Impacts of invasive alien plants on wild pollinators

The Anthropocene is characterised by a major decline in pollinator populations, particularly wild bees. At the same time, some plant species benefit from anthropogenic environmental change and expand their distribution as invasive alien species. By modifying floral resources and plant-pollinator interactions, these plants can act as strong ecological drivers of ecosystem change. This thesis aims to quantify how invasive alien plants reshape plant-pollinator systems, from pollen transfer and interaction networks to community functional traits and bee fitness, using a multi-scale and integrative framework combining field surveys, community-level analyses, and laboratory analyses.

Chapter 1 examines how native pollinators act as vectors of invasive pollen and how they contribute to the integration of invasive plants into native plant-pollinator networks. Based on the study of five invasive plant species in Belgium, we show that bees are key visitors and efficient pollen vectors of invasive plant species, although interaction patterns vary among species. Impatiens glandulifera stands out for its high pollen transfer efficiency and strong floral visitor fidelity, with most visitors carrying its pollen almost exclusively. Overall, invasive plants can dominate the pollen loads carried by pollinators, reducing the presence of native pollen and leading to a loss of interaction with native floral resources.

Chapter 2 investigates the consequences of invasive floral resource use on bee fitness under field conditions. Using sentinel colonies of Bombus terrestris, two invasive plants were assessed. Robinia pseudoacacia invasion negatively affected colony performance, increasing worker mortality and reducing offspring production (Chapter 2.1). In contrast, higher proportion of Impatiens glandulifera in the diet enhanced colony development and reproduction, but increased parasite prevalence (Chapter 2.2). These results highlight that invasive plants can simultaneously provide resources and impose hidden physiological costs or enhanced parasite transmission risk.

Chapter 3 explores how invasive plants affect plant and pollinator communities, their interactions, and functional traits. Using the butterfly bush, Buddleja davidii, as a model species, we investigated these effects across invasion gradients in two types of species-rich grasslands in Belgium. In a first study conducted across Belgian quarry grasslands, we show that increasing invasion reduces native floral resource availability and is associated with lower abundance of several common pollinator species, despite limited changes in community-level species richness (Chapter 3.1). We then investigated invasion impacts in Belgian slag heaps, combining field observations and pollen analyses to assess plant-bee interaction networks along invasion gradients and across flowering periods. Results revealed that invasion reduces wild bee and interaction abundance and richness, and that these negative effects intensify during the flowering peak of B. davidii (Chapter 3.2). Finally, we demonstrate that invasion also alters the functional traits of plant and bee communities, reducing bee functional diversity and reshaping community species composition, largely due to the increasing dominance of bumble bees (Chapter 3.3). Together, these studies show that B. davidii invasion consistently reshapes plant-pollinator systems across invaded habitats through cascading effects on floral resources, species interactions and community functional composition.

Overall, this thesis demonstrates that invasive plants reshape plant-pollinator systems through multiple and context-dependent mechanisms, affecting floral resource availability, pollen transfer dynamics, community structure, and bee fitness. These findings highlight the importance of considering both community, species and individual-level consequences to fully understand the impacts of biological invasions on plant-pollinator systems.