Tag Archives: climate change

Don’t treat soil like dirt!

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Soil is a precious resource, yet most of us pay little attention to the stuff under our feet.  It is the medium in which we grow our food and the foundation on which we build our cities. Soils filter our water, detoxify our pollutants, decompose our waste and hold vast reserves of the nutrients required for life. Soils are also fragile, taking thousands to million of years to develop but destroyed in minutes by human development.

For the past three years, myself and my fellow soil enthusiast Aurora have spent our Saturdays in December showing kids how awesome soil and the microbes that inhabit it actually are. We’ve developed a number of soil and microbiology activities to teach kids of all ages what soil actually is, who lives in it, and why we should value it. The take-home message? Don’t treat soil like dirt! Human beings (and nearly ever other species on earth) depend on soil for our very survival.

Here are some highlights from the last two weeks of the workshop:

Checking out some protozoa under the microscope!
Making a hypothesis before conducting an experiment!
Making a hypothesis before conducting an experiment!
MAKING SOIL! This is always a favorite. Want to make soil at home with your kids? Check out my homebrew recipe below...
MAKING SOIL! This is always a favorite. Want to make soil at home with your kids? Check out my homebrew recipe below…
Probably the coolest thing I've ever made in photoshop.
Probably the coolest thing I’ve ever made in photoshop.

We are even participating in an international, crowd-sourced science experiment known as the Tea Bag Index experiment to measure rates of decomposition in different soil types! This is a fun and easy experiment you can do in your backyard. All you need is a few teabags and a scale.  Decomposition, the breakdown of once-living organic matter and conversion into soil organic matter, is an important step in the global carbon cycle that is driven primarily by soil microorganisms. Ultimately, decomposed carbon is respired back to the atmosphere as carbon dioxide. Scientists are currently trying to understand how global climate change will affect decomposition and the microbial “respiration” of CO2 from soils. Projects such as the Tea Bag Index experiment provide scientists with valuable data that can be used to inform predictions about changes to the global carbon cycle. For more information on the Tea Bag Index experiment check out the website:


Or click here to access the protocol and get involved directly!


Most importantly, our workshop strives to underscore the importance of soils in our everyday lives.  Kids (and parents) often come unsure of what exactly soil is or why it should matter to them, and often enjoy the experience so much that they return week after week.

Live in the Philly area and got kids? Check us out, every Saturday for the rest of the month!

And since I can’t seem to stop geeking out about this stuff, here are some more cool resources to check out on soil science education:

USDA NRCS Healthy Soil Fact Sheets

microbes improve carbon cycle models

Microorganisms are key drivers of the global carbon cycle both on the land and in the ocean. Through a diverse array of metabolic strategies, microbes decompose organic and recycle organic carbon. Just as we respire a fraction of the carbon we consume as CO2, so do microbes. Globally, vast quantities of organic carbon are funneled through many billions of microbes every year: to be decomposed, recycled, and respired into the atmosphere. In spite of this, microbial activity has historically been ignored in attempts to model the global carbon cycle and predict climate change-related feedbacks. Instead, our models rely on untested assumptions that microbes and their carbon cycle activities will respond in uniform, predictable manners to increases in temperature, and as such, can essentially be ignored.

A recent paper in Nature Geoscience paper challenges this assumption by explicitly integrating microbial physiology into a new model of the soil carbon cycle. Compared with traditional models, the new model more accurately matches current observations of carbon stocks and fluxes across ecosystems. Regarding future carbon cycling in a warming world, the model produces several widely different scenarios that vary due to the potential response of microbes to rising temperatures. In short, if microbes respond negatively to warming by decreasing their “growth efficiency”; that is, if warming slows down their growth rates, no additional carbon is released to the atmosphere as the soil warms. But if microbes are able to adapt to higher temperatures and maintain their current growth efficiencies, higher temperatures will accelerate carbon decomposition rates and lead to potentially huge additional losses of carbon dioxide from soils.

The integration of biological mechanisms into earth system models is an important step forward in our ability to forecast future climates. Future research that empirically measures microbial community responses to long-term warming is desperately needed in order to accurately model and predict this potentially huge feedback to the global carbon cycle.





