Dotted along the length of Britain’s rivers are various obstacles – some as large as dams, others as small as weirs (which bisect a river like steps) – stopping creatures, sediment and plants from moving along the watercourse. Only 1% of the UK’s rivers are free of artificial barriers.

Moves are under way in many countries to remove such obstacles and let rivers “re-naturalise” and follow their own paths. But while many scientists agree that river barriers need to go in the UK too, other people are hesitant – concerned about creating unpredictable water flows in already flood-prone regions.

“Our rivers and our lakes are the most damaged of all ecosystems,” says Paul Kemp, a professor of ecological engineering at the University of Southampton. Globally, freshwater ecosystems are home to about a third of the planet’s vertebrate species, and these have been declining at twice the rate of marine and land-based animals. The situation in England is dire, with about 15% of rivers achieving good ecological status, according to the Rivers Trust.

Removing barriers “results in the greatest impact the most rapidly and for the least amount of money,” says Kemp.

In England, there are more than 50,000 barriers disrupting the passage of the country’s beleaguered rivers, according to the Environment Agency database. Scientists within the organisation suspect that there are many more. The overwhelming majority of these are relatively small: about 27,500 culverts, usually round concrete pipes, and around 16,300 weirs, which adjust the water level and effectively create a small dam.

These barriers are not only costly to maintain – they also cause avoidable environmental damage.

“A few decades ago, we thought only migratory species needed to move,” says Dr Perikles Karageorgopoulos, a senior technical specialist at the Environment Agency. “But we increasingly understand that all species need to move, from the tiniest ones that will have local migrations through to the others that migrate for many kilometres to overwinter or reach their spawning grounds.” Having so many barriers effectively creates a network of small dams or lakes, which offer plants and animals very different habitats from those found in free-flowing rivers, and are often not appropriate for river species.

Sediments also need to move. Barriers trap soil and geomorphological materials upstream, depriving downstream areas of sand and gravel, which are crucial for spawning creatures and many plants.

“Removing weirs is the most effective way of restoring a river,” says Karageorgopoulos. This is what happened in 2010, when a disused mill weir on the River Ouse in East Sussex failed and had to be removed. That stretch of the river had become a “ponded river with lilies”, he says. Now “there is a huge physical diversity that supports a large variety of species” – which also makes the river and its creatures more resilient to climate change.

“A diverse habitat is much more resilient to high or low [water] flows and extreme temperatures, and can provide a refuge for many species,” he explains. “In the past, the same river reach would have warmed up and become deoxygenated during low flows and warm weather. The warm water flowing downstream would have affected the ecology downstream too.”

In addition to that particular section, the weir removal had a positive impact on the river more than a kilometre upstream, Karageorgopoulos says. Within two years, the stretch of river was reclassified, moving from poor to good.

“It’s a good technique for focusing your efforts around restoration,” says Jesse O’Hanley, an environmental systems specialist and current associate dean of research and innovation at the University of Kent. “A lot of restoration efforts happen at a very small scale like planting some trees, bending the river in a little way or putting in some rocks. It’s a very hyper-localised solution that doesn’t really scale up and it’s expensive. It’s usually easier to let the river take care of it on its own.” This is what removing river barriers does: it lets rivers re-naturalise.

But letting a river choose its own path is a risky business, especially in flood-ravaged parts of England.

The Environment Agency estimates that about 3.4m properties in England are in areas at risk from surface flooding. Would removing river barriers make the situation worse, or improve it?

“Flooding is a natural process,” says Karageorgopoulos. “When you get really big floods, there just physically isn’t space in the river to contain the water.”

Rivers run in three dimensions: there is the flow we usually see in which water follows the channel of the river; but rivers also run laterally, breaching their banks and dispersing sideways on to flood plains, as well as vertically, linking the riverbed to the water table below. Obstacles and barriers have several consequences for a river’s passage in all three of those directions, both up- and downstream from the impediment.

The risk of flooding is always context specific, says Carlos Garcia de Leaniz, a professor of aquatic biosciences at Swansea University. Weirs and culverts can make flooding worse upstream, because they slow the water in the river, collecting it in mini-ponds and stopping it from flowing downstream. It is also quite easy for culverts to be blocked by trees and debris.

