The effect of GM crops on local insect life

A large study weighs up the existing evidence on the impact of GM crops on local insect life, providing some much-needed scientific rigour to the GM debate.

In Europe, the ‘GM debate‘ about the merits and dangers of genetically-modified (GM) crops is a particularly heated one. There is a sense of unease about the power of modern genetic technology, and a gut feeling that scientists are ‘playing God’. These discontents are stoked by the anti-GM camp, who describe GM crops with laden and fear-mongering bits of unspeak like ‘Frankenstein foods’ and ‘unnatural’.

Bt cotton is better for non-targeted insects than non-resistant crops sprayed with insecticdes.In a debate so fuelled by emotion and personal values, scientific research and a critical analysis of the evidence rarely gets a look-in. But science has to grudgingly admit some blame in this, because there is actually precious little research on the safety of GM crops. And many of the studies that have been done were short-term and poorly replicated.

A lack of research is dangerous. It provides opening for people on either side of the debate to quote single, small studies as canon and brushing aside any research that contrasts with their stances.

Adding evidence to the debate

Michelle Marvier and colleagues from Santa Clara University, California, are trying to change all that. They have analysed over 42 field experiments on GM crops to get an overall picture about their safety. The technique they used is called meta-analysis, a statistical tool that asks “What does everyone think?” It works on the basis that individual small studies may be far from conclusive, but pooling their results together can lead to stronger and more accurate results.

They looked at three strains of GM-crops that had been modified with genes from a soil-dwelling bacterium called Bacillus thuringiensis. The transferred genes are responsible for producing a number of biological (and therefore ‘natural’) insecticides. When moving them across to plants, geneticists typically try to match the insecticide to the pest they are trying to fight. (In the image on the right, Bt-peanut leaves are protected from the damaging European corn borer)Some GM crops are resistant to specific insect pests.

The toxins are delivered at high dosages to pests, but are restricted to the plant (and sometimes even to particular tissues). They can also be added to the chloroplast genome, which is quite separate form the plant’s nuclear DNA. This stops them from being transferred to other plants.

The hope is that these so-called ‘Bt crops’ can help to minimise the collateral damage of less targeted insecticide sprays. In theory, only pest insects that eat valued crops are killed, while the rest of the ecosystem is unharmed.

The results

That’s what Marvier set out to test. She looked at field experiments which tested the impact of caterpillar-resistant cotton and maize plants on the abundance of other groups of insects and invertebrates.

She found that these other creatures are found in greater numbers in fields containing the caterpillar-resistant GM plants, compared to those sprayed with conventional insecticides. However, the GM crops also led to slightly lower numbers of non-targeted insects compared to fields where no GM crops and no insecticides were used.

The results stayed the same even when Marvier analysed them in more detail. For example, she found much the same thing when she only looked at experiments that had been published in peer-reviewed scientific journals.

So assuming that Bt crops do indeed reduce the use of insecticides (and that’s far from proven), then they will also, as claimed, reduce the collateral damage caused by these chemicals. But they’re not as good for the environment as using no insecticides at all, be they engineered or sprayed.

The bigger picture

Bt-crops are better than large-scale insecticide spraying.At the local level, Marvier’s study provides some much-needed scientific backbone to the GM debate. But the real decisions need to be weighed up at a larger level. For example, it’s all well and good to say that a no-insecticide, no-GM field is the best solution, but that leaves farmers in a bit of a lurch.

One of the big criticism levelled against organic farming is that it leads to lower yield than other practices and requires more agricultural land to be viable and that deals a bad hand to farmers in the developing world. This in turn could lead to deforestation and habitat loss.

On the other hand, Marvier advises caution when interpreting her work. This study has revealed just one benefit of GM crops and even then, only for one specific type of genetic modification. Many of the studies involved isolated patches of land, rather than entire farming systems, where the situation is more subtle. Not all non-GM crops are sprayed with insecticides, while not all GM crops are free from them.

Any benefits must also be weighed against potential health or environmental risks, and again, these must be researched carefully.

To Marvier, the clearest message from her study is that we have started to accumulate enough data to look at this issue from an empirical, evidence-based point of view. If we are to make sound decisions, there is little room for anecdotal evidence or knee-jerk responses guided by personal philosophy.