A draft of the national climate report has been released for review by the American people. Please take a moment to look it over. It’s a lengthy report, but the message is clear from the opening letter: our climate is warming, faster than we anticipated even three years ago. We have many lines of evidence pointing to this conclusion. To name a few, hotter summers with periods of extreme heat lasting longer than any living American has ever experienced. More frequent, extreme weather events, such as superstorm Sandy that devastated coastal regions of the northeast this past year. Global sea level has risen approximately 8 inches since the end of the 19th century, and is projected to rise another 1-4 feet by the end of this one. The Greenland ice sheet is melting more rapidly than scientists have anticipated, and the north pole is expected to be completely ice free in the summer by mid-century. Massive die-offs of coral reefs are being observed, species distributions are shifting in time and space.

In deciding whether to try to massively reduce our carbon emissions and prevent some of the most dramatic consequences of climate change from being born out, or implementing a comprehensive global adaptation strategy, I believe that humanity faces its single greatest challenge yet as a species. The decisions we make now and in the coming decades will alter the environment we experience and the fundamental way our planet functions for thousands, if not hundreds of thousands of years to come.

Loss of top predators reduces Black Sea ecosystem’s resilience to eutrophication

Human activities are having profound effects on marine ecosystems. Since the mid 20th century, marine food webs have undergone dramatic changes due to the emergence of industrial fishing. Humans tend to feed on fish with “high trophic status”- those that are near the top of the food chain. In many systems, top predators have been severely reduced or even lost entirely due to overfishing. More recently, increased nutrient loads due to agriculture are fertilizing marine ecosystems, particularly in the more delicate and high-productivity coastal habitats. This often results in explosions of phytoplankton, the tiny photosynthetic bacteria that forms the bottom of most marine food chains. In recent years these phytoplankton blooms have been further facilitated by warmer sea surface temperatures and increased runoff levels due to climate change. Taken together, overfishing and anthropogenic nutrient loading are resulting in both top-down and bottom-up forcing that is fundamentally altering the the structure of marine communities.

A recent study in Global Change Biology examined the impacts of eutrophication and overfishing on a marine community in the Black Sea, a land-locked basin in Eastern Europe. Marine communities in the Black Sea are largely isolated, and offer ecologists the opportunity to study how removal of key organisms in a food web affect the broader community. Decades of overfishing have led to a loss of several traditional apex predators, which has disturbed the structure of the system from the top down. Recent eutrophication has further altered community structure. In particular, once-restricted jellyfish species have been able to make inroads, in some cases coming to dominate the oxygen-depleted waters that result from phytoplankton blooms.

The native marine community at this research group’s study site had difficulty adapting to increased productivity at the bottom of its food web. The researchers concluded that the community as a whole would likely have fared much better if top predators had not also been removed. The inclusion of top predators would have prevented jellyfish and other minor players from becoming dominant and in turn pushing out other mid-trophic level species. These results indicate that ecosystems-based fisheries management must take into account the role of top predators in structuring communities and offering resilience to other profound changes such as nutrient loading and warming. Whether the Black Sea will recover from the major disturbances it has suffered, or revert to a low-diversity, eutrophic state will depend in large part on the economic and political decisions of the countries whose fishing and agriculture practices have so profoundly affected it.

Llope et al. 2011. Overfishing of top predators eroded the resilience of the Black Sea system regardless of the climate and anthropogenic conditions. Global Change Biology 17: 1251-1265.


deep sea carbon cycling: more dynamic than we thought?

For years, scientists have speculated that deep sea carbon may have played an essential role in past climate change episodes. Specifically, it has been suggested that the C bound in seafloor sediment has undergone thermodynamic alterations in the past to due upwellings of molten magma from the mantle. Magma may have triggered the release of CO2 and methane into the upper ocean and eventually the atmosphere. Evidence suggests that the Paleocene-Eocene thermal maximum, which lasted approximately 100,000 years, may have been triggered in part by the release of greenhouse gases from the seaflooor.

Despite these speculations about deep sea carbon influencing past climate, little research has been done on the role of seafloor carbon in the present day C cycle. In some ways, this is surprising given the enormous amount of attention being payed to global C budgets and possible means of C sequestration. It is generally assumed that the deep sea represents a huge “carbon sink”, to which organic C from the upper ocean enters and does not emerge again for thousands to millions of years. This would suggest that whatever carbon-cycling processes are occuring at the seafloor are not powerful enough to cause a net carbon release.

