3 million years of a marine latitudinal diversity gradient

Perhaps the largest, most visible, macroecological trend on this planet is that there are generally more species in the tropics (low latitudes) than the poles (high latitudes). This pattern has been observed in both terrestrial and marine systems, across hugely varied groups of plants and animals. Despite the fact that this has been known for awhile now, no unified model exists that explains how this latitudinal species gradient came about or what mechanism(s) act to maintain it (intriguing hypotheses abound, however, including faster speciation in the tropics). What the structure of this gradient was like in the past is uncertain as well; for example, how stable this pattern is through time?

A recent paper in Ecology Letters by Dr. Yashuhara and colleagues uses paleotemperature information and diversity data of tiny planktonic critters called formaminfera (forams, if you’d like fit in) to investigate this latitudinal species gradient at various snapshots through time, over the past three million years.

Planktonic (residing in the water column while alive, rather than at the bottom of the sea in the benthos) forams as a groups are hugely abundant; deep-sea sediment in some parts of the world can comprise of almost 70% of forams, particularly their tests, or tiny calcium carbonate shells. Scientists have been studying forams in depth for some time now, in no small part due to their importance in studying past climate and oceanic conditions, and as a result their taxonomy is well-known. So if you’d like to take a detailed look at the latitudinal diversity gradient in the ocean over time, this is a good model system for that.

Using past datasets consisting of foram diversity and reconstructed sea-surface temperatures, the researchers investigated four specific temporal snapshots: modern, 18,000 years ago (Last Glacial Maximum), 120,000 years ago (Last Interglacial), and 3.3-3.0 millions years ago (in the Pliocene). They found that unimodal (think a relationship with a single hump-shaped peak) relationship between species diversity and latitude for all time points, with diversity generally peaking in the subtropics and falling off rapidly towards the poles. Additionally, used the reconstructed sea-surface temperature dataset, they found that species diversity tracks closely to temperature. For example, diversity was highest in the mid-Pliocene samples when it was the warmest of the time points considered here, and lowest during the last glacial maximum, the coldest. The authors even point out that the latitudinal diversity gradient was steeper during the last glacial maximum (i.e. the diversity in this set of samples decreased faster–at a steeper slope–towards the poles than the other time points), but this is likely due to the fact that the temperature is known to change more drastically with latitude during this time period.

This is important because it indicates that temperature has a big role to play in determining what species (and how many) live where. It also shows, as other studies have, that temperature is an important predictor of diversity in the ocean. This work’s central finding shows that this relationship has been robust at these various time points throughout the last three million years, even when examined at very high taxonomic resolution. Paleoecological work is really interesting and hugely useful in this vein; it can show us how things like climate can affect biodiversity over large timescales, and allow a different perspective of how large ecological patterns work.

Yasuhara, M., G. Hunt, H. J. Dowsett, M. M. Robinson, and D. K. Stoll (2012). Latitudinal species diversity gradient of marine zooplankton for the last three million years. Ecology Letters DOI: 10.1111/j.1461-0248.2012.01828.x

Debris patch from Japan’s tsunami en route to US

As if the tragic loss of life and ongoing nuclear woes weren’t enough, researchers Nikolai Maximenko and Jan Hafner at the International Pacific Research Center, University of Hawaii, predict that the massive deluge of debris that last month’s tsunami washed into the  sea is headed across the Pacific.  Using data from drifting oceanic buoys, the model predicts the debris will first spread out within the North Pacific Subtropical Gyre and start washing up in the Papahanaumokuakea Marine National Monument (NW Hawaiian islands) in a year.  In three years, the rest of the Hawaiian islands, the US West Coast, British Columbia, Alaska, and Baja California will see effects on their shorelines.  After the journey, the researchers predict that the debris will enter the North Pacific Garbage Patch and eventually get broken done into smaller particles.  In five years, Hawaii is expected to see another, more severe plume of oceanic trash.  Hopefully, these projections will help to inform clean-up responses.  The oceanic trash that does not either wash up on shorelines or sink, can end up in marine organisms.

