The stunted growth of coral reefs.

 
New York City shelters its 8.3 million residents within a complex, three-dimensional matrix of concrete and steel. Construction crews must work on the integrity of the city’s structural skeleton constantly just to keep up with growth and age. Like a living organism, the city is expanding up, down and outwards while repairing and replacing decrepit trusses and frames with new, high quality materials.

It is a common big-screen fantasy to imagine the physical decay of New York City if these construction efforts were to suddenly cease in some kind of post-apocalyptic madness. However, the structural collapse that would bring New York City back to nature is actually occurring within nature, and you don’t need CGI graphics to imagine it. Just last week a team of scientists shared new findings that warming oceans are reducing the size and strength of coral reefs in two different parts of the world.

A coral reef is not unlike a city. Reefs are massive underwater edifices that support a stunning diversity and density of organisms. But instead of using building materials like concrete and steel, coral reefs house their residents within and upon a skeleton of calcium carbonate. This calcium carbonate is continually laid down by the reef's construction crew – growing colonies of coral polyps.

Individual coral polyps are actually small, soft-bodied animals that grow affixed to a hard surface. For protection they secrete calcium carbonate near their base. This calcium carbonate accumulates as the polyps grow, clone themselves, and multiply into large colonies. Over many generations, the calcium carbonate left behind becomes the skeleton of the reef, which still consists of a living colony of coral polyps on the surface.

The ability of coral polyps to produce calcium carbonate, called calcification, depends on a host of environmental factors. For example, seasonal differences in water temperature cause calcification to increase in the summer and decrease in the winter, resulting in alternating layers of high and low calcium carbonate density. It creates a pattern similar to the annual growth rings of a tree. The scientists used this pattern to measure the extent of growth and the density of calcium carbonate produced every year by coral colonies. They focused on two genera, Porites and Montastraea, from the Great Barrier Reef and Mesoamerican Barrier Reef. They compared these annual growth data to existing records of warming sea surface temperatures spanning at least a decade.

One of the coral genera, Porites, showed the greatest sensitivity to warming temperatures regardless of location. In both the Great Barrier Reef and the Mesoamerican Barrier Reef, Porites experienced sharp declines in calcification rate as sea surface temperatures increased. If climate change models are correct, at the given rate of decline, Porites in the Great Barrier Reef will stop laying down calcium carbonate entirely by 2100. In the warmer Mesoamerican Barrier Reef, calcification by Porites will cease in 2060. The genus Montastraea has also experienced reduced rates of calcification, although not as extreme. Montastraea in the Mesoamerican Barrier Reef will experience a 40% reduction in calcification by 2100.

The consequences of reduced calcification will manifest differently in the two genera. As temperatures rise, Porites produces calcium carbonate at the same density, but compensates for reduced calcification by not extending as far. Therefore, Porites reefs will grow more slowly and could be outcompeted for space by other organisms. Montastraea continues to extend in warmer temperatures, but it does so at the expense of calcium carbonate density. Much like osteoporotic bones, a Montastraea reef with reduced calcium carbonate density is more susceptible to physical and biological damage. Warming water temperatures will compromise both types of reefs in their ability to support biodiversity, either in terms of space or strength.

As grim as they seem, these predictions are likely conservative. The scientists mention that they don’t consider other factors that affect calcification such as coral mortality, coral bleaching, disease, and the negative consequences of pollution, erosion and other environmental concerns. The slow decline of coral reefs may not be fodder for a disaster flick, but piles of stunted, brittle coral reef will be utterly disastrous for the world’s oceans in a time that is already considered a biodiversity crisis.

Carricart-Ganivet JP, Cabanillas-Tera´n N, Cruz-Ortega I, Blanchon P (2012) Sensitivity of Calcification to Thermal Stress Varies among Genera of Massive Reef-Building Corals. PLoS ONE 7(3): e32859. doi:10.1371/journal.pone.0032859 

Photo: A view of the Mesoamerican Barrier Reef, Belize en.mesoamericanreef.org

Wetland restoration: Is it worth the effort?

If forests are the lungs of our planet, then wetlands are surely the liver. Wetlands filter overland runoff through a unique suite of plants and soil microorganisms that break down harmful compounds, ensuring that waters reaching streams and coastlines are contaminant-free. Wetlands also store a massive amount of carbon in their soils, serve as a nursery ground for many fisheries, and protect shorelines from floods and storm surges.