Additionally, the barriers stop sediment from moving down the river, causing it to collect in specific places. This substrate acts as a blanket on the riverbed, cutting off the link between the river and the water table, as the water cannot filter through. “That means less water is able to reach the water table and more water needs to be carried by the river channel,” says Garcia de Leaniz. “That means the risk of flooding may actually increase.”

The reality is that flooding is necessary and will happen – it’s about deciding where that water will go.

All the scientists the Observer contacted agreed that it is possible to predict the outcomes of taking away a culvert or weir. “There will be localised changes in flooding, and that is one of the consequences [of barrier removal],” says Hannah Cloke, a professor of hydrology and co-director of Water@Reading at the University of Reading. “But you can predict where those will be, and those are the kinds of things we should be doing anyway – making space for water on the flood plain.”

In a 2019 report, the Environment Agency warned that if current development continues on flood plains, the number of properties at risk of flooding could double in the next 50 years.

“We can only engineer our way out of some level of flooding… We can’t keep building giant concrete walls and structures in our rivers to control the water – that’s impossible. Working with the landscape is a much more sensible way of managing our rivers, and also has benefits for ecosystems and water quality,” says Cloke.

Europe and the US are leading the global charge on barrier removal, particularly when it comes to dam removal. (Britain has about 2,800 dams.)

The European Union has decided that by 2030 it wants 25,000 kilometres of rivers in Europe to be free-flowing, says Garcia de Leaniz. He headed the EU-funded Amber (Adaptive Management of Barriers in European Rivers) project, which found that there are more than 1m barriers fragmenting rivers in Europe and the UK. “Europe is making great headway; the UK, not so much,” he says.

A significant reason for this is that the UK, particularly England, is much more densely populated. “It is much easier to restore rivers when few people live around,” says Garcia de Leaniz. He also has a pragmatic approach to barrier removal. “We have so many obstacles to choose from; let’s start with those that are obsolete and those that pose a flood risk or hazard,” he says. “And in the bigger scheme of things, some people would argue that for the cost of removing one big dam, you can remove 100 or more barriers, which is going to be much more beneficial.”

In England, the Environment Agency tends to work on a more ad hoc basis, says Karageorgopoulos. While it has a list of priority barriers to remove – such as those that are costing money to maintain and which serve no function – it has to move when opportunities present themselves. Although the Environment Agency owns or maintains the vast majority of these barriers, many of them are on private land or affect private land.

A major challenge, says Kemp, is land and river ownership. “You can’t just go and do a strategy on a river because you’ve got multiple owners of that land and you have to work with them collegiately, and try to find a solution,” he says.

Often, the Environment Agency “will be able to do things when they find a landowner who is receptive to the idea, so it is very opportunistic,” says Kemp.

Even if the owner is on board, the process still takes time. Many years of planning and stakeholder consultation are required, says Karageorgopoulos. “Money is the obvious [obstacle],” to barrier removals, but communities and anglers are often vocal opponents. “We are human – we like routine. We’ve even had opposition from people who like to feed the swans and ducks in a specific location,” says Karageorgopoulos. But when there are no land restrictions and “everyone is onboard, it is the easiest way” to improve river health.

It will not “fix” England’s rivers, though, warns Cloke. “Naturalising rivers is always a good idea because natural flows are what rivers are designed to do,” she says. “In the long term, it’s probably helpful and is one of the lowest hanging fruits.”

However, none of this will address the major threats to British rivers – namely huge quantities of pollution in the form of sewage and agricultural and industrial waste, and development. “Should we be building on floodplains? No. Should we be polluting our rivers? No. Those are the giant questions that need careful thinking from government,” she says.

Do you pay with crypto, dig the blockchain and drive a Tesla? You’re an early adopter, a future-chaser, right? So, naturally, you’ll vote jelly.

These animals are taking over the ocean. They are the great survivors of evolution – and of the Anthropocene. They are thriving in an era of warming oceans and algal blooms, and when humans have wiped out larger marine predators. Jellyfish can survive when human pollution reduces oxygen levels in the ocean. And so we will embrace their future, our grandchildren tucking into jellyfish and chips.

British seas are mostly too cold to be swarming with jellyfish species but the barrel jellyfish is Britain’s largest jelly, and most frequently found in south-western seas.

