Waterbirds dispersing native and alien plant species: new methods, new interactions, new insights

Waterbirds provide a variety of ecosystem services such as maintenance of biodiversity, pest control, and as bioindicator of natural conditions. Throughout this thesis, I have focused on a major ecosystem service they provided, which is seed dispersal. Waterbirds act as vectors for the dispersal of a wide variety of organisms that lack their own dispersal mechanisms, or are limited in their own dispersal capabilities. Seed dispersal is a process that greatly influences biogeography, gene flow, and community composition. Understanding the ability of alien and native organisms to disperse via birds is vital to understand how organisms can disperse in response to global change. Waterbird-mediated seed dispersal can be studied throughout different methods, including field-based studies and laboratorybased experiments. The combination of different methods allows a deeper understanding of the species interactions involving different vectors and plant species. Waterbirds connect aquatic and terrestrial habitats by their movements, and create a complex connectivity network based on their daily flights and diet content that makes them key vectors of native and alien plant species, weeds and also aquatic invertebrates. In this thesis, we address the role of two waterfowl species as seed vectors, the mallard Anas platyrhynchos and the greylag geese Anser anser, plus one predatory bird, the grey heron Ardea cinerea. We evaluated their role in vector-mediated dispersal using different methods. In Chapter 1, we combined GPS tracking of geese with the simultaneous collection of faecal samples. By combining these two methods we were able to show that greylag geese disperse a wide variety of angiosperm plant taxa with different traits, and that this included alien and weed plant species. We find evidence that waterbirds play a very important role in the colonization and expansion of both native and alien plant species (including weeds). We built a network using movement data based on GPS tracking, using direct flights as links and the habitats where they feed or roost as nodes. In nature there are also dispersal processes where at least two vectors are involved, termed secondary dispersal. Abiotic vectors such as wind, water or gravity may carry one propagule to one site, which may later be ingested by a biotic vector (e.g a mammal, or bird). In Chapter 2, we focused on the grey heron, a predatory bird with the potential to disperse plants and invertebrates that were first carried internally or externally by their prey animals. Herons regurgitated pellets that contained remains of their prey (e.g. bones, fur, feathers) but also plant seeds and aquatic invertebrates from aquatic and terrestrial ecosystems. By processing heron pellets from populations in the UK and The Netherlands, we showed that herons disperse plant seeds and aquatic macroinvertebrates widely in north-western Europe. In Chapter 3, we used experimental methods with captive birds to obtain this information as well as to study the importance of other plant attributes such as seed traits (e.g. seed size, seed shape, alien or native distribution status) for seed dispersal. We show that these seed traits can help explain the variation between plant species in endozoochory rates, including components of ingestion, seed survival, gut retention time, germination capacity and the time required for a seed to germinate. In Chapter 4, we simulated avian digestive processes to study interactions between seed traits and dispersal. We investigated dispersal síndromes based on seed morphology (Julve, 1998; Vargas et al., 2023) in relation to the ability of seeds to float (i.e. seed buoyancy), and studied this interaction for plant species differing in soil moisture requirements based on their Ellenberg F. In my thesis I conclude that using new methods to study new interactions between waterbirds and plants generates new insights about waterbird-mediated plant dispersal.

Las aves acuáticas proporcionan una gran variedad de servicios ecosistémicos, como ayudar a mantener la biodiversidad, controlar las plagas además de actuar como bioindicadores de las condiciones naturales de los ecosistemas. A lo largo de esta tesis, me he centrado en uno de los mayores servicios ecosistémicos que proporcionan dichas aves, la dispersión de semillas. Las aves acuáticas actúan como vectores para la dispersión de una amplia variedad de organismos que tienen limitadas sus capacidades de dispersión o carecen de ellas. La dispersión de semillas es un proceso que influye significativamente en la biogeografía, el flujo génico y la composición de las comunidades. Comprender la capacidad de los organismos nativos y exóticos para dispersarse a través de las aves es fundamental para entender cómo los organismos pueden dispersarse en respuesta al cambio global. La dispersión de semillas mediada por aves acuáticas se puede estudiar mediante diferentes métodos, incluidos estudios en el campo y experimentos en laboratorio. La combinación de diferentes métodos permite una comprensión más profunda de las interacciones entre especies que involucran diferentes vectores y especies de plantas. Las aves acuáticas conectan hábitats acuáticos y terrestres mediante sus movimientos, creando una red de conectividad compleja basada en sus vuelos diarios y en el contenido de su dieta, lo que las convierte en vectores clave tanto de especies de plantas nativas como exóticas, malas hierbas e invertebrados acuáticos. En esta tesis, abordamos el papel de dos especies de aves acuáticas como vectores de semillas, el ánade azulón Anas platyrhynchos y el ganso común Anser anser, además de un ave depredadora, la garza real Ardea cinerea. El objetivo principal será evaluar el papel de estas aves en la dispersión de plantas utilizando para ello diferentes metodologías.