Despite these benefits and more, humans allowed worldwide wetland degradation to continue unchecked for two hundred years. Instead of protecting existing wetlands, we gamble on restoration efforts, investing over $70 billion to bring back functional wetlands in North America alone. Until now the overall outcome remained unclear. But after investigating 621 wetland restoration projects around the world, a team of scientists led by Dr. David Moreno-Mateos and renowned ecologist Dr. Mary Power of the University of California Berkeley determined that wetland restoration is generally a losing bet. They published their results last month in PLoS Biology.

The team combed the scientific literature for studies that compare restored and newly created wetlands to adjacent undisturbed wetlands. They compiled all the data from these studies to find out how three characteristics of the restored or created wetlands recover over time: hydrology, biological community, and biogeochemistry.

Wetland hydrology includes water level, flooding patterns, and water storage. It recovers quickly because it depends on the surface features engineered by restoration teams. Before they place a single plant in the ground, restoration teams manipulate soil levels and permeability to achieve ideal patterns of wetland hydrology. The biological community, however, is much harder to recreate. Even after 100 years, biological communities in restored wetlands only recovered to 77% of their undisturbed counterparts. Moreover, plant communities recovered more slowly than animals.

Biogeochemistry, the movement and transformation of nutrients in a system, only recovered to 74% after 50 to 100 years. A healthy, functioning wetland stores large amounts of carbon and organic matter, because their flooded soils deprive decomposing microorganisms of necessary oxygen. If wetlands dry out during a disturbance, oxygen reaches decomposers, allowing them to break-down carbon compounds and larger pieces of organic matter. The decomposers eventually release carbon dioxide to the atmosphere as a byproduct of this process. Once restoration teams restore the proper wetland hydrology, decomposition shuts down and carbon begins to build up in the soil once again. However, even after twenty years, the amount of carbon and organic matter stored in wetland soils remains significantly less than levels in undisturbed wetlands.

Dr. Power's team suggests that restored wetlands do not resemble their original condition because they have shifted to alternative states. Much like building a house, successful ecosystem restoration requires that all the right pieces come together in the correct order. If a construction crew lays out the wrong foundation or neglects to build the second floor, the house will look completely different from the blueprints. The result of this botched construction project is like an alternative state. In ecosystems, the presence or absence of certain organisms determines which organisms come next. If a restoration team fails to introduce an important organism, or does so at the wrong time, a restored wetland may give rise to an alternative state that differs from the goal.

When it comes to wetland restoration, we may know which plant species to add, but the proper foundation remains unknown. Wetland plant communities and biogeochemistry depend on soil microorganisms. If the microorganism community shifts during wetland degradation, or if restoration requires imported soils, the soils may lack the microorganisms necessary for native wetland plants to grow. These unseen changes underground have cascading effects through the wetland ecosystem, producing an alternative state. Alternative states often host different organisms, function at reduced levels, and cannot confer the ecosystem benefits we depend on. Unfortunately, the only way to fix it is to start again from scratch.

Moreno-Mateos D, Power ME, Comı´n FA, Yockteng R (2012) Structural and Functional Loss in Restored Wetland Ecosystems. PLoS Biol 10(1): e1001247. doi:10.1371/journal.pbio.1001247

Photo: Vermont Department of Forests, Parks and Recreation

The ocean's most exclusive community.

The great thing about science is that questions lead to answers. The bad thing is that through this process, a subject that was once novel and strange slowly loses its mystique. When the thrill is gone and the mysterious becomes mundane, the jaded biologist longing for that delicious scientific buzz need only look down - way way down. And, oh my GOODNESS, a really thrilling bit of science was just pulled up from the uncharted ocean depths and published in PLoS Biology.

Deep-sea hydrothermal vents! Is there anything more amazing? They are remote like outerspace but with thriving communities of freaky biota. And as a team of researchers, led by Dr. Alex Rogers of Oxford University, recently found – if you’ve seen one you have NOT seen them all.

These ecosystems are so mystifying because they are fundamentally different from the ecosystems we are used to seeing. Whether you are in tropical rainforest, arctic tundra, or open ocean, nearly all food webs are built upon the plants and algae that harness sunlight to transform carbon dioxide into organic molecules. Photosynthesis is incredible, but also pedestrian. Things get really strange when you look into the darkness and find bizarre organisms that have capitalized on a different energy source – poisonous, smelly hydrogen sulfide gas.

