The Effects of Herbivory on interactions of pollinators and flowers in Acacia SPP. (Fabaceae,mimosoideae) in Laikipia

Abstract

Acacia trees are among the most dominant tree species found in semi arid ecosystems with a vast geographical distribution throughout the savanna habitats. The trees are leguminous and have remarkable ability to fix nitrogen in the soil, thereby, improving fertility. About all parts of an Acacia tree are edible to different types of animals and their leaves provide the only greenery in the dry season, so are bound to be eaten. As such, they support enormous pyramids of biomass in complex food chains with super predators at the apex. Economically, they are used as basic sources of fruits, firewood, medicine and secondary products for daily human livelihoods. Owing to their important, a lot of studies have focused on their growth ecology. Herbivory and reproduction are among the most studied aspects of Acacia. However, the interactive link between these two aspects and their effect on the growth success of Acacias is poorly understood. Understanding the link between herbivory and pollination, would not only provide useful information on the interaction of plants and diversity of visitors but also information on the floral morphometry and rewards to pollinators. The general objective of this study was to determine the effects of herbivory on interactions between pollinators and flowers in Acacia spp. The study was conducted at the Mpala Research Centre in Laikipia District of Kenya (0017'N, 36053'E) on one herbivore excluded plot (70m x 50m) and in the open grazed areas adjacent to the excusion plot. Acacia trees for investigation were randomly selected. Data were recorded on floral morphometric measurements and dehiscence in different browsing levels and simulated herbivory plots. The number and type of visitors to the flowers was also recorded besides determining the species diversity of wild flora within the study site. ANOVA was used to analyze variation in different flower attributes, SNKwas used to separate means with significant differences. Generally, unbrowsed trees produed significantly (P<0.05) more flowers, florets, anthers and larger floral diameters than naturally and heavilly browsed Acacia trees. Similarly, unbrowsed trees attracted more pollinators especially bees than naturally browsed trees. An earlier pollinator peak activity was recorded on unbrowsed trees than browsed trees. A significant shist of inflorescence dehiscence time occurred between unbrowsed and naturally browsed trees of A. nilotica, (KS=0.34, P<0.01), A. brevispica (KS=0.34, P<0.01) across the day. The unbrowsed trees attracted significantly more pollinators than naturally browsed trees probably because the unbrowsed trees had larger flowers thereby becoming more visible to pollinators from a distance than those of browsed trees. The delayed floral dehiscence of naturally browsed trees could have been caused by modification of the microclimate around the flowers making them open later in the day than unbrowsed trees. This study has shown than A. nilotica and A. brevispica vary their floral resources greatly when browsed, whereas A. etbaica tends to show compensatory floral growth. More studies therefore, need to be done on their reproductive success.