Thursday, 20 December 2012

A Word on Methodology: The Sporormiella Proxy

In my previous post, I mentioned the Sporormiella proxy used to determine abundance of Pleistocene megafauna in Madagascar. This is an analytical technique that has recently gained prominence in the study of the late Pleistocene megafauna extinction. There is an interesting paper by Feranec et al (2011) about the Sporormiella proxy and the problems associated with using it.


The Sporormiella Proxy
Sporormiella is a fungus that is present on the dung of herbivores. Sporormiella sporulating on dung release spores which adhere to nearby objects (usually plant matter). Herbivores then eat this plant matter and the spores, which pass through their digestive tracts, are released in their dung. The spores of this fungus are preserved readily in lake sediments, and stratigraphic changes in the abundance of this fungus in Pleistocene and Holocene sediment sequences have been used as a proxy to define megafaunal presence, decline and extinction globally.


Sporormiella Spores 
Problems
The presence of Sporormiella is not exclusive to large herbivore dung and has been found in the dung of small herbivores as well, such as hares. Thus, it is difficult to use Sporormiella as a sole and direct proxy for megafauna abundance unless specific species of Sporormiella associated only with large herbivores can be identified.

A stratigraphic decline in Sporormiella does not necessarily indicate a decline in megafauna. For example, Sporormiella is more abundant near lake shores than in the middle of lakes, so a decrease could simply mean a rise in the lake level. Sporormiella may also be preserved to varying degrees depending on type of lake sediment, lake levels, etc. A related point is that the absence of Sporormiella does not indicate the absence of herbivores – some modern day sites with abundant livestock have been shown not to contain Sporormiella in Davis and Shafer’s (2006) study. Thus, Sporormiella needs to be calibrated to other indicators of large herbivore population and is non-conclusive on its own.

Some academic papers must be viewed with some scepticism due to methodological over-reliance on this particular proxy. For example, in a Gill et al (2009) paper, a decline in Sporormiella in a Lake Appleman core in Indiania which starts from 14,800 years ago and which pre-dates a major change in the pollen assemblage is used to conclude that the late Pleistocene megafauna extinction was not caused by (usually climate-linked) vegetation changes. They also show that charcoal frequency increased at that site, indicating that human factors (like vegetation burning) were probably behind the extinctions. However, the tail end of the Sporormiella decline is also associated with a change in lake sediment size, which may reflect changes in the sediment input and hence catchment area of the Sporormiella source, rather than megafauna decline.

Conclusion  
While this analytical technique is certainly promising in contributing to research on Pleistocene megafauna extinction, it still needs to be refined. What is also important is to avoid complete reliance on just one proxy; the conclusions drawn from using this proxy should be calibrated to other indicators of megafauna abundance.

References

Davis, O. K. and Shafer, D. S. (2006) ‘Sporormiella fungal spores, a palynological means of detecting herbivore density’, Palaeogeography, Palaeoclimatology, Palaeoecology237, 1, pp. 40-50.

Feranec, R. S. et al (2011) ‘The Sporormiella proxy and end-Pleistocene megafaunal extinction: A perspective’, Quarternary International, 245, 2, pp. 333-338

Gill, J. L. et al (2009) ‘Pleistocene megafaunal collapse, novel plant communities, and enhanced fire regimes in North America’, Science, 326, pp. 1100-1103



2 comments:

  1. Hey there! Someone in my Myspace group shared this website with us so I came to check it out.I’m definitely loving the information. I’m book-marking and will be tweeting this to my followers!Terrific blog and fantastic design and style.

    Bubble
    www.gofastek.com

    ReplyDelete
  2. I love your blog. Keep it up.Visit my site too.

    n8fan.net

    www.n8fan.net

    ReplyDelete