Читать книгу Bats of Southern and Central Africa - Ara Monadjem - Страница 24

In southern Africa, all fruit bats have plant-based diets, while all other bats feed on a wide range of insect prey or, exceptionally in the case of Nycteris grandis, vertebrate prey as well (Fenton et al. 1990, 1993).

Frugivory and nectivory: The pteropodids feed on the fruits, leaves, flowers and nectar of a wide range of indigenous trees, often showing preference for the fruits of Ficus (Jacobsen and Du Plessis 1976, Bonaccorso et al. 2014) and Podocarpus. Soft-fleshed cultivated fruits are also favoured, including mangos, guavas, papayas, avocados, litchis, bananas, dates, and even Syringa berries. Bats prefer ripe fruits, and since most commercial fruits are picked under-ripe for shipping, fruit bats generally should pose no significant risk to fruit orchards (Smithers 1983, Fleming et al. 2009). However, there are apparently exceptions to this generalisation – Rousettus aegyptiacus has been shown to cause damage to litchi orchards in South Africa (Jacobsen and du Plessis 1976).

Carnivory: A few bats, including Nycteris grandis, are carnivorous. This species feeds on smaller vertebrates such as fish, frogs, mice, birds, and even other bats (Fenton et al. 1990, 1993). Elsewhere, notably in tropical America, frog-eating bats exploit the mating calls of frogs, while fish-eating bats have a specialised echolocation system to detect fine ripples on the water surface caused by fish, and sharp, hooked hind claws with which they can gaff their prey. In Europe, the greater noctule bat, Nyctalus lasiopterus, a vespertilionid, predates on high-flying migratory songbirds (Popa-Lisseanu et al. 2007, Estok and Siemers 2009). The possibility of large African bats, namely Scotophilus nigrita and Saccolaimus peli, exploiting this abundant, seasonal prey resource awaits investigation.

Insectivory: Some 70% of bat species worldwide eat insects (Jones and Rydell 2004). Depending to varying extents on their body size, jaw construction, wing shape, foraging behaviour, habitat use, and the nature of their echolocation calls, different insectivorous species feed on different groups of insects. Although studying the diet of insectivorous bats requires detailed microscopic or molecular examination of faecal remains, echolocation frequency, body size, and skull and mandible morphology are good predictors of diet (Jacobs 2000, Schoeman and Jacobs 2003, 2011). Strong robust jaws and long canines are indicative of hard-shelled prey, while a weak jaw with poorly developed coronoid processes is indicative of soft-bodied prey such as moths (Freeman 1979, 1981). In general, smaller bats are limited to smaller prey, while larger bats can take a wide size range of prey (Aldridge and Rautenbach 1987, Jacobs and Barclay 2009).

Aerial feeders, such as most Vespertilionidae, Emballonuridae and Molossidae, hunt flying insects exclusively on the wing; they are typically fast fliers and lack manoeuvrability (Aldridge and Rautenbach 1987). Gleaners, such as Hipposideridae and Rhinolophidae, capture stationary prey from branches or the ground and are typically capable of slow, manoeuvrable flight in confined spaces (Aldridge and Rautenbach 1987). Some species, such as Nycteris thebaica, Hipposideros caffer and certain Rhinolophus species, appear capable of both aerial feeding and gleaning. Some of these species are also perch hunters: they hang on a perch and make quick attacks when prey is detected moving past.

To a large extent, the design of the wing and the structure of the echolocation call together determine the prey that can be taken by a bat (Norberg and Rayner 1987, Schoeman and Jacobs 2008, 2011). This relationship is discussed in detail in the section Echolocation.

There is growing evidence that bats play vitally important ecological roles that may also have significant economic benefits, as bats are the major predators of night-flying insects (Jones et al. 2009, Boyles et al. 2011, Kunz et al. 2011, Maas et al. 2013, 2015, Roemer et al. 2019).

Every year, billions of corn earworm moths (Helicoverpa zea), fall armyworms (Spodoptera frugiperda) and other insects migrate in swarms from northern Mexico into Texas at altitudes of up to 3 km above ground. These insect swarms cause massive crop losses across the southern and central United States, costing the country billions of dollars annually (McCracken 1996, Boyles et al. 2011). Recent research using radar, weather balloons and bat detectors has estimated that the 100 million Tadarida brasiliensis bats occupying Bracken Cave and other major caves in central Texas can eat approximately 1,000 tons of insects each night. Even if only 10% of the bats’ diet were corn earworm moths (at 250 mg per moth), these bats would eat 340 million pest species moths each night, saving farmers millions of dollars (McCracken 1996, McCracken and Westbrook 2002, Kalko et al. 2008). Studies using bat detectors suspended on helium balloons in the Sengwa Wildlife Research Area, Zimbabwe, also demonstrated feeding by high-flying free-tailed bats (including Otomops martiensseni and Tadarida species) and Taphozous mauritianus at over 500 m above ground (Fenton and Griffin 1997). High-flying bats present unique challenges to conservation planners and managers, because these small mammals suffer impacts of aircraft strikes, pesticides and light pollution (Voigt et al. 2018).

