Assessing the impact of plastic waste on Hermit Crabs on remote Pacific Islands.

Plastics are designed to be light-weight, convenient, and durable; several characteristics that make them suitable packaging alternatives compared to other materials such as wood, glass or metal, but also makes them problematic in marine and terrestrial environments. Low manufacturing costs have contributed to huge demand for new plastic materials, with global production increasing by 6–8% per annum. Globally, less than 10% of the 348 million tonnes of plastic produced annually is ever recycled, with approximately 40% of plastic waste comprised of single-use packaging. The significant increase in disposal rates in the last half century, combined with inadequate or ineffective waste management, has led to huge quantities of plastic polluting ecosystems worldwide. Once in the ocean, plastic items can either sink or float, becoming dispersed over long distances via tides and currents Significant quantities of plastic are now recorded in all aquatic ecosystems, accounting for more than 95% of all debris items observed at-sea, on beaches, and along river banks. These synthetic materials persist for decades in the environment, posing a considerable threat to aquatic flora and fauna Mortality of wildlife from plastic debris can occur directly (e.g. through entanglement) or indirectly through exposure to plastic-associated toxins, which may contribute to reduced body condition or survival in some species. While evidence of harmful effects on individual organisms is increasing, there is currently little knowledge or agreement regarding whether plastic debris poses an ecologically relevant threat, affecting wildlife at the population level and contributing to an overall decline in species’ abundance. Establishing a clear link between debris interactions and population persistence is crucial, as loss of biodiversity contributes to the degradation of ecosystems and the valuable services they provide. While much of the focus of plastic impacts has understandably been on the marine ecosystem, increasing quantities of debris accumulating on beaches and adjacent vegetated areas has the potential to disrupt terrestrial species and ecosystems. In tropical ecosystems, Crabs, Malacostraca, play a crucial role in forest growth and development through aeration of soils and creation of carbon-rich soil microhabitats, therefore reductions in Crab abundance may impact plant recruitment.

In a paper published in the Journal of Hazardous Materials on 16 November 2019, Jennifer Lavers of the Institute for Marine and Antarctic Studies at the University of Tasmania, Paul Sharp and Silke Stuckenbrock of the Two Hands Project, and Alexander Bond of the Bird Group at the Natural History Museum, describe the results of a study into the effects of plastic waste on Hermit Crabs on two remote Pacific locations, the Cocos Islands and Henderson Island.

In order to understand the potential impact accumulating plastic may have on coastal crab populations, Lavers et al. recorded the number and frequency of Strawberry Hermit Crabs, Coenobita perlatus, entrapped in beach debris on individual beaches within the Cocos Islands and on Henderson Island, in the Pitcairn group, two remote areas where significant quantities of debris accumulate. We then estimate entrapment rates across both islands to provide an estimate of population-level impact of plastic beach debris on Crab populations.

The Cocos Islands are two small, mid-oceanic atolls (total land area 14 km²) located approximately 2760 km north-west of Perth, Western Australia. The southern atoll consists of a horseshoe chain of 26 islands around a shallow, central lagoon. The northern atoll (North Keeling, administered as Pulu Keeling National Park) is a relatively pristine, uninhabited island. Most of the Human population (around 600 people) reside on Home and West Islands. A range of marine resources are fished for food and tourism, including Crabs and other Crustaceans which are consumed or used for bait. Henderson Island is a raised coral atoll and UNESCO World Heritage Site (total land area 43 km²), administered as part of the Pitcairn Islands (UK). It is extremely remote, uninhabited, and located on the western boundary of the South Pacific Gyre, a known plastic-accumulation zone. Both Henderson and Cocos are very polluted, with about 38 million (239 items/m²) and 414 million debris items (713items/m²) deposited on beaches and throughout the beach-back vegetation, respectively.

Map of the study sites (blue circles): Cocos Islands (top; North Keeling not shown on inset map) and Henderson Island (bottom) with sampling regions shown in red. Lavers et al. (2019).