Recent research published in Nature Geoscience suggests otherwise. Several case studies have demonstrated dynamic processes occurring on the ocean floor can in fact lead to a net release of greenhouse gases. Spreading seafloor centers- regions where oceanic plates pull apart-  are a site of magma activity and hydrothermal venting. Hydrothermal vents release a variety of hot, mineral-rich fluids that can support a diverse microbial and invertebrate community. At one such spreading center in the Gulf of California, magma is intruding into thick organic basin sediments. These sediments have long been thought to sequester C, however, it now appears tht their heating is causing the release of methane into the upper ocean.

In the Northeast Pacific, another intriguing deep ocean C cycling system has been discovered. Here, microbes are converting ancient inorganic C into dissolved organic C, which is subsequently released to the overlying ocean. This discovery contradicts the general belief that ancient deep-sea C is highly stable and not accessible to microbes.

Other distinct seafloor C sources are rapidly emerging around the world, as improved technology and a heightened interest in seafloor processes are accelerate the pace of discovery. However, the contribution of such “point sources” to global C budgets is still highly uncertain and far more research is needed to come up with even a rough estimate of global deep sea C sources. Nonetheless, it would seem that we can no longer consider the deep ocean a black box of C sequestration, and that we should think carefully about the ramifications of introducing more carbon- either accidentally through the introduction of dissolved greenhouse gases to the ocean, or intentionally as part of a climate change mitigation strategy- to a system that is clearly more dynamic than we once thought.,

Reference : “Deep Sea Discoveries.” 2011. Nature Geoscience: Letters. Volume 4, Page 1.

Whats up with nitrogen at Hubbard Brook?

Buried deep in the White Mountains of New Hampshire, the Hubbard Brook experimental forest has provided ecologists with questions and answers for over four decades.  And more questions.

Hubbard Brook was founded on the principle that it was the ideal location for “small watershed” experiments: it is a forest ecosystem whose energetic inputs and outputs could be fully accounted for and directly measured. Ecologists have been able to ask many broad and profound questions at Hubbard Brook, and the forest has told them a story from which many fundamental ecological principles have been distilled. What causes forests to grow? Where do they get their nutrients? What sort of energy enters and leaves an ecosystem? How are human activities impacting forests? Are they resistant to change, and are they resilient in the wake of disturbance?

Before tackling any of these big questions, you may well wonder about my first statement: that the energetic inputs and outputs of the forest can be fully accounted for and measured. How can one possibly measure all the energy that enters and leaves a forest? In the case of Hubbard Brook, there is a deceptively simple answer: it’s all in the streams. Keep a record of what enters and leaves the streams, and you’ll have a good idea of what is entering and leaving the forest as a whole.

Plants need three things to grow: sunlight, nutrients and water. Of the plant essential nutrients, nitrogen is the most important in temperate deciduous forests, but phosphorous, calcium, magnesium and sulfur, among others, are also essential.  In describing where  nutrients and energy come from, ecologists like to talk about autochthonous and allochthonous inputs. Autochthonous inputs come from within the system. Allochthonous inputs come from elsewhere. In the case of nitrogen, an autochthonous source would be a leguminous plant, or a plant that can convert atmospheric nitrogen into a plant useable form through the biochemical process known as nitrogen fixation. An allochthonous input might be a nitrogen fertilizer sprayed on a field.

Hubbard Brook has plentiful rain and sunlight to support a healthy forest. The first unknown that ecologists tackled was nitrogen, and I’ll focus on it from here on out because understanding nitrogen at Hubbard Brook has proved more challenging than anyone ever imagined.

Where are the plants getting their nitrogen? It turns out that for the last 14,000 years (since the retreat of the North American glacier that allowed the northeastern US to re-vegetate), all nitrogen that has entered and fed Hubbard Brook has come from the atmosphere. From the rain. No nitrogen fixing plants, no weatherable rock-derived nutrients. All that plant nourishment has entered the forest silently, dissolved in the summer rains or the winter snow.

