In this new post, Dr. Filipa Coutinho Soares—a recent Ph.D. graduate from the University of Lisbon, Portugal—discusses her recently accepted paper, “Bird extinctions and introductions are causing taxonomic and functional homogenization in oceanic islands”.

About the paper

In our paper, we explore if bird extinctions and introductions driven by human activities are causing bird communities of oceanic islands to become taxonomically and functionally homogenized. In other words, we were looking to see if different insular bird communities are becoming taxonomically and functionally more similar to each other. This idea came to us after we found out that islands were becoming functionally poorer, in spite of having gained more species with introductions than they have lost through extinctions. The species that went extinct were functionally unique—like the dodo in Mauritius and the Hawaiian honeycreepers—whereas the species that were introduced by humans have been common widespread birds, including temperate gamebirds and cage pet birds.

The history of most oceanic islands has been largely shaped by humans. In fact, ever since humans arrived on islands, animal and plant populations have suffered severe declines, with many species going extinct soon after colonization. Historically, birds are by far one of the most impacted groups whose threats included predation by introduced species, habitat loss through land clearance, hunting for food or feather collection, and introduction of novel diseases. However, humans were also responsible for the introduction of several bird species—largely brought for aesthetic or hunting purposes.

To understand if these bird extinctions and introductions were causing taxonomic and functional homogenization of island bird communities, we explored the similarity between island communities by calculating changes in beta diversity. In other words, two islands became taxonomically homogenized if their bird species composition became more similar over time (decreasing beta-diversity), and, conversely, they differentiated if it became less similar (increasing beta-diversity). We focused on species to estimate taxonomic similarity and characteristics to estimate functional similarity. These characteristics are functional traits, being directly associated with the functions performed by the species in the ecosystem. To do this, we gathered information about all bird species—including extinct and introduced species, and their characteristics—across 64 oceanic islands belonging to 11 archipelagos. These islands are all larger than 100 km² and are distributed throughout three oceans: the Atlantic; the Pacific; and the Indian ocean. We measured taxonomic and functional similarity at the global scale (including all 64 islands), and at the archipelago scale (looking within and across archipelagos).

We found that islands were more similar both taxonomically and functionally after bird extinctions and introductions, which corroborated previous studies. This homogenization pattern is associated with the similarity of many introduced bird species that are often adapted to anthropogenic habitats, such as granivore birds that occupy open areas like weavers, quails, and pigeons.

Another interesting thing was the fact that we also saw that the widespread loss of species with similar traits often caused islands of different archipelagos to become less similar. Human colonization of oceanic islands has caused the extinction of bird species with similar traits, being typical of those species that evolved in isolation and with few-to-no predators. Many of these birds were flightless, ground-foraging, and had specialized diets, which of course made them highly vulnerable to anthropogenic threats like hunting, introduction of predators, and habitat loss.

Islands that had different bird communities before extinctions and introductions—such as those belonging to different archipelagos—became taxonomically more similar compared to islands with high original similarities. This is linked to the stronger effect of introductions on increasing similarity and often even counterbalancing the dissimilarity effect of extinctions.

To our knowledge, this study was the first to explore the effects of anthropogenic extinctions and introductions on both the taxonomic and functional homogenization of island assemblages. The results suggest that taxonomic homogenization does not always imply functional homogenization.

About the research

The impact of this paper can be seen as a warning message for the future. The paper highlights the importance of human activities as the main drivers of changes in biodiversity. Birds are not the only group that is being driven to extinction or introduced to islands—other animal groups and plants are also being reshuffled due to human activities. Likewise, this pattern of homogenization has also been observed on continents and at different scales.

Worryingly, bird introductions are expected to continue increasing on islands, which, considering the results presented here, suggests that island bird communities will likely continue to become more similar. In addition, homogenization might also be accelerated if species extinctions are not prevented, which is particularly concerning if those species perform unique functions in a given ecosystem.

Functional homogenization will likely have strong cascading effects on ecosystem functioning. A well-known example is the high species turnover in the Hawaiian Islands. This caused a significant decline of functional traits which were essential for seed dispersal, such as gape width and body mass, which ultimately threatened the future of native plant communities.

From a global perspective, functional homogenization will certainly increase the vulnerability of island ecosystems to global changes because it reduces the variability of responses to disturbances. As a consequence, the resilience of ecosystem services will also be impaired and this can end up posing a threat to human societies on islands. Our results provide important insights on how to evaluate the effects of anthropogenic changes on ecosystem functioning. We hope that this will contribute to the development of effective long-term conservation strategies. Nonetheless, future work aiming to identify which functional traits contribute the most to these changes would be vital. This work would enable us to link homogenization patterns to species traits and, eventually, to ecosystem functions and services.

A surprising limitation was the difficulty in obtaining detailed bird species lists for each oceanic island. Species distribution can be obtained from global databases (e.g., species range maps of the IUCN Red List), but these are often inaccurate at the island level. More specific databases exist but these are frequently incomplete, continuously updating, and usually omit extinct species (e.g., island field guides, specific databases). The number of extinct species across islands is most certainly underestimated since many newly extinct species are still being described every year. Therefore, our results are just a snapshot in time which reflect the current knowledge of original island bird assemblages. It is vital to continue documenting extinctions in the fossil record, thereby guaranteeing the correct identification of all persisting species and monitoring new introductions.

Similarly, information on species’ functional traits is often difficult to obtain—especially for extinct species—although extensive functional trait databases at the species level have been published in recent years for extant and extinct bird species. I want to take this opportunity to thank all of the people that have contributed to creating these incredible databases. Improving the detail of global-scale species distribution data and species-level functional information would greatly increase our understanding of the effects of extinctions and introductions on island bird functional diversity, subsequently contributing to improving global conservation strategies.

About the author

This paper is part of my PhD—a long four-year journey that ended just a week ago… I did my PhD at the University of Lisbon, Portugal with an amazing research group. Currently, I am looking for new opportunities to keep working on biodiversity conservation and ecology. I am most proud of my second PhD paper because it not only revealed a surprising and (very) alarming result but also raised other still unanswered questions, like the one discussed here.

I believe that the best thing about being an ecologist is the opportunity to answer the questions that drive you, but also that these answers contribute to the protection and conservation of biodiversity. With this being said, the only bad thing about being an ecologist (in my opinion) is the frequent instability of this career.

Apart from Ecology, I enjoy hiking in nature, especially if accompanied by friends and furry dogs. I have volunteered at an animal shelter ever since I was 13 years old and, more recently, I joined the national board of the first environmental NGO in Portugal.

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