Bt hypocrisy

For example, there is a certain irony to the opposition to Bt crops. Because its insecticides are ‘natural’, the bacterium is one of the few pesticides that organic farmers are allowed to spray onto their crops.

The bacteria of course use the exact same genes that are transplanted into Bt-engineered crops. Some may argue that this method is better because it is more ‘natural’, because the genes stay within the organism they were intended for. But is that really better?

Wholesale Bt spraying is a crude technique than the specific and targeted use of Bt-engineered crops. It means that the surrounding land is also covered in the bacteria and creatures other than pests are exposed to its entire gamut of toxins. And because farmers need constant supplies of the bacteria, it soaks up more money.

Reference: Marvier, McCreedy, Regetz & Kareiva. 2007. A meta-analysis of effects of Bt cotton and maize on nontarget invertebrates. Science 316: 1475-1477.

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Related stories on genetic modification:

Genetically-modified mosquitoes fight malaria by outcompeting normal ones
Magnifection: mass-producing drugs in record time
Feed the world – turning cotton into a food crop


Parasites can change the balance of entire communities

By changing the behaviour of their hosts, parasites can change the face of entire habitats. Now, scientists have demonstrated how a tiny flatworm can alter the structure of a tidal habitat by infecting small marine snails.

Conspiracy theories, TV thrillers and airport novels are full of the idea that the world is secretly run by a hidden society. We have come up with many names for this shadowy cabal of puppet-masters – the Illuminati, the Freemasons, and more. But a better name would be ‘parasites’.

The fluke Cryptocytole lingua affects the entire tidal habitat by infecting snails.Every animal and plant is afflicted by parasites. The vast majority are simple, degenerate creatures, small in size and limited in intelligence. They affect our health and development, and even our behaviour and culture. And by pulling the strings of key species, parasites can change the face of entire habitats.

In a typical school textbook, an ecosystem consists of plants that feed plant-eaters, who in turn, line the bowels of predators. But parasites influence all of these levels, and as such, they can change the structures of entire communities.

The idea that nature is secretly manipulated by these tiny, brainless creatures is unsettling but manipulate us, they do.

Chelsea Wood and colleagues from Dartmouth College found compelling evidence for the influence of parasites by studying small animals that live on the coasts of the North Atlantic.

Snails and flukes

These tidal boundaries are home to the common periwinkle (Littorina littorea; below), a type of marine snail. It invaded America’s shores about two centuries ago and has spread successfully along the eastern seaboard. It acts as this habitat’s equivalent of cattle, grazing the ephemeral algae that grows in rocky tidal pools.

But they are not alone. The periwinkles are unwitting passengers for the parasitic fluke Cryptocytole lingua (above), a kind of flatworm. In most parts of the coast, the flukes are relatively rare, but in some parts of New England where seabirds are plentiful as many as 90% of periwinkles can be infected.The common periwinkle, crowded on a rock. Many will be infected by flukes.

Like most parasites, the fluke has an amazingly complicated life cycle. Snails become infected by accidentally eating eggs spread in the droppings of seabirds. The larvae develop in their bodies by eating the snails inside out. They eventually give rise to a free-swimming creature that finds and infects fish, which are then eaten by seabirds, completing the cycle.

Infected snails lose their appetites

By devouring the snails’ internal organs, the flukes wreck their hosts’ digestive and reproductive systems. Wood reasoned that the neutered and malnourished snails must be have altered feeding habits and tested this idea in the lab and the field.

She found that the flukes put infected snails on an involuntary diet, and they ate far less algae than uninfected ones. While many parasites can directly alter their hosts’ behaviour, Wood believes that nixing the periwinkles’ appetities is not part of Cryptocytole’s plan.

It’s just an incidental side effect of infection and happens because the snails’ crippled digestive glands could not cope with the normal amount of food. And because the parasites trashed their reproductive systems and several other organs, they needed less energy to begin with.

