Ecology of the Aders' duiker (Cephalophus adersi) on Mnemba Island, Zanzibar

Abstract

Mnemba Island has been housing a population of the vulnerable Aders’ duiker (Cephalophus adersi) since 2005, sharing the small, forested area with a prolific population of Suni antelope (Neotragus moschatus) (both species re-introduced). Despite government attempts to control the population size of Suni antelope, the tiny antelope proliferates at a seemingly high rate. Additionally, no Aders’ duiker has been removed since introduction to the island. Together, the two antelope species have been negatively impacted the island’s natural forest condition over time. Due to the Aders’ duiker’s endangered status, this natural habitat is essential to the survival of the species. For the first time, a study was initiated to investigate the ecology of the Aders’ duiker on Mnemba island to aid in determining a feasible carrying capacity to better manage the protected species on this Island. The outcomes of the study would be instrumental in managing the sub population and creating more suitable conditions to provide a sustainable harvest for reintroduction of offspring into other conservation sites (to be identified by the Government) in the rest of the Zanzibar Archipelago. A variety of inquiries were planned and performed to obtain an overview of the natural habitat, interactions of the Aders’ duiker with its environment, each other, and with the Suni antelope. A lack of weather data for the north-eastern side of Unguja, Zanzibar also prompted an onsite weather station to gather baseline data for a better understanding of the island’s climate conditions. A comprehensive vegetation assessment confirmed suitable species diversity and phenology for the maintenance of the duiker population. A mark-recapture population survey using an adjusted Lincoln Peterson estimate was undertaken to infer the current population density. Then the collection of birth and death data over twelve months provided an instantaneous growth rate which was necessary for modelling a logistic population growth curve. From the population survey and the growth rate, an estimated 25 Aders’ duiker were estimated to be inhabiting the island and it was projected that a maximum sustainable harvesting density of 13 duikers would maintain an ecological carrying capacity for the island. An adjusted growth model suggested that the maintenance of the proposed ecological carrying capacity would result in a sustainable harvest rate of 40%, which aligns with the suggested rate for other antelope of this size. All behaviour activities for the Mnemba Aders’ duiker population was observed over twelve months, using a stratified purposeful focal sample survey methodology, and pooling data for all daylight hours for 300 focal observations. Theoretically, each of the 25 duikers had an equal opportunity to be observed 12 times over 12 months. The pooled data provided a time activity budget that indicated that the general Aders’ duiker population on Mnemba island foraged for 39% of daylight hours. Seasonal comparisons further indicated that males foraged 12% more in the dry season, while females foraged 10% less in the dry season. The species’ diel pattern was also confirmed as loosely crepuscular, with significant differences between male and female duiker activity patterns. Males presented a more matutinal pattern while females followed a more vespertine pattern. Further to the capacity assessment, a twelve-month GPS tracking survey of one male and one female duiker provided spatial data for estimating annual and seasonal home and core range sizes, using a kernel density estimation. The two pairs of duikers implicated in the tracking inquiry were found to use half the indigenous forest on the island as their combined home ranges, and based on the average core range sizes, the natural forest would have spatially housed 25 individuals with no core overlap, which happened to be the same number as the population estimate derived from the growth model. Comparisons between the population survey results and the spatial carrying capacity proposed that maximum density had been reached.