When animals become extinct, their functions in an ecosystem can be lost, often leading to the extinction of other species that depend on those functions. Can alien species, often viewed by conservationists as invasive pests, fill vacant roles due to extinction?
This question is easy to answer in the Hawaiian Islands, which is often considered a model system for studying extinction and invasion of species. Unfortunately, the chain of islands has undergone major ecological changes since the arrival of humans, with much of the species loss being caused by overhunting, deforestation, and the introduction of non-native predators, competitors, and disease.
Most of Hawaii’s native forest plants rely on birds for seed dispersal, a critical process in plant reproduction, and many also require birds for pollination. Still, at least 67 percent of the island’s native bird species – once a diverse collection of brightly colored honey snails, crows, and flightless waterfowl – are extinct. How do forests work without these birds – the missing gears and bolts in a broken system?
It turns out that with the disappearance of native species, humans introduced many alien birds for a variety of reasons, including recreational hunting and agricultural pest control, to name a few. Many of them have established themselves, including songbirds and wild birds that feed on nectar and / or fruit. So there is the potential for these introduced birds to replace the role of extinct birds by maintaining seed dispersal and pollination of native plants.
With overwhelming evidence that alien species can wreak havoc in Hawaii and around the world, it’s no surprise that conservationists often follow a rigid dichotomy: native is good and not native is bad. But in endangered systems like O’ahu, removal of all non-native birds would deprive forests of almost all birds and thus hinder seed dispersal for many native plants. So we need to consider how each intruder affects the ecosystem separately before drawing conclusions based on origin alone.
I am currently working with the Hawaiʻi VINE Project, a collaborative study of seed dispersal networks on Ohuahu, the most invaded Hawaiian island where all native fruit-eating birds have become extinct. We have found that alien birds actually maintain seed dispersal processes; In fact, they are the only seed dispersers for many of the native plants on Oahu – but there is a catch.
Through my doctoral thesis, I became interested in how historical birds differ morphologically from modern ones and how this can affect changes in ecosystem function. A particularly important bird characteristic is mouth size – also known as the gape width – which restricts the size of the seeds a bird can consume and distribute.
Many extinct Hawaiian birds are known only from subfossil remains, including skulls and bones excavated from sand dunes and lava tubes. After measuring these skulls, we used predictive models to estimate the animals’ mouth sizes, and with additional measurements from study skin samples, we examined how mouth sizes and other characteristics had changed with the extinction and introduction of birds.
We found that there are only four indigenous fruit-eating birds left in the entire archipelago, and compared to historical, mostly extinct, fruit-eating birds, the modern, mostly-introduced birds have much smaller mouths – about 40 percent smaller overall – which means that the Today’s birds cannot eat and scatter seeds as large as these ancient birds once could. In reviewing seed dispersal studies on the islands, we found that plants with larger seeds – those with a seed width of over 8.1 millimeters – are not dispersed by modern birds and may be at increased risk of extinction.
These results raise an obvious question: should we introduce birds with larger mouths? Currently, there isn’t enough previous research on “rewilding” – the deliberate introduction of non-native species to serve as functional surrogates for extinct species – to know if this is a good idea. There is a serious risk of unintended ecological impacts related to the introduction of species. However, we may be able to gain some insight into the approach by looking at Hawaii as an unintended “feral” experiment.
While alien birds on Oahu can be viewed as partial substitutes for the extinct birds, especially because of their distribution of small-seeded native plants, these benefits are offset by another important finding: They also spread a large number of small-seeded non-native plants. the fruit in large quantities all over the island. On the island of Maui, in areas where alien plants are less dominant, alien birds mainly spread native seeds; So, overall, it appears that these non-native birds are simply reacting to what plants are available to them, and their overall forest conservation impact can depend largely on the context of the plant community in which they live.
Therefore, in order to support the conservation benefits of alien birds that are currently established, we must eradicate alien plants, especially those that produce large numbers of fruits. Without the management of alien plants, a rewilding approach could backfire. The introduction of a great mouth bird could, for example, trigger a sudden spread of large-seeded alien plants, which is certainly undesirable for the preservation of native biodiversity, since alien plants often outperform native plants in terms of space and resource use.
In a world with increasing extinction and invasion rates, the reorganization of species communities will continue to pose challenges to conservation goals. In certain ecosystems, such as Hawaii’s forests, a classic paradigm that denigrates all non-native species can paradoxically hamper the preservation of native biodiversity. What is needed instead is an approach that takes into account the functional characteristics and ecological roles of different invaders in different circumstances in order to protect important ecosystem processes such as seed dispersal now and in the future.