Even so, by reducing the snails’ feeding, the parasites could potentially affect the entire tidal community. Wood demonstrated this by setting up cages in the tidal zone containing uninfected snails or infected ones. Sure enough, after a month or so, cages that housed infected snails had about 60% more algae than those with uninfected ones.

In the long-term, these effects are likely to be even larger as the fluke castrates its unfortunate hosts and lowers their life expectancy. Over time, this would reduce the size of the whole snail population and give the algae an even greater chance to grow.

Ripple effects

Periwinkles grazing on algae - they are less hungry if infected by Cryptocotyle.These snails’ lost appetites ripple out through the entire habitat. Infected snails mean more algae, which provide more food for other invertebrates. The algae also crowd out the rocky real estate that barnacles attach themselves too, and the loss of barnacles reduces the numbers of blue mussels that coexist with them.

Even though it never comes into contact with these other tidal players, the fluke indirectly influences all of them. Nestled within the body of a snail, it pulls the strings of the entire ecosystem.

This is one of the few instances where the effects of parasites on ecosystems have been carefully documented. Millions of similar dramas must play out all over the world, for half of the planet’s species are parasitic. It’s not our world, it’s theirs.

Find out more: Carl Zimmer’s superb book Parasite Rex is an amazing journey through the world of parasites, how they affect other animals, and how they change entire habitats.

Reference: Wood, Byers, Cottingham, Altman, Donahue & Blakeslee. 2007. Parasites alter community structure. PNAS 104: 9335-9339.

Images: by Chelsea Wood and James Byers

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Climate change responsible for decline of Costa Rican amphibians and reptiles

Amphibians around the world are facing extinction from habitat loss and a killer fungus. Now, climate change joins their list of enemies. In Costa Rica, warmer and wetter days have led to a loss of rainforest leaf litter that has sent amphibian and reptile populations crashing.

Miners used to take canaries into unfamiliar shafts to act as early warning systems for the presence of poisons. Today, climate scientists have their own canaries – amphibians.

The golden toad was one of the first casualties in the great amphibian decline.Amphibians – the frogs, toads and salamanders – are particularly susceptible to environmental changes because of their fondness for water, and their porous absorbing skins. They are usually the first to feel the impact of environmental changes.

And feel it they have. They are one of the most threatened groups of animals and one in three species currently faces extinction. The beautiful golden toad (right) was one of the first casualties and disappeared for good in 1989. Even though they are less glamorous than tigers, pandas or polar bears, amphibians are a top priority for conservationists.

The usual factors – introduced predators and vanishing habitats – are partially to blame, but many populations have plummeted in parts of the world untouched by pesky humans.

More recently, a large number of these deaths have been pinned on a fatal fungal disease called chytridiomycosis. Hapless individuals become infected when they swim in water used by diseased peers, and fungal spores attach to their skins. The disease had decimated amphibians across the Americans.

A third of the world’s amphibians face extinction, if not more.The extent of the damage may be even worse than we think. We have very little long-term data on the population sizes of many amphibian species, particularly in the tropics, where the greatest diversity exists. One of the few sites to buck the trend of ignorance is La Selva Biological Station in Costa Rica, which has been monitoring amphibian populations since the 1950s.

Steven Whitfield and colleagues from Florida International University used the La Selva data to analyse the populations of a species living among the leaf litter that covers the local rainforest floor. The team ran their census of about 30 species of amphibians, as well as many reptiles (lizards and snakes).

To their astonishment, the populations of these species had plummeted by 75% in 35 years. This massive decline is worrying for many reasons, the least of which is that La Selva sits within a protected area. Habitat destruction is non-existent here, so something else must be happening.

Nor is chytridiomycosis to blame. The fungus doesn’t tolerate high temperatures and only grows in temperate regions or mountainous ones. La Selva is neither. The killer fungus marks its presence with rapid falls in amphibian numbers within months, but these declines took place over decades.

And most tellingly of all, the reptiles suffered population losses as great as those of the amphibians. With their dry, scaly skins, reptiles lack the amphibian vulnerability to chemicals and chytridiomycosis. Something else is afoot.

