Evolution in island birds and the ‘insularity’ syndrome
Genomic data from Tristan’s Nesospiza finches collated by Bengt Hansson and Martin Stervander have provided novel insights into this recent adaptive radiation.Islands are important centres of endemism and key ‘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 the Gulf of Guinea, Cape Verde and Tristan islands as study systems.
The Theory of Island Biogeography, which predicts how species richness on an island is a function of its size and isolation, remains one of the most elegant of ecological theories. Although the theory’s architects, MacArthur and Wilson, were aware of the importance of speciation after colonisation, they were unable to include this variable in the model. In a project led by Luis Valente, we addressed this lack. By comparing sequences from almost 600 island birds from 41 archipelagos with those of their mainland relatives, we were able to estimate colonisation times and speciation rates. Amazingly, the empirical shape of the relationships between colonisation, extinction and speciation with island area and isolation is best explained by the original island biogeography model. These results were published in Nature in February 2020. That same month, Martim Melo and Luis Valente were on Bioko Island collecting blood samples from birds for a new project investigating how communities from continental islands (i.e. those that were formerly linked to the mainland) are assembled.
The Iago Sparrow Passer iagoensis is endemic to the Cape Verde archipelago where it occurs across a wide aridity gradient on most islands and islets. It constitutes an excellent study model for understanding how birds may adapt to an increasingly arid world. In addition, as humans only colonised the archipelago 500 years ago, and some islands remain uninhabited, the sparrow also makes a good model for the study of the evolution of commensalism. A collaboration led by Martim Melo, Ângela Ribeiro, Rauri Bowie and Mark Ravinet is combining fieldwork with genomics to untangle the micro-evolutionary processes occurring in historical times. Birds were sampled across 13 islands and islets, and across within-island aridity gradients to assess genetic structure using next generation sequencing.
Islands typically have impoverished communities, and our previous work confirmed that this extends to parasite diversity. Low parasite levels could lead to weaker immune systems, as suggested by the extinction of many Hawaiian birds after the accidental introduction of a malaria vector. This hypothesis is currently being studied through a collaboration between Claire Doutrelant, Rita Covas, Martim Melo, Claire Loiseau and Benoit Nabholz, which is investigating the genes underlying specific types of immune response. Additionally, human driven habitat change is likely to alter vector-parasite-host dynamics. Claire Loiseau and Martim Melo are leading a study to investigate whether land-use practices influence avian parasites on the endemic-rich São Tomé Island. Entomologists are documenting changes in the community of parasite vectors, a critical but often overlooked piece of this puzzle.
Activities in 2020
- During one month of fieldwork on Bioko Island, we sampled birds from lowland rainforest to Afromontane forest at 2 300 m, thanks to the support of the Bioko Biodiversity Protection Program and in particular Steve Miller and Amâncio Etingue.
- A survey along a human disturbance gradient on São Tomé was conducted, we sampled birds and their parasites and parasite vectors to assess the impact of human activities on bird-parasite interactions.
- José Cerca and Rosa Jiménez are running the bioinformatic analyses of 339 Iago Sparrow SNP RAD-sequences and 23 full genomes.
- Full genome sequences were constructed for several São Tomé endemic birds and their mainland counterparts to investigate whether insularity influences the evolution of the immune system by studying toll-like receptors. The genomes of range-restricted mainland species were also sequenced to compare with island species.
- PhD student Alois Robert showed that impoverished bird and insect communities affect the way birds sing on islands, and MSc student Louis Bliard showed that impoverished predator communities affect plumage colour of island birds.
- A paper published in Nature in February confirmed that, at least for birds, MacArthur’s and Wilson’s Theory of Island Biogeography is robust to the inclusion of evolution in the process of the assembly of island communities.
- A paper published in the International Journal of Parasitology reported the influence of land-use change in the bird parasite community of São Tomé Island.
- A paper published in Biology Letters examined the link between relaxed predation and bird coloration on islands.
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 abundant threats to these biotas, our work will help to understand and conserve island species.
Forever Principe, National Geographic, CNRS (PEPS), University of Montpellier (PhD grant to Alois Robert), LIA Biodiversity, Portuguese Science and Technology Foundation.
Research team 2020
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)
Rosa Jiménez (U. California Berkeley)
Dr Martin Stervander (U. Oregon)
Dr Luis Valente (Naturalis Biodiversity Center, Leiden)
Prof. Rauri Bowie (U. California Berkeley)
Prof. Bengt Hansson (Lund U.)
Prof. Peter Ryan (FIAO, UCT)
Students: Sandra Reis (PhD, CIBIO, U. Porto); Aloïs Robert (PhD, CNRS and U. Montpellier); Bárbara Freitas (MSc, CIBIO, U. Porto); Louis Bliard (MSc, CNRS and U. Montpellier).