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Quantifying the ecological impact of the straw-coloured fruit bat (Eidolon helvum) in West Africa

Quantifying the ecological impact of the straw-coloured fruit bat (Eidolon helvum) in West Africa


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ABEDI-LARTEY, Michael, 2016. Quantifying the ecological impact of the straw-coloured fruit bat (Eidolon helvum) in West Africa

@phdthesis{Abedi-Lartey2016Quant-35007, title={Quantifying the ecological impact of the straw-coloured fruit bat (Eidolon helvum) in West Africa}, year={2016}, author={Abedi-Lartey, Michael}, address={Konstanz}, school={Universität Konstanz} }

Abedi-Lartey, Michael eng 2016-08-11T12:01:35Z 2016-08-11T12:01:35Z Abedi-Lartey, Michael 2016 Quantifying the ecological impact of the straw-coloured fruit bat (Eidolon helvum) in West Africa Ecosystems worldwide provide beneficial services to humans and other life-forms. Fruit-eating and flower-visiting animals play a particularly important role in the regeneration of numerous plant species through their dispersal of pollen and seeds, thereby ensuring the maintenance or restoration of these ecosystems especially in tropical Africa. However, the ability of ecosystems to function effectively and to provide these services is being compromised by many factors, including defaunation, such as through bushmeat, trophy and pet utilization; and habitat degradation, fragmentation and loss. Therefore, animals capable of long-distance seed dispersal are particularly important in fragmented landscapes for maintaining gene flow and colonizing new sites for plants. Volant seed dispersers such as birds and fruit bats capable of utilizing fragmented landscapes are thus receiving increasing attention to quantify their seed disperser services. However, many seed dispersers that eat fleshy fruit do not cover large distances and/or leave the forest cover; even those that do usually drop most seeds under the source tree, or quickly defecate ingested seeds after feeding. Thus, animal vectors, such as the straw-coloured fruit bat (Eidolon helvum), that are capable of covering large distances and retaining seeds for long periods are particularly important for maintaining connectivity among plant populations in fragmented landscapes. I expected that the seasonal migration of E. helvum over vast distances across the African continent, probably following seasonal bursts of resource availability, would cause enormous fluctuations in population size, which in turn may influence the bats' impact on local ecosystems. I used a combination of state-of-the-art GPS/acceleration loggers and concurrent monitoring of the seasonal fluctuations in colony size, phenology of bat food resources and field-based feeding experiments to collect high spatio-temporal resolution data on the eco-physiological behaviour of fruit bats under different seasonal and landscape conditions. I quantified: 1) changes in foraging movements and colony population size in response to inter-seasonal environments; 2) changes in bat colony size, activity budgeting, movement ecology and body condition responses to intra-seasonal changes in food availability; and 3) dispersal distances for seeds of bat food resources. I found that in the urban landscape in and around Accra, located in the forest ecosystem in southern Ghana, bats foraged locally (3.5 -36.7 km) and in urban areas with low tree cover during the wet season, when colony populations were low (~ 4000 individuals); major food sources during this period were fruits of introduced trees. This was in contrast to a tripling of foraging distances (24.1 - 87.9 km) during the dry season, with population peak (~ 150,000 individuals), which was not compensated for by reduced resting periods. Dry season foraging areas were random with regard to urban footprint and tree cover, and food consisted almost exclusively of nectar and pollen of native trees. Fluctuations in food abundance potentially drive a variety of eco-physiological responses in frugivorous bats, including changes in body condition, activity budgets, diets and foraging areas. In view of earlier findings that pronounced inter-seasonal changes in E. helvum movement and foraging ecology, I next expected that if there were sufficiently strong environmental factors within a single season, the bats would exhibit distinct eco-physiological responses. In a savanna ecosystem in Ouagadougou, central Burkina Faso, I concurrently investigated intra-seasonal changes in food resource availability, colony size, bat activity budget and spatio-temporal movement patterns, and bat body condition during the mono-modal wet. I found that food availability was markedly different in the early (food high) and later (food low) stages of the wet season, with different composition of dominant food species during each period. The results indicate that E. helvum opportunistically shifted diets to capitalize on different