It also turns out that all nitrogen eventually leaves  Hubbard Brook by water as well, in a readily soluble form known as nitrate, or NO3-. Streams cut across the forest, forming discrete, independent watersheds over a relatively small space. (A watershed simply refers to a region whose hydrologic inputs and outputs are well defined and accounted for). Streams are an integral part of the Hubbard Brook forest. Much as your bloodstream carries essential nutrients to your tissue and allows your body to flush out toxins, streams transport nutrients to different parts of the forest and flush away excess chemicals that the system doesn’t use. Through long term monitoring of the stream chemistry at Hubbard Brook, including measurements of headwater inputs and lowland outputs, ecologists have developed an accurate record of how much nitrogen enters and leaves the system annually.

In an unpolluted forest, how much nitrogen enters, and how much nitrate leaves, tells ecologists a lot about the forest’s growth and nutrient requirements. At Hubbard Brook, scientists have found an annual cycle of stream nitrate that reflects a strategy the trees have evolved in response to a predictable environment.

When leaves  drop in autumn, nitrate export in streams peaks. Leaves fall both directly into the stream and onto the surrounding soil, where they are decomposed and their nutrients can be leached away. Over time, this process would lead to a dramatic reduction in the nitrogen held in the system, if not for the annual winter replenishment of nitrogen through snowfall. Snow contains nitrogen just like rain does, and as it builds up on the forest floor during the winter, a stockpile is created for the next growing season. Trees have timed their first buds to break in the spring just as this snow is melting and releasing a pulse of nitrogen into the earth. This is essentially how things work at Hubbard Brook, in the absence of human influence.

However, Hubbard Brook, situated just north of the Washington-New York-Boston urban corridor, is by no means isolated from human pollution. Nitrous oxide gases emitted from fossil fuel combustion dissolve in the atmosphere and are rained out across the world, often at great distances from the source. Since at least the 1960’s, excess nitrogen has been entering Hubbard Brook as nitrate through this process. As a negatively charged anion, nitrate often picks up a hydrogen cation, H+, to balance its charge. “Hydrogen cation” is basically a fancy term for acid (remember ocean acidification?) The greater the concentration of hydrogen in solution, the more acid a solution is. Nitrogen pollution, ecologists realized, was doing something very strange to rainwater chemistry at Hubbard Brook. The concept of acid rain was born.

Thus is should not be surprising that another more powerful trend overlies the annual nitrogen cycle at Hubbard Brook. From first measurements in the late 1950’s until 1970, scientists observed a steady, annual rise in stream water nitrogen export. Since 1970, however, nitrate concentrations have been dropping. This has been attributed in large part to the passage of the Clean Air Act and the subsequent reduction in nitrous oxide emissions from automobiles.

Here’s what is causing ecologists to scratch their heads. Nitrogen levels in Hubbard Brook streams have now been dropping steadily for forty years. They are approximately at their 1960 level, and this downward trend shows no indication of leveling off. This reduction is dramatic, unexpected, and not accounted for by the emissions reductions enforced through the Clean Air Act. It is becoming increasingly clear that something else about the ecosystem has fundamentally changed.

What exactly is happening to cause reduced nitrogen outflow from Hubbard Brook forests concerns ecologists for a variety of reasons. The main reason ecologists are concerned is because the forest has stopped growing. In fact, it is now thought that Hubbard Brook may be losing biomass.

Could reduced nitrogen loss reflect increased nutrient limitation in the forest, and a need for trees to hold tightly to the nutrients they have? Could it mean rainfall is no longer supplying the nitrogen the forest needs? Could the soil microbial community no longer be decomposing and transforming nitrogen in the manner that the forest has depended on for thousands of years? Could it possibly reflect some other fundamental hydrologic shift, perhaps induced by climate change?

The most compelling theories in my view are that the phenomenon of “missing nitrate” is climate change driven, or driven by changes in soil chemistry and loss of important soil cations. I’ll explain both of these ideas briefly and leave you to ponder.

Climate change is certainly causing other weird stuff to happen at Hubbard Brook. The forest typically experiences sub-freezing temperatures all winter and builds up a healthy snowpack. Snow insulates the soil and streams alike, preventing them from freezing. This allows frogs, salamanders and some fish to hibernate over winter without freezing. Recently, warmer and more varied winter weather patterns have lead to reduced snow cover, with the consequence that the soil and stream water are now freezing instead. (The number of frozen salamander bodies discovered in the winter of 2006 was apparently unprecendented). Loss of snow cover is also disrupting the annual nitrogen cycle, which could well be leading to a springtime nitrogen limitation for growing trees.

