Winning permanent protection for intact forests is a powerful tool for conserving bats and biodiversity, but it is rarely sufficient, especially in tropical rainforests that are shrinking at an alarming pace, to make room for agriculture. We must learn through scientific research exactly how these changes impact various species and then determine how agricultural landscapes can be designed and managed for the long-term conservation of biodiversity.
In heavily disturbed Mesoamerican rainforests, riparian corridors (forested areas bordering rivers, streams or other bodies of water) represent a vital refuge for many animals, as well as a protected pathway for moving among surviving forest fragments. Their role as reservoirs of bat species, however, is not clear. Knowledge of bats’ use of riparian habitats and the impact on bat-species diversity is crucial to conservation efforts.
With the support of a Student Research Scholarship from Bat Conservation International, I focused on the Lacandon Forest, a biodiversity hotspot that covers some 5,000 square miles (13,000 square kilometers) of mostly lowland forest in the southern Mexico state of Chiapas. The rainforests of Lacandon are heavily fragmented. It is estimated that 31 percent of the forested area was lost to agriculture, mostly cattle pastures, during the 1990s. And yet, as pastures and farm fields are cleared, riparian vegetation is typically left in place along streams.
My research attempted to determine the influence of riparian areas in a heavily agricultural region on the species composition and density of phyllostomid bats. Phyllostomidae (New World leaf-nosed bats) is an amazingly diverse family of at least 160 species that share little beyond a leaflike structure atop their noses. Seven subfamilies include species that feed on fruit, nectar and/or insects, plus carnivores that eat small frogs, birds, rodents and even other bats. All three species of blood-feeding vampire bats are also phyllostomids.
My team and I selected 12 sampling sites, three in each of four categories: riparian vegetation areas within the continuous mature forest, riparian vegetation within pastures, continuous mature forest away from riparian vegetation, and pastures without riparian vegetation. Streams in the riparian areas varied from 6.5 to 26 feet (2 to 8 meters) across. The pasture sites were located in the fragmented landscape of the Marques de Comillas municipality on the south side of the Lacantun River. Continuous forest sites were within the Montes Azules Biosphere Reserve on the north side of the river.
Over two years, we set five mist nets to sample bat diversity twice during the dry season and twice during the wet season at each site for a total of 70 nights. We captured 1,752 bats overall. Each bat was identified by species and classified by one of six “feeding guilds” – groups of species with similar feeding behaviors. These guilds are aerial insectivores, carnivores, gleaning insectivores, frugivores, nectivores and sanguivores (vampire bats).
The captured bats represented 28 species of Phyllostomidae. Ninety-one percent of all captures (1,598 individuals) were from 16 species of the subfamily Stenodermatinae, fruit-eating bats that sometimes also consume insects.
We used several analytical and statistical tools to confirm the adequacy of our results and to analyze the findings.
Our samples were dominated by five species: little yellow-shouldered bats (Sturnira lilium – 30%), great fruit-eating bats (Artibeus lituratus – 23%), Jamaican fruit bats (Artibeus jamaicensis – 11%), tent-making bats (Uroderma bilobatum – 10%) and Pallas’ long-tongued nectar bats (Glossophaga soricina – 6%). These five species represented more than 80 percent of the bat captures.
We captured 434 bats of 23 species in the riparian forested areas, 885 bats of 21 species in riparian pastures, 134 of 20 in non-riparian forest and 299 of 14 species in open pastures. We found three species exclusively in mature forests, two only in riparian pastures, and one species in riparian forests, while open pastures had no exclusive species.
Frugivores accounted for most of the species (59 percent) and more than 90 percent of individual captures, followed by gleaning insectivores with 18.5% of species, nectivores with 11.1 percent, and sanguivores with just 11 common vampire bats (Desmodus rotundus) and two hairy-legged vampires (Diphylla ecaudata). The only carnivorous bats in the sample were three fringe-lipped bats (Trachops cirrhosus).
The riparian mature and non-riparian mature forests contained all five feeding guilds. Riparian pastures had four guilds (carnivores were absent), while the open pasture had only three guilds (no carnivores or gleaning insectivores).
In summary, bat assemblages in open pasture were both less diverse and less abundant, while they were more diverse and numerous in riparian mature forests and riparian pasture habitats. This result is not unexpected, since many bats are found to prefer rivers with forested banks as commuting corridors that provide both foraging habitat and shelter from winds and predators.
Riparian pasture lands, meanwhile, share more species with mature forests than with open grasslands. Fruit-eating species were found in all habitat types, while gleaning insectivores and carnivores were missing from open pastures, perhaps due to a scarcity of food, shelter and roost resources.
As expected, our study suggests that different bat assemblages utilize riparian and non-riparian habitats. But we also found that riparian areas along pastures can be important habitat for some sensitive species, especially the gleaning insectivores of the subfamily Phyllostominae, which are usually absent from disturbed areas.
Our results demonstrate that retained riparian habitats within an agricultural matrix can serve as reservoirs for bat species, especially those that are mostly strongly associated with undisturbed landscapes. We plan to develop recommendations based on this research to help farmers retain and manage riparian zones to enhance bat conservation.
ERIKA DE LA PEÑA-CUELLAR is completing her Ph.D. in biological sciences at Centro de Investigaciones en Ecosistemas, National Autonomous University of Mexico.