Warmer and wetter days are diminishing the leaf litter that amphibians and reptiles call home.Whitfield believes that climate change is the answer. Over the past 35 years, La Selva has experienced wetter and warmer days. Temperatures have gone up by one degree Celsius, which slows the growth of local trees, and reduces the volume of leaves that they shed. The number of dry days has halved, and with more rainfall, the leaves that do fall decay faster.

So these combined climate changes have conspired to reduce the levels of leaf litter in the forest, robbing amphibians and reptiles alike of their homes. Even in this protected area, habitat destruction is going on right under our feet.

The climate change idea explains another odd finding. Whitfield saw that amphibian and reptile numbers had not declined in nearby abandoned cacao plantations. That’s because cacao trees shed their leaves throughout the year and provide a continual supply of new leaf litter.

The picture for the world’s amphibians seemed bleak enough, but it seems that we have been ignoring a larger simmering danger in the face of the immediate threat of chytridiomycosis. It is telling that all but one of the disappearing species in this study are listed as ‘least concern’ by the World Conservation Union (IUCN). Whitfield’s study should be a call to action for conservationists.

Reference: Whitfield, Bell, Phillippi, Sasa, Bolanos, Chaves, Savage & Donnelly. 2007. Amphibian and reptile declines over 35 years at La Selva, Costa Rica. PNAS doi.0611256104.

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Shark-hunting harms animals at bottom of the food chain

Overfishing has disproportionately reduces the numbers of the ocean’s ‘apex predators’ and large sharks are disappearing particularly fast. Their absence allows their prey to flourish and the consequences of that can be disastrous for animals at the bottom of the food chain, and the humans that depend on them.

On the surface, plummeting populations of sharks do not seem like much cause for concern for humans or, for that matter, other sea life. But this simple viewpoint relies on splitting animals into two groups – predators and prey.

The sand tiger shark - one of the victims of overfishing.In practice, this distinction is far too crude. Too put it bluntly, there are predators and there are predators. Those at the top kill those in the middle, and stop them in turn, from killing those at the bottom. As the old saying goes, the enemy of my enemy is my friend.

The rise in shark fishing is mainly driven by a growing market for their fins. Sharks’ fins soup is a delicacy in China, which is utterly ludicrous given that the fins themselves are tasteless and merely add texture.

China’s strong economy has put this expensive treat in the range of the expanding middle classes and the world’s sharks are paying the price for it.

Ransom Myers from Dalhousie University, Halifax, decided to study the effects of declining shark numbers by analysing a uniquely comprehensive shark census taken over the last 30 years on the American eastern seaboard.

In these waters, several shark species have all but vanished since 1972, including 99% of the bull, dusky and smooth hammerhead populations.

And because fishing expeditions tend to catch larger individuals, the average size of the survivors has plummeted. Mighty animals like the tiger and black-tip sharks are now up to half as long as they used to be.

The hammerhead shark helps to control numbers of bottom-dwelling predators like rays and skates.Unsuprisingly, as the sharks declined, their prey benefited. Great sharks mainly hunt smaller predators, including their close relatives skates, rays, and indeed, smaller sharks, whose numbers surged in their absence.

The cownose ray, for example, is now ten times more common than it was in the mid-70s.

And here’s where the domino effects begin.

It turns out that large sharks inadvertently carry out a sort of protection racket for animals at the bottom of the food chain.By taking out the mid-level predators, they prevented these lesser hunters from decimating stocks of small fish and invertebrates.

The cownose ray feeds mainly on shellfish like scallops, clams and oysters. In the 80s, their small numbers made little dent on the local scallop population which sustained the economies and stomachs of local seaside towns.

In 1996, the ray explosion started to spell the end for the scallops. By 2004, the local North Carolina scallop fisheries which had thrived for centuries were forced to close and remain closed to this day. Little did the locals imagine that the disappearance of dangerous sharks from their waters could have such strongly felt economic consequences.

The ludicrous demand for shark’s fin soup will wreak irrevocable damage on oceanic ecosystems.This is far from an isolated incident. In Ariake Sound off Japan, shark fishing is particularly intense, and a booming population of long-headed eagle rays has decimated shellfish populations just like their cownose cousins in the Atlantic.

This is one of the first times that the removal of apex predators has been so thoroughly studied in the ocean. On land, the consequences are well-known.