Its hefty, translucent, mushroom-shaped bell can grow to the size of a dustbin lid. A pretty dustbin lid, sometimes shaded in yellow, pink or blue. The bottom edge of the bell is fringed with violet, and this contains the jelly’s sense organs. Below are eight frilly, stocky arms, which look a little like elongated cauliflowers. These contain small, stinging tentacles, which deliver food to surrounding tiny mouths, hundreds of them. The stings are designed to disable zooplankton – tiny sea creatures – and so don’t harm us.

Watch this gentle giant swim, pulsing in the blue depths, mesmerising us with its sensuous curves and extraterrestrial strangeness.

If you’re a traditionalist and lover of history, the barrel jellyfish deserves your vote too. Jellyfish have floated through Earth’s oceans for more than 500m years, miraculous beings with bodies made up of 90% water and living without brain, heart or blood.

Sadly, we tend to encounter them washed up, dead, on our beaches in early summer months. These jellies have underestimated their own size, following their tiny prey into shallow water and become stranded, washed in, washed up by the waves and tides.

In some parts of the world, people have eaten similar species from the Rhizostomae order for thousands of years. Its bland flesh is livened up with strong sesame oil and soy, and served in sushi, noodles and even ice-cream.

But this 35kg behemoth is the favourite food of the world’s largest sea turtle, the leatherback. It, of course, is struggling to survive in an ocean filled with plastic bags that can, fatally, resemble its favourite jellyfish prey.

So vote jelly. Vote for the great the survivor, food for the gods of the sea, and the real deal – definitely not a plastic bag.

• Welcome to the Guardian’s UK invertebrate of the year competition. Every day between 2 April and 12 April we’ll be profiling one of the incredible invertebrates that live in and around the UK. Let us know which invertebrates you think we should be including here. And at midnight on Friday 12 April, voting will open to decide which is our favourite invertebrate – for now – with the winner to be announced on Monday 15 April.

It sounds like a set-up for an ecological horror film – to save one species of owl, US wildlife officials want to shoot down half a million of its cousins.

The federal government’s latest proposal to save the endangered spotted owls has raised complicated questions about the ethics of killing one species to save another, and the role of humans to intervene in the cascading ecological conundrums that they have caused.

The spotted owl – an elusive icon of the American west – has lost most of its habitat in the old growth forests of the Pacific north-west and Canada due to logging and development. The species has also faced increasing competition from the barred owl – a slightly larger, more successful cousin which was lured west over the last century as settlers and homesteaders reshaped the North American landscape.

Now, to save the spotted owls, the US Fish and Wildlife Service has finalised a proposal to cull hundreds of thousands of barred owls across California, Washington and Oregon over the next 30 years.

The plan has pitted animal welfare and conservation groups against each other. The proposal was published in November, but it drew renewed attention last week after 82 animal welfare organisations based around the US signed a letter calling it “colossally reckless”. Researchers and wildlife officials who support the plan have said that if the barred owls are not culled, the northern spotted owl’s demise is ensured.

“This is a case that poses a genuine ethical dilemma,” said Michael Paul Nelson, a professor of environmental ethics and philosophy at Oregon State University. “You’re either going to kill a bunch of individual living beings, or you’re going to let a species disappear. No matter what, harm is done.”

An invasive species, or natural competition?

Spotted and barred owls look very similar to the untrained eye, and they can interbreed to birth offspring that are called “sparred owls”. But the barred owls are more adept survivors. They hunt a greater variety of prey, are slightly less discerning about where they nest, and tend to reproduce more quickly. And over the past few decades, biologists have noticed that the barred owls are edging the spotted owls out of their territory.

“Barred owl removal is not something the Service takes lightly,” said Jodie Delavan, a public affairs officer with USFWS in Oregon. “However, the Service has a legal and ethical responsibility to do all it can to recover northern spotted owl populations. Unless invasive barred owls are managed, the federally listed northern spotted owl will be extirpated in all or a significant portion of its range.”

The northern spotted owls were listed as threatened in 1990 after fierce campaigning by environmentalists who fought to protect the ancient forests where the birds nest from the logging. But the protections came too late – 70% of their habitat is already gone. The climate crisis and increasingly fierce megafires now threaten to destroy what little remains of their forest habitats.