The animation from the International Pacific Research Center shows likely debris path and timeline (press release, PDF).  Top image: Debris offshore of Honshu, Japan.  Image:  US Navy.  Hat tip:  Emmet Duffy at SeaMonster.

Is the Earth’s sixth mass extinction looming near and large?

Fossil fish sculpture. Rae Allen. CC BY 2.0.
Ideally, your expenses are offset by your paycheck, so as you spend money for say, rent and food, you have cash coming in.  On the surface at least, this is similar to the dovetailing of extinction and speciation.  The vast majority (~99%) of all the species ever to have existed on Earth are extinct, never to be seen alive again.  However, this process is balanced by new species evolving, a process known as speciation.  So what happens when species extinctions far outpace species creation?  Mass extinctions, timeframes during which 75% of species are lost in a relatively short period (usually less than two million years and sometimes significantly less), have occurred five times (the Big Five) in the past 540 million years.  They are unique, singular events that stand out above the background level of extinction that is constantly ongoing.  However, more and more modern extinctions are being observed, amidst the myriad of human-derived disturbances, such as rapid climate change, invasive species introductions, habitat fragmentation, directly killing species, among others.

It is not a straightforward task to ascertain whether or not we are on track to another mass extinction—data from the fossil record must be comparable to historic and modern species assessments.  In a paper recently published in Nature, Dr Anthony Barnosky and colleagues point out why data comparisons of this type are difficult and proceed to broadly get around them by looking at the global picture.

The fossil record is not evenly distributed across taxa or geographies.  Fossils are particularly meager in some broad swathes of Earth, such as the tropics.  On the other hand, distributions are known for many modern species.  In terms of taxa, or groups of species, usually only animals with hard bits fossilize well.  Studying modern species is easier, because they’re still around, but less than 2.7% of known species have been assessed for risks (e.g. endangered, extinct in the wild, etc.) by the International Union for the Conservation of Nature (IUCN).  There’s also trouble with the species concept.  Fossils are usually identified at the genus, rather than species, level; modern work frequently uses genetic approaches to identify individuals to species.  Fossils are also not distributed evenly through time.  Fossil extinctions are recorded when a certain group of animals vanishes from the fossil record, the extinctions known are likely underestimates since most species have no fossil record.

In spite of these caveats, the researchers evaluated the existing data and show that it is possible to circumvent these various data comparison issues by taking a ‘big-picture’ global approach.  Conservatively, mass extinctions occur when the extinction to speciation ratio becomes so unbalanced that three quarters of species disappear, usually within less than two million years.  If two million years sounds like a leisurely long time, bear in mind that the Earth is ~4.54 billion years old. Most living things forever blinking of out existence in roughly 0.04% of that time is a colossally unique situation indeed.

Using a rate-based method, the researchers compared extinctions per million species-years (E/MSY)1 from throughout the fossil record and modern time.  By using various paleontology databases and accounting for data biases, they were able to establish a background rate of extinction.  Through this approach, it is clear that the maximum extinction rates since about 1,000 years ago are much higher than the average fossil rate and the recent average extinction rates are also significantly higher when compared to pre-anthropogenic (that’s pre-us, mind you) averages.

Another way to consider this is by splitting up the fossil record into 500-year intervals and calculating the likelihood that extinction rates were as high in many of these 500-year intervals as they were in the most recent 500 years. In the case of mammals, which have an average of 1.8 extinctions per million species-years, only 6.3% of these 500-year segments could have extinction rates comparable to the current 500-year interval in order to preserve the background E/MSY.  So no, it is supremely unlikely that many of these past 500-year bins had extinction rates that were as high as they are today.

