Assessment of Mangrove Phenology and the Role of Insect Pollinators in Fruit Production at Nyeke and Michamvi Mangrove Forests, Zanzibar
Ali, Abdalla Ibrahim
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Mangrove forests are evergreen estuarine and open systems which receive nutrients, fresh water and sediments from terrestrial environments. They vary both in their salinity tolerance and the degree to which salinity may be necessary to maintain their growth and competitive dominance. Mangroves grow throughout the tropics wherever the average monthly minimum temperature is at least 200C. The ecological importance of mangroves are due to the ecosystems’ ability to maintain marine life, their high productivity and role in supplying organic material to other coastal marine ecosystems as reported by many studies. Mangroves trees have been proven to be very important in the mangroves ecosystem. Anthropogenic activities have been shown to be the primary cause of mangrove depletion worldwide. Rising mangroves forest destruction has negatively impacted on pollinator diversity and fruit set significantly. However, little is known about the magnitudes of these issues in East Africa. This research was therefore designed to assess diversity and abundance of mangrove insect pollinators and their role in fruit set in four mangrove species at Nyeke and Michamvi mangrove forests, Zanzibar. The study was conducted in two mangrove sites in South region of Zanzibar, Nyeke mangrove forest located between 60 19’ and 60 24` S and 390 25` E, and Michamvi mangrove forest located between 60 14’ S and 390 49’ E. The distance between the two sites is approximately 25km. Four mangrove species which are pollinated by insects (Rhizophora mucronata, Bruguiera gymnorhiza, Ceriops tagal and Avicennia marina) selected from Nyeke and Michamvi mangroves forests were used in the study. The reproductive phenology, reproduction relationships of mangroves, pollinator species diversity and richness, and effect of pollination on fruit set were investigated. The study found that reproductive phenology varied among species and sites. The peak fruit set varied among species and sites. There was a positive relationship between temperature and reproduction but not with rainfall and relative humidity. In both sites the findings showed a weak relationship between fruit set and number of fruits. The study also revealed that increase in number of insect flower visitors and visits did not result in increased fruit sets. However, increase in number of flowers increased the number of insect flower visitors and visits. A total of 18029 insect flower visitors representing 70 species in 7 orders and 40 families were observed visiting flowers of the four mangrove species in both sites. Family Apidae of the order Hymenoptera was the most common and insects of this order were found in all four mangroves species. Apis mellifera was the most dominant flower pollinator for Bruguiera gymnorhiza, Ceriops tagal and Avicennia marina. Hypotrigona gribodoi was predominantly found on RM and is potentially the flower pollinator of this species. Higher number of Apis mellifera 721 (32.2%) was recorded in Bruguiera gymnorhiza at Nyeke site. Bagged experiment that prevented most pollinators accessing the flower, showed a high percentage of flower abortion and lowest fruits produced than other treatments in this study. A. marina had confirmed lower fruit set compared to the other species. Pollen Supplement (PS) (hand cross pollination) produce higher percentage of fruits set and fruits in some mangroves species in both sites. This not only shows that additional pollen enhances fertilization but also that pollination is necessary for fruit production. The study concludes that, in depth research on various variables of mangroves including inventory of pollinators, biodiversity, social economic significance, potential threats and phenology for other species and climate alteration are important for strengthen biodiversity conservation and mitigation.