Lavers et al. recorded visible macro-debris located on the surface within randomly-placed quadrats. In the beach-back, significant quantities of debris accumulate amongst the vegetation, creating an obvious hazard for Crabs. On Cocos, four quadrats were established on Direction Island and four on West Island from 20 to 29 March 2017, one on Pulu Blan Madar, and two on Home Island, from 1 to 2 September 2019, and 20 quadrats along the East Beach of Henderson Island during 12 to 16 June 2019. The boundary of each quadrat was located along the top edge of the beach and extended into the vegetation towards the centre of the island. On Cocos, the dimensions of each quadrat were 5 × 3 m (2017) or 6 × 4 m (2019), and on Henderson 6 × 6 m, reflecting differences in accessibility at each site. The size was reduced slightly for some quadrats (2/8 on Cocos in 2017 and 2/20 on Henderson) to enable navigation through thick forest and to protect sensitive habitats.

(A) Accumulated plastic debris creates an obstacle for crabs on the beaches of the Cocos Islands. (B) A Hermit Crab inside a green bucket along the high tide of South Island. (C) Accumulated plastic debris in the beach-back vegetation on West Island. (D) Crabs that became trapped and died inside a plastic drink bottle that washed up on Cocos. (2019). Lavers et al. (2019).

The location of the beach-back quadrats and timing of surveys overlapped periods when a range of Crab size classes were present on both islands and encompassed a diversity of habitats (e.g., areas dominated by Velvetleaf Soldierbush, Heliotropium foertherianum, or Small-leaved Mangrove, Pemphis acidula). However, the density of Crabs within these habitats was not recorded and no attempt was made to survey across seasons due to the remote nature of each site and limited access.

Within each quadrat, all intact plastic containers (e.g., drink, commercial, and industrial bottles) were recorded. Containers were then assessed for whether they posed a potential entrapment hazard to crabs based on meeting both of these criteria: (1) The lid was missing or the container was damaged such that it allowed Crabs access to the inside of the container, and (2) the container was positioned with the opening facing an upward angle, such that a Crab would have difficulty exiting and would therefore become entrapped. Lavers et al. then counted the number of Crabs (dead or alive) that had become entrapped in each container.

Lavers et al.used the density of bottles available to entrap Crabs across the eight quadrats on Cocos, 20 quadrats in the beach-back vegetation of Henderson, and four transects along Henderson’s East Beach (totalling 1139 m) to extrapolate the total number across the archipelago by resampling the values, with replacement, 10,000 times and scaling this to the area of beach-back vegetation (defined as the length of the vegetation line and extending 10 m inland). Beach length and beach-back dimensions were obtained using Google Earth Pro (version 7.3.2) and satellite imagery from 2016 to 2018 for beaches that were ocean-facing. Beaches that faced into the lagoon on Cocos (e.g., away from prevailing currents, sheltered by other islands) or small unnamed and potentially ephemeral sand bars were excluded as they do not likely accumulate significant quantities of plastic debris.

The estimated mean number of bottles on each beach was then used to predict the total entrapment using the probability and intensity values.

On Cocos we recorded 218 bottles that could potentially entrap Crabs across eight quadrats. Of these, 190 (87%) contained no Crabs, and the probability of entrapment was 0.128 (i.e. 12.8% of bottles contained Crabs). Of bottles that contained Crabs, the mean entrapment intensity was 7.857 Crabs per bottle. The overall entrapment rate was therefore 1.009 Crabs per bottle.

The density of plastic bottles in beach back ranged from 0.13 to 3.67 bottles per m². Across the 454 720 m² of ocean-facing beach back habitat, Lavers et al. estimated there were 562 352 bottles that could potentially entrap Crabs, producing an estimate of 507 938 Crabs entrapped in bottles across the archipelago.

In the beach-back vegetation on Henderson Island, Lavers et al. recorded 77 bottles across 20 quadrats covering 690 m², of which 65 (84%) contained no Crabs, and the probability of entrapment was 0.156 (i.e. 15.5% of bottles contained Crabs). There were 106.25 individuals in those containers with Crabs, resulting in an overall entrapment rate of 16.55 Crabs per bottle.

