Around Bwindi Impenetrable National Park (BINP) the “front-line communities” often loose substantial portions of their crops to wild animals. To mitigate Human-Wildlife Conflicts (HWCs) several interventions have been tried around BINP, these included hedges of Mauritius thorn (Caesalpinia decapetala (Roth) Alston), tea crop, keeping buffer areas open for livestock grazing, and selective killing of baboons.

However, there is no information on the effectiveness of such HWC mitigation interventions. Secondly, Caesalpinia, an exotic plant with a history of invading habitats elsewhere require careful investigation for promotion as a hedge along the park boundary. The objectives of this study were to investigate the invasive potential of Caesalpinia and the effectiveness of Caesalpinia hedge and other interventions in alleviating HWCs.

Mauritius thorn hedge (in middle of photo)

Community perceptions on the effectiveness of Caesalpinia hedge and other HWC interventions, and institutional mechanisms for managing HWC were also investigated. Data were collected on reproductive ecology of Caesalpinia, factors limiting seed germination, seedling survival, growth, population and impacts of Caesalpinia on regeneration of indigenous vegetation. Growth and survival features of Caesalpinia were compared with that of the indigenous congeneric shrub species, Mezoneuron angolense (Olive). The levels of effectiveness of Caesalpinia hedges and other interventions were determined. Questionnaire was administrated to assess community perceptions regarding Caesalpinia hedges and other HWC interventions.

The result indicated that Caesalpinia had both autogamous and cross-pollination breeding systems. Cross-pollination was promoted by a variety of agents such as sunbirds (Nectirinii), bees and Papilio species. Caesalpinia also reproduced vegetatively by sprouting from exposed roots. This meant that reproduction in Caesalpinia was not pollen limited. Flower abortion and loss was high due to infection by fungus, insect and bird predation. Caesalpinia fruit dispersal was not favoured by humid and cool conditions in BINP. Fungal infestation of seeds rapidly decreased viability. Rodent species predated on immature Caesalpinia fruits, leading to 48 – 85% fruit loss. There was highly significant correlation (R² = 0.987, p < 0.005) between amount of sunlight and seed germination. Hot water treatment increased percentage germination while direct fire damaged seeds. Caesalpinia started flowering within nine months of planting. Compared to the indigenous congeneric species of Mezoneuron angolense (Oliver), which grew well under various natural conditions, growth of Caesalpinia was limited by poor soil nutrient status. Caesalpinia seedling survival were also suppressed by combined effects of canopy cover, ground vegetation cover and root competition (R² = 0.852, p < 0.005). Exotic boundary plants of Cupressus and Eucalyptus similarly suppressed growth of Caesalpinia, probably through allelopathy, reduced nutrients, and light. Caesalpinia did not negatively affect soil nutrient status and regeneration of indigenous tree species. Population structure of Caesalpinia indicated retrogressive trend, with over 80 % reduction in area coverage over 9-year period.

Baboons and bush pigs were the main problem animals and contributed to over 90% to the crop depredation in this study. The local perception was that HWC had increased since the park was gazetted in 1991. Regarding mitigation measures, Caesalpinia hedge and tea resulted in significant reduction in baboon raids. Using a buffer zone for non-attractive activities reduced raiding by bush pigs (over 60%) and could work for Mountain gorillas.  

Although Caesalpinia is invasive elsewhere, factors ranging from suppression by indigenous vegetation, fruit, seed predation and fungal infections provided biological control to its expansion in BINP. The findings suggest that Caesalpinia may not invade natural habitats in BINP or other habitats with similar habitat characteristics.

Ranger at baboon trap in the bufferzone of Nkuringo, on the south side of BINP