Aim We analyse the consequences of species extinctions and introductions on the functional diversity and composition of island bird assemblages. Specifically, we ask if introduced species have compensated the functional loss resulting from species extinctions. Location Seventy‐four oceanic islands (> 100 km2) in the Atlantic, Pacific and Indian Oceans. Time period Late Holocene. Major taxa studied Terrestrial and freshwater bird species. Methods We compiled a species list per island (extinct and extant, native and introduced), and then compiled traits per species. We used single‐trait analyses to assess the effects of past species extinctions and introductions on functional composition. Then, we used probabilistic hypervolumes in trait space to calculate functional richness and evenness of original versus present avifaunas of each island (and net change), and to estimate how functionally unique are extinct and introduced species on each island. Results The net effects of extinctions and introductions were: an increase in average species richness per island (alpha diversity), yet a decline in diversity across all islands (gamma diversity); an average increase in the prevalence of most functional traits, yet an average decline in functional richness and evenness, associated with the fact that extinct species were functionally more unique (when compared to extant natives) than introduced species. Main conclusions Introduced species are on average offsetting (and even surpassing) the losses of extinct species per island in terms of species richness, and they are increasing the prevalence of most functional traits. However, they are not compensating for the loss of functional richness due to extinctions. Current island bird assemblages are becoming functionally poorer, having lost unique species and being composed of functionally more redundant species. This is likely to have cascading repercussions on the functioning of island ecosystems. We highlight that taxonomic and functional biodiversity should be assessed simultaneously to understand the global impacts of human activities.
Habitat loss and non-native species are two key drivers of biodiversity decline. The importance of their interactions is widely recognized, but remains poorly understood. We used the endemic-rich bird assemblage of São Tom e Island to study this type of interactions. We built species-specific binomial generalized linear models for 33 terrestrial bird species, based on presence-absence data from 2398 sampling points. Meta-analysis techniques revealed that land use was more important in explaining distribution than topographic variables, rainfall, or distance to coast for the studied bird species. Native species were more likely to occur in remote rainy forests at higher altitudes, whereas non-native species were more likely to occur in non-forested ecosystems, being associated with humanized lowland areas in the drier flat regions of São Tom e. The susceptibility of anthropogenic ecosystems to the establishment of non-native birds suggests that disturbance favours these species. An analysis of species feeding guilds further suggests that these habitat associations might be linked to resource availability, as a large proportion of non-native species rely on seeds, which are clearly more available outside the best-preserved forests. Contrarily, very few native birds seem to be using this resource, and instead rely on more complex ecological interaction, such as carnivory and frugivory. This difference may reduce the chance of negative interactions between native and non-native species (e.g. competition), but this should not be misunderstood for a lack of negative effects of the latter. Our results indicate that land-use intensification is the key driver of biodiversity changes on São Tom e, potentially facilitating the expansion of non-native species and demoting ecological complexity. Therefore, protecting the best-preserved forests is the single most important measure to ensure the conservation of native species. Animal Conservation.
Aim The world's islands support disproportionate levels of endemic avian biodiversity despite suffering numerous extinctions. While intensive recent research has focused on island bird conservation or extinction, few global syntheses have considered these factors together from the perspective of morphological trait diversity. Here, we provide a global summary of the status and ecology of extant and extinct island birds, the threats they face and the implications of species loss for island functional diversity. Location Global. Taxon Birds. Methods We provide a review of the literature on threatened and extinct island birds, with a particular focus on global studies that have incorporated functional diversity. Alongside this, we analyse IUCN Red List data in relation to distribution, threats and taxonomy. Using null models and functional hypervolumes, in combination with morphological trait data, we assess the functional diversity represented by threatened and extinct island endemic birds. Results and main conclusions We find that almost half of all island endemic birds extant in 1500 CE are currently either extinct or threatened with extinction, with the majority of threatened extant species having declining population trends. We also found evidence of 66 island endemic subspecies extinctions. The primary threats to extant island endemic birds currently are agriculture, biological resource use, and invasive species. While there is overlap between the hotspots of threatened and extinct island endemics birds, there are some notable differences, including the Philippines and Indonesia, which support a substantial number of threatened species but have no recorded post‐1500 CE bird extinctions. Traits associated with threatened island endemic birds are large body mass, flightlessness, aquatic predator, omnivorous and vertivorous trophic niches, marine habitat affinity, and, paradoxically, higher dispersal ability. Critically, we find that threatened endemics (i) occupy distinct areas of beak morphospace, and (ii) represent substantial unique areas of the overall functional space of island endemics. We caution that the loss of threatened species may have severe effects on the ecological functions birds provide on islands.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Aim: We analyse the functional consequences of the changes in species composition resulting from extinctions and introductions on oceanic island bird assemblages. Specifically, we ask if introduced species have compensated the functional loss resulting from species extinctions. Location: Seventy-four oceanic islands (>100 km2) in the Atlantic, Pacific and Indian Oceans. Time period: Late Holocene. Major taxa studied: Terrestrial and freshwater bird species. Methods: We compiled a species list per island (extinct and extant, native and introduced), and then compiled traits per species. We used single-trait analyses to assess the effects of past species extinctions and introductions on functional composition. Then, we used probabilistic hypervolumes in trait space to calculate functional richness and evenness of original versus present avifaunas of each island (and net change), and to estimate functional originality of extinct and introduced species. Results: The net effects of extinctions and introductions were: an increase in average species richness per island (alpha diversity), yet a decline in diversity across all islands (gamma diversity); an average increase in the prevalence of most functional traits (23 out of 35) yet an average decline functional richness and evenness, associated with the fact that extinct species were functionally more original (when compared to extant natives) than introduced species. Main conclusions: Introduced species are on average offsetting (and even surpassing) the losses of extinct species per island in terms of species richness, and they are increasing the prevalence of most functional traits. However, they are not compensating the loss of functional richness due to extinctions. Current island bird assemblages are becoming functionally poorer, having lost original species and being composed of functionally more homogeneous species. This is likely to have cascading repercussions on the functioning of island ecosystems.
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