Friday, August 31, 2012

Top five on Friday

Today's list is the top five most studied organsims. Well, kind of...

This top five list comes with data. I used Web of Science to search for Latin species names, not common names, listed on the model organism page of Wikipedia. So, values for some species, like Homo sapiens and Mus musculus, are grotesquely lower than they should be. Hence, species that should probably be listed, don't even make the list. In fact, like humans, they wouldn't even make the top ten.

Escherichia is king. A graph of the number of studies on each species.
For fun, I also looked the number of studies on each species where the word evolution was used in the title or abstract. It changes the list a little. Humans still do not make the list, but we make it into 6th at least.

Escherichia still king, but its power is weakened. A graph of the number of studies on each species where the word 'evolution' was used in the abstract or title.
No marine species make either list and don't even make the top tens, they probably don't even make the top twenties.

Thursday, August 30, 2012

Lowest ever Arctic sea ice cover

In the northern summer of 2007 the extent of Arctic sea ice reached a low of 4.17 million square kilometers, the lowest since satellite measurements began in 1979. A few days ago the extent of Arctic sea ice was just 4.10 million square kilometers, just breaking the 2007 record. But, the ice is likely to keep melting well into September. 

The minimum sea ice coverage of 2007, recorded on September 18 (left) next to the new record set on August 26 this year (right). Previous years suggest that ice will continue to melt until mid-September (image NSDIC).
Of concern is that this year was cooler than 2007. However, the extent of sea ice coverage measures just two of the three important dimensions. Like its area, the thickness of sea ice has also been falling. This means that the loss of ice area is accelerated because there is a smaller volume that needs to melt. 

Arctic sea ice volume estimated from ice extent and thickness data (image Wikipedia).
The ice loss exposes a greater expanse of open water, which feeds back via two mechanisms to further accelerate ice loss. The sea ice reflects sunlight back into space and as more dark water is exposed more energy is absorbed, which raises temperature. Larger areas of open ocean also allow storms to generate larger waves that break up the ice into smaller pieces, which melt faster.

Of note is that the six years with the lowest Antarctic ice covers are the last six years. Listed from lowest to highest ice cover, they are 2012, 2007, 2011, 2008, 2010 and 2009. To round out the top ten years with the least ice cover add 2005, 2006, 2002 and 2004 to the list. That's right, the top ten years of lowest sea ice cover have occurred in the last eleven years. And 2003 sits at number twelve after 1995.

Friday, August 24, 2012

Top five on Friday

On a whim, I've decided to start writing some top five lists. And, because five and Friday both start with 'f', I'm going to post them each Friday. Today it'll be my top five favorite popular science books that I have read. 

3. The world without us, Alan Weisman

5. Trilobite: Eyewitness to evolution, Richard Fortey

Friday, August 17, 2012

Australia's new marine parks revisited

Last month I wrote about the release of a draft plan for a set of marine parks in Australian Federal waters. I expressed concern that the large area and their distance from shore would make enforcement of the fully protected areas difficult. It seems I'm not alone in this concern. Three marine scientists in Western Australia have written an article in The Conversation expressing similar concerns and others about the design of the marine parks, including the under-representation of some habitats within the parks.

Tuesday, August 14, 2012

Shark week

Apparently it's the 25th year of the Discovery Channel's shark week. So, you can listen to deep, manly voices leaving dramatic pauses between words as you watch sharks all this week. Well, assuming you have a Discovery Channel subscription...

Perhaps in celebration, but more likely as coincidence, Ed Yong has an interesting piece on discovering the past shark biodiversity of a Central Pacific island by examining cultural artifacts.

Saturday, August 11, 2012

Selection on beak size in birds

ResearchBlogging.orgThe rapid evolution in the size and shape of bird beaks provide some of the best demonstrations of evolution in action. A classic example is the work of Rosemary and Peter Grant in the 1980's on Darwin's finches in the Galapagos. They showed that both competition and climatic conditions influenced beak size through changes in the availability of different sized seeds.

Gould's famous drawing of four of the fifteen species of Darwin's finches ([1] the large ground finch, [2] the medium ground finch, [3] the small tree finch and [4] the green warbler-finch). The Grants work primarily on the medium ground finch population on the island of Daphne Major, which they've visited every year since 1973.
Larger seeds are more difficult to crack. In drought years when they become proportionally more common, the finches with larger beaks did better because they had access to food that other birds didn't. Similarly, when a large seed eating competitor arrived, the birds with smaller beaks did better because smaller seeds became proportionally more common as competition for them was weaker.