Agricultural pests also featured prominently in the diet of bats occupying the fertile Sacramento Valley (Long et al. 1998). In another study reported in New Scientist magazine (Anonymous 1999), California pear farmers suffered crop losses of less than 5% to the corn earworm when a bat colony was situated within 2 km; when the bat colony was situated over 4 km away, crop losses of 60% were reported.

Closer to home, South Africa’s largest colony of 300,000 bats at the De Hoop Guano Cave assists farmers in the Bredasdorp area by consuming approximately 100 tons of insects annually, including many crop pests (McDonald et al. 1990b). South Africa is the largest exporter of macadamia nuts in the world. Insectivorous bats in macadamia orchards in the Soutpansberg prey on a range of insect pests, including two moths (Thaumatotibia batrachopa, Cryptophlebia peltastica) and two stink bugs (Nezara viridula, Bathycoelia distincta) that cause the greatest damage to this crop (Weier et al. 2019a). Using an avoided cost model based on consumption estimates, bat predation of stink bugs (the most damaging of pests in this industry) reduces total direct and indirect avoided costs to South African macadamia farmers by around US$150 per hectare per annum (Taylor et al. 2018c). However, these values are apparently underestimated: exclusion experiments in macadamia orchards in the Levubu area of Limpopo demonstrated that combined bat and bird exclusion could result in economic losses (due to decreased yield and quality from insect damage) of up to US$5,000 per hectare per annum or 60% of yield (Linden et al. 2019). These studies also demonstrated that by maintaining patches of natural vegetation within and around orchards, farmers were able to benefit from the pest consumption services of bats, whose foraging is concentrated in these patches (Weier et al. 2018). In Eswatini, molossid bats forage preferentially over sugarcane fields (Noer et al. 2012) where they consume a number of pest insects, including the most serious sugarcane pest, Eldana saccharina (Bohmann et al. 2011). Sugarcane fields typically support higher bat activity than neighbouring native savanna, albeit by a lower diversity of species (Mtsetfwa et al. 2018, Shapiro et al. in press).

With a growing emphasis on biological control and integrated pest management to reduce environmental impacts, more and more farmers are taking bats seriously as allies, especially given the evidence that a single colony of bats may consume millions of insects, including crop pests, each growing season. For this reason, farmers throughout southern Africa are now exploring ways of attracting bat colonies to their fields; these include erecting bat houses (Weier et al. 2019b).

Bats may also be important in mosquito control. In a laboratory experiment, bats of the genus Myotis were recorded capturing up to 600 mosquitoes in an hour (Griffin et al. 1960). Mosquitoes have been found in the diet of certain species of bats worldwide, but a priority for research is to establish their importance in the diet of southern African bats.

Fruit bats also bring important ecological and economic benefits. Research sponsored by Bat Conservation International has shown that the seed dispersal and pollination activities of fruit-eating bats are vital to the survival of equatorial and tropical rainforests. Some 300 plant species in the Old World tropics alone depend on bats for pollination or seed dispersal or both, providing more than 450 economically important products valued at hundreds of millions of US dollars annually (Fujita and Tuttle 1991). Seeds dropped by tropical bats are estimated to contribute towards some 95% of forest regrowth on cleared land in the African tropics (www.batcon.org). Certain bat-dependent trees, such as the baobab, Adansonia digitata (whose white flowers may help attract bats at night), are ecologically crucial, supporting dozens of other species. Extinction of baobabs resulting from the extinction of the Epomophorus species that pollinate them would trigger a cascade of linked extinctions. While recent observation anecdotally recorded Epomophorus sp. individuals visiting baobab flowers for nectar in Zimbabwe, efforts involving citizen scientists monitoring flowering baobabs (from dusk to midnight) in the Limpopo Province of South Africa recorded no fruit bat flower visitors during 32 tree-nights of observations (Taylor 2018). A range of insect flower visitors, including moths and beetles, were recorded and these attracted relatively high insectivorous bat activity.