Loss of big mammals breaks alliance between ants and trees

The natural world is full of alliances forged between different species, cooperating for mutual rewards. The relationship between ants and acacia trees was one of the first of these to be thoroughly studied. But new research suggests that this lasting partnership may be sundered by the unlikeliest of reasons – the decline of Africa’s large mammals.

Acacias are under constant attack from hungry animals, from tiny caterpillars to towering giraffes. In response, many species like the whistling-thorn tree (Acacia drepanolobium) recruit colonies of ants as bodyguards. Any hungry herbivores eager to chomp on the acacia’s leaves quickly get a mouthful of biting, stinging ants. The tree is a fair employer. In return for their services, its ant staff receive a sugary and nutritious nectar as food and hollow swollen thorns called ‘domatia’ as board.

But this pact is a fragile one. Todd Palmer from the University of Florida and colleagues from the USA, Canada and Kenya have found that it rapidly breaks down if the large animals that graze on the acacia disappear. Without the threat of chomping mouths, the trees reduce their investments in bodyguards to the detriment of both partners.

Palmer demonstrated this with plots of land in Kenya’s Laikipia Plateau, where fences have kept out large plant-eaters for over a decade. Since 1995, no herbivore larger than a small antelope has entered the four-hectare “exclosures” in an attempt to study the effect of these animals on the local ecology.

Within these 10 years, Palmer found that the majority of trees produced fewer domatia and less nectar and unexpectedly, the strongest alliances were hit the hardest. What were once happy partners quickly became selfish rivals.

Solving the San Francisco plankton mystery

Look into the oceans past the sharks, seals and fish and you will find the tiny phytoplankton. These small organisms form the basis of life in the seas but if their populations get to big, they can also choke the life from it by forming large and suffocating algal blooms.

The waters of San Francisco Bay have never had big problems with these blooms and if anything, scientists worried that the waters didn’t have enough phytoplankton. All that changed in 1999, when the phytoplankton population started growing. It has doubled in size since.

Now, scientists from the United States Geological Survey (USGS) have found that the blooms are the result of a long chain of ecological changes in the area. The plankton are just players in a large ensemble drama involves clams, mussels, fish, crabs and a cold snap.

How soil imprisons ancient carbon

Deep underground, there lies a sleeping giant that we would do well to avoid waking. The giant is a massive, dormant amount of carbon, and it’s better for us that it remains trapped in the ground rather than circulating in the atmosphere as carbon dioxide.

Many of the most crucial debates of the 21^st century will involve reducing the amount of carbon dioxide being pumped into the atmosphere. The many possible solutions include trapping carbon, either in the trunks of trees or in underground vaults. The irony is that a massive amount of carbon is already locked safely away underground.

The world’s soil acts as a carbon prison and it holds more of it than the earth’s atmosphere and all of its living things combined. Over three trillion tonnes of the element are incarcerated in soil and about 80% of this is found at depths of up to 3 metres. At these levels, carbon is very stable and plays no part in the carbon cycle, the process where the element is exchanged between the land, air and sea.

Now, Sebastien Fontaine and other scientists from the French National Institute for Agricultural Research have found that deep soil retains carbon so well because it lacks enough fuel for the microbes that decompose organic matter.

Genetic study puts damper on gray whales’ comeback

The eastern Pacific gray whale has bounced back from the brink of extinction to a healthy population of 22,000 individuals. But by measuring the genetic diversity of these whales, scientists have estimated that the original population was up to five times larger. The whales aren’t out of the danger zone yet, and climate change may explain why.

Twenty-two thousand sounds like a huge number. It’s happens to be number of eastern Pacific gray whales currently swimming off the coast of North America. It’s certainly much larger than 140, the number of whales that aboriginal people of this area are allowed to hunt. And it’s far, far bigger than zero, the population size that the whales were rapidly approaching in the mid 20^th century.

Obviously, it’s all relative. Twenty-two thousand is still much less than ninety-six thousand. That’s the size of the original gray whale population and it’s three to five times the current count. Not exactly cause for conservational complacency, then.

Previously, conservationists and whalers alike could only speculate on the number of whales that lived before their flirtation with extinction. But now, Elizabeth Alter and Stephen Palumbi from Stanford University have managed to pin down a figure by looking at the genetic diversity of living whales. And their results suggest that despite a rebound that Hollywood would envy, the grays are still a pale shadow of their former strength.

Icebergs are hotspots for life

Icebergs are hotspots for Antarctic life. The water around them teems with nutrients, plankton and animals – a mobile community dragged along by the drifting ice. Together, they enrich over a third of Antarctic waters.