On East Beach, Crabs were found in 8 of 33 bottles (24%) across 12 762 m² of the beach. The probability of entrapment was 0.242 (i.e. 24.2% of bottles contianed crabs), and the entrapment intensity 60.0 Crabs per bottle. The overall entrapment rate was therefore was 14.55 Crabs per bottle.

The density of bottles ranged from 0.083 to 1.103 bottles per m² in the beach-back, and was 0.035 bottles/m² on East Beach of Henderson Island, resulting in a potential 2046 bottles in 7600 m² of beach-back vegetation and 865 bottles on 24 908 m² of East Beach where Crabs could become entrapped. Combining the entrapment values, Lavers et al. estimate 33 922 Crabs entrapped on the beachback, and 28 003 Crabs on the beach, for a total of 60 961 entrapped Crabs on Henderson Island.

Overall Hermit Crab entrapment rates were extremely high on both Henderson and Cocos, with nearly 61 000 (2.447 Crabs/m²) and 508 000 crabs (1.117 crabs/m²) becoming entrapped, respectively. Though overall mortality on Henderson is lower, the beach area is much smaller than that on Cocos, and both the rate and severity of entrapment and mortality is much higher. These estimates are liberal, as the rate of degradation of Crab carcasses is unknown, therefore some shells may have been present in the bottles for more than 12 months. Furthermore, Lavers et al.'s analysis does not account for temporal patterns, such as localised abundance during the breeding season, which could influence entrapment rates, and must be considered as point estimates rather than a temporal rate (e.g., annual mortality). Such rates should be a research priority on sites that are heavily polluted and can be visited regularly.

At a temperature of 28–29 °C and relative air humidity of 75 % (similar to conditions at both field sites), reported average survival of Hermit Crabs was 5–9 days when the Crabs lacked access to water. Thus, once entrapped in plastic containers, mortality of Hermit Crabs likely occurs over a very brief period, depending on rainfall. Hermit Crabs, including Coenobita perlatus, use the odour of dead conspecifics to locate available shells, increasing shell-acquisition behaviour by up to 10 times, which are a limiting resource and both live and freshly Dead Crabs were occasionally observed together inside plastic containers. This suggests entrapments occur on a regular basis and conspecific attraction, the very mechanism that evolved to ensure Hermit Crabs could replace their shells, has resulted in a lethal lure. Accumulation of more than 20 Crabs in containers suggests a threshold, or dose response, may exist whereby the chemical signals of decaying Crabs act additively or multiplicatively with a maximum of 526 Crabs observed in a single container on Henderson Island.

(A) A Strawberry Hermit Crab navigates through natural and anthropogenic debris on East Beach, Henderson Island. (B) Accumulated debris on East Beach, Henderson Island. (C) 526 Hermit Crabs trapped inside a single container on Henderson Island in June 2019. (D) Some of the 526 Hermit Crab shells from the container shown in panel (C). Lavers et al. (2019).

The significant entrapment rate has the potential to negatively impact Hermit Crab populations. While no population size data exist for any Hermit Crab species on Henderson or Cocos, and estimates of adult or juvenile survival are not available, existing pressure on these Crabs is appreciable on Cocos as small Crabs are used as bait in recreational and artisanal fishing and there are localised depletions of Crabs around populated areas. Concerns have been raised regarding the current recreational fishery bag limit on Cocos, 9 l per day for mixed, small Crabs, and a no-take regulation was considered as part of a Parks Australia review of recreational fishing regulations. Information on longevity of Crabs is sparse, but suggests Anomuran Crabs (the group which includes Hermit Crabs) are long-lived (5–30 years in the wild). Entrapment in debris along beaches, and in the beach back vegetation, therefore presents an additional, significant threat to Crab populations which are already under pressure and likely rely on high survivorship of breeding adults to maintain populations. On Henderson, Crab populations are likely under predation pressure from introduced Pacific Rats, Rattus exulans, which can modify coastal ecosystems greatly.

