Diversity of plants, bee pollinators, and their interaction networks along an elevation gradient of the Eastern Afromontane biodiversity hotspot in Kenya

Abstract

Insect-mediated pollination is an important ecosystem service in terrestrial ecosystems, vital for the reproductive success of both wild and crop plants. Under the current global change scenario, ecological influences in mutualistic communities will provide different levels of environmental tolerance and mode of interaction, with species acting differently. This might eventually lead to devastating effects on biodiversity and the functioning of both managed and natural ecosystems. However, there is a lack of knowledge on the actual impacts these ecological changes might have on species assemblages across different climatic gradients in tropical ecosystems. In order to shed more light on this, this thesis was set to explore the effects of environmental changes on bee species (regarded as the most important pollinator group) diversity (Chapter 2), plant-pollinator interactions (Chapter 3), functional trait aggregations (Chapter 4) and resilience in plant-bee interaction networks (Chapter 5) across different elevation gradients of the Eastern Afromontane Biodiversity Hotspot of Kenya (EABH).

In Chapter 2, seasonal trends and drivers of bee diversity along an elevation gradient of the EABH were assessed to understand the influence of seasonality on elevational biodiversity patterns. Thereafter, an examination of how local species richness (α-diversity) and seasonal changes in local species communities (β-diversity) contribute to species richness across seasons (γ-diversity) along the elevation gradients was also assessed. Findings revealed that α-, β- and γ-diversity all decreased with elevation and high turnover of species across seasons contributed to a considerably higher γ- than α-diversity on study plots. Furthermore, a combination of seasonality in climate and the seasonal turnover of floral resources best explained the seasonality in bee species communities (β-diversity) observed across the elevation gradient. In Chapter 3, the impact of elevation and seasonality on plant-pollinator network assemblages, climate, floral resource availability, and bee diversity were analysed and quantified. Results showed a generally increasing pattern for nestedness. Specialisation in plant-bee interaction networks increased with elevation in the cold-dry and warm-wet seasons respectively. The findings further revealed that link rewiring increased in the warm-wet season with elevation but remained indifferent in the cold-dry seasons. Conversely, network modularity and plant species were more specialised at lower elevations in both cold-dry and warm-wet seasons, with higher values observed during the warm-wet seasons. Finally, it was revealed that flower species diversity had a positive relationship on modularity and bee species specialisation. On the other hand, bee species diversity had a negative relationship with specialisation, but was positively related to link rewiring, modularity, and plant species specialisation. Moreover, results showed that increased temperature positively influenced modularity, network, and plant species specialisation. To further understand the structural dynamics of bee activities across the elevation gradients, (Chapter 4), the impact of elevation on two bee functional traits (Intertegular distance and proboscis length) at the species (intra) and community (inter) levels of biological organisation was assessed. These traits are proxies for bee body size and feeding resources respectively. After finding patterns in bee functional traits with elevation, further analysis revealed the role of temperature and food resource availability as potential factors driving the observed variations in bee functional traits.

Finally, to understand how the smooth flow of ecosystem services is sustained across the elevation gradient, resilience in bee and plant networks (Chapter 5) was analysed and the effects of climate, bee and plant diversity, bee functional diversity, and network structures on bee and plant resilience were quantified. Results showed that mutualistic networks were more resilient to changing climates at higher elevations, such that bee and plant species exhibited high levels of resilience at higher elevations. Further analyses showed that bee species abundance, plant and bee species richness, temperature, its seasonality component, and precipitation play important roles in structuring ecosystem resilience.

In a nutshell, the results presented in this thesis clearly show the impact of ecological changes on species assemblages along the elevation gradients of the EABH. Here, the role of climate change is very prominent and expeditiously changing the provisioning of ecosystem services with pronounced negative effects on species assemblages. This study, therefore, emphasises the urgent need for conservation and restoration efforts aimed at reducing climate change effects and harnessing the ability of mutualists to restore broken links in order to improve ecosystem resilience and functioning along the slopes of the Eastern Afromontane Biodiversity Hotspots in Kenya and East Africa as a whole.