Say the word iceberg, and most people are likely to free-associate it with ‘Titanic’. Thanks to James Cameron (and, well, history too), the iceberg now has a reputation as an cold murderous force of nature, sinking both ships and Leonardo DiCaprio. But a new study shows that icebergs are not harbingers of death but hotspots of life.

In the late 1980s, about 200,000 icebergs roamed across the Southern Ocean. They range in size from puny ‘growlers’, less than a metre long, to massive blocks of ice, larger than some small countries.

They may be inert frozen lumps, but icebergs are secretly in the business of nutrient-trafficking. As the ice around Antarctica melts in the face of global warming, some parts break free from the parent continent and strike out on their own. As they melt, they release stored minerals into the water around them, and these turn them into mobile homes for a variety of life.

A tale of two icebergs

Kenneth L. Smith Jr, from the Monterey Bay Aquarium Research Institute, and other scientists from San Diego discovered the true extent of these icy ecosystems by studying two icebergs floating in the Antarctic Weddell Sea.

Even the smaller of the two, W-86, has a surface area larger than 17 football pitches. The larger one, A-52 was over a thousand times bigger, with a surface area of 300 km² and extending 230 metres into the freezing waters.

Smith and crew identified the duo through satellite imaging, and tracked them down by boat. Their ship spiralled around the blocks of ice collecting water samples as it went, from a dangerously close distance of a few hundred feet to a safer five miles away.

Hotspots for life

The skies above the two icebergs were patrolled by seabirds, including Cape petrels and Antarctic fulmars. Below the water, Smith explored the icebergs’ undersides with a remote-operated vehicle and found them teeming with life.

Below W-86, he saw a lattice-like surface, and the ridges of these were home to diatoms (right). These single-celled algae are part of the phytoplankton, microscopic creatures that make their energy from the sun and form the basis of the ocean’s food web. In between these diatom-covered ridges, baby icefish and segmented worms swam among the lattice’s nooks and crannies.

A-52 was even more varied, with large caves extending deep into the iceberg’s core. The team found diatoms here too, along with Antarctic krill (below), small shimp-like animals with a taste for diatoms. Among these were various invertebrates – comb jellies, colonial jellyfish-like animals called siphonophores, and predatory torpedo-shaped worms called chaetognaths.

Further out, the area immediately around the iceberg was void. But just beyond that, the ice was encircled by another halo of phytoplankton. These creatures, along with the diatoms on the ice itself, were thriving on the nutrients released by the melting ice, such as iron.

When Smith exposed diatoms to tiny mineral-rich particles filtered from his collected water samples, they grew slowly and steadily, while other diatoms cultured in normal water did not.

These drifting islands of ice were dragging entire communities along with them. As they drift and melt, they release small amounts of important nutrients. That triggers the growth of creatures at the bottom of the food chain and provides the foundations for larger animals like krill and seabirds. A-52 alone enriched a massive ring of water about the size of the Isle of Man.

To estimate the effect of other icebergs, Smith used satellites to count the number of bergs in a sample area. Within this space, the satellites spotted almost a thousand individual icebergs that, together, covered less than 0.5% of the ocean’s surface. But even this small amount was enough to enrich over 39% of the Southern Ocean!

Icebergs and climate change

By providing support for phytoplankton, the icebergs were also inadvertently helping to mitigate the effects of climate change. Just like land plants, phytoplankton make their own energy through photosynthesis. And just like land plants, they absorb carbon dioxide to do so. By eating the phytoplankton and excreting the remains, krill cause carbon to fall down into the ocean depths in a rain of droppings.

So even as their parent continent melts and releases carbon into the atmosphere, icebergs serve to draw planet-warming carbon away from the air and transfer it to the deepest sea. Smith believes that climate modellers need to take this into account to better predict the effects of melting Antarctic ice.

The disappearing ice can reveal underlying rock which absorbs more heat, hastens melting and releases even more trapped carbon – this is known as ‘positive feedback’. But as the ice melts, icebergs break off and these help to suck in carbon from the atmosphere – this is ‘negative feedback’. The next task is to understand how these two processed balance out.

Reference: Smith Jr, Robison, Helly, Kaufmann, Ruhl, Shaw, Twining and Vernet. 2007. Free-drifting icebergs: hot spots of chemical and biological enrichment in the Weddell Sea. Science doi.10.1126/science.1142834.

