by
- Sabine Rech – Centro ESMOI-UCN & Marine and Environmental Sciences Centre, Regional Agency for the Development of Research, Technology and Innovation, Portugal.
- Benjamin Aguila, Pamela Averill & Emilia Palma Tuki – Unidad de Estudio y Planificación de acciones para el Espacio Marino, Municipalidad de Rapa Nui.
- Dennis P. Gordon – National Institute of Water and Atmospheric Research, New Zealand.
- Leandro M. Vieira – Laboratório de Estudos de Bryozoa , Depto de Zoologia, Universidade Federal de Pernambuco, Brazil & Department of Life Sciences, Natural History Museum, UK.
- Martin Thiel – Centro ESMOI-UCN & MarineGEO, Smithsonian Environmental Research Center, USA.
Rapa Nui’s marine ecosystem is unique, with many endemic species that occur nowhere else in the world. This ecological distinctive community has been able to form due to the island’s extreme remoteness: Rapa Nui is situated 2000 km from the closest Polynesian Islands and 4000 km off the South American continent, in the middle of the South Pacific Ocean. But can geographic isolation protect the island and its vulnerable ecosystems from the arrival of invasive species? Unfortunately, not.
Despite Rapa Nui’s remote location, vessel-based connectivity with other South Pacific islands dates back at least 1200 years to Polynesian voyages and was later expanded by European vessel traffic starting in the 18th century. Today, sailing boats, cruise ships and international fishing fleets are the commonest vessels found in the Rapa Nui ecoregion. This long history of overseas vessel traffic has introduced many terrestrial non-indigenous species with devastating consequences for the island’s ecosystems (Hunt 2007). However, despite over 800 years of maritime traffic, few marine invaders have been reported on the island so far.
Rapa Nui has a high percentage of endemic marine species, which are particularly threatened by the impact of invasive species. Global climate change will even increase this problem, as it weakens many native species and facilitates the establishment of invasive species in new habitats. On Rapa Nui, coastal marine habitats are scarcely monitored, which makes it probable that invasions will only be noticed when they are already in an advanced state of spread. This is highly problematic, because once an invasive species is established, it is almost impossible to eradicate. Therefore, it is important to detect these invaders before they can establish in natural environments and spread along the island. Regularly monitoring the entire island would be a monumental task. However, if you know where to look and what to look for, early detection is possible with relatively little effort. For example, marine invasive species are often transported by vessels and are first introduced to ports or marinas. Moreover, these species are usually found on floating artificial structures, like buoys, pontoons or boats, because those cannot be reached by seafloor predators. This is especially important in temperate mid-latitude ecosystem like Rapa Nui, where natural predators are mostly found on the seafloor, but not in the water column or at the sea surface. Moreover, recent studies have shown that the larvae of some invasive species actively chose plastic substrata to attach to.
In 2022 we visited Rapa Nui with the goal of detecting invasive species that may have arrived recently or that may have been overlooked previously. We identified Hanga Piko as a likely entry point for invasive species‘ larvae from boats entering the harbour, as well as from cruise ships anchoring offshore. Therefore, we installed floating devices with bare plastic settlement plates in Hanga Piko, where we left them for four months, because that should be sufficient time for the larvae of any hidden invader to attach to them (see Pictures 1 and 2).
When we came back in early 2023, we made an astonishing discovery: Hidden beneath the dirt that had accumulated over the last months, on the lower side of our plastic plates, we found some tiny colonies of a strangely looking encrusting organism, which we had never found before (Pictures 3 and 4). When we later analysed it under a special microscope, we found that our first suspicion was true: The colonies belonged to Watersipora subtorquata, a notorious hull-fouling bryozoan species and global invader. Watersipora can grow large colonies (up to 25 cm in diameter) and, like many other bryozoans, compete for space with native species. It is also a nuisance to fishers and other vessel-based activities, as it commonly fouls boat hulls and is resistant to anti-fouling paints.
We cannot be sure about the origin of the colonies found in Hanga Piko. However, given that Watersipora is typically transported on ship hulls, it seems likely that it came to Rapa Nui through vessel traffic, for example from cruise ships or private sailing boats. It is also unclear if or how far the species may already have spread along the island. In fact, we found another colony of invasive Watersipora sp. on a piece of a broken plastic bottle that had been stirred up from the seafloor and stranded on Anakena beach (Picture 5). It was not possible to determine if this colony belonged to the same species (Watersipora subtorquata) or to another, similar species. In any case, this second finding indicates that Watersipora may already be present on the natural seafloor along Rapa Nui’s coasts.
Invasive species like Watersipora could be spread along the island’s coasts through vessels that transit between Rapa Nui’s small fishing harbours and coves, as well as by larger boats that anchor off Anakena. Floating plastic litter is another possible vector that can spread encrusting species between the island’s coastal habitats. The transport of attached organisms on floating plastics is very common in the global oceans. On Rapa Nui, another bryozoan species, Jellyella eburnea, can be commonly found on plastic pieces on Anakena and Ovahe. However, Jellyella is not an invasive species. Watersipora, on the other hand, is a very resistant invader that can even survive in the harsh conditions of exposed rocky shore habitats, like those of Rapa Nui. Therefore, it poses a considerable risk to the island’s natural marine ecosystem.
Despite Rapa Nui’s small size and remoteness, there is frequent vessel traffic from other parts of the world. For example, about 45 recreational vessels (sailing boats or yachts) arrive there per year, most of which come from the Juan Fernandez or Galápagos Archipelagos and usually stay for several days or even weeks. At present, the Galápagos Archipelago, at a distance of more than 3500 km to Rapa Nui, is the only location with known records of W. subtorquata in the Southeast Pacific. Given the frequent connectivity between the two sites via recreational vessels, those can be regarded as a possible, if not likely, vector of dispersal. Apart from that, large cruise ships arrive at Rapa Nui about once a month, mainly from French Polynesia or the South American continental coast (Chile, Peru) and cargo vessels arrive from the Chilean continental coast about twice a month.
To prevent the introduction and spread of invasive species through ships, strict rules for hull cleaning of both commercial and private vessels are already in place for a few oceanic islands and archipelagos, such as Galápagos, New Zealand and parts of the Hawaiian Islands and Australia. For Rapa Nui, the cleaning of boats before they arrive to the island would be crucial to protect its vulnerable ecosystems from marine invasions. Ideally, this should also apply for local vessels, to avoid the secondary spread of invasive species along the island. Additionally, regular monitoring of high-risk sites, such as harbors and marinas could help to detect invasions in early stages. These are relatively simple measures that could help to conserve Rapa Nui’s pristine and unique marine nature.
BOX 1: Bryozoa, also known as moss animals, are a phylum of small, colonial, aquatic invertebrates. The phylum contains several wide-spread invaders. Some invasive bryozoan species disrupt local ecosystems by overgrowing native organisms, such as kelp or coral, and altering habitat structures. Their rapid reproduction and ability to form dense colonies allow them to outcompete native species for space and resources. Invasive bryozoans can also negatively impact aquaculture by fouling nets, cages, and other equipment, increasing maintenance costs.