Significant reductions in crab populations have the potential to harm islands in several ways. On Cocos, tourism is a major source of employment, providing substantial economic and social benefits, and receiving widespread community support. On the main islands of Cocos, Seabirds no longer breed, therefore charismatic species like Hermit Crabs may provide an important opportunity for tourists to observe native wildlife. For example, on Christmas Island, the diversity and abundance of Crabs is a well-known tourist attraction. Cocos and Henderson Island lack native ground predators, therefore Crabs play a critical role in seed dispersal, removing detritus, and provide a range of benefits, such as soil turbation through burrow excavation and collection of leaf litter. Entrapment and mortality of large numbers of Crabs could therefore affect ecosystem function of coastal areas, which would have consequences for other biota as well as for tourism.

The accumulation of plastic debris alters water movement and heat transfer through beach sediments. Accumulated debris can also create a physical barrier, reducing the accessibility of beaches for breeding and hatchling Sea Turtles. Limited information is available for other species, especially invertebrates, however the presence of beach debris smothers benthic communities resulting in fewer Polychaete Worms and reduces the number of burrows constructed by Crabs. Significant annual losses of Crabs could lead to reduced breeding, and consequently lower recruitment. The larval duration and transport distance of most small Decapods, including Hermit Crabs, is relatively short with populations maintained through a combination of allochthonous (long distance) and autochthonous (local) recruitment. However, with the increasing isolation of an island, it becomes difficult for shallow water species to traverse the open ocean and establish a viable population, and Crab species richness on Cocos and Henderson is markedly lower than other island and mainland populations in the region. Similarly, Henderson’s remoteness would significantly impede successful larval dispersal to the island. Successful recruitment of Crabs therefore relies on considerable new individuals being released into the environment. Depleted populations, or those located on smaller, isolated islands therefore have less resilience to acute stressors than mainland ones, since they do not have the diversity of habitats to act as refuge for populations of species under pressure.

The increasing urbanisation and pollution of much of the world’s coasts with plastic debris threatens increasing and irreversible damages to beach ecosystems. Over the last three decades, plastic drink bottles have shown the fastest growth rate of all debris types reported on some remote islands. When such widespread changes are overlaid with the broad distribution of hermit crabs throughout the subtropics and tropics, it becomes clear the negative interactions between Crabs and debris are set to increase. This is of particular concern in areas of high Hermit Crab abundance, diversity, and endemism.

The mortality of Hermit Crabs attributed to beach debris, documented here for the first time, is significant, and likely a key factor contributing to the reported declines in hermit crabs on Cocos. Unfortunately, Cocos and Henderson are not unique, with similarly high concentrations of debris reported on beaches and in coastal vegetation worldwide. Other beaches with high debris load and Hermit Crabs may well experience similar mortality. The global mortality of Hermit Crabs is undocumented, likely to be substantial, and requires urgent investigation.

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Cetacean sightings within the Great Pacific Garbage Patch.

The Great Pacific Garbage Patch is an area of the Pacific Ocean between Hawai'i and California, and is one of a series of patches where an ocean gyre (rotating current) tends to trap floating plastics, resulting in an ever growing patch of discarded nets, ropes, lighters, toothbrushes, water bottles, pens, baby bottles, cell phones, plastic bags, and microplastics. The area is known to be on one of the major sea routes used by migrating Whales, and a variety of Whale species have washed up dead on the Californian coast with plastics in their stomachs, leading to concerns that the debris may be causing Whale fatalities, both through ingestion and entanglement, though there has to date been no direct studies of the impact of the garbage patches on Whales.

In a paper published in the journal Marine Biodiversity on 9 April 2019, Susan Gibbs of the Ocean Cleanup Foundation, Chandra Salgado Kent of the Centre for Marine Science and Technology at Curtin University, Oceans Blueprint, and the Centre for Marine Ecosystems Research at Edith Cowan University, Boyan Slat, also of the Ocean Cleanup Foundation, Damien Morales, again of the Ocean Cleanup Foundation, and of Blue Planet Marine, Leila Fouda, once again of the Ocean Cleanup Foundation, and of the School of Biological and Chemical Studies at Queen Mary University of London, and Julia Reisser of the Minderoo Foundation, and the Oceans Institute at the University of Western Australia, describe the results of an aerial survey of the Great Pacific Garbage Patch, which specifically targeted Whales present within the area.