The arrival of barred owls in the west appears to have hastened the spotted owl’s decline. It’s unclear why exactly the barred owls migrated westward, but researchers agree that it coincided with the arrival of European settlers in the east, and their reshaping of the owls’ native landscape. Previously, a scarcity of tree habitats in the Great Plains may have prevented the barred owls from venturing west until homesteaders planted trees for lumber, which provided new habitats. They also abandoned or outlawed Indigenous forest management practices, trapped beavers, over-hunted deer and elk, and drove away bison – all of which caused forests to overgrow.

That’s one of the reasons that the Fish and Wildlife Service, and biologists, consider the barred owls to be an invasive species – human intervention led to their arrival in the west. And that is why many believe it is humans’ responsibility to remove them.

“I grappled with it constantly. It’s not an easy thing,” said David Wiens, a wildlife biologist with the US Geological Survey who has spent his career studying interactions between spotted owls and barred owls. Several years ago, he and fellow researchers ran an experiment that involved shooting more than 2,400 barred owls across the north-west – and found that over five years, culling the barred owls helped spotted owl populations stabilise.

Even as the researchers culled barred owls, however, more of them moved in. In order to truly control their populations in the west, hunters would have to keep shooting them over a long period of time. “It’s a very tough decision,” he said. “Do you use lethal removal techniques? Or do you do nothing – just throw up your hands and let the cards fall where they will?”

Many conservationists have – squeamishly – agreed that the barred owls should be culled. But animal rights activists, some wildlife groups and the editorial board of the Los Angeles Times remains sceptical.

“The United States is targeting a native species not ever hunted for simply engaging in normal range expansions,” said Wayne Pacelle, president of the Center for a Human Economy and its lobbying arm, Animal Wellness Action, who co-authored the letter opposing the culling proposal. “If the US Fish and Wildlife Service is now going to start to manage social conflicts between animals, where does this end?”

Pacelle disputes the idea that the barred owl is invasive – as it is, after all, native to North America. And killing hundreds of thousands of them, over three decades, in an area where they are guaranteed to keep returning is “unworkable and inhumane”, he said.

The trouble is, he added, “we don’t have an easy fix for the spotted owl.”

Fraught questions for the Anthropocene

For Lisa Sideris, a professor of environmental studies at the University of California, Santa Barbara, who specialises in environmental ethics and the intersections of science and religion, the case of the two owls inspires introspection about the follies of anthropocentrism. “Some would argue that humans have altered ecosystems and the whole planet to such an extent that it becomes very hard to discern what it would mean to restore something back to natural conditions and whether that’s even possible.”

This isn’t the first time the coy spotted owls have pushed people to grapple with fraught philosophical ideas. “The spotted owl has been the poster animal for environmental conflicts for decades,” said Sideris.

The species found itself at the centre of what became known as the Timber Wars in the 1980s and 1990s. Loggers and environmentalists seeking to save old growth forests in California and the Pacific north-west clashed – in the courtroom and in the woods. In Oakland, California, Judi Bari and Darryl Cherney – two anti-logging activists campaigning to save the spotted owl – were critically injured by a pipe bomb that exploded under their car. In 1990, amid escalating conflict, the spotted owl was listed as threatened under the Endangered Species Act – and made the cover of Time magazine.

Still, tensions between timber industry leaders – who said that efforts to save the owl would cost tens of thousands of jobs – and environmentalists continued to build. In 2021, the Trump administration drastically slashed protections for the spotted owl. Joe Biden reversed the decision, but conceded 200,000 acres in owl habitat as part of the settlement of a timber industry lawsuit.

The spotted owl and the barred owl remain caught in the political crossfire. And all the while, wildlife officials and biologists are left with fraught questions about how best to save the species under strained circumstances.

Getting rid of the barred owls is ultimately a “triage” – a way to give the spotted owl some more time, and a fighting chance at survival, said Tom Wheeler, executive director of the conservation group Epic. “Does this just mean that there will always have to be somebody with a shotgun in our forest killing owls?” said Wheeler. “I think that we have to – as supporters of this – somewhat acknowledge that that is a possibility. And we have to be OK with that.”

Preventing extinction has become a sisyphean task, said Nelson, and despite government, scientists and conservationists’ best efforts, it remains impossible to predict or control exactly how nature will react.

“There is a hubris that underlies this idea that we’re just going to engineer our way out of these situations,” he said. “Because that is the same attitude that created these problems in the first place.”