The medium ground finch, Geospiza fortis. The Grants showed that its beak rapidly evolved to larger sizes in drought years and smaller sizes when faced with greater competition from the bigger beaked large ground finch, G. magnirostris (photo Wikipedia).
Although bird's beaks are clearly strongly shaped by diet, they are not just used for getting and eating food. The size and shape of beaks has also been linked to their use in preening and song production. More recently, studies have found that beaks may also play an important role in thermoregulation. Bigger beaks radiate more heat and could also help birds to conserve water in drier environments.

My pet kakariki (Cyanoramphus novaezelandiae). His beak is perfectly adapted for causing trouble.
In 2010, Symonds and Tattersall found that bird beaks conformed with Allan's rule, which posits that the relative size of body extremities should be smaller in colder environments to reduce heat loss. Bird beaks are highly vascularised and uninsulated extremities that can exchange a substantial amount of heat with the environment. Symonds and Tattersall found that beaks were relatively smaller in birds living at higher latitude and elevation.

In a recently published paper, Greenberg et al. develop this idea further. They argue that in drier environments birds could dissipate heat through their beaks rather than using evaporative cooling, thus conserving water. They test the hypothesis by examining the beaks of two subspecies of song sparrow, Melospiza melodia, that occupy habitats that contrast markedly in summer temperatures.

The song sparrow, Melospiza melodia (photo Wikipedia)
The Atlantic song sparrow lives in dune scrub and salt marsh edges along the mid-Atlantic coast, while the eastern song sparrow is widespread in gardens and wild scrublands. The coastal habitat of Atlantic song sparrows has higher summer temperatures and the availability of freshwater is limited relative to the habitat of the eastern song sparrow. And, consistent with the hypothesis, Atlantic song sparrows have beaks that are ~13% larger than eastern song sparrows.

To further evaluate the hypothesis, Greenberg et al. brought song sparrows into the laboratory where they could measure heat loss by thermally imaging the birds at constant ambient temperatures. Both subspecies maintained their beaks at higher than ambient temperature and higher than their body temperature. Heat lost through the beak was 5.6 - 10% of total heat loss, despite it making up less than 2.5% of total surface area.

A thermal image of an Atlantic song sparrow at an ambient temperature of 29°C (image Greenberg et al. 2012)
--> The beaks of the Atlantic song sparrows dissipated 33% more heat than the eastern song sparrow. Most of this difference can be explained by beak size, but Atlantic song sparrows may also maintain their beaks at higher temperatures than eastern song sparrows. Greenberg et al. estimate that the greater "dry" heat loss means an Atlantic song sparrow would conserve 7.7% more water than an eastern song sparrow of similar size.

In another study, Greenberg and Danner surveyed the beak size of song sparrows across California. They found that differences in beak size were strongly explained by the climatic conditions in which the birds lived. As summers became hotter and drier, song sparrow beaks became larger. Contrary to Allen's rule, winter temperatures poorly explained beak size differences, suggesting that heat dissipation is under stronger selection than heat conservation in the song sparrow.

Taken together these three studies support the hypothesis that climate is a significant selective pressure on the evolution of bird beaks. They highlight a little appreciated fact of evolution, that traits are often under multiple selection pressures and phenotypes are likely to reflect comprises between them. However, unlike the work on Darwin's finches, it has not been established that climate related beak size differences influence fitness variation. So, the work is strongly suggestive, but it's not a closed case.


Symonds, MRE, & Tattersall, GJ (2010). Geographical Variation in Bill Size across Bird Species Provides Evidence for Allen’s Rule The American Naturalist, 176 (2), 188-197 DOI: 10.1086/653666  

Greenberg R, Cadena V, Danner RM, & Tattersall G (2012). Heat Loss May Explain Bill Size Differences between Birds Occupying Different Habitats. PloS one, 7 (7) DOI: 10.1371/journal.pone.0040933  

Greenberg R, & Danner RM (2012). The influence of the California marine layer on bill size in a generalist songbird Evolution DOI: 10.1111/j.1558-5646.2012.01726.x

Monday, August 6, 2012

Playing Gaia Phytoplankton doesn't need much to keep it happy. Just some sunlight and a few important macronutrients, such as nitrate, phosphate and silicic acid. However, in some areas of the ocean the abundance of phytoplankton is substantially lower than would be expected from the plentiful supply of nutrients. Such areas are known as "high-nutrient, low-cholorophyll" (HNLC) regions and make up 20% of the ocean surface.