Calcium, which typically takes the form Ca2+ in soils, turns out have a high affinity for nitrate. The two ions balance each other’s charge, and when nitrate leaches out of soil, it often brings a calcium ion along with it. Actually, it often requires the charge-balancing association provided by calcium to leave the soil in the first place. Well, calcium turns out to be another very important plant nutrient, and unfortunately is not as quickly replenished through rain as nitrogen. Declining soil calcium levels will, over time, produce soil that leaches nitrate less readily. Nitrate loss in streams could thus be indicative of a serious calcium deficiency. Incidentally, sugar maple, a common species in New Hampshire forests and a calcium-accumulating tree, is dying out and may be entirely lost these forests within the next several decades.

What’s up with nitrogen at Hubbard Brook? No one really knows yet, but we’d sure like to figure it out.

In trying to paint a picture of the dynamic nature of nitrogen at Hubbard Brook, I’ve left quite a bit of detail out and haven’t discussed some of the fascinating experimental manipulations that have been conducted at the forest. Hopefully I’ve touched on enough of the important ideas that this ramble provides some sense of the many layers of complexity that ecologists must piece through to understand a system or solve a problem.


The title and much of the content of this article I’m crediting to Dr. Gene Likens, a senior ecologist at the Hubbard Brook experimental forest who provided fascinating information on the forest during a conversation and a guest lecture at the University of Pennsylvania.

Earthworms play key role in regulating carbon storage in tropical ecosystems

A principle frontier in our understanding of global carbon budgets is tropical forests, on which research is historically scarce. At temperate and high latitudes, a warmer climate is predicted to increase the rate of decomposition and soil carbon turnover, resulting in a positive feedback on atmospheric carbon as CO2 is released from soils at increasing rates. A better understanding of the mechanisms regulating tropical carbon storage is needed in order to develop a holistic picture of global carbon cycling and feedbacks due to climate change.

Earthworms are important regulators of many ecological properties of soils. Their burrowing activity increases soil pore space and contributes to soil structure and drainage. Most importantly, earthworms can digest a huge quantity of dead and partially decomposed plant material. This digestion causes chemical transformations that ultimately produce nutrient-rich soil organic matter, or SOM. SOM helps ensure soil fertility, and contributes to numerous physical and chemical soil properties such as soil structure, porosity, water retention, and the capacity of soils to buffer pH changes. SOM’s aggregate structure causes it to have high water stability. This is an essential property in tropical forests, which have the highest rainfall levels of any biome on Earth.

SOM produced by earthworms is also rich in both carbon and nitrogen. A detailed biochemical and molecular analysis of earthworm casts suggests that these creatures may in fact play a key role in controlling tropical carbon storage.

Casts are clumps of digested organic matter excreted by earthworms that aggregate into large and distinctive structures. Researchers working in the rain forest neighboring the Dong Cao village in Northeast Vietnam studied the effect of cast production by Amynthas Khami on soil C storge. A. Khami is a species of tropical earthworm that can grow up to 50 cm long and produce tower-like casts. The researchers first used a “simulated rainfall” experiment to determine the relative stability of casts versus control soils. They then measured total carbon content, lignin and mineral-bound SOM content of casts and control soils.

An earthworm cast produced by A. Khami, a large tropical species found in Northeast Vietnam.

The study found striking differences in the chemical composition of earthworm casts versus control soils that ubiquitously indicate higher carbon storage in casts. Casts are more structurally stable and can withstand at least twice as long a rainfall event as control soils without compromising their structural integrity. They are enriched in carbon compared with controls, and particularly in carbon compounds such as lignin that have a high “carbon storage” potential. Lignin, a primary constituent of woody plant tissue, is a complex and heterogeneous molecule that is both carbon-rich and difficult for microbes to decompose. Earthworms probably excrete high quantities of lignin after obtaining the more digestible carbon sources from the roots and leaves that they eat. Finally, high levels of mineral associated-SOM were found in casts. Soil minerals bind to organic matter through electrostatic interactions, and in doing so make it unavailable for decomposers.

