Loss of big mammals breaks alliance between ants and trees

Blogging on Peer-Reviewed ResearchThe 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.

Giraffe next to whistling-thorn acaciaAcacias 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.

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Solving the San Francisco plankton mystery

Blogging on Peer-Reviewed ResearchLook 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.

Solving the San Francisco plankton mysteryThe 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.

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How soil imprisons ancient carbon

Blogging on Peer-Reviewed Research

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.

Soil changes characteristics with depthMany of the most crucial debates of the 21st 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.

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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 20th century.

The gray whale hasn’t fully recovered from a century or more of huntingObviously, 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.

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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.

Icebergs are hotspots for Antarctic life.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

Diatoms cover the underside of icebergs and form a ring of life around them.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.

Diatoms fed by meltwater nutrients from icebergs, act as food for krill.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.

The brown trout increases the numbers of its prey by eating it!At the turn of the 20th 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.

By growing too large, the Arctic char muscled the trout out of Lake Takvatn.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

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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.

Industrial exhausts pump huge amounts of nitrogen into the atmosphere.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.

The world’s forests act as massive carbon stores.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

By fertilising forests, nitrogen emissions could offset carbon emissions.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.

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