Islands are becoming more similar after losing unique birds and gaining common widespread bird species

Filipa C. Soares, Jorge M. Palmeirim, Ana S. L. Rodrigues, Pedro Cardoso, Ricardo F. de Lima

This is a plain language summary of a Functional Ecology research article which can be found here.

Ever since humans arrived on oceanic islands, animal and plant populations have suffered severe declines, with many species even going extinct soon after colonization. Birds were by far one of the most impacted groups, threatened by 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, which were mostly brought to the islands for aesthetic or hunting purposes.

We tried to understand if bird extinctions and introductions are causing bird communities of oceanic islands to become more similar. We explored similarity by looking at the species, i.e.  taxonomic similarity, but also at their characteristics – functional similarity – because these are directly associated with their functions in the ecosystem. To do this, we gathered information about all bird species, including extinct and introduced, and their characteristics, across 64 oceanic islands. Then, we measured taxonomic and functional similarity at the global scale, and within and across archipelagos.

Islands are becoming taxonomically and functionally more similar after losing unique bird species and gaining common widespread birds. Islands are represented by the freeform and are coloured by their archipelago (two archipelagos are represented). The different coloured circles surrounding each species represent different functional traits. Native species are represented by a full line circle and introduced species by a dashed line circle. The numbers above the arrows between islands represent the similarity values, which range from 0, if islands have become fully differentiated, to 1, when islands have become fully homogenized. Introduced species are highlighted above the arrow ‘Time’, whereas extinct species are below. Islands of different archipelagos were originally less similar compared to islands of the same archipelago (0.3 < 0.7), but these became more homogenized after extinctions and introductions (from 0.3 to 0.6 compared to 0.7 to 0.8). Image produced by Filipa C. Soares.

We found that islands were more similar both taxonomically and functionally after bird extinctions and introductions. This result agrees with previous studies and is associated with the similarity of many introduced bird species, which are often adapted to anthropogenic habitats (e.g., granivore birds that occur in open areas like weavers, quails, and pigeons). Surprisingly, the widespread loss of species with similar traits, namely large flightless birds, often caused islands of different archipelagos to become less similar. Islands that had different bird communities before extinctions and introductions, such as those belonging to different archipelagos, became more taxonomically similar compared to islands with high original similarities. This is linked to the stronger effect of introductions on increasing similarity, which often counterbalanced the dissimilarity effect of extinctions.

Islands are already taxonomically and functionally more similar to each other after losing unique bird species and gaining common widespread birds. Knowing that introductions are expected to continue, islands will likely continue to become more and more similar, which will probably have strong cascading effects on ecosystems.

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