Other posts on ecology:
Shark-hunting harms animals at bottom of the food chain
The fox and the island – an Aleutian fable
Parasites can change the balance of entire communities
Human nitrogen emissions indirectly capture carbon by fertilising forests

Restoring predator numbers by culling their prey

Helping out a threatened predator by culling their prey seems like a really stupid idea. But Scandinavian scientists have found that it might be the best strategy for helping some of our ailing fish stocks.

Lennart Persson and colleagues from Umeå University came up with this counterintuitive idea by running a 26-year natural experiment with the fish of Lake Takvatn, Norway.

At the turn of the 20^th century, the top predator in Lake Takvatn was the brown trout. Over-fishing sent its numbers crashing, and it was virtually gone by 1980.

In its place, a smaller fish – the Arctic char ­– was introduced in 1930. Char should make a good meal for trout, so it was surprising that when the trout were reintroduced they failed to flourish despite an abundance of food.

It was only in the 1980s, when the researchers removed over 666,000 char from the lake that the trout started bouncing back. While their prey population fell by 80%, the trout have increased in number by 30 times. The lake’s temperature and nutrient levels were mostly constant during this time, so why did the trout do better when they prey was culled?

Persson believes that it’s not the numbers of the char, but their size that matters, and that changed irrevocably when the trout first vanished.

More predation means more prey?

Paradoxically, predators like trout, can actually increase the numbers of small prey by eating them. It seems like a strange idea, but it happens because the remaining prey face less competition for food. As a result, they grow more rapidly, mature faster and give rise to more young.

This means the population becomes, on average, smaller, since individuals spend less time growing and fill the water with baby fish. And that’s good news for predators. But take the predator out and the whole system grinds to a halt.

Conservationists often like to believe that an over-hunted predator will just bounce back into its original niche once hunting is stopped. But things are rarely that simple. In the predator’s absence, other species will rush in to fill the gap and the entire system can settle down into a new balance, which the predator can find very hard to slot back into.

Practicalities

In Lake Tyvatn, the absence of the trout meant that the char population faced no threats and competed heavily for resources. They grew and reproduced slowly, reaching sizes too large for the trout to tackle. The proportions of small char fell to a level which could not support reintroduced trout. Essentially, while the predator was away, the prey took it easy and locked the door so it couldn’t get back.

When the char were culled, this mimicked the effects of trout predation by removing the largest individuals. As a result, the numbers of smaller, trout-friendly char doubled and began to dominate the lake. And that shift finally allowed the trout to regain a foothold (or finhold) in the lake. The two fish have now established a balance in numbers for over 15 years.

Persson’s study clearly shows that removing a predator from its habitat (an all-too common occurrence) doesn’t create a predator-shaped hole in the ecosystem, ready to be filled again. Instead, it causes drastic changes to local food webs, that can only be reversed with counter-intuitive and ingenious strategies.

Stocks of predatory fish, including sharks, salmon, cod and trout, are threatened by over-fishing all over the world, and Persson believes that his strategy could help them to recover.

For example, the falling cod population in the Baltic sea could potentially be restored by fishing for its prey, like herring or sprat. It’s so crazy, it just might work.

 

Reference: Persson, Amundsen, de Roos, Klemetsen, Knudsen & Primicerio. 2007. Culling prey promotes predator recovery – alternative states in a whole-lake experiment. Science 316: 1743- 1746.

Related post on over-fishing:
Shark-hunting harms animals at bottom of the food chain
and others on changing ecosystems:
Attack of the killer mice – introduced rodents eat seabird chicks alive
The fox and the island: an Aleutian fable
Farmed salmon decimate wild populations by exposing them to parasites

Technorati Tags: trout, char, over-fishing, Lennart+Persson, Paul+Primicerio, ecology, food+webs

Human nitrogen emissions indirectly capture carbon by fertilising forests

Human activity has greatly increased the levels of active nitrogen in the environment. By acting as a fertiliser and speeding the growth of forests, this extra nitrogen has indirectly locked up more carbon dioxide in the world’s trees.

There is no doubt that of all the elements in the entire periodic table, carbon is currently hogging the limelight. As it cycles through our environment, the policy decisions and economic futures of entire countries hang in the balance.

For all its media-whoring, you might be forgiven for forgetting that carbon is not the only element we are belching into the environment. Over the last century, we have greatly overwhelmed the natural nitrogen cycle too.

Nitrogen – the neglected element

Through the manufacture of nitrogen-based fertilisers and the exhausts of our cars, power plants and factories, we have more than doubled the natural levels of active nitrogen in the atmosphere.