Though it well known that earthworm digestion initially speeds up decomposition, this new study suggests that casts may in fact contribute to long-term carbon stabilization. In tropical soils, which tend to cycle carbon quite rapidly, this mechanism should not go unappreciated. Future tropical land-use decisions may want to account for the welfare of this often-unappreciated soil organism.

Hong et al. 2011. How do earthworms influence organic matter quantity and quality in tropical soils? Soil Biology and Biochemistry 43: 223-230.

Fossil forams provide surprising insight into ice age oceans

In the North Atlantic, ocean water circulation patterns have far-reaching effects on global climate. Convective mixing is a dominant process due to thermal stratification of the water column. At low latitudes, warm, low-density surface waters float over a mass of much colder, high-density subsurface water. As warm surface water travels north, the temperature difference between surface and subsurface is diminished. Nutrient-depleted surface water cools and sinks, forcing deep water to rise. As deep water rises to the ocean surface, it brings a fresh pulse of nutrients that causes enhanced ocean productivity near the poles.

The formation of North Atlantic deepwater, or NADW, and the continual circulation of warm, subtropical water, play an important role in moderating Arctic climates. In colder intervals of Earth’s history such as the Last Glacial Maximum (LGM) 20,000 years ago, diminished thermal stratification reduces open ocean convection. Less surface water is transported poleward, and the water that is does not have the same warming effect on the local atmosphere and land surfaces.

This much about the interaction between North Atlantic circulation and climate is well understood. However, the timing of changes in NADW circulation and corresponding changes in climate remains something of a mystery. Scientists essentially face a chicken and egg problem- do climate changes shut down this oceanic conveyor belt, or does the shutdown of the conveyor belt occur first, by some other means entirely, but cause subsequent feedbacks on climate?

Currently, the climate change-induced NADW breakdown theory is popular and has been used to explain a number of abrupt climate reversals. The most prominent example is the Younger Dryas (YD), a brief cold-snap that occurred some 12 millions years ago following the end of the LGM and the retreat of continental glaciers. Proponents of this theory argue that glacial melting caused huge pulses of low-density freshwater into the north Atlantic, in precisely the region where vertical stratification is weak today and convective mixing occurs.  This influx of low-density water effectively shut down NADW formation, leading to a rapid cold reversal and a brief but dramatic rebound of continental glaciers.

A recent study using carbon isotopes found in fossil foraminifera, or forams, to date ocean water columns suggests otherwise. 14C is a heavy isotope of carbon that is produced in the upper atmosphere due to cosmic ray activity, and enters the surface ocean as a dissolved gas.  It is a popular isotope for radiometric dating, as it decays to 12C over a known period of time. The quantity of 14C remaining in a sample can thus be used to determine the sample’s age. A decreased 14C/12Cratio indicates an older sample. Indeed, numerous studies suggest that 14C depleted water is associated with decreases in convective mixing.

Fossil foraminifera, a popular organism for radiometric dating studies to reconstruct past climates

But how does one find 10,000 year old water to date and study in the first place? Scientists can’t simply put a bucket into the ocean and pull up 20,000 year old water to- they need a fossil or preserved object from the time period of interest. Some planktonic organisms such as forams leave behind a calcareous exoskeleton when they die. If buried quickly, these can be preserved for thousands or millions of years. While many planktonic organisms preferentially take up 12C over 14C, skewing the natural ratio of the two isotopes in their body tissue, forams do not significantly alter the natural 14C abundance. Examining fossil forams buried in ocean sediments thus provides a window into the past, allowing an accurate date to be ascribed to the ocean that the tiny creature existed in.

What are fossil forams from the North Atlantic telling us about ice age oceans? Proponents of the glacial melt water-induced NADW shutdown theory, and fans of “The Day After Tomorrow”, will no doubt be surprised by the finding that deepwater from the YD era actually dates to 600 years prior to the cold reversal. The shutdown of the oceanic conveyor belt prior to global cooling suggests that an unknown mechanism may in fact be driving ocean circulation, and in doing so exerting a powerful control on global climate.

Thornalley et al. 20110. The Deglacial Evolution of North Atlantic Deep Convection. Science 331: 202-205.

What makes an urban bird?

Globally, industrialization is causing human populations to migrate into cities. As a result, urbanization is among the fastest growing land use changes. The necessity to see urban environments as true ecosystems in order to understand what these environments can provide for displaced species is a topic of growing importance.

