Islands are important centres of endemism and key ‘natural laboratories’ for the study of ecology and evolution. However, some aspects of island ecology and evolution remain poorly understood. This programme studies patterns of adaptation and speciation on islands worldwide and conducts detailed studies using birds from the Gulf of Guinea, Cape Verde and Tristan islands as study systems.

MacArthur and Wilson’s Theory of Island Biogeography provided a theoretical framework to explain how island communities are assembled. It is particularly applicable to oceanic islands, with its predictions recently validated with empirical data from a worldwide analysis of birds, to which we contributed (Valente et al. 2020, Nature). Land-bridge islands, which have been connected to a mainland in the past, are quite distinct. In contrast to oceanic islands, land-bridge islands are formed with a complete biota – from which species can go extinct after isolation, and others may be added by migration or speciation in situ. In a study led by Luis Valente and Martim Melo, we are using Bioko Island (Gulf of Guinea) to investigate how parameters of the Theory of Island Biogeography play out in such setting. Our results will have implications for understanding an increasingly fragmented world and in managing protected areas – which are more similar to land-bridge islands than to oceanic ones. 

Organisms on islands often exhibit convergent evolution on a wide suite of traits, which are together termed the “island syndrome”. These common evolutionary outcomes are linked to the isolation, small size and stable climate associated with oceanic islands. Isolation reduces species richness on islands relative to mainland areas, limiting inter-specific competition and allowing for ecological release. Low species richness also results in fewer predators and parasites. During the last decade, our team has systematically investigated multiple aspects of the island syndrome, exploring a diverse array of traits (e.g. size, color, song, life-history) and at the role of different potential explanatory factors (e.g. competition, parasites, immune system). 

Reduced parasite levels on islands could underlie the evolution of weaker immune systems, as suggested by the extinction of many Hawaiian birds after the introduction of an avian malaria vector. Since 2015, we have been documenting how humans impact the ‘parasite landscape’ of São Tomé Island, in a project led by Claire Loiseau, from the University of Montpellier (France). Collaborators Mathilde Barthe and Benoit Nabholz, also from Montpellier, used a large-scale genomic approach to measure the diversity of hundreds of immune response genes from our Gulf of Guinea dataset. The results show that both genetic drift and relaxed selection lead to a decrease in immune function among island birds. This confirms our worries, as human-driven habitat change is likely to alter vector-parasite-host dynamics, together with increasing the likelihood of introducing new hosts and parasites. 

The Iago Sparrow Passer iagoensis, endemic to the Cape Verde archipelago, makes an excellent model for the study of adaptation in the wild. It occurs across a wide aridity gradient on 13 islands and islets, allowing insights into how birds may adapt to an increasingly arid world. It also is a good model to study the evolution of commensalism, as humans only colonised the archipelago 500 years ago, and some islands remain uninhabited. A collaboration bringing together Fitz research associates Martim Melo and Rauri Bowie, together with Ângela Ribeiro, Mark Ravinet and José Cerca, is combining fieldwork with genomics to untangle the sparrow’s recent micro-evolutionary history.

Activities in 2022

  • Raquel Ponti and research associates Rita Covas and Claire Doutrelant tested the overlooked role of a decrease, or absence, of predation in the emergence of the ‘island rule’, when small species evolve to become larger on islands, and large species become smaller. They found that the island rule is only detectable in the absence of predators.
  • Ana Leitão is using an experimental approach to examine the relative roles of the reduction in predation and reduced inter-specific competition in the evolution of colour of island birds.
  • The last field season for the project on the human impacts on the avian parasite communities of oceanic islands took place on São Tomé in June 2022. The team was joined by a group of artists from France (choreographer, dancer, illustrator, photographer, video). The output from their visit will include an art video, a short documentary about the research, and an exhibition integrating different media (video, drawings, photos, sounds).
  • A one-month expedition to Cape Verde took place in September with the objective of obtaining samples towards the study of the evolution of commensalism, and to obtain high quality DNA for assembling the full genome of this species. 
  • A three-week expedition to Bioko took place in November-December. Blood samples were obtained in natural habitat from lowland forest at sea-level to Afromontane forest at 2700 m. The expedition was organised as a ringing course – six beginner students attended a one week training session, whereas one intermediate and one advanced student were trained for the full three weeks.

Highlights:

  •  Amancio Motove Etingue, from the Bioko Biodiversity Protection Programme, completed his SAFRING training. He is the first SAFRING ringer in Equatorial Guinea and will now lead ringing programs there, including the setting up of a constant effort ringing programme.
  • Genomic analyses on the Iago Sparrow are providing a wealth of results demonstrating, beyond our expectations, the value of this system for the study of adaptation. As in other ‘seed-eating’ models, genes underlying bill size and shape are under strong selection.

Impact of the project
This project is uncovering novel patterns of adaptation in island birds and investigating the mechanisms underlying these adaptations. The findings contribute to our understanding of the ecology and evolution of island environments. Given the large number of species endemic to islands worldwide and the numerous threats they face, our work will inform conservation strategies for island species.

Key co-supporters
Forever Principe, National Geographic, CNRS (PEPS), University of Montpellier (PhD grant to Alois Robert), Portuguese Science and Technology Foundation.

Research team 2022
Dr Martim Melo (FIAO, UCT/CIBIO, U. Porto)
Dr Claire Doutrelant (CNRS/FIAO, UCT)
Dr Rita Covas (FIAO, UCT/CIBIO, U. Porto)
Dr Claire Loiseau (CIBIO, U. Porto)
Dr Mark Ravinet (U. Nottingham)
Dr José Cerca (U. Oslo)
Dr Martin Stervander (Natural History Museum, London)
Dr Luis Valente (Naturalis Biodiversity Center, Leiden)
Prof. Rauri Bowie (U. California Berkeley)
Prof. Bengt Hansson (Lund U.)
Prof. Peter Ryan (FIAO, UCT)
Dr Raquel Ponti (CIBIO, U. Porto)
Dr Ana Leitão (CIBIO, U. Porto)