Nitrogen is a valuable commodity in many parts of the world, and restricts the growth of local plant life. As such, the recent man-made influx has led to large increases in plant growth. In some cases like algal blooms that choke rivers and lakes, it’s too much of a good thing. But there is a silver lining.

Federico Magnani from the University of Bologna, together with an international team of scientists, have found that the changes in the nitrogen cycle may have been inadvertently fertilising our forests.

Carbon and nitrogen

The world’s forests act as massive carbon sinks, delaying the global warming effects of carbon dioxide by trapping it in prisons of wood and leaves. And larger forests mean more trapped carbon. The temperate forests of the Northern Hemisphere alone could store a massive 600 megatonnes of carbon every year.

The carbon and nitrogen cycles dance around each other in complex ways. When nitrogen levels increase, forests respond by channelling growth from roots to leaves and trunks. These above-ground organs are more enduring than roots and retain sequestered carbon for a longer time. More leaves also means increased photosynthesis, which serves to draw more carbon dioxide in from the air.

The extra nitrogen also delays the decay of leaf litter, further halting the release of organic carbon into the atmosphere.

Magnani’s colleagues are not the first group to try and look at the interplay between nitrogen levels and carbon capture. But other studies have found it difficult or impossible to account for the effects of nitrogen alone.

Carbon balance

A forest’s carbon balance – the amount of carbon trapped versus the amount released – depends on a variety of factors, including its age, logging, fires, and more. Some of these are easy to account for at a small scale. For example, when logging or fires kill off patches of forest, they become net sources of carbon as they start to regrow.

But after a couple of decades or so, the mature forest turns into a carbon sink, and the amount it stores outweighs the amount it releases. Clearly, a forest’s carbon balance changes as it matures, but real forests consist of patches of vegetation are very different ages.

To look at the overall picture, Magnani’s group took direct measurements of the carbon balance over a long period of time, from a network of forest sites in Western Europe and the USA. This allowed them to account for short-term sources of variation. And by using direct measurements, they have surpassed the models and simulations of previous studies.

The group found that carbon balance corresponds well with nitrogen levels in the area. In fact, the prowess of some forests at carbon capture seem to be overwhelmingly driven by their extra nitrogen boost. Our effects on the nitrogen cycle may have been acting like an unexpected carbon offset scheme.

Practicalities

So should we start pumping nitrogen in our forests to trap more carbon dioxide? Certainly, Magnani’s results suggest that small extra amounts of nitrogen can cause unexpectedly large levels of carbon capture. But his view and those of other commentators is a resounding “Not yet”.

There are still many questions left to be answered, particularly about the exact relationship between nitrogen addition and carbon levels. There is some evidence that some temperate forests are suffering from nitrogen saturation. Could adding more nitrogen damage them, or prevent them from returning to a situation where nitrogen is limited and not free-flowing?

And what of the other risks and benefits? The extra wood from the faster-growing trees could find a use as a replacement for concrete, a notoriously eco-unfriendly building material. But additional nitrogen could affect other animals and plants in the local environment. Would biodiversity suffer if certain species monopolise the newfound nitrogen bonuses?

As future research addresses these questions, those involved in forest management would do well to heed the importance of the world’s forests in sequestering carbon dioxide. There are other ways of increasing forest coverage besides mass-fertilisation, and the most obvious one is safeguarding the forests that we already have!

Reference: Magnani, Mencuccini, Borghetti, Berbigier, Berninger, Delzon, Grelle, Hari, Jarvis, Kolari, Kowalski, Lankreijer, Law, Lindroth, Loustau, Manca, Moncrieff, Rayment, Tedeschi, Valentini & Grace. 2007. The human footprint in the carbon cycle of temperate and boreal forests. Nature 447: 848-850.

Technorati Tags: carbon cycle, nitrogen cycle, nitrogen emissions, forests, carbon sequestration, science

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’.

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)

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

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.

Technorati Tags: GM crops, genetic modification, GM safety, GM environment, GM debate, insecticides, Michelle Marvier, Peter Kareiva, Bt crops,

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’.

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.

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

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

Technorati Tags: parasites, periwinkle, Cryptocytole, fluke, trematode, parasite effects

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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.

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.

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.

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.

Technorati Tags: reptiles, amphibians, amphibian extinction, Costa Rica, amphibian conservation, amphibian climate change, chytridiomycosis, science

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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.

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.

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.

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.

Technorati Tags: shark, top predators, conservation, shark fishing, shark hunting, overfishing, marine life, shark fin soup, science

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

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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.

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.

Salmon 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.

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