Combined effects of bird extinctions and introductions in oceanic islands: decreased functional diversity despite increased species richness

Abstract

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.

Threatened and extinct island endemic birds of the world: Distribution, threats and functional diversity

“…In addition, an analysis of avian extinctions in the Hawaiian Islands found that species that went extinct in prehistoric times (i.e., prior to European contact) tended to be large‐bodied, whereas those in historic times (i.e., after European contact) tended to be mid‐sized species, possibly because the most vulnerable large‐bodied species had already been lost (Boyer, 2008). However, in our null model and individual trait comparison analyses, we found that larger‐bodied island endemic species were more likely to be threatened and to have gone extinct (see also Fromm & Meiri, 2021; Soares et al, 2022). This result could illustrate that hunting, which typically targets larger‐bodied bird species (e.g., Duncan et al, 2002, 2013), is the most pervasive threat on islands, or it could be that the traits that tend to correlate with body size (low reproductive rates, low rates of population growth, small population sizes, small clutch size, long intervals between clutches, larger home ranges; Boyer, 2008; Gaston & Blackburn, 1995) are driving this pattern.…”

“…First, it does not include species that went extinct as a result of human actions prior to 1500 CE. Pre‐1500 CE human communities are known to have caused a large number of avian extinctions on islands through hunting, the introduction of non‐native species and habitat loss (Boyer, 2008; Duncan et al, 2013; Hume, 2017; Milberg & Tyrberg, 1993; Russell & Kueffer, 2019; Sayol et al, 2020; Soares et al, 2021, 2022; Steadman, 2006; Szabo et al, 2012; Whittaker & Fernández‐Palacios, 2007; Table 1). Indeed, we found evidence of at least 307 pre‐1500 CE island endemic extinctions (i.e., the 8% extinct figure increases to 22% if we include all known island endemic bird extinctions).…”

“…It was not possible to test this idea using the datasets we collated as we did not have trophic niche data for extinct species. However, a recent study by Soares et al (2022) did determine the primary diet type (invertivore, carnivore, frugivore, granivore, omnivore and herbivore) for 759 native bird species (including 214 extinct species; both pre‐ and post‐1500 CE extinctions) across 74 oceanic islands. Interestingly, a simple analysis of the data in Soares et al reveals that the proportion of extinct species in each of the six diet categories is roughly similar to the proportion of extant native species in each category (see Table 2), with invertivores being the only group with a greater than 5% difference between extinct and extant species proportion (26% of extinct species were invertivores compared with 39% of extant species).…”

“…This is largely because continuous functional trait data are hard to collect, particularly for extinct species. However, specifically for birds, recent advances (e.g., Sayol et al, 2020, 2021; Soares et al, 2021, 2022; Tobias et al, 2022; Triantis et al, 2022) have sourced and synthesised extensive functional trait data for extant and extinct island birds at the species level. The collection of these data now allows for more in‐depth analyses of avian functional diversity—the morphological and ecological characteristics that influence fitness and the effects of organisms on the environment—on islands (Tobias et al, 2022).…”

Collective and harmonised high throughput barcoding of insular arthropod biodiversity: toward a Genomic Observatories Network for islands

“…When climate change is combined with other impacts of increasing human population size and economic development, such as habitat modification and degradation, the challenge for managing and conserving insular biodiversity presents itself as being immediate and largescale (Russell & Kueffer, 2019). It is not only the loss of species and their interactions that is particularly relevant on islands, but also the loss of unique evolutionary history (phylogenetic and functional diversity), reflecting the loss of unique adaptations to the environment (Sayol et al, 2021;Soares, de Lima, Palmeirim, Cardoso, & Rodrigues, 2021).…”