Ornithologists are now attempting to determine what life history traits can predict a particular avian species’ probability of surviving an urban transition. To identify traits that make a species vulnerable to urban development, one group of researchers used a data set comprising avian responses to towns and cities for a <3,000 square kilometer, highly urbanized British region.

Previous research has found that migrating birds tend to disappear more quickly than permanent residents. Migrants may be disadvantaged when competing for more limited urban resources with permanent residents that have established territories. Migrants also seem to be more susceptible to mismatches in the timing of breeding and peak food availability. Such mismatches are becoming more common due to climate warming and increased climate variability, and are exacerbated in urban regions due to urban heat-island effects.

This recent study determined two new factors driving survival patters. The first important ‘survival factor’ is diet. Birds that feed on plant material have higher survival rates in urban environments than insectivores. High levels of “supplementary” vegetarian food supplies, including seeds and processed plant material, often characterize urban regions. Species that can exploit an entirely vegetarian diet may therefore be at an advantage over insect-dependent species.

The second important survival factor determined in this study is nesting location. Birds that nest above ground fare much better in urban environments than ground nesters. Obvious reasons for this include increased danger of ground nest destruction due to pedestrians, bicycles and automobiles. Ground nesters may also be at much higher risk of predation, due principally to the fact that they tend to produce broader, more open-cupped and vulnerable nests.

Studies quantifying traits that increase an organism’s survival chances due to urbanization are important not only in predicting which species will survive but in developing more conservation-friendly urban landscapes. For example, this study indicates that maximizing the availability of insect food sources, which could be achieved by increasing urban green space, may increase avian diversity. In addition to increased green space, the implementation or larger patches of green space may reduce intense resource competition that occurs on small, fragmented habitat patches. Improved suitability for ground-nesting species may be more challenging to implement, but is an important goal for city planners to keep in when designing new urban developments.

Evans et al. 2001. What makes an urban bird? Global Change Biology 17: 32-44.

Ice crystal formation in clouds stimulated by marine diatoms

The formation of ice crystals in the atmosphere is often facilitated by the presence of small, airborne particles that serve as a “nucleation” site for the growing crystal. Ice nucleation, with or without airborne particles, plays a large role in cirrus cloud formation. However, airborne particles allow ice crystals form in warmer, mixed-phase clouds that would otherwise have been ice-free.

A recent study published in Nature Geoscience reports that a common planktonic diatom, Thalassiosira pseudonana, can actually serve as a nucleation site for ice crystals. Diatoms are single-celled, marine photosynthetic organisms that are most famous for their often beautiful, glassy, silica-rich shells. They are found worldwide and are particularly abundant in cold, nutrient-rich ocean waters, such as the northern Pacific and Antarctic. Samples of T. pseudonana were exposed to water vapor and a supercooled salt solution under “typical tropospheric conditions” (ie, conditions that diatoms would be exposed to in the region of the atmosphere where cirrus-cloud formation takes place). The researchers found that the presence of diatoms in water allowed ice to form at substantially higher temperatures, and that the rate of ice nucleation in the presence of diatoms was generally rapid.

Thalassiosira pseudonana, a planktonic diatom

Small organisms that they are, the ability of diatoms and possibly other phytoplankton to initiate ice nucleation in clouds may have profound effects on climate. Increased ice crystal production due to diatoms could mean more incoming solar radiation reflected away from the earth by clouds (remember albedo effects?). Thus, diatom fragments in clouds may in fact increase the cooling potential of clouds (clouds are also important climate warmers, the water vapor contained within them is a powerful greenhouse gas).

A warming climate has been linked to changes in diatom populations. Warming is expected to lead to selection for smaller species of diatoms, which could be more easily aerosolized. Furthermore, warming may increase diatom populations due to enhanced ocean nutrient availability and decreased Arctic sea ice cover. These processes would both result in an increased concentration of diatomaceous aerosol material in clouds, leading to increased ice-crystal formation. Tiny glass cells, swept up unwittingly and unwillingly from their oceanic homes, may prove an important climate driver as they build icy shelters in the clouds.

Knopf et al. 2010. Stimulation of ice nucleation by marine diatoms. Nature Geoscience 2: 1037.