Gibbs et al. made two survey flights over the Great Pacific Garbage Patch using a Hercules C-130 aircraft flying out of Moffett Airfield in California, during October 2016, during each of which a series of transects of the patch were made. During each flight eight people were used to record Whale sightings, working in pairs from each of the plane's paratroop doors, with one person acting as a spotter and the other a recorder. Where possible Whales were also photographed.

A total of 14 Whales were spotted in seven groups, plus 1280 large plastic items (fishing nets etc.). The Whales were not evenly distributed across the patch, five of the seven sighting having occured in a brief period of time, large items of plastic were quite often seen in close proximity to Whales.

The first sighting comprised a group of four small Toothed Whales, Odontocetes, of an unknown species. The second sighting was of three Sperm Whales, Physeter macrocephalus, a mother, calf and an escort; the calf was about four and a half metres in length, suggesting it was vert young (Sperm Whale Calves are about four metres long when born, and grow very rapidly). The third sighting was of a large dark Whale of uncertain species, possibly another Sperm Whale. The fourth sighting was of a single 'relatively large' Whale.the fifth sighting was of two large Baleen Whales, Mysticeti, identified by the observation of two large blows with shapes consistent with those produced by the double blowholes of Baleen Whales. The sixth sighting was of a single Beaked Whale, Ziphiidae, of uncertain species. The seventh sighting was of two further Beaked Whales. The final three Whales were probably Cuvier’s Beaked Whales, Ziphius calvirostris, but could not be confidently identified, and may not all have been of the same species.

Cetaceans and ocean plastics within the Great Pacific Garbage Patch. In the map, background colour levels represent predicted plastic pollution gradient (red = highest levels, blue = lowest levels); grey lines show the survey transects (~665 km each) and black dots indicate locations of the seven Cetacean sightings. Photographs above the map show some of the Cetaceans observed in this study: Sperm Whales (sighting 2, and sighting 3) and Beaked Whales (sighting 6, and sighting 7); red circles in sighting 3 indicate debris locations. Photographs in the right side of the figure give examples of debris types sighted: ‘ghostnets’, ropes, crates and buoys. Gibbs et al. (2019).

The most common identifiable large items of plastic were abandoned fishing nets, followed by containers, buoys, lids, and ropes. Only three of the Whales could be identified to species level, but the observed specimens included Sperm Whales, Beaked Whales and Baleen Whales, all of which have been found washed up on Californian beaches with plastics in their stomachs, implying that they are affected by the plastic waste in the garbage patch.


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Counting floating plastics in the world’s oceans.

Floating plastic is considered to be a major pollutant in the world’s oceans. It enters the oceans in large quantities from shipping, coastal communities, the watersheds of river systems and even wind distribution from inland communities. Once it enters the sea plastic is both durable and highly buoyant, enabling its distribution over long distances within the oceans. Plastic eventually breaks down at sea, releasing synthetic polymers into the water, and can also absorb, transport and then release other pollutants, causing some scientists to believe it should be regarded as hazardous waste. Plastic is readily consumed by organisms in the ocean both large and small, with some organisms, such as Seabirds, known to suffer high levels of direct mortality as a result of plastic ingestion. Many organisms also consume plastics secondarily (by eating other organisms that have themselves been feeding on plastic), causing a build-up of plastic derived chemicals in the tissues of animals at the top of many marine food chains. In addition floating plastic has been shown to be capable of transporting many organisms to parts of the world where they do not usually occur, causing a profound reshaping of many marine ecosystems.

This has led to scientists and environmentalists taking a strong interest in plastics in the world’s oceans, though calculating the amount of plastic in the oceans and mapping its distribution has proved to be extremely difficult.

In a paper published in the journal PLoS One on 10 December 2014, a team of scientists led by Marcus Eriksen of the Five Gyres Institute, publish a new estimate of the floating plastic content of the world’s oceans, based upon survey data of the five ocean gyres (ocean gyres occur between the ocean’s tropical currents, which flow east-to-west, and temperate currents, which flow west-to east, creating areas of circular currents in the North and South Atlantic, North and South Pacific, and southern Indian Ocean), as well as coastal regions and enclosed seas along the coasts of Australia, the Bay of Bengal and Mediterranean Sea.