Deep-sea hydrothermal vents, found at an average depth of 2100 meters, spew plumes of hot water from the earth’s crust. This water can be as hot as 400˚C and contains high concentrations of hydrogen sulfide. The surrounding water is nearly freezing and dark as night with pressures so great it keeps the hot plumes from boiling. Still some organisms have managed to thrive in this oppressive environment. Not surprisingly, it all comes down to the microbes. Bacteria and archaea living in and around the vent utilize the energy stored in the bonds of hydrogen sulfide to fix carbon dioxide into organic molecules. This process, known as chemosynthesis, was only a theory until it was observed in action at the hydrothermal vents of the Galapagos Ridge in 1977.

The unusual properties at the base of the vent food web radiate up through all the animals it supports. The giant tube worm that hosts chemosynthetic bacteria within its body is the most familiar image. In many ways it has assumed the role of the community’s iconic species. That is until Dr. Rogers and his team restored the mystique of the hydrothermal vent ecosystem.

Departing from the vents of the tropics and subtropics that are relatively easy to access, the team examined the communities on the East Scotia Ridge (ESR), 500 km to the east of Cape Horn between South America and Antarctica. At a depth of more than 3000 meters in the Southern Ocean, the ESR has two ridge segments with hydrothermal activity, E2 and E9. A deep-sea drive by the remotely operated vehicle Isis revealed that these areas are completely devoid of the tubeworms, polychaetes, clams, and shrimp that we’ve come to expect in hydrothermal vent communities. Rather, they host a complex community of endemic organisms – organisms that haven’t been seen anywhere else – notably a new species of crab, stalked barnacles, limpets, snails, sea anemones, and a seven-armed starfish.

The biological diversity of these areas is built upon the diverse landscape. In some spots chimneys as tall as 15 meters release concentrated plumes of mineral-rich water from the Earth’s crust. This water emerges at temperatures exceeding 300˚ C, and when it hits the near-freezing water of the ocean floor, the minerals fall out of solution and create that black smoker appearance. In other areas there is more diffuse vent flow with temperatures closer to the surroundings. Even between the two sites there is variation in the chemical composition of the vented liquid. These differences could affect the microorganism populations at the two sites which would have cascading affects up the food web.

The truly thrilling thing about the ESR discovery is not the strange biota, because, let’s be honest – finding new species in a remote habitat is old hat. The amazing thing is WHY the species are so strange and why the ESR community is different from the ones we see in similar ecosystems. While they seem inhospitable to us, hydrothermal vents are the only suitable habitat for these organisms. In that way they are just like islands out at sea or parks in an urban landscape. Biogeography is the study of species distributions across space – the traits of an organism that lead it to new areas and the barriers that stand in its way. And remarkably, when you consider all the geologic, hydrologic, and biologic pieces of the puzzle, it appears that hydrothermal vent communities suggest the same patterns of biogeography that govern terrestrial communities.

Deep ocean organisms face unimaginable hurdles to dispersal. Larvae might catch a ride on an ocean current, but many of them won’t last long before passing by another hydrothermal vent. These vents are found only at the boundaries of tectonic plates, which would serve as a great dispersal corridor if they corresponded with the currents. They don’t. Even more daunting is the surface to sea-bed Polar Front, which encloses the Southern Ocean and effectively blocks the entry of outside organisms. At the Polar Front water temperatures and salinity levels change abruptly, creating an insurmountable physiological challenge to most organisms attempting to cross. Knowing this it’s really no shock that the ESR has so many endemic species and so few of the usual suspects. With these barriers preventing migration, the populations of the ESR have been held in reproductive isolation for millions of years with the forces of evolution at work.

However, over geologic time scales ocean currents and plate movements are not even constant, which adds a whole new twist to the story! The hydrothermal vents appeared when the ESR began to spread – around 15 million years ago. That period corresponded with climatic conditions that made the Polar Front less intense, meaning that organisms dispersing from other vent communities actually had a chance to colonize this brand new environment. But the gates closed around 13.8 million years ago when the climate changed and the Polar Front strengthened.