Just last year, Australian scientists found that in some areas, persecution of the local top dog – the dingo – has allowed introduced predators like cats and rats to kill off up to two thirds of ground-dwelling mammal species.

As for the great sharks, the responsibility for preserving these great animals lies with the only predators they themselves face – the fishermen who kill them, and the Asian restaurant-goers who create the demand for their fins.

Surely, the price of irrevocably altering an ecosystem is too high to pay for the right to eat textured soup?

Reference: Myers, Baum, Shepherd, Powers & Peterson. 2007. Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science 315: 1846-1850.

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Related posts on conservation and introduced animals:

The fox and the island: an Aleutian fable
Farmed salmon decimate wild populations by exposing them to parasites
Attack of the killer mice – introduced rodents eat seabird chicks alive

Farmed salmon decimate wild populations by exposing them to parasites

Salmon migrations serve to protect newly hatched youngsters from the parasites that afflict their parents. But salmon farms undermine this protection and jeopardise wild stocks by exposing young salmon to large numbers of parasitic sea lice.

The next time you buy salmon from your local supermarket, think about the hidden costs in each succulent fillet. Compared to wild fish, farmed salmon is far less likely to burden your wallet. But by buying it, you may be placing a much larger burden on the environment. Fish stocks around the world are declining due to over-fishing and ‘aquaculture’ – the farming of fish – was originally thought to help. But farming brings with it a host of ecological problems.

Wild salmon suffer fewer parasites than their farmed cousins. If the farmed fish are meat-eaters, as salmon are, they must be fed on the proteins and oils of wild fish, which does nothing to alleviate the stress on wild populations. Domesticated farmed fish are also genetically different to their free counterparts, and escapees risk spreading their genes and replacing local genetic diversity.

But the biggest and most immediate problem may be to do with the spread of parasites. Large, crowded and trapped animal populations are an easy target for parasites, and salmon farms are no exception. Farmed salmon are often infested with sea lice, parasitic relatives of prawns and shrimp, that cause direct damage, starve their host and increase vulnerability to disease. But the real problem comes when infected farmed salmon pass their parasites onto wild fish.

Martin Krkošek and colleagues from the University of Alberta believe that salmon farming may be disrupting behaviour that evolved in salmon to protect their young from parasites.

Newborn salmon are especially vulnerable to parasitesSalmon are known for their massive and demanding migrations in order to mate and lay eggs. Because of these treks, the young salmon enter the ocean several months before the adults and their parasite passengers return. In this way, the youngsters gain precious months’ respite from infections during which they can develop unhindered. It’s the same strategy that human parents use when they move to the country to raise their children in safer surroundings.

But this safe period is shattered if the ocean is crammed with lice-ridden farmed populations. Across the North Atlantic from Canada to Norway, wild juveniles are being infested with sea lice. The farms are providing the lice with new routes for infecting even more hosts, and the young salmon are not ready for them.

To adults, sea lice are irritating but tolerable, but to the much smaller young, carrying more than two lice is always lethal. Not only do they take up valuable nutrients the juveniles need to grow but they make them more vulnerable to predators and weaken their immune systems.

Krkošek analysed data on salmon and lice populations off the western coast of Canada and revealed disturbing trends. The lice were decimating many local populations of salmon, killing up to 95% of juveniles in some regions.

As aquaculture continues to spread, this study provides us with a harsh reality check and consumers around the world have the power to reverse the trend by protesting with their money. Choosing wild fish over farmed varieties sends a message to the fishing industry that the benefits of buying cheaper fish are outweighed by the costs to wild populations.

The fox and the island – an Aleutian fable


Island-dwelling animals across the world have been devastated by predators introduced by man. In the Aleutian islands, this age-old problem has gone one step further. There, the introduction of Arctic foxes has changed the very nature of the land itself.


Nizki island has changed. If you had visited Nizki in the 19th century, you would have been greeted by the chorus of massive colonies of seabirds, and set foot on verdant grassland. But travel to the island now and you would find a land transformed. The tall grasses and most of the seabirds have gone. The landscape is now tundra, dominated by low-lying shrubs and suffering from poor soil quality.

