pteropus niger subpopulations on Mauritius as it reduces production of flowers and fruits among natives (Monty et al. 2013), including those that produce fruits taken by P. niger (Nyhagen et al. 2005, Baider and Florens 2006, Krivek 2017). Introduced Crab-eating Macaques (Macaca fascicularis) further reduce the abundance of native fruits available to P. niger, taking fruits of key species before they ripen enough for bat consumption (Nyhagen 2004). For example, Baider and Florens (2006) reported losses > 95% of all the fruits of Sideroxylon grandiflorum (Sapotaceae), an endemic canopy tree reliant upon P. niger for seed dispersal. They observed similar levels of damage to unripe fruit for several ebony species, e.g., the common Black Ebony (Diospyros tessellaria) and the rarer D. nodose, and for Mimusops maxima (Sapotaceae), another important canopy tree of wet forests. These trees are important native fruit trees for P. niger, that tends to eat mostly ripe fruit (Krivek 2017). The population of macaques was reduced to an estimated 8,000 animals between 2006-2007, as wild animals were trapped for export for medical research (Sussman et al. 2011). However, macaque numbers have increased over the last decade, with censuses suggesting a wild population of 30,000–40,000 (Trask et al. 2013), and more recent estimates of 50,000.
The immediate and cumulative effects of invasive plants and monkeys combined limit the abundance of native fruits available to bats in the forest remnants. Moreover, only 500 ha or so of Mauritian native vegetation has been freed from alien plants out of the 90-km square or so (i.e., less than 4% of native vegetation is recovering from alien plant invasion – but not of monkey impacts). The vast majority of the native vegetation (96%) continues to degrade under the impact of invasive plants (Florens et al. 2017b).
Cyclones cause direct mortality to island fruit bats, destroy roost trees, and reduce fruit and flower resources resulting in starvation and/or compromised reproduction (McConkey et al. 2004). Dramatic population declines/extirpation following cyclones have been observed in island Pteropus populations in Tonga (McConkey et al. 2004), Samoa (Craig and Syron 1992, Craig et al. 1994), Christmas Island (Richards and Hall 2012), and neighboring Rodrigues. Cheke and Dahl (1981) report previous declines of P. niger on Mauritius following cyclones Carol (1960) and Claudette (1979), based on accounts from hunters and a report by Jones (1980). Mauritius has been hit by at least 38 cyclones (winds > 120 km/hr) in the last 86 years (one cyclone every 2.26 years). This gives a mean of 8-9 such cyclones during three generations (taken as 20 years) among which an average of at least two have devastating winds exceeding 210 km/hr. In the previous 20 years (three generations), the Mauritius Meteorological Services reported 11 cyclones/severe tropical storms, of which four were classified as intense (sustained winds > 159 km/hr) or very intense (> 210 km/hr). Although the last very intense tropical cyclone was in 2002, the risk remains high and recent trends and climate change forecasts suggest that frequency of strong cyclones is on the increase (Webster et al. 2005, Kuleshov et al. 2010, Kuleshov 2014). Maximum wind speeds are also likely to increase (Elsner et al. 2008, Deo and Ganer 2014) as is the rate at which cyclones intensify (Kishtawal et al. 2012). Climate change simulations for Mauritius further suggest a decrease in rainfall (Howells et al. 2013), which would likely negatively affect the availability of fruit and flower resources and lower the island’s carrying capacity.
d). Persecution in response to conflict over fruit crops:
Foraging on soft fruits has brought P. niger into conflict with fruit growers, both “backyard” and commercial, particularly during the litchi season in November-early January. Pressure from fruit growers resulted in a mass cull of the species by the Government from November-December 2015. The cull was carried out by Special Mobile Force using rifles of 0.22 calibre, and 26,225 cartridges 12 bore were used. Although the original target figure was 20% of the population (Anon. 2015), the total reported by the Commission of Police, as communicated by Honourable M. Seeruttun, the Minister of Agro-Industry and Food Security in Parliamentary Debate of 24 May 2016 (Anon. 2016) was 30,938. Per the Minister’s response, “the culling of bats was carried out in various regions throughout the island where bats exist, and these regions are mainly forest areas on both State and private lands. The island was split into four regions: the northern, southern, western and central eastern regions, and the culling exercise was carried out on different sites in those regions”. The December 2016 cull killed 7,380 bats (Anon. 2017). During the 2015 cull, although bats were still protected from private action, illegal take of bats for sport or control increased from an estimated 2,000 bats per year (Hutson and Racey 2013) to about 5,000 individuals (www.mauritian-wildlife.org), and possibly as many as 6,000 (V. Tatayah, pers. comm). Collectively, losses since October 2015 – January 2017 are c. 50% ((38,318 bats culled + 10,000 illegal take + 2,000 powerline fatalities)/2015 pre-cull population of 99,438) (Vincenot et al. 2017).
Although P. niger was protected under law by the Wildlife and National Park Act of 1993, in the 20+ years of legal protection, no fines were levied on those taking bats illegally (Florens 2013). The Wildlife and National Park Act of 1993 was repealed by the Native Terrestrial Biodiversity and National Parks Act (2015). Although the species is still protected from hunting, Clause 36 allows establishment of a Special Technical Committee specifically for “the purpose of controlling wildlife” that has attained “pest status”, and it was under this Clause that the culls of 2015 and 2016 were enacted. This erosion of protection for P. niger increases the ease and hence probability that the government will continue to implement future culls. Assessment of the species will be implemented should other culls take place.
The Sugar Protocol, which gave African, Caribbean and Pacific Group of States (ACP) countries preferential access to the EU market, expired in 2009. Consequently, there has been some conversion of land from sugar cane production to other uses, including fruit crops (Ministry of Agro-Industry and Food Security 2015). This suggests that fruit grower-bat conflicts are likely to persist (and possibly increase) over the next three generations, unless mitigation measures are adopted (netting of trees, planting dwarf varieties, pruning). Given the weakened legal protection, and continuing conflict with fruit-growers, future government-implemented culls are considered probable, and illegal killing of bats by the public likely to remain at current levels (at least 5,000 per year) without explicit intervention.
Hutson and Racey (2013) estimated that illegal take of bats for sport or control was c. 2,000 bats per year. Since the 2015 cull, although bats were still protected from private action, illegal take of bats for sport or control increased to an estimated 5,000 individuals (www.mauritian-wildlife.org), and possibly as many as 6,000. This offtake by hunters contributes to a sizeable annual mortality rate that will influence the long-term viability of the population. Therefore, it is important to devise methods to estimate the figure with greater accuracy and to reduce the number through enforcement and management of public perceptions.
f). Accidental deaths:
It is estimated that approximately 1,000-2,000 bats land and die on power lines each year (www.mauritian-wildlife.org, V. Tatayah, pers. obs.). In addition, several hundred to a thousand bats die each year in poorly-set nets used to exclude bats from fruit crops. Bats become entangled in slack nets and die of dehydration, starvation or are actively persecuted and killed by people.