Even more interesting is the phylogenetic history of one of the ESR endemics, which seems to corroborate the geologic and climatic stories. A new species of Kiwa crab, found in the vents of the ESR, is closely related to K. hirsuta of the nearby Pacific Antarctic Ridge. By looking at differences in their genetic markers, researchers loosely estimated that the two species diverged around 12.2 million years ago. Other ESR animals show similarity to species found in hydrothermal vents in the lower latitudes of both the Atlantic and Pacific. The dispersal of organisms from two oceans was likely aided by the Antarctic Circumpolar Current, which circulates around Antarctica, linking the Atlantic, Pacific, and Indian Oceans.

Dr. Rogers' team’s research adds another layer of complexity to the biogeography of vent ecosystems, even suggesting that the Antarctic vents comprise a new biogeographic province. For scientists and non-scientists alike it represents a whole new world of mysteries to be revealed, recharging our hope for big, exciting discoveries.

Rogers AD, Tyler PA, Connelly DP, Copley JT, James R, et al. (2012) The Discovery of New Deep-Sea Hydrothermal Vent Communities in the Southern Ocean and Implications for Biogeography. PLoS Biol 10(1): e1001234. doi:10.1371/journal.pbio.1001234

The Woods and Your Health

The opportunity to hike beneath the orange and red foliage of a deciduous forest in autumn is one of the greatest perks of living in New England. I spent a fair amount of time in the wooded margins of the Colby College campus, drawn out of bed at sunrise and enduring chilly mornings on the cusp of winter to collect field data. While studying the relative distribution and abundance of small mammals, I had several opportunities to walk beneath the fall foliage, wrapped in silence except for the sounds of our footsteps and our research subjects scurrying through discarded leaves. Despite the obvious ecological transition into dormancy, the New England woodland in autumn conjures feelings of freshness and vitality rather than decay. However, beneath the duff on the forest floor and within the variegated canopy lurks an invisible threat. Secretly, this serene setting could be a hotbed of disease.

Richard Preston’s The Hot Zone, painted a thrilling picture of an exotic disease festering in a remote jungle until it aggressively emerged into a vulnerable human population through an unconfirmed, but likely encounter with a reclusive animal. A more temperate, developed setting, however, is certainly not disease-free. Extremely virulent pathogens can lurk in your own backyard in natural reservoir species as common as the white-footed mouse. Zoonotic diseases, or those that can be transferred to humans from other vertebrate animals, are so common that the majority of the diseases that infect humans today originated in other animals. According to the CDC, approximately 75% of recently emerged infectious diseases, including West Nile virus, Eastern Equine Encephalitis, and Lyme disease, are zoonotic. The natural reservoir of these diseases is an animal that acts as a long-term host for the pathogen and for which an infection is non-lethal. The natural reservoir therefore acts as a continual source of a pathogen, which would otherwise eliminate itself by killing hosts too quickly to perpetuate the infection.

The emergence of new zoonotic pathogens in human populations has risen in recent years due to increased contact between humans and wildlife. As humans push back the margins of animal habitat for settlement and exploit animal resources through hunting and husbandry, we increase our contact with animals that may harbor pathogens, thereby increasing our exposure to zoonotic disease. Zoonotic pathogens do not require humans to complete their life cycle; under natural conditions the pathogen would stay within the animal population. If exposed to a disease reservoir or vector, however, humans can become an incidental host. Habitat destruction and resource exploitation increases our risk of infection by potentially deadly pathogens simply by accident.

My research partner and I were warned about disease exposure as we emptied Sherman traps, studying the relative abundance of the white-footed mouse (Peromyscus leucopus) and the deer mouse (P. maniculatus) in the margins of the woodland surrounding Colby College. Worldwide, rodents are natural reservoirs for over 35 diseases. The pathogens that cause these diseases can be spread to humans either directly through contact with saliva, urine, and feces; or indirectly via insect vectors. Lyme disease is a well-known and well-publicized disease that is caused by bacteria harbored in a mouse reservoir but spread by ticks as vectors. When summer ends and ticks overwinter on the forest floor, humans can still fall victim to the potentially deadly hantavirus pulmonary syndrome. Hantavirus is spread to humans by direct contact with the feces and urine of an infected mouse, causing renal failure and fatal hemorrhagic fever. While the symptoms sound like something straight out of The Hot Zone, there are no monkeys, fruit bats, or protective suits involved. Hantavirus can be contracted by inhaling aerosolized waste of common and abundant rodents, and hundreds of cases have been reported in the United States, especially in the western states.

