How much plastic does it take to kill a turtle? Typically just 14 pieces

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Plastic bags, balloons, and rope fragments were among more than 100 pieces of plastic in the gut of a single turtle.
Qamar Schuyler, Author provided

Britta Denise Hardesty, CSIRO; Chris Wilcox, CSIRO; Kathy Ann Townsend, University of the Sunshine Coast, and Qamar Schuyler, CSIRO

We know there is a lot of plastic in the ocean, and that turtles (and other endangered species) are eating it. It is not uncommon to find stranded dead turtles with guts full of plastic.

But we weren’t really sure whether plastic eaten by turtles actually kills them, or if they just happen to have plastic inside them when they die. Another way to look at it would be to ask: how much is too much plastic for turtles?

This is a really important question. Just because there’s a lot of plastic in the ocean, we can’t necessarily presume that animals are dying from eating it. Even if a few animals do, that doesn’t mean that every animal that eats plastic is going to die. If we can estimate how much plastic it takes to kill a turtle, we can start to answer the question of exactly how turtle populations are affected by eating plastic debris.

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In our research, published today in Nature Scientific Reports, we looked at nearly 1,000 turtles that had died and washed up on beaches around Australia or were found in nets. About 260 of them we examined ourselves; the others were reported to the Queensland Turtle Stranding Database. We carefully investigated why the turtles died, and for the ones we examined, we counted how many pieces of plastic they had eaten.

Some turtles died of causes that were nothing to do with plastic. They may have been killed by a boat strike, or become entangled in fishing lines or derelict nets. Turtles have even been known to die after accidentally eating a blue-ringed octopus. Others definitely died from eating plastic, with the plastic either puncturing or blocking their gut.

One of the first meals eaten by this sea turtle post-hatchling turned out to be deadly. It died from consuming more than 20 tiny pieces of plastic, many of which were about the same size as a grain of rice.
Kathy Townsend, Author provided

Some turtles that were killed by things like boat strikes or fishing nets nevertheless had large amounts of plastic in their guts, despite not having been killed by eating plastic. These turtles allow us to see how much plastic an animal can eat and still be alive and functioning.

The chart below sets out this idea. If an animal drowned in a fishing net, its chance of being killed by plastic is zero – and it falls in the lower left of the graph. If a turtle’s gut was blocked by a plastic bag, its chance of being killed by plastic is 100%, and it’s in the upper right.

The animals that were dead with plastic in their gut, but had other possible causes of death have a chance of death due to plastic somewhere between 0 and 100% – we just don’t know, and they can fall anywhere in the graph. Once we have all the animals in the plot, then we can ask whether we see an increase in the chance of death due to plastic as the amount of plastic in an animal goes up.

Conceptual framework for estimating the probability of death due to plastic debris ingestion. Figure provided by the authors.

We tested this idea using our turtle samples. We looked at the relationship between the likelihood of death due to plastic as determined by a turtle autopsy, and the number of pieces of plastic found inside the animals.

Unsurprisingly, we found that the more plastic pieces a turtle had inside it, the more likely it was to have been killed by plastic. We calculated that for an average-sized turtle (about 45cm long), eating 14 plastic items equates to a 50% chance of being fatal.

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That’s not to say that a turtle can eat 13 pieces of plastic without harm. Even a single piece can potentially kill a turtle. Two of the turtles we studied had eaten just one piece of plastic, which was enough to kill them. In one case, the gut was punctured, and in the other, the soft plastic had clogged the turtle’s gut. Our analyses suggest that a turtle has a 22% chance of dying if it eats just one piece of plastic.

A green sea turtle that died after consuming 13 pieces of soft plastic and balloons, which blocked its gastrointestinal system.
Kathy Townsend

A few other factors also affected the animals’ chance of being killed by plastic. Juveniles eat more debris than adults, and the rate also varies between different turtle species.

Now that we know how much is too much plastic, the next step is to apply this to global estimates of debris ingestion rates by turtles, and figure out just how much of a threat plastic is to endangered sea turtle populations.The Conversation

Britta Denise Hardesty, Principal Research Scientist, Oceans and Atmosphere Flagship, CSIRO; Chris Wilcox, Senior Research Scientist, CSIRO; Kathy Ann Townsend, Lecturer in Animal Ecology, University of the Sunshine Coast, and Qamar Schuyler, Research Scientist, Oceans and Atmospheres, CSIRO

This article is republished from The Conversation under a Creative Commons license. Read the original article.


It’s hard to spread the idiot fruit

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Idiospermum is otherwise known as “idiot fruit” or ribbonwood.
via Wikimedia Commons, CC BY-SA

Stuart Worboys, James Cook University

Sometimes, in rainforest research, the only way to go is up. Twenty years ago I chose the rare rainforest tree Idiospermum australiense as a research subject for my Master’s degree, and some months into the project I discovered it only produces flowers high in the canopy.