food resources as they became dominant in the landscape within the same season. Colony size correlated positively with food availability, showing a two-fold decrease with increasing food scarcity. Body condition declined significantly during food scarcity. Although bats were equally active at night in both food periods, they spent more time foraging and commuting during food low. Travel time away from day roost was lower during food high, but travel distance was similar for the two food periods. The space use in core and foraging areas did not differ in the two periods. Overall, intra-seasonal fluctuations in food resources are markedly different and apparently caused significant changes in body condition and foraging time budgets but not in distance and area use. In addition, I report for the first time the ability of E. helvum to relocate to new day roosts as solitary individuals. Individual excursions last for 1 – 6 consecutive nights before returning to the main colony during periods of food scarcity. I report for the first time that the highly gregarious, central-foraging E. helvum can spend time as solitary individuals at day roosts in foraging areas far away from colonies, apparently in response to food scarcity. Finally, I sought to translate the seasonal changes in E. helvum movements into a quantification of seed dispersal distances. By using a combination of high-resolution GPS-tracking of movements, field observations and gut retention time experiments, I modelled potential dispersal distances for small- and large-seeded fruits foraged by E. helvum during periods of colony population low (wet season) and high (dry season) in an urban and a rural landscape in the forest zone of Ghana. Nightly movements were generally longer in the urban than in the rural landscape and also longer in the dry than in the wet season. Gut passage time (mean 116 min; range 4-1143 min) was comparable to those of other fruit bats. Seed dispersal distances were similar for both large (median 42-67 m) and small (median 42-65 m) seeds, and reflected the fact that majority of seeds are dispersed only to feeding roosts through the foraging behaviour of shuttling between food trees and nearby feeding roosts. Estimated maximum seed dispersal kernel distance for small seeds was 75 km, four times further than previous maximum distance estimated for a frugivore. Furthermore, small seeds were dispersed over distances almost twice as long in the rural (49.7 km) compared to the urban (31.2 km) landscape. Maximum distances for small seeds in the urban landscape were three times longer during the dry season (75.4 km) versus the wet season (22.8 km); in contrast, distances in the rural landscape were three times longer in the wet season (67 km) compared to the dry season (24.4). For large seeds in the rural landscape, dispersal distance during the dry season (551 m) was almost twice that in the wet season (319 m). Phenology of food resources did not appear to influence seed dispersal distances. The maximum likelihood for seed dispersal beyond feeding roosts (99th percentile distance from food tree 263 m) was 4.7%. Small seeds were dispersed over even longer distances, > 500 and >1000 m, with a likelihood of 3.0 % and 2.3 % respectively. I show that E. helvum retains ingested seeds for very long periods and traverses large distances, along which both long and short-distance seed dispersal occur. This probably makes E. helvum the most important animal seed disperser currently known, at least in tropical Africa. These findings suggest that E. helvum is a keystone species for ecosystem functioning and urge its conservation. This study suggests that straw-coloured fruit bats are likely to disperse seeds in the range of hundreds of meters up to dozens of kilometres, and to pollinate trees for up to 88 km. Straw-coloured fruit bats forage over much larger distances compared to most other Old World fruit bats, thus providing vital ecosystem services across extensive landscapes. I recommend increased efforts aimed at maintaining E. helvum populations throughout Africa, since their suspected keystone role in various ecosystems is likely to increase due to the escalating loss of other seed dispersers as well as continued urbanization and habitat fragmentation. These findings provide additional evidence that (1) fruit bats may adapt their foraging movement patterns and diets within a single wet season in the savannah; similar changes were previously observed between seasons in the forest ecosystem; (2) the super-abundant production of shea and fig fruits in the savannah ecosystem may be an important factor in the annual northwards migration of E. helvum from the southern coastal forests in West Africa, thereby confirming the tight ecological inter-dependence of fruit trees and their seed dispersal via fruit bats.

Dateiabrufe seit 11.08.2016 (Informationen über die Zugriffsstatistik)

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