Eriksen et al. divided the plastics into four size categories, small microplastics (less than 1.00 mm), large microplastics (1.01-4.75 mm), mesoplastics (4.76-200 mm) and macroplastics (over 200 mm). The smaller plastic categories were sampled by slowly towing nets across the oceans, then sorting and weighing the samples (the nets had a mesh size of 0.33 mm, so very small particles are likely to have been missed). Macroplastics were observed by dedicated spotters from survey ships, and their weight estimated by comparison with macroplastic samples recovered from shorelines in northern-central Chile, South Africa, the Atlantic coast of North America and the Hawaiian Archipelago; this method may have missed macroplastics which were dark in colour or of marginal buoyancy (i.e. floating low in the water).

Eriksenet al.estimate that there are a minimum of 5.25 trillion plastic particles floating in the world’s oceans, with a minimum weight of 268 490 tons, with the North Pacific having at least 1.99 trillion particles weighing 96 400 tons, the Indian Ocean having at least 1.30 trillion particles weighing 59 130 tons, the North Atlantic having at least 0.93 trillion particles weighing 56 470 tons, the South Pacific having at least 0.491 trillion particles weighing 21 020 tons, the South Atlantic having at least 0.297 trillion particles weighing 12 780 tons and the Mediterranean having at least 0.247 trillion particles weighing 23 150 tons.

Model results for global count density in four size classes. Model prediction of global count density (pieces km^-2; see colour bar) for each of four size classes (0.33–1.00 mm, 1.01–4.75 mm, 4.76–200 mm, and >200 mm). Eriksen et al. (2014).

Microplastics were predicted to be the most abundant category, as larger plastics break down over time into smaller particles (microplastics are almost entirely fragments of larger objects), and the surveys found that 92.4% of the total particle count comprised microplastic particles. However within these categories the largemicroplastic particles were more abundant than the smaller particles, with a roughly 60:40 split. This was contrary to predictions, and suggests that smaller particles are being lost from the oceans at a greater rate than predicted, either through settling out (sinking or being washed ashore), consumption by animals or microbial breakdown (microbes attack the surfaces of all plastic particles in the sea, smaller particles have higher surface-area-to-mass rations, and are therefore broken down more rapidly).

The ratio of mesoplastic to macroplastic also exceeded expectations, with a predicted ratio of 16:1 and a detected ratio of ~24:1. This may be due to a greater than predicted rate of break-up of macroplastic particles or a greater than predicted rate of mesoplastic particles entering the sea, particularly plastic bottles and single-use packaging.

While the smaller plastic particles were far more numerous than the larger ones, the majority of the overall plastic mass was held in larger particles, with 75.4% of the mass inmacroplastic particles, 11.4% in mesoplastic particles, 10.6% in large microplsastic particles and 2.6% in small microplastic particles.

Model results for global weight density in four size classes. Model prediction of global weight density (g km^-2; see colour bar) for each of foursize classes (0.33–1.00 mm, 1.01–4.75 mm, 4.76–200 mm, and >200 mm). The majority of global weight is from the largest size class. Eriksen et al. (2014).

While there was more plastic in the more densely populated Northern Hemisphere, but the amount of plastic in the Southern Hemisphere was closer than expected, suggesting that more plastic is crossing the equator than previously realized, although it is possible that plastic is being lost from the northern oceans at a higher rate due to beach stranding or loss of buoyancy. Plastic was also found to be more abundant in the ocean gyres than in coastal environments, conforming the predicted expectation that plastics tend to accumulate in these environments, while plastics near the coast are more transient.

Finally Eriksen et al. note that according to the trade organization Plastics Europe, 288 million tons of plastic were produced globally in 2012. This suggests that the plastic content of the oceans is roughly the equivalent of 0.1% (one thousandth) of annual production. While it is not expected that all the plastic produced would end up in the oceans, this seems excessively low, as plastic is known to survive in the oceans for many years. Eriksen et al. note that their methodology only detected surface plastics, not plastics in the water column, on the ocean floor or incorporated in sediments, and suggest that future studies should attempt to incorporate these plastics.

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