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So, after a short course in single-rope technique, I found myself dangling 15 metres up in the rainforest canopy, surrounded by its sweetly fragrant, rose-like flowers. I followed the flowering process over a 24-hour period, taking photographs and catching potential pollinators. The tree is known locally as the “idiot fruit” (a loose translation of its scientific name) and there was I, dangling on a thin rope in its canopy, watching tiny insects. Oh, the irony.

The Conversation, CC BY

Intricate floral movements

Idiospermum australiense (also known as “ribbonwood”, or the “dinosaur tree”) makes for a fascinating and relatively approachable study subject. It is rare, with scattered populations covering a total of just 23 km². Known populations are mostly close to roads in very wet lowland tropical rainforests of Far North Queensland’s wet tropics.

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My research sites were idyllic locations close to crystal clear streams, and the tedium of solo field work would occasionally be broken by the wollock-a-woo call of the colourful wompoo pigeon or a wandering curious cassowary.

The hours of observations high in the forest canopy revealed an intricate process of floral movements that allow the plant to control their insect pollinators and prevent self-pollination.

The flowers of Idiospermum start as small spherical buds. Over a period of two days, the numerous cream-coloured, petal-like structures (called “tepals”) unfurl. They emit a fragrance that is sweet and fruity, and attract large numbers of small beetles and thrips (minute insects with fringed wings).

At the centre of the flower, the stamens are covered by a ring of hard rigid tepals, and the stigma – the female part of the flower – is accessible to pollinators via an open crater. But on the third day, things start to change. The stamens move and block the crater, while the ring of hard rigid tepals lifts and the stamens release their pollen. Pollinators can now feast on a reward of messy, sticky pollen, but are prevented from moving that pollen onto the flower’s stigma, thus preventing self-pollination.

Fertilisation only occurs if a pollen-covered insect enters the central crater in a newly opened flower elsewhere. Meanwhile, the ageing flowers start to change colour, first to a pale pink, then slowly deepening to crimson. If pollination has occurred, the flower will develop into a fruit containing one, rarely two, seeds.

The rose-like flowers of Idiospermum are cream-coloured when they first open, and fade to a deep crimson red over their 10-14 day life.
Wet Tropics Management Authority

The seeds themselves are remarkable. At up to 225 grams, they are probably the largest seed produced by any Australian plant (apart from the coconut). Unlike other rainforest plants with large fruits, they are not dispersed by cassowaries.

In fact, these enormous seeds have no known disperser: instead, they fall and germinate where they come to rest. The starchy reserves and protective poisons contained in the seed give the young seedling a great start in the dark and dangerous environment of the forest floor. But arguably, these seeds are the reason for the tree’s rarity. Their lack of a disperser, and reliance on a humid environment to prevent potentially fatal desiccation, may be the reason why their distribution is so restricted.

Refugees from deep time

Idiospermum occurs in just three widely separated populations, one in the Daintree, and two others 150km to the south, in the foothills of Queensland’s two highest mountains. They grow in “environmental refugia”: habitats, usually close to rain-attracting mountains, that have remained climatically stable for millions of years while the remainder of the continent has dried out. These refuges provide a safe and stable habitat for an extraordinary diversity of plants found nowhere else, including many that have been described as “primitive”.

“Primitive species” are modern species whose lineage branched off at a very early stage in the evolution of flowering plants, and who have retained primitive anatomical and genetic features that are similar to those seen in fossils of ancestral flowering plants.

The concentration of ancient flowering plant lineages within Queensland’s wet tropics makes the region internationally significant. With some 15 of the world’s 27 ancient plant families occurring within its 2 million hectares, it can be considered a living museum showcasing the evolution of the flowering plants.

The massive seeds, weighing up to 225 g, are probably the largest of any Australian plant (apart from the coconut).
Photo Neil Hewitt, Cooper Creek Wilderness, Daintree Rainforest.

Among the flora of the wet tropics, Idiospermum is truly iconic. It is the only member of its family (the Calycanthaceae) in the southern hemisphere: its closest relatives grow in China and North America. Its attractive, fragrant flowers retain a set of features seen in fossils some 88 million years old.

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It occurs in beautiful lowland rainforest locations, where it can often be easily found due to the scattering of seeds around its base. Idiospermum provides a focus for the region’s flora – its beauty, its rarity, its relictual nature, and its significance on a world scale.The Conversation

Stuart Worboys, Laboratory and Technical Support Officer, Australian Tropical Herbarium, James Cook University

This article is republished from The Conversation under a Creative Commons license. Read the original article.