While disease was not the primary focus of our research, it had very clear epidemiological implications. As humans encroach upon animal habitat, we upset the population balance of the ecological community by eliminating cover, introducing invasive species, and reducing food availability. Some species, such as the white-footed mouse, are generalists that can thrive in highly-disturbed, marginal habitat. Their populations tend to explode when specialist species populations decline. Therefore, as we push into deciduous and mixed forests, the white-footed mouse may benefit under the altered ecological conditions and increase in numbers, but overall biodiversity collapses. Incidentally, the white-footed mouse is a reservoir for both Lyme disease and hantavirus.

Zoonotic diseases are caused by pathogens that will readily infect a variety of different host species, but for which there are very few natural reservoir species. This means that only a few species can survive and spread the infection, thereby assisting the long-term survival of the pathogen. A high density of a natural reservoir species in a community would increase the presence of the pathogen and also increase the likelihood that the disease will be passed to another host or vector species. In contrast, high biodiversity would reduce the density of the natural reservoir species and increase the number of incompetent reservoirs that cannot spread the disease. It has therefore been hypothesized that biodiversity can prevent the spread of zoonotic disease and in this way protect human health.

In a ground-breaking 2000 paper in the journal Conservation Biology, Richard Ostfeld and Felicia Keesing applied this hypothesis to the spread of Lyme disease, calling it the “dilution effect.” They predicted that high species diversity within the host community would increase the likelihood that ticks would feed on animals that do not harbor the Lyme disease bacteria. As a result the infection power of the natural reservoir, the white-footed mouse, would be diluted. With a lower prevalence of infection in ticks, the vectors of the disease, Ostfeld and Keesing hypothesized that high species diversity would also reduce transmission of Lyme disease to humans. Sure enough, through analysis of state level ecological and epidemiological data, they found that states with a greater number of small mammal species in the host communities had fewer reported cases of Lyme disease per capita.

The work of Ostfeld and Keesling demonstrates that the dilution effect can apply to indirect infection through an insect vector. A recently published study in the journal PLoS One, however, took their theory one step further. While studying the relationship between small mammal biodiversity and the prevalence of hantavirus in Panama, scientists from the Museum of Southwestern Biology and the University of New Mexico found that the dilution effect holds up for direct infections as well. They theorized that when species diversity in a host community is reduced and the density of the natural reservoir species increases, infected reservoirs will have more encounters with uninfected reservoirs and therefore more opportunities to transmit the virus. In contrast, high species diversity would reduce the encounter rate and similarly reduce the transmission rate. After setting up experimental field plots, the researchers reduced species diversity within some of the plots by removing the non-reservoir species. When the number of species was reduced, both the density of the natural reservoir population and the prevalence of hantavirus among those individuals increased. From this finding one could hypothesize that high biodiversity would consequently reduce the rate of direct infection to humans who are exposed to the reservoir species.

The idea that biodiversity can offer a service to human health is not a new concept. In fact, when people mobilize to save large swaths of highly diverse habitat from destruction, human health is one of the most argued cases for its preservation. However, people usually argue that within these areas of high biodiversity may be un-studied organisms with compounds for potential pharmaceuticals or that can act as laboratory models. While certainly compelling, this case incentivizes the preservation of certain species of potential value, not necessarily biodiversity. The ability of a diverse community to dilute the presence of a zoonotic pathogen and reduce human exposure to some of the deadliest diseases is an even stronger correlation between high biodiversity and improved human health.



Ostfeld, Richard S. and Felicia Keesing. 2000. Biodiversity and Disease Risk: the Case of Lyme Disease. Conservation Biology. 14(3):722-278

Suzan, Gerardo, Erika Marce, J. Tomasz Giermakowski, James N. Mills, Gerardo Ceballos, Richard S. Ostfeld, Blas Armien, Juan M. Pascale, Terry L. Yates. 2009. Experimental Evidence for Reduced Rodent Diversity Causing Increased Hantavirus Prevalence. PLoS One. 4(5)

Photo: Peromyscus leucopus; Encyclopedia of Life