Diatoms make up a large part of the phytoplankton (photo Wikipedia)
A feature often associated with HNLC regions is the low bioavailability of iron. Iron is critical for phytoplankton as it is required to make chlorophyll and important enzymes like nitrate reductase. It has, therefore, been suggested that iron bioavailability is the most important factor limiting phytoplankton population size in HNLC regions. But, zooplankton grazing and deep mixing of surface water that reduces the time that phytoplankton has access to light have also been hypothesised.

Progression of the largest recorded natural phytoplankton bloom in the Southern Ocean, which occurred earlier this year. It was thought to be caused by the addition of iron blown into the sea by strong offshore winds. At its peak, it measured 100 km north-south and 200 km east-west (images Jan Lieser & NASA Terra Modis).
A study, published in Nature, investigated the effect of adding iron to the water on phytoplankton abundance. It was not the first experiment to do so, there have been several. But, unlike the others, this experiment showed that much of the dead phytoplankton was transported into the deep sea. Which is important for our understanding of the role that the ocean plays in regulating the climate.

A map showing the location of iron fertilization experiments. The experiment published in Nature is the EIFEX study, which is the second from the right (image taken from Baar et al. 2004).
During photosynthesis, phytoplankton consumes carbonic acid, which is formed when carbon dioxide dissolves into water. If sufficient quantities of the carbon that the phytoplankton take up can be transported into the deep ocean, away from the atmosphere, it can effectively reduce the amount of carbon dioxide in the atmosphere, cooling the planet. Indeed, phytoplankton blooms may have played a role in triggering past ice ages.

The study recorded phytoplankton abundance and particles falling beneath the bloom at various depths for 37 days, capturing the rise and fall of the bloom. The bloom peaked after 24 days; chlorophyll concentrations was almost four times those at control sites and the bloom covered nearly 800 square kilometers. As the bloom collapsed, at least half of the biomass sank below 1,000 meters, most of which is likely to have reached the seafloor.

Such evidence indicates that at larger scales and over longer periods, algal blooms could transport climatically significant amounts of carbon into the deep sea. In surface waters, the carbon could be returned to the atmosphere in a few months. But, in deep sea sediments the carbon might not reach the atmosphere for centuries. However, more work needs to be done to confirm the effect of phytoplankton blooms on climate.

A surprising finding of the study was that light availability did not seem to effect the plankton bloom as much as might of been expected. Chlorophyll concentrations remained high down to 100 meters where light availability should limit phytoplankton abundance. Indeed, the results showed that chlorophyll concentrations were similar or higher than those recorded at shallower depths in previous iron fertilization experiments.

Victor Smetacek, Christine Klaas, Volker H. Strass, Philipp Assmy, Marina Montresor, Boris Cisewski, Nicolas Savoye, Adrian Webb, Francesco d’Ovidio, Jesus M. Arrieta, Ulrich Bathmann, Richard Bellerby, Gry Mine Berg, Peter Croot, Santiago Gonzalez, Joachim Henjes, Gerhard J. Herndl, Linn J.Hoffmann, Harry Leach, Martin Losch, Matthew M. Mills, Craig Neill, Ilka Peeken, Rudiger Rottgers, Oliver Sachs, Eberhard Sauter, Maike M. Schmidt, Jill Schwarz, Anja Terbruggen, & Dieter Wolf-Gladrow (2012). Deep carbon export from a Southern Ocean iron-fertilized diatom bloom Nature, 487 (7407), 313-319 DOI: 10.1038/nature11229

Darwin wins again

When Darwin wrote the Origin of Species he couldn't have known about the huge advances that would come later. He was wrong on some points that creationists like to highlight. But, the reason we revere the genius of the man is that his insights were so often right. And so it has proven again in a new paper on the inner ears of sloths. I won't say anything more about it since Jerry Coyne has beaten me to the punch and written an extremely good and detailed account of the paper on his website Why Evolution is True.

Thursday, August 2, 2012

Carnival of Evolution #50

The newest edition of the Carnival of Evolution is up at Teaching Biology. I have not done anything about submitting my posts yet. Maybe I'll make it to the next one...