But would these current rates produce a large magnitude extinction event?  By using modern species assessments of very well-surveyed groups coupled with fossil data, Dr Barnoksy and his team calculate that extinction rates for mammals, birds, amphibians, and reptiles are as quick or quicker than all rates that would have been responsible for the previous five mass extinctions.  If all threatened species (defined by IUCN criteria) are lost within a century, and the current extinction rate continue, land-based amphibians, birds, and mammals would reach mass extinction thresholds in ~240 -540 years.  This slows down a bit if ‘only’ critically endangered species disappear within the next 100 years to ~890 – 2,265 years for those same groups of animals. Current extinction rates are higher or as high as those that preceded and caused the previous mass extinctions.  The researchers point out that while a pressing need for new research exists, the 75% species loss threshold could occur within the next three centuries.

Modern species losses are serious but does not pass the threshold for a mass extinction event yet.  Relatively small numbers of species surveyed historically have been lost, although scores of species have yet to be discovered and/or evaluated.  However, the researchers point out that losing critically endangered species would put us on the fast track to mass extinction, and losing endangered and vulnerable species would achieve the sixth extinction even faster, within a few hundred years. Sobering commonalities between the present-day and the past mass extinctions exist:

It may be of particular concern that this extinction trajectory would play out under conditions that resemble the ‘perfect storm’ that coincided with past mass extinctions: multiple, atypical high-intensity ecological stressors, including rapid, unusual climate change and highly elevated atmospheric CO2. [Barnosky et al.]

The diversity of life should be preserved while it’s still here.

 

1  Think of this as man-hours (or really person-hours).  On a purely mathematical basis, if you had one million species and an extinction rate of 1 per million species-years, one species would go extinct a year.

Barnosky AD, Matzke N, Tomiya S, Wogan GO, Swartz B, Quental TB, Marshall C, McGuire JL, Lindsey EL, Maguire KC, Mersey B, & Ferrer EA (2011). Has the Earth’s sixth mass extinction already arrived? Nature, 471 (7336), 51-7 PMID: 21368823

Image:  Rae Allen on Flickr (cc).

This article is also posted at The Urban Times.

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Tsunami in the Pacific: wave height and propagation models

The top image is a wave height model of the Pacific in the wake of the tsunami triggered by today’s 8.9 magnitude earthquake off Japan.  The waves are tallest near the earthquake epicenter and lessen with increasing distance, growing taller in coastal zones (but also decreasing in size with distance).  Tsunami waves extend through the entire water column to the ocean floor, unlike surface waves, so the faster the seafloor bathymetry changes from deep to shallow areas (second image shows 3D bathymetry offshore of Japan), the taller the waves could become.  The epicenter of this event was relatively close to shore, ultimately reducing the amount of water displaced, but still affecting much of the Pacific.

You can see NOAA’s animation of the tsunami propagation below:

For recent reports, maps, and a people finder, see Google’s Crisis Response site.  For ways to help, this is a good place to start.

Update:  For a narrated version of the NOAA animation, you can go here.

Images from the NOAA Environmental Visualization Laboratory (NOAA and NOAA Center for Tsunami Research).  See here and here for more information about how they were created and for some of the physics behind tsunamis.  Video from Youtube user ExWeather with NOAA data.

h/t pourmecoffee on Twitter for the NOAA images

Careening towards the sixth mass extinction

Even taking into account the difficulties of comparing the fossil and modern records, and applying conservative comparative methods that favour minimizing the differences between fossil and modern extinction metrics, there are clear indications that losing species now in the ‘critically endangered’ category would propel the world to a state of mass extinction that has previously been seen only five times in about 540 million years. Additional losses of species in the ‘endangered’ and ‘vulnerable’ categories could accomplish the sixth mass extinction in just a few centuries.

More thoughts to come.  In the meantime, please consider doing something about it.

From:  Barnosky AD, et al. Has the Earth’s sixth mass extinction already arrived? Nature 471: 51-57 doi:10.1038/nature09678

Deepwater Horizon Revisited

A recent study found oil and soot blanketing multiple areas of the seafloor in the Gulf of Mexico,  seemingly inundating the microbes that usually consume oil and leaving behind dead benthic animals.

Above is an absorbing and important lecture by Dr. Peter Roopnarine from last year on the ecosystem impact of the Deepwater Horizon disaster.  Dr. Roopnarine does really interesting work on mollusks, extinction, and food webs.