And if you looked carefully, you could probably spot the perpetrator behind the altered terrain – the Arctic fox. According to literature and folklore, the fox had exceptional powers of cunning and trickery. But science now reveals that they have another trick up their sleeve – the power to change entire landscapes.

The foxes arrive

The Arctic fox takes another Aleutian birdNizki Island is part of the Aleutian archipelago, a band of sub-Arctic islands that spans the gulf between Russia and Alaska.

In the late 19th century, the island chain was visited by fur traders. Seeking to forestall losses from declining sea otter numbers, the traders introduced Arctic foxes to the islands to act as a readily available future source of fur.

A century later, Donald Croll and James Estes from the University of California, Santa Cruz, were carrying out conservation work in the Aleutians. They noticed that islands infested by foxes had changed from grassland to low-lying tundra and wanted to work out how the furry predators had affected the Aleutian archipelago.

Thankfully, the fur traders had failed to introduce foxes to some of the islands, and many remain fox-free to this day. They had unwittingly set up a massive natural experiment, which Croll and Estes took advantage of. Backed by a team of researchers, they surveyed 18 islands, comparing those that were ridden with foxes and those that lacked them.

Goodbye seabirds, farewell gauno

They found that when foxes first invaded the islands, they began doing what natural selection had designed them to do – killing. Their prey were the local seabirds, and only species that nested on unreachable cliff faces escaped them.

Burrow-nesters like puffins and surface-nesters like gulls were easily taken and their populations were decimated. Today, seabirds are a hundred times more common on fox-free islands than on their fox-infested neighbours.

The landscape changes from grassland to tundra without guano

Bird droppings, or ‘guano’ were the main source of fertiliser for the Aleutian vegetation. By feeding in the productive ocean waters and defecating inland, the birds transferred nutrients from the rich sea to the poor land.

As the seabirds died, guano levels fell by over 60 times, and the soil was quickly rendered infertile. With foxes around, the levels of phosphorus – a key nutrient in most ecosystems – on an island plummeted by three times.

This newly depleted land could not longer support lush grassland, and shrubs became the dominant plant as the grasses died out.

As a final test of their theories, the scientists artificially added fertiliser to parts of fox-infested islands over three years, to mimic the effect of guano. On the fertilised terrain, grass rapidly re-established itself as the dominant plant group, increasing in numbers by 24 times.

Killer immigrants

The problem of introduced predators is, sadly, not uncommon. All around the world, people have transferred predators to places they don’t belong with devastating consequences. The problem is especially serious on islands, where the local wildlife is naïve about the threat of predators, or has lost defensive adaptations such as flight.

In Australia and New Zealand, foxes, cats and stoats have hunted their way through local bird populations, driving many to the brink of extinction. And as the Aleutian problem demonstrates, the effects of introduced killers can ripple out to affect more than just their prey. In this example, entire ecosystems can be changed over a very large area.

Deporting the problem

In the Aleutians at least, the problem seems solvable. The US Fish and Wildlife Service has been removing foxes from Aleutian islands for over 35 years. As a result, the seabirds are staging a comeback and the lush vegetation is returning. Even so, it may take several more decades for the islands to return to their former glory.

Until then, the Aleutians serve as a stark reminder of the disastrous effects of placing top predators where they don’t belong. Conserving these animals in their original homes is just as important – other studies have shown that removing top predators can wreak equally dramatic changes in an ecosystem.

Many of the world’s key predators – sharks, big cats, polar bears and many more – are facing extinction across a wide range of habitats. The need to conserve these decisive and often charismatic animals has never felt stronger.

Reference: Croll, Maron, Estes, Danner & Byrd. 2006. Science 307: 1959-1961.


Related posts on introduced predators:
Attack of the killer mice – introduced rodents eat seabird chicks
Shark-hunting harms animals at bottom of the food chain
Farmed salmon decimate wild populations by exposing them to parasites

Related posts on dogs:
Bone-crushing super-wolf went extinct during last Ice Age
Of dogs and devils: the rise of contagious cancer


Images: (Photos from Anthony DeGange and Donald Croll)