The forgotten fauna: Native vertebrate seed predators on islands

Abstract

Human colonization of islands has resulted in the reduction or loss of many native species, and the introduction of non‐native species, producing novel ecosystems. The impacts of these changes on mutualistic plant–animal interactions have received considerable attention, but the potential effects on some antagonistic interactions, such as seed predation, are less thoroughly understood, and often overlooked. Using three archetypal island groups—New Zealand, the Mascarenes and Hawaiʻi—we compare the taxonomic richness and functional diversity of vertebrate seed predators from prehuman and contemporary ecosystems. We scored species on several traits relevant to seed predation, then clustered species into functionally similar groups using hierarchical clustering. These archipelagos once supported between 19 and 24 species of exclusively avian seed predators (representing two to four orders) ranging from large, flightless herbivores to small, volant finches and parrots. Following human arrival, 63%–89% of these species went extinct, and between 12 and 23 non‐native seed predators were introduced. Contemporary seed predator faunas consist of between 14 and 26 species (representing six to seven orders), dominated by non‐native granivorous birds and omnivorous mammals. Our results reveal several examples in which non‐native species may be functionally similar to extinct seed predators, but most non‐native species are functionally different from extinct species, and therefore may be introducing novel seed predation pressures for insular ecosystems. Mammalian seed predators are especially functionally different from the native avian seed predators, as their teeth and widespread habitat distribution allow them to destroy a more diverse range of seeds, including the largest seeds. We highlight the need to understand how these altered seed predator communities are affecting native plant populations, particularly in the context of reduced pollination and seed dispersal. More broadly, we argue that antagonistic interactions are an integral part of any ecosystem, and therefore must be understood if we are to achieve more holistic restoration frameworks for insular ecosystems. A free Plain Language Summary can be found within the Supporting Information of this article.

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

“…These changes to island bird assemblages might disrupt well-established mutualistic plant-animal interactions and affect native plants, particularly through reduced pollination and seed dispersal (e.g. Caves et al, 2013; Carpenter et al, 2020). Herbivore birds introduced to islands that had no native browsers or grazers can greatly affect ecosystems, including by reducing food resources for pollinators and ultimately changing the phenotypic traits of plants related to pollination (e.g.…”

“…Flightless and weak flying birds can have important and sometimes irreplaceable ecological roles in key ecosystem functions (Boyer & Jetz, 2014), such as seed dispersal, pollination and herbivory (e.g. Carpenter et al, 2020) but they have been completely eradicated from almost all islands (Sayol et al, 2020;Fromm & Meiri, 2021). This proneness to extinction is linked to having evolved in the absence of mammalian predators, which were introduced to most islands (Milberg & Tyrberg, 1993;Russell & Kueffer, 2019).…”

“…In order to maintain functional diversity, we must prevent further loss of native ecosystems because their functioning depends on complex and irreplaceable ecological interactions (Aslan et al, 2013;Carpenter et al, 2020;Carmona et al, 2021). We also must avoid new introductions, especially of species that might affect ecosystem functions, either through predation (Milberg & Tyrberg, 1993;Sax & Gaines, 2008;Loehle & Eschenbach, 2011), competition (Soares et al 2021), or the disruption of mutualistic interactions (Caves et al, 2013;Carpenter et al, 2020). Lastly, we need to protect native species, focusing on those that have unique functional traits.…”

A roadmap to plant functional island biogeography

“…Invasive plants are superior competitors over natives mediated through traits linked to fast growth, high fecundity and resistance against external disturbances expressed by lower leaf mass per area, longer flowering periods and higher efficiency in capturing limiting resources (Baruch & Goldstein, 1999;Durand & Goldstein, 2001;Marx et al, 2016;Westerband, Knight & Barton, 2021). Invasive plants can also interfere with plant-animal mutualisms, likely leading to disrupted ecological networks (Heleno et al, 2013;Carpenter et al, 2020). Introduced herbivores like ungulates, rabbits or insects threaten native plants that have lost traits for defence due to the absence of herbivores, especially if predators of the herbivores are also absent (Bowen & Van Vuren, 1997).…”

European hedgehogs rear young and enter hibernation in New Zealand’s alpine zones

“…Kaiser-Bunbury et al 2010;Iles & Kelly 2014). These processes are inarguably important, however, antagonistic processes such as predation, herbivory, seed predation, and parasitism were also a part of historic ecosystems, and should be considered integral components of restored ecosystems (Carpenter et al 2020). Weka were historically a part of many ecosystems across the New Zealand mainland, so projects truly dedicated to ecosystem restoration should welcome their presence as an archetypal generalist that carries out many ecological processes.…”