[Video source:  California Academy of Sciencescc]

A Planetary Experiment: Ocean Acidification and Biology

Ocean acidification is a relatively newly recognized threat to marine ecosystems. Even coral reef scientists, many of whom are now feverishly investigating the effects of changing seawater chemistry, ranked ocean acidification as 36th out of 40th potential threats to coral reef ecosystems in 2004 [1]. Recently, the magnitude of the shifting chemical balance in the ocean has become strikingly apparent [2].

Atmospheric carbon dioxide concentrations  are predicted to exceed 500 ppmv by 2100 [3]. Today, atmospheric carbon dioxide concentration is above 380 parts per million, a value not seen in the past 740,000 years, conservatively [4]. The ocean functions as a massive carbon sink and absorbs up to a third of atmospheric carbon. As carbon dioxide dissolves into seawater, it reacts to form carbonic acid, which dissociates to form bicarbonate ions and protons.

Read the entire article at The Urban Times , a recently launched online magazine that I contribute to, and will be acting as editor of Seas and Ocean content.  Check out the Sea and Ocean archive here, and you can follow updates via Twitter here.

Trioxygen lovelies. An ozone update.

 

Know your O3NASA reports that the ozone hole over Antartica is in the midst of its annual size increase. Remember that ‘hole’ is a metaphor here.  There’s still ozone there, just much less than historical values. So far this year, the hole seems to be smaller than the average from 1979-2009, likely due to warmer stratosphere temperatures in the southern hemisphere.

September 16th was the International day for the Preservation of the Ozone Layer.  Better late than never, right?

Image: Ozone concentrations on September 21 2010 from NASA’s Ozone Hole Watch.  The Ozone Hole Watch has all sorts of nifty animations and data, so explore!

 

Symbiotic Foreclosure: coral bleaching predictions and a potential acclimation mechanism

NOAA—the National Oceanic and Atmospheric Administration—issued a press release on September 22nd declaring coral bleaching likely in the Caribbean.  NOAA reports that:

With temperatures above-average all year, NOAA’s models show a strong potential for bleaching in the southern and southeastern Caribbean through October that could be as severe as in 2005 when over 80 percent of corals bleached and over 40 percent died at many sites across the Caribbean.

Comparing future bleaching outcomes to 2005 is an especially bad sign. 1998 and 2005 were the most thermally destructive years for coral reefs since record keeping began in 1880, damaging reefs in the Indian Ocean, Western Pacific and the Caribbean1. Reefs in the Indian Ocean and in parts of the Pacific have begun to recover after the particularly well-known 1998 bleaching event, yet the situation in the Caribbean has worsened due to regional mass bleaching, and the general lowering of reef resilience through sedimentation, nutrient pollution, and over-exploitation1.


Shallow corals are capable of feeding heterotrophically, that is, snagging bits of organic matter and plankton out of the watercolumn, but they overwhelming make their living by playing host to photosynthetic, intracellular dinoflagellate algae, known collectively as zooxanthellae (Symbiodinium spp.). These algal symbionts can be biochemically compromised by heat stress and can then be expelled by the host, an event known as bleaching; the coral will inevitably starve unless it can recruit more symbionts.  Bleaching can be caused by other factors as well2, e.g. heat/cold shock, heavy metals, prolonged darkness, among others; however, thermal stress appears to be a main factor, especially in the regional-scale bleaching that NOAA is predicting. The connection between increased seawater temperature and bleaching has been confirmed by laboratory studies and observed in the field3, particularly after El Nino/Southern Oscillation climatic events.  A recent article published in the New York Times generated some discussion on the NOAA-administered Coral List in the past couple of days in regards to designating 2010 as an El Niño or La Niña year.  We’ve been in a La Niña (characterized by cool temperatures in the Pacific) since May, but earlier in 2010, an El Niño (characterized by warm temperatures) was present.

The planet, including the ocean, is warming4.  One possible mechanism for corals to adapt to hotter climate regimes is the idea of ‘symbiont shuffing.’  Not all types of zooxanthellae are created equal. Some groups—namely type D—are better at managing higher temperatures without expulsion from the host coral; more importantly, shuffling to type D has resulted in increased thermal tolerance in at least one species of scleractinian (stony, reef-building coral).  But don’t excited yet:  we do not know if this symbiont switching is widespread, if particular coral species are capable of this at all, or if this phenomenon may incur some sort of physiological cost.  A study in PLoS ONE5 on the Great Barrier Reef worked to clarify this last tenet.  Jones and Berkelmans found in lab experiments that Acropora millepora corals grew 29% slower with type D symbionts, than those with C2 symbionts—a thermally sensitive type.  In the field, this skeletal growth difference was 38%.  If reefs acclimate via this mechanism, growth rates would be depressed if this trend was widespread among reef-building species.  How quickly certain types of corals grow is an extremely important facet of recovery after a disturbance; the authors point out that without timely coral regrowth, phase shifts can occur to an algal-dominated states within coral reef ecosystems.  This has happened at locales in the Caribbean, albeit for different reasons (the dieoff of a dominant grazer).  So although potentially beneficial in terms of adapting to increased temperatures, a switch to type D does not seem to be a boon to coral physiology, at least in terms of growth. A bleaching event during this study dropped growth rates across all symbiont types by more than 50% for a year and a half, showing (not surprisingly) that loss of their zooxanthellae has a larger impact than shuffling zooxthanellae types. Thermal stress coupled with the physiological cost associated with this acclimation mechanism are expected to interact, possibly further compromising reef resilience in a warmer world. Something that I’d like to add is that this shuffling is not known to ameliorate the effects of ocean acidification, which refers to the alteration of seawater chemistry (really a change in the carbon cycle itself) in a way that causes coral calcification to become more energetically difficult.

CO2 levels are expected to exceed 500 ppmv by 21006.  This level of change has not been seen in at least the past 420 thousand years, perhaps longer6.  The long-term fate of these extremely biodiverse oceanic centres is yet unknown. These animals are enormously resilient in one form or another on evolutionary and geological timescales, but the sheer rate of this environmental change is huge (2-3 orders of magnitude faster than most changes in the past 420 kyr6) and we may see drastic changes in the reef ecosystem functionality well within this century.

This post was chosen as an Editor's Selection for ResearchBlogging.org

Image 1: The Batman Comic Generator (this particular representation was inspired by A Replicated Typo‘s icon at Research Blogging).  Image 2:  Bleaching fire coral at Flower Gardens (Gulf of Mexico). NOAA-FGBNMS).

References:

1.  Wilkinson C (ed) (2008)  Status of coral reefs of the world: 2008. Global Coral Reef Monitoring Network and Rainforest Research Centre, Townsville, Australia, 296 p.
2.  Douglas AE (2003). Coral bleaching–how and why? Marine pollution bulletin, 46 (4), 385-92 PMID: 12705909
3.  Hoegh-Guldberg O (1999) Climate change, coral bleaching, and the future of the world’s coral reefs. Marine and Freshwater Research 50:839-866
4.  Solomon SD et al. (eds) (2007) Climate change 2007:  The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge UK and New York USA.
5. Jones, A., & Berkelmans, R. (2010). Potential Costs of Acclimatization to a Warmer Climate: Growth of a Reef Coral with Heat Tolerant vs. Sensitive Symbiont Types PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010437
6. Hoegh-Guldberg O, Mumby PJ, Hooten AJ, Steneck RS, Greenfield P, Gomez E, Harvell CD, Sale PF, Edwards AJ, Caldeira K, Knowlton N, Eakin CM, Iglesias-Prieto R, Muthiga N, Bradbury RH, Dubi A, & Hatziolos ME (2007). Coral reefs under rapid climate change and ocean acidification. Science (New York, N.Y.), 318 (5857), 1737-42 PMID: 18079392